Cyclometallation and N=N bond cleavage of 2-(arylazo)phenols by osmium. Synthesis, structure and redox properties

284 J. Chem. Soc., Dalton Trans., 2001, 284–288 DOI: 10.1039/b007486l

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A

LTO

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FULL PAPER

Cyclometallation and N

᎐᎐N bond cleavage of 2-(arylazo)phenols

by osmium. Synthesis, structure and redox properties †

Kanchana Majumder,

Shie-Ming Peng

and Samaresh Bhattacharya *

a

_{Department of Chemistry, Inorganic Chemistry Section, Jadavpur University,}

Calcutta 700 032, India. E-mail: [email protected]

b

Department of Chemistry, National Taiwan University, Taipei, Taiwan, Republic of China

Received 15th September 2000, Accepted 13th November 2000

First published as an Advance Article on the web 11th January 2001

Reaction of 2-arylazo-4-methylphenol, (H2ap-R, where H2 indicates the two protons (the phenolic proton and

one phenyl proton at the ortho position of the arylazo fragment) that undergo dissociation upon complexation) with [Os(PPh3)3Br2] in the presence of a base afford two types of organometallic complexes of osmium(), viz.

red [Os(PPh3)2(ap-R)Br] and brownish-green [Os(PPh3)(ap-R)(N–O)][N–O is an iminosemiquinonate ligand

generated from the 2-(arylazo)phenol via N᎐᎐N bond cleavage]. The structures of [Os(PPh3)2(ap-Cl)Br] and

[Os(PPh3)(ap-Cl)(N–O)] have been determined by X-ray crystallography. In the [Os(PPh3)2(ap-R)Br] complexes, the

ap-R ligand coordinates to osmium as a tridentate C,N,O donor ligand forming two five-membered chelate rings and the two PPh3 ligands are trans. In the [Os(PPh3)(ap-R)(N–O)] complexes, osmium is bound to one PPh3, one ap-R

ligand coordinated as a tridentate C,N,O donor ligand and one iminosemiquinonate ligand. The [Os(PPh3)2(ap-R)Br]

complexes are paramagnetic (low-spin d5_{, S= 1/2) and show anisotropic EPR spectra at 77 K. The [Os(PPh} 3

)-(ap-R)(N–O)] complexes are diamagnetic and show sharp 1_{H and} 13_{C NMR signals. In acetonitrile solution all the}

complexes display intense charge-transfer transitions in the visible region. Cyclic voltammetry on these complexes in acetonitrile solution shows an osmium()–osmium() oxidation positive to SCE and an osmium()–osmium() reduction negative to SCE. Two irreversible oxidations are also displayed by all the [Os(PPh3)(ap-R)(N–O)]

complexes in the range 1.10–1.76 V vs. SCE.

Introduction

The chemistry of osmium has been receiving much current attention1 _{primarily because of the interesting properties}

exhib-ited by the complexes of this metal. As planned variation in the coordination environment around osmium brings about corresponding variation in the properties of its complexes, coordination of osmium by ligands of different types has been of significant importance. In the present work, which has origin-ated from our interest in the chemistry of osmium in different coordination spheres,2 _{we have chosen 2-(arylazo)phenols (1)}

as the principal ligand type. This ligand type is familiar as a bidentate N,O-donor forming six-membered chelate rings (2).3

However, in a series of osmium complexes of type [Os(bpy)2

-(L)]⫹[bpy= 2,2⬘-bipyridine; L = 2-(arylazo)phenolate], we have recently observed a rather uncommon coordination mode of this ligand type with formation of a five-membered chelate ring (3).2c _{The proximity of the phenyl ring, pendant from the}

uncoordinated azo-nitrogen, to osmium points to the possib-ility of formation of an Os–C bond at the ortho position (4). Such orthometallation could not take place in the [ Os-(bpy)2(L)]⫹ complexes, probably because of non-availability of

a vacant coordination site. However, this study clearly indi-cated that the necessary prerequisite for the expected C,N,O-coordination to occur, is to choose the right osmium starting material which can provide three vacant coordination sites and which preferably will retain some π-acid ligands, since form-ation of Os–C bonds is observed to be facile when π-acid co-ligands are present.4 _{With this strategy in mind we selected}

