Indi an Journal of Chcmistry Yol. 45A. April 2006. pp._858-863

Synthesis, spectroscopic and crystallographic studies on two copper(IJ) benzimidazole complexes: Supramolecular frameworks built from

N-H ... 0, N-H ... Cl and O-H ... Cl hydrogen bonds Md . Mijanuddin", Sumana Gangopadhyal' *, Swastik Mond 'l.l c

, Monika MukherjeeC & Mahallllllad Ali'" * "Department of Chemistry, J ~davpu r Universi ty . Kolkata 700 032. India

Email: m_ali2062 @yahoo.eom bDepartment of Chemistry. Gur:ldas College. Narikeldanga. Kolkata 700 054. India

<S.S. P. Dep'lItmcnt. Indian Association for the Cu lti vation of Science, Jadavpur. Kolkata 700 032 , Indi a

Received 3 1 May 2005; revised 30 Januarv 2006

The tridentate ligands 2,6-bis(benzimidazole-2'-yl)pyridine (LI) and N.N-bis(ben zimidazole-2'-yl-methyl )amine (Ll) with N3 donor set form the correspondi ng mononuclear [Cu ( LI )(CIO~)(C I )].H20J (1) and [C u(Lz)(C1 z)]. HzO (2) complexes, which have been characterized by elemental analysis. UY-Yis spectra. IR , EPR and magnetic moment studies. Single crysta l X-ray st ructure studies for both the complcxes show that coordination geometri es around the copper centre ,Ire best dcseribed as distorted sq uare pyramidal with three nitrogen atoms of tr identatc li gand and a chlorid(! ion defining thc eq uato l'ial Dlanes. The apical posi tions in the compl exes arc occupied by a perchlorate 0 atom in 1 and a chloride ion in 2. Metal directed assembly of infinite two-d imensional supramolecu lar network stabil izes the c rystal structures o f both complexes in the solid state. The global magnetic interactions in complexes 1 and 2 are found to be paramagnetic with ~dl values of 1.65 8M and 1.63 8M, respec ti vely, which are typica l for a Cu(lI) {(l system.

IPC Code: In t. CI 8 C07F I/08

The metal complexes of benzimidazole ligands are important displaying supramolecular self-assembly via inter-ligand hydrogen bonds '-

3. The bridging

moieti es between the benzimidazole groups in the li gand and the type of counter anions in the complexes seem to play important role in the supramolecular architecture. Among various benz­imidazole derivati ves. the planar 2.6-bis(benz­imidazole-2 '-yl)pyridine eLI) has been exclusively used and several diva lent metal complexes with LI are known4

-13

. But. only a few of them have been characterized structuralll9

"o,' 3. The planarity of li gand (L'), allows significant conjugation extending over adjacent atoms and thereby coordinating to a metal atom probab ly suits the req uirement for photolumi nescence beha viourll. N, N -bi s(benzi mida­zole-2'-y l-methyl)amine (e) is another important li gand where the pyridine ring between two benzimidazole groups in eLI) is replaced by an NH­(C H2h- moiety.

The syntheses. characterization, crystal and molec ular structures of two copper(II) complexes, IC u(L ') (C10~)(CI)].HzO (1) and [Cu(Lz)(Ch)].HzO (2). a long with possible supramolecular framework are descri bed here.

Materials and Methods The materials pyridine-2,6-dicarboxylic acid

(Lancaster), iminodiacetic acid (Lancaster) and a-phenylenediamine CA. R. Loba) were of reagent grade and used as received. The solvents acetone. methanol and acetoni tri Ie (Merck. India) were of reagent grade and dried by standard methods before use. Ligands L' and e were synthesized by known methods 10.14.

