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

† Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan
‡ Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada V6T 1Z1
§ Department of Chemistry, Daido University, Takiharu-cho, Minami-ku, Nagoya 457-8530, Japan
Inorg. Chem., Article ASAP
DOI: 10.1021/acs.inorgchem.5b00536
Publication Date (Web): July 2, 2015
Copyright © 2015 American Chemical Society

Synopsis

A new dinuclear dinitrogen Fe(II) complex with a tridentate iminophosphorane ligand (NpNPiPr), [Fe(NpNPiPr)]2(μ-N2), was synthesized and characterized structurally and spectroscopically. The resonance Raman spectrum gave the ν(N2) band at 1755 cm−1, and X-ray analysis revealed a longer N≡N bond (1.184(6) Å). These data indicate that the N2 molecule is the most activated among those reported hitherto.

Abstract

Abstract Image
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 ν(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 δ = 0.73 mm/s and ΔEQ = 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.
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