Chemistry of ruthenium with some phenolic ligands: Synthesis, structure and redox properties

Chemistry of ruthenium with some phenolic ligands: synthesis,

structure and redox properties

Falguni Basuli

_{, Anjan Kumar Das}

_{, Golam Mostafa}

_{, Shie-Ming Peng}

_,

Samaresh Bhattacharya

_*

a_{Department of Chemistry, Inorganic Chemistry Section, Jada6pur Uni6ersity, Calcutta700032, India} b_{Department of Physics, Krishnath College, Berhampur, West Bengal, India}

c_{Department of Chemistry, National Taiwan Uni6ersity, Taipei, Taiwan, ROC}

Received 11 January 2000; accepted 11 April 2000

Abstract

Reaction of three phenolate ligands, viz. salicylaldehyde (HL1_{), 2-hydroxyacetophenone (HL}2_{) and 2-hydroxynaphthylaldehyde} (HL3_{), (abbreviated in general as HL, where H stands for the phenolic proton) with [Ru(PPh}

3)3Cl2] in 1:1 mole ratio gives complexes of the type [Ru(PPh3)2(L)Cl2]. The structure of the [Ru(PPh3)2(L2)Cl2] complex has been solved by X-ray crystallogra-phy. The coordination sphere around ruthenium is O2P2Cl2 with a cis – trans – cis geometry, respectively. The [Ru(PPh3)2(L)Cl2] complexes are one-electron paramagnetic (low-spin d5_{, S = 1/2) and show rhombic ESR spectra in 1:1 dichloromethane – toluene} solution at 77 K. In dichloromethane solution the [Ru(PPh3)2(L)Cl2] complexes show several intense LMCT transitions in the visible region. Reaction between the phenolic ligands and [Ru(PPh3)3Cl2] in 2:1 mole ratio in the presence of a base affords the [Ru(PPh3)2(L)2] complexes in two isomeric forms.1H NMR spectra of one isomer shows that it does not have any C2symmetry and has the cis – cis – cis disposition of the three sets of donor atoms.1_{H NMR spectra of the other isomer shows that it has C}

2 symmetry. The structure of the isomer of the [Ru(PPh3)2(L1)2] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium is O4P2with a cis – trans – cis disposition of the carbonylic oxygens, phenolate oxygens and phosphorus atoms, respectively. The [Ru(PPh3)2(L)2] complexes are diamagnetic (low-spin d6, S = O) and show intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru(PPh3)2(L)Cl2] complexes shows a ruthenium(III)ruthenium(II) reduction near − 0.3 V versus SCE and a ruthenium(III)ruthenium(IV) oxidation in the range 1.08–1.24 V versus SCE. Cyclic voltammetry on both isomers of the [Ru(PPh3)2(L)2] complexes shows a ruthenium(II)ruthenium(III) oxidation within 0.09–0.41 V versus SCE, followed by a ruthenium(III)-ruthenium(IV) oxidation within 1.31 – 1.52 V versus SCE. © 2000 Elsevier Science Ltd. All rights reserved.

Keywords:Ruthenium; Phenolic ligands; Synthesis; Structure; Redox properties

1. Introduction

The chemistry of ruthenium has currently been re-ceiving a lot of attention [1 – 10] primarily because of the fascinating electron-transfer and energy-transfer properties displayed by the complexes of this metal. Ruthenium offers a wide range of oxidation states and the reactivities of the ruthenium complexes depend on the stability and interconvertibility of these oxidation

states, which in turn depend on the nature of ligands bound to the metal. Complexation of ruthenium by ligands of different types has thus been of particular interest. In the present study, which has originated from our interest in the chemistry of ruthenium in different coordination environments [11 – 18], we have chosen phenolic ligands of type 1 as the principal ligand, which are abbreviated in general as HL where H stands for the dissociable phenolic proton. The depro-tonated ligands are known to coordinate metal ions as bidentate O,O-donor forming six-membered chelate rings (2) [19,20]. Three different phenolic ligands have been used in this study, which are shown in 1 along with their specific abbreviations.

