Indian Journal of Chemistry Vol. 41 A, July 2002, pp. 1374- 1379

Isomeric mixed dioxolene 2-(arylazo)pyridine complexes of osmium(II): Synthesis, spectra and electrochemistry

Doyel Bose, Hafijur Rahaman, Jaya Banerjee, Naba Kumar Mondal & Barindra Kumar Ghosh*

Department of Chemi stry , The Uni vers ity of Burdw3n, Burdwan 7 13 104, Indi a

Received 22 Janlla l), 2002; revised 11 March 2002

The complcxes blue-violet (3) and red-v iolet (4) isomers of rOsX2L2] [X = CI or Br; L (2) = 2-(phenylazo)pyridine (LI) and 2-(III -tolylazo)py ri dine (L2)1 react smoothly with catechol and its substitu ted derivati ve [H2Q; (1)] ill boil ing aq ueous ethanol affording mi xed tri s complexes of the type [OsQL2] [(5) and (6), respect ively]. The new complexes have been charac terized by physicochcmi cal, spcctroscopic and magnctic methods. From IH NMR spectra (taki ng 8Mc of L~ as diagnosti c probe), the stercoretentivity of the reacti ons is proposed. The di amagnetic complexes display a number of spin­al lowed and spin-forbidden charge- transfer transitions in thc 1200-200 nm range. Both (5) and (6) are electroact ive and di spl ay six voltammetri c responses - two ox idati ons and fo ur reductions. EPR study of the coul ometri call y generated one­electron ox idation prod uct suggests the in volvement of semi quinone radical formation.

The quinone ligand system [H2Q; 1] has received much attention in the past few years l

.6 due to

structural chracteri zation, identi fication of effecti ve ox idation levels [hydroquinone (hq), la; semiquinone (sq), 1 b and quinone (q), Ie] , rich electrochemistry and their role as key compounds in photosynthesis and respiratory electron-transport chains. The results show3 that la stabili ses low ox idation state, Ib medium ox idati on state and Ie high oxidation state of metal. The ru thenium and osmium chemistry of Q in combination with 2,2'-bipyridine (bpy)3.5 containing diimine (-N=C-C=N-) function is well-established. Chemi try of Q in combination with 2-(arylazo)pyridines (L; 2) containing azoi mine (-N=N­C=N-) chromophore (i soelectronic with diimine function in bpy) in ruthenium6 bound state is reported. However, corresponding osmium chemistry remains unexplored. As a part of our programme7

.9 on the

reaction behaviour of isomeric [OsX2L2] (X = Cl or Br) [c is, trans, cis; ctc (3) and cis, cis, cis; ccc (4)] towards different mono- and bidentate ligands, we have examined here reacti vity of (3) and (4) towards two quinone li gands, catechol (H2Q 1

) and tetrachlorocatechol (H2Q2). Successful synthesis of the isomeric complexes of the type [OSQL2] (5 and 6) via stereoretenti ve reaction of [OsX2L2] [3 and 4, respec ti vely] with H2Q and their spectroscopic and electrochemical properti es are described below.

Materials and Methods Azoimine li gands (L) and isomeric [OsX2L2] (X =

Cl or Br) complexes were prepared by the literature

R'= H; Q1

R' = 3,4.5.6- tl'trach lora; Q 2

ctc

R2 = H ; L'

R 2 = Ml' i L2

L; 2

tee

3 X = CI. or Br It

etc ccc

5 6

BOSE el at .: ISOMERIC MIX ED COMPLEXES OF OSMIUM(II ) 1375

method7. Purification of solvent (MeCN) and

preparation of the supporting electrolyte ([Et4N][CI04]) for electrochemical work were executed following published procedure 10. Catechols and all other chemicals and solvents were Of analytical grade and were used without further purification. The synthetic reactions were carried out under dry dinitrogen and final work-up was done in I · air.

