Reactivity of the terminal oxo species ((tBu2PCH2SiMe2)2N)RhO

Abstract

Reactivity of the 4-coordinate molecule (PNP)RhO (PNP is (^tBu₂PCH₂SiMe₂)₂N) towards CO proceeds stepwise, first forming an η²-CO₂ complex, by a mechanism which involves a preliminary adduct of CO on Rh, then a second CO displaces CO₂. Reaction of the oxo complex with CO₂ occurs in time of mixing even at low temperature to form (PNP)Rh(η²-CO₃), with no intermediate detectable. DFT calculations indicate an initial bond formation between the oxo center and the CO₂ carbon. Reaction of (PNP)RhO with H₂ occurs only at a 1 : 2 molar stoichiometry, to ultimately form (PNP)Rh(H)₂ and free H₂O. No intermediate reaches detectable population even at −60 °C, but DFT mapping of various possible mechanisms on the singlet energy surface shows that the nearly equi-energetic (PNP)Rh(H₂O) and (PNP)RhH(OH) are formed, but only the latter readily adds the second molecule of H₂ to proceed to the observed products; these reactions thus both involve heterolytic splitting of H₂.