Production of Lactic Acid via Catalytic Transfer Dehydrogenation of Glycerol Catalyzed by Base Metal Salt Ferrous Chloride and Its NNN Pincer-Iron Complexes.

Abstract

NNN-Bis(imino) pyridine-based pincer-Fe(II) complexes with an expected trigonal bipyramidal (TBP) geometry equilibrated to a rearranged ion pair of an octahedral dicationic Fe complex containing two bis(imino)pyridine ligands that are neutralized by a tetrahedral dianionic-[FeCl4]2-. Single-crystal X-ray diffraction (SCXRD), high-resolution mass spectrometry (HRMS), and UV-visible (UV-vis) studies suggested that the equilibrium was dictated by the sterics of the R group on the imine N, with the less-crowded groups tilting the equilibrium to the ion pair and the bulky ones favoring the TBP geometry. Electron paramagnetic resonance (EPR) and Evan's magnetic moment measurements indicated that the complexes were paramagnetic with Fe(II) in a high-spin state. In solution, over a period of 7 days, these Fe(II) complexes oxidized to a mixture of low-spin and high-spin Fe(III) species. These pincer-Fe(II) were found to be highly active toward the transformation of biodiesel waste glycerol to value-added lactic acid (LA). Particularly, (Ph2NNN)FeCl2 (0.1 mol %) gave 91% LA with a 99% selectivity at 140 °C using 1.2 equiv of NaOH. With 0.0001 mol % (Ph2NNN)FeCl2, very high turnovers (74% LA, 98% selectivity, 740 000 turnover number (TON) at 4405 turnovers per hour (TOs/h)) were obtained after 7 days. EPR indicated Fe(III) species to be the active catalyst, a few of which were detected by HRMS. Experiments with Hg are suggestive of the mostly homogeneous molecular nature of the catalyst with a minor contribution from heterogeneous Fe nanoparticles.