Ruthenium Bidentate Phosphine Complexes for the Coordination and Catalytic Dehydrogenation of Amine- and Phosphine-Boranes

Addition of the amine-boranes H3B center dot NH(2)tBu, H3B center dot NHMe2 and H3B center dot NH3 to the cationic ruthenium fragment [Ru(xantphos)(PPh3)-(OH2)H][BAr4F] (2; xantphos= 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene; BAr4F=[B-{3,5-(CF3)(2)C6H3}(4)](-)) affords the eta(1)-B-H bound amine-borane complexes [Ru-(xantphos)(PPh3)-(H3B center dot NH(2)tBu)H]-[BAr4F] (5), [Ru(xantphos)-(PPh3)(H3B center dot NHMe2)H][BAr4F] (6) and [Ru(xantphos)(PPh3)(H3B center dot NH3)H]-[BAr4F] (7). The X-ray crystal structures of 5 and 7 have been determined with [BAr4F] and [BPh4] anions, respectively. Treatment of 2 with H3B center dot PHPh2 resulted in quite different behaviour, with cleavage of the B-P interaction taking place to generate the structurally characterised bis-secondary phosphine complex [Ru(xantphos)-(PHPh2)(2)H][BPh4] (9). The xantphos complexes 2, 5 and 9 proved to be poor precursors for the catalytic dehydrogenation of H3B center dot NHMe2. While the dppf species (dppf=1,1'-bis(diphenylphosphino) ferrocene) [Ru(dppf)-(PPh3)HCl] (3) and [Ru(dppf)(eta(6)-C6H5PPh2)H][BAr4F] (4) showed better, but still moderate activity, the agostic-stabilised N-heterocyclic carbene derivative [Ru(dppf)(ICy)HCl] (12; ICy= 1,3-dicyclohexylimidazol-2-ylidene) proved to be the most efficient catalyst with a turnover number of 76 h(-1) at room temperature.