CHAPTER-VI

EPR studies and ferromagnetic interactions of a dinuclear copper complex mediated by p-1,l bridging azido group of the ligand Hsalamp

VI. I INTRODUC'I'ION

The azide ion (Ni') is a versatile ligand that can coordinate to a metal ion in a

variety o f ways. In addition to monodentate behaviour, the azide ion can act as a bridging

ligand either in the end to end (j~-1.3) or in the end on (p-],I) mode to yield di and poly

nuclear colnplexes [ I - ] 71. 11 is u c l l established that the end on bridges give rise to a strong

ferromagnetic interaction via spin polarisation effect [6]. N3' is one o f the very best

ferromagnetic couplers, when it bridges in the 1, l fashion. In all the structurally

characterised compounds, in which I,l-azido groups only bridge the metal ions, the parallel

spin state ir thc ground state irrespective o f the nature o f these metal ions (provided

obviouslq that they are magnetic) [6.7,18-231. When a I,]-azido bridge along with another

bridge links two metal ions to each other. the situation is more complicated. Most often, the

overall coupling remains ferromagnetic. In a few cases, thc antiparallel spin state becomes

the ground state 124. 251. 'Thompson 1241 first suggested that such a situation could happen

in Cu(ll) chemistry whcn the Cu-N-Cu bridging angle on the wid0 side is larger then 108.5"

and the effects o f colriplemenrarity and countercomplementarity are also to be considered

126. 271. However, a ne\c I - D complex with simultaneous end-on azido and carboxylato

bridges is synthesiscd. sherein the Cu-N-Cu angle is found to be 11 1.9" and the authors has

claimed this to be an unexpected strong ferromagnetic coupling as a consequence o f ligand

HOMO'S counter complementarity 1261. Later on. it is suggested that according to the spin

distribution and ferromagnetic coupling, the coupling along the Cu(I1)-1.1-N3-Cu(II) linkage

is ferromagnetic irrespective o f the value o f Cu-N-Cu bridging angle [IZ]. For the a i d e

bridged complexes at small bridge angles (<=lOXO), the I'erromagnetic coupling associated

with azide more than compensates the antiferromagnetic effect o f the diazine bridge and the

systems exhibit net fcrrolnagnetism and at larger bridge angles net antiferrornagnetism is

obscrved and the azidc bridge exhibits an antiferromagnetic effect 124). A dominant

antiferromagnetic coupling with azide bridged angles exceeding 108" is reported by

Thompson et. al. 1281.

V1.2 PREPARATION 01: Cu-CU COMPLEX

To one equivalent o f copper perchlorate (1.8527 g, 0.005 moles) in methanol, one

equivalent o f sodium azide (0.325 g. 0.005moles) and one equivalent o f ligand (1.06 g,

0.005moles) arc added. 'The resulting solution is relluxed for 6 to 8 hours. To this (LCUNI).

one equivalent o f the ligand (1.06 g. 0.005molcs) and one equivalent copper perchlorate

(1.8527 g, 0.005moles) arc added and the uholc mass is further refluxed for about 6 hours.

when the colour is changed from greenish yellow to dark green. On evaporation o f the

solution, a green colour powder is obtained. This powder is further purified by dissolving i t

in methanol and the solution on evaporation. yielded amorphous solid and not good crystals

suitable Ibr EPR or X-ray studies. Scheme-VI.1 gives the systematic procedure for the

preparation o f the copper complex. the structure o f which is given below.

The analylical data for the complex is as follows: Molecular formula

C U ~ C ~ ~ I - I ~ ~ N ~ O ~ C I Calculated (Found): %C = 45.2(41.0). %H = 3.2(3.0). %N = 14.2(1 1.0).

%Cu = 18.39(18.1). The melting o f the complex is 510 K. The molar conductance in

methanol solvent (in units o f A,,/ S cm' mol") is 86, which indicates that the complex i s a

I : I electrolqte.

