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

Abstract

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.

Rhenium Complexes Bearing Tridentate and Bidentate Phosphinoamine Ligands in the Production of Biofuel Alcohols via the Guerbet Reaction

“…Raising the reaction temperature to 200 °C (Table , entry 5), increased both the isobutanol yield and selectivity to 28% and 88%, respectively, although the ethanol conversion also increased (81%). It is instructive to compare this to the analogous manganese complex ,which produced 9% isobutanol over 90 h under the same conditions; this corresponds to a 17-fold increase in TOF . At 180 °C complex 8 shows similar performance to 7 (Table , entry 6).…”

“…Complexes 4 – 6 show little activity for isobutanol production over 17 h (entries 1–3). We have reported in stoichiometric studies that the catalytically active analogous manganese complex undergoes ligand redistribution reactions to give a mixture of the cis and trans bis chelate complexes, free ligand, and a mono chelate species . In contrast, treatment of 4 with 100 equiv of NaOMe in refluxing methanol cleanly produces a cis bis chelate complex analogous to 5 that is then stable under these conditions for at least 3 days.…”

“…We have detailed the use of manganese bis chelate and bidentate catalysts for isobutanol formation, demonstrating that tridentate complexes are not a prerequisite for activity . Given these results, it appeared that analogous rhenium complexes would be compelling targets as potential catalysts.…”

“…Under more practical conditions, Jones was able to generate 22% n -butanol over just 24 h. More recently, Liu has used the same catalyst for isobutanol formation, reporting yields of 40% in 96% selectivity over 48 h . We have reported the use of a variety of manganese complexes supported by bidentate ligands for isobutanol formation (Figure ), with F giving yields of up to 21% over 90 h …”

Structural Evolution of MOF-Derived RuCo, A General Catalyst for the Guerbet Reaction

“…The effect of the concentration of the sodium alkoxide base promoter (sodium propoxide for 1-propanol upgrading, sodium butoxide for 1-butanol upgrading, and sodium pentoxide for 1-pentanol upgrading) on both the catalyst activity and the selectivity with which the desired product is formed shows a remarkably similar trend to that observed for 1-butanol. In addition to the expected carboxylate product commonly formed in base-promoted Guerbet reactions via a competing Cannizzaro or Tishchenko reaction, ,,− ,,, products of incomplete reduction can be formed alongside the desired Guerbet alcohol (see Figures e and S4). Notably, higher catalytic activity for 1 was observed in the upgrading of 1-propanol compared to that previously reported for 1-butanol, and the selectivity for the formation of the desired Guerbet product 2-methylpentanol was likewise increased, particularly in the presence of high loadings of base promoter.…”