· 41 The Reactivity of Bis(Benzamidinato) Yttrium Alkyl and Hydrido Compounds.∗ 3-1....
Transcript of · 41 The Reactivity of Bis(Benzamidinato) Yttrium Alkyl and Hydrido Compounds.∗ 3-1....
University of Groningen
Ancillary ligand effects in organoyttrium chemistryDuchateau, Robbert
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41
The Reactivity of Bis(Benzamidinato) Yttrium Alkyl
and Hydrido Compounds.∗∗
3-1. Introduction.
Recent reactivity studies on well-defined group 3 and lanthanide carbyl and hydrido
derivatives show a very rich and diverse chemistry.1 Dominant reaction pathways are insertion
of unsaturated substrates into Ln-X (X = C, H, N) bonds and X-H (X = C, H, N, O) bond
activation. In the latter case, the activation of saturated hydrocarbons like methane is amongst
the most intriguing.1c,e,2 Although some modifications of the ligand system have been
introduced,3,4 the most extensively studied system is Cp*2LnR (Ln = Sc, Y, lanthanides; R =
alkyl, hydride).1
Our interest in this field of chemistry is aimed at obtaining a better understanding of how
and to what extent electronic and steric properties of ancillary ligands influence the reactivity of
a complex. With the bis(N,N’-bis(trimethylsilyl)benzamidinato) yttrium alkyl (2.6, 2.7, 2.8) and
hydrido (2.9) complexes available (Chapter 2), we decided to explore their potential in C-X
activation and insertion reactions. To get a good picture of the similarities and differences in
reactivity of these new organoyttrium compounds compared with the known chemistry, the
choice of substrates was limited to those previously used to test the reactivity of the well
characterized yttrocene and analogous systems,1-3 Cp*[ArO]YR (ArO = O-2,6-
(CMe3)2C6H3)4d and [OEP]YR (OEP = octaethylporphyrin)4c (R = alkyl, hydride).
3-2. Thermolysis and Behavior of [C6H5C(NSiMe3)2]2YR (2.6 - 2.9) in Common
Solvents.
Before the reactivity of these compounds towards unsaturated substrates was investigated,
their behavior and thermolysis in common solvents was tested. When reactivity studies are
carried out and experimental data interpreted, the possibility of side reactions such as thermal
∗ Parts of this work were performed in collaboration with C. T. van Wee and E. A. Bijpost. The X-ray structure
determination described in this chapter was carried out by A. Meetsma (University of Groningen).
3
Chapter 3.
42
decomposition, C-X (X = H, hetero atom) of the solvent or ligand exchange should be taken
into account.
Aromatic and Aliphatic Solvents. At room temperature, the alkyl complexes
[C6H5C(NSiMe3)2]2YR ( = (µ-Me)2Li.TMEDA (2.6), CH2Ph.THF (2.7), CH(SiMe3)2 (2.8)) and
the hydride {[C6H5C(NSiMe3)2]2Y(µ-H)}2 (2.9) are stable in aromatic and aliphatic hydrocarbon
solvents (several months). Even at elevated temperatures (100oC), they remain intact for
prolonged periods of time (hours to days). This stability is quite surprising when compared with
their Cp*2LnR analogues (R = alkyl, H; Ln = Sc, Y, lanthanides) which undergo metalation of
the Cp* ligands upon thermolysis, affording fulvene species.1c,d-g,n,r, 5 Another interesting
feature of the bis(N,N'-bis(trimethylsilyl)benzamidinato) yttrium alkyls and hydride is their
complete lack of H/D scrambling and metalation of aromatic solvents; a dominant feature of
Cp*2LnR systems.1
Ethereal Solvents. The alkyls 2.6, 2.7 and 2.8 are stable in ethereal solvents (Et2O, THF;
room temperature). In contrast to {Cp*2Y(µ-H)}2, the hydride {[C6H5C(NSiMe3)2]2Y(µ-H)}2
(2.9) is stable in diethylether (2 weeks 25oC).1e In benzene, 2.9 does not react with THF (2
equiv.) but in neat THF (hours at room temperature), ethane and minor amounts (± 15 %) of
ethylene (Töpler pomp determination, GC-MS) are formed. Olefinic resonances in the 1H
NMR spectrum of the product mixture suggest the formation of yttrium enolate species as a
result of C-O bond activation.
Scheme 1.
RM +O
C
C
O H
H H
M OM MHM
- ethene (C)
O(CH2)4
HM OM MHM
- butane(B)
OM C
C
O H
H H
M- ethene (A)
- ethane
- R-H
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
43
Examples of C-O bond activation and metalation of THF by group 3 metal and lanthanide alkyls and
hydrides: (A): M = [C5H4Me]2Y, R = CH3.THF, CH2SiMe3.THF. (B): M = Cp*2Ln (Ln = Y, Sm), R = µ-
H. (C): M = Cp*2Ce, [C6H5C(NSiMe3)2]2Y, R = µ-H.
Cleavage of THF is known to proceed by a variety of reaction routes (Scheme 1). For BuLi6
and [C5H4Me]2YR.THF (R = Me, CH2SiMe3),7 ortho -metalation followed by extrusion of
ethylene has been proposed (Scheme 1A). The hydrides {Cp*2Ln(µ-H)}2 (Ln = Y, Sm) are
known to activate the C-O bond yielding the butoxo derivatives Cp*2Ln-O-nBu1n,8 In an
analogous reaction, [Cp*2Sm.THF2]+[BPh4]- reacts with Cp*K to form Cp*2Sm-O-
(CH2)4C5Me5.THF (Scheme 1B).9 In contrast, with THF the closely related {Cp*2Ce(µ-H)}2
yields a µ-oxo species with elimination of ethane (0.5 mol per mol Ce; Scheme 1C).8 An
unusual cleavage of THF was observed by Gambarotta et al. who isolated a vanadium
ynolate species on treatment of VCl3.THF3 with [OEPG]Li4.THF4 (OEPG =
octaethylporphyrinogen).10 The formation of ethane (quantitative) and of minor amounts of
ethylene (no hydrogen), in combination with the observed olefinic proton resonances in the 1H
NMR spectrum of the product mixture, suggest that the reaction of 2.9 with THF proceeds by
the same mechanism as proposed for {Cp*2Ce(µ-H)}2. The difference in product distribution,
compared to the reaction of {Cp*2Ce(µ-H)}2 with THF, indicates that the second C-O bond
activation, yielding the corresponding µ-oxo species, is slower in our system.
When reactivity studies are carried out and experimental data interpreted, the possibility of
side reactions, such as ligand exchange, should be taken into account. The reaction of the
chloride 2.2 with LiAlH4 demonstrated that under certain conditions, benzamidinato ligands
exchange rapidly (Cf. Chapter 2).
3-3. Reactivity Towards Alkenes.
Olefin Polymerization. Migratory olefin insertion into metal-carbon, metal-hydrogen1e,3a,e,4
and even metal-amido1o,3d bonds is a fundamental step, essential to catalytic processes
involving olefins. The capability of group 3 and lanthanide elements to catalyze these
transformations is clearly demonstrated by the fact that many of their well-defined
organometallic compounds are catalysts for olefin hydrogenation and polymerization with
activities approaching, or even exceeding, those of homogeneous systems based on other
transition metals.1c,3a,b
Chapter 3.
44
When benzene-d6 solutions of 2.9 were treated with ethylene (4 atm) no reaction was
observed. However, heating to 65oC resulted in the formation of polyethylene.11 1H NMR
spectra collected during the reaction showed the gradual consumption of ethylene and more
importantly 2.9 as the only organoyttrium compound present in detectable amounts. This
suggests that dissociation of 2.9 into the active monomer (initiation) is much slower then
propagation (Figure 1). This is supported by extensive studies of group 3 metal and f-element
catalyzed olefin insertion processes for which it has been found that unsolvated, monomeric
compounds are much more reactive than the corresponding oligomeric or solvated
compounds.1g,2,3e,4c,d Resonances attributable to ethylene oligomers or mixed hydrido-ethyl
species, [Y](µ-H)(µ-Et)[Y] ([Y] = [C6H5C(NSiMe3)2]2Y), as reported by Marks et al. for [Y] =
[µ,η5,η5-(C5Me4)SiR2(C5H4)]2Y3e and by Schaverien for [Y] = Cp*[ArO]Y,4d were not
observed. This indicates that propagation is fast relative to the rate of termination. Since the
affinity of 2.9 towards ethylene is low, reactions under more drastic conditions were carried out
(55oC, 70 atm ethylene). Although some increase in activity was found (4 g PE /
mmol(2.9).h),11 the total yield of polyethylene remained low.12
Y
N
N
NN R
Y
N
N
N THFN CH2Ph
Y
N
N
NN
HH
Y
N
N
NN
- THF
H2C CH2
R = H, CH2Ph
Polyethylene2.9
2.7
Figure 1. Slow dissociation of the dimeric hydride (2.9) or of the coordinated THF in 2.7, necessary
to create an open coordination site for an incoming ethylene molecule, is believed to inhibit the
reaction with ethylene.
To circumvent the inhibiting effect of hydrido bridges, the reaction of the monomeric
[C6H5C(NSiMe3)2]2YCH2Ph.THF (2.7) with ethylene was carried out. At 65oC (4 atm
ethylene), 2.7 polymerizes ethylene, although the rate seems to be lower than found for the
reaction of 2.9 with ethylene under the same conditions. Again, the 1H NMR spectra collected
during and after the reaction showed the gradual consumption of ethylene and 2.7 as the only
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
45
detectable organoyttrium derivative present. Like dissociation of 2.9 into the monomeric
hydride, replacement of the coordinated THF molecule in 2.7 by an incoming ethylene
molecule is assumed to be the rate determining step.
Surprisingly, with propylene (4 atm) and 1-hexene (neat), the precursors 2.7 and 2.9 were
left unreacted. Neither oligo-/polymerization or indication for the formation of an η3-allyl
species, as found for {Cp*2Ln(µ-H)}2 (Ln = Y, lanthanides),1c,f,13 was observed under the
conditions employed (days, 80oC).
Olefin Hydroboration. As part of a more general study on the hydroboration of alkenes
catalyzed by group 3 and 4 metals and lanthanides, the catalytic activity of the yttrium
bis(benzamidinato) compounds 2.7, 2.8 and 2.9 in hydroboration of 1-hexene with
catecholborane was investigated.14 All complexes proved to be catalytically active, but the
activity is about 5 - 10 % of that of the lanthanum complex, Cp*2LaCH(SiMe3)2, for which the
highest activity has been reported.1p
O
O
BH
HN
N
N
N
Y
Figure 2. Proposed intermediate for the hydroboration of 1-hexene, catalyzed by 2.7 - 2.9.
In all cases, fast catalyst deactivation occurs, probably due to the reaction of catecholborane
with the benzamidinato ligands. A possible explanation is transfer of the benzamidinato
ligands from yttrium to boron, comparable to the observed ligand exchange reaction of
[C6H5C(NSiMe3)2]2YCl.THF (2.2) with LiAlH4 (Chapter 2). Nevertheless, 2.8 and 2.9 showed
a substantially higher catalytic activity than Cp*2YCH(SiMe3)2.14 This is surprising, taking into
account that in the absence of catecholborane no reaction of 2.9 with 1-hexene was observed
at all. Possibly, catecholborane activates the yttrium catalyst by forming intermediates such as
shown in Figure 2. It could also be that transfer of ligands from yttrium to boron initially
activates the catalyst. A more extensive discussion concerning the hydroboration of α-olefins
catalyzed by the yttrium bis(benzamidinato) system, including experimental details, has been
given elsewhere.14
Chapter 3.
46
3-4. Reactivity Towards 1-Alkynes.
Selective dimerization of terminal alkynes, catalyzed by various transition metals has been
extensively studied.15 Until a few years ago, synthetic application of the catalytic dimerization
of 1-alkynes had been limited due to the lack of selectivity and the formation of higher
oligomers. Recently, a number of d-block metal and lanthanide complexes have been shown
to be catalysts for the selective dimerization of 1-alkynes. Typical examples are
[P(CH2CH2PPh2)3]RuH2,16 Cp*2TiR and Cp*2TiCl2/RMgX,17 [{Cp*2ZrMe}+{B(4-C6H4F)4}-],18
Cp*2LnR1d,e,k and Cp*[ArO]YR.4c The first step in the catalytic cycle is believed to be
formation of a metal alkynyl, [M]-C≡CR.
Since terminal alkynes are more reactive than olefins, it was anticipated that this substrate
would be ideal to probe the catalytic potential of our system. A study concerning the
dimerization of 1-alkynes catalyzed by bis(benzamidinato) yttrium complexes will be discussed
below (Section 3-5.). First the synthesis and characterization of some well-defined alkynyl
derivatives will be described.
Synthesis of Bis(N,N-Bis(Trimethylsilyl)Benzamidinato) Yttrium Alkynyl Derivatives.
[C6H5C(NSiMe3)2]2YCH(SiMe3)2 (2.8) reacts with one equivalent of several terminal alkynes,
HC≡CR, to give the corresponding dimeric acetylides, {[C6H5C(NSiMe3)2]2Y(µ-C≡CR)}2 (R =
H (3.1), Me (3.2), n-Pr (3.3), SiMe3 (3.4), Ph (3.5), CMe3 (3.6), eq. 1) in nearly quantitative
yield.
3.1, R = H; 3.2, R = Me;3.3, R = n-Pr; 3.4, R = SiMe3;3.5, R = Ph; 3.6, R = CMe3.
