Osmium–azopyrimidine chemistry. Part VII: synthesis, structural characterisation and electrochemistry

Osmium–azopyrimidine chemistry. Part VII: synthesis, structural characterisation and electrochemistry

Polyhedron 21 (2002) 753 /762 www.elsevier.com/locate/poly Osmium azopyrimidine chemistry. Part VII: synthesis, structural characterisation and ele...

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Polyhedron 21 (2002) 753 /762 www.elsevier.com/locate/poly

Osmium azopyrimidine chemistry. Part VII: synthesis, structural characterisation and electrochemistry /

S. Senapoti a, U.S. Ray a, P.K. Santra a, C. Sinha a,*, Alexandra M.Z. Slawin b, J. Derek Woollins b a b

Department of Chemistry, The University of Burdwan, Burdwan 713104, India Department of Chemistry, University of St Andrews, St Andrews KY16 9ST, UK Received 14 June 2001; accepted 10 December 2001

Abstract Reaction of 2-(arylazo)pyrimidines (R/C6H4 /N /N /C4H3N2, abbreviated aapm; R /H (papm), o -Me (o -tapm), m -Me (m tapm), p -Me (p -tapm), p -Cl (p -Clpapm)) with (NH4)2[OsCl6] in 2-methoxyethanol gives two isomers of composition OsCl2(aapm)2. They are structurally characterised by 1H NMR spectra and established as the cis /trans /cis (ctc ) and cis /cis /cis (ccc ) isomer. With reference to the coordination pairs of Cl; N(pyrimidine) (N) and N(azo) (N?), the structure of ctc -OsCl2(p -tapm)2 is confirmed by an X-ray diffraction study. The complexes exhibit multiple MLCT transitions in the visible to near-IR region. Redox studies show an Os(III)/Os(II) couple at 1.2 /1.3 V and Os(IV)/Os(III) couple at /2 V versus SCE. EHMO calculations and comparison with analogous ruthenium(II) complexes explain the spectral and redox properties of the complexes. # 2002 Published by Elsevier Science Ltd. Keywords: Arylazopyrimidines; Osmium(II) complexes; Geometrical isomers; X-ray structures; MLCT transitions; Redox property

1. Introduction The coordination complexes of heterocycles continues to be of attraction and their applications encompass catalysis, organic synthesis and bioinorganic chemistry [1,2]. Incorporation of an arylazo group into the heterocycle backbone gives arylazoheterocycles [3 /33]. They have been used to stabilise low-valent metal redox states [3 /20]; oxo-metal complexes [21 /23], for the purpose of colouring and dyeing agents [24,25], metallo-chromic indicators in analytical chemistry [26 /28] and some of them exhibit novel electrochemistry or nonlinear optical properties [4,10,29,30]. The heterocycle moiety may be hooked to polymeric bed by an azo



For Part VI, see Ref. [33]. * Corresponding author. Tel.: 91-342-557-683; fax: 91-342-564452. E-mail address: [email protected] (C. Sinha).

group and has been used in micro scale solid-phase extraction process [31,32]. Electronic property and pacidity of arylazoheterocycles have largely been dependent on the nature and number of heteroatoms, ring size and the substituent type in the rings [34,35]. With this background, we have initiated a programme to study the chemistry of the azoimine function in the pyrimidine heterocycle [16 /19]. It is chosen because of its higher pacidity than conventional widely used pyridine bases [34,35] and its biochemical importance [1,36,37]. The ruthenium chemistry of arylazopyrimidines has already been developed by our group [17]. In this report we describe the synthesis, spectral studies, redox property and single-crystal X-ray structure of one of the isomers of osmium(II) complexes of 2-(arylazo)pyrimidines (aapm). The electronic properties of osmium(II) complexes are different from that of ruthenium(II) /arylazopyrimidine [17] complexes and have been interpreted from theoretical calculations using EHMO approximation.

0277-5387/02/$ - see front matter # 2002 Published by Elsevier Science Ltd. PII: S 0 2 7 7 - 5 3 8 7 ( 0 2 ) 0 0 8 3 3 - 1

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S. Senapoti et al. / Polyhedron 21 (2002) 753 /762

