Chemistry of some ruthenium phenolates: Synthesis, structure and redox properties

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Polyhedron 19 (2000) 1673 – 1680

Chemistry of some ruthenium phenolates: synthesis, structure and

redox properties

Prasanta Kumar Sinha

a

_{, Larry R. Falvello}

b

_{, Shie-Ming Peng}

c

_,

Samaresh Bhattacharya

a,

_*

a_{Department of Chemistry, Inorganic Chemistry Section, Jada}_{6pur Uni6ersity, Calcutta}_{700032, India} b_{Department of Inorganic Chemistry, Faculty of Science, Uni}_{6ersity of Zaragoza, E-50009}_{Zaragoza, Spain}

c_{Department of Chemistry, National Taiwan Uni}_{6ersity, Taipei, Taiwan, ROC}

Received 16 March 2000; accepted 28 April 2000

Abstract

Reaction of three phenolate ligands, viz. 2,4,6-tribromophenol (HL1_{, where H stands for the phenolic proton), 2-nitrophenol} (HL2_{) and 2,4,6-trinitrophenol (HL}3_{O), with [Ru(PPh}

3)3Cl2] in a 2:1 molar ratio in the presence of a base gives complexes of type [Ru(PPh3)2(L)2] (L = L1, L2and L3). The 2,4,6-tribromophenolate ligand (L1) binds to ruthenium as a bidentate O,Br-donor, while the 2-nitrophenolate ligand (L2_{) acts as a bidentate O,O-donor. 2,4,6-Trinitrophenol (HL}3_{O) undergoes oxygen loss from one} nitro group at the ortho position and coordinates to ruthenium in the 2-nitroso-4,6-dinitrophenolate (L3_{) form through the nitroso} nitrogen and phenolate oxygen. The structures of the [Ru(PPh3)2(L1)2] and [Ru(PPh3)2(L3)2] complexes have been solved by X-ray crystallography. In [Ru(PPh3)2(L1)2] the coordination sphere around ruthenium is O2P2Br2with a trans – cis – cis disposition of the three sets of donor atoms, respectively. In [Ru(PPh3)2(L3)2] ruthenium has a N2O2P2 coordination sphere with a cis – cis – trans arrangement of the three sets of donor atoms, respectively. The [Ru(PPh3)2(L)2] complexes are diamagnetic (low-spin d6, S = 0) and in acetonitrile solution show intense MLCT transitions in the visible region. Cyclic voltammetry on the [Ru(PPh3)2(L)2] complexes shows a reversible ruthenium(II) – ruthenium(III) oxidation within 0.63 – 0.71 V versus SCE followed by an irreversible ruthenium(III) – ruthenium(IV) oxidation near 1.5 V versus SCE. © 2000 Elsevier Science Ltd. All rights reserved.

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

1. Introduction

There is much current interest in the chemistry of ruthenium [1 – 10], most of which is due to the fascinat-ing electron-transfer and energy-transfer properties dis-played by the complexes of this metal. The coordination environment around the metal primarily dictates properties of the ruthenium complexes. Com-plexation of ruthenium by ligands of different types has thus been of particular interest. In the present work, which has emerged from our continued interest in the chemistry of ruthenium in different coordination envi-ronments [11 – 25], our objective has been to explore the

chemistry of ruthenium bound to phenolate oxygens. Phenolate oxygen is a recognized hard donor and hence coordination of ruthenium by phenolate oxygen is of particular importance with regard to stabilization of the higher oxidation states of this metal [26 – 29]. The initial goal of the present study was to investigate the coordi-nating ability of simple phenols with no additional donor atoms. As a source of ruthenium, [Ru(PPh3)3Cl2] has been used, which is well known for its efficiency in binding to ligands of different types via dissociation of PPh3 and chloride ligands [29 – 33]. However, we have observed that until a second donor site is incorporated at the ortho position of the phenyl ring, no tractable phenolate complex is formed.

* Corresponding author. Tel.: + 91-33-483-6223; fax: + 91-33-473-4266.

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

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When phenols of type 1 (abbreviated in general as HL, where H stands for the dissociable phenolic pro-ton) are used, where D is the second donor atom linked to the ortho carbon directly or via one intervening atom, stable complexes of general formula [RuII_(PPh

3)2(L)2] are formed. The chemistry of these complexes is described in this paper with special refer-ence to synthesis, structure and redox properties.

