Solid State Collapse of a High-Spin Square-Planar Fe(II) Complex, Solution Phase Dynamics, and Electronic Structure Characterization of an Fe(II)2 Dimer

Square-planar high-spin Fe­(II) molecular compounds are rare, and until recently, the only four examples of non-macrocyclic or sterically driven molecular compounds of this kind shared a common FeO4 core. The trianionic pincer-type ligand [CF3-ONO]­H3 (1) supports the high-spin square-planar Fe­(II) complex {[CF3-ONO]­FeCl}­{Li­(Sv)2}2 (2). In the solid state, 2 forms the dimer complex {[CF3-ONO]­Fe}2­{(μ-Cl)2­(μ-LiTHF)4} (3) in 96% yield by simply applying a vacuum or stirring it with pentane for 2 h. A detailed high-frequency electron paramagnetic resonance and field-dependent 57Fe Mössbauer investigation of 3 revealed a weak antiferromagnetic exchange interaction between the local iron spins which exhibit a zero-field splitting tensor characterized by negative D parameter. In solution, 2 is in equilibrium with the solvento complex {[CF3-ONO]­FeCl­(THF)}­{Li2(Sv)4} (2·Sv) and the dimer 3. A combination of frozen solution 57Fe Mössbauer spectroscopy and single crystal X-ray crystallography helped elucidate the solvent dependent equilibrium between these three species. The oxidation chemistry of 2·Sv was investigated. Complex 2 reacts readily with the one-electron oxidizing agent CuCl2 to give the Fe­(III) complex {[CF3-ONO]­FeCl2}­{Li­(THF)2}2 (4). Also, 2·Sv reacts with 2 equiv of TlPF6 to form the Fe­(III) complex [CF3-ONO]­Fe­(THF)3 (5).