* Corresponding author. Fax: + 91-33-473-4266.

E-mail address:_{samaresh –[email protected] (S. Bhattacharya).}

0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 0 ) 0 0 4 0 4 - 6

While phenolate oxygen is a recognized hard donor, and coordination by it is known to stabilize the higher oxidation states of ruthenium [21 – 23], coordination of metals by carbonylic oxygen has also recently been of considerable interest in bioinorganic chemistry [24 – 28]. It may be mentioned here that ruthenium chemistry of these ligands has not been explored much [19]. Stability of different oxidation states of ruthenium is expected to depend on the number of phenolate and carbonylic oxygen in the coordination sphere and in the case of mixed-ligand complexes, on the nature of coligands also. Herein we have restricted our studies to some mono- and bis-phenolate complexes of ruthenium, which have been synthesized by reacting the phenolic ligands with [Ru(PPh3)3Cl2] under different experimen-tal conditions. The chemistry of complexes of the type [Ru(PPh3)2(L)Cl2] and [Ru(PPh3)2(L)2] has been de-scribed in this paper with special reference to synthesis, stereoisomerism and electron-transfer properties.

2. Experimental

2.1. Materials

Commercial ruthenium trichloride was purchased from Arora Matthey, Calcutta, India, and was con-verted into RuCl3·3H2O by repeated evaporation with concentrated hydrochloric acid. Triphenylphosphine (PPh3), triethylamine (NEt3) and salicylaldehyde (HL1) were obtained from SD, India. 2-Hydroxyacetophenone (HL2_{) and 2-hydroxynaphthaldehyde (HL}3_{) were} pur-chased, respectively, from Spectrochem, India and Aldrich. All other chemicals and solvents were reagent grade commercial materials and were used as received. [Ru(PPh3)3Cl2] was prepared following a reported pro-cedure [29]. Purification of acetonitrile and preparation of tetraethylammonium perchlorate (TEAP) for electro-chemical work were performed as before [30,31].

2.2. Preparation of complexes

2.2.1. [Ru(PPh3)2(L1)Cl2]

HL1 _{(15 mg, 0.12 mmol) was added to a suspension} of [Ru(PPh3)3Cl2] (100 mg, 0.10 mmol) in ethanol (40 cm3_{) and the mixture was stirred for 5 h to produce a} green solution. On partial evaporation of the solvent, [Ru(PPh3)2(L1)Cl2] separated out as a green crystalline

solid, which was collected by filtration, washed with ethanol and dried in air. The yield was 55 mg (64%).

2.2.2. [Ru(PPh3)2(L2)Cl2]

This was synthesized by following the same procedure above using HL2 _{instead of HL}1_{. The yield was 60 mg} (69%).

2.2.3. [Ru(PPh3)2(L3)Cl2]

Dichloromethane (40 cm3_{) was added to a solid} mixture of [Ru(PPh3)3Cl2] (100 mg, 0.10 mmol) and HL3 (20 mg, 0.12 mmol) and the solution was stirred for 3 h to afford a green solution. On evaporation of the solvent, a green solid was obtained, which was washed with ethanol and dried in air. Purification of this product was achieved by chromatography through a silica gel column. Using toluene as the eluent a green band resulted, which was collected. Evaporation of the eluate gave [Ru(PPh3)2(L3)Cl2] as a green microcrys-talline solid. The yield was 55 mg (61%).

2.2.4. ctc and ccc isomers of[Ru(PPh3)2(L1)2]

2.2.4.1. Method A. [Ru(PPh3)3Cl2] (100 mg, 0.10 mmol) was added to a hot solution of HL1_{(30 mg, 0.24 mmol)} in ethanol (40 cm3_{), followed by NEt}

3 (30 mg, 0.30 mmol). The mixture was refluxed for 2 h to produce a red solution. On partial evaporation of solvent, a micro-crystalline red solid precipitated, which was collected by filtration, washed thoroughly with water and dried in vacuo over P4O10. Purification was achieved by chro-matography through silica gel column. Using toluene and 1:4 acetonitrile – toluene as the eluents, two different red bands resulted, which were collected separately and evaporation of the eluates, respectively, gave the ctc and ccc isomers of [Ru(PPh3)2(L1)2]. The yield was 30 mg (33%) for the ctc isomer and 32 mg (35%) for the ccc isomer.

