Manganese 2-phosphinophosphinine precatalysts for methanol/ethanol upgrading to isobutanol

Abstract

Two Mn-phosphinophosphinine complexes were synthesised from reaction of the proligand with [MnBr(CO)₅] at 80 °C for 2 h; 2-diphenylphosphino-3-methyl-6-trimethylsilylphosphinine manganese tricarbonyl bromide (2^TMS) and 2-diphenylphosphino-3-methyl-phosphinine manganese tricarbonyl bromide (2^H). ³¹P{¹H} NMR spectroscopy revealed characteristic chemical shifts for the phosphinine and phosphine donors bound to Mn (255.4 and 23.7 ppm for 2^TMS; 234.2 and 24.8 ppm for 2^H), and single crystal X-ray diffraction established the structure of the chelating complex 2^TMS. Rapid reaction of both complexes with water was observed with 2^TMS reacting to eventually yield a single product, syn-3^TMS, from the syn-1,2-addition of water across the PC multiple bond on the bromide face, confirmed by X-ray diffraction for both an unsolvated and solvated structure, where MeOH was found to be H-bonding to the P-OH functionality. The reaction of 2^R with dry methanol gave multiple products that were not in equilibrium with each other, and the molecular structure of one isomer was definitively established by X-ray diffraction as an unusual 1,4-addition product (1,4-4^TMS). However, reaction of 2^R with methanol in the presence of trace water showed that hydrolysis products 3^R were formed preferentially. Both phosphinine complexes acted as pre-catalysts for the Guerbet upgrading of methanol/ethanol to isobutanol at 180 °C over 90 h, giving yields of isobutanol (based on moles of ethanol) of 22% for 2^TMS and 27% for 2^H. This is superior to known Mn dppm complexes [dppm = bis(diphenylphosphino)methane], including the 21% yield recorded for the best derivative [MnBr(κ²-PPh₂C(H)PhPPh₂)(CO)₃] shown to date.