www.elsevier.com/locate/poly

Polyhedron 24 (2005) 201–208

Synthesis, spectroscopic and electrochemical properties of someheteroleptic tris-chelates of ruthenium (II) involving

2,2 0-bipyridine (bpy) and N-(aryl) pyridine-2-aldimine(L):X-ray crystal structures of [Ru(bpy)(L2)2](ClO4)2 Æ H2O and

3-N(4-tolyl) imidazo [1,5a] pyridinium perchlorate

Dipankar Mishra a, Subhendu Naskar a, Bibhutosh Adhikary a,Raymond J. Butcher b, Shyamal Kumar Chattopadhyay a,*

a Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711 103, Indiab Department of Chemistry, University of Howard, 525 College Street, NW, Washington, DC 20059, USA

Received 28 September 2004; accepted 1 November 2004

Abstract

Four ruthenium (II) complexes of the formulae [Ru(bpy)(L1)2](ClO4)2 Æ H2O (A1), [Ru(bpy)(L2)2](ClO4)2 Æ H2O (A2),

[Ru(bpy)(L3)2](ClO4)2 Æ H2O (A3), [Ru(bpy)(L4)2](ClO4)2 Æ H2O (A4) (where L1 = N-(2-pyridylmethylene) phenyl amine, L2 =

N-(2-pyridylmethylene) 4-(methyl) phenyl amine, L3 = N-(2-pyridylmethylene) 4-(chloro) phenyl amine, L4 = N-(2-pyridylmethyl-

ene) 4-(fluoro) phenyl amine and bpy = 2,2 0-bipyridyl) have been synthesized. In addition to these ruthenium complexes, we have

also been able to isolate four imidazopyridinium perchlorate compounds B1–B4 from the same reactions. The X-ray crystal struc-

tures of one representative ruthenium complex (A2) and the imidazopyridinium perchlorate compound (B2) have been determined.

The Ru(II) center in the complex is coordinated by six N donors with a distorted octahedral geometry. The imine ligands (L) act as

bidentate N,N donors.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Ruthenium (II) complexes; Bidentate N,N donors; Imidazopyridinium perchlorate; Synthesis; X-ray structures

1. Introduction

Chemistry of ruthenium with a–a 0 diimine ligands is

an area of unabated interest during the past two decades

[1–16] because of the interesting photophysical and pho-

tochemical characteristics of these compounds and their

possible applications in energy research [7,17], in elec-troluminescent devices [18,19], as well as sensors and

0277-5387/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.poly.2004.11.011

* Corresponding author. Tel.: +91 33 2668 4561; fax: +91 33 2668

4564/2916.

E-mail address: shch20@hotmail.com (S.K. Chattopadhyay).

switches [20,21]. A number of these compounds are also

reported to exhibit an interesting non-linear optical re-

sponse [22,23]. The a–a 0 diimine ligands, most com-

monly investigated in these studies, are those where

both the donor nitrogen atoms are part of heterocyclic

rings like pyridine, pyrimidine and pyrazine. Compara-

tively less attention has been devoted [15,16,30] to sys-tems where at least one nitrogen atom is not part of a

heterocyclic ring. In this paper, we report on the synthe-

sis and spectroscopic properties of four heteroleptic

ruthenium (II) complexes (A1–A4), where Ru(II) is

coordinated by one bipyridyl and two N-(aryl) pyri-

dine-2-aldimine ligands (L). We also report here the

202 D. Mishra et al. / Polyhedron 24 (2005) 201–208

synthesis and spectroscopic properties of four 3-N(aryl)

imidazo [1,5a] pyridinium compounds (B1–B4), which

were obtained as side products during the synthesis of

the ruthenium complexes.

2. Experimental

2.1. Materials

Commercial ruthenium trichloride (RuCl3 Æ xH2O)

purchased from Arora Matthey (Kolkata, India) was

processed by repeated evaporation to dryness with conc.

HCl. �Ru(bpy)Cl3� was prepared by the published proce-dure [24]. Acetonitrile used for spectroscopic and elec-

trochemical studies was purified according to a

reported method [25]. Tetraethyl ammonium perchlo-

rate (TEAP), used for the electrochemical work, was

prepared as reported in the literature [25]. All other

chemicals were of analytical grade and used without fur-

ther purification. The ligands (L) were prepared by

reacting pyridine-2-aldehyde with the appropriate arylamine using the published procedure [8].

