AGOSTIC ACETYL COMPLEXES OF MOLYBDENUM - SOLUTION BEHAVIOR AND SOLID-STATE STRUCTURE

Molybdenum acetyl complexes of composition Mo(C(O)CH3)(L-L)CO(PMe3)2 containing different bidentate anionic S- and O-donor ligands have been prepared and characterized. The dithiocarbamate derivatives (L-LS2CNR2; R = Me (1a), i-Pr (1b), C5H10 (1c)) have been found to display the agostic acetyl structure Mo(C(O)CH3) both in solution and in the solid state, as confirmed by X-ray studies carried out on complex 1c. This compound crystallizes in the triclinic space group P1BAR with unit cell parameters a = 9.408 (3) angstrom, b = 10.972 (8) angstrom, c = 11.720 angstrom, 76.60 (2)-degrees, and D(c) = 1.44 g cm-3 for Z = 2. Least-squares refinement using 6459 independent observed reflections led to a final R value of 0.051. The related xanthate ligands form two types of complexes: the normal xanthates Mo(C(O)CH3)(S2COR)CO(PMe3)2 (R = Me (2a), Et (2b), i-Pr (2c)) and phosphonium xanthates Mo(C(O)Ch3)(S2C(PMe3)OR)CO(PMe3)2 (R = Me (3a), CF3CH2 (3b), i-Pr (3c)). Compounds 2 are agostic in the solid state but exist in solution as equilibrium mixtures of the agostic acetyl Mo(C(O)CH3) (S2COR)CO(PMe3)2, the bidentate acyl Mo(eta2-C(O)CH3)(S2COR)CO(PMe3)2, and the methyl dicarbonyl Mo(CH3)(S2COR)(CO)2(PMe3)2. For 3, the M and i-Pr derivatives readily lose PMe3 to convert into the normal xanthates 2a and 2c, respectively, but 3b is stable toward loss of PMe3 and has an agostic structure both in solution and in the solid state. Crystals of 3b are monoclinic, space group C2/c, with cell parameters a = 29.698 (3) angstrom, b = 9.368 (3) angstrom, c = 20.992 (3) angstrom, beta = 117.10 (1)-degrees, and D(c) = 1.46 g cm-3 for Z = 8. Least-squares refinement based on 4773 observed reflections led to a final R value of 0.027. Other related complexes containing the O-donor ligands acac and t-Bu-acac (acac = CH3C(O)CHC(O)CH3 (4a); t-Bu-acac = Me3CC(O)CHC(O)CMe3 (4b)) have also been prepared and characterized.