2.2.4.2. Method B. [Ru(PPh₃)₂(L1_)Cl

2] (50 mg, 0.06 mmol) was dissolved in a minimum volume of dichloro-methane and to it a solution of HL1_{(10 mg, 0.08 mmol)} in ethanol (30 cm3_{) was added, followed by NEt}

3 (10 mg, 0.10 mmol). The solution was refluxed for 4 h. Upon evaporation of the solution, a red crystalline solid was obtained, which was washed with water and dried in vacuo over P4O10. The solid was then purified as in Method A. The ctc and ccc isomers of [Ru(PPh3)2(L1)2] were obtained in 36 and 32% yields, respectively.

[Ru(PPh₃)₂(L2_)Cl

2] and HL2 were used instead of [Ru(PPh3)2(L1)Cl2] and HL1, respectively. Yields of the ctc and ccc isomers were 35 and 33%, respectively.

2.2.6. ctc and ccc isomers of [Ru(PPh3)2(L3)2]

These isomers have been prepared by following the above procedures (Section 2.2.4). In method A, HL3 was used instead of HL1_{. The ctc and ccc isomers of} [Ru(PPh3)2(L3)2] were obtained in 32 and 36% yields. In method B, [Ru(PPh3)2(L3)Cl2] and HL1 were used in-stead of [Ru(PPh3)2(L1)Cl2] and HL1, respectively. Yields of the ctc and ccc isomers were 32 and 34%, respectively.

2.3. Physical measurements

Microanalyses (C, H, N) were performed using a Perkin – Elmer 240C elemental analyzer. IR spectra were obtained on a Perkin – Elmer 783 spectrometer with samples prepared as KBr pellets. Electronic spec-tra were recorded on Shimadzu UV 240 spectrophoto-meter. Magnetic susceptibilities were measured using a PAR 155 Vibrating sample magnetometer fitted with a Walker scientific L75FBAL magnet. 1_{H NMR spectra}

with the help of DPPH (g = 2.0037). Electrochemical measurements were made using a PAR model 273 potentiostat. A platinum disc working electrode, a plat-inum wire auxiliary electrode and an aqueous saturated calomel reference electrode (SCE) were used in a three electrode configuration. Dinitrogen gas was purified by successively bubbling it through alkaline dithionite and concentrated sulfuric acid. All electrochemical experi-ments were performed under a dinitrogen atmosphere. All electrochemical data were collected at 298 K and are uncorrected for junction potentials. An RE 0089 X-Y recorder was used to trace the voltammograms.

2.4. Crystallography

Single crystals of [Ru(PPh3)2(L2)Cl2] were grown by slow diffusion of hexane into a dichloromethane solu-tion of the complex. Selected crystal data and data collection parameters are given in Table 1. Data were collected on a Siemens Smart CCD diffractometer using graphite monochromated Mo Ka radiation (l= 0.71073 A, ) by v scans within the angular range 1.57B uB25.00°. X-ray data reduction, structure solution and refinement were done using SHELXTL-PLUS package. The structure was solved by direct methods.

Single crystals of ctc-[Ru(PPh₃)₂(L1₎

2] were grown by slow diffusion of benzene into an acetonitrile solution of the complex. Selected crystal data and data collec-tion parameters are given in Table 1. The unit cell dimensions were determined by a least-squares fit of 25 centered reflections (10.805u520.94°). Data were col-lected on an Enraf – Nonius CAD-4 diffractometer us-ing graphite monochromated Mo Ka radiation (l=0.71073 A,) by u−2u scans within the angular range 3.0B2uB45.0°. Three standard reflections, used to check the crystal stability towards X-ray exposure, showed no significant intensity variation over the course of data collection. X-ray data reduction, and structure solution and refinement were carried out using the SHELXS-97 package. The structure was solved by the direct methods.