2.2. Physical measurements

Elemental analyses were performed on a Perkin–El-

mer 240 C, H, N analyzer. UV–Vis spectra were re-

corded using a JASCO7850 spectrophotometer.

Luminescence spectra were obtained using a Perkin–El-mer LS55 fluorescence spectrophotometer. Magnetic

susceptibilities were measured with a PAR model 155

vibrating sample magnetometer with Hg[Co(SCN)4] as

the calibrant. Cyclic voltammetry and differential pulse

voltammetry experiments were carried out using a

PAR Versastat-II instrument driven by E-chem soft-

ware. A three-electrode configuration with a Pt working

and auxiliary electrodes, Ag/AgCl reference electrodeand TEAP as the supporting electrolyte were used.

The potentials were calibrated against the ferrocene/ferr-

ocenium couple (0.44 V versus Ag/AgCl reference).

2.3. Synthesis of the complexes

CAUTION! Perchlorate salts of organic and metallo-

organic species are potentially explosive, and though we

have not encountered any problems in our work, neverthe-

less they should be handled with care.

The complexes A1–A4 and the organic products B1–

B4 reported in this work were synthesized by a general

procedure in almost similar yields; therefore, details

are given for A2 and B2 as a representative example.

To a 30 ml methanolic solution of L2 (1 mmol, 182

mg) solid �Ru(bpy)Cl3� (0.5 mmol, 181.75 mg) was addedand the reaction mixture was heated to reflux for 4 h.

The solution was allowed to come to room temperature;

a saturated methanolic solution (2 ml) of NaClO4 was

added and filtered. The filtrate was evaporated to dry-

ness and the crude product was washed with water

and dried over fused CaCl2. The crude mass was then

dissolved in chloroform and purified by column chroma-

tography using a silica gel (60–120 mesh) column. Apink band was eluted with CHCl3–CH3CN (4:1), fol-

lowed by a brown band eluted with CHCl3–CH3CN

(3:2). Evaporation of the pink fraction led to isolation

of B2, whereas A2 was obtained from the brown frac-

tion. The pink product B2 was recrystallized from etha-

nol–isopropanol (1:1) (yield 70–75 mg, 34–36%). The

brown product was recrystallized from dichloromethane

to acetonitrile (1:2) solution (yield 175–180 mg, 31%).The elemental analyses data of all the compounds are

in good agreement with the calculated values (Calc. val-

ues are in parentheses). RuC34H30N6O9Cl2 (A1): C,

48.87 (48.92); H, 3.52 (3.59); N, 9.98 (10.07)%.

RuC36H34N6O9Cl2 (A2): C, 49.67 (49.88); H, 3.77

(3.92); N, 9.69 (9.70)%; RuC34H28N6O9Cl4 (A3): C,

44.88 (44.98); H, 2.98 (3.08); N, 9.19 (9.26)%; RuC34H28-

N6O9F2Cl2 (A4): C, 46.53 (46.68); H, 3.13 (3.20); N, 9.54(9.61)%; C13H11N2O4Cl (B1): C, 52.89 (52.97); H, 3.66

(3.73); N, 9.43 (9.50)%; C14H13N2O4Cl (B2): C, 54.41

(54.43); H, 4.17 (4.21); N, 8.97 (9.07)%; C13H10N2O4Cl2(B3): C, 47.37 (47.41); H, 2.99 (3.03); N, 8.46 (8.51)%;

C13H10N2O4FCl (B4): C, 49.86 (49.92); H, 3.18 (3.20);

N, 8.89 (8.96)%.

2.4. X-ray crystallography

For both A2 and B2, data were collected on a Bruker

smart CCD area detector diffractometer at 93(2) K,

using graphite monochromated Mo Ka radiation. The

data were solved by direct methods [26] and refined by

a least square program [27] using SHELXSHELX 97. The non-

hydrogen atoms were refined with anisotropic displace-

ment parameters. All hydrogen atoms were placed atcalculated positions and refined as riding atoms using

isotropic displacement parameters. The refinement

converged with residuals summarized in Table 1. Cor-

rections for Lorentz polarization effects and semiempir-

ical absorption were applied.

2.4.1. Description of the structures

A2: The molecular structure of A2 is given in Fig. 1.In this complex, Ru(II) is in an octahedral N6 donor

environment with the average trans angles being.

