An evaluation of monovalent osmium supported by the PNP ligand environment

Nitrogen is essential to reaction of (PNP)OsI (PNP is N((SiMe2CH2PBu2)-Bu-t)(2))and Mg powder in THF, to give equimolar (PNP)OsH(N-2)and hydrido carbene [((Bu2PCH2SiMe2)-Bu-t)N((SiMe2CH2PBu)-Bu-t-(CMe2CH)]OsH. This reaction is attributed to H-2 evolution from solid magnesium, rather than high energy H atom transfer between molecules, but relies also on the strong pi-basicity of Os in favoring alpha-H migration from the metallated Bu-t group on Os to form the second product, the hydrido carbene species. The path to two different products begins because the simple N-2 adduct of (PNP)OsI undergoes spontaneous heterolytic H-C splitting of the Bu-t methyl group, to produce a secondary amine intermediate [((Bu2PCH2SiMe2)-Bu-t)N(H)((SiMe2CH2PBu)-Bu-t(CMe2CH2)]OsI(N-2)which can then be dehydrohalogenated by Mg. The analogous reaction for (PNP)RuCl shows production of only (PNP)RuH(N-2), with none of the hydride carbene dehydrogenation product. Comparative (Ru vs. Os) DFT calculations reveal the reaction steps where the Os analog is much more exothermic, accounting for certain reaction selectivities.