Thermolysis of (MeCp)(2)Nb(eta(2)-CO2)R complexes: Decarbonylation vs insertion

A series of carbon dioxide complexes of the type Cp(2)'Nb(eta(2)-CO2)R (Cp' = MeC(5)H(4); R = -CH(2)SiMe(3), 1a; -CH(2)CMe(3), 1b; -CH(2)Ph, 1c; -CH3, 1d) was prepared and the thermolyses of 1a-1c investigated. When heated at 60 degrees C in THF, complexes 1a and 1b are converted to the corresponding oxo derivatives (MeC(5)H(4))(2)Nb(=O)R (2a, 2b) and CO. The structure of 2a was established by X-ray diffraction and consists of a pseudotetrahedral Cp(2)MXY arrangement. That the oxo group of 2 is derived from coordinated CO2 was confirmed by C-13 labeling. Thermolysis of benzyl derivative Ic exhibits concentration dependent behavior, converting to the corresponding oxoalkyl derivative 2c in dilute solutions but forming a mixture of 2c and the carbonyl complex (MeC(5)H(4))(2)Nb(CO)CH(2)Ph (4) in more concentrated solutions. With extended reaction times 2c and 4 decompose producing PhCH(3), PhCH(2)CH(2)Ph, PhCH(2)OH, and PhCHO, the products of apparent M-C homolysis and oxidation.