2. Experimental 2.1. Materials Osmium tetroxide was purchased from Johnson Matthey. (NH4)2[OsCl6] and 2-(arylazo)pyrimidine were prepared following the literature procedures [6,38]. All other chemicals, solvents, their purification process used for different measurements, etc. were described in Ref. [17]. 2.2. Preparation of complexes: ctc and ccc -dichloro-bis[2-(phenylazo)pyrimidine]osmium(II), OsCl2(papm)2 (blue /violet ctc (2a); red /violet ccc (3a)) To brown /red solution of (NH4)2[OsCl6] (0.5 g, 1.14 mmol) in 2-methoxyethanol (50 cm3) nitrogen gas was bubbled continuously and refluxed on an oil-bath for 20 min. 2-(Phenylazo)pyrimidine (0.44 g, 2.4 mmol) in ethanol (10 cm3) was added in drops to this refluxing solution over another half-an hour. The mixture was refluxed under nitrogen with magnetic stirring for 24 h. During this period, the solution turned brown /violet to blue /violet. This was concentrated to about 20 cm3 by slow bubbling of N2 gas. The solution was cooled to room temperature and kept in refrigerator overnight. The shining dark coloured crystalline precipitate was collected by filtration and washed with ethanol /water (1:1, v/v) and dried over P4O10. The dried product was dissolved in a small volume of CH2Cl2 and was chromatographed on an alumina (neutral) column. A small portion of orange /red band was eluted with benzene and rejected. Blue /violet band was eluted using 4:1 (v/v) benzene /acetonitrile. Red /violet band was eluted by acetonitrile. The solutions were collected separately and evaporated slowly in air. The crystals so obtained were dried over P4O10. The yields were of blue /violet ctc -OsCl2(papm)2 (2a), 38% and red-violet, ccc -OsCl2(papm)2 (3a), 11%. All other complexes were prepared by following the identical procedure and the yields were varied between 30 /50% for the blue /violet isomers and 10 /15% for the red /violet isomers. 2.3. X-ray crystallography Crystal that was suitable for X-ray diffraction study of ctc -OsCl2(p-tapm)2 (2d) was grown by slow diffusion of hexane into a dichloromethane solution at 298 K (crystal size (mm): 0.2 /0.2 /0.1). Data were collected on a Siemens SMART CCD diffractometer with graphite monochromatised Mo Ka radiation (l/0.71073 ˚ ) at 293 K. Crystal data and collection parameters are A listed in Table 1. The data were corrected for absorption effects by empirical method using azimuthal scan data. Systematic absences led to the identification of space

Table 1 Crystal data and structure refinement parameters for ctc -Os(p tapm)2Cl2 (2d) Empirical formula Formula weight Crystal system Temperature Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a (8) b (8) g (8) ˚ 3) V (A ˚) l (A Dcalc (g cm 3) Z m (mm1) R a wR b Goodness-of-fit c

C22H20Cl2N8Os 657.56 triclinic 293 (2) P 1¯ 10.0300(4) 10.2587(4) 12.2855(4) 96.387(2) 101.029(2) 90.522(2) 1232.47(8) 0.71073 1.77 2 5.417 0.0434 0.0977 0.878

a

R ajFoFcj/aFo. wR  [aw (Fo2Fc 2)/awFo4]1/2, w 1/[s 2(Fo2)(0.0604P )2], P  (Fo22Fc 2)/3. c Goodness-of-fit is defined as [w (FoFc)/(nonv)]1/2 where no and nv denote the number of data and variables, respectively. b

group P1¯. Of 7753 collected reflections 3530 with I / 2s (I ) were used for the structure solution. The structure was solved by the conventional heavy-atom method and was refined by the full-matrix least-squares method on all Fo2 data using SHELXTL 5.03 package on silicon Graphics Indigo-R4000 computers. Hydrogen atoms were included in calculated positions and refined with isotropic thermal parameters which were 1.2 aromatic (H) times the equivalent isotropic thermal parameters of their parent carbon atoms.

3. Results and discussion

3.1. Synthesis and assignment of isomers 2-(Arylazo)pyrimidines (aapm, 1) possess the azoimine, /N /N/C /N/, functional group and the donor centres are N(pyrimidine), N and N?. Analogues that have been used in this work are 2-(phenylazo)pyrimidine (papm, R /H, 1a), 2-(o-tolylazo)pyrimidine (o -tapm, R /8-Me, 1b), 2-(m -tolylazo)pyrimidine (m -tapm, R / 9-Me, 1c), 2-(p -tolylazo)pyrimidine (p-tapm, R /10Me, 1d), 2-(p-chlorophenylazo)pyrimidine (p -Clpapm, R /10-Cl, 1e).

S. Senapoti et al. / Polyhedron 21 (2002) 753 /762

755

Fig. 1. Crystal structure and atom labelling scheme for ctc -OsCl2(ptapm)2 (2d).

The reaction of aapm with (NH4)2[OsCl6] in 2methoxyethanol/ethanol proceeds shortly under refluxing condition and upon concentrating dark-coloured crystals of composition OsCl2(aapm)2 (2/3) were isolated (Eq. (1)). OsCl 62 2aapm

0 OsCl2 (aapm)2 4Cl



(1)

