Cooperative B-H bond activation: dual site borane activation by redox active kappa(2)-N,S-chelated complexes

International audienceCooperative dual site activation of boranes by redox-active 1,3-N,S-chelated ruthenium species, mer-[PR3{kappa(2)-N,S-(L)}(2)Ru{kappa(1)-S-(L)}], (mer-2a: R = Cy, mer-2b: R = Ph; L = NC7H4S2), generated from the aerial oxidation of borate complexes, [PR3{kappa(2)-N,S-(L)}Ru{kappa(3)-H,S,S '-BH2(L)(2)}] (trans-mer-1a: R = Cy, trans-mer-1b: R = Ph; L = NC7H4S2), has been investigated. Utilizing the rich electronic behaviour of these 1,3-N,S-chelated ruthenium species, we have established that a combination of redox-active ligands and metal-ligand cooperativity has a big influence on the multisite borane activation. For example, treatment of mer-2a-b with BH3 center dot THF led to the isolation of fac-[PR3Ru{kappa(3)-H,S,S '-(NH2BSBH2N)(S2C7H4)(2)}] (fac-3a: R = Cy and fac-3b: R = Ph) that captured boranes at both sites of the kappa(2)-N,S-chelated ruthenacycles. The core structure of fac-3a and fac-3b consists of two five-membered ruthenacycles [RuBNCS] which are fused by one butterfly moiety [RuB2S]. Analogous fac-3c, [PPh3Ru{kappa(3)-H,S,S '-(NH2BSBH2N)(SC5H4)(2)}], can also be synthesized from the reaction of BH3 center dot THF with [PPh3{kappa(2)-N,S-(SNC5H4)}{kappa(3)-H,S,S '-BH2(SNH4C5)(2)}Ru], cis-fac-1c. In stark contrast, when mer-2b was treated with BH(2)Mes (Mes = 2,4,6-trimethyl phenyl) it led to the formation of trans- and cis-bis(dihydroborate) complexes [{kappa(3)-S,H,H-(NH(2)BMes)Ru(S2C7H4)}(2)], (trans-4 and cis-4). Both the complexes have two five-membered [Ru-(H)(2)-B-NCS] ruthenacycles with kappa(2)-H-H coordination modes. Density functional theory (DFT) calculations suggest that the activation of boranes across the dual Ru-N site is more facile than the Ru-S one