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

Abstract

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(C222)} 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 Mossbauer 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 selfconsistent 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′.