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~esuhs and discussion
3 P r o p e r t ~ e s o f nlcso-mono a n d n l e ~ o - t e t r a cupera ted porphyrln
c o ~ n p l e x e s
3 1 S y n t h c s ~ s a n d Chdracten7at ion
The complexes froni XV to X?i \\ere synthes~zed by the rcdctlnn of the
cnrrcrpond~ng n~etallo porphynn dnd B In the molar ratlo as discussed In chapter 2
and the propo\ed molecular structure 1s g l x n In F I ~ 3 1 The y~e ld IS found to be
very low ('ompounds XI: .YVI and X'VU are stable In alr and are soluble In almost all
nonpolar orgdnlc solvents In contrast to the parent porphynns VII, X and XYIII , w h ~ c h
are soluble only In polar +olvent+ The precursors are character17zd usrng electron~c
spectra. IR and I H N M R b AB mass analys~s has been performed for the complexes
and voltammetr~c analys~s and electron paramagnetic resonance (EPR) (at room
temperature dnd dt 77 K) of the complexes are d~scussed along w ~ t h the llgdnds
3 2 FAB M a s s A n a l y s ~ s
The FAB mass analysis formed the base for the confirmat~on of formabon of
complexes from XV to XX' In the absence of crystal structure Some of the complexes
resulted in the adduct Ion peaks and some resulted In molecular ion peak
B M
XVlll Cu
XIX VO
W Ni
Fig 3. 1. Proposed Molecular Structure of various mao-substituted porphyrins
From the FAB mass analysis, it is understood that all the complexes present
along with solvent molecules. Various drying procedures were not successful in
complete removal of solvent. The complcx XV shows the molecular ion pcak at 2082
as M+1 adduct peak ( C ~ S H I O ~ O ~ ~ N I I C U S C ' I ~ ) . 'The next fragment appears ar 1691 due
to loss of ( - C ~ H ~ J O S C I ~ C U ) unit.
A (Mt2) peak is obtained from the FAB mass study on XVI. Thus, mass
expected for XvI ( C ~ ~ I I I ~ ~ O ~ ! N I > C U ~ V C I ~ ) is at 2055 but the first adduct peak occur at
2057.
The complex XY1I.A ( C ~ ~ H ~ ~ O ~ ~ N I ~ C U A N ~ C ~ ~ ) resulted in [Mi] peak at 2041.
The loss of (-CsHl404CIl) frabmcnt from thc parent molecule results in thc nest peak
at 1834 as adduct peak.
The expected mass for the complex W I L B (CssI18dN1208CuZn4) is at 1759
however an adduct peak due to loss of (-CzHz) fragment at 1733 is obtalncd. Also the
peak at 1562 is assigned to the unit present aftcr the loss of (-CbHgNzCu)' and the
pcak at 460 is assigned to the fragment present after the loss of (-Cs~HaoN90~CuZtu)'.
The mono cuperated complex resulted in molecular ion peak unlike the tetra
cuperated porphyrins. Thus the complex X7'III (CbOHSo~~NOCu2C12) show first adduct
peak at 1099, which corresponds to M' ion peak.
n e complex X)( (CdlHb106NbC~NiC11) also show M' ion peak at 1249. The
next peak appears at 997 due to loss of (-C4H140jCIp) moiety.
Table 3.1. FAD-Mass account o f various arso-substituted porphyrins
S. No. Compound Mass Molecular formula Type of
Expeclud Observed ~ o n
2. XVI 2055 2057 C Y ~ H I O ~ O I ~ N I I C U ~ V C I ~ M t 2
3 . XVII-A 2041 2041 C'u4HlooOl2NI2Cu4NiCL M'
4. XVII-B 1729 1733 C X R H R ~ N I ~ O ~ C U Z ~ M-C2II2
The cntire mass spcchunl resulted in one identical peak at 391, which
corresponds to the common molecular structure ( C ~ ~ H I ~ N I ) . A FAB mass spectrum
of the complex is given in Fig 3 . 2 & 3. 3 and the Table 3 . I illustrates the
corresponding mass values.
3. 3. Nuclear Magnetic Resonance [NMR]
'H NMR spectra of l'rer porphyrins II, 111, V and precursor A were recorded on
AMX-300 MHz NMR sprctronleter in CDC13 using TMS as the internal standard,
while the porphyin [V was recorded on AMX-400 MHz NMR spectrometer in
DMSO-da.
The 'H NMR spectrum of A in CUCll is given in Fig 3. 4a. The spectrum
clearly shows the resonances characteristic of all the protons found in the ligand, A
broad singlet at 13.45 ppm corresponds to phenolic hydroxy proton. A sharp singlet
at 8.38 ppm corresponds to inlint. proton (.IIC=N) formed due to schiffs
condensation. The multiplet that c ~ ~ u r s in the range of 6.8 to 7.5 ppm can be
assigned to aromatic proton resonances. The alkyl S H 2 protons near imine group of
the precursor is found as a triplet at 3.7 ppm. Another triplet at 2.6 ppm arises from
alkyl <HI protons near terliary amine group. Resonances duc to N-methyl protons
of diamine appear at 2.3 ppm as a sharp singlet. I'hc observa[ion of phenolic proton
resonance and shift in the aldehydic proton from downfield to upfield suggest the
formation of Schiff s ligand A.
Thc '11 NMR spectrum of llclcarly reveals the resonances characteristic of all
the protons in the compound. The 'H NMK spectrum ofII in CDCI, is given in Fig 3.
4b and the resonance positions with the coupling constants are given in Table 3. 2.