Y
N
N
N
N
H
SiMe3
SiMe3
C Y
N
N
NN
CC
Y
N
N
NN
CR
CR
(1)
2.8
HC CR
- H2C(SiMe3)2
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
47
With the hydride 2.9 as starting material for 3.6, minor amounts of the intermediate
[{[C6H5C(NSiMe3)2]2Y}2(µ-H)(µ-C≡CCMe3)] (1H NMR: δ = 8.0 (t, µ-H, 1JYH = 28 Hz)) and
traces of H2C=C(H)CMe3 were observed in the 1H NMR. The presence of this intermediate
indicates a stepwise reaction of 2.9 with HC≡CCMe3 (eq. 2), comparable to the reaction of
{Cp*[ArO]Y(µ-H)}2 with HC≡CSiMe3.4d Traces of H2C=C(H)CMe3 indicate hydrogenation of
the terminal alkyne. When 3.6 was treated with dihydrogen no H2C=C(H)CMe3 could be
observed. Hence, the hydrogenation of 3,3-dimethyl-1-butyne is catalyzed by either 2.9 or
[{[C6H5C(NSiMe3)2]2Y}2(µ-H)(µ-C≡CCMe3)]. This is very similar to the reaction of
[C6H5C(NSiMe3)2]2AlH with HC≡CCMe3, where in addition to C-H bond activation of
HC≡CCMe3 affording [C6H5C(NSiMe3)2]2Al-C≡CCMe3, a competing insertion reaction of
HC≡CCMe3 into the Al-H bond was observed, yielding H2C=C(H)CMe3 and
[C6H5C(NSiMe3)2]2Al-C≡CCMe3. This aluminum alkynyl formed showed no further reactivity
towards HC≡CCMe3 and/or dihydrogen.19
Y
N
N
NN
HH
Y
N
N
NN
2.9
Y
N
N
NN
HC
Y
N
N
NN
C
Y
N
N
NN
CC
Y
N
N
NN
C
C
(2)
3.6
HC CCMe3
- H2
HC CCMe3
- H2
The 13C NMR spectra (benzene-d6) of the yttrium acetylides (3.1 - 3.6) at elevated
temperatures (70-80oC) did not show any sign of dissociation, suggesting stable dimeric
structures. However, the dimeric acetylides can be cleaved by a Lewis base. Addition of THF
to solutions of 3.1 or 3.6 resulted in instantaneous formation of the corresponding monomeric
THF adducts, [C6H5C(NSiMe3)2]2YC≡CR.THF (R = H (3.7); R = CMe3 (3.8), eq. 3). This
corresponds with the reaction of {Cp*2Ce(µ-C≡CCMe3)}n with THF, affording
Cp*2CeC≡CCMe3.THF.8 In contrast, the compounds [C5H4R]2Y(µ-C≡CCMe3)}2 (R = H, Me)
Chapter 3.
48
does not react with THF.1c The monomeric 3.8 could also be isolated when 2.7 was treated
with HC≡CCMe3 (eq. 4).
3.1, R = H;3.6, R = CMe3.
3.7, R = H;3.8, R = CMe3.
Y
N
N
N THF
N C CRY
N
N
NN
CC
Y
N
N
NN
CR
CR
THF(3)
Y
N
N
N THF
N C CY
N
N
N THF
N CH2Ph (4)
2.7 3.8
HC CCMe3
- toluene
Finally, when the ‘ate’ compound, [C6H5C(NSiMe3)2]2Y(µ-Me)2Li.TMEDA (2.6), was treated
with HC≡CCMe3 the corresponding [C6H5C(NSiMe3)2]2Y(µ-C≡CCMe3)2Li.TMEDA (3.9) could
be isolated. This compound is structurally comparable with Cp*2Y(µ-C≡CCMe3)2Li.THF and
[Cp*2Sm(µ-C≡CPh)K]n, prepared by Evans et al. ,20 and Cp*[C6H5C(NSiMe3)2]Y(µ-
C≡CCMe3)2Li.TMEDA which will be discussed in Chapter 6.
2.6 3.9
Y
N
N
N Me
N Me Li
N
N
Y
N
N
N C
N C Li
N
NC
C
(5)HC CCMe3
Characterization. The IR spectra of 3.1-3.9 show ν(C≡C) stretching vibration frequencies
ranging from 1914 to 2060 cm-1, comparable to those of other dimeric group 3 and lanthanide
acetylides1d,e,k, 4d,20,21 and notably lower than those of the corresponding free acetylenes.22
The 1H NMR spectra (25oC) of 3.1 - 3.9 show sharp singlets for the benzamidinato
trimethylsilyl groups, indicating fast fluxional behavior in solution. The 13C NMR spectra of 3.1
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
49
- 3.6 display a triplet (20-21 Hz) for the α-carbon of the acetylide. The splitting of the signal is
due to 89Y-13C coupling and indicates that each alkynyl ligand interacts with two (time-
averaged) identical yttrium centers. It is remarkable that the α-C resonance of 3.4 (δ = 172.3
ppm, 1JY-C = 19 Hz) is considerably shifted to low-field compared to 3.1 - 3.3 and 3.5 - 3.6 (δ
ranges from 136.0 to 146.6 ppm); why this should be so is, as yet, unclear. The 13C NMR
spectra of 3.7 - 3.9 show a doublet (3.7: δ = 111.6 ppm, 1JY-C = 11 Hz; 3.8: δ = 89.3 ppm,1JY-C
= 12 Hz; 3.9: δ = 121.1, 1JY-C = 15 Hz) for the α-carbon of the acetylide, proving that the
complexes are monomeric. Worth mentioning is that, due to the coupling with yttrium, the β-H
of the ethynyl ligand of 3.7 appears as a doublet in the 1H NMR spectrum (δ = 1.99 ppm, 3JY-H
= 1.3 Hz). This suggests a stronger interaction of yttrium with the ethynyl fragment in the
monomeric 3.7 than in the dimeric 3.1, for which no yttrium coupling is observed. However,
the yttrium-carbon coupling constants in these compounds (3.7: 1JY-C = 11 Hz, 3.1: 1JY-C = 20
Hz) suggest the opposite.
Table I. Selected Bond Distances and Angles for {[C6H5C(NSiMe3)2]2Y(µ-C≡CH)}2 (3.1).
Distances (Å).
Molecule A. Molecule B.
Y(3)-N(5) 2.395(4) Y(1)-N(1) 2.396(4)
Y(3)-N(6) 2.345(4) Y(1)-N(2) 2.350(4)
Y(3)-N(7) 2.335(4) Y(2)-N(3) 2.376(4)
Y(3)-N(8) 2.381(4) Y(2)-N(4) 2.343(4)
Y(3)-C29) 2.556(5) Y(1)-C(1) 2.503(6)
Y(3)a-C(29) 2.509(5) Y(2)-C(1) 2.558(6)
Y(3)-C(30 2.959(10) Y(2)-C(2) 2.970(10)
N(5)-C(31) 1.329(6) N(1)-C(3) 1.333(6)
N(6)-C(31) 1.329(6) N(2)-C(3) 1.325(7)
N(7)-C44) 1.320(6) N(3)-C(16) 1.337(6)
N(8)-C(44) 1.337(6) N(4)-C16) 1.318(6)
C(29)-C(30) 1.164(8) C(1)-C(2) 1.148(14)
Angles (deg)
Molecule A. Molecule B.
N(5)-Y(3)-N(8) 149.33(15) N(1)-Y(1)-N(1)b 148.78(15)
N(6)-Y(3)-N(7) 98.70(14) N(2)-Y(1)-N(2)b 101.32(14)
Chapter 3.
50
N(5)-Y(3)-N(6) 57.92(13) N(1)-Y(1)-N(2) 57.90(13)
N(7)-Y(3)-N(8) 58.12(13) N(3)-Y(2)-N(4) 58.00(13)
Y(3)-C(29)-C(30) 98.4(5) Y(1)-C(1)-C(2) 158.3(9)
Y(3)a-C(29)-C(30) 159.8(5) Y(2)-C(1)-C(2) 99.4(9)
C(31)-Y(3)-C(44) 119.75(15) C(3)-Y(1)-C(3)b 120.55(15)
C(16)-Y(2)-C(16)b 118.87(15)
N(5)-Y(3)-N(6)-C(31) -3.8(3) N(1)-Y(1)-N(2)-C(3) -3.2(2)
N(7)-Y(3)-N(8)-C(44) -3.3(3) N(3)-Y(2)-N(4)-C(16) -4.1(2)
Symmetry operations: a = -x, y, ½ -z; b = -x, y, ¾ -z.
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
51
Molecular Structure of {[C6H5C(NSiMe3)2]2Y(µµ-C≡≡CH)}2 (3.1). To determine the bonding
of the alkynyl fragment in 3.1, an X-ray crystal structure determination was carried out.23 The
unit cell contains two crystallographically independent molecules (both exhibiting C2
symmetry) that do not differ markedly in their structure. PLUTO drawings of both molecules
are depicted in Figure 3.
+-[Y][Y]
C
C
C
H
C
H
HC
C
[Y]
HC
C[Y]
3.1A 3.1B
Figure 3. PLUTO drawings and schematic presentations of both crystallographically independent
molecules of {[C6H5C(NSiMe3)2]2Y(µ-C≡CH)}2 (3.1). For clarity reasons, only the ethynyl hydrogens
are shown.
In molecule 3.1A (Figure 3A), two identical [C6H5C(NSiMe3)2]2Y-C≡CH units are related to
each other by a 2-fold axis perpendicular to the plane of the acetylide bridge. In the other
molecule (3.1B, Figure 3B), both yttrium atoms are on a special position along the 2-fold axis.
Chapter 3.
52
In molecule 3.1A, each yttrium is σ-bonded to one ethynyl ligands and π-bonded to the other.
As a result of the symmetry operation obeyed by molecule 3.1B, both alkynyl ligands are σ-
bonded to Y(1), whereas the π-system of these alkynyls interact with Y(2). Hence, molecule
3.1B is best described as a Zwitterion, {[C6H5C(NSiMe3)2]2Y(C≡CH)2}-
{Y[C6H5C(NSiMe3)2]2}+ (Figure 3), which is in agreement with the longer Y(2)-C(1) (2.558(6)
Å) compared to the Y(1)-C(1) bond (2.503(6) Å).
Since both the benzamidinato-SiMe3 and ethynyl groups emerge as single resonances in
the 1H and 13C NMR spectra, fast equilibration of the regio-isomers (3.1A, 3.1B) occurs in
solution (Figure 3). Erker et al. found a ∆Go of approximately 0.1 kcal.mol-1 for a similar
equilibrium for dimeric zirconium alkynyl derivatives, [C5H5]2Zr(µ-C≡CMe)2 Zr[C5H4CMe3]2.24
As the majority of the corresponding bond lenghts and angles are very similar in both
molecules, only notable features of molecule 3.1A will be discussed here. Molecule 3.1A is
formed by two distorted octahedral yttrium atoms, each surrounded by two chelating
benzamidinato ligands and two bridging acetylide fragments. The Y(3)-N(5) (2.395(4) Å) and
Y(3)-N(8) (2.381(4) Å) bonds are longer than Y(3)-N(6) and Y(3)-N(7) (2.345(4) Å and
2.335(4) Å respectively). The virtually identical C-N bonds (Table I) of the benzamidinato
fragments indicate the usual electron delocalization within the NCN fragment. All Y-N
distances are within the range observed for other yttrium N,N’-bis(trimethyl-
silyl)benzamidinato derivatives (Chapter 2) and suggest some π-interaction with the yttrium
center. The Y(3)-C(29) (2.556(5) Å) and Y(3)a-C(29) (2.509(5) Å) bonds are longer than the
Y-C bond in Cp*2Y(µ-C≡CCMe3)2Li.THF (2.38(2) Å),20a but very similar to the Y-C distances
in [Cp2Y(µ-Me)]2 (2.553(1) Å, 2.537(9) Å).25 The Y-C bonds for 3.1A are within the range of
Ln-C bonds of dimeric 4f element acetylide complexes when the differences in atomic radii are
taken into account.21 The Y(3)-C(29) and Y(3)a-C(29) bonds and the Y(3)-C(29)-C(30)
(98.4(5)o) and Y(3)a-C(29)-C(30) (159.8(5)o) angles show that the bridge is non-symmetric but
still with a strong interaction of the α-carbon atom, C(29), with both yttrium centers. The
acetylide stretching vibration of ν(C≡C) = 1915 cm-1 (cf. HC≡CH: ν(C≡C) = 1947 cm-1;
Cp3UC≡CH: ν(C≡C) = 2062 cm-1),26 the large 1JC-H coupling constant (218 Hz) for Y-C≡CH and
the Y(3)a-C(29)-C(30) angle of 159.8(5)o suggest an interaction of the triple bond π-orbitals
with Y(3). Since π-interaction of the acetylide is expected to result in elongation of the C≡C
bond, the observed value of the C(29)-C(30) bond (1.164(8) Å), which is considerable shorter
than in free acetylene (1.21 Å),26a is surprising. Comparable short C≡C bonds27 were also
observed in alkali-metal alkynyls (NaC≡CMe (1.09(2) Å), NaC≡CH (1.17(6) Å)),27b Cp*2Sm-
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
53
C≡CPh.THF (1.11(2) Å, 1.13(2) Å),27c and [OEPG]V(OC≡CH)Li2.THF3 (1.102(9) Å),10 but the
reason for this shortening of the C≡C bond is unclear.
Thermolysis of Dimeric Acetylide Derivatives. Thermolysis (24-72 h, 100oC) of the
dimeric alkynyl species (3.1 - 3.6) resulted in the formation of several unidentified products.
The rate of the thermolysis strongly depends on the alkynyl substituent. 3.1 decomposes
within hours at 100oC, while the characteristic triplet signal for the alkynyl α-carbon of 3.4 is
still present after two days at 100oC. Unequivocal assignment of resonances in the NMR
spectra arising from C-C coupled µ-trienediyl compounds, as formed upon thermolysis of
{Cp*2Ln-C≡CR}n (Ln = La, Ce, Sm),28 was not possible. Although the β-H signal in the 1H
NMR of 3.1 disappeared upon thermolysis, clean formation of an acetylenediyl species similar
to Cp*2Ln-C≡C-LnCp*2 as found by Bercaw (Ln = Sc)29a and Evans (Ln = Sm)29b was not
observed in our system. Since a complicated mixture of very soluble thermolysis products
was formed, isolation and characterization of the individual products proved to be impossible.