Electrical conductivities (in MeCN, solute _10-3 M), electronic (in MeCN, solute _ 10-3 M) , and IR spectra (KBr di scs, 4000-600 cm-l

; polyethylene discs, 600-300 cm-I) and IH NMR (in COCl3; Me4Si as internal standard) spectra were recorded using a Philips PR 9500 conducti vity bridge, Hitachi 330/Shimadzu UY 160A spectrophotometer and Perkin-Elmer 783 spectrometer and Brucker 270 MHz spectrometers, respectively . Magnetic susceptibilities were measured with a PAR 155 vibrating sample magnetometer. EPR spectra were run on a Yarian 109C E-line X-band Spectrometer fitted with a quartz Dewar for measurements at 77 K (liquid nitrogen); all spectra were calibrated with 1, I-dipenyl-2-picryl hydrazyl (g = 2.0037). Microanalyses (C, H and N) were done wi th a Perkin-Elmer 240C elemental analyser. Electrochemical studies were performed (in MeCN at 298 K) under dry N2 with a PAR 370-4 electrochemistry system as described elsewhere8

. In cyclic voltammetry (CY), the following parameters and relations were used: scan rate (v), 50 mYs-l; formal potential EO = 0.5 (Epo + Epc) where Epa and Epc are anodic and cathodic peak potentials, respectively; I':.E" is the peak-to-peak separation. In differential pulse voltammetry (DPY): scan rate (v), 10 mYs· l; modul ation amplitude (!1E), 25 mY ; ~ = Ep + 0.5(!1E) where EI' is the OPY peak potential. The agreement between ~ data obtained by the two techniques was good (within ±5 mY). The potentials are referenced to a saturated calomel electrode (SCE) and are uncolTected for the junction contributions.

Preparation of complexes The isomeric red-violet [OsQL2] [(5a-5d) and (6a-

6d)] co mplexes were prepared using a general procedure. Yields varied in the range 60-65%. Detail s for a particular compou nd are given below.

Cis-trans-cis-{OsQI LI2l (Sa) 0. 15 g (1.36 mmol) H2Q I was added to a

suspension of 0. 1 g (0.139 mmol) of cis, trans, cis­[OsBr2LI 2] and 0.14 g (1.4 mmol) ofCaC0 3 in 30 ml

EtOH-H20 (2: I). The mixture was heated to reflux under dinitrogen atmosphere for 6 h. The initial blue­violet solution changed to red which was filtered through a fine glass-frit and was evaporated under reduced pressure. The residue was washed copiously with Et20 and redi ssolved in a minimum of dichloromethane and was loaded on a silica gel (60-120 mesh) column (20 x 1 cm). A small blue-violet band was eluted first with dichloromethane. Finally, the slow moving red band of desired compound was collected with PhH-MeCN (4: I). Slow evaporation of thi s solution resulted in dark red crystals that were separated by filtration, washed with chilled H20 and dried in vacuo over P40 lO to yield analytically pure (5a); yield, 0.055 g (60%). Pure (5b)-(5d) and (6a)­(6d) were prepared similarly .

Results and Discussion Reaction of [OsX2L2] with an excess of H2Q, a

dibas ic acid, proceeds slowly but smoothly in boiling aqueous ethanol according to Eq. (1),

[OsX2L2] --------7) [OSQL2] (3/4) EtOH-H20 (5/6)

+ other products ... ( I)

The presence of CaC03 as a heterogeneous basel I allows completion of reaction 1 within a reasonable time. To hasten the reacti on H2Q was used in five-fo ld excess. Of the two 3 and 4, the latter is more reacti ve. With tetrachlorocatecholate, the reaction time is shorter and even in the absence of a base the reaction completes, which is ev identl y due to greater ac idity of H2Q2 over H2QI. By changing the solvent matrix fro m aqueous ethanol to aq ueous 2-methoxyethanol, reaction 1 proceeds much faster, but isomeric homogenity (from IH-NMR results ; see below) is lost; a mixture of 5 and 6 is obtainable from either 3 or 4. The presence of high boiling (b. p., 398 K) solvent (2-methoxyethanol) may presu mabl y be responsible fo r the reorganization 7 in OSL2 coordination frame.

The complexes were characteri zed uS1l1g microanalytical, spectroscopic, magnetic and electrochemical results (Tables 1-3). The air-stable moisture-insensitive complexes are soluble in a range of common organic solvents, such as dichloromethane, acetonitrile, methanol, ethanol but are insoluble in water. In acetonitri le solutions, they behave l2 as non-electro lytes as indicated by their very low AM values (0-5 n-I cm2 mOrl). Room-temperatu re

1376 INDIAN J CHEM, SEC A, JUL Y 2002

solid-phase magnetic susceptibil ity measurements show that all the complexes are uniformly diamagnetic (t2g (ref. 6) idealised, S = 0).