�175�, where as cis angles lie between 78� and 101�.The average Ru–N distance is 2.0602 A (Table 2), which

is quite comparable to that observed [28] for [Ru-

(bpy)3]2+. The average Ru–N distance (2.0585A) ob-

served for the Ru(II)-bpy fragment is also similar to that

of the Ru(II)–(L2)2 fragment, where it has a value of2.061 A. The Ru–Nimine distances (2.057 A) within the

Ru(II)–(L2)2 fragment are shorter than the Ru–Npy

Table 1

Crystallographic data for A2 and B2

A2 B2

Empirical formula C36H34Cl2N6O9Ru C14H13N2O4Cl

Formula weight 866.66 308.71

Temperature (K) 93(2) 93(2)

Wavelength (A) 0.71073 0.71073

Crystal system triclinic orthorhombic

Space group P�1 Pbca

Unit cell dimensions

a (A) 11.5049(11) 10.0776(7)

b (A) 13.2695(13) 7.7626(5)

c (A) 13.4267(12) 34.819(3)

a (�) 98.453(2) 90

b (�) 99.652(2) 90

c (�) 113.044(2) 90

Volume (A3) 1807.7(3) 2723.8(3)

Z 2 8

Density (calc.) (g cm�3) 1.592 1.489

Absorption coefficient (mm�1) 0.646 0.256

F(0 0 0) 884 1272

Crystal size (mm) 0.05 · 0.30 · 0.86 0.45 · 0.5 · 0.99

Index ranges �14 6 h 6 14, �16 6 k 6 17, �18 6 l 6 17 �13 6 h 6 13, �10 6 k 6 9, �46 6 l 6 46

Reflections collected 13 617 19 578

Independent reflections 8631 [Rint = 0.0290] 3494 [Rint = 0.0634]

Absorption correction semi-empirical semi-empirical

Refinement method full-matrix least-squares on F2 full-matrix least-squares on F2

Data/restraints/parameters 8631/0/516 3494/0/191

Goodness-of-fit on F2 1.053 1.090

Final R indices [I > 2 r(I)] R1 = 0.0407, wR2 = 0.0857 R1 = 0.0586, wR2 = 0.1460

R indices (all data) R1 = 0.0542, wR2 = 0.0907 R1 = 0.0696, wR2 = 0.1526

Largest difference peak and hole (e A�3) 0.728 and �0.719 0.794 and �0.433

Fig. 1. ORTEP diagram and atom numbering scheme for the cation of

A2, showing 50% probability ellipsoids.

D. Mishra et al. / Polyhedron 24 (2005) 201–208 203

distances (the average is 2.065 A). The fact that the Ru–

Nimine distances are shorter than the Ru–Npy distances

of Ru-bpy as well as the Ru(II)–(L2)2 fragment is prob-

ably due the greater back bonding acceptor capability of

the imine moiety compare to the pyridine moiety. The

bite angle of the imine ligand (78.02(9)� (Table 2)) is alsovery similar to that of bpy (78.61(9)�) and agrees well

with the values reported earlier [29,30]. One of the per-chlorate ions is disordered in the lattice.

B2: The ORTEP diagram along with the atom num-

bering scheme for the compound B2 is shown in Fig. 2.

The bond distances within the imidazo pyridine frag-

ment clearly indicate that there is extensive delocaliza-

tion within the imidazole ring and the C5–C4 and N1–

C1 part of the pyridine ring. However the C4–C3 and

C1–C2 bonds are more like double bonds. The aryl ringattached to the N2 atom is twisted by about 46� from

the plane of the imidazopyridine fragment (Table 3).

Fig. 2. ORTEP diagram and atom numbering scheme for the

compound B2, showing 50% probability ellipsoids.

