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.