The product was purified by chromatographic separation on alumina (neutral) bed to give blue /violet (isomer A) and red /violet (isomer B) complexes. The alcoholic solvent may be responsible [6] for the reduction of Os(IV) to Os(II) in Eq. (1). Microanalytical (Table 3) data support the composition of the complexes OsCl2(aapm)2. Blue /violet and red/violet isomers are assigned to 2 and 3, respectively. The complexes are diamagnetic in the crystalline state and are non-electrolytic in nitromethane and acetonitrile. Isomers are soluble in common organic solvents to give blue /violet (for 2) and red/violet (for 3) solution. The blue /violet isomer (OsCl2(p-tapm)2 (2d)) has been characterised by X-ray crystallographic method. 3.2. Structure of blue /violet OsCl2(p -tapm)2 (2d) A view of the molecule is shown in Fig. 1. The coordination around osmium is approximately octahedral. The atomic arrangement involves sequentially cis chlorine, trans -N(pyrimidine) and cis -N(azo) and is abbreviated cis /trans /cis (ctc ) configuration (C2-symmetry). Selected bond parameters are listed in Table 2. Two atomic groups Os, Cl(1), N(22), N(27), N(2) and Os, Cl(2), N(22), N(7), N(2) constitute two planes (mean ˚ ) and are orthogonal (dihedral angle deviationB/0.04 A 91.878). The atomic groups Os, Cl(1), Cl(2), N(7), N(27) do not constitute a good plane and is deviated by /0.16 A˚. The distortion from octahedral geometry is due to acute chelate bite angle 76.4(3)8. The azo nitrogens, N(7) (deviation, 0.16 A˚) and N(27) (deviation, /0.15 A˚) are

Table 2 ˚ ) and bond angles (8) for compound ctc Selected bond lengths (A OsCl2(p -tapm)2 2d ˚) Bond length (A Os N(2) Os N(22) Os N(7) Os N(27) Os Cl(1) Os Cl(2) N(1) N(7) N(21) N(27) C(1)  N(2) C(1)  N(6) C(21) N(22) C(21) N(26)

Bond angles (8) 2.029(7) 2.030(8) 1.974(7) 1.967(8) 2.389(2) 2.393(2) 1.320(10) 1.329(11) 1.368(11) 1.328(12) 1.350(12) 1.348(13)

N(2) Os N(7) N(22) Os N(27) Cl(1)  Os Cl(2) N(2) Os N(22) N(7) Os N(27) N(2) Os Cl(1) N(2) Os Cl(2) N(22) Os Cl(1) N(27) Os Cl(2) N(7) Os Cl(1) N(7) Os Cl(2) N(27) Os Cl(1) N(27) Os N(2) N(7) Os N(22) N(22) Os Cl(2)

76.4(3) 76.4(3) 88.48(9) 177.1(3) 100.7(3) 86.0(2) 94.1(2) 96.1(2) 84.9(2) 86.9(2) 169.7(2) 170.2(2) 101.7(3) 101.6(3) 88.0(2)

associated with this distortion. Each chelate ring constitutes a good plane and no atom deviating by more than 0.04 A˚. The dihedral angle between the chelate rings is 105.838. The pendant p -tolyl ring is twisted by 36 /548 from the respective chelate ring plane. The Os /N(pyrimidine) [Os /N(2) /2.029(7), Os / ˚ ] bond distance is longer than Os / N(22)/2.030(8) A ˚] N(azo) [Os /N(7) /1.974(7), Os /N(27)/1.967(8) A bond distance (Table 2). The N /N distances are 1.32 / ˚ elongated by 0.07 A ˚ compared to free azo 1.33 A ˚ distance (1.26 A) [6,39]. The coordination can lead to a decrease in the N /N bond order due to both s-donor and p-acceptor characters of the ligand. The elongation of the N/N distance and shortening of Os /N(azo) bond length are thus an indication of the existence of considerable Os /aapm p-bonding with major involvement of the azo group. Comparison of bond distances between ruthenium(II) [17] and osmium(II) complexes (Fig. 2) reveals that the

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S. Senapoti et al. / Polyhedron 21 (2002) 753 /762

Fig. 2. Dimensions of chelate rings with e.s.d. of: (a) ctc -OsCl2(p tapm)2 (2d) and (b) ccc -OsCl2(p -tapm)2 (3d).