The numbering sequence of protons is given in the structure. Tne p p ~ o l e protons
appear as singlet at 8.86 ppm. The H!,h protons of rr~eso-phenyl groups appear as
doublet at 8.13 ppm with = 8.4 Hz and I13,s protons appear at 7.28 ppm as
doublet with same J value. The methyl protons of 4-methoxy groups appear as a
singlet at 4.1 ppm and the N.H proton appear as bmad singlet at -2.76 ppm. All these
observations are in line with earlier studies.'"
Table 3. 2 'H NMR (3001400 MHz) information of ligands with TMS as internal
reference
Porphyrin's
Compound N-H protons /?-protons meso aryl Salen protons
in ppm in ppm groups' in ppm
protons
in ppm
II(CDCI!) -2.76, 2H 8.86, 8H H2.6 = 8.13
(2H, Jz.3 = 8.4
112)
H!,r = 7.28
(2H. J2.3 = 8.4
Hz)
~ O C H I = 4.10,
12H
III(CDCI3) -2.77, 2H 8.85, 8H H2,6,2..6, = 8.14-
8.08, 8 8
H,m5, = 7.55
(GH, J2.3, = 7.8
Hz)
HA,$ = 7.29
(2H, J2.3 = 8.4
Hz)
~ ~ C I I ! = 4.1,
3H
HI ,EH~ = 2.7,
9H
IUDMSO-do' -2.88, 2H 8.86, 8H HZ,' = 8.0 (8H,
12.3 = 8.4 Hz)
H,,r = 7.21
(8H, J2.3 = 8.4
Hz)
45 Table 3. 2 Contd ...
= 9.94,
4H
V(CDCI3) -2.77, 2H 8.85, 8H H2.6.28.6, = 8,10
- 8.04, 8H
H,.,i' = 7.55
(6H, J2,.3, = 8.1
Hz) H3.5 = 7.19
(2H, J2,3 = 8.4
Hz) H4.0~ = 8.95,
1 H
HCSH) = 2.7.
9H
A (CDCI,) l I~ , ." l l= 13.4S3
I H
H3 = 8.38, 1H
H3',4 , ,5 , ,6 , = 7.5
- 6.8. 4H
H3 ' 3.7, 2H
HZ = 2.6. 2H
Hi =2.3,6H
The resonances corresponding to all the protons present in 111 are observed in
the position in line with the earlier references. Fig 3 . 4 ~ illustrate the proton magnelic
resonance of Ill in CDCI3 corresponding to the numbering sequence of protons as
given in the structure. The Ppyrrole protons appear as singlet at 8.85 ppm similar to
that observed in 11. The doublets of and H2',6' protons of meso-phenyl groups
appear as multiplet due to overlapping of the two resonances in the range of 8.14 to
8.08 ppm. However the doublet from H 3 , , ~ , proton of 4-methylphenyl group appear at
7.55 ppm with J = 7.8 Hz and thc doublet from H3,5 proton of 4-methoxqphenyl p u p
appear at 7.28 ppm with J = 8.4 ppn). The methyl protons of 4-methoxy group appcar
as a singlet at 4.1 ppm and that of the methyl protons of 4-methyl group appear at
2.70 ppm as a sharp singlet corresponding to 9 protons, in analogous with the
~i tcrature. '~ ' The N-H proton appcar as broad singlet at -2.77 ppm.
It is found that the ' H NMR spectrum of IV in DMSO-d6 (Fig 3. 4d) appcarcd
comparable to the spectrum of / I except that the methoxy proton resonances disappear
and [he phenolic hydroxy proton appear. Thus the phenolic protons appear as a
singlet at 9.94 ppm. A small shift in the aromatic protons and N-H proton resonances
compared to I! is observed due to hydroxy groups and N-H appears as a broad singlet
at -2.88 ppm. However [hr Ppyrrole protons remained identical. All the resonance
positions and the coupling constants are given in Table 3 . 2 .
The 'H-NMR spectrum of Y in CDCI?, Fig 3 . 4 e clearly shows the conversion
of methoxy group to hydroxy group by the observation of resonance at 8.95 ppm as
singlet corresponds to one proton and disappearance of methoxy protons resonance.
It is notable that the 'H NMR spectrum of V, coincide with III, except that the
resonance positions of aromatic protons are slightly shifted due to hydroxy group
substitution. The observed resonances of all the protons along with the coupling
constants arc given in Table 3 . 2 .
3 .4 . Infrared spectra
Infrared spectral studies for all thc synthesized precursors and complexes wcre
undertaken however this study is not that much informative. IIence bricf discussions
of the significant funct~onal groups vibrations arc ~llustrated
Infrared spectrum of B givcs a broad band around 35 I6 cm" and is assigned to
coordinated water stretching vibration. Absorption at 2931 cm'l is recognked as C-H
stretching vibration and a s l ~ a ~ hbad at 1635 c n ~ " corresponds to C=N stretching
vibration. Vibrations around 1088 and 624 cm correspond to CI -0 stretching of per
chlorate ion.14'
All free base porphyrins and rnetalloporphyrins show respectivc vibrational
panern of their corresponding functional goups. Thus, the phenyl and pyrrole C-H
stretch is observed at 2940 cm", methyl stretch at 2831 cm-I, OH stretching at 3391
ern.', CO stretching at 1241 cm.', and N-H snctch as weak pattern overlapping in
aromatic CH stretching. The peripherally metallated complexcs show pattcrn
completely different from that of their corresponding parent precursors.
All the mono and tetra cuperated porphyrins show a sim~lar infrared spectrum.
