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

Ashley M King,Hazel A Sparkes,Duncan F Wass,Richard L Wingad

doi:10.1021/acs.organomet.0c00588

Ashley M King, Hazel A Sparkes + Show 2 more

Open Access

https://doi.org/10.1021/acs.organomet.0c00588

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Journal: Organometallics	Publication Date: Oct 26, 2020
Citations: 22	License type: cc-by

Affiliation: Cardiff University, University of Bristol

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.

Highlights

The search for alternative energy sources which are sustainable and meet concerns about energy security is a crucial scientific and technological goal.[1]
Biofuels offer an attractive alternative to liquid fossil fuels for transportation and can be economically viable and environmentally sound if the appropriate crops and agriculture methods are used.[2,3]
Bioethanol is widely used as a sustainable alternative fuel to conventional gasoline, often as a gasoline/bioethanol blend, but there are technological issues with this fuel: Ethanol has only 70% of the energy density of gasoline, readily absorbs water, can cause problems with separation and transportation in existing fuel infrastructure, and can be corrosive to current engine technology.[4,5]

Summary

■ INTRODUCTION

The search for alternative energy sources which are sustainable and meet concerns about energy security is a crucial scientific and technological goal.[1]. It remains inferior to the bis-chelate complexes of the same ligand suggesting its formation is detrimental It is not clear why dppm remains the most effective ligand for isobutanol production with both manganese and ruthenium. A recent paper by Kireev et al supports our hypothesis of dppm acting as a noninnocent ligand on manganese.[36] In this paper, monochelate 7 is reacted with KHMDS to form complex 7b, containing two highly strained 3-membered rings This can be converted to hydride complex 7c under 50 atm of hydrogen (Scheme 4). Complex 8 outperforms both monochelate 7 and the previously most effective catalyst, 2, by a significant margin, with turnover numbers in excess of 200 This shows the potential positive effects that substitution of the dppm backbone can have toward catalytic activity for manganese complexes.

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■ REFERENCES