[Os(PPh3)3Br2] as the starting material since in its reaction with

ligands containing dissociable protons, it is known to lose one

† Electronic supplementary information (ESI) available: microanalyt-ical and EPR data. See http://www.rsc.org/suppdata/dt/b0/b007486l/

PPh3 and at least one bromide while retaining the two PPh3

ligands2a,d,5 _{and thus it provides at least three vacant}

coordin-ation sites on osmium. This selection indeed turned out to be very successful and two families of organo-osmium complexes (viz. red and brownish-green complexes), both containing the 2-(arylazo)phenolate ligand coordinated in the expected fashion (4), have been obtained from the reaction of 2-(arylazo)phenols with [Os(PPh3)3Br2]. It may be mentioned here

that osmium is in the ⫹3 state in all these complexes and to our knowledge these represent the first6 _{group of cyclometallated}

complexes of osmium(). In the red complexes, osmium is bound to two PPh3 ligands, one 2-(arylazo)phonolate ligand (as

in 4) and one bromide. In the brownish-green complexes osmium is coordinated to one PPh3, one 2-(arylazo)phenolate

from the cleavage of one 2-(arylazo)phenol across the N᎐᎐N bond. These two groups of complexes appear to be of partic-ular interest because they represent two very interesting reac-tions: (i) formation of organometallic complexes of trivalent osmium, which is rare6 _{and (ii) metal promoted N}

᎐᎐N bond cleavage which is also not that common and of much current interest.7 _{An account of the chemistry of these organo-osmium}

complexes is presented here with special reference to synthesis, characterization and electrochemistry.

Experimental

Osmium tetroxide was purchased from Arora Matthey, Cal-cutta, India and was converted to [NH4]2[OsBr6] by reduction

with hydrobromic acid.8_[Os(PPh

3)3Br2] was synthesized,

start-ing from [NH4]2[OsBr6], by following a reported procedure.9

The 2-(arylazo)phenol ligands were prepared by reacting the respective diazotized aniline with alkaline para-cresol. Purific-ation of acetonitrile and preparPurific-ation of tetraethylammonium perchlorate (TEAP) for electrochemical work were performed as reported in the literature.10 _{All other chemicals and solvents}

were reagent grade commercial materials and were used as received.

Preparations

Each [Os(PPh3)2(ap-R)Br] complex and the corresponding

[Os(PPh3)(ap-R)(N–O)] complex were obtained from a single

reaction. Details are given below for a particular case.

[Os(PPh3)2(ap-H)Br] and [Os(PPh3)(ap-H)(N–O)]. To a

solu-tion of H2ap-H (43 mg, 0.20 mmol) in 2-methoxyethanol (30

cm3_{) was added [Os(PPh}

3)3Br2] (100 mg, 0.09 mmol) followed

by triethanolamine (0.03 cm3_{, 0.20 mmol). The mixture was}

heated at reflux for 6 h. The solvent was then evaporated on a water bath and purification of the crude product thus obtained was achieved by chromatography through a silica gel (60–120 mesh) column. Using a 1 : 1 hexane–benzene mixture as the eluent, a yellow band eluted first and this was rejected. With a 1 : 7 hexane–benzene mixture as the next eluent, a red band was eluted (leaving behind a brownish-green band) which was collected. Evaporation of the eluate gave [Os(PPh3)2

(ap-H)-Br] as a crystalline solid. The yield was 32%.

After eluting the red band with a 1 : 7 hexane–benzene mixture, the remaining brownish-green band was eluted using benzene as the eluent. Evaporation of the eluate gave

[Os(PPh3)(ap-H)(N–O)] as a crystalline solid. The yield was

58%.

Physical measurements

Microanalyses (C, H, N) were performed using a Perkin-Elmer 240C elemental analyzer. Electronic spectra were recorded on Shimadzu UV 1601 and Hitachi 330 spectrophotometers. Mag-netic susceptibilities were measured using a PAR 155 Vibrating Sample Magnetometer fitted with a Walker scientific L75FBAL magnet. EPR spectra were recorded with a Varian Model 109C E-line X-band spectrometer fitted with a quartz Dewar for measurements at 77 K (liquid dinitrogen). All spectra were calibrated with the aid of DPPH (diphenylpicrylhydrazyl, g= 2.0037). NMR spectra were recorded on a Bruker DRX-500 NMR spectrometer. Electrochemical measurements were performed using a PAR model 273 potentiostat. A platinum-disc or graphite working electrode, a platinum 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. Electro-chemical measurements were made under a dinitrogen atmos-phere. All electrochemical data were collected at 298 K and are uncorrected for junction potentials.

