The role of sterics and electronics of PNP pincer ligands in Iron(II) and manganese(I) chemistry

Abstract

Base metal catalysis is an emerging field in organometallic chemistry to replace precious metals by earth abundant metals. To achieve so, a suited ligand backbone to support the non-precious metals is needed, in order to get catalytic activity. The influence of well designed ligands on base metal chemistry is exampled on a series of iron(II) and manganese(II) compounds. The research bases on tridentate “pincer” ligands with a pyridine backbone connected with two phosphine donors (PNP-ligands). The ligands vary in the set-up of linkers (CH2, NH, NMe, O) and phosphine moiety PR2. A new class of iron(II) PNP pincer complexes, made up of two pincer ligands in different bonding modes (tridentate and bidentate) is described. The complexes of general formula κ3,κ2-[Fe(PNP)2X]+ are only observed when small phosphines (PR2; R = Me, Et, nPr, nBu, Ph) and a NH linker is apparent in the PNP ligands. In solution, the formation is inevitable, even when altering the stoichiometry. The 31P {1H} NMR gives rise to an A2B spin system for the coordinated phosphines, and a singlet for the vacant, non-coordinating phosphine. The X-ray structures reveal that a hydrogen bonding between NH linker and the pyridine nitrogen is stabilizing the coordination geometry. The distorted octahedral structure leads to a high degree of stress, which makes the bidentate ligand labile. Rearrangement to tridentate mode and displacement by carbon monoxide (CO) are possible. Carbonyl complexes of type [Fe(PNP)(CO)X2] are accessible, which are prone to CO release on thermal treatment, with increasing steric demand of the phosphine. Manganese(I) PNP pincer complexes of type [Mn(PNP)(CO)2H] were found to be pre-catalysts for the selective hydrogenation of aldehydes. Functional groups like ketones, nitriles, esters and olefins are tolerated. Among the selected PNP ligands, NH linkers delivered the best results. The results suggest that bifunctionality of the ligand along with ligand-metal cooperation is essential for the mechanism. Turnover numbers (TON) of up to 10.000 could be achieved. The hydrogenation proceeds at room temperature, without additives in protic media. Analogue rhenium(I) PNP pincer complexes [Re(PNP)(CO)2H] had inferior performance below 100 TON’s. Additionally, the hydrido complexes [Mn(PNP)(CO)2H] and [Re(PNP)(CO)2H] activate carbon dioxide (CO2) at ambient conditions. The 1,2-addition of CO2 leads to a series of formate complexes of the types [Mn(PNP)(CO)2(OCHO)] and [Re(PNP)(CO)2(OCHO)]. In summary, these results offer a guide for Mn(I) and Fe(II) pincer chemistry allowing to alter the chemical properties in a modular fashion.