In IR spectra, v(C=N) and v(N=N) modes of the complexes L are observed at -1590 cm' l and >1280 cm,l, respectively . The shift of v(N=N) towards higher frequency (Table 1) in going from [OsX2L2] to

[OSQL2] complexes suggests less osmium-azo IT-back bonding in the latter. The better IT-acceptance behaviour of Q over X results in less IT-back donation6

.13 towards azo fragment which is the

effective TC-acceptance centre l4 in L. Of types 5 and 6 complexes, the latter show v(N=N) frequency more towards lower energy reflecting better IT-acceptance of L in ccc geometry over that of etc geometry as was the case7a in 3 [v(N=N), - 1280 cm'l] and 4 [v(N=N), -1275 cm' l]. The typ ical metal bound v(C- O) frequency of Q is seen at -1100 cm'l. All other characteristic L vibrations are invariably seen in the 1600-600 cm' l range.

IH NMR investigations of selected complexes were made in order to solve the stereochemical questions in these mixed-tris complexes. The methyl signal (OMe) is particularly usefu l in diagnosing the geometry of [OsL2f+ moiety . Re levant resul ts are shown in Tab le I and Fig. I . A single sharp methyl signal at

-2.2 ppm is observed for [OsQL\ ] (Q = QI and Q2) complexes obtained from 3 via reaction I . Thus, the two-fold axis of 3 is retained in the pm duct 5 . Mixed­tris [OsQL\ ] (6) complexes resulting from the reaction of H2Q and 4 show two equally intense methy l signals at -2. 15 and -2.30 ppm (Fig. 1) demonstrating that [OSL\ ]2+ group has the same

symmetry as in 4. Thus, the transformations 3~5 and4~6 in reaction 1 proceed in a stereoretentive

( a) (b) ( c )

2.4 2.0 2.4 2.0 2.4 2.0

6( p pm)

Fig. I_ IH NMR methyl s ignal s (in CDCI}) of (a) clc-[OsQ IL221 (5b), (b) ccc- fOsQIL221 (6b) and (c) mixture of 5b and 6b

Table I- Analytical data. and selected IR (c m- I) and IH NMR resulis

Compd/ Found (Calcd)% IRa IH MRd

Mol formula C H N v(C=C) + v(N=N )C v(C-O)" OMc(ppm) v(C=N)"

clc- [OsQILI 21 (Sa) 50 .8 3.3 12.3 1592 1288 11 02

C2sHn N60 20 S (50.6) (3 .3) ( 12.6)

eee- [OsQILI 21 (6a) 50.5 3.2 12.6 1590 1285 1102

C2sHn N60 20 S (50.6) (3. 3) ( 12.6)

ele-IOsQ IL\ 1 (5b) 51.7 3.6 11 .8 1590 1290 1100 2.23

C.1o H2(,N60 2OS (52 .0) (3.8) (12.1 )

eee- /OsQIL221 (6b) 52.2 3.8 12.3 1588 1286 1102 2. 16,

C30H26 N60 20S (52 .0) (3.8) ( 12.1 ) 2.29

ele-I OsQ2L 121 (5c) 42 .2 2.5 10.4 1588 1288 1102

C2x H IHN60 2CI40 s (4 1.9) (2.3) ( 10.4)

ece-fO sQ2Ll 21 (6c) 41.7 2.3 10.1 1592 1285 1104

C2gH IxN602CI40s (41.9) (2.3) ( 10.4)

ele- fOsQ2L221 (5d) 43.4 2.9 10.4 1590 1290 1103 2.29

C.1o H22 N60 2CI40s (43.4) (2.7) ( 10. 1 )

eee- fOsQ2L221 (6d) 43.6 2.9 9.8 1590 1286 11 04 2 .2 1,

C30H22 N60 2C140 s (43.4) (2.7) (10.1 ) 2.35

"In KBr di scs; bMedium and sharp; cSharp and strong; dSolvent. CDCI}

BOSE et al.: ISOMERIC MIX ED COMPLEXES OF OSMIUM(II) 1377

manner. It is logical to assume that in L I complexes also, the gross geometry is same. Three signal s are invariably seen in the case of the products obtained from ei ther 3 or 4 and H2Q in aqueous 2-methoxyethanol. Two (at -2.15 and -2.3 ppm) of the three signals have equal peak heights (Fig. 1). This implies that OSL22+ moiety is certainly present both in trans, cis (one signal) and cis, cis (two signals) geometry in the orientation (W ,W) and (N",Na

