- CHAPTER V

ELECTRON SPIN RESONANCE El OPTICAL ABSORPTI% SPECTRAL STUDIES

A. ELECTRON PlN RESONANCE SPECTRAL STUDIES OF COPPER(I1)

-AMlrES

Electron spin resonance spectroscopy has been extensively employed for

the study147-152 of copper(i1) compounds. The main feature and point of

interest In all these studies has been the variation of g-values for dlfferent

coordinations and ligand structures around the cu2+ ion. Another point of

interest has been the different point symmetries a t the paramagnetic ion

site. For copper(I1) complexes g and A values which can be determined

accurately by studies on single crystals and frozen solutions show discre te

change in the point symmetries very effectively 152,153

The magnetic properties of the odd electron in cupric ion complexes

with nitrogenous ligands are of particular interest as they show the effects

of t he interatomic electron spin coupling between the cupric ion and Its

Iigands. Furthermore, availability of nitrogens for binding with the metal ion

in b io logica l compounds and metal nitrogen bonds a r e of in te res t in

connection with ESR spectroscopy.

The copper(11) ion with d9 configuration has an effective spin of S.+

and associated spin angular momentum of ms = +f leading t o a doubly

degenerate spin energy s t a t e in t he absence of a magnetic field. On

application of a magnetic field this degeneracy is removed and transitions

occur between the two levels given by the condition h 3 = g H. For a f ree

electron, the Lande splitting factor g has the value 2.0023, g is only

so tropic i n a cubic environment, i n axial and rhomblc crystal fields two and

three different g-values are obtalned respectively.

The expressions for these are given below.

6k i2 1. Octahedral = p= 2 - ------

l o 09 2. Tetragonal

2~~~ h a. Elongated gl = 2 - ----------

2 2 E' Eg- Big) 8 ~ g A

g, = 2.- -------- E( *B~, -~A~~)

6~~~ A b. Compressed g~ = 2 - -----------

2 2 E( Eg- Alg)

g, = 2.00

3. Rhombic

2~: A (Cosol - 3 Sin a )* a. Elongated g, = 2 - ---- ---------------------

E(~B,, - 2 ~ , ~

I n practice, the observed crystal g values for copper(I1) complexes w i l l

be determined by: a) the symmetry of the ligand field around the capper(I1)

ion, b) whether the tetragonal axes of the copper(I1) ions present i n the unit

cell are aligned parallel. This must be the case If the number of molecules

(2) i n the unit cell is one, but is not necessarily the case, if the number 1s

greater than one. If all the tetragonal axes are aligned parallel then the

crystal g-val~es accurately reflect the local copper(I1) ion environment of g-

values, and these may be measured using either single crystal techniques or

(slightly less accurately) by measurements upon poly-crystalline samples. For

the axial spectra, G Is ewal to (g,, -2)/($-2) which is approximately ecyal t o

4.0 and 1s an evidence for the presence of a d 2- 2 ground state. The g- x Y values for a regular trigonal bipyramidal steriochemistry have been shown t o

be eqdvalent to those for a compressed tetragonal distortion. The ESR

spectrum of [ C U ( N H ~ ) ~ A ~ ( S C N ) ~ I lS4 shows a low g-value which is slightly

greater than two consistent with a dZ2 ground s ta te in this trigonal

bipyramidal steriochemistry.

If the tetragonal axes in the unit cell are not aligned parallel, than

the observed crystal g-values are not simply related to the g-values of the

local copper(I1) ion e n v i r ~ n m e n t ' ~ ~ . If the complexes are of known crystal

structure then the observed g-values may be related to the local g-values155,

It is not possible t o specify precisely the local g-values if the crystal

structures is unknown, but three situations are observed in practice.

Isotropic spectra

These are predicted for a copper(I1) ion in a regular octahedral o r

te trahedral steriochemistry neither of which occur in practice. They can

occur w i t h a dynamically distorted octahedral steriochemistry, or one

involving pswdorotati(Jn of [CuL612+ chromophore. This situation is believed

t o occur in [ K ~ ~ c u ( N O ~ ) ~ I 151p1a7. An isotropic ESR spectrum would also

occur if a [CuL6]2+ chromophore of symmetry lower than octahedral was

unbrrgoing free rotation in the crystal la t t ice, t he [ C U ( N H ~ ) ~ I 2+ cat ion,

to be present in the pentammines and hexammlnes of t h e

copper(11) im, is considered to be freely rotating in these complexes and

exhibits an isotropic ESR spectrum. he observation of some type of

tetragoml ESR spectrum a t temperatures lower than room tempera ture Is

cons is ten t148 with the t e rmina t lon of f r ee ro ta t ion a t this lower

temperature. The most common reason for observing an isotropic ESR

spectrum is through extensive exchange coupling between grossly misaligned

local copper(I1) ions, as in [ & - C U ( N H ~ ) ~ B ~ ~ ] . In these cases the line width

is so broadlS3 that it is sometimes difficult even to observe an ESR

spectrum e.g., [ ~ - C U ( N H + ~ B ~ ~ ] .

