Manganese Diphosphine and Phosphinoamine Complexes Are Effective Catalysts for the Production of Biofuel Alcohols via the Guerbet Reaction

We report a variety of manganese-based catalysts containing both chelating diphosphine (bis(diphenylphosphino)methane (dppm: 1 , 2 , and 7 ) or 1,2-bis(diphenylphosphino)ethane (dppe: 3 )), and mixed-donor phosphinoamine (2-(diphenylphosphino)ethylamine (dppea: 4 - 6 )) ligands for the upgrading of ethanol and methanol to the advanced biofuel isobutanol. These catalysts show moderate selectivity up to 74% along with turnover numbers greater than 100 over 90 h, with catalyst 2 supported by dppm demonstrating superior performance. The positive effect of substituting the ligand backbone was also displayed with a catalyst supported by C-phenyl-substituted dppm ( 8 ) having markedly improved performance compared to the parent dppm catalysts. Catalysts supported by the phosphinoamine ligand dppea are also active for the upgrading of ethanol to n -butanol. These results show that so-called PNP-pincer ligands are not a prerequisite for the use of manganese catalysts in Guerbet chemistry and that simple chelates can be used effectively.