V1.3, iR SPECTRA

The IK spectrum o f the copper complex, rccorded In KBr pellet, in the region 600-

4000 cm'l is sliown in Figure V1.1. The spectrum shows peaks in the region 1389-1334 cm-I.

which are assigned for the stretching vibrations o f the aromatic skeleton[30]. The C=N

stretching frequent) at 16 10 c m . indicating a shifi o f about 13 cm-' compared to the ligand,

confirms the participation o f the nitrogen atom in the complex formation [3 I ] . The phenolic

C=O observed at 1300 cn i ' confirms the participation o f phenolic oxygen in the

coordination with the metal ion [19]. The deprotonation o f phenolic 01-1 and subsequent

coordination o f oxygen with the copper metal ion is confirmed by the reduction in the

intensity of the peak in the region 2915-2935 cm.', compared to the corresponding band in

the ligand (see Figure 111.4). Generally in principle the mode o f coordination o f the NI' is

identifiable by IR criteria 1321. 'l'he IK spectrum shows the azide asymmetric stretch as a

very sharp band at 2060 and 2080 cm-' in the complexes reported [29]. A similar azide

asymmetric stretch is observed in our copper complex at a value o f 2076 cm". The

perchlorate seems Lo be uncoordinated and it appears strongly at 1093 cm" 1281.

V1.4. ELECTRONIC SPECTIM

Electronic spectrum of the complex recorded in methanol solvent is given in Figure

V1.2 (a and b). Figure VI.2 (a) contains three bands at 394, 289 and 235 nm, wi th

absorbance values of 4320, 12,120 and 18.150 ( ~ i ~ m o l . ' c m " ) . When compared with pure

ligand spectrum (see Figure 111.3), a shift in bands is noticed, due to the formation o f the

complex. 'The peak observed at 394 nm corresponds to N3 + Cu( l l ) charge transfer

transition [33,34]. The other two bands observed at 289 and 235 nm are assigned to

intraligand transitions. The red shift in the ).,,, value is indicative o f distonion in its

coordination geometr). (see bclow). Figure V I . 2 (b) contains a single broad band around

671 nm, corresponding to d-d transition [28j. The broadness is due to the well-known Jahn-

Teller distortion.

V1.5. EPR STUDIES

As good single crystals are not obtained, EPR results for powder and solution

samples, at room and luw temperatures (77 K ) arc only discussed. The sample is nicely

powdered and the EPR spcctrum is recorded both at RT and 77 K . The EPR spectrum

obtained at KT is given in Figure V1.3. It consists of a low field line around 110 mT.

corresponding to the !+I> to ! - I> transition. Its intensity is low, since it is a forbidden

transition. As mentioned in Chapter IV, this line arises due to the half-field transition,

corresponding to the interaction of two copper nuclei. tiowever, the superhyperfine from the

two copper nuclei are not be seen at this room temperature due to dipolar broadening. In

addition, there are two strong lines at the centre of the spectrum. The first one wi th a g value

of 2.22 corresponds to the monomer impurity, while the other one is due to the dimer, with a

very sinall zero-field splitting. One can also notice, a pair o f lines on either side o f this line,

which corresponds to dimer, !kith a slightly higher zero-field splitting. Repeated

purifications do not remove the monomer impurity completely. The high field line,

corresponding to !O> to !*I> transition is not seen due to its very low intensity. The g and D

values obtained at this temperature are:

200 300 400 500

wavelength (nm) --+

Fig. V1.2 (a & b). UV-Visible spectra of the p-1,l-azido bridged dicopper

t 2 0 " I U , I u I f 1 e

2 $ 1 0'

complex in methanol solvent in the range (a) 200-500 nm and (b) 500-800 nm.

(b)

67 1

I 500 600 700 800

I wavelength (nm) 4

gl = 2.10 g2= 2.16

DI = 105(2) niT D2 = very small within lincwidth

Iiencc, the powder samplc is cooled to 77 K and the spectrum obtained at 77 K is given in

Figure V1.4. I t consists of more number of resonances than the RT spectrum. The low field

line obtaincd at KT still rcrnaincd at around 120 ml' and does not show any extra

superhyperfine lines. Howcvcr, thc two lines observed at K'I' slightly merged to a single line

with a g value o f 2.16. This may be due to the monomer g value slightly reduced. as noticed

in some cases. In addition, one can notice around two lines on either side o f this strong line,

which arc due to the copper dimer. Similar to room temperature analysis, we have

calculated the two g arid D values. As mentioned previously, the separation between the pair

o f resonances (one at low field and one at high ficld) corresponds to 2D. l'he calculated spin

Iiamiltonian parameters for the dimer were:

g l = 2.10 & = 2.16

Dl = 103 mT Dz = not resolved.

As mentioned in Chapter IV, the reason ]nay be that the interaction between the hvo copper

ions may be weak or the rcsonanccs are having v e y low intensity, coupled with high

broadness. In order to know the behaviour o f the system in a detailed way, solution spectra

o f the sample is mcasured both at room and 77 K.