Axial spectra

This type of spectrum is most commonly observed with tetragonal

local copper(I1) ion environments in which the tetragonal axes are aligned

parallel. For [CU(NH$~(SCN)~] gL =2.056, g,, =2.237 and G=4.23. his

type of spectrum may also be observed with systems of symmetry lower than

tetragonal , where the misalignment is small. The e f f ec t of exchange

coupling is then to decrease the spread of the observed g-values which

is ref lected in t h e obse rva t ion of a G-value lower t h a n 4.0; t h u s

[ C U ( N H ~ ) ~ ( N ~ ~ ) ~ ] has an ESR spectrum with gL ~2.068, gu ~2.231 and

~ ~ 3 . 4 . Consequently the value of G is an useful indication of the extent of

exchange coupling in tetragonal systems. The lower accuracy of the powder

techniwe156 for measuring ESR of rhombic Systems can result In the

observation of an axial spectrum even when no misalignment is present. For

example, [ c ~ ( N H ~ ) ~ ( c u L ~ ) ~ ] has an ESR s p e c t ~ m with =2.054, 4 =2.219

and G=4.21 measured as a powder but gives three g values as a single

crystal g1=2.054, g2=2.058, g3=2.z3, the two lowest g values are not beinp

resolved i n the powder method.

This spectrum can arise from the slight misalignment of rhombic or

axial local copper(11) ion e n v i r ~ n m e n t s l ~ ~ t ~ ~ ~ . The axial environments of

[ C U ( N H ~ ) ~ . ~ O . S O ~ ~ are misaligned by 61° and give rise t o three g-values,

g1=2.047, g2=2.126 and g3=2.172. The ef fect of exchange coupling is t o

reduce the spread of the local g-values.

Basically the interpretat ion of the ESR spectrum of an isolated

transit ion metal ion yields values for two kinds of parameters. The

s~ectroscopic spl i t t ing constants (g-values) describes the freq~encies (or

energies) required to bring about transition between the dif ferent electron

spin states. The hyperfine coupling constants (A) describes the electron spin

nuclear spin interactions. Both of these parameters may be anisotropic.

These parameters are affected by the presence of ligands around the central

metal ion.

The g-values of a complex are determined by the expectation values

of the components of the vector operator L+25. In many cases th@ orbltal

angular momentum i n the ground state of the molecule is completely

quenched and an isotropic g-value of 2.0023 is obtained to zero order

approximation. This is the case if the complex is of suff iciently low

symmetry and that the orbitals containing odd electrons are non-degenerate.

This approximation, however, does not usually suf f lce for transition me ta l

complexes. Spin orb i t coupling leads to a mixing of excl ted s t a t e

configuration with the ground state configuration and it is necessary to find

' the expected values of L+2S using these corrected ground s t a t e wave

functions. Typically, therefore, perturbation theory leads to g-values of the

form gll =2.(1-x h/Ah gL =2(1-y X/A) for an orbitally non-degenerate ground

state, where X is the spin-orbit coupling parameter, A is an energy difference

between the ground state and excited state configuratlons and x and y are

simple numerical factors x and A are obtainable from optical absorption

spectroscopy. In practice i t is found that the g-values measured do not

agree with those obtained for the free ions. Typically a 20 or 30% reduction

in the spin' orblt coupling constant is involved. This effect 1s attributed t o

covalency in the metal ligand bonding.

ESR studies have been used to investigate the covalent bonding of

transition metals in a variety of compounds. The work on copper complexes

was most highly developed by Maki and ~ c ~ a r v e ~ l ~ ~ in thelr analysis of the

ESR spectra of copper sal icylaldehydeimine and ace ty l a c e t o n a t e .

Considering the biological importance of copper and the blochemical and

analytical importance of phenylhydrazones and with a view that of metal ion

may vary their biochemical activity the present author prepared t h e

copper(I1) complexes of HAPPH, RAPPH, PPH and HWH and characterlsed

them by ESR spectral studies.