˚ in the latter N /N distance is elongated by 0.03 A complex, the C /N distances vary only slightly. The systematic shortening of M /N(azo) distance compared to M /N(pyrimidine) distance in Ru(II) and Os(II) complexes is a further indication of the presence of stronger M /azo p-bonding. The Os /N(pyrimidine) ˚ compared to bond distance is shortened by 0.02 A Ru /N(pyrimidine) bond distance suggesting the pinteraction order Os /Ru. 3.3. Spectral studies The infrared spectra of the isomers differ significantly in the region 4000/200 cm 1. All the complexes exhibit intense bands at 1530 /1560 and 1340 /1370 cm 1, corresponding to C /N and N /N stretching, respectively. It is observed that endocyclic C /N is shifted to lower frequency region by 60 /90 cm 1 and n(N /N) is reduced by 140 /150 cm 1 [6,17]. This supports the N ,N ?-chelation to osmium(II). The spectra are in agreement with the p-bonding scheme in the order Os /N(azo)/Os /N(pyrimidine) (vide supra). The N / N frequency of OsCl2(aapm)2 are systematically lowered (by 30 /60 cm 1) more than that of RuCl2(aapm)2 [17]. The spectra exhibit two medium intense bands at 300/ 310 and 320 /330 cm 1 corresponding to two Os /Cl bonds. This supports the cis -OsCl2 configuration in the complexes. The vibrational spectra of isomer A, OsCl2(aapm)2 and ctc -RuCl2(aapm)2 are nearly superimposable in the above region [17] except for frequency shifting of N /N and C /N stretching bonds to the lower frequency region. Isomer B is similar to the spectrum of ccc RuCl2(aapm)2. These observations suggest that the isomer A (blue /violet compound) has the cis /trans / cis (2) and isomer B (red /violet compound) has the cis / cis /cis (3) configuration. The solution electronic spectra of the complexes in CHCl3 exhibit multiple bands and shoulder in the region 250 /1200 nm (Table 3). The absorption below 400 nm is due to intraligand charge transfer transitions (n0/p*, p 0/p*) and are not considered further. High intense absorptions (o /104) are observed in the range 410/430,

510 /530, 560/640 nm along with two weak shoulders (o /103) at longer wavelength region 700 /730 and 900 / 1030 nm. Main absorption bands are assigned to t2 0/ p*charge transfer transitions where the p*orbital has a large azo character [6,35]. The spectral comparison (Fig. 3) show that the profile pattern is comparable and the bands are blue shifted by 10/100 nm on going from ctc OsCl2(aapm)2 to ccc -OsCl2(aapm)2 complexes. In d6metal complexes, multiple t2(Os)0/p* charge-transfer transition [6,38] can occur from low symmetry splitting of the metal level from the presence of more than one interacting ligands (each contributing p* orbital(s)) and from mixing of singlet and triplet configuration in the excited state via spin /orbit coupling. Comparison of the electronic spectra of the present series of complexes and those of RuCl2(aapm)2 reveal that the transitions are blue shifted by 30 /150 nm on going from ruthenium(II) to osmium(II) complexes. The spectral behaviour of OsCl2(aapm)2 is comparable to that of OsCl2(aap)2 [aap /2-(arylazo)pyridine]; the absorption positions are red shifted by 20 /35 nm on going from OsCl2(aap)2 to OsCl2(aapm)2. This is in agreement with the p-acidity order, arylazopyridine/arylazopyrimidine [34,35]. The 1H NMR spectra of the complexes were recorded in CDCl3 and were compared with the spectra of free ligands and analogous ruthenium(II) complexes. The proton-numbering patterns are shown in aapm (1) and are assigned on the basis of spin /spin interaction and the effect on substitution therein. Protons in two-aapm units of OsCl2(aapm) are separately assigned as H and H1. The spectral data are summarised in Table 4. The Ar /Me signal has been particularly useful in determining isomer configuration. ctc -OsCl2(tapm)2 (2b/2d) have C2-symmetry and are expected to exhibit a single Ar /Me signal while the ccc -OsCl2(tapm)2 (3b /3d) are C1-symmetric and are expected to exhibit two /Me signals of equal intensities. This is indeed observed (Fig. 4). The pyrimidine protons (4 /6-H) [16] appear at higher frequency region (7.4 /9.3 ppm) and the signals at 6.6 /7.4 ppm are due to azoaryl protons (8 /12-H). The doublet at approximately 9.3 ppm refers to 4-H because of the closest position to the metal centre followed by the 6-H (ca. 8.8 ppm) and 5-H shows multiplet at approximately 7.4 ppm. Aryl protons (8 / 12-H) in the present complexes have been perturbed in a peculiar manner. Usually 8,12-H appears at lower frequency side compared to 9/11-H signals because of the closest position of the electron withdrawing azo group. Besides, the substitution 10-R perturbs 9 and 11H more profoundly than 8,12-H. The electron donating substituent (/Me group in p -tapm) shifts 9- and 11-H to lower frequency region while the electron withdrawing group ( /Cl group in p -Clpapm) shifts them to higher frequency side relative to phenyl-H. This has been observed in the free ligand, aapm [16] and complexes of ruthenium(II) [17] and palladium(II) [18]. It is

Table 3 Elemental analyses a, UV /Vis spectral Compound

a b c d e

and cyclic voltammetric data

Elemental analyses

a

(%)

C

H

N

38.25 (38.14) 38.19 (38.14) 40.09 (40.17) 40.25 (40.17) 40.23 (40.17) 40.12 (40.17) 40.08 (40.17) 40.23 (40.17) 34.44 (34.37) 34.42 (34.37)

2.46 (2.54) 2.62 (2.54) 3.00 (3.04) 2.98 (3.04) 3.10 (3.04) 2.96 (3.04) 2.96 (3.04) 2.96 (3.04) 2.08 (2.01) 2.06 (2.01)