A stretching vibration around 2900 cm" was attributed to C-H stretching v~bration of
aromatic group. A vibration at around 2852 cm'l was attributed to C-H stretching
vibration of methyl group. A sharp band around 1650 cm.' corresponding to C=N
stretching vibration is noticed in all the spectra indicating that the metal precursor B is
coordinated to the porphyrin.
c
500 600 700 300 4tavelength (nm)
' n n h 300 400 500 600 700
Wavelength (nm)
0.0 300 4W 700 300 400 500 600 700
~avel t$h (nmfoO Wavelength (nm)
-- , I , I , I ' . - l . ? . i
300 400 500 600 700 800 Wavelength (nm)
Fig 3. 5 , Electronic absorption spectrum in lOmm pathlength (0.OlmM) methanolic
solution of (a) 1V (b) VI (c) LY (d) Xll and (e) B
3 5 Electronic spectra
Electron~c absorptron spectra are studled In methanol and I or d~chloromethane
as solvents I h e ahsorpt~on spectrum of B In methanol Ir glvcn In Fig 3 5 and the
absorptron maxlma are grven In Tahle 3 3 The spectrum ?how+ absorpt~on around
370 nm that anses due to llgand to metal charge transfer A broad pattern centered at
550 nm corresponding to metal d-d transrtlon ~q alqo obrerved 14' Frsc base
porphyrlns I I - V, Fig 3 5 show four vrs~hle bandr, Q band< between 500 and 600 nm
(aau(n) + e,(n') symmetry In D4h) and a strong near UV rcglon around 400 nm, H or
Soret band (al,(n) + e,(ne) symmehy In D4h) 1 9 " Table 3 3 yrve the abborptton
maxlma of I1 ln drchloromethane Hydrolya~s of methoxy group to hydroxy g o u p to
form IV results In a small sh~f t In the Soret band towards blue regron dnd Q-bands
show no observable s h ~ f t
The elechonlc absorpt~on spectra of metalloporphyrlns VI - XIJ,' reveal
transltrons In the expected llnes " I '
The observation of only two Q bands, sh~fted In compan\on to the free base
porphynns II - V, confirms the coordrnat~on of metal ion at the central lore of the
porphyrrn nng On coordrnatlon w ~ t h metal precursor, R, the Q band pattern In W,
XVI and XVII do not show any marked change, suggesting that the porphyrln structure
has not perturbed drastically However, the Q band absorption energy IS found to
shlft sl~ghtly towards lower energy and w ~ t h Increased broadness Thls IS due to
meso- posltlon perturbahon of the porphynn nng l9 "
Table 3. 3 Electronic absorption spectral statistics of various meso-subst~tuted
Porphyrins (0.OlmM) in dichloromethane.
S. No. Compound Absorption maxima (in nm)
B(0,O) Qd1 .0) Qy(0,O) C',(l70) Qx(0,O) Others
... I . B * .- --- ..- --- 353 377 554
2 . IV* 410 508 545 584 643 --- --- ---
... - . . . . . . 5 . VfII 407 -- 525 --- -
6. X 421 --. 543 587 .- -. - .- --.
... ......... 7. XI 414 --- 530 .-.
... 8. XIII 405 518 .- --. -. . -. -. -
... ... ......... 9. XIV 408 514 -.-
... 10. XV 410 -- 535 583 352 381 ---
... 1 1 . XVf 419 --- 540 581 358 377 --- ... ... ... 12. X Y l l A 410 520 347 376 ---
... 13 XVIl-8 416 --- 53 1 566 384 ---
... ...... 14 XI'III 41 1 527 571 -- 305
... ... ... ...... 15 xrx 408 515 322
...... 16. XX 413 --- 529 572 -. 268
* - Methanol IS used a s thc solvcnt
The Soret band region in XV, XYI and XVII show additional features in
comparison with their respective parent metalloporphyrins. In VII, X and XIII the
Soret bands appear as usual at 406, 421 and 405 nm, respectively. Howcver, on
complexation with B, the corresponding Soret bands in XV, XVI and XVlI show more
broadness and also shift to lower energy. Additionally, one could also see additional
broad shoulder like panerns in the blue side of the Sorct band maximum. We believe
that these new features originate from B chromophore. As in pure A, we observe
strong featureless charge transfer transitions at 377 nm, and a broad weak ligand field
transitioil of copper ion at 550 nm. Thus the charge transfer transitions overlap with
the porphyrin Soret transitions resulting in broadening and appear as shoulder like
pattern. Also, we believe that the ligand field transition of copper in B chromophore
is buried under s&onger Q bands of metallopolphyrins as one could see more
broadening nnd increased baseline intensity in the region greater than 600 nm, which
is not the case in the parent metalloporphyrins. Since, the electronic transitions of the
individual units in XV, XVI and XVII neither show any major change nor any other
new transitions, it is clear that the electronic interactions between porphyrin ring and
B units are negligible. The absorption spectra of XV, XVI and XVII are shown in Fig
3 . 6.
