Synthesis of Aryl-Substituted Bis(imino)pyridine Iron Dinitrogen Complexes

The synthesis and characterization of dimeric, aryl-substituted bis(imino)pyridine iron dinitrogen complexes is described. In contrast to reduction with sodium amalgam where bis(chelate) iron compounds were isolated, stirring (ArPDI)FeBr2 or (MeBPDI)FeBr2 (PDI = 2,6-(ArNCMe)2C5H3N; Ar = 2,6-Et2-C6H3N (EtPDI), 2,6-Me2-C6H3N (MePDI), 2-iPr,6-Me-C6H3N (Me,iPrPDI); MeBPDI = 2,6-(2,6-Me2-C6H3NCPh)2C5H3N) with sodium naphthalenide resulted in isolation of the desired iron dinitrogen compounds as diamagnetic solids. Two examples, [(EtPDI)Fe(N2)]2(μ2-N2) and [(MeBPDI)Fe(N2)]2(μ2-N2), were characterized by X-ray diffraction. The solid state metrical parameters, in combination with infrared and Mössbauer spectroscopic data, establish ferrous compounds with doubly reduced chelates. Each new bis(imino)pyridine iron dinitrogen compound was screened for the catalytic hydrogenation of ethyl-3-methylbut-2-enoate, and the compound bearing the smallest aryl substituent, [(MePDI)Fe(N2)]2(μ2-N2), offers significant improvement over the original (iPrPDI)Fe(N2)2 pre-catalyst and is one of the most active iron pre-catalysts known.