3. Results and discussion

3.1. Preparation and characterization

3.1.1. [Ru(PPh3)2(L)Cl2] complexes

Reaction of [Ru(PPh3)3Cl2] with an equimolar quan-tity of each phenolic ligand (HL) proceeds smoothly in dichloromethane solution at ambient temperature to Table 1 Crystallographic data [Ru(PPh3)2(L2)Cl2] ctc-[Ru(PPh3)2(L1)2] Formula C44H37Cl2O2P2Ru C50H40O4P2Ru Formula weight 831.65 867.87 orthorhombic, Pnma

Space group triclinic, P1(

a (A, ) 10.5844(10) 10.7368(19) b (A, ) 23.817(2) 11.637(4) 15.407(2) c (A, ) 16.732(7) 96.18(3) 90 a (°) 90 b (°) 91.78(3) g (°) 90 99.44(3) V (A,3_{) 3884.0(8) 2047.7(12)} 4 Z 2 Crystal size (mm) 0.50×0.30×0.15 0.40×0.30×0.20 293(2) T (K) 298 6.60 m (cm−1_{) 4.951} R1= 0.0522a R Rf= 0.046d wR2= 0.1390b Rw= 0.047c 1.082e GOF 2.01f a_R 1= Fo−Fc /Fo. b_wR 2= [[w(Fo2−Fc2)2]/[w(Fo2)2]]1/2. c_{GOF = [[w(F}

o2−Fc2)2]/(M−N)1/2, where M is the number of

reflections and N is the number of parameters refined.

d_R

f= Fo−Fc /Fo. e_R

w= [w(Fo−Fc)2/w(Fo)2]1/2. f_{GOF = [w(F}

o−Fc)2/(M−N)]1/2, where M is the number of

Table 2

Microanalytical, electronic spectral and cyclic voltammetric data

Compound Microanalytical dataa _{Electronic spectral data}b _{Cyclic voltammetric date}c

lmax(nm) (o, M−1cm−1) E1/2(V) (DEp, mV) %H %C 4.4 [Ru(PPh3)2(L1)Cl2] 63.5 656 (423), 492d(966), −0.28 (120), (63.2) (4.3) 396 (4700), 304 (19300), 1.16 (120) 272d_{(22 000) 240 (25 600)} 4.9 624 (907), 388 (5200), 62.6 −0.37 (120), Ru(PPh3)2(L2)Cl2] (63.5) (4.5) 300d_{(15 100), 268}d_{(23 100), 1.08 (120)} 236 (38800) 65.3 [Ru(PPh3)2(L3)Cl2] 4.5 588 (855), 400d(2000), −0.30 (120), (65.1) (4.3) 312d_{(3300), 260}d_{(10700), 1.24 (120)} 232 (19300) 69.7 ctc-[Ru(PPh3)2(L1)2] 4.7 504 (2300), 404d(7000), 0.23 (80), (4.6) 364 (10 400), 232 (78 300) (69.2) 1.35 (100) 4.5 500 (1300), 408d_(3700), ccc-[Ru(PPh3)2(L 1₎ 2] 69.5 0.28 (80), (4.6) 356 (5000), 232 (38600) (69.2) 1.41 (100) ctc-[Ru(PPh3)2(L2)2] 70.1 5.0 492 (1800), 388d(5000), 0.09 (80), (4.9) 340 (7800), 260 (22 900), (69.7) 1.31 (110) 236 (23600) 69.9 ccc-[Ru(PPh3)2(L2)2] 4.9 500d(2300), 400 (6300), 0.21 (80), (4.9) 336 (8200), 232 (64 600) (69.7) 1.52 (120) 72.1 ctc-[Ru(PPh3)2(L3)2] 4.7 490 (3000), 390 (9600), 0.35 (80), (4.6) 340d_{(12 200), 320}d_{(17 700),} (72.0) 1.46 (100) 280 (32 700), 232 (77 600) 4.6 490 (3000), 398 (7400), 72.2 0.41 (80), ccc-[Ru(PPh3)2(L3)2] (4.6) 340 (11 200), 320d_{(17 200), 1.52 (110)} (72.0) 276 (30 000), 232 (61 300)

a_{Calculated values are in parenthesis.} b_{Dichloromethane solution.}

c_{Solvent, acetonitrile; supporting electrolyte, TEAP; reference electrode, SCE; E}