Crystallography

[Os(PPh3)2(ap-Cl)Br]. Single crystals of [Os(PPh3)2(ap-Cl)Br]

were grown by slow diffusion of hexane into a dichloromethane solution of the complex. Selected crystal data and data collec-tion parameters are given in Table 1. Data were collected on a SMART CCD diffractometer using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) using ω-scans. X-Ray data reduction and structure solution and refinement were per-formed using the SHELXTL-PLUS package.11 _{The structure}

was solved by direct methods.

[Os(PPh3)(ap-Cl)(N–O)]. Single crystals of [Os(PPh3

)(ap-Cl)-(N–O)] were grown by slow diffusion of hexane into a dichloro-methane solution of the complex. Selected crystal data and data collection parameters are given in Table 1. Data collection and structure solution and refinement were done similarly as above.

CCDC reference number 186/2268.

See http://www.rsc.org/suppdata/dt/b0/b007486l/ for crystal-lographic files in .cif format.

Results and discussion

Five 2-(arylazo)phenol ligands (1) have been used in the present study. The ligands are abbreviated in general as H2ap-R, where

H2 corresponds to the two protons (one phenolic proton and

one phenyl proton at the ortho position of the arylazo frag-ment) which undergo dissociation upon complexation (vide infra) and R corresponds to the substituent. Reaction of each of these ligands with [Os(PPh3)3Br2] in 2 : 1 mole ratio proceeds

smoothly in refluxing 2-methoxyethanol in the presence of a base to afford two complexes, one red and the other brownish-green, which have been subsequently separated by chrom-atography. The combined yield of the two complexes was high. These two types of complexes turned out to be significantly different in composition and properties (vide infra) and hence they are discussed below under two separate headings.

The red complexes [Os(PPh3)2(ap-R)Br]

The molecular structure of a representative member of this family, viz. that obtained from the reaction with H2ap-Cl, has

been determined by X-ray crystallography. A view of the com-plex molecule is shown in Fig. 1 and selected bond parameters are presented in Table 2. In the complex molecule, one ap-Cl ligand, two triphenylphosphines and one bromide are bound to

Table 1 Crystallographic data for [Os(PPh3)2(ap-Cl)Br] and

[Os(PPh3)(ap-Cl)(N–O)]

[Os(PPh3)2(ap-Cl)Br] [Os(PPh3)(ap-Cl)(N–O)]

Empirical formula Mw Crystal system Space group a/Å b/Å c/Å β/⬚ V/Å3 Z λ/Å Crystal size/mm T/K µ/mm⫺1 R indices GOF C49H39 BrClN2OsP2 1039.32 Monoclinic P21/n 12.2703(2) 19.1253(1) 19.2075(3) 107.742(1) 4293.1(1) 4 0.71073 0.40 × 0.20 × 0.4 295(2) 4.0776 R1= 0.0571a wR2= 0.1494b 1.045c C38.50H31.50Cl2N3O2OsP 859.23 Monoclinic C2/c 40.4327(2) 10.9195(2) 17.5688(3) 112.731(1) 7154.2(2) 8 0.71073 0.30 × 0.22 × 0.02 295(2) 3.797 R1= 0.0565a wR= 0.1355b 1.022c a_{R1= Σ |F} o|⫺ |Fc| /Σ|Fo|. bwR2=[Σ{w(Fo2⫺ Fc2)2}/Σ{w(Fo2)}]1/2. c_GOF=[Σ{w(F