The procedures fo r the preparation and analysis of metal complexes

are described in chapter 2.

ESR spectra of the solid complexes ( C u m = 2 9 8 ) a t 300' and 9 0 ' ~

were recorded using Poland X-Band ESR SE/x-2514 spectrometer whkh was

operated in the range 8.8-9.6 GHz. DPPH was used as the g-marker.

E S U T S A19 DISCUSSION

All t he complexes a r e powders which a r e insoluble In water and

soluble in dioxane, dimethylformamide, pyridine, piperldlne and ethoxy

ethanol.

The complexes are found to be stable in air and non-hygroscopic. All

the complexes were analysed for the metal, carbon, hydrogen and nitrogen

contents and the results showed a 1:2 metal-ligand composition. Nitrobenzene

and DMF solutions of these complexes gave low conductance values indicating

a non-electrolytic nature. Elemental analysis data also revealed that the

complexes are dihydrated. The presence of water has also been confirmed by

infrared spectral studies. IR spectral data also reveal that the complexes a re

formed by the replacement of the hydrogen of the phenolic OH group with

nitrogen of the atomethlne of the phenylhydrazone group coordinating t o it.

The electron spin resonance spectra of the poly crystalline specimens

of the four copper-phenylhydrazone complexes diluted in the respective zinc-

~ h e n ~ l h ~ d r a z o n e complexes (Cu:Zn = 2:98) are shown in figs.36-39, which

showed interesting features of resolution of the copper hyperfine structures.

In all the complexes the low field part of the spectrum consisted of three of

the four expected hyperfine lines (the fourth line overlapping with the high

f i e l d l i n e ) ( c u ~ ~ , S = i a n d n u c l e a r sp in 1=3/2). T h e r e s p e c t r a are

characteristic of axial symmetry. The spectrum is composed of two parts,

the O=oO (gl~ ) Part and the 0=90° (gL ) part, where 0 is the angle between

H and the axis perpendicular t o the coordinating plane formed by t h e ligand.

Therefore the copper hyperflne line of the m ~ = -(3/2) of the gl absorption

overlapped with the gl part . The spectra a r e f i t t e d Into an axial spin

Hamiltonian given below159*1a.

H = B [ gU Hzsz + gL (HxSx + HySy) I + [ A IzSz + AL (IxSx + IySy)I (35)

with S=i and nuclear spin I = 312, the parameters in the spin Hamiltonian a s

calculated f rom t h e spec t ra a r e given in table 22,23. The quadrupolar

interactions of the copper nucleus have been neglected here. The value for

gU is obtained from the recorded spectrum by taking the magne t ic f i e ld

value corresponding to the cen t re of the second and third hyperflne line.

The value of yL is obtained from the magnetic field value corresponding t o

t h e point of intersection of the high field deviation shaped line with the

horizontal line underlying the low-field hyperfine peaks. The analysis of ESR

spectrum of copper-phenylhydrazone complexes is straight forward when w e

compare this spec t ra with t h a t of t h e ESR spec t ra of severa l copper

complexes reported in the literature 161-164. In the spectra of the present

complexes the line widths a r e larger and t h e resolution IS su f f ic ien t t o

measure all the ESR parameters (Table 22,231 accurate l~. Table 22923 gives

the 9, A and optical absorption fretyencies and orbital reduction factors Km

and K~ evaluated for the complexes using the expressions given below.

g~~ = ge - 8 ~ 1 ~ A E~ (36)

gA = ge - 2 ~ z h / A E2 (37)

where is the spin orbit coupling constant for cu2+ ion (-828 cm-'1, 9, is

the free spin g-value (2.00231, El and E2 are the absorption frequencie for

2 2 the g rand state 2 ~ 1 g ( X -Y >) to 2 ~ 2 g and 2 ~ g transitions respectively.