1/2= 0.5(Epa+Epc), where Epaand Epcare anodic and cathodic

peak potentials, respectively;DEp= Epa−Epc; scan rate, 50 mV s−1. d_Shoulder.

afford complexes of the type [Ru(PPh₃)₂(L)Cl₂]. It is interesting to note here that ruthenium has undergone a one-electron oxidation during the course of this syn-thetic reaction. In view of the ruthenium(III) – rutheniu-m(II) reduction potentials in these complexes (vide infra), oxygen in air appears to have served as the oxidant. Compositions of the complexes have been confirmed by their microanalytical data (Table 2). As all the three phenolate ligands used in the present study are asymmetric bidentate in nature, the [Ru(PPh3)2(L)Cl2] complexes may exist in three geomet-ric isomegeomet-ric forms (3 – 5).

To distinguish between the possible three isomers, molecular structure of a representative complex, viz. [Ru(PPh3)2(L2)Cl2], has been determined by X-ray crys-tallography. The structure is shown in Fig. 1 and

selected bond distances and angles are presented in Table 3. The O₂P₂Cl₂ coordination sphere around ruthenium is distorted octahedral in nature, which is

Cl(2) C(1) C(1)_C(2) 2.415(2) 1.405(14) Ru_P(1) C(2)_C(3) Ru_O(1) 1.995(5) 1.45(3) C(3)C(4) 2.025(6) 1.39(5) RuO(2) C(4)C(5) 1.22(4) C(5)C(6) 1.44(2) C(6)C(7) 1.42(2) C(1)C(6) 1.38(2) C(7)C(8) 1.494(13) O(2)_RuCl(2) 176.9(2) 179.41(8) P(1)_RuP(1A) O(1)RuCl(1) 173.2(2) O(1)RuCl(2) 86.4(2) 90.5(2) O(1)RuO(2) C(2)RuCl(1) O(2)RuCl(1) 86.9(2) 96.27(11) O(2)RuP(1) 89.73(4) 90.10(5) O(1)RuP(1) Cl(1)RuP(1) Cl(2)RuP(1) 89.88(5) 90.28(4) 3 2 (L2_)Cl 2].

Infrared spectra of the [Ru(PPh3)2(L)Cl2] complexes show many sharp and strong vibrations in the 1600 – 300 cm− 1_{region, of which the vibrations near 520, 700 and} 745 cm− 1_{are also observed in [Ru(PPh}

3)3Cl2] and hence these are attributable to the Ru(PPh3)2 fragment of the [Ru(PPh3)2(L)Cl2] complexes. One new band, observed near 1580 cm− 1_{in all these complexes, is assigned to the} n(CO) vibration of the coordinated phenolate ligand. The n(RuCl) stretching vibration appears as a strong band in all the complexes near 330 cm− 1_{. The infrared} spectral data thus correspond to the composition of the complexes. The [Ru(PPh₃)₂(L)Cl₂] complexes are soluble in common polar organic solvents, like dichloro-methane, chloroform, acetonitrile, etc., producing green solutions. Electronic spectra of these complexes have been recorded in dichloromethane solution. A selected spectrum is shown in Fig. 2 and spectral data are listed in Table 2. Each complex shows few intense absorptions in the visible region together with some very intense absorptions in the ultraviolet region. The absorptions in the ultraviolet region may be assigned to transitions occurring within the ligand orbitals. To have an insight into the nature of transitions appearing in the visible region, qualitative EHMO calculations have been per-formed [35,36] on a model of the [Ru(PPh₃)₂(L)Cl₂] complexes, which was computer generated from [Ru(PPh₃)₂(L1_)Cl