Table 2

Selected bond distances (A) and bond angles (�) for A2

Bond distances

Ru–N(2) 2.053(2) Ru–N(2A) 2.059(2)

Ru–N(2B) 2.057(2) Ru–N(1) 2.064(2)

Ru–N(1B) 2.057(2) Ru–N(1A) 2.071(2)

Bond angles

N(2)–Ru–N(2B) 96.38(9) N(1B)–Ru–N(1) 98.46(9)

N(2)–Ru–N(1B) 174.03(9) N(2A)–Ru–N(1) 97.99(8)

N(2B)–Ru–N(1B) 78.02(9) N(2)–Ru–N(1A) 94.23(9)

N(2)–Ru–N(2A) 85.82(9) N(2B)–Ru–N(1A) 101.60(8)

N(2B)–Ru–N(2A) 177.80(9) N(1B)–Ru–N(1A) 88.97(9)

N(1B)–Ru–N(2A) 99.79(9) N(2A)–Ru–N(1A) 78.02(9)

N(2)–Ru–N(1) 78.61(9) N(1)–Ru–N(1A) 172.11(9)

N(2B)–Ru–N(1) 82.64(8)

Table 3

Selected bond distances (A) and bond angles (�) for B2

C(1)–C(2) 1.339(3) C(2)–C(1)–N(1) 118.4(2)

C(1)–N(1) 1.390(3) C(1)–C(2)–C(3) 121.3(2)

C(2)–C(3) 1.439(3) C(4)–C(3)–C(2) 120.7(2)

C(3)–C(4) 1.349(3) C(3)–C(4)–C(5) 119.1(2)

C(4)–C(5) 1.419(3) C(6)–C(5)–N(1) 106.09(18)

C(5)–C(6) 1.367(3) C(6)–C(5)–C(4) 135.5(2)

C(5)–N(1) 1.409(3) N(1)–C(5)–C(4) 118.4(2)

C(6)–N(2) 1.379(3) C(5)–C(6)–N(2) 106.97(18)

C(7)–N(1) 1.337(3) N(1)–C(7)–N(2) 107.50(18)

C(7)–N(2) 1.341(3) C(9)–C(8)–N(2) 119.03(19)

C(8)–C(9) 1.386(3) C(14)–C(8)–N(2) 119.67(19)

C(8)–C(14) 1.392(3) C(7)–N(2)–C(6) 110.14(18)

C(8)–N(2) 1.429(3) C(7)–N(2)–C(8) 124.94(18)

C(9)–C(10) 1.391(3) C(6)–N(2)–C(8) 124.92(18)

C(10)–C(11) 1.397(3) C(7)–N(1)–C(1) 128.70(19)

C(11)–C(13) 1.392(3) C(7)–N(1)–C(5) 109.30(18)

C(1)–N(1)–C(5) 121.98(19)

Ru

NN

N

CH

R

N

CH

N

R

N

H

N

NN

R ClO4

(ClO4)2N

C

reflux in MeOH,

+

NaClO4 soln.

( L )

(A)

(B)

L A B

R = H L1 1 1

R = Me L2 2 2

R = Cl L3 3 3

R = F L4 4 4

Ru(bpy)Cl3 + R

Scheme 1.

204 D. Mishra et al. / Polyhedron 24 (2005) 201–208

3. Results and discussion

Reaction of �Ru(bpy)Cl3� with L in refluxing metha-

nol produces a mixture of the Ru(II) complex A and

the imidazopyridine derivative B, both of which can be

separated by column chromatography and on recrystal-

lization, crystalline compounds can be isolated for both

species. Crystal structure determinations were made for

A2 and B2 as representative members of A and B,

respectively.The imidazopyridinium derivatives B1 to B4: The for-

mation of the imidazopyridinium derivatives in the reac-

tion described in Scheme 1 is very interesting. Though

imidazopyridine compounds and some of their simple

salts are well known [31,32], the 3-N(aryl) imidazopyrid-

inium derivatives are rare. Thus a simple single step syn-

thesis of these compounds starting from readily

available laboratory chemicals, as described above, isattractive. It has been reported [31] that imidazopyridine

is formed in the reaction of 2-picolylamine with formic

acid in presence of POCl3. Probably, a similar mecha-

nism is operative here. The �Ru(bpy)Cl3� (where the oxi-dation state of ruthenium is closer to +4 than +3) first

oxidizes the solvent methanol to formaldehyde, which

then reacts with the schiff base in the presence of �Ru(b-

py)Cl3� to produce B, as described in Scheme 2.

