Indian Journal of Chemistry

Vol. 38A. September 1999. pp.900-90S

X-band electron paramagnetic resonance spectra of some Cu(II) mononuclear and Cu(II)-Cu(II)/Cu(II)-Zn(II)/Cu(II)-Ni(II)

binuclear complexes

Ram N Patel *, Ayodhya P Patel & Krishna B Pandeya Department of Chemistry, A P S University, Rewa (MP) 486003, India

Received 11 September 1 998; revised 10 May 1999

X-band EPR spectra of polycrystalline samples of some Cu(II) mononuclear and Cu(II)-Cu(II)/Cu(II)-Zn(II)/Cu(II)-Ni(II) bi-nuclear complexes using diethylenetriamine and imidazole are reported. All the complexes show EPR spectral characteristic of S= I , "l whieh yield the value of the zero field parameter 'D' in the range O.OI 2±O.OOI em') .

Diethylenetriamine (I,dien) is a potential terdentate l igand capable of coordinating through the two amino and one imino group. Such terdentate ligands are known to form binuclear copper(H) complexes, which have been a subject of extensive studies, especially with regard to the nature of their spin-spin interactions ' . This l igand is known to form imidazolate-bridged polynuclear com­plex of the type [Cu(im)(dien)] n(X) n' where im is an anion obtained by deprotonation of imidazole (imH) . Sato and coworkers ' have reported the crystal structure of above complex. In this complex, the copper atoms are bridged by the imidazolate ring in two alternative orientations. The geometry around copper(II) is trigo­nal-bipyramidal, comprising the tridentate dien and two imidazolate ions . The copper atoms are bridged by imidazolate ring in two alternative orientations leading to two polynuclear UI�its in each unit cell .

Imidazole(II, imH) i s a monodentate l igand. Its ring contains two n itrogen atoms. The N-3 nitrogen is termed as pyridine n itrogen while N- l is termed as pyrrole ni­trogen. Imidazole i s a ligand of biological importance involved in active sites of metalloenzymes as a part of histidine moiety2. Imidazolate, the deprotonated form of imidazole, is involved in active site of boving erythro­cyte superoxide dismutase3 . To explore the properties of imidazolate-bridged binuclear complexes, model com-/ plexes of the pairs copper-copper".5, copper-cobalt\ and copper-zinc 7- " , have been synthesized and characterized using various physico-chemical methods.

This paper describes the synthesis and X-band EPR spectra of some mononuclear copper(II) and homo/

"HNQ(S) /(4' N

" ( I ) (3)

(2) H II

heterobinuclear complexes with diethylenetriamine and imidazole.

Materials and Methods

Diethylenetriamine (s.d. Fine-chern.), imidazole (s.d. Fine-chern), copper chloride dihydrate (Aldrich) and n ickel chloride hexahydrate (Aldrich) were used as sup­plied. Other chemicals used were of reagent grade.

[Cu(dien)Hp](CI)2 - In methanol-acetonitrile medium (5 : 1 ), solutions ofCuCl2 .2Hp (34 1 mg, 2 mmol, 50 ml) and dien (206 mg, 2 mmol, 50 ml) were mixed together, stirred well and the contents were left overnight. B luish green crystals of [Cu(dien)Hp](CI)2 thus formed were collected by filtration and were washed with ethanol and dried in vacuo at room temperature, yield 65%.

{Cu(dien)imH](CI)l - In methanol-acetonitrile medium (5 : I ) , solutions of CuCI2 .2Hp (34 J mg, 2 mmol, 50 ml), dien (206 mg, 2 mmol, 50 rnI) and imH ( 1 36 mg, 2 mmol, 50 ml) were mixed together, stirred well and the con­tents were left overnight. Dark bluish green crystals of [Cu(dien)imH](Cl)2 thus formed were collected by fi l­tration and were washed with ethanol and dried in vacuo at room temperature, yield 60%. {( dien)Cu(im)Ni( dien)(Cl)3 - In methanol-acetonitrile medium (5: 1 ), solutions ofCuCI2 .2HP (34 1 mg, 2 mmol,