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

osmium indicating the molecular composition to be [Os(PPh3)2

-(ap-Cl)Br]. Elemental (C,H,N) analytical data of all the red complexes agree well with the [Os(PPh3)2(ap-R)Br]

com-position. The ap-Cl ligand is coordinated in a tridentate fashion (4), the two PPh3 ligands are mutually trans and the bromide

ligand is trans to the coordinated azo-nitrogen. The CNOP2Br

coordination sphere around osmium is distorted octahedral in nature, which is reflected in all the bond parameters around osmium. Valid comparison of the observed Os–C distance with others has not been possible owing to the lack of prece-dence of such OsIII_{–C bonds. However, the observed Os}III_–C

length is slightly shorter than known OsII_{–C lengths,}12 _which

may be attributed to the difference in oxidation states of osmium. The Os–N, Os–O, Os–P and Os–Br distances are all normal, upon comparison with structurally characterized osmium() complexes containing these bonds.13 _{The spectral}

and electrochemical properties of all the [Os(PPh3)2(ap-R)Br]

complexes are similar (vide infra) and hence they are all assumed to have the same structure as [Os(PPh3)2(ap-Cl)Br].

Magnetic susceptibility measurements show that all the

[Os(PPh3)2(ap-R)Br] complexes are one-electron paramagnetic

species (µeff= 1.85–1.97 µB), which corresponds to the ⫹3 state

of osmium (low-spin d5_{, S= 1/2) in these complexes. It may be}

noted here that osmium has undergone a one-electron oxid-ation during the course of the synthetic reaction and in view of the osmium()–osmium() potential (vide infra) aerial oxygen appears to be the probable oxidant. Considering the

coordin-Fig. 1 View of the [Os(PPh3)2(ap-Cl)Br] molecule.

Table 2 Selected bond distances (Å) and bond angles (_{⬚) for}

[Os(PPh3)2(ap-Cl)Br] Os–P(1) Os–P(2) Os–C(1) Os–N(2) Os–O(1) Os–Br C(1)–C(2) C(2)–C(3) P(2)–Os–P(1) O(1)–Os–C(1) P(2)–Os–Br P(2)–Os–C(1) P(2)–Os–N(2) P(2)–Os–O(1) O(1)–Os–Br C(1)–Os–Br 2.400(2) 2.408(2) 2.034(8) 2.005(6) 2.130(6) 2.4686(14) 1.410(11) 1.367(12) 174.08(7) 153.6(3) 85.79(6) 91.7(2) 95.7(2) 88.8(2) 100.9(2) 105.5(2) C(3)–C(4) C(4)–C(5) C(5)–C(6) C(6)–C(1) C(6)–N(1) C(7)–N(2) C(13)–O(1) N(1)–N(2) N(2)–Os–Br P(1)–Os–Br P(1)–Os–C(1) P(1)–Os–N(2) P(1)–Os–O(1) N(2)–Os–O(1) N(2)–Os–C(1) 1.385(13) 1.382(13) 1.386(12) 1.422(11) 1.357(10) 1.427(10) 1.329(10) 1.301(9) 178.3(2) 88.29(6) 90.2(2) 90.2(2) 92.1(2) 78.2(2) 75.4(3)

ation sphere around osmium in the [Os(PPh3)2(ap-R)Br]

com-plexes, consisting of three non-equivalent P–Os–P, C–Os–O and N–Os–Br axes, rhombic EPR spectra may logically be expected. However, EPR spectra of all these complexes, recorded in 1 : 1 dichloromethane–toluene solution at 77 K, show anisotropic spectra with only two distinct resonances (g1

and g2). A representative spectrum is shown in Fig. 2. Though

the presence of the third resonance has not been detectable, the observed spectra do show a broad feature near 4200 G, which could be due to the third signal (g3). Non-appearance of the g3

signal in its usual shape is quite common in complexes of osmium().14 _{Electronic spectra of these complexes, recorded}

in acetonitrile solution, uniformly show two intense absorp-tions in the visible region together with one weak absorption near 700 nm (Table 3). The intense absorptions in the visible region are probably due to ligand-to-metal charge-transfer transitions. The weak transition near 700 nm may be assigned to one of the two possible transitions within the three split metal orbitals (components of the t2 orbitals).