The 3-N(aryl) imidazo [1,5a] pyridinium compounds

(B1–B4) are deep pink in colour due to a strong absorp-

tion at 520–535 nm. The solvatochromism (Fig. 3)exhibited by this band clearly suggests it is a charge

transfer transition. It was also observed that for a given

solvent the energy of the longest wavelength band fol-

lowed the order B3 < B1 = B2 < B4. The same trend

was followed in the E0 value of the first oxidation peak

for the compounds (Table 7). In fact, a plot of energy of

the transition (cm�1) versus E0 value yields a good linear

relationship (R = 0.99; supplementary Figure. 1). How-ever, this trend is different from that observed for the

HOMO–LUMO gap for these compounds (in the gas-

eous phase) as calculated by the PM3 method [33a,b],

which follows the order B3 < B4 � B1 < B2. This indi-

cates that solvation plays a very important role in deter-

mining the energy of the frontier orbitals of these

compounds, which is quite expected because of their

charged structures and appreciable ground state dipolemoment (Table 5). Interestingly, the transition energy

follows a roughly linear relationship (Table 6, supple-

mentary Figure. 2) with the change in dipole moment

(Dlge = le � lg) between the ground state (lg) and the

first excited singlet state (le), assuming the excited state

has the same geometry as the ground state i.e., a

Franck–Condon excited state. It was also observed that

the intensity of the first charge transfer transition fol-lows a linear correlation (Table 6, supplementary

Figure 3) with Dl2ge. All the four compounds B1–B4 ex-

hibit luminescence, with a strong emission at �380 nm

(excited at 280 nm or 240 nm) at room temperature

(Table 4). These compounds with a strong charge trans-

fer transition near the visible region, appreciable dipole

moment in the ground state, moderate value of Dl, as

R NC

N

CH H

O

H

RC

NC

OH

H2

H

N

R

C

NC

H

N

H

RN

C

H

N

H H

C

:+

..

+

+

+

H+

-H+

-OH-

Scheme 2.

5 10 15 20 25 30 35 40

18360

18540

18720

18900

19080

19260

5 10 15 20 25 30 35 40

18360

18540

18720

18900

19080

19260--- for B1 (R=0.94)--- for B2 (R=0.96)--- for B3 (R=0.96)--- for B4 (R=0.80)

Ene

rgy(

cm-1)

D

Fig. 3. Plot of energy of CT transition vs. Dielectric constant (D) for

compounds B.

Table 5

HOMO and LUMO energies and calculated dipole moments of the

imidazopyridinium derivatives

Compounds B1 B2 B3 B4

EHOMO (eV) �13.09 �13.23 �12.77 �13.17

ELUMO (eV) �4.99 �5.05 �5.16 �5.11

ELUMO (eV) � EHOMO (eV) 8.10 8.18 7.61 8.06

lg (debye) 2.70 3.70 7.15 6.53

le (debye) 4.29 1.42 2.06 8.36

Dlge (i.e., le � lg) (debye) 1.59 �2.28 �5.09 1.83

(Dlge)2 2.53 5.20 25.91 3.349

D. Mishra et al. / Polyhedron 24 (2005) 201–208 205

well as stable excited state, are good candidates for

screening of their possible non linear optical properties

[23]. Besides, the imidazopyridinium cephalosporinsare known to exhibit good Gram positive activity, as

Table 4

UV–Vis spectra and luminescence spectral data

Compounds kmax/nm (e/lit mol�1 cm�1)a

A1 631 (641), 481 (5938), 440 (5792), 388c

A2 631 (525), 482 (6472), 438 (6220), 315c

A3 631 (924), 485 (8375), 440 (7883), 390c

A4 631 (957), 481 (10 209), 437 (9444), 38

B1 525 (886), 285 (6203), 270 (5907), 232

B2 525 (1824), 310c (6566), 282 (12 787),

B3 535 (3499), 288 (7607), 265c (7150), 23

B4 520 (759), 285 (7290), 270 (6683), 232

a Acetonitrile solution.b Qualitative spectra recorded in acetonitrile solutions at room temp., for A

and B4 it was 285, 280, 265 and 285 nm, respectively.c Shoulder.

