Bis(trimethylsilyl)ethyne as a Two-Electron Alkyne Ligand in Group 6 Carbonylmetal(O) Complexes: Photochemical Syntheses and Comprehensive Characterization of M(CO)_5(eta~2-Me_3SiC ident to CSiMe_3) (M = W, Mo,Cr) and trans-Mo(CO)_4(eta~2-Me_3SiC ide

摘要

Continued irradiation of W(CO)_6 in the presence of excess bis(trimethylsilyl)ethyne (btmse) in n-hexane solution yields W(CO)_5(eta~2-btmse) as the sole product,with quantum yields of 0.66 and 0.69 at lambda_(exc) = 365 and 313 nm,respectively.Cr(CO)_6 behaves analogously,while with Mo(CO)_6 the initially generated Mo(CO)_5(eta~2-btmse) undergoes further CO substitution to form trans-Mo(CO)_4(eta~2-btmse)_2 as the second product.All four compounds were isolated in crystalline form and fully characterized by elemental analysis,MS,IR,and NMR spectroscopies as well as by single-crystal X-ray crystallography.They assume a quasi-octahedral coordination geometry with the alkyne triple bond being eclipsed to one OC-M-CO axis and,in the case of trans-Mo(CO)_4(eta~2-btmse)_2,in trans-orthogonal orientation to the second alkyne.Both Mo(CO)_5(eta~2-btmse) and Cr(CO)_5(eta~2-btmse) are labile toward alkyne displacement by CO,whereas W(CO)_5(eta~2-btmse) and trans-Mo(CO)_4(eta~2-btmse)_2 undergo spontaneous ~(12)CO/~(13)CO exchange in the dark under mild conditions.Partially ~(13)CO labeled samples generated in this way provide complementary CO stretching vibrational data needed for thorough analyses at the level of the energy-factored CO force field approximation.From all the structural features and spectroscopic data it is evident that bis(trimethylsilyl)ethyne acts as a two-electron donor ligand in this series of d6 carbonylmetal(O) complexes.