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

Cooperative dual site activation of boranes by redox-active 1,3-N,S-chelated ruthenium species, mer-[PR(3){κ(2)-N,S-(L)}(2)Ru{κ(1)-S-(L)}], (mer-2a: R = Cy, mer-2b: R = Ph; L = NC(7)H(4)S(2)), generated from the aerial oxidation of borate complexes, [PR(3){κ(2)-N,S-(L)}Ru{κ(3)-H,S,S′-BH(2)(L)(2)}] (trans–mer-1a: R = Cy, trans–mer-1b: R = Ph; L = NC(7)H(4)S(2)), 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 BH(3)·THF led to the isolation of fac-[PR(3)Ru{κ(3)-H,S,S′-(NH(2)BSBH(2)N)(S(2)C(7)H(4))(2)}] (fac-3a: R = Cy and fac-3b: R = Ph) that captured boranes at both sites of the κ(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 [RuB(2)S]. Analogous fac-3c, [PPh(3)Ru{κ(3)-H,S,S′-(NH(2)BSBH(2)N)(SC(5)H(4))(2)}], can also be synthesized from the reaction of BH(3)·THF with [PPh(3){κ(2)-N,S-(SNC(5)H(4))}{κ(3)-H,S,S′-BH(2)(SNH(4)C(5))(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 [{κ(3)-S,H,H-(NH(2)BMes)Ru(S(2)C(7)H(4))}(2)], (trans-4 and cis-4). Both the complexes have two five-membered [Ru–(H)(2)–B–NCS] ruthenacycles with κ(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.