Efficient net transfer-dehydrogenation of glycerol: NNN pincer–Mn and manganese chloride as a catalyst unlocks the effortless production of lactic acid and isopropanol

Abstract

Herein, a series of pincer–Mn complexes based on bis(imino)pyridine ligands of the type ^R2NNN (R = ^tBu, ⁱPr, Cy and Ph) were synthesized and characterized using various spectroscopic techniques. SCXRD studies revealed a trigonal bipyramidal geometry around the metal center in all the complexes. EPR spectroscopy confirmed the presence of high-spin Mn(II) centers with the consistent observation of sextets in EPR spectra. Additionally, solution magnetic moment measurement exhibited values ranging from 5.8 to 6.2 BM for all the complexes, which are in accordance with the theoretical value of 5.92 BM. HRMS analysis complemented structural characterization, showing fragments corresponding to various solvent adducts and derivatives of the complexes. Subsequently, the synthesized complexes were investigated for their catalytic activity in the transfer dehydrogenation of glycerol to lactic acid in the presence of acetone. Among the considered complexes, the catalyst ^Ph2NNNMnCl₂ was found to be highly efficient. Remarkably, a yield of 92% LA was observed with >99% selectivity at 0.5 mol% loading of ^Ph2NNNMnCl₂ in the presence of 1 equivalent of NaOH at 140 °C in 24 h, surpassing the yield obtained from its precursor MnCl₂·4H₂O, where a yield of 72% LA was observed with 96% selectivity under similar reaction conditions. This catalytic system was further investigated with a range of acceptors, and good to moderate yields were observed in most cases. Moreover, several control experiments, including reaction with PPh₃, CS₂ and Hg, highlighted the major involvement of molecular species in the reaction medium. Deuterium labelling studies indicated the involvement of C–H bond activation in the catalytic cycle but not in the rate-determining step (RDS), with a secondary kinetic isotope effect (KIE) of 1.25.