N2 Activation by an Iron Complex with a Strong Electron-Donating Iminophosphorane Ligand

Abstract

A new tridentate cyclopentane-bridged iminophosphorane ligand, N-(2-diisopropylphosphinophenyl)-P,P-diisopropyl-P-(2-(2,6-diisopropylphenylamido)cyclopent-1-enyl)phosphoranimine (NpNPiPr), was synthesized and used in the preparation of a diiron dinitrogen complex. The reaction of the iron complex FeBr(NpNPiPr) with KC8 under dinitrogen yielded the dinuclear dinitrogen Fe complex [Fe(NpNPiPr)]2(μ-N2), which was characterized by X-ray analysis and resonance Raman and NMR spectroscopies. The X-ray analysis revealed a diiron complex bridged by the dinitrogen molecule, with each metal center coordinated by an NpNPiPr ligand and dinitrogen in a trigonal-monopyramidal geometry. The N-N bond length is 1.184(6) Å, and resonance Raman spectra indicate that the N-N stretching mode μ(14N2/15N2) is 1755/1700 cm-1. The magnetic moment of [Fe(NpNPiPr)]2(μ-N2) in benzene-d6 solution, as measured by 1H NMR spectroscopy by the Evans method, is 6.91μB (S = 3). The Mössbauer spectrum at 78 K showed d = 0.73 mm/s and ?EQ = 1.83 mm/s. These findings suggest that the iron ions are divalent with a high-spin configuration and that the N2 molecule has (N-?N)2- character. Density functional theory calculations performed on [Fe(NpNPiPr)]2(μ-N2) also suggested that the iron is in a high-spin divalent state and that the coordinated dinitrogen molecule is effectively activated by π back-donation from the two iron ions (dπ) to the dinitrogen molecule (πx and πy). This is supported by cooperation between a large negative charge on the iminophosphorane ligand and strong electron donation and effective orbital overlap between the iron dπ orbitals and N2 π∗ orbitals supplied by the phosphine ligand.