Hathaway pointed out that for pure CF -bonding Kll=KI 0.77, for I n

plane, p i bonding, KII < KL and for out of plane pi-bondrng KL < 4 . I n al l

the present complexes K ~ I < K l which suggests that i n these chelates i n

plane pi-bonding 1s present. The parameters presented in table 23 show some

interesting trends. The gtl values are almost same for al l the complexes

indicating that the type of bonding is same i n al l the complexes and CU-N

and Cu-0 bond-lengths do not vary from complex to complex. Kivelson and

~ 1 e m a 1 - 1 ~ ~ ~ have shown that g,l is a moderately sensitive funct ion fo r

Indicating covalency. Normally gll is 2.3 or more for ionic environment and

it is less than 2.3 for more covalent environment. The present ESR results

show that g,, is less than 2.3 in all the cases suggesting that the copper-

phenylhydrazone complexes are more covalent in nature. Similar observations

were reported by Wasson and ~ r a ~ ~ ~ ~ ~ . Massacesi et al.lb4 reported that

g,, is 2.3-2.4 for copper-oxygen bonds (octahedral and planar respectively),

2.2-2.3 for copper-nitrogen bonds and mixed copper-nitrogen and oxygen

systems (with variatlon i n the point symmetry from octahedral and planar

among them) and 2.1-2.2 for the c o p p e r - ~ ~ l f u r bonds. For the present

copper-phenylhydrazone complexes gl,= 2.212 -2.221, in conf i rm i t y w i th the

presence of mixed copper-nitrogen and oxygen bonds in these chelates. The

data presented in table 22 also reveal that there is no much var iat ion i n

values of A,, and isotropic 9[90'ln(9,1 + and A[AO=1/3(Au +2& )I

values. Furthermore the high field perpendfcular t ransi t i~n shows no signs of

9 1

W I ~

any f u r t h e r resolution i n t o t h e individual compounds t o yield gx and gy

[(where g~ = 4 (gx+gy)l. This would mean that in a11 the complexes t h e

ni t rogens and oxygens provide e f fec t ive ly the same ligand field strength.

Under this symmetry the unpaired electron is considered t o be in t h e ground

state

~ a t h a w a y ~ ~ ~ f ~ ~ ~ stated that two types of axial spectra are observed

depending on the value of lowest g-factor. 1. Lowest g > 2.01 such a

spectrum can be observed for a copper(I1) ion in axial symmetry with all the

principle axis aligned paral le l and would be consis tent with e longa ted

te t ragona l , oc tahedra l and square planar steriochemlstries. In these axial

spectra the' g-values a re related by the expression,

and is a n evidence that a d 2- 2 ground state is present. If G > 4.0, then X Y

the local tetragonal axes a r e aligned parallel or only slightly misaligned, if

G < 4.0, significant exchange coupling is present and t h e misal ignment 1s

appreciable. Consequently the value of G is a useful indication of t h e extent

of exchange coupling in te tragonal systems. 2. Lowest g < 2.03 - such

spectra can be observed for a copper(I1) ion in: axial symmetry with all t h e

pr inciaal axes aligned paral le l and would be consistent with compressed

tetrapnal-octahedra1 or trigonal-bipyramidal steriochemistries. Fur ther , if

the g-value is less than 2.03 it is an indication that the unpaired electron is

p reseq t in t h e d,2 ground s t a t e o r b i t a l and in t h e s e c o m p r e s s e d

s ter ic~hemistr ies the value of G has no significance.

copper-phenylhydrazone complexes gL r2.052-2.057, gll =29212-2.221,

4.27 snd gl ) gL (Table 22). These values suggest

have symmetry lower than regular octahedral and the misalignment h these

cases is not appreciable. Further, for these complexes the lowest g value is

greater than 2.041 and G > 4.0 consistent with a d,Zwy2 ground state and - with a distorted octahedral geometry.

--band W parameten

ESR and optical absorption spectra have been used many times t o

determine the covalent bonding parameters for the cu2+ Son l n varlow ligand

field environments. The ESR parameters g,, , gL , AN and AL and the

separation of the d-orbitals 2~ lp ---> corresponding to x2-yZ > to 2g XY > transition and 2~ --> 2~ corresponding to x2-Y* > -4' xz, yz

lg 9 > are used'to evaluate the bonding parameters a2, P:, p2 and the Fermi

contact hyperfine interaction term K. 01' measures the covalency of the

inplane bonds, p2 of the inplane pi-bonds and l2 of the out-of-plane

pi-bonds. These parameters are close to 1.0 for ionic bonds and become

smaller with increasjng covalent bonding.

For a cu2+ complex w i th Oh symmetry the ground state can be

expressed by the molecular

Big= ~ / X ~ - Y ~ > - I R D I ' I - ~ X + ~ ~ Y + ~ % - $ Y (39)

where oc'is the ligand group orbital coefficient for the ground state and d is

the metal d-orbital coefficient. The larger the square of the $, the more

covalent is the bonding.