The monocuperated metalloporphyrins XYIII, XLY and XX result in an
absorption spectrum as shown in Fig 3 . 6. Similar to the tetracuperated complexes,
monocuperated complexes, XVIII, XIX and XI', also do not show any marked change
in the Q band pattern due to coordination with metal precursor B at the peripheral site,
suggesting that the porphyrin structure has not perturbed drastically. However, the Q
band absorption energy is found to shifi slightly towards lower energy and peak
o
b a a n 0 5 n 0 5 c
C
0 0 0 0 300 400 500 600 700
300 400 500 600 700 Wavelength (nm) Wavelength (nm)
00- 300 400 500 600 700
Wavelength (nm)
250 300 350 400 450 500 550 600 Wavelength (nm)
Wavelength (nm)
0.0 250 300 350 4CO 450 500 550 600
Wavelength (nm)
Fig 3. 6. Electronic absorption spectrum in lOmm pathlength (0.OlmM)
dichloromethane solution o f (a) XY (b) XVl (c) X Y I I A (d) W I I I (e)
XYU(and (f)H
Fig 3. 7 . EPR spectrum (a) ,17'(b) XVl ( c ) MI-A (d)XVlll (e) XlXat room
temperaturc .
brt~adness is not to the extent as observed in the tetracuperated complexes. The
difference in the broadness between tetracuperated and monocuperated complexes
arise due to difference in the number of metal ions coordinated at the periphery of the
porphyrin. The electronic absorption data are tabulated in Table 3. 3. The Soret
region also shows similar bchavluur revealing additional shouldcr like pattern. All
these results suggest that the coordination of copper has taken place at the rtzeso-
phet~yl site.
3 . 6. Electron Paramagnetic Rcsonance [EPK]
'I'he EPR spectra are recorded for solution and polycrystalline powder of the
prepared complexes at room temperature and at liquid nltrogen temperature. The
EPR spectra of polycrystalline powder of all the metalloporphyrins at room
temperature show featureless broad lines Also, the solution spectmm in ethanol or
dichloromethane at room temperature shows isohopic p and A values. Since, these
spectral patterns do not reveal any addit~onal ~nformation, no further discussions are
made. However, for the sake of completeness, the spectral patterns are given in Fig
3. 7
The EPR spectrum of precursor B at 77K, (Fig 3. R), shows four line pattern
and spin Hamiltonian parameters are estimated through simulation as g1=2.260.
Allcu'18.5 mT, AI1"=1.30 m7.. gl=2.072, A ~ ~ ~ = 1 . 6 8 mT, Al"l.30 mT. The
observed g and A values suggest that the copper exists in a more distorted
geometry.'48 This geometrical distortion arises due to the flexibility of the SchifFs
jigand.
Fig 3. 8. ESR spectrum of B In ethanol at 77 K. (a) expenmental, (b) simulated. 7he
simulated parameters are g11.2.260, &=2.072, ~ ~ ~ ' ~ = 1 8 . 5 r n ~ , A,('=] .68
mT, AIIN=1.3 mT, A~'= I .3 mT. Line width: parallel =1.6mT,
perpendicular = 1.6 mT. v = 9.053 GHz
The spectrum of VII obtained from ethanol glass at 77 K IS given In Fig 3. 9.
shows a typical axially symmetric copper(I1) ion spectrum coordinated a[ thr center of
the porphyrin core."' The unpaired electron in d?.; orbital of copper(l1) itrn, with
electronic ground state '81, gives rise to two sets of metal hyperfino lines
corresponding to gl and g~ values. The first two components of the four copper
hyperfine lines in the parallel rcgion are well resolved in low Reld and Ihe third
component is slightly merged with the much stronger pcrpendicular Iinc to the
different extent while the fourth parallel line is completely overlapped. The
appearance of super hrperfine lines from four-nitrogcn atom ind~cales the
coordination of copper at the center of the porphyrin core. I'he simulation of EPR
spectrum (Fig 3. 9) allowed us to estimate accurately, the spin Hamiltonian
parameters as g1=2,190 ~ # ~ ~ = 1 9 . 6 3 mT, ~ ~ ' = 1 . 9 8 mT, g,=2.045, ~ - " " = 3 . 4 mT,
~ ~ ~ = 1 . 4 2 mT and the values are summarized in Table 3 .4 .
The EPR behaviour of X in frozen ethanolic solution is reproduced in Fig 3.
10. The spectrum clearly shows two sets of eight vanadyl (I = 712) hypcrfine splitting
patternb and further simulation (Fig 3. 10) confirms the assignment and the spin
Hamiltonian values are given in Table 3. 4.
Table 3 . 4 EPR Spin Hamiltonian parameters of various n~eso-suhstituted
porphyrins at 77 K
S No. Compound gll gl Allin mT A! in mT A j Y in ml' ~ ; ? n mT
1. B 2.260 2.072 18.5 1.68 1.3 1.3
2 . V11 2.190 2.045 19.63 3.4 1.98 1.42
3. X 1.965 1.982 16.92 5.64 0.29 0.28
4. XV 2.247 2.081 19.53 2.8 1.18 1.08
2.222 2.056 19.31 3.02 1.16 1 .08
5 XVI 1.959 1.982 17.02 5.78 0.29 0.28
2.210 2.055 18.7 1.88 1.3 1.3
6 . XVII-A 2.230 2.054 18,O 1.7 1.025 1.025
7, XYII-B 2,180 2.040 19.6 3.3 1.97 1.42
8, XVIII 2,198 2,055 19.59 3,2 I .8 1.36
2.222 2.067 18.6 1.7 1.25 1.1
9. XIX 1.962 1.982 16.89 5.65 0.29 0.28
2.205 2.053 18.5 1.68 1.3 1.3
10. XI: 2.226 2.065 18.0 1.78 1.03 1,03
Complex XIIldo not show any EPR signals since, nickel(I1) in a square planar
geometry exhibiting a low spin diamagnetic state.lsO
The EPR spechum of XVII-A recorded at room temperature and at 77 K is
shown in Fig 3. 11 which reveals a typical axially symmetrical four line pattern of
copper ion. The spin Hamiltonian values are estimated from simulation. Thus, the
\.slues are found to be gll=2.230, ~ ~ " ~ = 1 8 . 0 mT, ~ ~ " 1 , 0 2 5 mT, gL=2.054, A ~ ~ " = I , ~
rnl', A:'=l.025 mT. In comparison with, pure B, we could see a slight lowering of g
values without much change in copper hyperfine values indicating that the copper
adapts more planar geometry than in pure B. Since, nickcl(I1) in a square planar
geometry exhibits diamagnetic statelso, the resultant spectrum of XVII-A should
reflect the spin interactions beween the peripheral copper ions. Since the spectrum
does not show any such effect, as peripheral copper centers do not interact with each
other. I'his is not surprising, as the copper centers are dispossessed at a large distance
from cach other. (In t h ~ s context, zinc porphyrin could have been an ideal cho~ce than
nickel porphyrin. However, our efforts in synthesizing tetracuperated zinc porphyrin
failed and invariably compound XV resulted. This must have occurred by trutis
metallation of zinc by copper at the porphyin center'").
