Trinuclear [Fe III2–MgII] compounds with aminopyridines as precursors for supported CC bond hydrogenation catalysts at atmospheric pressure in a plug-flow reactor

Abstract

The development of precursors for hydrogenation processes based on heterometallic iron(III) complexes will provide a relevant alternative to “non-platinum” catalysts. Methods for the synthesis of pivalate heterometallic compounds {Fe^III–Mg^II} with aminopyridine (3-aminopyridine (β-ampy) and 4-aminopyridine (γ-ampy)) compositions [Fe₂Mg(O)(piv)₆(L)H₂O]n·solv (L = β-ampy (1); γ-ampy (2), piv⁻ – pivalic acid (Hpiv) anion, ^tBuCOOH; solv = 1.5CH₃CN (1)) have been developed. According to X-ray data, the structural unit of 1 and 2 is a trinuclear motif {Fe₂MgO}, which is connected into polymer chains through spiro fragments of ampy. The positions of metal atoms are clearly localized, which made it possible to unambiguously determine the geometric characteristics of the metal fragment. The formation and stabilization of supramolecular levels occur due to intermolecular hydrogen bonds, which form a clathrate system of a mesh type with included CH₃CN solvent molecules for 1. Mössbauer spectroscopy data for 1 and 2 contain exclusively paramagnetic Fe³⁺ ions in a high-spin state (S = 5/2), located in an octahedral environment of nitrogen and oxygen atoms. Magnetic measurements of complex 1 demonstrate slow relaxation of magnetization induced by a constant magnetic field, occurring through Raman and direct relaxation mechanisms. Based on complexes 1 and 2, catalysts supported on γ-Al₂O₃ were obtained, which were tested in the processes of styrene (vinylbenzene, VB) and dicyclopentadiene (DCPD) hydrogenation in a flow-type reactor at atmospheric hydrogen pressure. The catalyst sample obtained using 1 demonstrated satisfactory efficiency in the studied processes, which made it possible to obtain yields of the corresponding hydrogenation products with a conversion of up to 80%.