)

respectively. Evidently, cis, cis is the major component. This is in consonance with the fact that during substitution (X by 0 ), extensive stereochemical rearrangement occurs. A similar observation was found for tris chelate [OsL3f + formation?b from 3 in such severe reaction condition .

The complexes exhibit several absorption bands and shoulders in the 1200-200 nm region (Table 2). Though soluti on colour of the complexes 5 and 6 with a particul ar Q is the same (orange-red), the recorded spectra have sharp difference. A characteristic feature of the orange-red solutions is an intense band at -550 nm with a shoulder at -425 nm. Weak absorption

bands at lower energies are also observable in all complexes . A blue shift of these absorptions is found in going from 5 to 6. The transitions are ass igned to metal-to-ligand charge-transfer (MLCT) transitions within the framework of pseudo-octahedral osmium(lI) geometry. The more intense bands at hi gher energies are presumably due to spin-allowed transitions, whereas the weaker bands at hi gher wavelength could be due to spin-forbidden transiti ons being allowed as a result of strong spin-orbit couplingls.l? in heavy metals like osmium. The transitions below 400 nm could be due6.? to

intraligand n-n* and n-n* transiti ons. The electroactivi ty of the complexes was examined

in MeCN solutions using voltammetry (CY and DPY) and coulometry at platinum and glassy carbon electrodes. The DPY technique is particularly useful for observing responses close to so lvent cut-off regions. The results are summari zed in Table 3. The complexes show two ox idati ve responses at -0.4-0.9 V and -0.9-1 .3 V ranges. The two responses rEqs (2) and (3)] are reproducible with no trace of

Compd

Table 2-Electronic spectral data,,·b at 298 K

Am"" nm (E, U l cm· l)

Sa

6a

5b

6b

5c

6c

5d

6d

111 0 (790), 960c ( 1,620), 805 (2,510), 715c (2,090), 560 (7,480), 440c (6,950), 372 (14.4 10), 310 (2 1,250)

I 102 (820), 955c ( 1,680), 795 (2,560). 710" (2,250), 555 (7,520), 425c (7, 190), 358c ( 14.6 10), 3 10 (2 1,290)

II IS (840), 965c ( 1,670), 805 (2,670), 7 15c (2,270), 560 (7 ,700), 440c (7,300), 375c ( 14,700), 3 10 (2 1,780)

11 05 (860) , 958c ( 1,770) ,795 (2,780), 71 Oc (2,340), 555 (7,790), 430c (7,420), 360c ( 14,870), 3 10 (2 1,9 10)

1075 ( 1,050), 942c ( 1,930). 773 (3,0 10), 690c (2,570), 540 (7,870) , 420c (7 ,520). 350 ( 14,8 10), 3 10 (2 1,950)

1066 (1,160), 935c (2,040), 762 (3,350), 678 (2,650), 535 (7,840), 406 (7,640), 350c ( 14.920), 309 (22,2 10)

1080 (1,120), 945< (2,040) , 775 (3,400), 690c (2,690), 540 (8,0 I 0), 420c (7,800). 355< ( 15, 120), 3 10 (22 ,800)

1070 (1,240), 940c (2, 110), 765 (3,520), 680c (2,730), 535 (8, 190). 410c (7 ,890), 350c ( 15 ,300), 308 (22,840)

"Solvent, MeCN ; bSolute concentration, - 10.3 M; cShoulder.