To prove unequivocally, microwave power saturation recovery technique is
employed to determine the relaxation parameters of the synthesized romplexes. This
technique involves the study of the intensity of EPR transition lines as a function of
microwave power.152 The dependency of peak area with microwave power is shown
in Fig 3 . 12a and the related saturation EPR spcctnm are given in Fig 3. 12 (b - 0.
Fig 3.9. ESK spectrum of VII in ethanol at 77 K, (a) experimental; (b) simulated,
I h e simulation parameters arc gl.2.19, ~ ~ 2 . 0 4 5 , ~ ~ ~ ~ = 1 9 . 6 m T ,
~ p = 3 . 4 m T , ~ ~ ~ = 1 . 9 8 ml', ~ 1 ~ = 1 . 4 2 mT. Line width: parallel = 0.55
mT, perpendicular = 0.4 mT. v = 9.062 GHz
Fig 3. 10. ESR spectrum ofXin ethanol solution at 77 K. (a) experimental; @)
simulated. The simulation parameters are g14.965, ~ ~ 1 . 9 8 2 , ~1"=16.9
mT, AIV=5.64 mT, ~ ! ~ = 0 . 3 mT, A?=0.3 mT, Line width: parallel = 0.6
mT, perpendicular =0.45 mT. v = 9.052 GHz
1"ig 3. 11. ESR spectrum of XL'll-A in ethanol at 77 K. (a) experimental; (b)
simulated. The simulation parameters are g1=2,23, b=2,054, A ~ ~ ~ = I S . O
m'T, A ? = I . ~ mT, ~ ~ " 1 . 0 3 mT, ~:=1.03 mT. Line width: parallel =
1.6 mT, perpendicular = 1.6 mT. v = 9.052 GHz
$ 4 , '
m am (m .m lam Ilmo (am lbm]
POI- ("WI
F I ~ 3 I2a EPR microwave power saturation recovety plot at 77 K of 0 5 niM lor the
peak of (a) B at 261 mT (b) XVII-A at 261 mT (c) X at 354 mT (d) SI'I at
354 mT (e) XVII-B at 272 mT Solid l ~ n c represents theoretical fit to the
equat~on I = [a ( P ) ' ~ I ( I + P I ~ ) ] ~
The experimental data are fined to the following equation, ernploylng non-hear least
square fitting
afhcrc a' Ir a proport~onal~ty constant, 'b' IS the value of microwave power
' p ' sucli thdt 111tenslty I. measured exper~mentally, falls half the value that should be
attatned in the abscnce of saturation and r md~cates the degree of homogene~ty of the
sp111 systems Thc parameter ' c ' assumes values between 1, completely
homogcneous, and 2, completely ~nhomogeneous, at low mrcrowave power Slnce,
the valuc of b relates to the measure of sptn-spin relaxat~on times"' by the relatton h
= y 2 ~ ' T I T 2 , where y l i & y o magnetic ratlo, H 1s the appl~ed magnetic field, TI IS the
spln l an~ce rclaxat~on tlrne and TI IS the tlme constant of the rate of attainment of
equll~brlum, when the spln relaxes w~thln the spln system, the valuc of b muqt reflect
the spin relaxat~on tlme of the copper Ions and any change In ~ t s value can bc
correlated to the extent of spin-spln lnteractlons between the paramagnerlc centers
I h e estimated b value B 1s 1 88 mW and c 1s found to be 0 99 lnd~cattng near
homogcneous spln system The b value for XVII-A 1s found to be 1 54 mW, which 1s
qulte sllnllar to thar of B indlcat~ng no ~nteractlon between the copper ceniers The
small d e v ~ a t ~ o n In the value In X'VII-A may be due to some structural changes to artaln
Although we were not able to synthesize zlnc at the core of the porphynn tn a
tetracuperated porphynn system, we have succeeded In syntheslzlng a tetrazlnc-
Pig 3 . I?b. Representative picture of EPR Power Saturation spectrum of B (0.5 mM)
in ethanolic solution at 77 K in the X-band frequency of 9.05252 GHz in
the range of 200 to 400 rnl'. * represents the peak at 261 rnT used for
saturation plot.
Fig 3. I2c. Representative picture of EPR Power Saturation spectrum of'X7'll-A (0.5
mM) in ethanolic solut~on at 77 K in the X-band hquency of 9.0518 GHz
in the range of 250 to 350 mT. * represents the peak at 261 mT used for
saturation plot.
I..
