Synthesis of Molybdenum Dihapto Carbon Dioxide Complexes via Oxidation of a Carbonyl Ligand

A carbonyl ligand can be thermally liberated from TpRMo­(NO)­(CO)2 in the presence of a potential σ-donor ligand (L) to give TpRMo­(NO)­(L)­(CO), where TpR = hydridotris­(pyrazolyl)­borate (Tp) or hydridotris­(3,5-dimethylpyrazolyl)­borate (Tp*). For Tp, complexes featuring L = 1-methylimidazole (1-MeIm), 4-(dimethylamino)­pyridine (4-DMAP), PMe3, PMe2Ph, PPh3, and P­(OMe)3 have been prepared. In addition, an N-heterocyclic-carbene complex, where L = 1,3-dimethylimidazol-2-ylidene (NHC), has also been prepared using 1,3-dimethylimidazolium-2-carboxylate as the carbene-transfer agent. For Tp*, complexes featuring L = 1-MeIm, 4-DMAP, PMe3, pyridine, and P­(OMe)3 have been prepared. Oxidation of the more electron-rich TpRMo­(NO)­(L)­(CO) complexes using a hydroperoxide gives η2-CO2 complexes, TpRMo­(NO)­(L)­(η2-CO2). For Tp, η2-CO2 complexes where L = 1-MeIm, NHC, PMe3, and PMe2Ph have been isolated. For Tp*, η2-CO2 complexes where L = 1-MeIm and PMe3 have been isolated. Carbonyl stretching frequencies indicate the susceptibility of TpRMo­(NO)­(L)­(CO) complexes to oxidation to form an η2-CO2 complex. Carbonyl complexes featuring νCO values of >1885 cm–1 did not result in observable η2-CO2 complexes upon oxidation. The η2-CO2 complexes are most often formed as mixtures of two coordination diastereomers that can interconvert in solution. For the PMe3 complexes, a single diastereomer in which the uncoordinated oxygen of the CO2 ligand points toward PMe3 is heavily favored at equilibrium (>30:1). These PMe3 η2-CO2 complexes have also been analyzed by X-ray diffraction. The η2-CO2 complexes are air stable and thermally stable at room temperature in solution. Reduction of TpRMo­(NO)­(L)­(η2-CO2) back to TpRMo­(NO)­(L)­(CO) can be accomplished using chemical reductants, including LiBH4, LiAlH4, and a mixture of magnesium powder and P­(NMe2)3.