Unusual electronic structures of square-planar iron-nitrosyl complexes

Abstract

Metal-nitrosyl species have attracted considerable attention due to their crucial roles in NO transformation chemistry. Although a plethora of {M–NO}ⁿ complexes have been reported, square-planar {M–NO}⁷ complexes remain unknown. This work describes the synthesis and characterization of an unprecedented square-planar {Fe–NO}⁷ species, [Fe(NO)(PCP)][BAr^F₄] (3, PCP = 2,6-bis(di-tert-butylphosphinomethyl)phenyl), and its one-electron reduced {Fe–NO}⁸ congener, [Fe(NO)(PCP)] (4), both derived from a square-pyramidal {Fe–NO}⁷ precursor, [FeBr(NO)(PCP)] (2). Those complexes were thoroughly characterized using single-crystal XRD, Mössbauer, EPR as well as IR spectroscopies, coupled to wavefunction based ab initio calculations. Our results reveal that the electronic structures of 2 and 3 are best interpreted as an S_Fe = 3/2 ferric center antiferromagnetically coupled to an S_NO = 1 NO⁻ ligand, yielding an overall S_t = 1/2 ground state. Conversion of 3 to 4 is metal-centered reduction, and hence 4 is comprised of an S_Fe = 1 Fe(II) ion antiferromagnetically bonded to an S_NO = 1 NO⁻, leading to a diamagnetic ground state. Complex 3 differs from all other {M–NO}⁷ systems in its ground level having an orbital two-fold near-degeneracy, manifested by g_z ≫ 2 > g_x,y, a spectroscopic signature that can be used to identify low-spin square-planar {M–NO}⁷ complexes.