< .*
bhl;T--/pip. - ' 1
."I
;ypp-
Fig 3. 12d. Representative picture of ESR Power Saturation spectrum of A?/ (0.5
mM) in chanolic solution at 77 K in the X-band frequency of 9.0589
GHz in the range of 340 to 420 mT. * represents the peak at 354 mT
used for saturation plot.
copper porphyrin complex XVII-B. The EPR spectrum ofXYII-B in room temperature
and at 77 K is given in Fig 3. 13. The spectrum resembles that of VII with almost
same spin Hamiltonian parameters. Since Zn(l1) is a diamagnetic metal and hence hy
substitution of it at the periphery of VU do not alter the spectrum of copper ion at the
porphyrin corc. This confirms that the modification at the peripheral site alone do nut
alter the nature of the magnetic propeny of the central copper ion. The relaxation
tlmc measurements also confirm this prediction as we obscrve b as 6.12 mW. which is
similar to that obsewed in VII (6.08 mW). Such obsenaations arc quite common in
paramagnetic mctal centre in diamagnetic hosts 154,155 and in macro-cycle ligand
The EPR spectrum of XV shows interesting results. The room temperature
solution spectrum in dichloromethane contains four isotropic lines with g,,, and Ai,, as
2.1 I9 and 7.4mT, respectively. However, the solution spectrum at 77 K, (Fig 3. 14),
clearly reveals transitions corresponding to two types of copper ions with nearly
identical spin Hamiltonian values but with different intensity distribution. These two
types of transitions must arise from the VII and B units present in XV. Moreover, the
spectrum reveals complete loss of nitrogen super hyperfine lines of copper at the
porphyrin core suggesting extensive broadening of spectrum. In contrast, the
cthanolic frozen solution spectrum of a mixture, consisting of 1:4 molar ratios of VII
and 5, shows super hyperfine splitting pattern from nitrogen and the resulting pattern
(spectrum not shown) could be best described as superposition of the spectrum of the
two components. This means that the loss of nitrogen super hyperfine lines in XV
must arise from the interaction between central copper and peripheral copper centers.
No additional transitions are noticed in the frequency region below or above the main
Fig 3. 126. Representative picture of EPR Power Saturation spectrum of X (0.5 mM)
in ethanolic solution at 77 K in the X-band frequency of 9.0521 5 GHz in
the range of 340 to 420 mT. * represents the peak at 354 mT used for
saturation plot.
Fig 3. 12f. Representative picture of ESR Power Saturation spectrum of IX (0.5 mM)
in ethanolic solution at 77 K in the X-band frequency of 9.051 8 GHz in the
range of 266.0 to 278.0 mT. * represents the peak at 272 mT used for
saturation plot.
resonance position, indicating that the nature of interaction is strongly dipolar.
Assuming that the peripheral copper sites do not interact with each other, as proposed
earlier, we simulated the spectrum considering two types of copper ions with line
broadening due to dipole interaction between cenbal copper ion and peripheral copper
ions. The simulated spectrum is given in Fig 3. 14 and there is a good agreement
between the experimental and simulated spectrum. We notice many features in the
spectral pattern firstly, the g l values are found to be 2.247 and 2.222 for VII and B
units respect~vely. Similar changes in g, values are also noted and the values are
2.081 and 2.056, respectively. In comparison with the values of the respective pure
parent compounds, the central copper ion shows an increase in g[ and g, values while
the peripheral copper ions exhibit a decrease. Since, the g values of both the moieties
approach each other, 11 is clear that both the set of copper ions stabilize in a nearly
similar geometry Secondly, in comparison with the parent compound B, the Ay
value of peripheral copper ion in XV increases from 18.5 lo 19.3 mT, which strongly
suggests that copper ion, adapts more planar geometry than in pure B. Also, the AL
value increases from 1.68 to 3.02 mT, and this large increase indicates more axially
symmetric nature of peripheral copper in XY than in B. This further means that the
electronic density is more delocalised in the peripheral copper ion."' However, on
expected lines, the A values of VII fragment do not show any marked change as the
copper ion is already exists in a stable planar ~onfi~uration. '~ ' Thirdly, the true line
width of VII in XV, in comparison with the parent VII, estimated through simulation,
is found to increase h m 0.5 and 0.4 mT to 1.25 and 1.35 mT for parallel and
perpendicular lines respectively. This incnase in the line widths is responsible for the
loss of nitrogen super hypertine lines of the potphyrin core copper ion in XO.
However, the line width of B in XV is found to be nearly same at 1.6 mT, as observed
in B. Since, it is clear that the peripheral copper sites do not interact between
themselves, as shown in the case of XVII-A, and peripheral modification alone do not
bring about spectral broadening of the central copper ion, as shown in the case of
X W - B , the increased broadness in the spectrum of XV is a clear indication of strong
dipolar interaction between the central copper and peripheral copper ions. The
measurement of spin relaxation times TI, of copper ions in XV would give direct
cvidcnce of dipolar interaction. However, the overlap of both central copper and
pcripheral copper ions' peaks forbids us to evaluate any reasonable relaxation time
data Since, this interaction requires a shift in the electron density from copper ion
towards the porphyrin n system, measurement of degree of covalency would throw
somr light on the extent of interaction. Thus, we estimated the change in the electron
density character in terms of degree of covalency, a', in various bonds through the
well-known
in which A[ values are taken in units of cm". The a values are found to be 0.88 for
VII and 0.92 for Ayindicating that the Cu-N bonds at the porphyrin core are slightly
more covalent in XV than in VII. This increased covalency must be occuning by
adapting more planar geometry resulting in more delocalization of metal non-bonding
electron density towards the porphyrin n structure. The corresponding a values for
precursor B and peripheral copper ion in XY are 0.92 and 0.91, respectively,
indicating no major change. In this context, it is worthwhile to take a note of similar
dipolar interaction observed in bi-copper complex of bi-cyclam ligand
Fig 3. 13. ESR spechum ofdYI1-B in ethanol at 77 K. The Hamiltonian observed
61 .Z.i8, &=2.040, ~ ~ ~ ~ = 1 9 . 6 m ~ , ~ ~ " = 3 . 3 m7; ~[ '=1 .97 mT,
~:=1,42 mT. Line width: parallel = 0.5 mT, perpendicular ~0.45 mT.