well as excellent activity against K1 b-lactamase produc-

ing Klebsiella aerogenes 1082 E strain [34].The redox behaviors of the pink compounds (B) have

been investigated using cyclic voltammetry (CV), as well

as differential pulse voltammetry (DPV) (Table 7). Both

B1 and B2 exhibit one quasi reversible oxidative re-

sponse at around 1.02 V; in the DPV one sharp peak

is obtained at the same potential demonstrating that

only one redox couple is involved. However for B3

and B4 no resolvable peak was observed in the CV,

kem/nmb

(1833), 283 (21 994), 235 (13 196) 733

(13 448), 282 (23 534), 231c (19 331) 725

(3941), 282 (39 413), 240c (24 633) 710

7c (3190), 283 (36 499), 237 (21 695) 722

(12 582) 384

245 (23 345) 382

7c (10 345) 378

(13 001) 383

1, A2, A3, A4 the excitation wavelength was 480nm and for B1, B2, B3

Table 6

Correlation between the lowest energy absorption band and Dlge forthe imidazopyridinium derivatives

Solvent Energy (cm�1) vs.Dlge Molar extinction

coefficient vs. (Dlge)2

Slope Intercept Ra Slope Intercept Ra

Dichloromethane 64.19 18 819 0.79 231.52 394.27 0.99

Acetone 46.21 18 879 0.71 59.65 824.40 0.99

Ethanol 74.68 18 999 0.79 94.45 521.39 0.98

Acetonitrile 59.89 19 063 0.87 108.27 740.82 0.95

a R = correlation coefficient.

Table 7

Cyclic voltammetry and differential pulse voltammetry (DPV) data

Compounds E0/V (DEp/mV) in MeCN solution

A1 1.45 (71), �0.86 (208), �1.61 (CV)/�1.46 (DPV),

�1.74 (DPV)

A2 1.45 (90), �0.86 (217), �1.54 (CV)/�1.41 (DPV),

�1.69 (DPV)

A3 1.49 (92), �0.83 (145), �1.50 (CV)/�1.41 (DPV),

�1.57 (DPV)

A4 1.48 (105), �0.82 (132), �1.56 (CV)/�1.42 (DPV),

�1.70 (DPV)

B1 1.02 (145)

B2 1.02 (286)

B3 1.07, 0.91 (DPV)

B4 1.26, 1.06 (DPV)

206 D. Mishra et al. / Polyhedron 24 (2005) 201–208

but in the DPV two well-defined oxidation peaks are ob-

served at 0.91 and 1.07 V for B3 and at 1.06 and 1.26 Vfor B4.

The ruthenium complexes A1 to A4: The heteroleptic

tris chelate complexes are brown in colour with a strong

absorption around 480nm. For tris-chelate complexes

such as these three isomers are possible [16]. However

determination of the X-ray crystal structure of one rep-

resentative complex A2, shows that in the isolated com-

plex only one isomer is present in the solid state wherethe two Npy of the two ligands (L) are cis to each other,

whereas the two Nimine are trans to each other, which

may be called a cis–trans isomer. The similarity of spec-

troscopic, electrochemical and chromatographic behav-

iors of A1, A3 and A4 with that of A2 led us to

believe that in these complexes the same isomer also

predominates.

The redox behavior of the complexes was studied bycyclic voltammetry (CV) and differential pulse voltam-

metry (DPV). All the complexes undergo a quasi-revers-

ible oxidation at �1.3 to 1.4 V which may be assigned to

Ru(II) to Ru(III) oxidation. In the reductive side all the

complexes show a two-electron quasi-reversible reduc-

tion at �0.8 V and two one electron quasi-reversible

reductions near �1.4 to �1.7 V. The last two peaks

are not resolved in the CV, but they can be well identi-fied in the DPV (supplementary Figure 4). As our PM3

calculations show that the LUMO of the ligand L is

about 0.2 eV lower in energy than that of bpy, the first

reduction is assigned to electron addition to the p* orbi-

tal of coordinated L. In fact the EHMO calculation

shows that LUMO and LUMO + 1 have large contribu-

tions from Lp* orbitals and are separated from each

other by only 0.03 eV, so that the first two electronreduction is probably due to simultaneous electron addi-

tion to LUMO and LUMO + 1. The second and third

reductions are probably due to addition of electrons to

LUMO + 2 and LUMO + 3, where the former is mainly

based on bpy, and the later have contributions from all

the three ligands (see supplementary Figure 5 for pic-

tures of the HOMOs and LUMOs).