"1

�

PATEL et at.: EPR SPECTRA OF CU(l1) COMPLEXES 90 1

Table I - Elemental analysis for the Cu(II) complexes

S. Complex Colour % Found !Calcd.2 No. C H N Cu Ni Zn

[Cu(dien)H,O)(CI), Bluish Green 1 8 .80 ( 1 8.79) 5.93 (5.9 1 ) 1 6.45 ( 16.44) 24.87 (24.85)

2 [Cu(dien)imH)(CI), Dark Bluish Green 27.52 (27.50) 5.61 (5 60) 22.93 (22.9 1 ) 20.79 (20.78)

3 [Ni(dien)H,o)(CI), Violet 1 9 . 1 6 ( 19. 1 5) 6 03 (6 02) 1 6.78 ( 1 6.75) 23.43 (23.41 )

4 [Ni(dien)imH)(CI), Dark violet 27.96 (27.94) 5.69 (5.69) 23.29 (23.28) 19.54 ( 1 9.5 1 )

5 [(dien)Cu(im)Cu(dien»)(Cl).l Blue 26.08 (26.06) 5.78 (5.76) 22. 1 2 (22. 1 1 ) 25.09 (25.07)

6 [(dien)Cu(im)Zn(dien»)(CI), Light Blue 25.98 (25.97) 5.75 (5.74) 22.05 (22.03) 1 2.50 ( 1 2.49) 1 2.88 ( 1 2.85)

7 [(dien)Cu(im)Ni(dien»)(CI).l Bluish violet 26.32 (26. 3 1 ) 5.84 (5.82) 22.35 (22.32) 1 2.68 ( 1 2.65) I I . 70 ( 1 1 .69)

Table 2 - EPR data of the Cu(II) complexes

S.No. Complex Temp. HTt."Nf: H" H"

ICu(dien)H,OJ(CI), RT J390 JJ9 1 J l XO

LNT J220 J220 J020

ICu(dien)imH I(CI), RT 33XO 3JXO

LNT 3240 J240 3025

l(dien)ClI(im)Cu(dien)I(CI)., RT J390 JJ9 1

LNT 3230 J23 1

4 l(dien)Cu(im)Zn(dien)I(CI), RT J395

LNT J240

( dien)ClI(im)Ni(dien)I(C1h RT 3390 339 1 LNT 3240 3241

50 ml), dien (206 mg, 2 mmol, 50 ml) and imH ( 1 36 mg, 2 mmol, 50 ml) were mixed together and stirred well (Set A). Similarly, in methanol-acetonitrile medium (5: 1 ), solutions of NiCI2.6Hp (475 mg, 2 mmol, 50 ml) and dien (206 mg, 2 mmol, 50 m)) were mixed together and stirred well (Set B) . Contents of Set A and Set B were then mixed together and stirred wel l . pH of the solution was raised to - 1 2 .0 by adding 1 M NaOH solution and the contents were left overnight. B luish violet crystals of [(dien)Cu(im)Ni(dien)] (Cl)2 thus formed were col­lected by filtration and were washed with ethanol and dried in vacuo at room temperature, y ield 60%.

The complexes , v i z . , [N i (d ien)H20] (CI ) 2 ' [Ni(dien)imH] (CI)2' [(dien)Cu(im)Cu(dien)](Cl)� and [(dien)Cu(im)Zn(dien)](Cl)3 were prepared by similar methods.

The EPR spectra were recorded on a Varian E-Jine Century Series EPR sepctrometer equipped with a dual

H1.2

J220

3095

J220

30X5

Hzl H" x"' t:1.2 liz D(G) E(G) R G ,-XII (em" ) (1).)