2] by replacing the phenyl groups of the PPh₃ligands with hydrogen. A partial MO diagram is shown in Fig. 3. The highest occupied (singly occu-pied) molecular orbital (MO-1) and the next two filled orbitals (MO-2 and MO-3) of this model are predomi-nantly (]80%) ruthenium t2 in character. There are two filled molecular orbitals (MO-4 and MO-5) below these metal t2 orbitals, which are localized almost en-tirely (]96%) on the phenolate ligand. The intense absorptions observed in the visible region may therefore be assigned to the allowed ligand-to-metal charge-trans-fer transitions occurring from the filled orbitals of the phenolate ligand (MO-4 and MO-5) to the half-filled ruthenium t₂ orbital (MO-1).

Magnetic susceptibility show that all the three [Ru(PPh₃)₂(L)Cl₂] complexes are one-electron paramag-netic, which is in accordance with the + 3 oxidation state of ruthenium (low-spin d5_{, S = 1/2) in these} com-plexes. Electron spin resonance spectra of the [Ru(PPh3)2(L)Cl2] complexes, recorded in 1:1 dichloromethane – toluene solution at 77 K, show rhom-bic spectra with three distinct signals (g1, g2and g3in the order of decreasing magnitude). A representative spec-trum is shown in Fig. 4 and the spectral data are Fig. 2. Electronic spectra of (a) [Ru(PPh3)2(L3)Cl2] and (b)

ctc-[Ru(PPh3)2(L1)2] ( — ) and ccc-[Ru(PPh3)2(L1)2] (---) in

dichloromethane solution.

reflected in the bond parameters. The two bulky PPh3 ligands are in trans positions, as is usually observed in complexes of ruthenium(III) having the Ru(PPh3)2 moi-ety [32 – 34], while the two chloride ligands have occu-pied cis positions. The observed bond distances are all quite normal as observed in structurally characterized complexes of ruthenium containing similar ligands

Fig. 3. Qualitative molecular orbital diagram of [Ru(PPh3)2(L)Cl2].

presented in Table 4. The observed rhombicity of the ESR spectra is understandable in terms of the gross molecular symmetry of these complexes containing the three non-equivalent PRuP, O(phenolic)–RuCl and O(carbonylic)RuCl axes. The rhombic distortion can be thought of a combination of axial distortion (D, which splits t₂ into a and e) and rhombic distortion (V, which splits e). The splitting pattern is illustrated in Fig. 4. Spin-orbit coupling causes further changes in the energy gaps. Thus two electronic transitions (transition energies DE1 and DE2; DE1BDE2) are possible within these three levels. All these energy parameters have been computed (Table 4) using the observed g values, the g tensor theory of low-spin d5 _{complexes and a} reported method [37 – 39]. The axial distortion is ob-served to be much stronger than the rhombic one. The DE1 transition falls in the infrared region (3200 – 3700 cm− 1_{) and could not be detected. The DE}

2 transition, which is expected to occur near 6000 cm− 1 _(1667 nm), could not be verified either because of the non-transparency of the solvent in this region. However, the ESR data analysis shows that the [Ru(PPh3)2(L)Cl2] complexes are significantly distorted from ideal octahe-dral geometry, which was also observed in the struc-tural analysis of [Ru(PPh3)2(L2)Cl2].

3.1.2. [Ru(PPh3)2(L)2] complexes

These complexes have been prepared in two different ways (Scheme 1). Direct reaction of the phenolic

lig-ands (HL) with [Ru(PPh3)3Cl2] in 2:1 mole ratio pro-ceeds smoothly in refluxing ethanol in the presence of a base to afford the bis complexes of type [Ru(PPh3)2 -(L)₂]. These complexes can also be prepared from [Ru(PPh₃)₂(L)Cl₂] by reacting them with one equivalent of the respective phenolic ligand (HL) in the presence of a base. Thin layer chromatographic experiments indicated the presence of two isomers (isomer-I and isomer-II) in all [Ru(PPh3)2(L)2] complexes, which have

