Axial ligand effects on the geometric and electronic structures of nonheme oxoiron(IV) complexes

A series of complexes [Fe^IV(O)(TMC)(X)]⁺ (where X = OH^-, CF₃CO₂^-, N₃ ^-, NCS^-, NCO^-, and CN^-) were obtained by treatment of the well-characterized nonheme oxoiron(IV) complex [Fe^IV(O)(TMC)-(NCMe)]²⁺ (TMC = tetramethylcyclam) with the appropriate NR₄X salts. Because of the topology of the TMC macrocycle, the [Fe^IV(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 Mössbauer data collected for these complexes conclusively demonstrate that the characteristic spectroscopic features of the S = 1 Fe^IV=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 [Fe^IV(O)(TMC)(X)]⁺ species have Fe=O bond lengths similar to that of [Fe^IV(O)(TMC)(NCMe)]²⁺ (1.66 ± 0.02 Å). The mechanisms by which the trans ligands perturb the Fe ^IV=O unit were probed using density functional theory (DFT) computations, yielding geometric and electronic structures in good agreement with our experimental data. These calculations revealed that the trans ligands modulate the energies of the Fe=O σ- and π-antibonding molecular orbitals, causing the observed spectroscopic changes. Time-dependent DFT methods were used to aid in the assignment of the intense near-UV absorption bands found for the oxoiron(IV) complexes with trans N₃^-, NCS^-, and NCO^- ligands as X^--to-Fe ^IV=O charge-transfer transitions, thereby rationalizing the resonance enhancement of the ν(Fe=O) mode upon excitation of these chromophores.