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

A carbonyl ligand can be thermally liberated from Tp(R)Mo-(NO)(CO)(2) in the presence of a potential sigma-donor ligand (L) to give Tp(R)Mo-(NO)(L)(CO), where Tp(R) = 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 Tp(R)Mo(NO)(L)(CO) complexes using a hydroperoxide gives eta(2)-CO2 complexes, Tp(R)Mo(NO)(L)(eta(2)-CO2). For Tp, eta(2)-CO2 complexes where L = 1-MeIm, NHC, PMe3, and PMe2Ph have been isolated. For Tp*, eta(2)-CO2 complexes where L = 1-MeIm and PMe3 have been isolated. Carbonyl stretching frequencies indicate the susceptibility of Tp(R)Mo(NO)(L)(CO) complexes to oxidation to form an eta(2)-CO2 complex. Carbonyl complexes featuring nu(CO) values of >1885 cm(-1) did not result in observable eta(2)-CO2 complexes upon oxidation. The eta(2)-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 eta(2)-CO2 complexes have also been analyzed by X-ray diffraction. The eta(2)-CO2 complexes are air stable and thermally stable at room temperature in solution. Reduction of TpRMo(NO)(L)(eta(2)-CO2) back to Tp(R)Mo(NO)(L)(CO) can be accomplished using chemical reductants, including LiBH4, LiAlH4, and a mixture of magnesium powder and P(NMe2)(3).