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

A new tridentate cyclopentane-bridged iminophosphorane ligand, N-(2-diisopropylphosphinophenyl)-P,P-diisopropyl-P-(2-(2,6-diisopropylphenylamido)cyclopent-1-enyl)phosphoranimine (NpNP), was synthesized and used in the preparation of a diiron dinitrogen complex. The reaction of the iron complex FeBr(NpNP) with KC under dinitrogen yielded the dinuclear dinitrogen Fe complex [Fe(NpNP)](μ-N), 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 NpNP 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(N/N) is 1755/1700 cm. The magnetic moment of [Fe(NpNP)](μ-N) in benzene-d solution, as measured by H NMR spectroscopy by the Evans method, is 6.91μ (S = 3). The Mössbauer spectrum at 78 K showed d = 0.73 mm/s and ?E = 1.83 mm/s. These findings suggest that the iron ions are divalent with a high-spin configuration and that the N molecule has (N-?N) character. Density functional theory calculations performed on [Fe(NpNP)](μ-N) 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 (π and π). 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 N π∗ orbitals supplied by the phosphine ligand.