Ruthenium(II) complexes containing a phosphine-functionalized thiosemicarbazone ligand: synthesis, structures and catalytic C-N bond formation reactions via N-alkylation

A series of ruthenium(II) complexes incorporating a thiosemicarbazone chelate tethered with a diphenylphosphine pendant have been studied. Thus, [(PNS- Et) RuCl(CO)(PPh3)] (1), [N, S-(PNS- Et) RuH(CO)(PPh3)(2)] (2) and [(PNS- Et) RuCl(PPh3)] (3) were synthesized by reactions of various RuII precursors with 2-(2-(diphenylphosphino) benzylidene)- N- ethylthiosemicarbazone (PNS- Et). However, complexation of PNS- Et with an equimolar amount of [RuCl2(dmso)(4)] resulted in two different entities [(PNS- Et) RuCl(dmso)(2)] (4) and [(PNS- Et)(2)Ru] (5) with different structural features in a single reaction. All the RuII complexes have been characterized by analytical and various spectroscopic techniques. Compounds 1- 5 were recrystallized, and the X- ray crystal structures have been reported for 1, 2 and 5. In the complexes 1 and 3- 5 the ligand coordinated in a tridentate monobasic fashion by forming PNS five- and six- membered rings, whereas in 2, the ligand coordinated in a bidentate monobasic fashion by forming a strained NS four- membered ring. Furthermore, compounds 1- 5 showed catalytic activity in N- alkylation of heteroaromatic amines. Notably, complexes 1- 3 were found to be very efficient catalysts toward N- alkylation of a wide range of heterocyclic amines with alcohols. In the presence of a catalytic amount of 2 with 50 mol% of KOH, N-1, C-5- dialkylation of 4- phenylthiazol- 2- amine has been investigated. Reaction of in situ generated aldehyde with amine yields the N-1, C-5- dialkylated products through the hydride ion transformation from alcohol. Complexes 1- 3 also catalyzed a variety of coupling reactions of benzyl alcohols and sulfonamides, which were realized often with 99% isolated yields. Advantageously, only one equivalent of the primary alcohol was consumed in the process.