v = 9.062 GHz
1;ig 3. 14 ESR spectrum ofXV in ethanol at 77 K. (a) experimental; (b) simulated
The simulation parameters arc g1=2.247,2.222, &=2.081,2.056, AHCU=
19.5mT, 1 9 . 3 m ~ , ~ ~ = 2 . 8 m ~ , 3 . 0 m ~ , ~ ~ ~ = 1 . 1 8 r n ~ , i . I6mT,
ALN=l ,08 mT, 1.08 mT, Line width: parallel =1.25mT, 1.6mT,
perpendicular = 1.35 mT, 1.6 mT. v = 9.056 GHz
big 3. 15. ESR spectrum of XVI in ethanol at 77 K. (a) experimental; (b) simulated.
'Ihe simulation parameters are gll=1.959, 2.210, a=1.982, 2.055, A["=
17.0 mT, ~ / ~ ~ = 1 8 . 7 mT, .AIV=5.78 mT, A?=] .88 mT, ~ ~ ~ = 0 . 1 ml', 1.3
mT. ~ ~ ~ = 0 . 3 mT, 1.3 mT. Line width: parallel = 0.8 mT, 1.4 mT,
perpendicular = 0.65 mT, 1.4 mT. v = 9.059 GHz
The EPR bchav~our of XYI is found to be different from that o f W . In the Fig
3. 15 one could see copper hyperfine lines in addition to vanadyl hyperfine lines with
approximately 4:1 intensity ratio in the low lield region. In the high ficld region, the
spectrum is complex due to overlapped resonance. Knowing that the peripheral
copper centers do not interact w ~ t h each other, simulation of EPR spectrum of XVl
was performed. 'lhr simula~cd spectrum is given in Fig 3. 15 and it matches
exceedingly well with thc experimental spectrum. The evaluated spin Hamiltonian
values are gll=1.959, Ali\-17.0 mT, All"=0.3 mT, g~=l .982 , AIV=5.64 mT, AIh=0.3
m l , l 'he corrcspond~rlg values for B units are gl=2.210, ~ ~ ' ~ = 1 8 . 7 mT, ~ ~ ~ ~ = 1 . 3 0
niT, gl=2.055. A , ' " = I . ~ R mT. ~ ~ " 1 . 3 0 mT. The values corresponding to vanadyl
ion In XI// are nearly same as that in X. However, the spin Hamiltonian values of
peripheral copper ions are similar to that observed in either XV or XVIII, Thus, it is
clear that the interaction between vanadyl and meso- copper ions are much weaker
than between copper ions in XY.
The high magnetic field parallel lines of vanadyl ion in XVI, which are free
from any copper hyperfine lines, helped us to evaluate a qualitative measure of the
spill relaxation timc of vanadyl ion through microwave power saturation recovery
technique. The dependency of peak area with microwave power is shown in Fig 3.
12. l l e value of h obtain from the equation 1 is found to be 460 and 510 mW for XVI
and A', respectively and c is found to be 0.99 indicating near homogeneous spin
system. The nearly same value of b in both XVI and X indicates that the spin-spin
interaction is nearly zero; otherwise one would have seen large reduction in the b
value ofXV1.
It is interesting to csplaln why Ihc estent of interactions different in XV and
XVI. In thc case of all coppcr systems, the unpaired electron occupies the d,2.,'
orbital becausc of the square planar geometry around copper ions. In contrary. the
unpaired eleclro~l ol'square pyr;~midal vanadyli1I) ion In the porpllyrin occupy out of
pla~ic. d,, or d,, orliital, thus ni in~mi~inp the ititrract~on with the copper(l1) ion.
'l'lle EPR study of .\1711iii. .YIX and .CY resulted in a spectrum similar to that of
the corresponding mebl precursor porphynn As the numbcr of copper coordinated at
1111. ~tlrvo posilion ul'rhu porp11yr111 d~ffers berjvetn the complexe~ -171 XVI. .YVIl and
,Yl'ill. .4'1,\: .KY. t l~c I:PR sig~ial signtlicant of the peripheral copper are weak and the
coordination of the pcriphcral position is understood from thc EPK signal oblalncd for
A'.\' The spill Harnil1~)nian values ohta~ned for XVllland XIX (Table 3. 4), are similar
to thar of thc prccursors. hforrc~\rr , i4e could not scc complete loss of nitrogen super
hyperline line in .Yl'lll suggesttng that the interacrion berween thc copper centers 1s
weak. This is duc to the presence of only one copper at the periphery, which makes
the interaction to be weaker than that observed inA7. Similar behaviour is observed
In all othcr monocupcratcd metallo porphyrias
3. 7 . Electrochem~cal bchaviour
'She electrochemical proputlies of .YV - fl and respecrive precursors are
investigated through cyclic voltammetric and square wave voltammetric techniques.
The electrochemical behaviour of 11 to V, results in a voltammogram as a
representative voltammogram of11 is given in Fig 3. 16 and the related parameters are
given in Table 3. 5 The voltammogram clearly represents two step one electron
reversible oxidation and two step one electron red~ction.'"".'~' The redox processes
can be summarized as
0.972 V 1.224 \' .. Porphyrin,,l: P?' ======= pT- I) - P' - P. .I .62R \' -1.266 \'
0 .743V 1.178V Porphyrin 11': "' - P ' -- P?
