Photoinduced hydrometalation and hydrogenation of activated olefins with molybdenum and tungsten dihydrides (Cp2MH2)

Article abstract of DOI:10.1021/om00156a024

The early-transition-metal hydrides Cp₂MoH₂, Cp₂WH₂, and Cp₂ReH rapidly form a series of electron donor-acceptor (EDA) complexes with various activated olefins as shown by the spontaneous appearance of vivid colors, the absorption energies of which correlate with the electron affinity of the olefinic acceptor and the oxidation (ionization) potential of the hydridometal donor in accord with Mulliken theory. Deliberate excitation of the charge-transfer absorption band leads to the quantitative hydrometalation of fumaronitrile by Cp₂MoH₂ at 25°C, and the structure of the σ hydrido alkyl adduct Cp₂Mo(CHCNCH₂CN)H (I) has been established by X-ray crystallography, (space group P2₁, monoclinic, with a = 8.090 (3) A?, b = 10.282 (4) A?, c = 8.316 (3) A?, β = 116.92 (3)°, V = 617 A?³, Z = 2, R = 0.028, R_w = 0.028 for 1802 reflections with I > 3σ having 2θ ≤ 60°). Under the same photochemical conditions, the tungsten analogue Cp₂WH₂ effects quantitative hydrogenation and leads to succinonitrile together with the olefinic π-adducts to tungstenocene in high yields. (In both cases, the thermal (dark) processes are nonexistent) The charge-transfer mechanism for olefin hydrometalation and hydrogenation stemming from charge separation in the EDA complex (i.e. [Cp₂MH₂^?+,>C^--C ^?<]) is delineated in terms of the one-electron-oxidation potential E°_ox of the hydridometal species and the subsequent facile proton transfer from the labile cation radical Cp₂MH₂^?+ (M = Mo, W) to the acceptor moiety. The close similarity of the photoinduced process for olefin hydrometalation and hydrogenation of various activated olefins with those effected thermally at higher temperatures is discussed.