Axial Coordination of Pseudohalides Modulates the Physical Properties of a Non-Heme Fe=O Unit

High valent non-heme iron-oxygen species, such as Fe=O adducts, have been proposed as key intermediates for numerous non-heme iron enzymes. However, only one such Fe=O adduct has been detected in an enzyme system. In contrast, synthetic complexes containing non-heme Fe=O units have been generated using a variety of supporting ligands, and these studies have already demonstrated that the coordination environment of Fe=O complexes influences spectroscopic and reactivity properties. The best characterized of these non-heme Fe=O adducts is [Fe(O)(TMC)(NCCH3)] 2+ (1-NCCH3), where TMC is 1,4,8,11-tetramethyl-1,4,8,11tetraazacyclotetradecane. Recently it has been shown that the axially coordinated NCCH3 ligand that is trans to the oxo ligand may be substituted by trifluoroacetate (CF3CO2 ), yielding a new complex, [Fe(O)(TMC){OC(O)CF3}] , that is a more reactive one-electron oxidant. To further explore the effects of axial ligation on the spectroscopic and reactivity properties of Fe=O adducts supported by TMC, the abilities of the pseudohalides azide (N3 ) and thiocyanate (NCS) to displace NCCH3 were examined. Treatment of 1-NCCH3 with one equivalent of N3 or NCS yielded [Fe(O)(TMC)(N3)] + (1-N3) and [Fe (O)(TMC)(NCS)] (1NCS), respectively. These complexes both have altered electronic absorption spectra relative to 1-NCCH3, particularly in regards to the near-UV spectral region (~410 nm) where the 1-N3 and 1-NCS complexes exhibit moderately intense features that likely arise from ligand-to-metal charge transfer transitions. 1-N3 and 1-NCS were further characterized using X-ray absorption spectroscopy, and an extended X-ray absorption fine structure (EXAFS) analysis of the corresponding X-ray absorption spectra yielded metal-ligand bond lengths for these species. In regards to stability, both 1-N3 and 1-NCS display shorter half-lives than 1-NCCH3. The reactivities of these complexes towards substrates are discussed.