Electrochemical properties of the [Os(PPh3)2(ap-R)Br]

com-plexes have been studied in acetonitrile solution (0.1 M TEAP) by cyclic voltammetry. Each complex shows a reductive response negative to SCE and an oxidative response positive to SCE (Table 3). The oxidative response, observed in the range 0.75–0.91 V (all potentials are referenced to SCE), is irreversible and is assigned to the osmium()–osmium() oxidation. The reductive response, observed in the range ⫺0.68 to ⫺0.74 V, is reversible with a peak-to-peak separation(∆Ep) of 60–70 mV

and is assigned to the osmium()–osmium() reduction. The one-electron stoichiometry of these responses has been estab-lished by comparing their current heights with that of the standard ferrocene/ferrocenium couple under identical exper-imental conditions. The potentials of both the metal-centered oxidation and reduction are found to increase linearly with increasing electron-withdrawing character of substituent R in the 2-(arylazo)phenolate ligand expressed in terms of Hammett substituent constant (σ)15 _{[σ values of the substituents are:}

OMe= ⫺0.27, Me = ⫺0.17, H = 0.00, Cl = 0.23, NO2= 0.78]. It

is interesting that a single substituent which is four bonds away from the electroactive metal center can influence the metal-centered redox potentials in a predictable manner.

Fig. 2 EPR spectrum and t2-splitting of [Os(PPh3)2(ap-H)Br] in 1 : 1

Table 3 Characterization data for the [Os(PPh3)2(ap-R)Br] complexes

Compound Electronic spectral dataa_λ

max/nm (ε/M⫺1 cm⫺1) Cyclic voltammetric dataa,b E₂₁/V (∆Ep/mV)

[Os(PPh3)2(ap-OMe)Br] [Os(PPh3)2(ap-Me)Br] [Os(PPh3)2(ap-H)Br] [Os(PPh3)2(ap-Cl)Br] [Os(PPh3)2(ap-NO2)Br] 676 (120), 480 (3900), 380 (3500), 328c _{(4800), 312 (6500)} 676 (530), 468 (4800), 400 (5000), 336 (6100), 316 (7400) 688 (380), 460 (3300), 400 (3800), 324c _{(7100), 308 (8600)} 684 (570), 460 (2300), 404 (2900), 304c _{(5300), 312 (5800)} 748 (370), 512 (1900), 432 (2300), 368c _{(3700), 312 (5900)} 0.75,d_{⫺0.74 (60)} 0.76,d_{⫺0.73 (60)} 0.79,d_{⫺0.72 (70)} 0.83,d_{⫺0.71 (60)} 0.91,d_{⫺0.68 (70)} a_{In acetonitrile solution.} b_{Supporting electrolyte TEAP; reference electrode SCE; E}

₂₁= 0.5(Epa⫹ Epc), where Epa and Epc are anodic and cathodic

peak potentials respectively; _∆Ep= Epa⫺ Epc; scan rate 50 mV s⫺1. cShoulder. dEpa value.

The brownish-green complexes [Os(PPh3)(ap-R)(N–O)]

The identity of these complexes has been revealed by structure determination of a representative member (obtained from reaction with H2ap-Cl) of this family. The structure is shown in

Fig. 3 and selected bond parameters are listed in Table 4. The structure shows that two 2-(arylazo)phenol ligands have inter-acted with the osmium center, but in different fashions. While one 2-(arylazo)phenol ligand is coordinated as a tridentate C,N,O donor ligand forming two five-membered chelate rings (4) as above, the second 2-(arylazo)phenol has undergone cleav-age across the N᎐᎐N bond and the iminophenol fragment, thus generated, is coordinated as a bidentate N,O donor ligand forming a five-membered chelate ring (5). The sixth coordin-ation site is occupied by one triphenylphosphine ligand. From the bond parameters around osmium, it is clear that the CN2O2P coordination sphere around osmium is distorted

octa-hedral in nature. Bond distances within the Os(C–N–O) chelate compare well with the structure of [Os(PPh3)2(ap-Cl)Br]. In the

bidentate iminophenolate fragment, the C(14)–O(2) and C(19)– N(3) distances lie between those expected for localized single and double bonds, while the C(15)–C(16) and C(17)–C(18) bonds are much shorter than the other four C–C bonds within the phenyl ring. All these data clearly indicate that this ligand is coordinated in the iminosemiquinonate form (5).16 _{All five}

complexes belonging to this family are therefore formulated as

[Os(PPh3)(ap-R)(N–O)], where N–O is the iminosemiquinonate

ligand. The observed microanalytical data for all these com-plexes are consistent with this formulation. As all the

[Os(PPh3)(ap-R)(N–O)] complexes display similar properties

(vide infra), the other four [Os(PPh3)(ap-R)(N–O)] complexes

are assumed to have the same structure as [Os(PPh3

)(ap-Cl)-(N–O)].