~h~ magnitude of the a2 can be estimated using the fol lowing

approximate

a2 = All /P + (gU - 2.0023 + 317 (gL - 2.0023) + 0.04 (40)

where P = 0.036 cm'l for cu2+ free ion, and All 1s expressed i n cm-l. The

a2 values obtained by this expression (Table 23) tend t o be slightly less

than those obtained using the more elaborate and exhaustive molecular

orbi ta l theory. This leads to a small discrepancy i n the value of ( d112

derived from a2 uslng the expression for normalisation of the Big orbital

a2 + ( u ~ ) ~ - z s ~ S = i (41)

where S is the overlap integral between the metal d 2- 2 and the normalised x Y

ligand orbitals. we have assumed the overlap integral value calculated by

~ s s o u r l ~ ' S = 0.092 and the valuer of (CS,~)~ were presented In table 23.

In order to estimate the molecular orbital coefficients (a, $ and 1,

Hathaway and ~ o m l i n s o n l ~ l have assumed the value of pl as one and

calculated oC and $ for several copper(I1) ammonia complexes employing the

expressions K,,* d Pi and KLe d$ . The present author determined the

values of 6 for the four copper-phenylhydrazone complexes f rom the

expression (5). Substituting the values of oc i n the expresslon KLE OL j3

the parameter is obtained and these values are also given i n table 23.

The estimated values of the bonding coefficient Indicate a large amount of

in-plane -bonding and very l l t t le out of plane pi-bonding.

Giordano and ~ e r e r n a n l ~ ' have treated the free ion dipolar term P as

a variable to absorb the effect of electron delocalization by the use of the

expression.

These suggest that identification of bonding groups may be

obtained from values of P, e.g., for bonding of two nitrogens and two oxygen

atoms the value of P is i n the range of 0.022-0.029 cm-l . The values

obtained for the Present phenylhydrozone complexes by using the above

expression consistent w i th bonding of copper t o two nitrogens and t w o

oxygens. Smaller values for P also indicate slightly stronger in-plane pi-

bonding In the systems agreeing with a higher ligand field.

The Ferml contact hyperfine interaction term K may be obtained from

the expression169.

where

- *is0 gav - 2.0023 . K r - --me---- + --------- A2

(43) p p2

the free ion dipolar term P = ge gN p, p~ < r-3 > w 0.036, K Is a

dimensionless quantity which is a measure of the contribution of S electrons

to the hyperfine interaction and is generally found to have a value of about

0.3 substituting the values of Ai,,, gav and P i n the above expression the

value of K for four complexes were obtained which are also given in table

23. In these calculations P2 is assumed as one. The values obtained for K

are is good agreement with those estimated by ~ s s o u r ~ ~ ' and Abragam and

prycel'O.

ELECTRON SPIN RESONANCE SPECTRAL STClDIES OF COPPER(I1)-

n m IN DIFFERENT SOLVENTS

Extens ive s t u d i e s on ESR spectroscopy of copper complexes in

different solvents have been made to investigate the structural changes and

nature of metal-ligand bond.

Paramagnetic resonance spectral studies of dilute solutions of copper(I1)

acetylacetonate in chloroform, dioxane, pyridine and toluene was investigated

by M z ~ a r v e ~ l ' ~ . He found tha t dloxane and pyridine had e marked

influence on the gyromagnetic ratio due to the formation of weak complexes

between the solute and the solvent.

M c ~ o n n e l l ~ ~ ~ has developed a simple microcrystalline model to discuss

the e f f ec t of molecular motion in the liquid s ta te on the paramagnetic

relaxation of ions or molecules. He found that the observability of hyperfine

structure is strongly dependent upon complex formation and licwid viscosity.

Kivelson and ~ e i m a n l ~ ~ have interpreted ESR spectra of several

copper complexes including those of biological importance using molecular

orbital theory with particular attention to CU-phthalocyanine and i t s

der1va:Ives.

The ESR spectral properties of several amlne complexes of CU(II) in

Solution and vitreous s tate were studied by CerSmann and walle en^^^. The

authors found that hyperfine splitting due t o the magnetic moment of nitro-

gen ligand atoms could often be observed In the vitreous s tate which are not

observed in the liq~id state. The theory of spin-lattice relaxation .through

spirrrotational interaction has been used by Wilson and ~ i v e l s o n ~ ~ ~ - ~ ' ~ for

explaining the ESR line wldths in solution. Kuska e t investigated the

e f f ec t of substituents on the anisotropic ESR parameters of copper acetyl

acetonates in chloroform, dimethylformamide and pyridine to get the correct

ordering for the relative covalencies for the series.

