Axial Ligand Effects on the Geometric And Electronic Structures of Nonheme Oxoiron(IV) Complexes
A series of complexes [Fe{sup IV}(O)(TMC)(X)]{sup +} (where X = OH{sup -}, CF{sub 3}CO{sub 2}{sup -}, N{sub 3}{sup -}, NCS{sup -}, NCO{sup -}, and CN{sup -}) were obtained by treatment of the well-characterized nonheme oxoiron(IV) complex [Fe{sup IV}(O)(TMC)(NCMe)]{sup 2+} (TMC = tetramethylcyclam) with the appropriate NR{sub 4}X salts. Because of the topology of the TMC macrocycle, the [Fe{sup IV}(O)(TMC)(X)]{sup +} 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 Moessbauer data collected for these complexes conclusively demonstrate that the characteristic spectroscopic features of the S = 1 Fe{sup IV} {double_bond} O unit, namely, (1) the near-IR absorption properties, (2) X-ray absorption pre-edge intensities, and (3) 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{sup IV}(O)(TMC)(X)]{sup +} species have Fe {double_bond} O bond lengths similar to that of [Fe{sup IV}(O)(TMC)(NCMe)]{sup 2+} (1.66 {+-} 0.02 {angstrom}). The mechanisms by which the trans ligands perturb the Fe{sup IV} {double_bond} 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 {double_bond} O {sigma}- and {pi}-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 N{sub 3}{sup -}, NCS{sup -}, and NCO{sup -} ligands as X{sup -}-to-Fe{sup IV} {double_bond} O charge-transfer transitions, thereby rationalizing the resonance enhancement of the {nu}(Fe {double_bond} O) mode upon excitation of these chromophores.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC02-76SF00515
- OSTI ID:
- 953128
- Report Number(s):
- SLAC-REPRINT-2009-156; TRN: US200914%%203
- Journal Information:
- J. Am. Chem. Soc. 130:12394,2008, Vol. 130, Issue 37
- Country of Publication:
- United States
- Language:
- English
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