The mechanism of the N᎐᎐N bond cleavage is, as yet, not clear. However, coordination of the 2-(arylazo)phenolate ligand to osmium as a bidentate N,O-donor forming five-membered chelate ring (3) seems to be a probable step

Fig. 3 View of the [Os(PPh3)(ap-Cl)(N–O)] molecule.

preceding the N᎐᎐N bond cleavage. Coordination of 2-(arylazo)-phenolate ligands to osmium in this mode has been observed by us.2c _{While cleavage of the N}

᎐᎐N bond is a four-electron reduc-tion process, osmium underwent only one-electron oxidareduc-tion during the course of the synthetic reaction. The solvent (2-methoxyethanol) appears to serve as the source of necessary reducing equivalents. Indirect evidence for the involvement of solvent in the redox reaction comes from the fact that reaction carried out in tert-butyl alcohol does not yield the brownish-green product.

From the composition of these [Os(PPh3)(ap-R)(N–O)]

complexes it is clear that osmium is in the ⫹3 oxidation state (low-spin d5_{, S= 1/2). However, magnetic susceptibility}

meas-urements show that the [Os(PPh3)(ap-R)(N–O)] complexes are

diamagnetic. Antiferromagnetic interaction between the unpaired electron on osmium() and that on the iminosemi-quinonate radical appears to be responsible for the observed diamagnetism. Such antiferromagnetic interaction is quite common in transition metal complexes of quinones and related ligands.17 1_{H NMR spectra of the [Os(PPh}

3)(ap-R)(N–O)]

complexes have been recorded in CDCl3 solution. All five

com-plexes uniformly display two distinct methyl signals at δ ca. 2.3 and 1.2 which are assigned, respectively, to the methyl group in the p-cresol fragment of the tridentate ap-R ligand and the methyl group in the iminosemiquinonate ligand. The N–H sig-nal is observed in all the [Os(PPh3)(ap-R)(N–O)] complexes as

an isolated resonance at δ ca. 12. The aromatic region (δ 5.0– 8.0) is rather complex in nature owing to overlap of signals arising from all the three types of ligands. Therefore assignment of these signals to specific protons has not been possible. How-ever, intensity measurement of the signals corresponds well with the total number of aromatic protons present in the respective complexes. 13_{C NMR spectra of the [Os(PPh}

3

)-(ap-R)(N–O)] complexes have also been recorded in CDCl3

solution. The expected number of signals are observed in all the complexes. The two methyl carbons, one in the p-cresol fragment of the ap-R ligand and the other in the N–O ligand,

Table 4 Selected bond distances (Å) and bond angles (_{⬚) for}

[Os(PPh3)(ap-Cl)(N–O)]ⴢ0.5CH2Cl2 Os–P(1) Os–O(1) Os–N(1) Os–C(12) Os–O(2) Os–N(3) C(19)–N(3) C(19)–C(18) C(18)–C(17) P(1)–Os–O(2) O(1)–Os–C(12) P(1)–Os–O(1) O(1)–Os–N(1) N(1)–Os–C(2) C(12)–Os–O(2) O(2)–Os–N(3) N(3)–Os–P(1) 2.341(2) 2.075(5) 2.045(6) 2.056(7) 2.074(5) 1.933(7) 1.414(10) 1.397(12) 1.36(2) 166.2(2) 150.4(3) 85.7(2) 77.3(2) 75.5(3) 101.5(3) 78.9(3) 92.0(2) C(17)–C(16) C(16)–C(15) C(15)–C(14) C(14)–O(2) C(12)–C(7) C(7)–N(2) N(2)–N(1) C(1)–O(1) C(6)–N(1) N(1)–Os–N(3) P(1)–Os–N(1) P(1)–Os–C(12) P(1)–Os–O(2) P(1)–Os–N(3) C(19)–N(3)–Os C(14)–O(2)–Os 1.42(2) 1.34(2) 1.419(11) 1.303(10) 1.434(11) 1.405(10) 1.303(8) 1.315(10) 1.426(10) 161.3(2) 103.3(2) 89.3(2) 166.2(2) 92.0(2) 117.0(6) 113.7(5)