JOXO JJ40 2. 135 2. IOX 2. 1 1 5 1 37 46.92 6.07 0.94

(0.0 1 3) 29JO 3 1 95 2 . I JX 2 .0X6 2 . 1 05 1 39 52.08 6.05 0.93

(0.0 1 3)

3075 3330 2. 102 2. 1 1 3 1 34 54.XJ 6.23 · 1 . 10

(0.0 12) 2945 3 1 95 2. 145 2 . 1 03 2. 1 1 3 1 3 2 4 1 .90 6.23 0.91

(0.0 12)

3090 3330 2. 1 1 5 1 27 46.87 6.23

(0.0 1 2) 2955 3 1 85 2. 107 1 2 1 55. 1 2 6.40

(0.01 I ) 3260(HlSo) 2.01!5(x,,,,,)

3 1 00(H,s<,) 2.093(x,,,,,)

30X5 3350 2. 1 1 0 1 39 54.43 6.06

(0.0 1 3) 2955 3 2 1 0 2. 105 134 50.60 6.2 1

(O.o J 2)

cavity and operating at X-band with 1 00 kHz modula­tion . TCNE (g = 2.00277) was used as a field marker. The frozen samples were placed in a Varian liquid nitro­gen Dewar flask for measurement at 77K. The glJ- and AIJ- values were measured according to the standard pro­cedure l2 .

Electronic spectra were recorded on a Shimadzu UV­VIS recording spectrophotometer UV - 1 60 using matched I cm path length quartz cell .

Results and Discussion

All the <;ynthesized complexes gave satisfactory el­emental analyses (Table I ) .

[Cu( dien)Hp j(Cl)2 - Dien forms a powdery bluish green complex with copper(II) . We have recorded X-band EPR spectra at room temperature and at l iquid nitrogen tem­perature (Fig. 1 ) . At l iquid nitrogen temperature the spec-

902 INDIAN J CHEM, SEC. A, SEPTEMBER 1 999

w o I .j2 (a) L N T -' 0.. � <t

k ) LNT

2 60 0 G

, , : , : ,

:; , . : . r ... ! . . ' . " ... "

. t!

! r :: .. . , . " . . " : . " " : 1 , ' , ' , , , , � I ,. ' .. , ,. , " c " , : : . , ,

I i : T C NF . S Id(: . . • ! . ,

.. .. . . ... .. . __ ..... :

" , � ........... - .... . .. . ..

� 200 G

3 000 G 3400 G 3800 G Mc:xgnetic fitld

Fig. 1 - X-band EPR spectra of (a) & (b) [Cu(dien)H,O](CI), and (c) & (d) [Cu(dien)imH](CI )2'

. .

trum is better resolved. The spectra are unlike that of a tetragonal coppef(ll) complex and indicate the presence of spin-spin interaction. The value 13 of G(=gll-2Ig1. - 2) comes out to be - 1 .0, which again suggests spin-spin interaction.The interaction between unpaired spins of two copper ions (each 5=112) gives rise to a singlet (5=0) and triplet (5= 1 ) state. The spectrum under discussion, shows four l ines at room temperature : H 3 1 80 G' H

1. 1 ' ' 1.2' 3220 G; Hzl ' 3080 G and H'l ' 3340 G (Table 2). From these data the g-values are obtained as : g1. I=2. 1 35 , gJ.2 = 2 . 1 08, gz = 2 . 1 1 5 (Table 2). Kivelson and Neiman 14 have reported that the gil value in a copper(ll) complex, can be used as a measure of the covalent character of the metal':chelate bond. If this value is greater than 2.3, the environment is essentially ionic and values less than this limit indicate a covalent environment. The value for this complex is 2. 1 1 5 , which being less than 2.3, indicates

w o ::> I­:::; a. � <{

LNT �200G .

�! .. -, ...... _ .. _-_ .•. _

.. -.-

1 6 0 0 G .......

