~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.