Table 5 Characterization data for the [Os(PPh3)(ap-R)(N–O)] complexes

Compound Electronic spectral dataa_λ

max/nm (ε/M⫺1 cm⫺1) Cyclic voltammetric dataa,b E₂₁/V (∆Ep/mV)

[Os(PPh3)(ap-OMe)(N–O)]

[Os(PPh3)(ap-Me)(N–O)]

[Os(PPh3)(ap-H)(N–O)]

[Os(PPh3)(ap-Cl)(N–O)]

[Os(PPh3)(ap-NO2)(N–O)]

600 (2700), 393 (9500), 362 (11500) 600 (3900), 401 (10300), 353 (12900) 585 (3400), 410 (10200), 345 (11600) 590 (4800), 406 (12200), 353 (14300) 569 (5400), 436 (9700), 366 (10800) ⫺0.96 (60), 0.58 (60), 1.10,c _1.60c ⫺0.95 (60), 0.60 (60), 1.10,c _1.68c ⫺0.92 (60), 0.62 (60), 1.12,c _1.68c ⫺0.88 (60), 0.67 (60), 1.12,c _1.70c ⫺0.78 (60), 0.78 (60), 1.30,c _1.76c a_{In acetonitrile solution.} b_{Supporting electrolyte TEAP; reference electrode SCE; E}

₂₁= 0.5 (Epa⫹ Epc), where Epa and Epc are anodic and cathodic

peak potentials respectively; _∆Ep= Epa⫺ Epc; scan rate 50 mV s⫺1. cEpa value.

are respectively observed at δ ca. 20.4 and 20.9. The aromatic carbons are observed in the range δ 115–175, of which the most downfield signal (δ ca. 174) is assigned to the metallated carbon. Electronic spectra of the [Os(PPh3)(ap-R)(N–O)]

complexes have been recorded in acetonitrile solution. Each complex shows intense absorptions in the visible region (Table 5) which are probably due to allowed charge-transfer trans-itions involving both metal and ligand orbitals.

The electrochemical properties of the [Os(PPh3)(ap-R)(N–

O)] complexes have been studied by cyclic voltammetry in acetonitrile solution (0.1 M TEAP). Each complex shows three oxidative responses positive to SCE and a reductive response negative to SCE (Table 5). The first oxidative response, observed in the range 0.58–0.78 V, is reversible in nature (∆Ep= 60 mV) and is tentatively assigned to the osmium()–

osmium() oxidation. The reductive response, displayed within ⫺0.96 to ⫺0.78 V, is also reversible (∆Ep= 60 mV) and is

assumed to be an osmium()–osmium() reduction. The one-electron nature of these couples has been established by com-paring their current heights with those of the standard ferrocene/ferrocenium couple under identical experimental conditions. The potential of both the metal-centered oxidation and reduction is found to correlate linearly with the electron-withdrawing character (σ) of the substituent R in the ap-R ligand. Two irreversible oxidations are also displayed by all the

[Os(PPh3)(ap-R)(N–O)] complexes in the range 1.10–1.76 V,

but the sites of these oxidations are not clear.

Conclusion

The present study shows that 2-(arylazo)phenols (1) undergo two types of interesting chemical transformation upon reac-tion with [Os(PPh3)3Br2], viz. osmium()–carbon bond

form-ation yielding cyclometallates of osmium (2) and N᎐᎐N bond cleavage affording iminosemiquinonate chelates of osmium (3). The organoosmium fragment (2) appears suitable for studying reactivities of the Os–C bond and such studies are currently in progress.

Acknowledgements

Financial assistance received from the Council of Scientific and Industrial Research, New Delhi [Grant No. 01(1408)/96/ EMR-II] and the Department of Science and Technology, New Delhi [Grant No. SP/S1/F33/98] is gratefully acknowledged. We thank the Third World Academy of Sciences for financial support enabling the purchase of an electrochemical cell system. Thanks are also due to Dr Falguni Basuli for her help.

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