The visible region of the electronic spectra of thecomplexes consists of a weak transition at �630 nm

and a broad envelope from 600 to 375 nm consisting

of two peaks at �480 and �440 nm and a poorly re-

solved shoulder around 380 nm. The weak intensity of

the first transition suggests that it is probably a charge

transfer transition to a 3MLCT state [4,35,36]. The

bands near 480nm and 440nm are charge transfer tran-

sitions to 1MLCT states. Under the C2 symmetry of thecomplex, the metal dp orbitals will transform as a + 2b.

The lowest energy p* orbital of bpy will transform as b

while the lowest energy p* orbitals of the two imine li-

gands will transform as a + b. Thus nine MLCT transi-

tions are possible in principle, six of them are charge

transfer from metal to imine ligand Lp* orbitals, while

three of them are charge transfer from metal to bpy p*orbital. Overlapping of these bands is expected to giverise to a broad envelope. As mentioned earlier, the li-

gand Lp* orbitals are of lower energy than the bpy p*orbital, so the MLCT transition to the later orbital is ex-

pected to be of higher energy than those involving the

Lp* orbital. Earlier workers [15] have assigned the peak

around 480 nm to that involving the Lp* orbital, while

the peak near 440 nm to that involving the bpy p* orbi-

tal. However the comparable intensity of both the bandsprecludes such an assignment, as only one bpy ligand is

present, compared to two L ligands. Moreover it was

pointed out by Lever [37,38] that for a MLCT transition

involving the bpy p* orbital, a molar extinction coeffi-

cient of �4000 M�1 cm�1 per bpy ligand is expected.

As the molar extinction coefficients of the transition

near 440nm for the present complexes are much larger

than that predicted by Lever, so the band �440 nm aswell as that �480 nm are assigned to MLCT transitions

involving the Lp* orbital, while the shoulder around

380nm is probably due to that involving the bpy p* orbi-tal. This is also supported by the large difference of the

observed reduction potential in the complexes between

the L based reduction and bpy based reduction. The

other higher energy transitions are probably intraligand

in nature. An EHMO calculation on the compound A2supports this conclusion. Since the actual symmetry of

the molecule in this case is less than C2 (it is C1), all

D. Mishra et al. / Polyhedron 24 (2005) 201–208 207

the orbitals are of �a� symmetry. The three d orbitals are

106A (�12.25 eV), 105A (�12.31 eV) and 104A (�12.38

eV), where 106A and 105A have appreciable contribu-

tion from schiff base ligands and 104A has similarly

appreciable contribution from bpy. The LUMO,

LUMO + 1 and LUMO + 2 are 107A (�10.01 eV),108A (�9.97 eV) and 109A (�9.68 eV), respectively;

again the first two are schiff base ligand based where

as the last one is bpy based. Thus the first two bands

are probably due to the transitions between

106A ! 107A, 108A and 105A ! 107A, 108A, where

as the charge transfer to bpy 104A ! 109A is expected

to lie at higher energy. A preliminary luminescence

study shows that all the complexes are luminescent atroom temperature. Excitation at 480 nm leads to a

broad emission at 700–730 nm.

4. Computations

The PM3 and EHMO calculations were carried out

with the HYPERCHEMHYPERCHEM 7.01 program [39] using the de-fault parameters of the program. Single point PM3 cal-

culations (which includes configuration interaction

involving single, excited states arising from the 10 high-

est occupied and 10 lowest unoccupied orbitals) on the

ligands were made on the geometries obtained directly

from the crystal structure or when the structure was

not available, the geometry was optimized by the

MM+ method. An extended Huckel calculation wasdone on A2 using the geometry taken directly from the

crystal structure after removing the perchlorate counter

ions.

Acknowledgments

D.M. and S.N. acknowledge CSIR, UGC respec-tively, for their fellowships. S.K.C. acknowledges

AICTE and UGC, New Delhi, for financial support.

We thank Dr. A. Barbieri for her comments and

suggestions.

Appendix A. Supplementary data

The structural data for A2 and B2 have been depos-

ited at the Cambridge Crystallographic Data Centre

with the deposition numbers 240405 and 240406 respec-

tively. Copies of this information may be obtained free

of charge from The Director, CCDC, 12 Union Road,

Cambridge CB2 IEZ, UK (Fax: +44-1223-336033;

e-mail: deposit@ccdc.cam.ac.uk or www: http://

www.ccdc.cam.ac.uk). Supplementary data associatedwith this article can be found, in the online version, at

doi:10.1016/j.poly.2004.11.011.

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