Cooperative B-H bond activation: dual site borane activation by redox active κ-,-chelated complexes.

Cooperative dual site activation of boranes by redox-active 1,3-,-chelated ruthenium species, -[PR{κ-,-(L)}Ru{κ--(L)}], (-2a: R = Cy, -2b: R = Ph; L = NCHS), generated from the aerial oxidation of borate complexes, [PR{κ-,-(L)}Ru{κ-,,'-BH(L)}] (--1a: R = Cy, --1b: R = Ph; L = NCHS), has been investigated. Utilizing the rich electronic behaviour of these 1,3-,-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 -2a-b with BH·THF led to the isolation of -[PRRu{κ-,,'-(NHBSBHN)(SCH)}] (-3a: R = Cy and -3b: R = Ph) that captured boranes at both sites of the κ-,-chelated ruthenacycles. The core structure of -3a and -3b consists of two five-membered ruthenacycles [RuBNCS] which are fused by one butterfly moiety [RuBS]. Analogous -3c, [PPhRu{κ-,,'-(NHBSBHN)(SCH)}], can also be synthesized from the reaction of BH·THF with [PPh{κ-,-(SNCH)}{κ-,,'-BH(SNHC)}Ru], --1c. In stark contrast, when -2b was treated with BHMes (Mes = 2,4,6-trimethyl phenyl) it led to the formation of - and -bis(dihydroborate) complexes [{κ-,,-(NHBMes)Ru(SCH)}], (-4 and -4). Both the complexes have two five-membered [Ru-(H)-B-NCS] ruthenacycles with κ-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.