Ruthenium bidentate phosphine complexes for the coordination and catalytic dehydrogenation of amine- and phosphine-boranes

Addition of the amine–boranes H₃B ? NH₂tBu, H₃B ? NHMe₂ and H₃B ? NH₃ to the cationic ruthenium fragment Ru(xantphos)(PPh₃)(OH₂)H]BAr^F₄] ( 2 ; xantphos=4,5‐bis(diphenylphosphino)‐9,9‐dimethylxanthene; BAr^F₄=B{3,5‐(CF₃)₂C₆H₃}₄]^?) affords the η¹‐B? H bound amine–borane complexes Ru(xantphos)(PPh₃)(H₃B ? NH₂tBu)H]BAr^F₄] ( 5 ), Ru(xantphos)(PPh₃)(H₃B ? NHMe₂)H]BAr^F₄] ( 6 ) and Ru(xantphos)(PPh₃)(H₃B ? NH₃)H]BAr^F₄] ( 7 ). The X‐ray crystal structures of 5 and 7 have been determined with BAr^F₄] and BPh₄] anions, respectively. Treatment of 2 with H₃B ? 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)₂H]BPh₄] ( 9 ). The xantphos complexes 2 , 5 and 9 proved to be poor precursors for the catalytic dehydrogenation of H₃B ? NHMe₂. While the dppf species (dppf=1,1′‐bis(diphenylphosphino)ferrocene) Ru(dppf)(PPh₃)HCl] ( 3 ) and Ru(dppf)(η⁶‐C₆H₅PPh₂)H]BAr^F₄] ( 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.