Synthesis of [Cu(L1)(CI04)(CI)].I1 20 (1)

A suspension of 1 mmol (0.311 g) of ligand (L') in acetonitrile solution at 50°C was added to a stoichiometric quantity (0.171 g) of CuCI 2.2HzO. The mixture was stirred on a water bath unti 1 a clear deep green solution was obtained. Then. a few drops of a saturated solution of NaCl04 were added and the mixture was stirred for another 4 h on a water bath. The solution was then allowed to evaporat.e slowly at ambient temperature to get the crystall ine products. Green single crystals suitable for X-ray diffraction were obtained by re-crystallization from acetonitrile. Composition of the complex determined by elemental analysis : C, 42.90; N. 12.69: H 3.27 %. Calculated

based on C'9H'5C12CuN50 5: C, 43.23 ; ?'-I. 13.26; H. 2.86%.

MIJANUDDIN el nt. : SYNTHESIS & CHARACTERIZATION OF Cu(lI ) BENZIMIDAZOLE COMPLEXES 859

Synthesis of [Cu(L z)(C lz) ] H zO (2)

In a typi ca l procedure, a suspension of 0 .277 g ( 1.0 mmol ) of ligand (L2) in methano l at SO°C was mixed with a stoichi o metric quantity (0. 17 1 g) of CuCI 2.2H20 and the mixture was stirred on a wate r bath until a clear deep green solution was obtained . Crysta lline solid material was obtained by slow evaporat ion of the solvent. Single crystals suitable for X-ray diffraction were obta ined by re-crystallization from acetonitri Ie . Compositi on of the complex dete rmined by e lementa l analysis: C , 43 .97 ; N. IS . 18 ; H. 3.61 %. Calculated based on C'6H '7ChCuNsO: C. 44.71 ; N, 16.29; H. 3.99%.

Physica l measurments

Elemental ana lyses were carried out usi ng a Perkin-Elmer 240 e lemental analyzer. lR (400-4000 cm") was recorded fro m KBr pellets on a Nico let Magna IR 7SO series-II FTlR spectrophotometer. E lectronic spectra were recorded on Agilent 84S3 UV -Vis di ode array spectrophotometer. Room temperatJre magnetic suscept ibilities were recorded on a PAR vibrating sample magnetometer using [Hg[Co(NCSL] as the calibrant. EPR were recorded from Vari an X-Band EPR spectrophotometer (Model E 109).

Crystal data collection and r'clinement

Single crystal X-ray diffraction data for 1 and 2 were collected at room temperature using a DIP image plate (MacScience DIPLabo 3200 I) with graphite monochromated MoKa radiation . T he structures were solved by direct methods and refined by full-matrix least-squares techniques on p2 using SHELXS97 and SHELXL97 (Sheldrick. 1997). For both structures. a ll H-atoms located fro m the

J Ol ' ~

Fig. ) - ORTEP diagram ofCu-complcx I with 30% probability elli psoid s.

difference Fourier map were treated as riding (except for the solvent water H-atoms which were he ld fixed ) with N-H distances = 0.86 A, C-H distances = 0.93 A for CH groups and 0.97 A for CH2 groups. The thermal parameters for the H-atoms were taken as Uiso(H) = 1.2 Ucq(CIN) for CH. CH2 and NH groups. and Uiso(H) = I. S Ueq(O) for water hydrogen atoms .

Results and Discussion The asymmetri c units of complexes 1 and 2 with

atom numbering scheme are given in F igs 1 and 2. Table 1 summarizes the crystallographic data and refinement parameters. Se lected bond lengths and bond ang les are li sted in Table 2.

Molecular' structurc of [Cu(L 1)(CI04)(CI)].HzO (1)

In complex 1, the coordination polyhedron about the copper(II) cente r can be described as approxi mate ly square pyramidal wi th a CuN30 C I chromophore. The three nitrogen atoms N(l) , N(2). N(3) , of the tr identate li gand and the chloride atom CI(l ) define the basal plane; the apical position is occupied by O(S) atom of the pe rchlorate anion. The metal atom is displaced by 0 . 111 (2) A towards the perchlorate ion. The degree of di storti on of the coordinati on polyhedron from ideal square planar geometry can be asses!:;ed from the trigonality index parameter ('I) 0.24 (Ref. IS), for a perfect square pyramidal coordination the va lue oft is ze ro. Si m il ar distortion in metal coordination polyhedron has been observed in fi ve coordi nated copper(II) systems where t values lie in the range 0.03 to 0.30 (Ref. 16). The Cu- N bond lengths in 1 ranging between 1.982(2) to 2.02 1(2) A agree we ll with the reported va luess. It is interesting to note that both chloride and perchlorate ions are coordinated to copper(ll) center in 1 leaving the H20 molecu le, a better donor than CI04 -, uncoordinated. T he ax ial Cu- 0(5) bond distance, 2.SIO(3) A is sign ificantly longer than

