Nitrogen-based analogues

of the uranyl ion

Nik Kaltsoyannis

Department of ChemistryUniversity College London

Introduction

Uranyl UO22+ is the most common functional unit in the chemistry of U(VI).

Imido ligand NR2- is isoelectronic with the oxo O2- ligand, and the two groups can often be interchanged in transition metal complexes.

The alkyl or aryl R group provides a variable unavailable in oxo chemistry synthesis of the isoelectronic imido analogues of uranyl is highly desirable.

However, synthesis of imido uranyl analogues has proved very difficult. In 1996 Denning speculated that the isolation of U(NR)2 is not possible because U(VI) is too oxidising.

Possible analogues - 1

“Synthesis and structure of the first U(VI) organometallic complex”

D.S.J. Arney, C.J. Burns and D.C. Smith, JACS 114 (1992) 10068

“Stable Analogues of the Uranyl Ion Containing the -N=U=N- Group”

D.R. Brown and R.G. Denning, Inorg. Chem. 35 (1996) 6158

How good is this analogy?

PR3

N

U

N

PR3

O

U

O

4+2+

PR3

N

U

N

PR3

ClCl

ClCl

O

U

O

ClCl

ClCl

2-

i.e.

Possible analogues - 2

On the valence electronic structure of UO22+

In 1999….what was generally agreed upon:UO2

2+ has 12 valence electrons (coming from the oxygen 2p and uranium 5f, 6d and 7s orbitals)These electrons are accommodated in four molecular orbitals, of g, u, g and u symmetry

5f non-bonding (and)

6d anti-bonding (and)

O(2p) combinations(6 in total)

5f anti-bonding (and)

6 bonding orbitals

(filled in UO22+)

6d non-bonding ()

U(5f)

U(6d)

u

g

g

u

U6+ UO22+ O2

4-

In 1999….what was generally not agreed upon:The ordering of these four molecular orbitals

Reference Computational method Molecular orbital ordering

Cornehl et al., Angew. Chem. Int. Ed. Engl. 35 (1996) 891.

CCSD/quasi-relativistic pseudopotentials

g < g < u < u

K.G. Dyall, Mol. Phys. 96 (1999) 511.

DHF/all electron g < u < g < u

Zhang et al., J. Phys. Chem. A 103 (1999) 6880.

CISD/relativistic ECPs g < u < u < g

De Jong et al., J. Mol. Struct. (THEOCHEM) 458 (1999) 41.

DHF+CCSD(T)/all electron g < u ~ g < u

g

u (HOMO)

u+g

(order not certain)

Summary of experimental studies

(from R.G. Denning, Struct. Bonding 1992, 79, 215.)

two questions:Is this the correct orbital ordering?If so, why is the u orbital so much less stable than the others?

Possible reasons for the destabilisation of the u MO of UO22+

1 Overlap between the U f and O 2p orbitals is small because the position of the angular nodes of the f orbital gives rise to extensive overlap cancellation in regions of different phase.

y

x

z

O

U

O

Figure 5.10 from The f elements by N. Kaltsoyannis and P. Scott

2 The “pushing from below” mechanism. The u orbital is destabilised by a filled-filled interaction with the U 6p semi-core orbitals.

C.K. Jørgensen and R. Reisfeld, Struct. Bonding 50 (1982) 121

-19

-20

-21

-22

-23

-24

eV

-25

u

g

g

u

u

g

g

u (HOMO)

u

U 6p in core U 6p in valence

20% U 6d79% O 2p

35% U 5f64% O 2p

13% U 6d, 2% U 7s77% O 2p, 8% O 2s

56% U 5f, 8% U 6p34% O 2p, 2% O 2s

100% U 5f

19% U 6d80% O 2p

36% U 5f63% O 2p

14% U 6d, 2% U 7s76% O 2p, 8% O 2s

49% U 5f47% O 2p, 3% O 2s

100% U 5f

u (HOMO)

Molecular orbital energy level diagrams for UO22+

N. Kaltsoyannis, Inorg. Chem. 39 (2000) 6009. R.G. Denning, JPCA 111 (2007) 4125.

Don’t believe everything you read in textbooks….

Overlap between the uranium valence orbitals and the oxygen p levels decreases in the order

f > f > d > d

y

x

z

O

U

O

A better qualitative molecular orbital energy level diagram for UO2

2+

Always include the actinide 6p orbitals in your calculations!

1u U f

3u U-O

3g U-O

2u U-O

1g U-O

1 U f

-0.44

1.751

+2.12-0.50

+2

+1

0

-1

-2

U OO

6 U-N

3 U-N

2 U-O

5 U-O

1u U f

3u U-N

3g U-N

1g U-N

U ON

2u U-N

1.7160.94 2+

+2.88

1.62

DhCv

-3

eV

1.659

Dh

U NN+

0.90

-0.62

1.7341.62

+1.34-0.67

20% U d79% O p

35% U f64% O p

34% U d66% N p

41% U f58% N p

22% U d/f75% O p

43% U d/f55% N p

Molecular orbital energy level diagrams for UN2, UON+ and UO22+

U NO PH3 U NPH3N PH3

(HOMO)(HOMO)4e U-N /P-H

1a2+8a1 U f

7a1 U-O

3e U-O

6a1 O-U-N-P-H 2e#

3eu U-N /P-H

1eg##

5a2u+1a1u U f

4a2u U-N-P-H

4a1g U-N-P

## U-N / between U-N and P-H

2eu##

# O lone pair/ between N p and P-H

2eg U-N /P-H

+2

+1

0

-1

-2

-3

eV

1.824Å

0.92

3+

+3.06-0.44

1.781Å1.711Å

1.42 0.57

-0.66 -0.59

1.823Å4+

+3.08

1.822Å

1.36 0.59

-0.66 -0.63

Molecular orbital energy level diagrams for OUNPH33+ and U(NPH3)2

4+

Why is below in OUNPH33+ and U(NPH3)2

4+?