0.789 \' 1.151 V Pi' = p -- p P' P?' I'orphyrin I/: .I 624 v -1.258 y
The cyclic voltammogram of metal precursor B in acetonitrile (Fig 3. 17)
results in single oxidation process in the potential range of 0 to 1.5 V and peak at
1.229 V is attributed to the oxidation couple arising due to oxidation of copper(I1) to
copper(1ll). Similarly, in the range of O to -1.5 V the compound B shows an
irreversible reduction processes at 4 . 4 7 2 V and the reverse scan shows an anodic
peak at 4 . 2 V which is due to reoxidation of copper(0) to copper(l1) It is well
known that copper(1) to copper(0) occurs at -0.8 V in acet~nitr i lc . '~ ' Further more,
the copper(l1) to copper(1) is irreversible due to large geometrical change involved
during to the process of r e d ~ c t i o n . ' ~ The electrochemical processes occurring can be
summarized as
Fig 3 . 16. Cyclic Voltammogram (a) Oxidation (b) Reduction of lI(0.1 mM) at the
scan rate of 50 mV, in dichloromethanc with ImM TBAP as supporting
electrolyte and SCE as the reference electrode.
Table 3. 5 Votammehic data of various meso-substituled porphyrins (0.lmM) with
ImM TBAP as supporting electrolyte in a three electrode system with
2mm pt disc as working electrode. All potentials arc with respect to SCE
S No Complex Oxidation Potential in V Reduction Potent~al in V
& p* p-= $' CU?'+ cu" Cu"-tCu' p = p- p-= ,3
8. XVIIl 0.920 1.173 1.373 -0.489 -1.073 -1.411
9. XIX 0.667 0.989 1.48 -0.569 -1.104 -1.393
10. XY 0.673 0.945 1.270 -0.558 -0.618 -1.274
Fig 3. 17. Cyclic Voltammogram (a) Oxidation (b) Reduction of B (0.1 mM) at the
scan rate of 50 mV, in methanol with ImM TBAP as supporting
electrolyte and SCE as the reference electrode.
The electrochemical behaviour of other precursor involved in the multinuclear
porphyrin complex is understood from the voltammetric analysis o f the metal
porphyrins Vl, H a n d XI1 in dichloromethane The cyclic voltammogram results in a
weak current flow; due to low polarity dicllloromethane solvent and hence square
wave voltammetric technique was employed to understand the redox processes o f the
metalloporphyrins.
A l l metalloporphyrins behavcd similar to free base porphyrins. l'hus, two
reversible one-electron oxidation and two reversihje one electron reduction processes
can be observed. Fig 3. 18 represent the voltammogra~n obtained for VI in
dichlorornethane. The redox properties show shiR as compared to their free base
counterpart, which is due to metal substitution.
The electrochemical study o f XI', see Fig 3. 19, shows three one electron
oxidation processes in the range o f 0.5 to 1.5 V. The oxidation peak at 0.76 V and 1.0
V are assigned to the oxidation of porphyrin while that around 1.49 V is assigned to
the oxidation of copper at the periphery. Various kinetic criterions are employed to
understand the nature o f the charge transfer process and are summarized as below
I'lg 3. 18. Cyclic voltammogram, anodic (a) Square wave voltammogram, cathodic (b) of
V/ (0.1 mM) at the scan rate of 50mV, in dichloromethane with ImM TBAP
as supporting electrolyie and SCE as the reference electrode.
Fig 3. 19. Square wavcvoltammogram representing reduction ofXV(O.1 mM)
at the scan rate of SOmV, in dichloromethane with 1mM 'TBAP as
supporting electrolyte and SCE as the reference electrode
I h e electrochemical study o f W , in the range of -0.5 to -1.5 V, result in three
cathodic processes. The peak at -0.653 V is assigned to the reduction process taking
place in the peripheral copper ion and the while the other two potentials at around -
0.99 and -1.37 V are assibned to the reduction processes taking place at the porphyin
n cloud. The redox potentials are given Table 3. 5. It is clear from the redox
potentials, the oxidation and reduction of the copper(I1) at the porphyin periphery is
difficult in XV in comparison with that of the precursor B. During the electrochemical
process a change in the geometry from distorted to planarity occurs which becomes
more difficult in the coordinated state than in the free precursor R. This difficulty to
undergo change in geometry makes the oxidation and reduction processes difficult in
XV than in precursor. On the other hand oxidation and reduction of porphyrin ring is
easier in comparison with that of VI.
The electrochemical study of XVIII behaves similar to that of XV except that
the ratio of the currents for periphery copper(I1) ion and porphyrin ring is close to I .
This is consistent with the molecular formalism of single copper(l1) coordinated at the
pcriphcry. Tlic voltammetric study of XVII result in three oxidation peaks in the
range of 0.5 to 1 . 5 V as similar to that observed in XVand only two reduction peaks in
the range of 4 . 5 to -1.8 V. ' I le redox potentials are given in Table 3. 5 . The
anodic peaks at 0.7 and 0.98 V are assigned to oxidation of porphyin unit while that
at 1.35 V is assigned to the oxidation of metal at the periphery. The redox potentials
show a trend similar to that observed in XY, for all precursors, also the other
cuperated porphyrin complexes show similar oxidation and reduction processes.
A representative voltammogram of unsymmehical copper complex, XLY, is given in
Fig 3. 20.
E I V
Fig 3. 20. Square wave voltammogram representing the oxidation ofXX (0.1 mM) ac
the scan rate of 50mV, in dichlorornethane with ImM TBAP as supporting
electrolyte and SCE as the reference electrode.