3-5. Catalytic Dimerization of Terminal Alkynes.
To probe the catalytic activity of bis(benzamidinato) yttrium compounds in 1-alkyne
dimerization, a variety of substrates were examined and the results compared with those
obtained from Cp*2Ln-R (Ln = Sc,1e,29a Y,1d,k La,1k,28b Ce,1k,28b Sm;20b R = alkyl, alkynyl, H)
and Cp*[ArO]YR (R = alkyl, alkynyl)4d systems.
Scope of the Catalytic 1-Alkyne Dimerization Reaction. The dimerization of 1-alkynes
catalyzed by bis(N,N’-bis(trimethylsilyl) benzamidinato) yttrium alkyl and hydrido derivatives
(2.8, 2.9) strongly depends on the alkyne substituent. For small 1-alkynes, HC≡CR (R = H,
Me, nPr), no dimerization is observed (days, 80oC) and the reaction stops on formation of the
dimeric acetylides (3.1 - 3.3). It is only with acetylenes containing bulky substituents, HC≡CR
(R = Ph, SiMe3, CMe3), that catalytic dimerization takes place (vide infra ). This observation
strongly contrasts with the Cp*2LnR catalyzed dimerization of 1-alkynes, for which high activity
was observed for all alkynes applied regardless the size of the alkyne substituents. The
assumption that the first step in the dimerization is formation of the acetylide, is supported by
the fact that the reaction rate is independent from whether 2.8, 2.9 or the acetylides 3.4 - 3.6
are used as precursors.
Chapter 3.
54
Compared to the Cp*2LnR (Ln = La, Ce) catalyzed dimerization of 1-alkynes (Nt (25oC) ≥
1900 mol.mol-1.h-1),1k,28b the observed catalytic activity of [C6H5C(NSiMe3)2]2Y-R (R =
CH(SiMe3)2, µ-H, µ-C≡CR) in 1-alkyne dimerization, is much lower (Nt (80oC) = 20 - 40
mol.mol-1.h-1). Although the coordinated THF retards the reaction considerably, the THF
adduct 3.8 still does catalyze the dimerization of HC≡CCMe3 (Nt (50oC) ≈ 1 mol.mol-1.h-1),
which implies that the coordinated THF can be replaced by an incoming acetylene.
Table II. Product distribution of the oligomerization of terminal alkynes, HC≡CR, catalyzed by group
3 metal and lanthanide based complexes.
Compound R 2,4-dimer 1,4-dimer higher oligomers Ref.
[C6H5C(NSiMe3)2]2YR’ CMe3 100 -- -- This work
(R’ = CH(SiMe3)2, µ-H) Ph 100 -- --
SiMe3 -- 100 --
Cp*2ScMe H -- -- 100 1e
Me 100 -- --
Cp*2YCH(SiMe3)2 CMe3 100 0 -- 1k
Ph 89 11 --
SiMe3 20 80 --
Cp*2LaCH(SiMe3)2 Me 78 -- 21 1k
CMe3 100 -- --
Ph -- 86 14
SiMe3 4 45 51
Cp*2CeCH(SiMe3)2 Me 74 -- 25 1k
CMe3 100 -- --
Ph -- 82 18
SiMe3 7 61 32
Cp*Ce[CH(SiMe3)2]2 CMe3 100 -- -- 30
Cp*[ArO]YCH(SiMe3)2 SiMe3 -- 100 -- 4d
1,4-substituted 1-buten-3-yne C C
HR
H CC
R
2,4-substituted 1-buten-3-yne C C
RH
H CC
R
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
55
The regio- and stereoselectivity of the reaction depends strongly on the acetylene
substituent and catalyst applied. For Cp*2LnCH(SiMe3)2, the only other system for which the
dimerization of a variety of 1-alkynes has been investigated, only HC≡CCMe3 is selectively
coupled to the head-to-tail trans -H2C=C(CMe3)C≡CCMe3. For all other 1-alkynes tested,
HC≡CR (R = Me, Ph, SiMe3), a dramatic drop in selectivity was observed yielding either
mixtures of head-to-tail and head-to-head isomers (Ln = Y, La, Ce) or the additional formation
of higher oligomers (Ln = La, Ce, Table II). In our system, HC≡CMe, HC≡CPh and
HC≡CCMe3 are selectively coupled to head-to-tail dimers: 2,4-disubstituted trans -1-buten-3-
ynes (Scheme 2). However, for HC≡CSiMe3 a dramatic change in regioselectivity takes place,
exclusively yielding the head-to-head coupled product: trans -Me3SiC(H)=C(H)-C≡CSiMe3
(Scheme 2). This remarkable change in regioselectivity was also observed by Schaverien and
Heeres who found that Cp*[ArO]YCH(SiMe3)2 and Cp*2YCH(SiMe3)2 preferentially couple
HC≡CSiMe3 to trans -Me3SiC(H)=C(H)-C≡CSiMe3 (Table II). 1k,4b,28b The origin of this
change in regioselectivity is thought to be electronic (vide infra ).28b,31 That other factors also
play a decisive role is clearly illustrated by the [Cp*2ZrMe(B(4-C6H4F)4)] catalyzed coupling of
HC≡CSiMe3, which exclusively yields head-to-tail coupled dimers and trimers.18
Proposed Mechanism for the Dimerization of 1-Alkynes. The generally accepted
mechanism for the dimerization of 1-alkynes, catalyzed by group 3 metals and lanthanides is
plausible for our system as well.1d,e,k, 4b,20b,28b This mechanism consists of two interfering
processes: dissociation of the precursor into the catalytically active, unsolvated, monomeric
acetylides and the sequence of insertion and C-H bond activation reactions (Scheme 2). Den
Haan et al. 1d and Heeres et al. 1k proposed that the unsolvated monomeric acetylides are the
active species. These monomers leave sufficient space for an incoming alkyne, whereas the
free coordination site of the corresponding oligomeric compounds is blocked by bridging
ligands.1g,2,3e,4c,d If this assumption is correct, the catalytic activity should strongly depend on
the tendency of the dimeric acetylides to dissociate, resulting in catalytically active species.
Dissociation of the dimeric precursors is expected to be both sterically and electronically
controlled. When the steric strain within the dimeric acetylide complexes increases, the
dimeric structures become less favorable which will result in a higher concentration of
monomer (larger Keq, Scheme 2) and consequently in a higher observed catalytic activity. In
addition, the tendency to form dimeric species increases with increasing electrophilicity of the
metal center. Assuming that the electronic properties of {Cp2Y(µ-C≡CCMe3)}2 and {Cp*2Ln(µ-
Chapter 3.
56
C≡CCMe3)}n are comparable, the observation that {Cp2Y(µ-C≡CCMe3)}2 is inactive for the
dimerization of HC≡CCMe3,32 while the sterically more crowded {Cp*2Ln(µ-C≡CCMe3)}n is
very active in dimerizing HC≡CCMe3, suggests that the tendency to form inactive dimers is
mainly sterically controlled. The yttrium center in the bis(N,N’-bis(trimethylsilyl)benzamidinato)
system is more electrophilic than in the bis(pentamethylcyclopentadienyl) coordination
environment. Furthermore, the latter system is slightly more bulky.33 Hence, on the basis of
both steric and electronic arguments the gross catalytic activity of {[C6H5C(NSiMe3)2]2Y(µ-
C≡CR)}2 (3.1 - 3.6) in alkyne dimerization is expected to be lower than for the corresponding
{Cp*2Y(µ-C≡CR)}n compounds.
Scheme 2.
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
57
R = Ph, CMe3
HC CR
THF
R = SiMe3
Y
N
N
N THF
N CCR
Y
N
N
NN
CC
Y
N
N
NN
CR
CR
Y
N
N
N
N
H
SiMe3
SiMe3
CY
N
N
N THF
N CH2Ph
Keq. Keq.
HC CR
Y
N
N
N
NC CR
H
R
R
NN
N
N
YSiMe3
H
Me3Si
NN
N
N
Y
HC CR
HC CRHC CR
C
C C
H
HR
C
R
C
CC
H
R H
C
R
The observed catalytic activity of the complexes 3.4 - 3.6 to dimerize the corresponding 1-
alkynes (HC≡CR, R = SiMe3, Ph, CMe3 respectively), is indeed much lower than found for the
{Cp*2Y(µ-C≡CR)}n analogues, while the sterically less demanding acetylides 3.1 - 3.3 are
even inactive for the dimerization of the corresponding alkynes (HC≡CR, R = H, Me, n-Pr
respectively). This observation supports the earlier made assumption that the corresponding
monomeric acetylides are the active species.
Chapter 3.
58
During the dimerization of HC≡CR (R = SiMe3, Ph, CMe3), notable amounts of the dimeric
acetylides (3.4 - 3.6 respectively) are present, while no monomeric yttrium acetylides could be
detected (1H NMR spectroscopy). This indicates that for 3.4 - 3.6 Keq is small. Assuming that
less than 3 % of the catalyst is active (detectability limit for 1H NMR ≈ 3 %), a minimum Nt
(80oC) of 1300 mol.mol-1.h-1 can be estimated, which is in good agreement with the observed
Nt (25oC) of ≥ 1900 mol.mol-1.h-1 for 1-alkyne dimerization catalyzed by {Cp*2Ln(µ-
C≡CCMe3)}n. Hence, the intrinsic catalytic activity of the bis(benzamidinato) yttrium acetylides
is similar to that observed for the corresponding {Cp*2Ln(µ-C≡CR)}n complexes (vide supra ).
The cause of the low tendency of the dimeric acetylides to dissociate cannot simply be
assigned to either steric or electronic effects, but is probably a combination of both. Since the
steric bulk of the bis(benzamidinato) ligand system is slightly less than of the bis(pentamethyl-
cyclopentadienyl) ligand set, it is not very convincing to attribute the differences in reactivity to
steric reasons alone. On the other hand, the fact that bulky 1-alkynes are selectively dimerized
while for small 1-alkynes the reaction stops on formation of the dimeric acetylide complexes
proves that electronic effects are not the only reason either. If this was the case, the rate of
dimerization should be the same for both small and bulky 1-alkynes, which is clearly not true.
The same arguments can be used to explain the low turnover frequency observed when the
THF adduct 3.8 is used as precursor. Like dissociation of the dimeric acetylides, the tendency
of 3.8 to release the coordinated THF is very low.
During the insertion step, the regioselectivity is determined. On the basis of steric
considerations, the head-to-tail dimer is expected, as found for the dimerization of HC≡CR
(R= Me, Ph, CMe3; Scheme 2). To form the head-to-head product, as observed for
HC≡CSiMe3, the acetylene substituent has to point in the direction of the bulky ancillary ligand
system during insertion (Scheme 2). Sterically this will be unfavorable. Therefore, the
exclusive formation of trans -Me3SiC(H)=C(H)-C≡CSiMe3 has to be electronically and not
sterically controlled.4d,28b,31 To complete the catalytic cycle, C-H bond activation of the
insertion product results in release of the 1-buten-3-yne and formation of the monomeric
acetylide (Scheme 2). Insertion, instead of C-H bond activation, of another alkyne into the
yttrium-1-buten-3-ynyl species is clearly unfavorable, since higher oligomers (GC-MS) were
not found.
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
59
Time (min).
0
10
20
30
40
50
60
70
80
90
100
0 100 200 300 400 500
326.0 K
332.6 K
338.6 K
344.3 K
350.4 K
Figure 4. Normalized ratio of substrate to yttrium concentration as a function of time and
temperature for the dimerization of HC≡CCMe3 using [C6H5C(NSiMe3)2]2YCH(SiMe3)2 as
precatalyst.
One question that remains is which step of this process is rate-determining. Kinetic
experiments carried out for the dimerization of HC≡CSiMe3 and HC≡CCMe3 showed clean
frist order dependence in 1-alkyne (Figure 4). The fact that the amount of the dimeric
acetylides (3.4 - 3.6) remains constant during the dimerization reactions, suggests that
insertion of an incoming acetylene into the Y-C≡CR bond is rate-determining, with the
assumption that equilibration (Keq, Scheme 2) is rapid. If C-H bond activation was rate-
limiting, build-up of the intermediates [Y]-C(H)=C(CMe3)-C≡CCMe3 or [Y]-C(SiMe3)=C(H)-
C≡CSiMe3 would be expected. However, since Keq is very small, only a very small amount of
active catalyst is present determining the rate-determining is tricky.
Apparently, the same process that reduces the catalytic activity of the bis(N,N’-
bis(trimethylsilyl)benzamidinato) yttrium compounds in olefin polymerization (slow dissociation
of the dimeric precursors into the active catalyst) is also responsible for the low activity in the
catalytic dimerization of 1-alkynes. Nevertheless, these results show that the intrinsic catalytic
activity of yttrium bis(benzamidinato) compounds in 1-alkyne dimerization is very similar to
that observed for the analogous Cp*2YR systems.
3-6. Reactivity Towards Nitriles.
Chapter 3.
60
The reactivity of group 3 and 4 metals and lanthanides is dominated by their propensity to
form strong bonds with hard donor atoms such as oxygen and nitrogen. Therefore, it has
generally been assumed that early transition metal complexes containing these bonds are too
inert to function as catalysts. Recently however, several examples appeared in the literature
reporting reactive M-O and M-N bonds, which illustrates that amido and oxo compounds can
function as catalysts or stoichiometric reagents.1o,q,s, 3d,20b,34 Therefore, we argued that
oligo/polymerization and catalytic hydrogenation of nitriles may be possible. The reaction of
[C6H5C(NSiMe3)2]2YR (R = CH2Ph.THF (2.7), CH(SiMe3)2 (2.8), µ-H (2.9)) with N≡CCMe3
and N≡CMe has been studied in more detail and the results are presented in this section.
