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.