17.74 (17.80) 17.76 (17.80) 17.11 (17.04) 17.00 (17.04) 16.97 (17.04) 17.11 (17.04) 17.11 (17.04) 16.95 (17.04) 16.14 (16.04) 16.10 (16.04)

c

lmax (nm) (10 3 o , M 1 cm1)

Os(III)/Os(II) E 11/2 (V) Os(IV)/Os(III) E 21/2 (V) Ligand reductions E L (V) (DE , mV) (DE , mV) (DE , mV)

1016 (1.137) d, 719 (1.119) d, 572 (7.496), 521 (11.033), 414 (9.774) 907 (1.007) d, 604 (3.922), 513 (9.815), 414 (11.367)

1.224 (80)

2.41 (140)

1.198 (80)

1.996 (160)

1005 (0.876) d, 676 (4.183), 578 (8.660), 538 (10.321), 1.182 (80) 492 (8.041), 408 (5.490) 1.158 (70) 890 (0.674) d, 662 (4.783), 560 (7.883), 405 (6.709)

1.987 (160)

1020 (0.986) d, 708 (1.286) d, 570 (6.486), 522 (9.038), 1.185 (80) 414 (9.715) 886 (0.710) d, 687 (0.854) d, 518 (6.128), 414 (7.270) 1.152 (80) 1011 (1.035) d, 722 (0.988) d, 635 (3.204), 586 (5.655), 522 (8.817), 415 (12.966) 903 (0.720) d, 719 (1.135) d, 610 (3.60), 514 (6.154), 429 (10.067) 1023 (1.061) d, 736 (0.984) d, 525 (9.344), 414 (12.242) 782 (1.872) d, 702 (3.744), 561 (8.186), 425 (7.924)

1.903 (170) 1.978 (170) 1.888 (160)

1.189 (70)

2.008 (140)

1.170 (70)

1.964 (160)

1.290 (80)

2.106 (180)

1.257 (78)

2.000 (170)

0.24 (90), 0.63 (100), 1.259 e, 1.582 e 0.318 (80), 0.678 (90), 1.303 e, 1.644 e 0.319 (90), 0.704 (100), 1.468 e, 1.809 e 0.584 (100), 0.860 (120), 1.612 e, 1.864 e 0.311 (80), 0.696 (100), 1.455 e, 1.721 e 0.577 (100), 0.844 (110), 1.598 e, 1.818 e 0.308 (100), 0.648 (120), 1.418 e, 1.725 e 0.588 (100), 0.813 (120), 1.558 e, 1.805 e 0.212 (90), 0.603 (120), 1.164 e, 1.523 e 0.284 (100), 0.674 (110), 1.157 e, 1.592 e

S. Senapoti et al. / Polyhedron 21 (2002) 753 /762

ctc -OsCl2(papm)2 (2a) ccc -OsCl2(papm)2 (3a) ctc -Os(o -tapm)2Cl2 (2b) ccc -OsCl2(o -tapm)2 (3b) ctc -OsCl2(m -tapm)2 (2c) ccc -OsCl2(m -tapm)2 (3c) ctc -OsCl2(p -tapm)2 (2d) ccc -OsCl2(p -tapm)2 (3d) ctc -OsCl2(p Clpapm)2 (2e) ccc -OsCl2(p Clpapm)2 (3e)

b

Calculated values are in parentheses. In CHCl3. Solvent, CH3CN; supporting electrolyte, [NBu4][ClO4] (0.01 M), solute concentration, 10 3 M, scan rate, 0.05 V s 1. Shoulder. Cathodic peak potential, Epc, V.

757

758

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Fig. 3. Electronic spectra of ctc -OsCl2(p -clpapm)2 (2e) ( */) and ccc -OsCl2(p -clpapm)2 (3e) (- - -) in CHCl3 at 298 K.

observed that a doublet (2H) appears at lowest frequency side in the ctc -OsCl2(p-Raapm)2 (av. 6.6 ppm) which feels little perturbation on substitution at aryl ring (p-R) and second doublet perturbes significantly. The former doublet has been assigned to 8,12-H and the second doublet refers to the signal corresponding to 9,11-H. In OsCl2(o-tapm)2 (2b/3b) and OsCl2(m -tapm)2 (2c/3c) aryl-H are unsymmetrical. In OsCl2(m tapm)2(2c/3c) a singlet (1H) resonance is observed at 6.55 ppm that is unambiguously assigned to 8-H and is followed by a doublet (6.60 ppm) for 12-H. This unusual proton movement may be concluded on considering the charge delocalisation, t2(Os)0/p* orbitals. Thus, 8-H and 12-H protons are affected first and feel severe perturbation. This observation mismatches with our experience on isoelectronic ruthenium(II) complexes [17]. Because of the relativistic effect [40], 5d orbitals in Os(II) have been perturbed more effectively than that

of 4d-orbitals of Ru(II) by the p acidic ligands and hence the reverse swings of charge densities. 3.4. Electrochemistry The electrochemical properties of the complexes have been examined by cyclic volammetry on a platinum milli working electrode in super dry MeCN using [Bu4N][ClO4] (0.1 M) as supporting electrolyte. Electrochemical data are given in Table 3 and a representative voltammogram is shown in Fig. 5. In the potential range /0.5 to /2.4 V at a scan rate 50 mV s 1 a nearly reversible osmium(III) /osmium(II) couple (2) is observed with peak-to-peak separation of 70 /80 mV. OsCl 2 (aapm)2 e XOsCl2 (aapm)2 E 0298,