Fig. 4. ESR spectrum of [Ru(PPh3)2(L2)Cl2] in 1:1 dichloromethane –

2.428 2.233 1.828 4.574 1.93 −1.976 [Ru(PPh3)2(L2)Cl2] 3.691 5.830 2.407 2.258 1.795 [Ru(PPh3)2(L3)Cl2] 1.97 4.065 −1.264 3.515 5.029 a_{In 1:1 dichloromethane–toluene solution at 77 K.}

b_{Spin-orbit coupling constant (l) for complexed ruthenium(III) is 1000 cm}−1_.

been separated by column chromatography. Microana-lytical data of these complexes are in good agreement with their compositions (Table 2). Both isomers of these complexes are diamagnetic, which corresponds to the bivalent state of ruthenium (low-spin d6_{, S = O) in} these complexes.

As all three phenolate ligands used in the present study are asymmetric, the [Ru(PPh₃)₂(L)₂] complexes may exist in five geometrical isomeric forms (6 – 10). Isomers 6 – 9 have a C₂ axis, while isomer 10 does not have any C2 symmetry. 1H NMR spectra have been recorded on both isomers of all three [Ru(PPh3)2(L)2] complexes in CDCl3 solution. Isomer-I of all three complexes clearly shows the presence of a C2axis, while isomer-II indicates the absence of C2 symmetry. For example, isomer-I of [Ru(PPh3)2(L1)2] shows only one aldehydic proton signal at 8.68 ppm, while isomer-II of [Ru(PPh3)2(L1)2] shows two aldehydic proton signals (1H each) at 8.36 and 8.53 ppm. This shows that isomer-II has structure 10, where the three sets of donor atoms are in cis positions and henceforth this isomer will be labeled as the cis – cis – cis or ccc isomer.

It has not been possible to assign specific stereochem-istry of isomer-I on the basis of 1_{H NMR spectral} results alone. However, as complexes of ruthenium with the Ru(PPh₃)₂ are known to Prefer cis-disposition of PPh₃ ligands [33,34,40], these [Ru(PPh₃)₂(L)₂] com-plexes may be assumed to have either structure 8 or 9. To sort out this problem of stereochemistry of isomer-I,

the molecular structure of isomer-I of [Ru(PPh3)2(L1)2] has been determined by X-ray crystallography. The structure is shown in Fig. 5 and selected bond distances and angles are presented in Table 5. Ruthenium has a distorted octahedral O4P2 coordination sphere with the two PPh3 ligands in cis positions, the two phenolate oxygens in trans positions and the two carbonylic oxy-gens in cis positions. Therefore isomer-I of [Ru(PPh₃)₂(L1₎

2] has structure 9 (henceforth referred to as the ctc isomer to indicate the cis – trans – cis disposi-tions of the carbonylic oxygens, phenolate oxygens and phosphines, respectively). The observed bond parame-ters are all quite normal. In view of the similarity in synthetic procedure and properties, isomer-I of the other two [Ru(PPh3)2(L)2] complexes are assumed to have similar ctc structure.

Scheme 1.

Table 5

Selected bond distances (A, ) and bond angles (°) for [Ru(PPh3)2(L 1₎ 2] O(1)C(1) 1.295(6) RuP(1) 2.2929(18) O(2)C(7) 2.3031(17) 1.239(7) RuP(2) O(3)_C(8) Ru_O(1) 2.057(3) 1.313(6) O(4)_C(14) 2.090(4) 1.262(7) Ru_O(2) Ru_O(3) 2.052(3) 2.128(4) RuO(4) P(1)RuO(2) 169.48(11) 169.69(11) P(2)RuO(4) O(1)RuO(3) 173.61(14) P(2)RuO(3) 99.22(6) 87.90(11) P(1)RuP(2) O(1)_RuO(2) P(1)_RuO(1) 90.17(11) 90.75(14) O(1)_RuO(4) 93.05(11) 83.48(14) P(1)_RuO(3)