The copper sample is dissolved in dry ethanol and the solution EPR spectrum o f the

solution sample is recorded at RT. l 'he spectrum thus obtained is shown in Figure V1.S.

This spectrum is slightly broader than the Cu-Zn dimer solution spectrum (see Figure V1.14).

This EPK spectrum clearly indicates a four-line pattern due to the interaction o f the single

unpaired electron on copper atom with the copper nuclear spin (1-312). This is due to the

monomer impurity prescnt in the complex. Generallj, this monomer impurity overtakes a

dimer in solution state due to the more solubility nature. The calculated isotropic g and A

values were:

g,, = 2.16; Also = 5.1 1 1n.T

The copper hyperfine, not seen in powder spectrum due to dipolar-dipolar

broadening, has appeared in solution due to the ver). low concentration o f the solution. As

mentioned earlier, here also the concentration of the solution is increased in steps, and the

EPR spectrum is recorded at each concentration. At higher concentrations, the four lines

disappeared resulting in a single broad line. The weak resonances for the dimeric copper are

disappeared due to the change in coordination due to solvent effect or might have dissociated

into monomers. However, the powder obtained after the evaporation o f the solution, still

gives a broad line at RT. tlencc, no conclusive evidence can be made from the solution

spectrum (for example, the same rcsult is obtained for Cu-Zn dimer, see below). No further

information is obtained from solution spectrum, since the zero-field splitting tensor averages

to zero in solution. Ilence, the sample is cooled to 77 K and the EPR spectrum has recorded

at 77 K. A portion o f the EPK spectrum in the region of magnetic field 210-410 mT is

shown in Figure V1.6. This clearly differs from the RT powder EPR spectrum. As

mentioned earlier in RT powder spectrum discussion, this also represents a clear triplet

spectrum with orthorhombic g tensor. As the lines are very broad at RT, the lines are not

well resolved at that tcmperaturc. As the sample is cooled to 77 K, the two perpendicular

components are splined, resulting in onhorhombic g values. The central sharp line may be

due to the monomcr impurity. Repeated crystallisations with the solvent do not remove the

monomer completely. The resonances are numbercd from I to 9. The three resonances

numbered by 3, 4, 6 and 7 correspond to the parallel component o f copper monomer

impuriy and 5 corresponds to perpendicular component. The two g values are 2.10 and 2.06

and parallel hyperfine component is 18.9 mT. The average of these two g values (2.14) has

matched fairly well k i th the g value of the solution (2.1 1) (Figure VI.5). The remaining

resonances, i.e, I. 2. 5. 6, 8 and 9 are obtained form the triplet nature o f the EPR spectrum.

The three pairs are (1.9). (2.8) and (5.6). The spin Hamiltonian parameters calculated from

these pairs are:

These values are comparable to the values obtained for the d~meric complex discussed in

Chapter IV (that complex also had orthorhombic symmetv). The values reported for the

dimeric complex in Chaptcr 1V arc:

g, = 2.07 g2= 2.13 g ~ = 2.05

Dl = 77.5 mT D2 = 44.6 1nT DI= 1.6mT

Fig

. VI.

6. E

PR so

lutio

n sp

ectr

um o

f the

dic

oppe

r co

mpl

ex in

eth

anol

sol

vent

at 7

7 K

. T

hree

g a

nd D

val

ues a

re

notic

ed in

the

spec

trum

. v

= 9

.047

74 G

Hz.

However, large D values are reported for dimeric copper complexes [7]

An interesting observation o f the present study is the presence o f resonance lines at

half-field transitions. This kind of result is reported very rarely in the EPR literature. A

similar observation is noticed in our previous study (Chapter IV). The EPR spectrum o f the

solution sample recorded at 77 K in the region 115-165 mT is shown in Figure V1.7. One

can clearly notice a seven line pattern with a rough intensity ratio o f 1:2:3:4:3:2:1. The

average hyperfine coupling constant is 6.2 mT. In addition, i f the dihedral angle between the

two copper atoms and the connecting ligand atoms would also play an important role in

deciding the extent of the interaction.

Magnelic susceptibility o f powdered sample of the complex is measured in the

temperature rangc 77 -300 K using vibrating sample magnetometer. Analysis is carried

out using the modified Bleaney - Bowers equation 1351

where the parameters have their usual meaning [36]. A best fit to the experimental value

for the co~nplexes are summarized in Table VI.1. The temperature dependence o f the

t~iagnetic property for the complcx is given in Figure V1.8. The observed J value is about

18 cm- . The rcsults suggest a weak intramolecular ferromagnetic interaction between

the two Cu(ll) ccnlers and confirms the observation of EPR spectrum.

