Axial Ligand Effects on the Geometric and Electronic Structures of Nonheme Oxoiron(IV) Complexes

A series of complexes [FeIV(O)(TMC)(X)]+ (where X = OH−, CF3CO2−, N3−, NCS−, NCO−, and CN−) were obtained by treatment of the well-characterized nonheme oxoiron(IV) complex [FeIV(O)(TMC)(NCMe)]2+ (TMC = tetramethylcyclam) with the appropriate NR4X salts. Because of the topology of the TMC macrocycle, the [FeIV(O)(TMC)(X)]+ series represents an extensive collection of S = 1 oxoiron(IV) complexes that only differ with respect to the ligand trans to the oxo unit. Electronic absorption, Fe K-edge X-ray absorption, resonance Raman, and Mossbauer data collected for these complexes conclusively demonstrate that the characteristic spectroscopic features of the S = 1 FeIV═O unit, namely, (i) the near-IR absorption properties, (ii) X-ray absorption pre-edge intensities, and (iii) quadrupole splitting parameters, are strongly dependent on the identity of the trans ligand. However, on the basis of extended X-ray absorption fine structure data, most [FeIV(O)(TMC)(X)]+ species have Fe═O bond lengths similar to that of ...