Ruthenium Dihydrogen Complexes with Wide Bite Angle Diphosphines

The wide bite angle diphosphines homoxantphos (10,11-dihydro-4,5,-bis(diphenylphosphino)dibenzo[b,f]oxepine), sixantphos (4,6-bis(diphenylphosphino)-10,10-dimethylphenoxasilin), and thixantphos (2,8-dimethyl-4,6-bis(diphenylphosphino)phenoxathiin) were used to prepare cis[MH2(diphosphine)2] complexes (1a-f) by reaction of [Ru(cod)(cot)] (cod = cyclo-octa-1,5-diene, cot = cyclo-octa-1,3,5-triene) with 2 equiv of the diphosphine under dihydrogen pressure. The electronic properties of the thixantphos ligand were varied. Complexes 1a-f can be protonated with HBF4 or CFsCOOH to yield hydrido(dihydrogen) complexes c/s[MH(H2)(diphosphine)2 (2a-f), which were characterized by VT (variable temperature) NMR and 7i measurements. These complexes show fast hydrogen atom exchange between the -H and the terminal hydride at all temperatures studied. They are thermally unstable toward dihydrogen loss yielding the cationic monohydride complexes c/s[MH(diphosphine)2 (3a-f). Coordination of the -H2 is dominated by — d donation, and hence, the H-H distance is hardly influenced by the electronic properties of the ligands.