(2)

lies close to 1.2 V and is The formal potential, the highest potential amongst the known complexes of

Table 4 1 H NMR data of OsCl2(aapm)2 in CDCl3 Compound

d (ppm)/J (Hz)

2a 3a 2b 3b 2c 3c 2d 3d 2e 3e a b c d e

b

[4?-H]

9.25 (6.0) 9.46 (6.0) [8.91 (6.6)] 9.30 (6.0) 9.44 (6.0) [8.83 (6.0)] 9.32 (6.0) 9.30 (6.0) [8.81 (6.0)] 9.32 (6.0) 9.42 (6.0) [8.86 (6.0)] 9.28 (6.0) 9.41 e [8.91 e]

5-H

b

[5?-H]

7.38 (6.0) 7.82 (6.0) [7.41 (6.0)] 7.37 (6.0) 7.78 (6.0) [7.39 (6.0)] 7.38 (6.0) 7.75 (6.0) [7.42 (6.0)] 7.40 (6.0) 7.77 (6.0) [7.07 (6.0)] 7.46 (6.0) 7.80 (6.0) [7.10 (6.0)]

6-H

b

[6?-H]

8.84 (6.0) 8.86 (6.0) [8.30 (6.0)] 8.81 (6.0) 8.80 (6.0) [8.25 (6.0)] 8.82 (6.0) 8.75 (6.0) [8.23 (6.0)] 8.83 (6.0) 8.79 (6.0) [8.24 (6.0)] 8.89 (5.5) 8.86 e [8.28 e]

8-H

b

[8?-H]

6.65 (7.5) 7.72 (7.0) [7.40 (7.0)]

6.55 6.83

9-H [9?-H] c

7.12 (6.0) 7.16 (6.0) [6.59 (6.0)] 6.81 b (6.0) 7.09 (6.0) [6.64 (6.0)]

d d

(6.34 d)

6.56 b (6.5) 7.72 (9.0) [7.17 (9.0)] 6.59 c (7.5) 7.77 (7.0) [7.37 (7.0)]

6.87 b (6.0) 6.97 (6.0) [6.55 (6.0)] 7.09 c (6.0) 7.18 (6.0) [6.76 (6.0)]

10-H [10?-H] c

7.30 (7.0) 7.38 (7.0) [7.23 (7.0)] 7.09 c (7.0) 7.24 (7.0) [6.88 (7.0)] 7.00 b (7.0) 7.31 (7.0) [6.69 (7.0)]

11-H [11?-H] c

7.12 (6.0) 7.16 (6.0) [6.59 (6.0)] 7.09 c (7.0) 7.24 (7.0) [6.88 (7.0)] 7.05 c (7.0) 7.40 (7.0) [6.77 (7.0)] 6.87 b (6.0) 6.97 (6.0) [6.55 (6.0)] 7.09 b (6.0) 7.18 (6.0) [6.76 (6.0)]

12-H

b

[12?-H]

6.65 (7.5) 7.72 (7.0) [7.40 (7.0)] 6.61 (6.0) 6.95 (6.0) [6.42 (6.0)] 6.60 (6.5) 6.94 (7.0) [6.49 (7.0)] 6.56 (6.5) 7.72 (9.0) [7.71 (9.0)] 6.59 (7.5) 7.77 (7.0) [7.37 (7.0)]

Me [Me?]

2.44 2.66 [2.45] 2.30 2.61 [2.40] 2.23 2.40 [2.24]

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4-H

a

For ccc -Os(aapm)2Cl2 respective protons of two aapm are distinguished as H and H?. Doublet. Triplet. Singlet. Broad.

759

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potential coulometry of one complex ctc -OsCl2(papm)2 at potential 1.4 V versus SCE using Pt-net working electrode. The solution colour changes from blue/violet to brown /red. Reported potentials are higher than those of osmium(II) /arylazopyridines which may be due to p-acidity order of pyrimidine /pyridine [34,35]. The azopyridine ligands thus stabilise (with respect to oxidation) osmium(II) better than the azopyridines. Reductions are observed in the potential range 0 to / 2.0 V using glassy-carbon working electrode. Four successive redox couples are observed (Table 3); the first two couples are quasi-reversible (DEp /100/140 mV) in nature. The remaining two responses at negative to SCE do not exhibit reasonable anodic peaks on scan reversal. These reductions are believed to be the successive electron addition into two azoimine functions [4,6,17]. 3.5. EHMO calculation

Fig. 4. 1H NMR spectra of: (a) ctc -OsCl2(p -tapm)2 (2d) and (b) ccc OsCl2(p -tapm)2 in CDCl3 at 298 K.