2. Experimental

2.1. Materials

Commercial ruthenium trichloride was purchased from Arora Matthey, Calcutta, India and was con-verted to RuCl₃·3H₂O by repeated evaporation with concentrated hydrochloric acid. Triphenylphosphine, triethylamine, 2,4,6-tribromophenol (HL1_), 2-nitrophen-ol (HL2_{) and 2,4,6-trinitrophenol (HL}3_{O) were} ob-tained from SD Fine Chemicals, Mumbai, India. [Ru(PPh3)3Cl2] was prepared following a reported pro-cedure [34]. Purification of acetonitrile and preparation of tetraethylammonium perchlorate (TEAP) for electro-chemical work were performed as reported in the litera-ture [35,36]. All other chemicals and solvents were reagent grade commercial materials and were used as received.

2.2. Preparation of complexes 2.2.1. [Ru(PPh₃)₂(L1₎

2]

[Ru(PPh₃)₃Cl₂] (100 mg, 0.10 mmol) and 2,4,6-tribro-mophenol (85 mg, 0.26 mmol) were taken up in 30 cm3 methanol and triethylamine (42 mg, 0.42 mmol) was added. The resulting mixture was stirred for 2 h. A dark-red microcrystalline solid separated out, which was collected by filtration, washed with methanol and dried in vacuo. Recrystallization from 1:4 di-chloromethane – hexane solution afforded shiny red crystals of [Ru(PPh3)2(L1)2]. The yield was 94 mg (70%).

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

This compound was prepared by following the same above procedure using 2-nitrophenol instead of 2,4,6-tribromophenol. The yield was 70 mg (75%).

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

To a solution of 2,4,6-trinitrophenol (50 mg, 0.22 mmol) in ethanol (30 cm3_{) was added [Ru(PPh}

3)3Cl2] (100 mg, 0.10 mmol) and triethylamine (42 mg, 0.42 mmol). The resulting mixture was heated at reflux for 2.5 h to produce a red solution. Upon cooling the solution to room temperature, a dark red crystalline solid separated out, which was collected by filtration,

washed with ethanol and dried in air. Recrystallization from dichloromethane – benzene afforded shiny red crystals of [Ru(PPh₃)₂(L3₎

2]. The yield was 34 mg (30%).

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 a Shimadzu UV 240 spectropho-tometer. Magnetic susceptibilities were measured using a PAR 155 vibrating sample magnetometer fitted with a Walker scientific L75FBAL magnet. 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. A RE 0089 X-Y recorder was used to trace the voltammograms. Dinitrogen gas was purified by successively bubbling it through alkaline dithionite and concentrated sulfuric acid. All electro-chemical experiments were performed under a dinitro-gen atmosphere. All electrochemical data were collected at 298 K and are uncorrected for junction potentials. 2.4. Crystallography

Single crystals of [Ru(PPh3)2(L1)2] 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. The unit cell dimensions were determined by a least-squares fit of 25 centered reflections (11.45u525.5°). Data were col-lected on an Enraf – Nonius CAD-4 diffractometer us-ing graphite-monochromated Mo Ka radiation (l=0.71073 A,) by v scans within the angular range 2.15BuB27.49°. Three standard reflections were used to check the crystal stability towards X-ray exposure and they showed no significant intensity variation over the course of data collection. X-ray data reduction, structure solution and refinement were done using the SHELXS-97 andSHELXL-97 packages. The structure was solved by the direct method.

Single crystals of [Ru(PPh₃)₂(L3₎

2] were grown by slow diffusion of benzene into a dichloromethane solu-tion of the complex. Selected crystal data and data collection parameters are given in Table 1. The unit cell dimensions were determined by a least-squares fit of 25 centered reflections (10.84BuB19.80°). 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 2u max 50.0°. Three standard reflections, used to check the crystal stability towards X-ray exposure, showed no significant

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intensity variation over the course of data collection. X-ray data reduction, and structure solution and refine-ment were done using theNRCVAXpackage. The struc-ture was solved by the direct method.

3. Results and discussion

Three phenolic ligands have been used in the present study which are shown, along with their individual abbreviation and coordination mode, in Fig. 1. The phenolic ligands react smoothly with [Ru(PPh3)3Cl2] in the presence of a base to afford complexes of type [Ru(PPh3)2(L)2]. It may be noted here that unlike in the reactions of the first two phenolic ligands, heating of the reaction mixture at reflux was necessary in the reaction of 2,4,6-trinitrophenol with [Ru(PPh3)3Cl2] for obtaining the phenolate complex. It is also interesting to note that 2,4,6-trinitrophenol undergoes oxygen loss from one nitro group during the course of the synthetic reaction and coordinates in the nitrosophenolate fash-ion. Hence 2,4,6-trinitrophenol is abbreviated as HL3_O (see Fig. 1) to underline this oxo-transfer reaction. Mechanism of this oxygen loss reaction is not yet clear. However, oxygen from the nitro group is probably transferred to the PPh3, dissociated from [Ru(PPh3)3Cl2]. Indirect evidence of this oxo-transfer comes from detection of OPPh3 in the residue of the synthetic reaction (after isolation of [Ru(PPh₃)₂(L3₎

