SYNTHESIS, STRUCTURE, AND REACTIVITY OF THE REMARKABLY STABLE EARLY-LATE BINUCLEAR METAL HYDRIDE CP2TA(MU-CH2)2IR(CP-ASTERISK)(H) - A WARNING CONCERNING CATALYST IDENTIFICATION IN HOMOGENEOUS CATALYTIC REACTIONS

The early-late heterobimetallic complex Cp2Ta(mu-CH2)2Ir(Cp*)(Cl) (1) was isolated in 68% yield from the reaction of 0.5 equiv of [Cp*IrCl2]2 with LiN(SiMe3)2 and Cp2Ta(CH2)(CH3). The complex mechanism of this reaction, investigated by low-temperature NMR and ESR spectroscopy, involves paramagnetic intermediates. Treatment of 1 with LiBH4 led to the isolation of Cp2Ta(mu-CH2)2Ir(CP*)(H) (2), which was characterized by X-ray crystallography (monoclinic space group P2(1)/n (Z = 4) with a = 9.034(2) angstrom, b = 13.603(4) angstrom, c = 15.974(2) angstrom, and beta = 101.287(16)degrees). The deuterium analogue of 2, CP2Ta(mu-CH2)2Ir(Cp*)(D)(3), was prepared by treating 1 with LiAlD4. Complex 2 could be converted back to the chloride 1 upon reaction with CCl4. Surprisingly, the thermally stable 2 was unreactive toward phosphines and D2, as well as several other reagents, but reacted slowly with an excess of CO at 135-degrees-C to form Cp2-Ta(mu-CH2)2Ir(CO)2 (5) and 1 equiv of Cp*H. Treatment of 2 with MeI at 105-degrees-C led to the slow formation of Cp2Ta(mu-CH2)2Ir(Cp*)(I) (6) (which exhibited temperature dependent NMR spectra in solution) together with 1 equiv of CH4. Despite the marked stability of 2, we found that ethylene was readily hydrogenated in the presence of 2 and H-2 at room temperature. Although this at first appeared to be a well-behaved homogeneous reaction, further investigation of this reaction revealed that 2 itself cannot be the hydrogenation catalyst.