CI2

. N I2

-\C13 ~

N2 (;~ ' 4-14 CI 8 } . CI5

Jt­~rv

ifC17 ~1C 1 6

Fig. 2 - ORTEP diagram of complex 2 with 30% probab il ity ell ipsoids.

860 INDIAN J C HEM, SEC A. APRIL 2006

Table I - Crysta l data and st ructure re finement fo r complcxes 1 and 2

Parameters

Empi ri ca l fo rmula

Formu la weight

Temperatu re

Wavc!ength

Crystal system. space group

C I9 H I 5 Clz C u Ns Os

527.80

293(2) K

0.7 1073 A

T ric linic. P T

429.79

293(2) K

0 .7 1070 A

2

Orthorhombic , Pbea

Unit cell dimens ions

Volume

(/ = 8.8 16(3 ) A. a = 88. 734(6)°

b = 10.59 1 (4) A, ~ = 85 .7 13(6)°

c = 11 .264(4) A, y = 76.892(6)°

102 1.4 (7)AJ

(/ = 13.269(7) A.

b = 14.680(9) A,

c =1 7.578( 10) A,

3424(3 ) P Z. Calculated densit y

Abso rpti on coefficient

F(OOO )

Crysta l size

o range for data coll ection

Limiti ng indi ces

2 , 1.7 16 mg/mJ

1.375 mm·1

534

0 .20 x 0 .15 x 0 . 13 mm

1.8 10 to 24.990

- 10< =11 < =1 0

- 12< =k < =1 2

- 13< = I < = 13

8, 1.667 mglmJ

1.603 mm·1

1752

0 . 16 x 0. 12 x 0 . 10 mIll

0< =h < =1 5

0< = k < =17

0< = I < = 20

Re llec ti ons co llec ted/uni que

Completeness o f El

9943/3594 I R(int) =0.0 175]

24 .99: 99 .8%

2656/2656 [R(illl ) = 0 .0000]

25 .09 : 87. 1%

none none Absorption correc tion

Refinement methode Fu ll -matri x least- squares on Fl Fu ll -matrix least-squ areson F2

Oat ail"Cst ra in ts/parameters

Goodness-of-fi t-on /-"2

3594/ 0 /289

1.035

2656 / 0 ,. 226

1.094

Final R indi ces[1>2a(I) 1

R ind ices(all dat a)

Rl = 0 .03 27, wR2 =0.089 1 R I = 0 .0326, wR2 =0.OS62

R I = 0 .033 8. wR2 =0.0&67

0.569 and - 0 . 649 e. AJ Largest diiT.peak and ho le

HI = 0.03 75, 1V!?2 =0.0930

0.449 and - 0 .274 e. A'>

equatori al bond lengths l1. 923(3)-1. 980(2) A I observed in copper(ll) compl exes with oxygen containing ligands ' 7.' 8. Thi s lengthening of Cu- O (perchl orate) di stance is. howeve r. consistent with the correspond ing values reported earlier' Y and presumab ly due to steri c requirement of the ligand. bul kier ClO-l- ani on which prevent s any closer approac h of the perchl orate ion.

The molecular structure of ] exhi bits diffe rent i ntra- and i nter-molecu lar hydrogen bonds Crable 3 ) .