V1.7. CYCLIC VOL1'AMMETRIC STUDY

Complexes of the type(a)[Cu2(0Ac)3(bipy)~]CIO4 and (b) [Cu2(0H)

(li20)(OA~)(bipy)~](CIO4)2 reported[37] has shown interesting results. While complex (b)

displayed only broad, ill-defined and grossly irreversible feature. complex (a) displayed

some interesting behaviour. 'The complete scan for complex (a) showed two cathodic

processes o f comparable peak currents at E, values of -0.81 V and -1.80 V and a large

anodic feature at -0.57 V on the reverse scan, characterist~c o f an electrochemically active

species adsorbed on the electrode. This suggested that the cathodic features are both two

! Temperature (K) I L -_ _ _ _ _ _ _ _ _ I _ _ __ ._ _ _ _ _ -

Fig V18 Temperature dependence of Magnetic Susceptibility for the dlcopper complex

[Cu2(Hsalamp)2 (p-I, l -N3)]CIO4

TABLE. VI. 1

Magnetic Susceptibility Data for the dicopper complex [C~~(Hsalarnp)~ (p-[,I-NI)]CIO~

Temperature (K)

I

I o3 XU

Observed

5.69

4.94

4 40

3.96

3 31

2 84

2 23

1 84

1 56

1 42

( c ~ ~ M.~)

Calculated

5.72

4.97

4.43

4.00

3.35

2.89

2 26

1.86

1.58

I .44

kff

Observed

1.897

1.887

1.877

1.868

1856

1.847

1.842

1.841

1.837

1.835 I

(EM)

Calculated

1.901

1.891

1.883

1.877

1 867

1.860

1.852

1.848

1.847

1.847 I

electron reduction waves and peak currents are certainly larger than expected for a one

electron process. In the CV thc bchaviour o f the following equation is observed.

The cyclic voltammetric studies are performed in dried acetonitrile solvent for the

azide bridged Cu-Cu complex, The response from the cathodic reduction is not that much

good as shown in Figure V1.9. The value o f E, is around -1.00 V. The differetial pulse

voltammogram recorded is given in Figure V1.10. From these figures, i t is concluded that

the process o f reduction o f two copper ions from +2 state to + I state and subsequent

reduction to 0 state is achieved at the same EN value o f about -1.00 V. This conclusion is

arrived based on the broad feature observed in the differential pulse voltammogram. As the

complex is also prepared from the same ligand environment [37] as complexes (a) and (b) 1, any drastic change in the E, valuo are nut noticed.

V1.8. PREPARATION OF Cu-Zn COMPLEX

7'0 one equivalent o f zinc perchlorate (1.862 g. 0.00Jmoles) in methanol one

equivalent ol' sodium u i d c (0.325 g, 0.005moles) and one equivalent o f ligand (1.06 g,

0.005moles) are added and renuxed for 6 to 8 hours. To the resulting product o f this

compound, one equivalent o f the ligand (1.06 g. 0.005moles) and one equivalent copper

perchlorate (1.8527 g. 0.005moles) are added and the whole mass is refluxed for about 6

hours. A colour change is observed from greenish yellow to light green. The whole mass is

evaporated, while a pale green colour powder is obtained. This powder is dissolved in

methanol for further c~stall isation. We are not able to obtain good crystals suitable for X-

ray and EPR studies. Scheme-VI.2 gives the systematic procedure for the preparation of the

copper-rinc complex, thc struclure o f which is given below.

The analytical data for the complex is as follows: Molecular formula

C U Z ~ C ~ ~ H ~ ~ N ~ O ~ C I Calculated (Found): %C = 45.0(42.0), %H = 3.17(2.9), %N =

14.15(12.0) , %Cu = 9.18(9.0l) %Zn = 9.44(9.20). The melting o f the complex i s found to

be 490 K. The molar conductancc Ibr the complex measured in methanol solvent (in units of

A, S cm2 mol") is 80.0, which indicates that the complex is a 1: I electrolyte.

