Metal ion-mediated selective activations of C-H and C-Cl bonds. Direct aromatic thiolation reactions via C-S bond cleavage of dithioacids

The reactions of potassium salt of dithiocarbonate, R'OCS2K, 4 (R' = Me, Et , Pr-n, Bu-n, Pr-i, Bu-i, -CH2Ph) with the low-spin ctc-Ru-II(L)(2)Cl-2 1, ctc-Os-II(L)(2)Br-2 2 and mer-[Co-II(L)(3)](ClO4)(2). H2O 3 [L = 2-(arylazo )pyridine, NC5H4-N=N-C6H4(R), R-H, o-Me/Cl, m-Me/Cl, p-Me/Cl; ctc: cis-tran s-cis with respect to halides, pyridine and azo nitrogens respectively) in boiling dimethylformamide solvent resulted in low-spin diamagnetic Ru-II(L' )(2), 5, Os-II(L')(2) 6 and [Co-III(L')(2)]ClO4 7 respectively (L' = o-S-C6 H3(R)N=NC5H4N). In the complexes 5, 6 and 7 ortho carbon-hydrogen bond of t he pendant phenyl ring of the ligands (L') has been selectively and directl y thiolated via the carbon-sulphur bond cleavage of 4. The newly formed tri denate thiolated ligands (L') are bound to the me;al ion in a meridional fa shion. In the case of cobalt complex (7), during the activation process the bivalent cobalt ion in the starting complex 3 has been oxidised to the tri valent Co-III state. The reactions are highly sensitive to the nature and t he location of the substituents present in the active phenyl ring. The pres ence of electron donating Me group at the ortho and para positions of the p endant phenyl ring with respect to the activation points can only facilitat e the thiolation process. The complexes (1c, 2c and 3c) having chloride gro up at the ortho position of the active phenyl ring underwent the thiolation reaction selectively via the carbon-chloride bond activation process. The rate of carbon-chloride activation process has been found to be much faster compared to the C-H bond activation. The reactions are sensitive to the na ture of the solvent used, taking place only in those having high boiling an d polar solvents. The rate of the reactions is also dependent on the nature of the R' group present in 4, following the order: Me similar to Et > Pr-n > Bu-n > Pr-i > Bu-i >> -CH2Ph. The molecular geometry of the complexes in solution has been established by H-1 and C-13 NMR spectroscopy. The thiola ted complexes (5, 6, 7) exhibit metal to ligand charge-transfer transitions in the visible region and intraligand pi-pi* and n-pi* transitions in the UV region. In acetonitrile solution the complexes display reversible M(III) reversible arrow M-II reductions at 0.43 V for Ru (5a), 0.36 V for Os (6a) and -0.13 V for Co (7a) vs saturated calomel electrode (SCE).