Osmium mediated selective aromatic thiolation reaction in the complexes [Os-II{o-SC6H3(R)N=NC5H4N}(2)]. Synthesis, spectroscopic characterization, electron-transfer properties and crystal structure of the complex where R=H

The reactions of potassium salt of xanthates, R'OCS2-K+ 2 (R'=Me, Et, Pr-n, Pr-i, Bu-n, Bu-i, benzyl) with the low-spin ctc-Os-II(L)(2)Br-2, 1 [L=NC5H 4-N=N-C6H4(R), R=H, m-Me; cfc: cis-trans-cis with respect to the bromides,p yridine and azo nitrogens, respectively] in boiling N,N-dimethylformamide s olvent resulted in low-spin diamagnetic Os-II[o-SC6H3(R)-N=N-C5H4N](2), 3 c omplexes. In complex 3 the o-carbon hydrogen bond of the pendant phenyl rin g of both the parent ligands L, has been selectively and directly thiolated via carbon-sulfur bond cleavage of the xanthate. The reaction is selective to the nature of the solvent used, taking place only in those having high boiling points and relative permittivities. The rate of the reaction is dep endent on the nature of the R' group present in the thiolating agent 2, fol lowing the order: Et > Me > Pr-i > Bu-i > Pr-n > Bu-n > benzyl. The formati on of the thiolated product (3) is authenticated by the single-crystal X-ra y structure for the complex where R=H. Structure of the complex has reveale d that the OsN4S2 coordination sphere has a meridional configuration, cis-t rans-cis with respect to sulfur, azo and pyridine nitrogens, respectively. The molecular geometry of the complexes in solution has been established by H-1 and C-13 NMR spectroscopy. When one methyl group is present at the met a position of the pendant phenyl ring of L the reaction resulted in two iso meric complexes due to free rotation of the singly bonded meta-substituted phenyl ring with respect to the azo group. LH NMR study indicated the prese nce of an intimate mixture of the two isomers in solution in a 3:4 ratio. I n dichloromethane solution, complex 3 exhibits two metal-to-ligand charge-t ransfer transitions in the visible region and intraligand (pi-pi*/n-pi*) tr ansitions appear in the UV region. In dichloromethane solution the complexe s display one reversible osmium(II)right arrow over left arrow osmium(III) oxidation couple near 0.35 V and an irreversible oxidative response near 1. 0 V vs SCE due to the oxidation of the coordinated thiol group. Two success ive one-electron reductions of the coordinated azo groups take place at the negative side of SCE. (C) 1999 Elsevier Science Ltd. All rights reserved.