1 400G . 1800 G

(a) lNT

(b) RT /

........... /

TeNE ,/

f: ! ! i 1 I : 1 \ i \

i . 200G 1---"-4

2 2 0 0 G 2 60 0 G 3 00 0 G 3400 G 3800 G

Ma9netic f i eld

Fig. 2 - X-band EPR spectra of [(dien)Cu(im)Cu(dien)](C1)y

considerable covalent bonding character. From these value the zero field splitting parameter 'D' has been obtained as 1 37 G. Fol lowing Chasteen et ai. 15, the cop­per-copper distance has been evaluated as 6.07 A.

[Cu(dien)imH](Ci)2- Fig. 1 also contains spectra of [Cu(dien)imH](CI)2 ' The spectra are simi lar to those of [Cu(dien)Hp] (CI)2 and are unlike that of a tetragonal co�p�r(II) c�mplex spectra indicating presence of spin­spm mteractlOn. Again for this system the value of G

�omes out to be - 1 .0, which again suggests spin-spin mteraction. At l iquid nitrogen temperature observed tran­sitions are : H1. I ' 3025 G; HJ.2' 3085 G; Hzl' 2945 G and HZ2, 3 1 95 G (Table 2). The values obtained from these data are: g1.l = 2. 1 45, g1.2 = 2. 1 03 and gz (-gil) = 2. 1 1 3 . The value of gil of this complex is less than 2.3 (Table 2), which again indicates covalent character. The evaluated copper-copper d istance comes out to be 6.23A.

[(dien) Cu(im) Cu( dien)](Cl)3 - The X-band spectra of this compound provide additional convincing support for the presence of a weak exchange interaction. The spectra are recorded at room temperature and at l iquid n itrogen t�mperature (Fig. 2). At both temperatures, the complex gl:es a broad signal in the low field region indicating spm- exchange interactions between two copper(ll) ions,

t

PATEL et al.: EPR SPECTRA OF CU(II) COMPLEXES 903

RT ... -.. .,.-..-.-------.-�

1 6 00 G 200G I-----<

w 1400 G 1 a OO G -g (a ) t- L N T :; Il. � <l (b)

RT

2 600 G 3 0 00G

!\/TCNE f �

.

200 G �

3400 G 3 8 00 G Masne.tic field

Fig. 3 - X-band EPR spectra of [(dien)Cu(im)Zn(dien)](CI)I'

and, therefore, hyperfine splitting is not seen. The �Ms = 2 transition is weaker at room temperature than that at I iquid n itrogen temperature and is located at - 1 650 G (inset Fig. 2) . This can be attributed to a weak magnetic interaction between two copper ions through an imida­zole ring and offers a most definitive proof of the pres­ence of an intradimer magnetic exchange interaction. The two expected spin-allowed EPR transitions occur (at RT) HzI' 3090 G and HZ2 3330 G (Table 2), yielding g, value 2 . 1 1 5 (Table 2), from these values the zero field split­ting parameters D and E, have been obtained as 1 27 G and 46.87 G (Table 2). Again, following Chasteen et al. 1 5 , copper-copper distance has been evaluated as 6.23A. However, this value is slightly higher than that for some other imidazolate bridged binuclear copper(II) complexes ( -6.oA) lo. l 7.

[(dien)Cu(im)Zn(dien)}(Cl)3 - The spectra of this com­plex at room temperature and at l iquid nitrogen tempera­ture are presented in Fig. 3. The spectra are isotropic. On cooling to liquid nitrogen temperature, no change was observed . No �M = 2 transition was observed at s half-field at either temperature in the spectrum. The ab-sence of �M = 2 signal (inset, Fig. 3) in both tempera-s tures may be due to lack of Cu-im-Cu impurity9. This shows that complex comprises only of a heterobinuclear Cu/Zn complex .

w o ;:J l­S Q. (a ) L NT � 1---__ <t .

( b) RT -- - - --- ---- - ---

I j !

!

2 2 00 G 2 6 00 G 30QO'G 3 40 0 G Magnetic fi.eld

200G I-----'

3 8 00 G

Fig. 4 - X-band EPR spectra of [(dien)Cu(im)Ni(dien)](CI\.

