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 ] ( S ) and quasi square-planar {K( C 222)} 2 [Fe(pin F ) 2 ] ( S' ) and trigonal-planar {K(18C6)}[Fe(OC 4 F 9 ) 3 ] ( T ) where pin F = perfluoropinacolate and OC 4 F 9 = tris -perfluoro- t -butoxide. The zero-field splitting (ZFS) and hyperfine structure parameters of the S = 2 ground states were determined using field-dependent 57 Fe Mössbauer 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 S has a small rhombicity, E / D = 0.082, and a positive D = 15.17 cm -1 , T 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 T 's Fe site are determined by the interaction of [Fe(OC 4 F 9 ) 3 ] - with {K(18C6)} + . In contrast, two distinct countercations of S / S' have a negligible influence on their [Fe(pin F ) 2 ] 2- moieties. Instead, the distortions in S' are likely induced by the chelate ring conformation change from δλ, observed for S , to the δδ conformation, determined for S' .