Spectroscopic and Theoretical Investigation of High-Spin Square-Planar and Trigonal Fe(II) Complexes Supported by Fluorinated Alkoxides

The electronic structures and spectroscopic behavior of three high-spin Fe-II complexes of fluorinated alkoxides were studied: square-planar {K(DME)(2)}(2)[Fe(pin(F))(2)] (<bold>S</bold>) and quasi square-planar {K(C222)}(2)[Fe(pin(F))(2)] (<bold>S '</bold>) and trigonal-planar {K(18C6)}[Fe(OC4F9)(3)] (<bold>T</bold>) where pin(F) = perfluoropinacolate and OC4F9 = tris-perfluoro-t-butoxide. The zero-field splitting (ZFS) and hyperfine structure parameters of the S = 2 ground states were determined using field-dependent Fe-57 M & ouml;ssbauer and high-field and -frequency electron paramagnetic resonance (HFEPR) spectroscopies. The spin Hamiltonian parameters were analyzed with crystal field theory and corroborated by density functional theory (DFT) and ab initio complete active space self-consistent field (CASSCF) calculations. Whereas the ZFS tensor of <bold>S</bold> has a small rhombicity, E/D = 0.082, and a positive D = 15.17 cm(-1), <bold>T</bold> exhibits a negative D = -9.16 cm(-1) and a large rhombicity, E/D = 0.246. Computational investigation of the structural factors suggests that the ground-state electronic configuration and geometry of <bold>T</bold>'s Fe site are determined by the interaction of [Fe(OC4F9)(3)](-) with {K(18C6)}(+). In contrast, two distinct countercations of <bold>S</bold>/<bold>S '</bold> have a negligible influence on their [Fe(pin(F))(2)](2-) moieties. Instead, the distortions in <bold>S '</bold> are likely induced by the chelate ring conformation change from delta lambda, observed for <bold>S</bold>, to the delta delta conformation, determined for <bold>S '</bold>.