2]), identified by its characteristic infrared spectrum (nPO observed at 1185 cm− 1_{). Triphenylphosphine is well} known to act as an oxygen-scavenger in many oxo-transfer reactions [37 – 40]. The role of [Ru(PPh3)3Cl2] in this oxo-transfer reaction is not yet clear, but that it does not act as a mere supplier of PPh3is clear from the fact that a mixture of PPh₃ and 2,4,6-trinitrophenol is unable to bring about any oxo-transfer reaction. Prior coordination of the 2,4,6-trinitrophenolate ligands to ruthenium in the nitrophenolate fashion followed by oxo-transfer from the metal-bound ligands appears to be probable. It may also be noted here that the yield of 2[Ru(PPh3)3Cl2] + 2HL3O

[Ru(PPh3)2(L3)2] + 2HCl + 2OPPh3+ [Ru(PPh3)2Cl2] (1) [Ru(PPh3)2(L3)2] is rather low, which might be at-tributed to the fact that [Ru(PPh3)3Cl2] dissociates in solution yielding free PPh₃ and an unstable [Ru(PPh3)2Cl2] species [41,42]. Hence 2 mol of [Ru(PPh3)3Cl2] (which supply 2 mol of PPh3) are re-quired for the reduction of 2 mol of 2,4,6-trinitrophen-ol. The 2 mol of reduced ligand (L3_{) utilize only 1 mol} of [Ru(PPh3)2Cl2] for complex formation (Eq. (1)). The other mole of [Ru(PPh3)2Cl2] probably undergoes de-composition [41,42]. Some characterization data of the complexes are given in Table 2. Elemental (C, H, N) analytical data agree well with the compositions of the [Ru(PPh3)2(L)2] complexes. All three complexes are dia-magnetic, which corresponds to the bivalent state of ruthenium (low-spin d6_{, S = 0) in these complexes.} Table 1 Crystallographic data [Ru(PPh3)2(L3)2] [Ru(PPh3)2(L1)2] C54H40N6O12P2Ru Formula C48H34Br6O2P2Ru 1285.22 Formula weight 1049.0

Space group monoclinic, P21/c orthorhombic, Pccn 11.766(4) 12.8954(18) a (A, ) 16.334(2) b (A, ) 14.613(3) c (A, ) 21.572(6) 28.395(5) 97.071(15) b (°) 90 4509.2(15) V (A,3₎ _4882.1(21) 4 4 Z 0.42×0.27×0.02 Crystal size (mm) 0.50×0.10×0.10 298 299 T (K) 5.775 4.487 m (cm−1₎ R1= 0.0740a Rf= 0.038c R indices Rw= 0.037d wR2= 0.1300b GOF 0.993e _1.15f a_R 1=SFo−Fc/SFo. b_wR 2= [S[w(Fo2−Fc2)2]/S[w(Fo2)2]]1/2. c_R f=SFo−Fc/SFo. d_R w= [Sw(Fo−Fc)2/Sw(Fo2]1/2. e_{GOF = [S[w(F} o 2 −Fc 2₎2

]/(M−N)]1/2_{, where M is the number of}

reflections and N is the number of parameters refined.

f_{GOF = [Sw(F}

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

reflections and N is the number of parameters refined.

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Table 2

Microanalytical, electronic spectral and cyclic voltammetric data

Electronic spectral datab _{Cyclic voltammetric data}c

Compound Microanalytical dataa

lmax(nm),o (M−1cm−1) E1/2(V),DEp(mV) %H %C %N 2.68 [Ru(PPh3)2(L1)2] 44.82 495(1200), 310(6200), 0.63(60), (2.65) 250(27 900), 220(43 300) (44.83) 1.48d [Ru(PPh3)2(L 2₎ 2] 63.94 4.16 3.06 490(5900), 390(8800), 0.64(70), (63.93) (4.22) (3.11) 250(28 700), 224(46 000) 1.50d 3.24 7.98 [Ru(PPh3)2(L3)2] 54.91 530(11 300), 340(21 520), 0.71(60), (3.25) (8.02) 285(34 440), 224(55 960) (54.99) 1.55d

a_{Calculated values are in parentheses.} 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_E

pavalue.