Insertion Versus C-H Bond Activation. An NMR tube reaction showed that N≡CCMe3 (1
equiv., benzene-d6) does not react or even coordinate with 2.8. This is against expectation,
since the sterically more hindered Cp*2YCH(SiMe3)2 reacts smoothly with N≡CCMe3 to yield
the imido, Cp*2Y(N=C(CMe3)CH(SiMe3)2).N≡CCMe3. When 2.8 is treated with N≡CMe, C-H
bond activation rather than insertion takes place, yielding H2C(SiMe3)2 and the novel dimeric
crotonitrileamido complex, {[C6H5C(NSiMe3)2]2Y(µ-(N,N’)-N(H)-C(Me)=C(H)-C≡N)}2 (3.10,
eq. 6). This contrasts with the reactions of {[C5H4R]2Y(µ-H).THF}2 (R = H, Me) and Cp*2ScR
(R = p-Me-C6H4, Me) with acetonitrile: both resulted in clean insertion.1c,34a Metalation was
also observed for Cp*2LnCH(SiMe3)2 (Ln = La, Ce) with acetonitrile, affording dimeric
cyanomethyl complexes, {Cp*2Ln(CH2C≡N)}2.1i For our system, this compound could not be
detected. However, it is likely that it is formed as an intermediate which reacts further by
insertion of another equivalent of acetonitrile, subsequently followed by a 1,3-H shift, to give
3.10 (eq. 6). The reaction compares well with the alkali metal catalyzed dimerization of
acetonitrile to crotonitrileamides,35,36 and is analogous to the reaction of the corresponding
bis(N,O-bis(tert -butyl)alkoxysilylamido) yttrium bis(trimethylsilyl)methyl compound,
[Me2Si(NCMe3)(OCMe3)]2YCH(SiMe3)2 (4.6), with N≡CMe. In the latter case,
{[Me2Si(NCMe3)(OCMe3)]2Y(µ-(N,N’)-N(H)-C(Me)=C(H)-C≡N)}2 (4.8) was isolated and
structurally characterized (Chapter 4). The IR and NMR data for 3.10 are in good agreement
with those of Na[N(H)-C(Me)=C(H)-C≡N]35,36 and {[Me2Si(NCMe3)(OCMe3)]2Y(µ-(N,N’)-
N(H)-C(Me)=C(H)-C≡N)}2 (4.8, Chapter 4). The lowfield shift of the C(Me)=C (δ = 131.3 ppm)
and the highfield shifts of the C(H)=C (δ = 48.1 ppm) and C(H)=C (δ = 3.50 ppm) resonances
in the 13C and 1H NMR spectra of 3.10 clearly demonstrate the extensive charge
delocalization within the crotonitrileamido fragment.
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
61
N CMe
C-H activation
Insertion
1,3-H shift
N CMeY
N
N
N
NC
SiMe3
SiMe3
H
C
N:
CH2
Me
CN
C
:N
CH2 Me
CN
Y
N
N
NN Y
N
N
NN
C C
Me
NC
H
HN
CC
Me
N C
H
H N
Y
N
N
N
N
Y
N
N
N
N
CH2
CN
NC
CH2
Y
N
N
N
N
Y
N
N
N
N
(6)
2.8
3.10
It appears that the bulky carbyl group in 2.8 acts as a classical Brønsted base, while the
sterically less hindered N≡CCH2 fragment of the intermediate cyanomethyl complex is a better
nucleophile which leads to insertion of another acetonitrile. When the reaction of 2.8 with
exactly one equivalent of acetonitrile was carried out, a 1 : 1 mixture of 2.8 and 3.10 was
obtained. Hence, the insertion of the second equivalent of N≡CMe is faster than the preceding
metalation step. In contrast, for Cp*2LnCH(SiMe3)2 (Ln = La, Ce) C-H bond activation is faster
as is illustrated by the selective formation of the cyanomethyl complex.
Nitrile insertion was observed for 2.7 with N≡CCMe3, initially yielding
{[C6H5C(NSiMe3)2]2Y(µ-N=C(CMe3)CH2Ph)}2 (3.11a), which completely rearranges to
{[C6H5C(NSiMe3)2]2Y(µ-NH-C(CMe3)=C(H)Ph)}2 (3.11b, eq. 7) as a result of imine-enamine
tautomerism. In a similar manner, 2.7 reacts with acetonitrile but here a mixture of compounds
is obtained. Besides the formation of the imido {[C6H5C(NSiMe3)2]2Y(µ-N=C(Me)CH2Ph)}2
(3.12a, 50 %) and enamido {[C6H5C(NSiMe3)2]2Y(µ-NH-C(Me)=C(H)Ph)}2 (3.12b, 40 %)
derivatives, competitive deprotonation of acetonitrile is observed yielding the crotonitrileamido
(3.10, 10 %, eq. 8).
Chapter 3.
62
Y
N
N
N THF
N CH2Ph insertion
N CCMe3
2.7
3.11a
Y
N
N
N
NY
N
N
N
N
CMe3
C
PhCH2
N
N
CH2Ph
C
Me3C
1,3-H shift (7)
3.11bC
Ph
H
N
CCMe3
H
CPh
H
N
CMe3C
HY
N
N
N
N Y
N
N
N
N
Remarkably is the influence of the Me versus CMe3 substituent on the N≡CR: In contrast to
the complete conversion of 3.11a into 3.11b, the imido 3.12a does not completely rearrange
into the enamido 3.12b, but remains present even after prolonged periods of time.
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
63
Y
N
N
N THF
N CH2Ph+
2.7
C C
Me
NC
H
HN
CC
Me
N C
H
H N
Y
N
N
N
N
Y
N
N
N
N
N CMe
(8)+
C
Ph
H
N
CMe
H
CPh
H
N
CMe
HY
N
N
NN Y
N
N
NNY
N
N
N
NY
N
N
N
N
Me
C
PhCH2
N
N
CH2Ph
C
Me
(3.10, 10 %)
(3.12a, 50 %) (3.12b, 40 %)
Bis(N,N’-bis(trimethylsilyl)benzamidinato yttrium hydride (2.9) reacts instantaneously at
room temperature with nitriles, N≡CR (1 : 1 ratio; R = Me, CMe3, eq. 9).
2.9
(9)Y
N
N
NN
HH
Y
N
N
NN N CR
R
C
H
N
N
HC
R
Y
N
N
N
N Y
N
N
N
N
3.13, R = Me;3.14, R = CMe3.
Resonances characteristic for insertion products {[C6H5C(NSiMe3)2]2Y(µ-N=C(H)R)}2 (R =
Me (3.13), CMe3 (3.14)) were observed in the 1H NMR spectra, although the products are
unstable and decompose rapidly at room temperature. The 1H NMR spectra of both
compounds are consistent with those of the corresponding {[C5H4R]2Y(µ-N=C(H)R’)}2 (R = H,
R’ = Me, CMe3; R = Me, R’ = Me) species, prepared in similar fashion by Evans et al. 1b
Chapter 3.
64
These results show that the competition between nucleophilicity and Brønsted basicity of
the R-group in [C6H5C(NSiMe3)2]2YR (R = CH(SiMe3)2, CH2Ph, µ-H) has a strong influence
on the reaction pattern of these compounds with α-hydrogen containing nitriles. The bulky
bis(trimethylsilyl)methyl substituent tends to react as a classical Brønsted base such as M2-
naphthalene (M = Li, Na, K, MgCl)35 and NaN(SiMe3)2,36 deprotonating acetonitrile. With only
10 % deprotonation of acetonitrile, the benzyl group in 2.7 clearly shows a reduced Brønsted
base character, and resembles Cp*2ScR (R = p-Me-C6H4, Me),34a [Cp’2MR]+ (Cp’ = C5H5,
C5H4Me; M = Ti, Zr; R = alkyl)37a-c and Al(NEt2)Et2.38 As a result of its highly nucleophilic
character, the hydride 2.9 only shows insertion of nitriles, analogous to Cp*2ScH,34a
Cp*2ZrH2,34a and [Cp’2ZrH]+ (Cp’ = C5H5, C5H4Me).37c,d
Hydrogenation of Nitriles. The fact that the Ln-N (Ln = Sc, Y, lanthanides) bond is less
inert as previously assumed1o,s,3d,20b,34a is clearly emphasized by the Cp*2ScH catalyzed
hydrogenation of nitriles.34a To investigate the reactivity of the Y-N bond in our system, the
reaction of 2.9 with N≡CCMe3 in the presence of hydrogen (4 atm.) was studied.
Following the reaction by 1H NMR spectroscopy revealed that initially a mixture was
formed amongst which 3.14 could clearly be identified. After 24 hours at room temperature,
the hydrogenated product [C6H5C(NSiMe3)2]2Y-N=C(CMe3)N(H)CH2CMe3 (3.15) had been
formed in 90 % yield (eq. 10).39 This compound does not react further with dihydrogen or
N≡CCMe3. Even after days at 60oC, no free H2NCH2CMe3 had been formed. This is not
surprising, since formation of the corresponding scandium compound, Cp*2Sc(N=C(CMe3)-
N(H)CH2CMe3), is responsible for catalyst deactivation during the scandium catalyzed
hydrogenation of nitriles. The mechanism of the hydrogenation-dimerization reaction (eq. 10)
is assumed to be analogous to that observed for Cp*2ScH with N≡CCMe3 in the presence of
hydrogen and will not be further discussed.38
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
65
CMe3
C
H
N
N
H
C
Me3C
Y
N
N
N
N Y
N
N
N
NY
N
N
NN
HH
Y
N
N
NN
3.14
3.15
(10)2 Y
N
N
NN N
CN
CMe3
H2C CMe3
H
2.9
N CCMe3
H2
N CCMe3
3-7. Reactivity Towards Activated Arenes.
C-H bond activation or ortho -metalation of aromatic molecules C6H5X (X = H, Me, OMe,
SMe, NMe2, CH2NMe2, PMe2), P(=CH2)Ph3, mesitylene, pyridine and α-picoline is a
common process for Cp*2LnR (Ln = Sc, Y, lanthanides; R = alkyl, H) species; it provides a
very clean route towards new carbyl derivatives.1a,d,e, 40
As discussed above, heating of benzene or toluene solutions of 2.7 - 2.9 did not lead to
metalation or H/D scrambling. Furthermore, the stability of 2.8OMe and 2.9OMe and the lack of
reactivity of 2.9 towards C6H5X (X = OMe, CH2NMe2) demonstrates a much lower tendency
of the bis(benzamidinato) yttrium compounds to ortho -metalate compared with their Cp*2LnR
congeners (Chapter 2). This can be assigned to the different character of the Y-C or Y-H bond
which is a clear result of the different electronic properties of the benzamidinato ligands.
However, with pyridine or α-picoline fast reactions are observed.
C-H Bond Activation Versus Insertion. When [C6H5C(NSiMe3)2]2YCH(SiMe3)2 (2.8)
was treated with pyridine, a complicated reaction occurred. Quantitative formation of the
alkane, H2C(SiMe3)2, indicated metalation of pyridine. However, the expected pyridyl species
could not be detected by 1H NMR spectroscopy. Vinylic C-H resonances (1H NMR: δ = 4 - 6
Chapter 3.
66
ppm) suggest partial reduction of pyridine. It is probable that insertion of another equivalent of
pyridine into the Y-C bond of the initially formed pyridyl derivative occurred, similar as
proposed for the reaction of Cp*2Y(η2-(C,N)-2-NC5H4) with pyridine (eq. 11).40
Y
N
N
N
N
H
SiMe3
SiMe3
C
2.8
N
N
(11)
Y
N
N
N
NN
Y
N
N
N
N N
N H
While {Cp*2Y(µ-H)}2 is an excellent precursor for the pyridyl derivative, Cp*2Y(η2-(C,N)-2-
NC5H4),1d,40 reaction of {[C6H5C(NSiMe3)2]2Y(µ-H)}2 (2.9) with pyridine resulted in insertion
rather than C-H bond activation, as the 1,2-inserted product [C6H5C(NSiMe3)2]2Y-NC5H6
(3.16, eq. 12) was obtained. Hence, the reactivity of 2.9 towards pyridine, resembles that of
{[C5H4R]2Y(µ-H).THF}21b and the in situ prepared bis(alkoxysilylamido) yttrium hydride,
[Me2Si(NCMe3)(OCMe3)]2YH.Py (Chapter 5). Subsequent thermal isomerization to the 1,4-
addition product, as observed for the latter two compounds, did not occur in this system (days,
100oC).
3.16, R = H,3.17, R = CH2Ph.
(12)Y
N
N
N
N R
N
Y
N
N
N
N N
RH
2.9, R = H;2.7, R = CH2Ph.THF.
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
67
Since the reactivity of the benzyl derivative, [C6H5C(NSiMe3)2]2YCH2Ph.THF (2.7) towards
acetonitrile was found to be intermediate between that of the bis(trimethylsilyl)methyl
compound 2.8 (C-H bond activation) and the hydride 2.9 (insertion), we were interested to see
how it would react with excess pyridine. Compound 2.7 reacts instantaneously with pyridine
(2.5 equiv.) by insertion to form exclusively the 1,2-insertion product, [C6H5C(NSiMe3)2]2Y-
NC5H5-2-CH2Ph (3.17, eq. 12). Unfortunately, its extreme solubility precluded purification,
although it could conclusively be identified (1H, 13C NMR, COSY, NOESY). Hence, the
reactivity of 2.7 towards pyridine is very similar to that of 2.9 and alkyllithium reagents. The
Ziegler alkylation of pyridines by alkyllithium reagents involves intermediate 1,2-insertion
products which subsequently undergo elimination of LiH.41 However, analogous formation of
the yttrium hydride {[C6H5C(NSiMe3)2]2Y(µ-H)}2 (2.9) and release of 2-benzylpyridine was not
observed in this system.
Y
N
N
N
N R
N
3.18
(13)Y
N
N
N
NN
CH2
2.7, R = CH2Ph.THF;2.8, R = CH(SiMe3)2;2.9, R = H.
