PHOTOINDUCED HYDROMETALATION AND HYDROGENATION OF ACTIVATED OLEFINS WITH MOLYBDENUM AND TUNGSTEN DIHYDRIDES (CP2MH2)

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 CpFMo(CHCNCH2CN)H (I) has been established by X-ray crystallography, (space group P2ltmonoclinic, with α = 8.090 (3) Â, b = 10.282 (4) Â, c = 8.316 (3) Â, β = 116.92 (3)°, V = 617 Â3, Ζ = 2, R = 0.028, Rw= 0.028 for 1802 reflections with/> 3σ having 2Θ < 60°). Under the same photochemical conditions, the tungsten analogue Cp2WH2 effects quantitative hydrogénation 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 hydrogénation stemming from charge separation in the EDA complex (i.e. [Cp2MH2,+,>C“-JC<]) is delineated in terms of the one-electron-oxidation potential E”ol 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 hydrogénation of various activated olefins with those effected thermally at higher temperatures is discussed. © 1990, American Chemical Society. All rights reserved.