The ESR of some Cu(I1) complexes of p -ketoenolates and their Schiff

bases ih toluene-chloroform (1:4) solutions a t 300° and 7 7 ' ~ were studied by

Chappin and ~ u d e k l ~ ~ . The covalency was found to increase In the order:

p -ketoamide < B -ketoenolates < transbidentate Schiff bases < c ig te t ra -

dentate schiff bases.

Adato e t a1.l" carried out ESR spectral studies of @ -ketoenolates,

d -dioximates, -dioxlmates and Schiff bases in chloroform solution. The

e f f e c t of substitution, ring size and nature of the heteroatom on metal-

ligand bonding parameters were investigated. Subs t i tu t ion in t h e six

membered chelate ring by electron withdrawing groups caused decrease of

the covalent character of the Cu(1I) ligand a -bonding and an increase of the

out of plane n-bonding whereas replacement of the oxygen donar atom by

nitrogen donar atom caused increase of the covalent character of the 6 - bonding. Enlargement of ring Site of CU(II) chelates from 5 to 6 entailed a

decrease of tne covalent bonding.

The magnetic and spectal parametrs of CU(II) amino acid complexes

were used by Zyzyck e t a l , lBO to calculate molecular orbital coefficients

using simple Huckel molecular orbital calculations of t h e lmine model and

carbanion model the observed trends were rationalised

Charry e t a1.181 studied ESR and optical absorption spectra of five

coordinate copper(11) complexes in water, acetone and pyrldlne and found that

pyridhe was more interacting through nitrogen. The study was also extended

to polyaffline complexes in various solvents. The formation of compressed

octahedron in frozen aqueous solutions was attributed to the formatlon of

clusters of copper(l1) ion pairs. Seshadri Naidu and Raghava ~ a i d u l ~ ~ and

Sure* Babu e t al.lB3 have carried out ESR spectral studies of polycrystalline

copper-chalcone and copper-oxirne complexes respectively. ~ e s a v u l u e t a1.lw

Ramakrishnaiah et a1.1e5 and Mohan Rao et a1,186 have investigated and

reported ESR studies of copper-oxime and copper chalcone complexes

respectively in various organic solvents.

The present chapter describes the electron spin resonance spectral

studies of copper(I1) complexes of phenylhydrazones in d i f fe ren t organic

solvents, vlz., dioxane, dimethylformamide, pyridine and piperidine.

EXPERIMENTAL

PrepratIan of copper(11) ccmxdexes

The copper complexes of phenylhydrazones were prepared according t o

the procedures described in chapter 2.

The ESR spectra of copper(I1) HAPPH, CU(II) RAPPH, Cu(I1) PPH and

Cu(11) HNPH chelates in dloxane, dimethylformamide, pyridine and piperidins

solvents were recorded by using Poland X-Band ESR SE/X 2544 spectrometer

which was operated at 8.8-9.1 G H ~ at 3 0 0 0 ~ and ~ O O K at a concentration of

~ O ~ M using DPPH as the 9-marker

Spectroscopic grad dioxane, dimethylformamide, pyridlne and piperidine

were used as solvents

The ESR spectra of copper(1I) chelates in dioxane, dimethylformamide,

pyr id lne and piperidlne consist of four absorption l ines (except

[CU(II)(HNPH)~]) (figs. 40-55). These four lines in the multiplet correspond

to the (21+1=4) spin orientations of the copper nuclei with I=3/2. ESR

spectra of the most copper(I1) complexes in solution at room temperature

normally exhibit four-hyperfine lines with spin dependent line widths due to

the tumbling motion of the microcrystalline units. This tumbling motion in

solution will average the anisotropic g and A values observed in the solids.

I n the spectra of a l l the chelates presently studied in four solvents the

expected nuclear interaction between the unpaired electron and each copper

isotope ( c u ~ ~ and C U ~ ~ ) is not detected. It may be due to the fact that at

room temperature ESR signal of the less abundant copper isotope is obscured

by the random motion of the paramag~t ic

The hyperfin structure of the present ~0mplexeS obtained h different

orpanlc solvents Is resolvable at room temperature and is of interest since

Bagguley and ~ r i f f i t h s l ~ ~ found it impossible t o resolve t he hyperfine