Fig. 5. Cyclic voltammogram of ctc -OsCl2(p -tapm)2 in MeCN using Pt-disc working electrode. The solute concentration and scan rate are 10 3 M and 50 mV s 1, respectively.

the azoimine system. A second one-electron response observed at approximately 2 V (close to solvent cutoff limit) is quasireversible (DEp /140/180 mV) in nature. This is assigned to the osmium(IV)/osmium(III) redox couple [6]. The data reveal that between the two isomers the order of E 0298 is tc /cc. The one-electron stoichiometry of the first couple was established by controlled

Approximate composition studies of the frontier orbitals in osmium(II) complexes of 2-(arylazo)pyrimidine were performed by the extended Hu¨ckel calculation on a model complex of ctc -OsCl2(p-tapm)2. Similar calculation has also been carried out to ruthenium(II) analogues. Since crystallographic data for ctc -RuCl2(ptapm)2 are not available with us; herein, we are using crystal data of ccc -RuCl2(papm)2 [17] for gross comparison of spectral and electrochemical properties with osmium(II) complexes. The calculated molecular orbitals, have been used in order to assess the solution spectra and redox properties of the osmium(II) complexes and their comparison with ruthenium(II) complexes. The HOMO and LUMO of the complexes are depicted in Fig. 6. In osmium(II) complex, the HOMO (EHOMO //11.152 eV) is constituted by 81% metal dorbitals and the LUMO (ELUMO //10.768 eV) is composed of 68% ligand orbitals. The azoimine function contributes 42% to the LUMO. ? In ccc -RuCl2(p apm)2, the HOMO (E HOMO / /11.316 eV) is composed of 77% d(Ru) and the ? LUMO (E LUMO //10.510 eV) is 59% ligand orbitals. In ruthenium complex the HOMO is laying 0.164 eV below than the HOMO of osmium complex and the LUMO is stabilised by 0.258 eV in osmium complex compared to ruthenium complex. Thus, the energy difference between HOMO and LUMO is higher in RuCl2(aapm)2 than OsCl2(aapm)2. This is also observed in the ordering of MLCT transitions. The EHMO results suggest that the HOMO, HOMO1, HOMO-2, etc. are predominantly metal character and LUMO, LUMO/1, LUMO/2 are basically p*-ligand character. So, multiple charge transfer transitions may be observed. This is indeed observed. Energy difference determines the MLCT transitions and explains the shorter wavelength absorption of osmium complexes

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761

Fig. 6. Frontier orbitals, HOMO and LUMO of: (a) ctc -OsCl2(p -tapm)2 and (b) ccc -RuCl2(p -papm)2.

than that of ruthenium complexes. In cyclic voltammetric experiment the complexes exhibit both metal oxidations and ligand reductions. The oxidation involves electron extraction from HOMO which is metal orbital dominated and the reduction involves electron accommodation at LUMOs. Because of higher energy of HOMO of osmium complex (/11.152 eV) compared to

ruthenium complex (/11.316 eV) the Os(III)/Os(II) and Os(IV)/Os(III) couples appear at higher potentials than that of Ru(III)/Ru(II) and Ru(IV)/Ru(III) complexes, respectively. The energy of LUMO of osmium complex is lower than that of ruthenium complex. Thus reduction of ligand is observed at lower potential in osmium complexes than ruthenium complexes (Table 3).

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4. Supplementary material Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 168654 for compound 2d. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: /44-1223-336033; email: [email protected] or www: http:// www.ccdc.cam.ac.uk).

Acknowledgements Financial assistance from the University Grants Commission, and Department of Science & Technology, New Delhi is gratefully acknowledged.