From the composition of the complexes and also from the fact that ruthenium(II) prefers to be hexacoordi-nated, the phenolic ligands appear to have served as bidentate ligands. Five geometrical isomers (2 – 6) are then possible for the [Ru(PPh3)2(L)2] complexes.

To sort out the problem of stereochemistry and also to authenticate coordination mode of the phenolic lig-ands, molecular structures of [Ru(PPh3)2(L1)2] and [Ru(PPh3)2(L3)2] have been solved by X-ray crystallog-raphy [43]. The structures are shown in Fig. 2 and selected bond parameters are given in Table 3. In [Ru(PPh3)2(L1)2], 2,4,6-tribromophenol is coordinated to ruthenium via loss of the phenolic proton, as a bidentate O,Br-donor ligand forming a five-membered chelate ring with a bite angle of 80.55°. While the phenolate oxygens are mutually trans, the PPh₃ligands are mutually cis and the two bromines are also mutually cis. Hence the O2P2Br2 coordination sphere around ruthenium has a

trans – cis – cis geometry (6) with regard to mutual

dispo-sitions of the three pairs of donor atoms. The coordina-tion sphere around ruthenium is distorted octahedral in nature, which is reflected in the bond parameters around ruthenium. While the RuO and RuP distances are quite normal [18,21,44,45], the RuBr lengths are a bit longer than is usually observed [46 – 48]. The CBr lengths corresponding to the coordinated bromines are slightly longer than the other two CBr distances in the same ligand and this may be attributed to coordination of the bromine to ruthenium(II). The phenolate CO lengths are also quite usual [44,45]. The [Ru(PPh3)2(L2)2] complex could not be characterized crystallographically [43]. However, the 2-nitrophenolate ligand is known to coordinate as bidentate O,O-donor forming a six-mem-bered chelate ring [49] as shown in Fig. 1. The same coordination mode of the 2-nitrophenolate ligand is assumed in [Ru(PPh3)2(L2)2].

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Structure determination of [Ru(PPh3)2(L3)2] shows that 2,4,6-trinitrophenol is coordinated, via loss of the phenolic proton and reduction of one nitro group at the

ortho position, as a 2-nitroso-4,6-dinitrophenolate

an-ion. Chelation takes place from the nitroso nitrogen and phenolate oxygen forming a five-membered chelate ring with a bite angle of 80.85°. The coordinated nitro-gens are mutually cis, the phenolate oxynitro-gens are also mutually cis and the two coordinated PPh3 ligands are mutually trans. The coordination sphere around ruthe-nium is slightly distorted octahedral N2O2P2 with a

cis – cis – trans geometry, respectively (3). While the

RuP distances are normal [50,51], the bond distances

within the NO chelates are rather unusual. The RuN bonds are significantly shorter than usually observed [15,16,18]. Such a short RuN distance is known to result only from very strong p-interaction [52]. The RuO bonds are longer than usually observed [21,45]. The observed CO lengths are shorter than usual phen-olate CO distances [44,45]. The unusual nature of observed bond distances in [Ru(PPh3)2(L3)2] appears to result from the combined effect of the possible resonance in the nitrosophenolate ligand (7) and the p-bonding interaction [52–54] between ruthenium(II) and the imine-oxo moiety of the nitrosophenolate lig-and.

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Fig. 3. Cyclic voltammogram of [Ru(PPh3)2(L1)2] in acetonitrile

solu-tion (0.1 M TEAP) at a scan rate of 50 mV s− 1_.

This strong p-interaction is also reflected in the cis disposition of this imine nitrogens [53,54]. In complexes containing the RuII_(PPh

3)2 moiety, the two triphenylphosphines usually take up cis positions [15,18,45,55,56], which has also been observed in [Ru(PPh3)2(L1)2] (vide supra). In [Ru(PPh3)2(L3)2] the imine-oxo group appears to be a stronger p-acceptor than PPh3 and thus forces the bulky PPh3 ligands to take up trans positions for less steric hindrance.

The IR spectra of these complexes show many sharp bands of different intensities in the 1700 – 400 cm− 1 region due to vibrations arising from the coordinated PPh₃and phenolic ligands and are therefore complex in nature. No attempt has been made to assign individual bands. However, the strong vibrations near 520, 700, and 740 cm− 1 _{displayed by all the complexes indicate} the presence of the Ru(PPh3)2 fragment [15,18,44,45]. The [Ru(PPh3)2(L)2] complexes are soluble in polar solvents like acetonitrile, dichloromethane, chloroform, etc, producing intense red solutions. Electronic spectra of the complexes have been recorded in dichloro-methane solution. All these complexes show several

intense absorptions in the visible and ultraviolet region. Spectral data are presented in Table 2. The intense absorptions in the ultraviolet region are attributable to transitions within the ligand orbitals and those in the visible region are probably due to allowed metal-to-lig-and charge-transfer transitions. Multiple charge-trans-fer transitions in such mixed-ligand complexes are known to result from lower symmetry splitting of the metal level, the presence of different acceptor orbitals and from the mixing of singlet and triplet configura-tions in the excited state through spin-orbit coupling [57 – 60].