structure i n dilute copper Tutton salts at room temperature due t o shor t

spin- lat t ice re laxat ion t ime o f about 2 x 10-9 sec. ESR spectra of

[ C U ( W ~ ~ 12+ and [cu(H~o)~ 12' ions in aqueous solutions have also shown na

hyperfine structures. In (CU(NH~)?+ spectrum the peak t o peak w id th i s

300+_10 gsus and g=2.101+005 and i n [cu(%o)~]~+ peak to peak widths is

l l 5 + 5 - gaus and g=l.&cr~O.OOS. Apparently the spin latt ice relaxation tIme in

the present copper-phenylhydrazone chelates i s greater than i n the ionic

Tutton salts. This may be due t o the presence of much stronger bonds

be tween the copper and I t s nerghbours (oxygen and n i t r o g e n o f

phenylhydrazone) with the result that the oscillatIona1 frequencies are much'

higher giving a smaller Fourler component in the region of the resonance

fr-ency.

The ESR spectra o f copper(I1) complexes i n glassy s tate g ive t h e

principal g and A values 1849189-192. I f the solvent is non-interacting, the cj

and A values correspond to the molecular g and A values of the complex i n

solid state. On the other hand, if electron donar solvents l ike pyridine

interact with the complexes, complexes different from the parent ones w i l l

be formed and the g and A values wil l also be different from those of the

respective parent co-nplexes. The changes in the resonance spectra can be

viewed in terms of molecular orbital theory of stevenslS2 and 0wen193 by

comidering the effect of the solvent molecule as equivalent t o t ha t of a

small negative charge placed along the Z axis. From the tetragonel f ie ld

f rom the m~ lecu le ifself, there wi l l be four energy level l with the C* ~ 2 -

y2 stale the lowest and the d some what above it0 The T?xy and ( l f x z ,

nayl) states wi l l be higher i n energy and not too far apart i f the tetng0m.I

field not to0 f a r from a cubic field. The doublet ( Tfxz- d y z ) s ta te will

have the higher energy of the two. The cylindrically symmetric field from a

2 2 negative charge on the Z axis will affect the energy of the d x -y and

$ xy levels t o the same extent since dxZ-y2 can be converted into dXy by

a 45' rotation about the Z axis. Further the ( lfxz and d y z ) s ta te rfmains

degenerate and will approach the $xy s ta te since the additional negative

charge along Z axis will have t h e effect of making the tetragonal field

sorromding the copper more nearly cubic and both bf these levels a r e

degenera te and g should increase as the solvating interaction with copper

increases 150,197

In the present investigation the complexes studied in solutIon HNPH

contain an additional fused benzene ring and i t is a bulky ligand when

compared t o o ther ligands. C U ( H N P H ) ~ che la te did not give four l i n e

spectrum in solvents. However, a normal spectrum i.e., four lines is obtained

when the chelate was diluted with diamagnetic zinc complex of this ligand.

I t would therefore appear that the spectra shown in fig.39 can be attributed

t o anisotropic molecular reorientation of the chelate in solution 195,196

The isotropic g values (Tables 24,251 of the copper-phenylhydrazones

a r e higher in dioxane and lower in piperidine and they fall in the order

dioxane > dimethylformamide > pyridine > piperidine s u g g e s t i n g t h a t --

- -_ pipsr idine is in te rac t ing more with copper than the other solvents. The

greater interaction of piperidine is also evidenced from the higher solubility

of t h e copper c h e l a t e in this solvent and also t h e s h i f t of t h e optical

absontion peak to a lower wave number region.

The ESR spectra of the present copper(I1)-phenylhydrazone complexes

in ~ ~ r i d i n e and piperldine solvents are interesting. It showed, apart f rom

copper hyperfine lines, a multiplet of a superhyperfine lines corresponding t o

(21+1=9) due to nearly e q a l splitting from four coordhating 14N atoms (1=1),

two nitrogen atoms are from two phenylhydrazone ligands and the remaining

two from the two solvent molecules. The superhyperfine 1s seen only on the

high field line because of MI dependent line width (due t o slow tumbling).

From the spectra i t is clear that the intensity of al l the superhyperfine lines

are not same. The reason for high intensity of 1st and 9 th l ines In th i s

mul t ip le t may be due to strongly bonded nitrogens present in the phenyl-

hydrazone ligands to the copper(I1) ion in the chelate.

Two sets of AN values have been derived f rom the superhyperfine

lines of each spectrum. One set having higher AN values may be due t o

two strongly bonded N atoms from two ligand molecules i n the chelate.