N(S) atom of benzimidazole li gand at (x, y, z) acts as a hydrogen bond donor. via H(S). to perchl orate 0 (2)

and 0(3) at (-x. l -y, l -z) forming a centrosymmetric R:/ (l 6) ring (Fig. 3). The sol vent water molec ules augmented ind ividual R/ (l 6) supramolecul ar motifs by strong N- H . .. 0 , O- H ... O and O-H .. Cl hydrogen bonds, whi ch expand the supramolecul ar aggregati on to tw~ dimensional parallel sheets in the plane (0 1 I ). Similar types of hydrogen bonded supramolecul ar assembli es in metal compl exes with water molecules have already been reported20

Molecular stru cture of [Cu(l} )(Clh l.ll z0 (2)

The crystal structure of 2 is si mi lar to that described fo r 1 with the metal atom displayin g an

MIJANUDDIN e/ (fl.: SYNTHESIS & CHARACTERIZATION OF Cu(II ) BENZIMIDAZOLE COMPLEXES 861

Table 2 - Selected bond lengths (A) and angles (0) i'or complexes 1 and 2

Bond lengths (A) Bond angles (0)

COlllfi lex I

Cu-N( I ) 1.982(2) N(3) -Cu- CI ( I ) 10047(6)

Cu-N(2) 2.02 1 (2) N( I ) -Cu - N(3) 7964(8)

Cu-NO) 2.02 1 (2) N( I) -Cu- O(S) 92.5 1( 10)

Cu-Cl( I ) 2.207( 1) N(2) -Cu- 0 (5) 91.18(9)

Cu-0(5) 2.5 10(3) NO) -Cu- O(S) 9344(8)

N( I ) -Cu- N(2) 79.29(8) CI ( I ) -Cu- 0(5) 9444(S)

N(2) -Cu- CI( I ) 99.99(6)

COlllfilex 2

Cu( I )-N( I ) 1.884(2) N(3) -Cu( 1)- CI(2) 9S.62(5)

Cu( I )-N(2) 1.993(2) N( I) -Cu( I )- N(3) 78.54(9)

Cu( I )-N(3) 1.948(2) Nel ) -Cu( I )- CI ( I ) 91 . IS(6)

Cu( I )-CI(2) 2240( I ) N(2) -Cu( I )- CI( I ) 95.77(5)

Cu( I )-CI( I ) 2.76 1(2) NO) -Cu( 1)- CI( I ) 9238(6)

( 1) - Cu( I )- N(2) 82.77(8) CI(2) -Cu( 1)- CI( I ) 101.10(2)

N(2) - Cu( 1)- CI (2) 97.97(5)

Fig. 3 - Supramolccular mati i' o r I exh ib iting R/( 16) rin g. The atoms marked with a (#) arc at symmetry position (-x . I-y. I -z).

approximate square pyramidal coordination geometry (1 = 0.13) . The basal plane in 2 is defined by the three nitrogen atoms [N(l ). N(2) , N(3)] of the ligand and one chlorine atom [CI (2)J; the other chlorine atom [CI(I)J occupies the axial site . The C u atom li es 0 .199(1 ) A above the basal plane. The axial lC u­CI(l ). 2.76 1 (2) ], is long compared to equatorial C u­CI(2) di stance r2.240( I) AJ due to lahn-Te ller di stortion. Si milar long C u- C I di stance is reported in lite rature21

.22

. As a consequence of C I- CI charge repulsion , the IC I(apical)- C u-Cl (equatorial)] angle deviates considerably from [CI(apical)-Cu­N(equatorial)] angles (Table 2).

