Synthesis and Characterization of Hydrido Carbonyl Molybdenum and Tungsten PNP Pincer Complexes

In the present study the Mo(0) and W(0) complexes [M­(PNP)­(CO)3] as well as seven-coordinate cationic hydridocarbonyl Mo­(II) and W­(II) complexes of the type [M­(PNP)­(CO)3H]+, featuring PNP pincer ligands based on 2,6-diaminopyridine, have been prepared and fully characterized. The synthesis of Mo(0) complexes [Mo­(PNP)­(CO)3] was accomplished by treatment of [Mo­(CO)3(CH3CN)3] with the respective PNP ligands. The analogous W(0) complexes were prepared by reduction of the bromocarbonyl complexes [W­(PNP)­(CO)3Br]+ with NaHg. These intermediates were obtained from the known dinuclear complex [W­(CO)4(μ-Br)­Br]2, prepared in situ from W­(CO)6 and stoichiometric amounts of Br2. Addition of HBF4 to [M­(PNP)­(CO)3] resulted in clean protonation at the molybdenum and tungsten centers to generate the Mo­(II) and W­(II) hydride complexes [M­(PNP)­(CO)3H]+. The protonation is fully reversible, and upon addition of NEt3 as base the Mo(0) and W(0) complexes [M­(PNP)­(CO)3] are regenerated quantitatively. All heptacoordinate complexes exhibit fluxional behavior in solution. The mechanism of the dynamic process of the hydrido carbonyl complexes was investigated by means of DFT calculations, revealing that it occurs in a single step. The structures of representative complexes were determined by X-ray single-crystal analyses.

本研究合成并全面表征了Mo(0)与W(0)配合物[M(PNP)(CO)₃]，以及通式为[M(PNP)(CO)₃H]⁺的七配位阳离子氢羰基Mo(II)和W(II)配合物，这类配合物的配体为基于2,6-二氨基吡啶的PNP钳形配体（PNP pincer ligands）。[Mo(0)配合物[Mo(PNP)(CO)₃]的合成通过将[Mo(CO)₃(CH₃CN)₃]与相应PNP配体反应实现。类似的W(0)配合物则通过钠汞齐还原溴羰基配合物[W(PNP)(CO)₃Br]⁺制得。该类中间体可由已知双核配合物[W(CO)₄(μ-Br)Br]₂得到，该双核配合物由W(CO)₆与化学计量比的Br₂原位制备。向[M(PNP)(CO)₃]中加入HBF₄可在钼、钨中心发生无副反应的质子化，生成Mo(II)与W(II)氢化物配合物[M(PNP)(CO)₃H]⁺。该质子化过程完全可逆，加入三乙胺（NEt₃）作为碱后，可定量再生[M(PNP)(CO)₃]型Mo(0)和W(0)配合物。所有七配位配合物在溶液中均表现出流变行为。本研究通过密度泛函理论（DFT）计算探究了氢羰基配合物的动态过程机理，结果显示该过程为单步进行。代表性配合物的结构经X射线单晶衍射分析确定。