Another set having small AN values may be due to weakly coordinated two

nitrogens of two solvent molecules (pyridine or piperidine). The copper

hyperf ine l ines of these complexes in these solvents are not fully resolved

perhaps because of very low tumbling effect of the copper complex molecule

i n the solvents. The Copper hyperline lines can be resolved if the rotation

of the molecule is increased. This can be done by increas ing t h e

temperature. The Acu values of the present copper-phenylhydrazone

complexes are in the order dioxane > dimethylformamide > pyridine >

piperidine. The variat ion of spl i t t ing with solvents thus indicates that

stronger the solvated complex the smaller the value of the unpaired

electronss wave function at the Apparently the bonding molecule

pulls the electron away from copper*

Banding nature and magnetic parameters

The metal-ligand bond nature can be calculated using molecular orbital

calculation given by Maki and ~ c ~ a r v e ~ l ~ ~ . This theory was originally

Proposed for planar complexes having tetragonal symmetry. For t h e sake

of simplicity a number of workers in ESR have applied this theory t o

and also for five coordination These

~a l cu l a t i on~ require a detailed knowledge about the orbltal splitting. But in

the present case only one optical absorption peak is observed in all solvents

for all copper-phenylhydrazones. This will not give complete information

about orbital splitting to estimate the nature of a - and TT -bonds. In a

spectrum taken in solution the i~otro~ic'nuclear hyperfine splitting <A> is

related to the covalency of the unpaired electron by the equation194.

<A> = P (-*' ko + g - 2.m23 + 0.04)

where or2 is the fraction of time the unpaired electron present on t h e

Copper atom, p is 0.036 cm-l and ko is about 0.43. If a2 = 1, the bond

should be completely ionic and if ** = 0.5 the bond would be completely

covalent165. The d 2 values (Tables 26,271 for all copper-phenylhydrazones

In the solvents were calculated using the above ewation. The values of &2

a re higher in dloxane and lower in piperidine Indicating that the covalent

character of copper-ligand (phenylhydrazones) a -bond is in dioxane than in

piperidine. In the present study of copper-phenylhydrazones in pyridine and

piperidine the degree of covalency calculated from the copper hyper-fine

s tructure was found to be smaller than that calculated from the nitrogen

hyper fine structure167.

Maki and Carvey158 used the fol lo~ing expression for d estimation of

out-of-plane Q -bond strength.

aL1 = (1 - d2)# + dS where S = 0.092.

The Fermi contact hyperfine interaction term K may be obtained from

the e ~ r e 5 ~ i m ~ ~ ~ K = A F + (g) - 2.0023, where the free ion dipolar term P

= 9.9~ Be PN (rm3) 6 0.036. The values of the K for the present chelates

(Table 25) follow the same trend as those of a2 values. The contribution of

Fermi contact term to the hyperfine interaction i n different solvents is in

the order dioxane > DMF > pyridine > piperidine.

Magnetic moment values are calculated with the help of g values by

uslng the following expression194.

p2 = 3/4 (g)2

The magnetic moment values for the present complexes (Tables 26, 27)

l i e above the spin only value of 1.73 BM and %cur i n the range of 1.82-1.83

BM normal ly found fo r the copper che la tes i n t he absence o f

ant i ferromagnetic coupling. The values are consistent with orbitally non-

degenerate ground state201 for the Cu(I1) ion. The increase i n the magnetic

moment values above the spin only value arises probably from the mixing up

of orbitally degenerate excited state into the ground state via spin-orbit

coupling. The values further indicate that the copper phenylhydrazones are

mono nuclear and posses distorted octahedral symmetry in a l l the solvents.

TaMe - 24: ESR Parameten of CdI1) complexes of phenylhydrazanes h pyridine end piperidhe at !dk

I- Complex . ?nt

Table - 25: ESR Paremeters of Cali) cunplexa of @enyIhydramnes In dioxane 8nd dirnethylfodde at 90' K

Sol- Complex vent

'Jc-26: W i n g cud orbital reduction parameters of Cu (11) cmplexes of.phenylhydramrrr h pyridine and piperidom at

- Complex nt

'yddine; B: Piperidine

structure of 4th Line

Fig .43: ESR Spectrum of C u(~r)Z-hydroxy -1 -naphthaldeh yde phenylhydrazona chelate in pyridine at 300° K

Spectrum a t 90" K

Fig -47: ESR Spectrum of Cu(1r12-hydroxy-I- naphthaldehyde phenyl hydrazona chelate in dimethyl formcunida a t 3 0 0 " ~

-

Fig 5 5 : ~ ~ R Spectrum of Cu (11)-2-Hydroxynophthaldehyde phenylhydrazone complex in piperidine