Table 3 - Hydrogen bonding geometry (A) i'or 1 and 2

Complex I

D-HA D-H H A D A D-H A

01 -- HI 9 .. CII 0.8900 2.3800 3243(3) 164.00

N4 -- H4 .. 0 1#1 0.8600 1.9400 2.792(4) 17300

NS -- HS .. 02#2 0.8600 2. 1400 2.986(4) 166.00

NS -- H5 .. 03#3 0.8600 2.5700 3.238(S) 135.00

01 -- H2O .. 01#4 0.8100 2 1300 2.928(4) 167.00

C9 -- H9 .. 02#5 0.9300 24200 3304(4) 15S.00

COlllfilex 2

D-H A D-H H" A D' A D-H A

N7 -- H7 .. 01 OS600 2.0900 2.(3 10) 167.00

C2 -- H2 .. CI2 0.9300 2.6800 H05(3) 13500

C 17 -- H 17 .. CI2 0.9300 2.6700 3404(3) 136.00

01 -- HIA .. CII#6 I .OSOO 26200 3.393(3 ) 128.00

N I2 -- HI 2 .. CII #7 OS600 2.3600 3. 177(3 ) 158.00

C5 -- H5 .. C12#8 0.9300 2.8000 3672(3) 15 8.00

Symmetry codes: # I -I +x.y.z #2 -x. l-y. l-z #3 -x. l-y. l -z #4 I -x.-y .-z #5 -x. l -y. l -z #6 112-x, l -y.-1I2+z #7 112-x.1I2+y.z #8 x. 1/2-y. -1I2+z

Complex 2 shows two C- H ... CI and one N- H . . . O intra-molecular hyd rogen bonds (Table 3) . Lnte r­ligand hydrogen bonds involving the water molecules . chloride ions and the NH group of the benzimidazo le moi ety linked the individua l components to form infinite two-dimensional supramolecular frameworks. The solvent water molecul e 0(1) at (x . y, z) acts as donor, via HI A. to the CI(l) ion at (l12-x. I-y. z- I 12). and N( 12) atom of benzimidazole moiety .at (x . y. z) acts as a hydrogen bond donor. via H(l 2). to another adjacent chloride ion CI(l) at (l12-x. 1/2+y. z) forming the infinite supramolecular wavy ladders propagating along 10 1 0] directi on as shown in Fig. 4 .

IR analyses The IR spectra of the li gands were compared with

the spec tra of the complexes in o rde r to identi fy the binding mode of the li gands to the Cu atom and the important lR data of the ligands and complexes are li sted in Table 4. The free li gands L I and L 2 showed strong absorption bands at 3 199 and 3 180 cm·1

respectively, which are characteristic of the v(N-H) group o f benzimidazole ring. On complexation. these bands shift to 3 194 and 3 110 cm- I for 1 and 2.

IN DI AN J CHEM. SEC A. APRIL :2006

Fi g. 4 -- Inrinite sliprailloiecli lar wavy ladders propagat ing alung 10 I 01 direction rur the cOlllplex 2.

respecti ve ly; ~ lth ough N-H group of benzimidazole ring does not parti c ipate in bonding to the metal center. T his small shifting is probab ly due to the ex istence of hydrogen bonds between N-H group and coordi nated-anions (C10~- and Clf Strong bands observed at 1600 cm-t in L t and 1649 cm-t in L 2 have

been ass igned to v(C=N) of benzimidazo le ring. which are shifted to 1605 and 1625 cm·t in 1 and 2. respect ive ly indicat ing their pa rticipa tion in bondi ng through thi s nit rogen atom to the copper atom. T he IR band corresponding to v(C-N) at 1274 cm-t of f ree l igand (L2) is shifted to hi gher frequency (1280 cm-t) in complex 2 showi ng that the other coordinati on is th rough thi s ali phat ic ni trogen atom. T he spectrum of 1 ex hibits splitting of the v(C I O~) at 1157 and 1086 cm·t indicating the coordinati on of perchlorate ani ons directl y to the copper atom.

Elt-dJ'Oni c spectn l

T he U V spectra of the comp lexes 1 and 2 are similar. but di ffe rent from that of the li gands. T he bands at 3 13-3 \ I nm reg ion ass igned to n--7n* tran si tions in the l igands are spli t into two bands: ( i ) n"--7n * at 3 14 nm (£ = 22980. dm' mol' l cnf l). and

Tab le 4 -IR Spectral data ror the ligands and l' rnplexes hand Illax illla (C Ill-I