Since its methyl protons are rather acidic, for 2.7 and 2.9 metalation of picoline was
expected to become competitive to insertion. NMR tube reactions of 2.7, 2.8 and 2.9 with α-
picoline indeed showed the formation of toluene, H2C(SiMe3)2 and H2 respectively, together
with the exclusive formation of the α-picolyl derivative, [C6H5C(NSiMe3)2]2Y(η2-(C,N)-CH2-2-
NC5H4) (3.18, eq. 13). For 2.7, the reaction is complete within hours at room temperature,
whereas for 2.8 and 2.9 heating to 100oC (hours) was required. While this reaction is strictly
analogous to that of 4.6 (Chapter 5) and alkyllithium reagents with α-picoline,42 it fully
contrasts that of the corresponding Cp*2YR (R = Me.THF, CH(SiMe3)2) systems, which
afforded the corresponding pyridyl derivative, Cp*2Y(η2-(C,N)-2-NC5H3-6-Me).1d This again
clearly demonstrates the difference in Y-C bond character as a result of electronic changes in
the ancillary ligand system. The NMR data of 3.18 are nearly identical to those of the
corresponding, structurally characterized, bis(alkoxysilylamido) yttrium α-picolyl species,
Chapter 3.
68
[Me2Si(NCMe3)(OCMe3)]2Y(η2-(C,N)-CH2-2-NC5H4) (5.2, Chapter 5), illustrating the high
similarity between both systems.
3-8. Concluding Remarks.
The results presented in this chapter clearly illustrate similarities and differences in
reactivity between [C6H5C(NSiMe3)2]2YR and the corresponding Cp*2YR (R = alkyl, H)
complexes as a result of the different character of the Y-R bond. Like Cp*2YR,
[C6H5C(NSiMe3)2]2YR is a catalyst for ethylene polymerization and 1-alkyne dimerization,
although for the bis(benzamidinato) yttrium complexes, reaction rates are much lower. This
seems to be a result of their high tendency to form stable, inactive dimers. The cause for this
behavior cannot simply be assigned to either steric or electronic effects, but is a combination
of both. Since the steric bulk of the bis(benzamidinato) ligand system is slightly less than that
of the bis(pentamethyl-cyclopentadienyl) ligand set, it is not very convincing to attribute the
differences in reactivity to steric reasons alone. On the other hand, the fact that bulky 1-
alkynes are selectively dimerized while for small 1-alkynes the reaction stops with the
formation of the dimeric acetylide complexes, shows that electronic effects are not the only
reason either. If the latter was true, the rate of dimerization should be the same for both small
and bulky 1-alkynes, which is clearly not the case.
A fundamental difference between the bis(benzamidinato) yttrium alkyl and hydrido
complexes and the corresponding (permethylated) yttrocene alkyl and hydrido compounds is
the Brønsted basic behavior of the former towards nitriles and (substituted) pyridines. Hence,
the reactivity of bis(benzamidinato) yttrium alkyl complexes resembles that of alkyllithium
reagents and suggests a highly polarized/ionic system. This is supported by the easy loss of
benzamidinato ligands to other Lewis acids as was observed during the reaction of
[C6H5C(NSiMe3)2]2YCl.THF with LiAlH4. This is characteristic for ionic systems (Chapter 2).
3-9. Experimental Section.
Material and Methods. All compounds are extremely oxygen- and moisture-sensitive.
Manipulations were therefore carried out under nitrogen by using glovebox (Braun MB-200) and
Schlenk techniques. Hydrogen (Hoek-Loos 99.9995 %), ethylene, propylene (DSM Research B.V.),
acetylene (Ucar, C.P.) and propyne (Matheson, C.P.) were used as purchased. 1-Hexene, alkynes,
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
69
nitriles, pyridine and picoline (Janssen) were dried over 4Å molecular sieves and distilled prior to
use. Benzene-d6 was degassed and dried over Na/K alloy. All solvents were distilled from Na
(toluene), K (THF) or Na/K alloy (ether, pentane) and stored under nitrogen. [C6H5C(NSiMe3)2]2YR
((µ-Me)2.TMEDA (2.6), CH2Ph.THF (2.7), CH(SiMe3)2 (2.8), µ-H (2.9) were prepared according to
procedures described in Chapter 2. Experimental details of the [C6H5C(NSiMe3)2]2YR (R =
CH2Ph.THF (2.7), CH(SiMe3)2 (2.8), µ-H (2.9)) catalyzed hydroboration of 1-hexene is described
elsewhere.14
Physical and Analytical Measurements. NMR spectra were recorded on a Varian VXR 300 (1H
NMR at 300 MHz, 13C NMR at 75.4 MHz) spectrometer. 1H and 13C NMR spectra were referenced
internally using the residual solvent resonances. IR spectra were recorded on a Mattson-4020
Galaxy FT-IR spectrophotometer. Elemental analyses were carried out at the Analytical Department
of this laboratory. Quoted data are the average of at least two independent determinations. The
polyethylene high temperature GPC was performed by Polymer Laboratories LTD. Polystyrene was
used as calibrant.
NMR Tube Reaction of [C6H5C(NSiMe3)2]2YR (R = CH2Ph.THF (2.7), µµ-H (2.9)) with Ethylene.
NMR tubes containing benzene-d6 (0.5 mL) solutions of 2.7 or 2.9 (0.01 - 0.02 mmol) were charged
with ethylene (4 atm) and monitored at fixed time intervals. Heating was required for the reaction to
proceed. At 65oC the ethylene was gradually consumed with formation of polyethylene. With 2.7, the
ethylene was consumed after several hours. For 2.9 the reaction is complete within 30 minutes (1H
NMR). The contents of the NMR tubes were quenched with methanol. The polyethylene was filtered
off, dried and characterized by IR.
Ethylene Polymerization Experiment. A 200 mL autoclave was charged with 100 mg (0.16
mmol) of 2.9 dissolved in 75 mL toluene. The autoclave was closed, pressurized with ethylene (70
atm) and heated to 50oC. After 1.5 hour, the autoclave was cooled to room temperature and the
pressure released. After quenching the reaction mixture (methanol), the polyethylene was filtered and
washed with methanol and pentane. After drying, 1.0 g polyethylene was obtained. The product was
characterized by IR spectroscopy and high temperature GPC: Mn = 46300, Mw = 239900, Mw/Mn =
5.2.
Preparation of {[C6H5C(NSiMe3)2]2Y(µµ-C≡≡CR)}2 (3.1, R = H; 3.2, R = Me). 3.1: (a) A 100 mL
Schlenk flask charged with a solution of 2.9 (1.4 g, 1.1 mmol) in benzene (10 mL) was treated with
acetylene (1.5 mmol) and allowed to stand for three days at room temperature, upon which large
colorless crystals of 3.1 were formed (1.1 g, 0.86 mmol, 78%). (b) A 100 mL Schlenk flask charged
with a solution of 2.8 (1.95 g, 2.52 mmol) benzene (10 mL) was filled with acetylene (2.6 mmol). The
flask was allowed to stand for three days at room temperature, upon which large colorless crystals of
3.1 were formed (1.3 g, 1.0 mmol, 80%). IR (KBr/Nujol, cm-1): 3262(w), 2926(vs), 2855(s), 1914(w),
1445(vs), 1387(vs), 1242(s), 1003(m), 982(s), 841(vs), 785(w), 760(s), 725(w), 700(w), 679(w),
482(w). 1H NMR (benzene-d6, δ): 7.33 (m, 4H, Ar), 7.10 (m, 6H, Ar), 3.22 (s, 1H, C≡CH), 0.19 (s,
Chapter 3.
70
36H, C6H5C(NSi(CH3)3)2). 13C NMR (benzene-d6, δ): 184.5 (s, C6H5C(NSiMe3)2), 146.7 (dt, Y-
C(≡CH)-Y, 1JY-C = 20 Hz, 2JC-H = 27 Hz), 143.2 (s, Ar), 128.1 (d, Ar, 1JC-H = 218 Hz), 126.5 (d, Ar,1JC-H = 160 Hz), 115.4 (d, C≡CH, 1JC-H = 218 Hz), 3.19 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 118 Hz).
Anal. Calcd. (found) for C56H94N8Si8Y2: C: 52.47 (52.83); H: 7.39 (7.52); Y: 13.87 (13.91). 3.2:
Compound 3.2 was prepared by a similar procedure from 2.8 (1.00 g, 1.29 mmol) and propyne; it was
isolated as microcrystalline material (0.60 g, 0.46 mmol, 71 %). IR (KBr/Nujol, cm-1): 2924(vs),
2855(s), 2060(w), 1445(vs), 1429(vs), 1402(vs), 1246(s), 1005(m), 982(s), 843(vs), 785(w), 760(s),
727(w), 702(w), 480(w). 1H NMR (benzene-d6, δ): 7.37 (m, 4H, Ar), 7.10 (m, 6H, Ar), 2.01 (s, 3H,
C≡CCH3), 0.21 (s, 36H, C6H5C(NSi(CH3)3)2). 13C{1H} NMR (benzene-d6, δ): 184.8 (s,
C6H5C(NSiMe3)2), 143.3 (s, Ar), 136.0 (t, Y-C(≡CMe)-Y, 1JY-C = 21 Hz), 128.1 (s, Ar), 126.8 (s, Ar),
8.0 (s, C≡CCH3), 3.9 (s, C6H5C(NSi(CH3)3)2). Anal. Calcd.(found) for C58H98N8Si8Y2: C: 53.18
(53.85); H: 7.54 (7.56); Y: 13.57 (13.48).
NMR Tube Reaction of {[C6H5C(NSiMe3)2]2Y(µµ-H)}2 (2.9) with 1-Pentyne. To an NMR tube
charged with a benzene-d6 (0.7 mL) solution of 2.9 (32 mg, 0.052 mmol), 1-pentyne (5.5 µL, 0.056
mmol) was added. After 24 hours at room temperature, all 1-pentyne had been consumed to give
{(C6H5C(NSiMe3)2)2Y(µ-C≡C-n-Pr}2 (3.3) as the only product. 1H NMR (benzene-d6, δ): 7.40 (m, 4H,
Ar), 7.07 (m, 6H, Ar), 2.60 (t, 2H, C≡CCH2), 1.91 (dt, 2H, CH2CH2CH3), 1.03 (t, 3H, CH2CH3), 0.26
(s, 36H, C6H5(NSi(CH3)3)2). 13C NMR (benzene-d6, δ): 185.1 (s, C6H5C(NSiMe3)2), 143.3 (s, Ar),
136.5 (t, Y-C(≡C(CH2)2CH3)Y, 1JY-C = 21 Hz), 132.4 (s, Y-C≡C), 128.4 (d, Ar, 1JC-H = 155 Hz), 127.7
(d, Ar, 1JC-H = 159 Hz), 126.5 (d, Ar, 1JC-H = 173 Hz), 25.1 (t, C≡CCH2CH2, 1JC-H = 129 Hz), 22.8
(m, CH2CH2CH3), 14.4 (q, CH2CH3, 1JC-H = 117 Hz), 4.1 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 117 Hz).
Preparation of {[C6H5C(NSiMe3)2]2Y(µµ-C≡≡CR)}2 (3.4: R = SiMe3; 3.5: R = Ph; 3.6: R = CMe3).
3.4: At room temperature, HC≡CSiMe3 (0.34 mL, 2.47 mmol) was slowly diffused into a pentane (5
mL) solution of 2.8 (1.65 g, 2.13 mmol). After 24 h, the colorless microcrystalline material formed
was washed with pentane (5 mL) and dried in vacuo , yielding 1.00 g (1.4 mmol, 66%) of 3.4. IR
(KBr/Nujol, cm-1): 2957(vs), 2926(vs), 2855(s), 1983(w), 1952(w), 1904(w), 1447(vs), 1435(vs),
1389(vs), 1246(s), 1005(m), 986(s), 922(w), 843(vs), 783(w), 760(s), 729(w), 702(w), 677(w),
650(w), 567(w), 480(w), 438(w). 1H NMR (benzene-d6, δ): 7.49 (m, 4H, Ar), 7.10 (m, 6H, Ar), 0.54 (s,
9H, C≡CSi(CH3)3), 0.28 (s, 36H, C6H5C(NSi(CH3)3)2). 13C{1H} NMR (benzene-d6, δ): 185.6 (s,
C6H5C(NSiMe3)2), 172.3 (t, Y-C(≡CSiMe3)-Y, 1JY-C = 20 Hz) 143.1 (s, Ar), 140.3 (s, C≡CSiMe3),
128.3 (s, Ar), 127.9 (s, Ar), 125.6 (s, Ar), 4.8 (s, C6H5C(NSi(CH3)3)2), 2.8 (s, C≡CSi(CH3)3). Anal.
Calcd.(found) for C62H110N8Si10Y2: C: 52.21 (52.35); H: 7.77 (7.81); Y, 12.47 (12.57).
Compounds 3.5 and 3.6 were prepared by a similar procedure and isolated as colorless crystals in
70% (3.5) and 77% (3.6) yield. 3.5: NMR data and elemental analysis showed that 3.5 contained 0.25
equivalent pentane per dimer in the crystal lattice. IR (KBr/Nujol, cm-1): 3040(w), 2912(vs), 2857(vs),
2049(m), 1883(w), 1769(w), 1597(w), 1576(w), 1441(vs), 1414(vs), 1381(vs), 1072(m), 982(m),
843(vs), 785(s), 756(vs), 721(vs), 702(s), 689(s), 544(m), 482(s), 438(m). 1H NMR (benzene-d6, δ):
7.92 (d, 2H, Ar), 7.48 (m, 4H, Ar), 7.07(m, 9H, Ar), 1.23 (m, 1.5H, pentane), 0.88 (m, 1.5H, pentane),
0.16 (s, 36H, C6H5C(NSi(CH3)3)2). 13C NMR (benzene-d6, δ): 184.8 (s, C6H5C(NSiMe3)2, 143.0 (s,
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
71
Ar), 141.8 (t, Y-C(≡CPh)-Y, 1JY-C = 21 Hz), 133.7 (dt, Ar, 1JC-H = 162 Hz, 2JC-H = 7 Hz), 131.1 (s,
C≡CPh), 129.3 (dt, Ar, 1JC-H = 158 Hz, 2JC-H 7 Hz), 124.6 (t, Ar, 2JC-H = 9 Hz), 22.7 (t, pentane, 1JC-H
= 124 Hz), 14.2 (q, pentane, 1JC-H = 124 Hz), 4.0 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 119 Hz). Anal.
