We have developed new synthetic routes to hydroxycyclopentadienyl complexes of cobalt and group 6 metals (Cr, Mo, W), analogs of the Ru Shvo type complexes. The reaction of commercially available tetraphenylcyclopentadienone with Co2(CO)8 led to the formation of rare mixed-valence tricobalt complex [Ph4C4CO]Co(CO)2]2 [ μ-Co(MeCN)2], where the central Co(II) bridges the carbonyl groups of two cyclopentadienone ligated Co(I) species. In contrast to the conventional Pauson-Khand reaction to generate cyclopentadienone-ligated cobalt complexes, this method benefits from both mild reaction conditions and affords the products in high yields. Treatment of this tricobalt cobalt complex with proton source such as water or alcohols yields monomeric cobalt complex {[(Ph)4CpOH]Co(CO)2}. Phosphite substituted monomeric cobalt complexes {[(Ph)4CpOH]Co(CO)[P(OR)3]} were also synthesized, and protonation of these complexes yields the corresponding cobalt hydrides. The previously reported hydroxy-cyclopentadienyl Mo complex {[(Ph)4CpOH]Mo(CO)3[AcCN]} was found to be an effective precatalyst for the transfer hydrogenation of ketones, aldehydes, imines and allylic alcohols with 2-propanol or formic acid. A newly designed bidentate cyclopentadienone ligand was prepared and ligated to Cr, Mo, W, Fe, and Ru carbonyl complexes. The phosphine-tethered Mo complex showed superior catalytic activity in transfer hydrogenation than {[(Ph)4CpOH]Mo(CO)3[AcCN]}. Furthermore, enantiomers of axially chiral Ru analog were isolated and shown to mediate the asymmetric hydrogenation of secondary imines with modest enantioselectivities.

Synthesis of Co and Mo hydroxycyclopentadienyl complexes and their catalytic activities in transfer hydrogenation

253rd National Meeting (2017)

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