)

v / (cm-I) C I9H 13Ns CI 9H ISNsOs C IGHIS Ns CI 6 H I7N)OC I ~ C I~C lI Cli

v(N-H) 3182 3194 3 199 3110

v(C-H)",o," 3061.735 3064.747 3060.740 3065.747

v(C-H )"I;1'1o 2965 :29 17

v(C=N),"oll' 1600 1605 1649 16:25

v(C=C)"roll' 1460 1469 1440 1455

v(C-N),,;l>Il' 13 19 132 1 133:2 13:21

v(C-N),,;;plo 1:274 1:280

v(CIO.) 11 57, 1086

v(HP ) 3433 343-'

(ii ) n lJ--7n"* at 359 nm (£ = 17400. dm' mol' l cm-I) for complex 1; and (i) n"--7n * at 3 15 mn (£ = 22980. dm' mor t cm·I). and ( ii ) nb--7n" * at 360 nm (£ = 17400. dm' mol' l cm·t) for comp lex 2. T he elec tronic spectra of these complexes in MeC and M eOH show low intensity bands at 665 11 m (£ = 107. dm' mol' l cm-I) for 1 and 702 nm (£ = 106. dm' mol' l

cn,-I) fo r 2. respec ti ve ly due to d-d transiti on}' . T his

indica tes that the geometry around copper(l l) is penta­coordinated square pyramidal.

Magnetic propel·ties

T he magnetic moments for the two complexes ha ve been measured at room temperature ;v i th ~lcll values 1. 65 8M and 1.63 8M for 1 and 2. respective ly indica tin g that copper atom in both the complexes are paramagnetic wi th one un paired elec tron .

EI'R spednl

T he room temperature EPR spectra of po lyc rystall i ne powdered samp les of Cu( II ) complexes 'vvith one un pa ired elec tron were recorded at the X-band frequencies and the resul t were treated by the reported method2~.25 . For complex I . the

ani sotropic spectrum has an axial symmetry. w ith four hyperfi ne peaks in the para llel region due to the interac tion of the unpaired electron with nuclear pin of the copper nucleus. T he EPR parameters are:

gil = 2.04 , g1. = 1.94. A" = 130 G and A 1. = 10 G (v = 9_5 13 GH z). T he fact gil > g1. and two g va lues (g1. > 1.93 and gil > 2) indicates the square pyramida l geometry around copper atom with ax ial elongation in

MIlA UDDI N 1'1 af.: SYNTHES IS & CHARACTERI ZATION OF Cu(lll BENZIM IDAZOLE COM PLEX ES S(ll

compl ex 1 (Refs 26. 27) whi ch is confirmed by the longer Cu-O (Cl O~) bond length .

Supplementary Material Crystallographi c data for the structures reported in

thi s paper have been deposited with the Cambridge Crystallographi c Data center with the depositi on numbers CCDC 268380 and 26838 1. Copi es of the informati on may be obtained free of charge from the Director. CCDC 12 Uni on Road. Cambridge. CB2 IEZ. UK (Fax: +44- 1223-336033 ; Email : deposit @ ccdc.cam.ac. uk or httpllwww.ccdc .cam.ac. uk).

Acknowledgement SG and MA gnttefu ll y acknowledge the finan cial

ass istances from the UGC and CSIR.

References I Sun W-Y . Xic J. Mei Y- I-I & Yu K-B. Ne ll ' ./ Chelll. 24

(WOO) 519 .

2 Raj S B. MU lhi ah PT. Boce ll i G & Ri ghi L. I\ era Crl'sl . E57 (200 I ) m59 I .

-' Akuwga\\"a T. I-Iasegawa T . Nakamura T & Sai to G. Crrsl /,'lI g COIIIIII . 5( 10). (2003) 54.

4 Nelson S M . Esho F S & Drew M G B . ./ Chl'llI Soc Daltoll Trail S. ( 1982) 407.

5 Sanni S B. Bchm H J . Bcurskcns P T. Van A lbada G A. Rcedj ik: J. Lcnstra A I-I T. Add ison A W & Pal aniandava r M J . ./ Cllelll Soc DallOIl TrailS, ( 1988) 1-l29.