Calcd.(found) for C68H112N8Si8Y2(C5H12)0.25 : C: 57.60 (57.44); H: 7.40 (43); Y: 12.09 (12.25). 3.6: IR
(KBr/Nujol, cm-1): 3057(w), 2957(vs), 2926(vs), 2870(s), 2855(s), 2037(m), 1443(s), 1429(vs),
1414(vs), 1397(vs), 1248(vs), 1003(w), 980(s), 839(vs), 785(m), 758(s), 748(s), 721(s), 704(s),
480(s), 438(m), 415(m). 1H NMR (benzene-d6, δ): 7.39 (4H, Ar), 7.04 (m, 6H, Ar), 1.52 (s, 9H,
C(CH3)3), 0.19 (s, 36H, C6H5C(NSi(CH3)3)2). 13C NMR (benzene-d6, δ): 185.7 (s, C6H5C(NSiMe3)2),
143.1 (s, Ar), 141.8 (t, Y-C(≡C(CMe3))-Y, 1JY-C = 22 Hz), 140.2 (s, C≡C(CMe3)), 128.2 (dt, Ar, 1JC-H
= 160 Hz, 1JC-H = 7 Hz), 127.4 (dd, Ar, 1JC-H = 117 Hz, 1JC-H = 7 Hz), 31.8 (q, C(CH3)3, 1JC-H = 126
Hz), 4.8 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 119 Hz). Anal. Calcd.(found) for C64H110N8Si8Y2: C: 55.14
(55.23); H: 7.95 (8.01); Y: 12.75 (12.82).
NMR Tube Reaction of 3.1 with THF; Preparation of [C6H5C(NSiMe3)2]2YC≡≡CH.THF (3.7). THF
(9 µL, 0.1 mmol) was added to an NMR tube charged with 3.1 (15 mg, 0.023 mmol) in benzene-d6
(0.5 mL). The 1H NMR collected after 5 minutes at room temperature showed the quantitative
formation of 3.7 and the presence of free THF. 1H NMR (benzene-d6, δ): 7.25 (m, 4H, Ar), 7.07 (m,
6H, Ar), 3.70 (m, 17.2 H, THF-α-CH2), 1.99 (d, 1H, C≡CH, 3JY-H = 1.3 Hz), 1.45 (m, 17.2H, THF-β-
CH2), 0.16 (s, 36H, C6H5C(NSi(CH3)3)2). 13C{1H} NMR (benzene-d6, δ): 183.5 (s, C6H5C(NSiMe3)2),
143.4 (s, Ar), 128.2 (s, Ar), 128.0 (s, Ar), 126.2 (s, Ar), 89.2 (d, C≡CH, 1JY-C = 12 Hz), 68.4 (s, THF-
α-CH2), 25.6 (s, THF-β-CH2), 2.5 (s, C6H5C(NSi(CH3)3)2.
Preparation of [C6H5C(NSiMe3)2]2YC≡≡CCMe3.THF (3.8). At room temperature, THF (50 µL, 0.62
mmol) was added to a pentane (20 mL) solution of 3.5 (0.8 g, 0.57 mmol). After 10 minutes, the
solution was concentrated until crystallization started. Cooling to -30oC yielded 3.8 (0.63 g, 0.82
mmol, 72 %) as a white microcrystalline solid. IR (KBr/Nujol, cm-1): 3061(w), 2955(vs), 2922(vs),
2854(s), 2043(w), 2022(w), 1446(vs), 1431(vs), 1410(vs), 1366(s), 1244(s), 1005(m), 984(s), 916(w),
841(vs), 787(m), 758(s), 723(m), 700(m), 480(m), 457(w). 1H NMR (benzene-d6, δ): 7.17 (4H, Ar),
7.02 (m, 6H, Ar), 4.14 (m, 4H, THF-α-CH2), 1.44 (m, 4H, THF-β-CH2) 1.41 (s, 9H, C(CH3)3), 0.13 (s,
36H, C6H5C(NSi(CH3)3)2). 13C NMR (benzene-d6, δ): 182.8 (s, C6H5C(NSiMe3)2), 143.7 (s, Ar),
143.1 (s, Y-C(≡CCMe3)), 127.9 (dt, Ar, 1JC-H = 160 Hz, 2JC-H = 7 Hz), 127.8 (dd, Ar, 1JC-H = 160 Hz,2JC-H = 7 Hz), 126.2 (dt, Ar, 1JC-H = 160 Hz; 2JC-H = 7 Hz), 111.1 (d, Y-C(≡CCMe3), 1JY-C = 11 Hz),
71.1 (t, THF-α-CH2, 1JC-H = 150 Hz), 32.7 (q, C(CH3)3, 1JC-H = 126 Hz), 28.1 (s, CMe3), 25.3 (t,
THF-β-CH2, 1JC-H = 133 Hz), 2.56 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 118 Hz). Anal. Calcd. (found) for
C36H63N4OSi4Y: C: 56.22 (56.31); H: 8.26 (8.34), Y: 11.56 (11.65).
Preparation of [C6H5C(NSiMe3)2]2Y(µµ-C≡≡CCMe3)2Li.TMEDA.(hexane) (3.9). To a solution of 2.6
(1.4 g, 1.8 mmol) in ether (40 mL), 3,3-dimethyl-1-butyne (0.45 mL, 3.7 mmol) was added. After 16
hours at room temperature, the solution was concentrated until crystallization started and cooled to -
30oC. Yield: 1.4 g (1.4 mmol, 75 %) of 3.9 as large colorless crystals. NMR data showed that 3.9
contains one equivalent of hexane in the crystal lattice. IR (KBr/Nujol, cm-1): 2956(vs), 2934(vs),
Chapter 3.
72
2826(vs), 2047(m), 1460(vs), 1454(vs), 1379(s), 1290(m), 1242(s), 1036(w), 1022(w), 1005(m),
984(s), 949(m), 847(s, br)785(s), 756(s), 735(s), 702(s), 683(m), 602(m), 478(m), 421(m). 1H NMR
(benzene-d6, δ): 7.42 (m, 4H, Ar), 7.14 (m, 6H, Ar), 2.33 (s, 12H, TMEDA-CH3), 2.02 (s, 4H,
TMEDA-CH2), 1.40 (s, 18H, C(CH3)3), 0.87 (m broad, 14H, hexane), 0.26 (s, 36H,
C6H5C(NSi(CH3)3)2. 13C NMR (benzene-d6, δ): 182.3 (s, C6H5C(NSiMe3)2), 144.2 (s, Ar), 127.8 (dd,
Ar, 1JC-H = 162 Hz, 2JC-H = 6Hz), 127.4 (d, Ar, 1JC-H = 160 Hz), 126.6 (d, Ar, 1JC-H = 166 Hz), 121.1
(d, Y-C, 1JY-C = 15 Hz), 57.1 (t, TMEDA-CH2, 1JC-H = 134 Hz), 46.9 (q, TMEDA-CH3, 1JC-H = 134
Hz), 32.4 (q, C(CH3)3, 1JC-H = 126 Hz), 3.3 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 118 Hz). The hexane
from the crystal lattice could be removed by heating the compound in vacuo at 50oC. Anal. Calcd.
(found) for C44H85LiN6Si4Y: C: 58.31 (58.39); H: 9.45 (9.51); Y: 9.81 (9.92).
Catalytic Dimerization of Alkynes HC≡≡CR (R = Ph, CMe3, SiMe3) by [C6H5C(NSiMe3)2]2YR (R
= CH2Ph.THF (2.7), CH(SiMe3)2 (2.8), µµ-H (2.9), µµ-C≡≡CR’ (R’ = SiMe3 (3.4), Ph (3.5), CMe3 (3.6)).
Alkynes (100 µL, 0.9 - 0.7 mmol) were added to NMR tubes charged with benzene-d6 solutions (0.5
mL) of 2.7 - 2.9 (0.02 (± 5) mmol; catalyst to substrate ratios ranging from 1 : 35 to 1 : 60). The 1H
NMR spectra collected after 1 hour at 80oC indicated the quantitative formation of the 1-buten-3-
ynes, traces of unreacted 1-alkyne and characteristic resonances of {[C6H5C(NSiMe3)2]2Y(µ-
C≡CR)}2 (3.4 - 3.6), when 2.8, 2.9 or 3.4 - 3.6 were used as precursors. Nt(80oC) = 20 - 40 mol.mol-
1. h-1. For the dimerization of HC≡CMe3, catalyzed by 2.7 or 3.8, the 1H NMR spectra collected after
1 hour at 80oC showed the presence of H2C=C(CMe3)-C≡CCMe3, resonances attributable to 3.8 and
considerable amounts of unreacted HC≡CCMe3 (Nt (80oC) ≈ 1 mol. mol-1 .h-1 ).
NMR Kinetic Experiments for the Catalytic Dimerization of HC≡≡CCMe3 and HC≡≡CSiMe3. In a
typical kinetics experiment, an NMR tube was charged with a standard solution of 2.8 (0.17 mM) in
benzene-d6 (0.7 mL) and 1-alkyne (100 µL, 0.7-0.8 mmol). After the NMR tube was filled and sealed,
an array of 1H NMR spectra was recorded at constant temperature (± 0.2oC) and appropriate time
intervals until the reaction was completed. The kinetics were monitored by following the difference in
integral of the vinylic-protons of the 1-butene-3-ynes formed and the 1-alkyne acetylenic-proton. The
measurements were repeated at different temperatures ranging from 53-77oC.
Preparation of {[C6H5C(NSiMe3)2]2Y2(µµ-(N,N’)-N(H)-C(Me)=C(H)-C≡≡N)}2 (3.10). To a solution of
2.8 (1.5 g, 1.9 mmol) in pentane (15 mL), acetonitrile (200 µL, 3.8 mmol) was added at room
temperature. After stirring for 16 hours at room temperature, the formed yellow solution was
concentrated until crystallization started and the mixture was cooled to -30oC. After isolation (1.0 g),
the crude product was recrystallized from pentane (10 mL). Cooling to -30oC yielded 3.10 (0.8 g, 1.1
mmol, 60 %) as large colorless crystals. IR (KBr/Nujol, cm-1): 2953(s), 2924(vs), 2854(s), 2152(s),
1523(s), 1429(vs), 1313(m), 1246(s), 1180(m), 1074(w), 1030(w), 1004(m), 986(s), 920(m), 839(vs),
785(m), 760(s), 731(m), 702(s), 684)m), 638(w), 569(w), 482(m), 414(w). 1H NMR (benzene-d6, δ):
7.22 (m, 4H, Ar), 7.03 (m, 6H, Ar), 6.84 (s, 1H, NH), 3.50 (s, 1H, CH), 2.16 (s, 3H, CH3), 0.16 (s,
36H, C6H5C(NSi(CH3)3)2. 13C NMR (benzene-d6, δ): 183.6 (s, C6H5C(NSiMe3)2), 179.1 (s, C≡N),
143.2 (s, Ar), 131.3 (d, C(CH3)=C, 2JY-C = 5 Hz), 128.1 (d, Ar, 1JC-H = 161 Hz), 126.3 (d, Ar, 1JC-H =
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
73
159 Hz), 48.1 (d, CH, 1JC-H = 175 Hz), 25.1 (qd, CH3, 1JC-H = 127 Hz, 3JY-C = 10 Hz), 2.5 (q,
C6H5C(NSi(CH3)3)2, 1JC-H = 118 Hz). Anal. Calcd. (found) for C60H102N12Si8Y2: C: 51.70 (51.64), H:
7.38 (7.44), Y: 12.75 (12.78).
NMR Tube Reaction of [C6H5C(NSiMe3)2]2YCH2Ph.THF (2.7) with N≡≡CCMe3. An NMR tube was
charged with a solution of 2.7 (23 mg, 0.030 mmol) in benzene-d6 (0.5 mL). Then N≡CCMe3 (2.8 µL,
0.30 mmol) was added. The 1H NMR spectrum of the reaction mixture monitored after 10 minutes at
room temperature showed that all 2.7 was consumed and that {[C6H5C(NSiMe3)2]2Y(µ-
N=C(CMe3)CH2Ph)}2 (3.11a) was formed. Within hours at room temperature, compound 3.11a
gradually rearranged into the enamine isomer {[C6H5C(NSiMe3)2]2Y(µ-NH-C(CMe3)=C(H)Ph)}2(3.11b). 1H NMR (benzene-d6, δ) of 3.11a: 7.55 (d, 2H, Ar, 3JH-H = 10 Hz), 7.30 (m, 6H, Ar), 7.16 (m,
7H, Ar), 3.93 (2H, CH2), 1.37 (s, 9H, C(CH3)3), 0.14 (s, 36H, C6H5C(NSi(CH3)3)2). The 1H NMR
spectrum indicated that the THF was released. For NMR data of 3.11b see below.
Preparation of {[C6H5C(NSiMe3)2]2Y(µµ-NH-C(CMe3)=C(H)Ph)}2 (3.11b). A solution of 2.7 (1.2 g,
1.5 mmol) in pentane (10 mL) was treated with N≡CCMe3 (0.14 mL, 1.5 mmol) at room temperature.