[(dien)Cu(im)Ni(dien)](CI)3 - The EPR spectra for this complex at room and at liquid n itrogen temperature (Fig. 4) were also recorded. The spectra are nearly iso­tropic. Again on cool ing to l iquid nitrogen temperature, no change was observed. The metal-metal distance at RT in the system under discussion works out to be 6.06 A and is identical with the earlier reported values 1 0. 17 . The observed transitions and EPR parameters are pre­sented in Table 2. �Ms = 2 signal was not detected in spectra, indicating the absence of Cu-im-Cu impurity9.

Electronic spectra - We have also recorded the electronic spectra of this series of complexes in nujol. The elec­tronic spectra for al l the complexes are presented in Fig. 5 and Am"x values are l isted in Table 3 . The s imple copper(II) binary and ternary complexes exhibit a strong band at 1 5970 and 1 6550 cm-I respectively. These are assigned to the superimposed transition

2B ,g � 2B2g

+ 3Eg, suggesting a distorted octahedral configuration in 11I1jollx . However, in ternary copper(II) complexes with imidazole, the Amax shifted toward the higher energy. The electronic spectra of the nickel(II) binary and ternary complexes show three bands. An examination of these bands indicates that the complexes have octahedral ge­ometries and might possess D4h geometryl9 . The various

904 INDIAN J CHEM, SEC. A, SEPTEMBER 1 999

(9)

• 100 nm•

100nm �

100nl1\-; I •

WAVELENGT H

:.& 01>

1100

1 1 00

900

900

1 100

Fig. 5 - Electronic spectra of : (a) [Cu(dien)Hp](CI)2 . (b) [Cu(dien)imH](CI)2: (c) [Ni(dien)Hp](CI)2 ' (d) [Ni(dien)imH](CI)2 (d) [(dien)Cu(im)Cu(dien)](CI\ '

(f) [(dien)Cu(im)Zn(dien)](Cl)J ' and (g) [(dien)Cu(im)Ni(dien)](CI)J

ligand field parameters for the complexes have been cal­culated. v, directrly gives the value of 10Dq. The l ODq value of the ternary complex is greater than that of the binary nickel(II) complex. This may be attributed to higher l igand field of complex. The nephelauxetic pa­rameter � comes out to be 0.84 and 0.7 1 for binary and ternary nickel(II) complexes respectively. The � value, indicates that the covalent character of M-ligand '0" is low.

Table 3 - A.m•x values of the M(II) complexes

S.No. Complex An ... in Nujol

(cm·l)

[Cu(dien)Hp](CI)2 1 5970

2 [Cu(dien)imH](CI)2 1 6550

3 [Ni(dien)Hp](CI)2 27030, 1 6720, 10250

4 [Ni(dien)imH](CI)2 27400, 15970, 1 0800

5 [(dien)Cu(im)Cu(dien)](CI)J 26660, 1 6660

6 [( dien)Cu(im)Zn( dien) ](CI)J 26880, 1 6000

7 [(dien)Cu(im)Ni(dien)](CI)J 26800, 1 6280

The electronic spectra of homo/heterobinuclear com­plexes show two bands (Table 3) . The A.max of these two bands fall in the ranges 26660-26880 cm" and 1 6000-1 6660 cm" . The bands in the range 1 6000- 1 6660 cm" are assigned to dod transitions. In case of heterometallic binuclear compiexes, a red shift of the electronic transi­tions may arise from the influence of the i midazolate bridging of heteronuclear atom. This indicates that these heterometallic complexes9 are a Cu/Zn species and not a mixture of a CulCu, Zn/Zn and NilNi complexes. Also these binuclear complexes possess a band in the 25000-27000 cm,l region, which i s possibly due of M-im-M bonding20•

Acknowledgement

We thank the R S I C, lIT Bombay for EPR facilities. Financial assistance from CSIR (Scheme No. 1 525), New Delhi is gratefully acknowledged.

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