6 Piguet C. Bocquct B, Mulcr E & Willi ams A F, lIe/l ' Chilli lIera. 72 ( 1989) 323.

7 Addison i\ W. Burman S, Wahl grecn C B, Rajan 0 A. Rowe TM & Sinn E, ./ Chl'llI Soc 00110 11 Trail S. ( 1987) 262 1.

8 Shashikala N, Gayalhri V, Gowda N M N & Rcddy G K. .//IIr1 ClII'!II Soc , 66 (1 989) 537 .

9 RUllimann S. MorcJu C M , Willi ams A F, Bcrnardinclii G & Addison A W, Polvhedron , II ( 1992) 635.

10 Wan g S, Zhu y, Zhang F, Wang Q & Wang L. Polrh erlron, I I (1992) 1909.

I I Wang S, Luo Q, Zhou X & Zeng Z. Polyhedroll. 12 ( 1993 ) 1939.

12 Vaidyanathan V G & Nair B U. Ell r./ /ll org Chelll. 0003 ) 3633.

13 Wang S. Cui Y. Tan B, Luo Q. Shi J & Wu Q. l 'olr/Il'ilr(JII .

I 3 ( 1994) 166 1. 14 Bcrcnds I-I P & Stcphan D W. /ll o rg Chilll /\ Clil. 93 ( 198-l )

173.

15 Addison A W, Rao l' N. Rccd ijk J. van Rijn J & Verschoor G c. ./ ChI'lli Soc Daholl ·( mllS. ( 1984) 1349.

16 Herman G G. Lippcns W. Goeminnc A M . Steenl and M & Blalon N M. IIc/([ Cr l'Sl, C59 (2003) m294.

17 Mukhelj ee M . Mukherj ec A K. DUlla S K , Nanda K K & Helli well M , I\cla C rys l , C50. ( 1994) 5-l2.

18 Grove H. Slcllen J. Jul ve M & Liorct F,./ 0/(' 111 Soc DullOlI

'(rail s. (2000) 515. 19 Bianchi A . Bologni L. Dapporto P. Micheloni M & Paolelli

P. /llorg ClJelll. 23 ( 1984) !201: Add ison A W, Rao T \Y.

Sinn E. /ll org Chelll . 23 ( 1984) 1957: Pavkovic S F & Brown J N. II c/({ Crvswllog r , Seer /J. 38 ( 1982) 27-l: Ballagli a L P. Corrad i A B, Macrotr igiano C. Menabue L & Pcl lacani G C. ./ e helll Soc 0011011 TraIlS. ( 198 1) 8: Fawcell T G. Rudi ch S M. Toby B H. LalancellC R A . Potcnza J A & Schugar H J . /ll org ClI 1" 11 , 19 ( 1980) 940: Pell V B. Diaddario L LOr). Dockal E R. Corrield P W. Ceccarell i C. Glick M D. Ochrymowyez L A & Rorabaeher D B. /ll org ClJelll . 22 ( 1983 ) 940.

20 Dcak A. Kalman A. Park5nyi L & I-Iaiduc I. Aero Crrsl . 1357 (2001 ) 303.

2 1 Li -Ping Lu. Miao-Li Zhu & Pin Yang. AeI({ Crnl. C60 (2004) m2 1.

22 Yu -B in Dong. Mark D Smith , Hans-Conrad zur Loyc. Solid

Slate Sci. 2 (2000) 86 1. 23 Murthy N N & Karlin K D. in Mechollislic lJioillo rg({lIic

Cll ellli5lr.l'. cdilcd by H I-I Thorp & V L Pecoraro (A merin n Chcmica l Society. Washington. DC). 1995. Chap. 6.

24 BlcJney B. Phil Mag . 42 ( 195 I ) 44 1.

25 Vanngard T & Aasa R. P({ralllag ll etic i?eso ll oll ce. Vo l II. Ed itcd by W Low (Acadcmic Press. London), 1963. p.509 .

26 Evans D A . Kozlowski M C. Murry J A. Burgey C S. Campos K R. Con nell B T & Staples R J . ./ 1\111 Chelll SoC, 12 1 ( 1999) 669-685

27 Batra G & Mathur p, TraIlS M el Chelll . 20 ( 1995) 26.