An evaluation of monovalent osmium supported by the PNP ligand environment

Abstract

Nitrogen is essential to reaction of (PNP)OsI (PNP is N(SiMe₂CH₂P^tBu₂)₂) and Mg powder in THF, to give equimolar (PNP)OsH(N₂) and hydrido carbene [(^tBu₂PCH₂SiMe₂)N(SiMe₂CH₂P^tBu(CMe₂CH)]OsH. This reaction is attributed to H₂ evolution from solid magnesium, rather than high energy H atom transfer between molecules, but relies also on the strong π-basicity of Os in favoring α-H migration from the metallated ^tBu group on Os to form the second product, the hydrido carbene species. The path to two different products begins because the simple N₂ adduct of (PNP)OsI undergoes spontaneous heterolytic H–C splitting of the ^tBu methyl group, to produce a secondary amine intermediate [(^tBu₂PCH₂SiMe₂)N(H)(SiMe₂CH₂P^tBu(CMe₂CH₂)]OsI(N₂) which can then be dehydrohalogenated by Mg. The analogous reaction for (PNP)RuCl shows production of only (PNP)RuH(N₂), with none of the hydride carbene dehydrogenation product. Comparative (Ruvs. Os) DFT calculations reveal the reaction steps where the Os analog is much more exothermic, accounting for certain reaction selectivities.