The yellow solution formed was allowed to stand overnight, after which it was concentrated to
approximately 5 mL and cooled to -80oC. The crude product was isolated and recrystallized from
pentane (5 mL). Crystallization at -80oC yielded 3.11b (0.9 g (1.1 mmol, 73 %) as a microcrystalline
material. 1H NMR (benzene-d6, δ): 7.80 (d, 2H, Ar, 3JH-H = 10 Hz), 7.25 (m, 6H, Ar), 7.09 (m, 7H,
Ar), 5.88 (s, 1H, NH), 5.25 (s, 1H, CH), 1.58 (s, 9H, C(CH3)3), 0.10 (s, 36H, C6H5C(NSi(CH3)3)2.13C NMR (benzene-d6, δ): 183.7 (s, C6H5C(NSiMe3)2), 170.1 (s, C(Me)=CH), 143.0 (s, Ar), 142.7(s,
Ar), 131.0 (dd, Ar, 1JC-H = 157 Hz, 2JC-H = 8 Hz), 128.0 (dd, Ar, 1JC-H = 161 Hz, 2JC-H = 6 Hz), 127.7
(dd, Ar, 1JC-H = 160 Hz, 2JC-H = 7 Hz), 126.5 (dt, Ar, 1JC-H = 157 Hz, 2JC-H = 7 Hz), 124.8 (d, Ar, 1JC-
H = 154 Hz), 122.1 (dt, Ar, 1JC-H = 156 Hz, 2JC-H = 7 Hz), 85.1 (d, CH, 1JC-H = 160 Hz), 37.8 (s,
C(CH3)3), 29.9 (q, C(CH3)3, 1JC-H = 125.5 Hz), 2.9 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 117 Hz).Anal.
Calcd. (found) for C76H124N10Si8Y2: C: 57.76 (58.10), H: 7.91 (8.04), Y: 11.25 (11.13).
NMR Tube Reaction of [C6H5C(NSiMe3)2]2YCH2Ph.THF (2.7) with N≡≡CMe. To an NMR tube
containing a solution of 2.7 (21 mg, 0.027 mmol) in benzene-d6 (0.5 mL), acetonitrile (3 µL, 0.05
mmol) was added at room temperature. The 1H NMR spectrum recorded after 10 minutes showed
that all 2.7 had reacted to give a mixture of compounds: 3.10 (10 %), {[C6H5C(NSiMe3)2]2Y(µ-
N=C(Me)CH2Ph)}2 (3.12a, 50 %) and {[C6H5C(NSiMe3)2]2Y(µ-NH-C(Me)=C(H)Ph)}2 (3.12b, 40 %)
along with resonances of uncoordinated THF. An 1H NMR spectrum recorded after two days at room
temperature showed no further changes. 1H NMR (benzene-d6) for 3.12a: 3.47 (s, 2H, CH2), 1.86 (s,
3H, CH3), 0.07 (s, 36H, C6H5C(NSi(CH3)3)2). 1H NMR (benzene-d6, δ) for 3.12b: 6.60 (s, 1H, NH),
4.78 (s, 1H, CH), 2.16 (s, 3H, CH3), 0.16 (s, 36H, C6H5C(NSi(CH3)3)2).
NMR Tube Reaction of {[C6H5C(NSiMe3)2]2Y(µµ-H)}2 (2.9) with N≡≡CMe. To an NMR tube
containing a benzene-d6 (0.5 mL) solution of 2.9 (20 mg, 0.032 mmol) N≡CMe was added (1.8 µL,
0.03 mmol). The 1H NMR spectrum monitored directly after the NMR tube was filled, showed
resonances characteristic for {[C6H5C(NSiMe3)2]2Y(µ-N=C(H)Me)}2 (3.13), δ: 9.3 (m, 1H,
Chapter 3.
74
N=C(H)Me), 7.28 (m, 4H, Ar), 7.08 (m, 6H, Ar), δ 2.40 (d, N=C(H)CH3, 3JH-H = 3.4 Hz), 0.11 (s, 36H,
C6H5C(NSi(CH3)3)2 and uncoordinated THF. Compound 3.13 is unstable and decomposes gradually
at room temperature to form a mixture of unidentified products.
NMR Tube Reaction of {[C6H5C(NSiMe3)2]2Y(µµ-H)}2 (2.9) with N≡≡CCMe3. To an NMR tube
containing a benzene-d6 (0.5 mL) solution of 2.9 (25 mg, 0.041 mmol), N≡CCMe3 was added (15 µL,
0.12 mmol). After 5 minutes at room temperature, the 1H NMR spectrum of the reaction mixture
showed characteristic resonances of uncoordinated THF and {[C6H5C(NSiMe3)2]2Y(µ-
N=C(H)CMe3)}2 (3.14). 1H NMR (benzene-d6, δ) for 3.14: 9.53 (m, 1H, N=C(H)Me, 7.31 (m, 4H, Ar),
7.07 (m, 6H, Ar), 1.25 (s, 9H, N=C(H)C(CH3)3), 0.19 (s, 36H, C6H5C(NSi(CH3)3)2. The product
(3.14) decomposes at room temperature to form a mixture of unidentified products.
NMR Tube Reaction of {[C6H5C(NSiMe3)2]2Y(µµ-H)}2 (2.9) with N≡≡CCMe3 and H2; Preparation
of [C6H5C(NSiMe3)2]2Y(N=C(CMe3)N(H)CH2CMe3) (3.15). N≡CCMe3 (16 µL, 0.14 mmol) was added
to a solution of 28 mg (0.045 mmol) of 2.9 in 0.5 mL of benzene-d6. The reaction mixture was
transferred into an NMR tube. At a vacuum line, the NMR tube was cooled to -198oC, degassed and
exposed to 1 atm. of H2. The progress of the reaction was followed by 1H NMR spectroscopy.
Initially, the same products (including 3.14) were formed as during the same reaction in the absence
of hydrogen. Gradually, the 1H NMR spectrum changed and after 1 day at room temperature, the
hydrogenated insertion product, [C6H5C(NSiMe3)2]2Y(N=C(CMe3)N(H)CH2CMe3) (3.15), had been
formed in 90 % yield. 1H NMR (benzene-d6, δ): 7.28 (m, 4H, Ar), 7.08 (m, 6H, Ar), 5.52, (s, 1H, NH),
3.40 (s, 2H, CH2), 1.19 (s, 9H, C(CH3)3), 1.17 (s, 9H, C(CH3)3), 0.12 (s, 36H, C6H5C(NSi(CH3)3)2.13C NMR (benzene-d6, δ): 186.6 (s, N(H)-C(CMe3)=NCH2CMe3), 183,5 (d, C6H5C(NSiMe3)2, 2JY-C =
3 Hz), 143.6 (s, Ar), 128.1 (dd, Ar, 1JC-H = 161 Hz, 2JC-H = 6 Hz), 127.8 (dd, Ar, 1JC-H = 161 Hz, 2JC-
H = 7 Hz), 126.4 (dt, Ar, 1JC-H = 157 Hz, 2JC-H = 7 Hz), 60.9 (d,CH2, 1JC-H = 133 Hz), 38.8 (s,
C(CH3)3), 32.9 (s, C(CH3)3), 29.2 (q, C(CH3)3, 1JC-H = 125 Hz), 28.6 (q, C(CH3)3, 1JC-H = 125 Hz),
2.7 (q, C6H5C(NSi(CH3)3)2, 1JC-H = 118 Hz).
NMR Tube Reaction of [C6H5C(NSiMe3)2]2YCH(SiMe3)2 (2.8) with Pyridine. Pyridine (6.6 µL,
0.08 mmol) was added to an NMR tube charged with 25 mg (0.032 mmol) of 2.8 in benzene-d6 (0.5
mL). The reaction was followed by 1H NMR. At room temperature, no reaction occurred. The NMR
tube was warmed to 50oC, upon which the solution changed from colorless to red. After one week at
50oC, the resonances of 2.8 had disappeared under formation of H2C(SiMe3)2 resonances and
signals in the vinyllic (3-6 ppm) and aromatic (7-9 ppm) regions.
NMR Tube Reaction of {[C6H5C(NSiMe3)2]2Y(µµ-H)}2 (2.9) with Pyridine. Pyridine (2.6 µL, 0.032
mmol) was added at room temperature to a solution of 2.9 (10 mg, 0.016 mmol) in benzene-d6 (0.5
mL). Upon addition, the solution changed from colorless to yellow. The reaction mixture was
transferred into an NMR tube and an 1H NMR spectrum was recorded. This showed that all 2.9 had
reacted under exclusive formation of the 1,2-insertion product [C6H5C(NSiMe3)2]2YNC5H6 (3.16). 1H
NMR (benzene-d6, δ): 6.54 (dd, 1H, H4 NC5H6, 3JH-H = 9.0 Hz, 3JH-H = 8.6 Hz), 5.44 (ddd, 1H, H5
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
75
NC5H6, 3JH-H = 5.6 Hz, 3JH-H = 6.0 Hz, 4JH-H = 2.4 Hz), 5.22 (dt, 1H, H3 NC5H6, 3JH-H = 9.4 Hz, 3JH-H
= 4.3 Hz), 4.52 (d, 2H, H2, H2’ NC5H6, 2JH-H = 4.3 Hz), 0.04, (s, 36H, C6H5C(NSi(CH3)3)2).
Preparation of [C6H5C(NSiMe3)2]2Y-NC5H5-2-CH2Ph. (3.17). Pyridine (0.15 mL, 1.8 mmol) was
added at room temperature to a solution of 2.7 (1.3 g, 1.7 mmol) in toluene (10 mL). The yellow
reaction mixture formed was stirred for 5 hours, after which the solvent was evaporated. The
remaining oil was stripped (3 x 5 mL) with pentane. Removal of all the volatiles in vacuo , resulted in
a sticky oil. The 1H NMR spectrum of the oil showed that 3.17 had been formed in 90 % yield.
Attempts to purify by sublimation or vacuum distillation failed and led to decomposition of the
product. 1H NMR (benzene-d6, δ): 7.02 (m, 1H, H6), 6.44 (dd, 1H, H4, 3JH-H = 9.0 Hz, 3JH-H = 8.6
Hz), 5.27 (dd, 1H, H5, 3JH-H = 6.0 Hz, 3JH-H = 4.7 Hz), 5.20 (dt, 1H, H3, 3JH-H = 6.0 Hz, 4JH-H = 2.6
Hz), 4.93 (dt, 1H, H2, 3JH-H = 6.0 Hz, 3JH-H = 5.1 Hz), 3.91 (d, 1H, C(H)H, 2JH-H = 11.5 Hz, 3JH-H =
11.0 Hz), 2.74 (dd, 1H C(H)H, 1JH-H = 12.0 Hz, 3JH-H = 8.1 Hz), 0.07 (s, 36H, C6H5C(NSi(CH3)3)2).13C NMR (benzene-d6, δ): 183.9 (s, C6H5C(NSiMe3)2), 143.1 (s, ipso-C ), 140.4 (d, Ar, 1JC-H = 161
Hz), 107.2 (d, CH NC5H4, 1JC-H = 160 Hz), 94.9 (dd, CH NC5H4, 1JC-H = 167 Hz, 3JC-H = 6 Hz), 55.4
(dt, C(H)CH2Ph, 1JC-H = 34.7 Hz, 3JC-H = 6 Hz), 41.6 d, CH2, 1JC-H = 29 Hz), 2.5 (q,
C6H5C(NS(CH3)3)2, 1JC-H = 119 Hz).
Preparation of [C6H5C(NSiMe3)2]2Y(ηη2-(C,N)-CH2-2-NC5H4) (3.18). To a solution of 2.7 (1.4 g,
1.8 mmol) in toluene (20 mL), α-picoline (180 µL, 1.8 mmol) was added at room temperature. After 4
hours, the volatiles were removed in vacuo and the remaining orange oil was stripped with pentane
(3 x 5 mL) but the product did not solidify. An 1H NMR spectrum of the isolated crude product (1.2 g,
1.5 mmol, 84%) showed some traces of 2.7 and α-picoline as the only contaminations. Since
crystallization, sublimation and vacuum distillation failed, the product could not be isolated
analytically pure. 1H (benzene-d6, δ): 7.92 (d, 1H, Ar, 3JH-H = 5.6 Hz), 7.30 (m, 4H, Ar), 7.02 (m, 6H,
Ar), 6.87 (ddd, 1H, H4 NC5H4, 3JH-H = 8.6 Hz, 3JH-H = 7.1 Hz, 4JH-H = 1.2 Hz), 6.68 (d, 1H, H3
NC5H4, 3JH-H = 8.3 Hz), 6.07 (dd, 1H, H5 NC5H4, 3JH-H = 6.8 Hz, 3JH-H = 5.6 Hz), 2.71 (d, 2H, YCH2,2JY-H = 0.9 Hz), 0.05 (s, 36H, C6H5C(NSi(CH3)3)2). 13C NMR (benzene-d6, δ): 183.8 (s,
C6H5C(NSiMe3)2), 166.3 (s, NC5H4), 146.3 (d, NC5H4, 1JC-H = 172 Hz), 135.6 (dd, NC5H4, 1JC-H =
158 Hz, 3JC-H = 7 Hz), 127.7 (d, Ar, 1JC-H = 161 Hz), 126.3 (d, Ar, 1JC-H = 157 Hz), 120.4 (d, Ar, 1JC-
H = 163 Hz), 106.9 (d, Ar, 1JC-H = 163 Hz), 52.4 (dt, YCH2, 1JC-H = 143 Hz, 1JY-H = 6 Hz), 2.6 (q,
C6H5C(NSi(CH3)3)2, 1JC-H = 118 Hz).
References and Notes.
1 (a) Watson, P. L. J. Chem. Soc., Chem. Commun. 1983, 276-277. (b) Evans, W. J.;
Meadows, J. H.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1984, 106, 1291-1300. (c)
Jeske, G.; Lauke, H.; Mauermann, H.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am.
Chem. Soc. 1985, 107, 8091-8103. (d) Den Haan, K. H.; Wielstra, Y.; Teuben, J. H.;
Chapter 3.
76
Organometallics 1987, 6, 2053-2060. (e) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.;
Burger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem.
Soc. 1987, 109, 203-219. (f) Bunel, E.; Burger, B. J.; Bercaw, J. E. J. Am. Chem. Soc. 1988,
110, 976-978. (g) Evans, W. J.; Chamberlain, L. R..; Ulibarri, T. A.; Ziller, J. W. J. Am. Chem.