-27

-26.5

-26

-25.5

-25

-24.5

-24

-23.5

1.68 1.7 1.72 1.74 1.76 1.78 1.8 1.82 1.84 1.86U-N bond length (Å)

6a1 O-U-N-P-H

4e U-N

Walsh diagram for elongation of the U-N bond in OUNPH33+

U NO PH3 U NPH3N PH3

4e U-N /P-H

1a2+8a1 U f

7a1 U-O

3e U-O

6a1 O-U-N-P-H

2e*

3eu U-N /P-H

1eg**

5a2u+1a1u U f

4a2u U-N-P-H

4a1g U-N-P

2eu**

2eg U-N /P-H

4e U-N /P-H

1a2+8a1 U f

7a1 U-O

3e U-O

6a1 O-U-N-P-H

2e*

3eu U-N /P-H

1eg**

5a2u+1a1u U f

4a2u U-N-P-H

4a1g U-N-P

**U-N / between U-N and P-H

2eu**

*O lone pair/ between N p and P-H

2eg U-N /P-H

+2

+1

0

-1

-2

-3

eV

1.8240.92

3+

+3.06-0.44C3v

1.7811.711

1.42 0.57

-0.66 -0.59

1.823 4+

+3.08D3d

1.8221.36 0.59

-0.66 -0.63

U 6p in core U 6p in coreU 6p in valence U 6p in valence

Molecular orbital energy level diagrams for OUNPH33+

and U(NPH3)24+ with U 6p in core and valence

Density functional theory calculations on UO2

2+ confirm the valence MO ordering proposed by Denning (on the basis of experimental data) and indicate that the u HOMO is destabilised with respect to the other valence MOs on account of a filled‑filled interaction with the uranium 6p semi‑core orbitals (the pushing from below mechanism).

Comparison of the isoelectronic series UO22+, UON+ and UN2 indicates that the uranium–

element bonding MOs are in all cases more stable than the uranium–element bonding levels, and that U–N bonding is significantly more covalent than U–O.

The U–N bonds in OUNPH3+ and U(NPH3)2

4+ are longer and less covalent than their equivalents in UON+ and UN2, and the U–N bonding MOs are more stable than the U–N levels. This reversal of the U–N / MO ordering with respect to UON+ and UN2 is due to a combination of two factors:(a) stabilising N–P(–H) contribution to the U–N MO(s)(b) increased U–N distance which destabilises the U–N bonding levels.

As with UN2 (and UO22+) the d MO of U(NPH3)2

4+ is much more stable than the f. This is partly due to the destabilising influence of the pushing from below mechanism on the f MO, which is found to operate in the iminato systems to a similar extent as in UO2

2+.

Summary

PR3

N

U

N

PR3

O

U

O

4+2+

So how good is this analogy?

“It is certainly correct of Denning to describe uranium bis iminato complexes as structural analogs of the uranyl ion, it is not clear that the

analogy can be fully extended to the electronic structure”

N. Kaltsoyannis, Inorg. Chem. 39 (2000) 6009

Never say never….

T.W. Hayton, J.M. Boncella, B.L. Scott, P.D. Palmer, E.R. Batista and P.J. Hay, Science 310 (2005) 1941T.W. Hayton, J.M. Boncella, B.L. Scott, P.D. Palmer, E.R. Batista and P.J. Hay, JACS 128 (2006) 10549

Comparison of UO22+, UN2 and U(NMe)2I2(THF)2

U OOU NMeMeN

(HOMO)

U NN

(HOMO)3u U-O

3g U-O

2u U-O

1g U-O

U-N

U-N

U-N

3u U-N

3g U-N

1g U-N

2u U-N

1.716Å 2+

+2.88 -0.44

1.844Å

+1.50

+2

+1

0

-1

-2

-3

eV

1.734Å

+1.34 -0.67

20% U d79% O p

35% U f64% O p

34% U d66% N p

41% U f58% N p

32% U f27% U f39% U f, 23% U d24% U d

24% U f

10% U d

+1.27 +2.84

blue = Mullikenred = NBO

“Overall, we can say that the U-N bonding orbitals in U(NMe)2I2(THF)2 are of the same type as those of the UO2

2+ fragment…although the ordering is different. This difference in order is an indication that the “pushing from

below” mechanism proposed for uranyl does not exert a strong influence in the present case due perhaps to a smaller involvement of the uranium 6p

orbital in the binding MOs”

T.W. Hayton, J.M. Boncella, B.L. Scott, P.D. Palmer, E.R. Batista and P.J. Hay, JACS 128 (2006) 10549