Manganese-Mediated Formic Acid Dehydrogenation.

A robust and rapid manganese formic acid (FA) dehydrogenation catalyst is reported. The manganese is supported by the recently developed, hybrid backbone chelate ligand PNNOP ( PNNOP=2,6-(di-tert-butylphosphinito)(di-tert-butylphosphinamine)pyridine) (1) and the catalyst is readily prepared with MnBrCO to form [( PNNOP)Mn(CO) ][Br] (2). Dehydrohalogenation of 2 generated the neutral five coordinate complex ( PNNOP)Mn(CO) (3). Dehydrogenation of FA by 2 and 3 was found to be highly efficient, exhibiting turnover frequencies (TOFs) exceeding 8500 h , rivaling many noble metal systems. The parent chelate, PONOP ( PONOP=2,6-bis(di-tert-butylphosphinito)pyridine) or PNNNP ( PNNNP=2,6-bis (di-tert-butylphosphinamine)pyridine), coordination complexes of Mn were synthesized, respectively affording [( PONOP)Mn(CO) ][Br] (4) and [( PNNNP)Mn(CO) ][Br] (5). FA dehydrogenation with the hybrid-ligand supported 2 exhibits superior catalysis to 4 and 5.