Electrochemical properties of all the complexes have been studied in acetonitrile solution (0.1 M TEAP) by cyclic voltammetry. A selected voltammogram is shown in Fig. 3 and the voltammetric data are presented in Table 2. Each complex shows two oxidative responses on the positive side of SCE. The first oxidative response is observed within 0.63 – 0.71 V versus SCE and is assigned to the ruthenium(II) – ruthenium(III) oxidation (Eq. (2)) This oxidation is reversible in nature with a peak-to-peak separation (DEp) of 60 – 70 mV, which does not change when the scan rate is changed. [RuII_(PPh

3)2(L)2]X [RuIII(PPh3)2(L)2]++ e− (2) [RuIII_(PPh

3)2(L)2]+[RuIV(PPh3)2(L)2]2 ++ e− (3) The anodic peak current (ipa) is also equal to the cathodic peak current (ipc) as expected for a reversible electron-transfer process. The one-electron nature of this oxidation has been verified by comparing its cur-rent heights with those of the standard ferrocene – fer-rocenium couple under identical experimental conditions. The second oxidative response, which ap-pears near 1.5 V versus SCE, is irreversible and is tentatively assigned to ruthenium(III) – ruthenium(IV) oxidation (Eq. (3)). One-electron stoichiometry of the oxidation is verified by comparing its current height (ipa, calculated after deduction of solvent contribution from the observed current at Epa) with that of the ruthenium (II) – ruthenium(III) oxidation.

Table 3

Selected bond distances (A, ) and bond angles (°) [Ru(PPh3)2(L1)2] [Ru(PPh3)2(L3)2] RuO(1) 2.091(6) RuP 2.4205(18) 2.100(4) RuO1 2.086(6) RuO(2) RuBr(1) 2.6354(13) RuN1 1.927(5) RuBr(4) 2.6343(12) O1C6 1.269(7) 2.282(3) RuP(1) O2N1 1.243(6) 2.299(2) RuP(2) O3N2 1.208(8) 1.281(10) O(1)_C(1) O4_N2 1.224(8) O5_N3 1.230(7) 1.299(10) O(2)_C(7) 1.914(9) C(2)_Br(1) O6_N3 1.208(8) C(4)Br(2) 1.899(11) N1C1 1.437(7) 1.895(11) N2C3 1.457(8) C(6)Br(3) 1.465(7) C(8)Br(4) 1.906(8) N3C5 1.359(8) C(10)Br(5) 1.893(9) C1C2 1.382(9) C2C3 C(12)Br(6) 1.895(9) 1.385(9) C3C4 C4C5 1.363(2) 1.428(8) C5C6 C6C1 1.426(8) 171.59(6) 171.6(2)

O(1)RuO(2) PRuPa

168.12(7)

P(1)RuBr(1) O1RuN1a 177.93(17) P(2)RuBr(4) 169.08(7) O1aRuN1 177.93(17)

96.16(9) 80.85(17) P(1)RuP(2) O1RuN 80.85(17) O1aRuN1a Br(1)RuBr(4) 83.54(4) O(1)RuBr(1) 80.55(17) O(2)RuBr(4) 80.93(16)

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4. Conclusions

The present study shows that coordination of phen-ols to ruthenium as simple monodentate oxygen donor ligands is probably not possible, but phenols having a second donor site linked to the ortho position can bind to ruthenium as a bidentate ligand affording stable complexes.

5. Supplementary data

Copies of supplementary data can be obtained free of charge from The Director, Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: + 44-1223-336-033; e-mail: deposit@ccdc. cam.ac.uk or www: http://www.ccdc.cam.ac.uk), quot-ing the deposition numbers CCDC 141332 and CCDC 141333.

Acknowledgements

Financial assistance received from the Council of Scientific and Industrial Research, New Delhi [Grant No. 01(1408)/96/EMR-II] and the Directorate General for Higher Education (Spain) under Grant PB98-1593 is gratefully acknowledged. Thanks are also due to the Third World Academy of Sciences for financial support enabling the purchase of an electrochemical cell system.

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