Soc. 1988, 110, 6423-6432. (h) Burger, B. J.; Thompson, M. E.; Cotter, W. D.; Bercaw, J. E.
J. Am. Chem. Soc. 1990, 112, 1566-1577. (i) Heeres, H. J.; Meetsma, A.; Teuben, J. H.
Angew. Chem., Int. Ed. Engl. 1990, 29, 420-422. (j) Heeres, H. J.; Heeres, A.; Teuben, J. H.
Organometallics 1990, 9, 1508-1510. (k) Heeres, H. J.; Teuben, J. H. Organometallics 1991,
10, 1980-1986. (l) Sakakura, T.; Lautenschlager, H.-J.; Tanaka, M. J. Chem. Soc., Chem.
Commun. 1991, 40-41. (m) Forsyth, G. M.; Nolan, S. P.; Marks, T. J. Organometallics 1991,
10, 2543-2545. (n) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. Organometallics 1991, 10, 134-
142. (o) Gagné, M. R.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 275-294. (p)
Harrison, K. N.; Marks, T. J. J. Am. Chem. Soc. 1992, 114, 9220-9221. (q) Yasuda, H.;
Yamamoto, H.; Yokota, K.; Miyake, S.; Nakamura, A. J. Am. Chem. Soc. 1992, 114, 4908-
4910. (r) Booij, M.; Deelman, B.-J.; Duchateau, R.; Postma, D. S.; Meetsma, A.; Teuben, J. H.
Organometallics 1993, 12, 3531-3540. (s) Li, Y.; Fu, P.-F.; Marks, T. J. Organometallics
1994, 13, 439-440. (t) Hajela, S.; Bercaw, J. E. Organometallics 1994, 13, 1147-1154.
2 Watson, P. L. J. Am. Chem. Soc. 1983, 105, 6491-6493.
3 (a) Jeske, G.; Schock, L. E.; Swepston, P. N.; Schumann, H.; Marks, T. J. J. Am. Chem.
Soc. 1985, 107, 8103-8110. (b) Jeske, G.; Lauke, H.; Mauermann, H.; Schumann, H.; Marks,
T. J. J. Am. Chem. Soc. 1985, 107, 8111-8118. (c) Coughlin, E. B.; Bercaw, J. E. J. Am.
Chem. Soc. 1992, 114, 7606-7607. (d) Giardello, M. A.; Conticello, V. P.; Brard, L.; Gagné,
M. R.; Marks, T. J. J. Am. Chem. Soc. 1994, 116, 10241-10254. Decreasing the steric bulk of
the ancillary ligand system does not always lead to an increase in reactivity: (e) Stern, D.;
Sabat, M.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 9558-9575.
4 (a) Piers, W. E.; Shapiro, P. J.; Bunel, E. E.; Bercaw, J. E. Synlett 1990, 74-84. (b) Shapiro,
P. J.; Bunel, E.; Schaefer, W. P.; Bercaw, J. E. Organometallics 1990, 9, 867-869. (c)
Schaverien, C. J.; Orpen, G. Inorg. Chem. 1991, 30, 4968-4978 and references cited therein.
(d) Schaverien, C. J. Organometallics 1994, 13, 69-82. (e) Fryzuk, M. D.; Haddad, T. S.;
Rettig, S. J. Organometallics 1991, 10, 2026-2036. (f) Fryzuk, M. D.; Haddad, T. S.; Rettig,
S. J. Organometallics 1992, 11, 2967-2969. (g) Bazan, G. C.; Schaefer, W. P.; Bercaw, J. E.
Organometallics 1993, 12, 2126-2130. (h) Jubb, J.; Gambarotta, S.; Duchateau, R.; Teuben,
J. H. J. Chem. Soc., Chem. Commun. 1994, 2641-2642.
5 Den Haan, K. H.; Teuben, J. H. J. Chem. Soc., Chem. Commun. 1986, 682-683.
6 (a) Bates, R. B.; Kroposki, L. M.; Potter, D. E. J. Org. Chem. 1972, 37, 560-562. (b) Jung, M.
E.; Blum, R. B. Tetrahedron Lett. 1977, 43, 3791-3794
7 Evans, W. J.; Dominguez, R.; Hanusa, T. Organometallics 1986, 5, 1291-1296.
8 (a) Booij, Ph. D. Thesis , University of Groningen, 1989. (b) Deelman, B.-J.; Booij, M.;
Meetsma, A.; Teuben, J. H.; Kooijman, H.; Spek, A. L. Organometallics in press.
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
77
9 Evans, W. J.; Ulibarri, T. A.; Chamberlain, L. R.; Ziller, J. W.; Alvarez, D. Jr.;
Organometallics 1990, 9, 2124-2130.
10 Jubb, J.; Gambarotta, S. J. Am. Chem. Soc. 1993, 115, 10410-10411.
11 The polyethylene was characterized by IR.
12 Mw = 239900, Mn = 46300, Mw/Mn = 5.2. Note that the broad Mw/Mn value cannot be explained
by slow initiation relative to propagation alone: Gold, L. J. Chem. Phys. 1958, 28, 91-99.
Termination by β-H elimination will exacerbate the Mw/Mn broadening. Furthermore, for
ethylene polymerization Mw/Mn broadening can be attributed to heterogenization due to
precipitation of long-chain Y-alkyl species from the solution.
13 (a) Watson, P. L.; Roe, D. C. J. Am. Chem. Soc. 1982, 104, 6471-6473. (b) Eshuis, J. J. W.
Ph. D. Thesis , University of Groningen, 1991.
14 Bijpost, E. A.; Duchateau, R.; Teuben, J. H. J. Mol. Catal. 1995, 95, 121-128.
15 (a) Yamazaki, H. J. Chem. Soc., Chem. Commun. 1976, 841-842. (b) Yoshikawa, S.; Kiji, J.;
Furukawa, J. Makromol. Chem. 1977, 178, 1077-1087. (c) Ishikawa, M.; Ohskita, J.; Ito, Y.;
Minato, A. J. Organomet. Chem. 1988, 346, C58-C60.
16 Bianchini, C.; Frediani, P.; Masi, D.; Peruzzini, M.; Zanobini, F. Organometallics 1994, 13,
4616-4632. and references cited therein.
17 Akita, M.; Yasuda, H.; Nakamura, A. Bull. Chem. Soc., Jpn. 1984, 57, 480-487.
18 Horton, A. D. J. Chem. Soc., Chem. Commun. 1992, 185-187.
19 Duchateau, R.; Meetsma, A.; Teuben, J. H. manuscript in preparation.
20 (a) Evans, W. J.; Drummond, D. K.; Hanusa, T. P.; Olofson, J. M. J. Organomet. Chem.
1989, 376, 311-320. (b) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics 1993, 12,
2618-2633.
21 For example see: (a) Evans, W. J.; Wayda, A. L. J. Organomet. Chem. 1980, 202, C6-C8.
(b) Evans, W. J.; Engerer, W. J.; Coleson, K. M. J. Am. Chem. Soc. 1981, 103, 6672-6677.
(c) Atwood, J. L.; Hunter, W. E.; Wayda, A. L.; Evans, W. J. Inorg. Chem. 1981, 20, 4115-
4119. (d) Boncella, J. M.; Tilley, T. D.; Andersen, R. A. J. Chem. Soc., Chem. Commun.
1984, 710-712. (e) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. Organometallics
1983, 2, 709-714. (f) Shen, Q.; Zheng, D.; Lin, L.; Lin, Y. J. Organomet. Chem. 1990, 391,
307-312.
22 HC≡CH: ν(C≡C) (Raman) = 1947 cm-1; HC≡CMe: ν(C≡C) = 2140 cm-1; HC≡CPh ν(C≡C) = 2110 cm-
1; HC≡CCMe3: ν(C≡C) = 2110 cm-1; HC≡CSiMe3: ν(C≡C): 2037 cm-1.
23 Crystal data for (C28H47N4Si4Y)2: Space group: monoclinic, C2/c with a = 27.088(2) Å, b =
27.102(2) Å, c = 20.964(1) Å, β = 113.40(1)o, V = 14125(2) Å3, dcalc = 1.206 g.cm-3, and Z = 8.
Data were collected on an Enraf-Nonius CAD-4F diffractometer at 130 K with Mo K-α (λ =
0.71073 Å). The structure was solved by Patterson methods and refined to R(F) = 0.044,
Rw(F) = 0.036 for 7813 unique reflections with I ≥ 2.5 σ(I) and 1035 parameters.
24 Erker, G.; Frömberg, W.; Benn, R.; Mynott, R.; Angermund, K.; Krüger, C. Organometallics
1989, 8, 911-920.
Chapter 3.
78
25 Holton, J.; Lappert, M. F.; Ballard, D. G. H.; Pearce, R.; Atwood, J. L. Hunter, W. E. J. Chem.
Soc., Dalton Trans. 1979, 54-61.
26 (a) Morrison, T. T.; Boyd, R. N. In Organic Chemistry ; Allyn and Bacon: Boston, 1980, p 250.
(b) Tsutsui, M.; Ely, N.; Gebala, A. Inorg. Chem. 1975, 14, 78-81.
27 (a) C≡C distances for free acetylenes range from 1.19-1.21 Å: Dale, J. In Chemistry of
Acetylenes , Viehe, H. G., Ed., Marcel Dekker: New York, 1969, pp 3-96. (b) Weiss, E.; Plaso,
H. Chem. Ber. 1968, 101, 2947-2955. (c) Evans, W. J.; Ulibarri, T. A.; Chamberlain, L. R.;
Ziller, J. W.; Alvarez, D. Jr. Organometallics 1990, 9, 2124-2130.
28 (a) Evans, W. J.; Keyer, R. A.; Ziller, J. W. Organometallics 1990, 9, 2628-2631. (b) Heeres,
H. J.; Nijhof, J.; Teuben, J. H. Organometallics 1993, 12, 2609-2617. (c) Forsyth, C. M.;
Nolan, S. P.; Stern, C. L.; Marks, T. J., Reingold, A. L.Organometallics 1993, 12, 3618-3623.
29 (a) St. Clair, M.; Schaefer, W. P.; Bercaw, J. E. Organometallics 1991, 10, 525-527. (b)
Evans, W. J.; Rabe, G. W.; Ziller, J. W. J. Organomet. Chem. 1994, 483, 21-25.
30 Heeres, H. J. Ph. D. Thesis , University of Groningen, 1990.
31 Stockis and Hoffman have performed calculations on the polarization of the π* orbitals of
HC≡CMe and HC≡CSiMe3. The acetylene substituent appears to have a pronounced influence
on the electron density of the alkyne sp-carbon atoms and distinct differences in polarization
were found for both alkynes. These electronic effects are believed to be responsible for the
difference in regioselectivity of the catalytic dimerization of 1-alkynes. Stockis, A.; Hoffman,
R. J. Am. Chem. Soc. 1980, 102, 2952-2962.
32 Cp2YCH(SiMe3)2, prepared from {Cp2YCl}n and LiCH(SiMe3)2 in ether, was treated with 10 fold
excess of HC≡CCMe3. Heating to 80oC for two weeks showed the formation of {Cp2Y(µ-
C≡CCMe3)}2 (ref: 1b) as the only product, which does not show any reactivity with excess of
HC≡CCMe3. Duchateau, R.; Teuben, J. H. unpublished results.
33 The steric bulk of the bis(benzamidinato) ligand system is slightly smaller than of the
bis(pentamethylcyclopentadienyl) ligand set but significantly larger than of the
bis(cyclopentadienyl) ligand environment. The yttrium center in bis(benzamidinato) complexes
was found to be considerable more electrophilic than in the corresponding yttrocene
derivatives. For details see Chapter 7.
34 (a) Bercaw, J. E.; Davies, D.; Wolczanski, P. T. Organometallics 1986, 5, 443-450. (b)
Patten, T. E.; Novak, B. M. J. Am. Chem. Soc. 1991, 113, 5065-5066.
35 (a) Binev, I. G.; Todorova-Momcheva, R. I.; Juchnovski, I. N. Doklady Bolgarskoj Academii
Nauk. 1976, 29, 1301-1304. (b) Juchnovski, I. N.; Binev, I. G. J. Organomet. Chem. 1975,
99, 1-10.
36 Krüger, C. J. Organomet. Chem. 1967, 9, 125-134.
37 (a) Guram, A. S.; Jordan, R. F.; Taylor, D. F. J. Am. Chem. Soc. 1991, 113, 1833-1835. (b)
Borkowsky, S. L.; Baenziger, N. C.; Jordan, R. F. Organometallics 1993, 12, 486-495. (c)
Alelyunas, Y. W.; Guo, Z.; LaPointe, R. E.; Jordan, R. F. Organometallics 1993, 12, 544-553.
(d) Jordan, R. F.; Taylor, D. F.; Baenziger, N. C. Organometallics 1990, 9, 1546-1557.
38 Hirabayashi, T.; Itoh, K.; Sakai, S.; Ishii, Y. J. Organomet. Chem. 1970, 21, 273-280.
Bis(Benzamidinato) Yttrium Compounds: Reactivity.
79
39 3.15 is assumed to be similar to the scandium analogue. However, on the bases of NMR data
only, no distinction can be made between the following two possible structures:
Y
HR
H
H
C
RC
N
N
Y
H
R
H
C
RC
N
N
H
40 (a) Deelman, B.-J.; Stevels, W. M.; Lakin, M. T.; Spek, A. L.; Teuben, J. H. Organometallics
1994, 13, 3881-3891. (b) Deelman, B.-J. Ph. D. Thesis , University of Groningen, 1994.
41 (a) March, J. Advanced Organic Chemistry, Reactions, Mechansims and Structure , 3rd
ed.; John Wiley and Sons: New York, 1985. (b) Joule, J. A.; Smith, G. F. Heterocyclic
Chemistry ; Van Nostrand Reinhold Company: London, 1972.
42 Colgan, D.; Papasergio, R. I.; Raston, C. L.; White, A. H. J. Chem. Soc., Chem. Commun.
1984, 1708-1710.