E I V

Figure V1.9. Cyclic Voltammogram of the azido bridged dicopper complex

[Cu2(Hsalamp)z ( P I . I -N3)]C104 (I mM)tTBAP (0.1 M) in acetonitrile

Figure VI. 10. Differential pulse Voltammogram of the azido bridged dicopper complex [Ct~~(HsaIamp)~ @-I , I-N3)]CI04 (I mMpTBAP (0.1M) in acetonitrile

V1.9. IR SPECTRA

The IR spectrum o f the copper-zinc complex, recorded in KBr pellet, in the region

600-4000 ern.' is shown in Figure VI.1 I . The complex shows a peak at 1438 cm-I, which is

assigned to the stretching vibrations o f the aromatic skeleton [30]. The presence o f a peak at

1099 cm-' confirms the uncoordinated perchlorate ion [28]. The C=N stretching frequency

occurred at 1606 cm", indicates a shift o f about 7 cm.', confirms the participation o f the

nitrogen in complex formation [30j. In our complex the azide absorption is found at the

characteristic frequency o f 2072 cm-' [24, 321.

VI. 10. ELECTRONIC SPECTRA

Electronic spectrum o f thc complex recorded in methanol solvent is given in Figure

V1.12 (a and b). Figure VI.IZ(a) contains three bands at 395, 281 and 239 nm, with

calculated absorbance values o f 4312, 11\80, and 18250 (dL mol"cm.'). When compared

with pure ligand spectrum (see Figure 111.3), a shift in the bands is observed, due to the

formation o f the complcx. In the copper-zinc complex, the peak observed at 395 nrn

corresponds lo NJ+CU(II) charge transfer transition. The other two bands observed at 287

and 239 nm are assigned to intraligand transitions (33.341. The red shift in the I,,, value is

indicative o f distortion in its coordination geometry (see below). Figure VI. I 2 (b) contains

a single broad band at 657111~1, indicating the d-d transition [28].

IV.l I . EPR STUDIES

Similar to the Cu-Cu dimer, here also, we are reporting the results obtained from

powder and solution samples recorded at RT and 77 K. Thc EPR spectra obtained at RT and

77 K are almost identical, except for a change in intensit) o f the resonance lines. Hence, the

EPR spectrum ob~ained at K T is given in Figure V1.13. I t contains a broad line at g =2,

corresponding to the single copper ion. As Ihe other ion is zinc(ll), which is diamagnetic,

wi l l not interfere with the coppcr EPR spectrum. The calculated spin Hamiltonian

parameters are:

RT: gl = 2.19; LN1': gj= 2.19;

methanol solvent in the range (a) 200-500 nm and (b) 500-800 nm.

Also, in the figure a very weak resonance around 135 mT is noticed, similar to the above-

mentioned Cu-Cu dimer. 'This can be attributed to a small amount ofdimeric impurity in the

complex. The intensity of this line does not change by lowering the temperature to 77 K.

The copper-zinc sample is dissolved in dry ethanol and the EPR spectrum o f the

solution sample is recorded at RT. The spectrum thus obtained is shown in Figure V1.14.

This is similar to the solution EI'K spectrum obtained Ibr Cu-Cu dimer (see Figure VI.5).

'The spectrum indicates a four-line pattern due to the interaction o f the single unpaired

clectron on copper atom with the copper nuclear spin (l=3/2). However, four hypertine lines

arc noticed, which are broadened in other copper-copper dimers. The third and fourth

copper hyperfine lines arc ucl l rcsolved in copper-zinc complex compared to other copper-

copper complcxcs, This clearly indicates the presence of a diamagnetic ion in the dimetallic

complex. The calculated isotropic g and A values for copper-zinc complex are:

These values arc close to other solution spectra values recorded earlier. The sample is

cooled to 77 K and the E P K spectrum recorded at 77 K is shown in Figure VI. 15 in the

region of magnetic field 225-375 ml'. This clearly diff'crs l'rom the RT powder EPR

spectrum. 'The LNT spectrum given in Figure VI.15 is a typical copper(l1) EPR spectrum

with axially symmetric g values. 'The extra lines marked hi th * are due to the dimeric

copper co~nplcs. \rhich i s also detected in room temperature powder spectrum. An

ekpandcd version of'the EPK spectrum In the perpendicular region is given in Figure V1.16.

The spin Hariiiltonian parallicters calculated are:

g~ = 2.054; AL = 2.58 m'l

These are normal values found Ibr a copper ion in distorted environments, This also

confirms the presence of single copper in our copper-rinc wide complex. As expected, no

resonance lines arc noticed at half-field position.

Fig

. VI.

16. E

PR

sol

uti

on sp

ectr

um o

f the

cop

per

-zin

c co

mp

lex

in e

than

ol s

olve

nt

at 7

7 K

ind

icat

ing

the

pre

sen

ce o

nly

fou

r li

nes

in th

e pe

rpen

dicu

lar r

egio

n.

v =

9.0

6833

GH

z

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