Synthesis and Catalytic Reactivity of Bis(alkylzinc)-hydride-di(2-pyridylmethyl)amides

Direct zincation of dipicolylamin (DPA) and alcohols (MeOH, iPrOH, tBuOH) with dialkylzinc gives bis(alkylzinc)alkoxide-di(2-pyridylmethyl)amides {alkyl = methyl (1, 2), trimethylsilylmethyl (3), bis(trimethylsilyl)methyl (4), alkoxide = OMe, OiPr, OtBu} possessing a central four-membered Zn2NO unit. Treatment of these alkoxides 1−4 with arylsilanes leads to an exchange of the (μ-OR′) moiety, yielding the corresponding hydrides [(RZn)2(μ-H){μ-N(CH2Py)2}] {R = Me (6), CH2SiMe3 (7), CH(SiMe3)2 (8)}. Hydride 6 reacts with tBuNH2 to form the corresponding amide [(MeZn)2{μ-N(H)tBu}{μ-N(CH2Py)2}] (5) and adds acetone to yield 2 again. The trimethylsilyl-substituted derivative 7 undergoes spontaneous conversion to form the pentanuclear zinc hydride-bridged dimer [{Me3SiCH2Zn}4{Zn(μ-H)4}{μ-N(CH2Py)2}2] (9). The new hydride complexes were characterized in solution and in the solid state including single-crystal X-ray analysis of 8 and 9. Both hydrides 6−8 and alkoxides 1−4 were found to catalyze the hydrosilylation of aldehydes and ketones effectively. The variation of the zinc-bound alkyl group facilitates control over the catalyst reactivity by steric and electronic means. In order to achieve a deeper insight into the mechanism and the role of the cocatalytic Zn(II) centers, extensive DFT calculations were performed. In the two-step catalytic process the ketone first coordinates to one catalytic center of 6 and thus cleaves a Zn−(μ-H) bond. A subsequent intramolecular hydride transfer leads to the formation of the bridged dinuclear zinc alkoxide being the most stable species in this cycle. In the second half of the cycle, possessing the highest activation barrier, the silane inserts into a Zn−O bond, forming a six-membered ring with a Zn[μ-(H−Si−O)]Zn moiety. Consecutive cleavage of the Si−HZn and Zn−O bonds regenerates the zinc hydride 6 along with the formation of the silyl ether. NMR spectroscopic studies support these findings.