Table 3-Electrochemical result s,,·b in MeCN at 298 K

Compd £lm ,Y(tJ.Ep, mY) Os"ll" c q/sqC d d r d rl c rl r1 3

Sa 1.0 I (100) 0.39(60) -0.43(60) -0.80(90) -1 .48( 120) - 2.02

6a 0.99( 100) 0.39(60) -0.4 1 (60) -0.78(90) - 1.46( 120) -2.00

5b 1.00( I 00) 0.39r.g(60) -0.45(60) -0.82(90) - I A9( 120) - 2.0 1

6b 0.98( I 00) 0.39(60) -OA3(60) -0.80(90) -IA6( 120) -2.03

5c 1.20( I 10) 0.8i·h(60) -0.36(60) -0.75(90) - I A2( 11 0) - 1.9 1

6c 1.1 8( I 10) 0.82(60) -0.34(60) -0.72(90) - IAO( 11 0) -1.92

5d 1.1 9( II 0) 0.82(60) -0.36(60) - 0.74(90) - IA I ( 11 0) -1.90

6d 1.16( I 10) 0.82(60) -0.33(60) -0.73(90) -1.39( 110) - 1.9 1

' Meaning and units of symbols are the same as in the tex t; bUn less otherwi se stated both CY and DPY results are set; cWork ing eleclrode. platinum; dWorking electrode, glassy carbon; COnly DPY results; fConstant potenti al coulometry (oxidation done at pOlential EO + 200 mY) : n = Q/Q' where Q' is the calculated coulomb cou nt for one-electron transfer and Q is the coul omb count found after exhaustive electro lysis of 0.01 mmol of the solute; gn = 1.0 I; hn = 1.03

1378 INDIAN J CHEM, SEC A, JUL Y 2002

decomposition after a number of cycles. One-electron nature of couple 2 was confirmed from.

[OSQL2t + e' ~ [OSQL2J

[OsQL2J2+ + e' ~ [OsQL2t

... (2)

... (3)

constant-potential electrolysis and that of 3 from a comparison of current height with couple 2 in DPY experiments. The substituent on Q has strong effect on the potential of couple 2 reflecting involvement of hydroquinone to semiquinone oxidation . The EPR spectrum of coulometrically one-electron oxidised product of 5/6 in frozen (77 K) dichloromethane glass displays almost isotropic nature with g value close to 2 (Fig. 2) that corroborates this assignment. It is well-established5d that complexes containing semiquinone ligands that are not magnetically coupled with other paramagnetic centres in the molecule generally have g values close to 2. Here, the

IDPPH

3200 3300 3400

Fi g. 2- EPR spectrum of cou lometrica lly one-electron ox idi sed product of [OSQI L 221 in CH2Ci2 g lass (77 K) (DPPH = 1, 1-di phcnyl-2-picry Ihydrazy I)

excellent n-acceptor L stabilises osmium(II) state. The potential of couple 3 is not so sensitive (Table 3) from a change of substituent on Q network.

The negative side of SCE was scanned using a glassy carbon electrode. The LUMO (n* level) of L can accommodate3 up to two electrons. Four successive one-electron reductions are, therefore , expected for OSL22+ moiety . All the one-electron (on comparison of current height with couple 1) reductions (rl - r4) are, however, observable within the available potenti al window. These involve azo reductions. The first two are reversible in CY whereas lower current heights of the responses at more negative potentials indicate that three/four-electron reduced products are not stable and some kind of chemical assistances 18 are connected with the charge­transfer steps (electrochemical-chemical, EC) . The substituents in Q have almost no effect on the potentials of these electrode processes indicating reaction centres far from Q.

We conclude that we are successfu l in isolating isomeric [OSQL2J complexes. The compounds are formed in stereoretentive routes. They show rich electronic spectra - very much characteristic of two different isomeric entities. Within the avail able potential window the complexes display rich electrochemistry pertaining electron transfers from/to Q, L and the metal: (a) hydroquinone to semiquinone oxidation, (b) osmium(II) to osmium(lII) transformation, and (c) four successive L-based reductions. Thus, a nearly-complete electron transfer series3 is formed. The presence of strong n-acceptor L stabilises osmium(II)-hydroquinone state; however, osmium(II)-semiquinone can be generated in solution and is characterised by conjunctive benefit of spectroscopic and electrochemical studi es.

Acknowledgement Financial support from the CSIR and the UGC,

New Delhi is gratefully acknowledged . We are also indebted to Prof. A Chakravorty, Kolkata for spectroscopic and electrochemical work . DB and HR are respectively thankfu.l to CSIR and UGC for their fellowships.

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BOSE et al.: ISOMERIC MIXED COMPLEXES OF OSMIUM(I1 ) 1379

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