Homoleptic Fe(III) and Fe(IV) Complexes of a Dianionic C₃-Symmetric Scorpionate

High-valent ironspecies have been implicated as key intermediatesin catalytic oxidation reactions, both in biological and syntheticsystems. Many heteroleptic Fe(IV) complexes have now been preparedand characterized, especially using strongly & pi;-donating oxo,imido, or nitrido ligands. On the other hand, homoleptic examplesare scarce. Herein, we investigate the redox chemistry of iron complexesof the dianonic tris-skatylmethylphosphonium (TSMP2-) scorpionate ligand. One-electron oxidation of the tetrahedral,bis-ligated [(TSMP)(2)Fe-II](2-) leads to the octahedral [(TSMP)(2)Fe-III](-). The latter undergoes thermal spin-cross-over bothin the solid state and solution, which we characterize using superconductingquantum inference device (SQUID), Evans method, and paramagnetic nuclearmagnetic resonance spectroscopy. Furthermore, [(TSMP)(2)Fe-III](-) can be reversibly oxidized to the stablehigh-valent [(TSMP)(2)Fe-IV](0) complex.We use a variety of electrochemical, spectroscopic, and computationaltechniques as well as SQUID magnetometry to establish a triplet (S = 1) ground state with a metal-centered oxidation andlittle spin delocalization on the ligand. The complex also has a fairlyisotropic g-tensor (g (iso) = 1.97) combined with a positive zero-field splitting (ZFS) parameter D (+19.1 cm(-1)) and very low rhombicity,in agreement with quantum chemical calculations. This thorough spectroscopiccharacterization contributes to a general understanding of octahedralFe(IV) complexes. Theredox chemistry of bis-ligated homoleptic complexesof iron with a dianionic C-3-symmetric tris-skatylmethylphosphonium(TSMP2-) scorpionate ligand is investigated. Thepseudotetrahedral [(TSMP)(2)Fe-II](2-) complex can undergo two consecutive one-electron oxidations to givelocally octahedral Fe(III) and Fe(IV) compounds. The Fe(III) complexundergoes thermal spin-cross-over between the doublet and sextet states.The Fe(IV) compound has a triplet ground state. Its electronic structureis extensively analyzed by a combination of spectroscopic and computationalmethods.