References [1] D.J. Brown, in: Comprehensive Heterocyclic Chemistry, A.R. Katritzky, C.W. Rees, A.J. Boulton, A. McKillop (Eds.), vol. 3, 1984, p. 57. [2] J. Reedijk, in: Comprehensive Coordination Chemistry, G. Wilkinson, R.D. Gillard, J.A. McCleverty (Eds.), vol. 2, Pergamon Press, Oxford, 1987, p. 73. [3] D. Datta, A. Chakravorty, Inorg. Chem. 22 (1983) 1085. [4] B.K. Ghosh, A. Chakravorty, Coord. Chem. Rev. 95 (1989) 239. [5] S. Goswami, A.R. Chakravarty, A. Chakravorty, Inorg. Chem. 20 (1981) 2246. [6] B.K. Ghosh, A. Mukhopadhyay, S. Goswami, S. Ray, A. Chakravorty, Inorg. Chem. 23 (1984) 4633. [7] S. Ganguly, S. Karmakar, C.K. Pal, A. Chakravorty, Inorg. Chem. 38 (1999) 598. [8] A. Bharath, B.K. Santra, P. Munshi, G.K. Lahiri, J. Chem. Soc., Dalton Trans. (1998) 2643. [9] M. Heilmann, F. Baumann, W. Kaim, J. Fiedler, J. Chem. Soc., Faraday Trans. 92 (1996) 4227. [10] K. Pramanik, M. Shivakumar, P. Ghosh, A. Chakravorty, Inorg. Chem. 39 (2000) 195. [11] R. Samanta, P. Munshi, B.K. Santra, N.K. Loknath, M.A. Sridhar, J.S. Prasad, G.K. Lahiri, J. Organomet. Chem. 31 (1999) 579. [12] B.K. Santra, P. Munshi, G. Das, P. Bharadwaj, G.K. Lahiri, Polyhedron 18 (1999) 617.

[13] A.K. Ghosh, P. Majumder, L.R. Falvello, G. Mostafa, S. Goswami, Organometallics 18 (1999) 5086. [14] V.W.W. Yam, V.C.Y. Lan, L.X. Wu, J. Chem. Soc., Dalton Trans. (1998) 1461. [15] T.K. Misra, D. Das, C. Sinha, Polyhedron 16 (1997) 4163. [16] P.K. Santra, D. Das, T.K. Misra, R. Roy, C. Sinha, S.-M. Peng, Polyhedron 18 (1999) 1909. [17] P.K. Santra, T.K. Misra, D. Das, C. Sinha, A.M.Z. Slawin, J.D. Woollins, Polyhedron 18 (1999) 2869. [18] P.K. Santra, R. Roy, C. Sinha, Proc. Indian Acad. Sci. (Chem. Sci.) 112 (2000) 523. [19] P.K. Santra, C. Sinha, W.-J. Sheen, F.-L. Liao, T.-H. Lu, Polyhedron 20 (2001) 599. [20] T.K. Misra, D. Das, C. Sinha, P.K. Ghosh, C.K. Pal, Inorg. Chem. 37 (1998) 1672. [21] K.S.Y. Leung, Y. Li, Inorg. Chem. Commun. 2 (1999) 599. [22] N.C. Pramanik, S. Bhattacharya, Transition Met. Chem. 22 (1997) 524. [23] S. Banerjee, S. Bhattacharyya, B.K. Dirghangi, M. Menon, A. Chakravorty, Inorg. Chem. 39 (2000) 6. [24] S. Goswami, A.R. Chakravarty, A. Chakravorty, J. Chem. Soc., Chem. Commun. (1982) 1288. [25] N. Ertan, Dyes Pigments 44 (1999) 41. [26] T.G. Deligeorgiev, D. Simov, Dyes Pigments 38 (1998) 115. [27] I. Sing, R. Saini, Anal. Chim. 85 (1995) 193. [28] T.C.B. Saldanha, M.C.U. de Araujo, B.B. Neto, H.C. Chame, Anal. Lett. 33 (2000) 1187. [29] G.A. Kochelaeva, V.M. Ivanov, G.V. Prochorova, J. Anal. Chem. 55 (2000) 14. [30] G.B. Luo, T.T. Liu, L.M. Ying, X.S. Zhao, Y.Y. Huang, D.G. Nu, C.H. Huang, Langmuir 16 (2000) 3657. [31] P. Chattopadhyay, C. Sinha, D.K. Pal, Fresenius’ J. Anal. Chem. 357 (1997) 368. [32] (a) D. Das, A.K. Das, C. Sinha, Talanta 48 (1999) 1013; (b) D. Das, A.K. Das, C. Sinha, Anal. Lett. 32 (1999) 567. [33] P.K. Santra, U.S. Ray, S.N. Pal, C. Sinha, Inorg. Chem. Commun. 4 (2001) 269. [34] E.C. Constable, Coord. Chem. Rev. 93 (1989) 205. [35] F. Casalboni, Q.G. Mulazzani, C.D. Clark, M.Z. Hoffman, P.L. Orizondo, M.W. Perkovic, D.P. Rillema, Inorg. Chem. 36 (1997) 2252. [36] F. Jolibois, J. Cadet, A. Grand, R. Subra, N. Raga, V. Barone, J. Am. Chem. Soc. 120 (1998) 1864. [37] M. Louloudi, Y. Deligiannakis, J.-P. Tuchagues, B. Donnadieu, N. Hadjiliadis, Inorg. Chem. 36 (1997) 6335. [38] S. Pal, T.K. Misra, P. Chattopadhyay, C. Sinha, Proc. Indian Acad. Sci. (Chem. Sci.) 111 (1999) 687. [39] D. Das, PhD Thesis, Burdwan University, Burdwan, India, 1998. [40] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, Wiley, New York, 1972.