P(1)_RuO(4) 91.07(11) O(2)_RuO(3) 85.08(14) O(2)RuO(4)

97.04(11) 78.62(14)

P(2)RuO(1)

91.08(11)

P(2)RuO(2) O(3)RuO(4) 90.94(14)

because of the lower symmetry splitting of the metal orbitals and presence of different accepting orbitals. For proper assignment of the absorptions in the visible region, qualitative EHMO calculations have been per-formed as before on models of the ctc and ccc isomersa of [Ru(PPh₃)₂(L1₎

2], where phenyl groups of the PPh3 ligands have been replaced by hydrogen. The results of these calculations are qualitatively very similar for both the isomers. A partial MO diagram for one isomer is shown in Fig. 6. The highest occupied molecular orbital (HOMO) and the next two filled orbitals (HOMO-1 and HOMO-2) are basically (\75%) ruthenium t2 or-bitals. There are two relatively close vacant molecular orbitals above these filled orbitals, the lowest pied molecular orbital (LUMO) and the next unoccu-pied orbital (LUMO + 1), which are primarily (\95%) p-orbitals of the phenolate ligands. The intense absorp-tions observed in the visible region may therefore be assigned to the charge-transfer transitions occurring from the filled ruthenium t₂ orbitals to the vacant p-orbitals of the phenolate ligands.

3.2. Electron-transfer properties

Electron-transfer properties of the [Ru(PPh3)2(L)Cl2] and [Ru(PPh3)2(L)2] have been studied in acetonitrile solution by cyclic voltammetry. Representative voltam-mograms are shown in Fig. 7 and voltammetric data are presented in Table 2.

Each [Ru(PPh3)2(L)Cl2] complex shows a reductive response on the negative side of SCE and an oxidative response on the positive side. Both responses are quasi-reversible in nature. The reduction, observed near − 0.3 V (all potentials are referenced to SCE), is tenta-tively assigned to ruthenium(III)-ruthenium(II) reduc-tion and the oxidareduc-tion, which occurs within 1.08 – Infrared spectra of ctc and ccc isomers of the

[Ru(PPh₃)₂(L)₂] complexes are very similar. Each shows characteristic vibrations near 500, 700, 750 cm− 1_, indi-cating the presence of the Ru(PPh3)2 moiety. The n(CO) vibration is observed as a strong band near 1580 cm− 1_{in all these complexes. The [Ru(PPh}

3)2(L)2] complexes are soluble in common organic solvents like dichloromethane, chloroform, acetone, acetonitrile, etc., forming intense red solutions. Electronic spectra of these complexes, recorded in dichloromethane solution, show several intense absorptions in the visible region and few absorptions of very high intensity in the ultra-violet region (Fig. 2, Table 2). The latter absorptions are assigned to transitions within the ligand orbitals. The former absorptions in the visible region are proba-bly due to allowed metal-to-ligand charge-transfer tran-sitions. Multiple charge-transfer transitions are common in such mixed ligand complexes, primarily

Fig. 7. Cyclic voltammograms of (a) [Ru(PPh3)2(L3)Cl2] and (b)

ctc-[Ru(PPh3)2(L1)2] ( — ) and ccc-[Ru(PPh3)2(L1)2] (---) in

acetoni-trile solution (0.1 M TEAP) at a scan rate of 50 mV s− 1_.

Thanks are also due to the Third World Academy of Sciences for financial support for the purchase of an electrochemical cell system. F.B. thanks the University Grants Commission, New Delhi, for her fellowship.

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1.24 V, is tentatively assigned to ruthenium-(III) – ruthenium(IV) oxidation. Both potentials have shifted to a little more negative values in [Ru(PPh3)2(L2)Cl2] relative to [Ru(PPh3)2(L1)Cl2], which may be attributed to the electron donating character of the methyl group in L2_.

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4. Supplementary data

Supplementary data are available from the Cam-bridge Crystallographic Data Center, 12 Union Road, Cambridge, CB2 1EZ, UK, on request, quoting the deposition numbers CCDC 138541 and 138542.

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