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.

Automated summary

High-valent iron species are important intermediates in catalytic oxidation reactions, and their study contributes to the understanding of both biological and synthetic systems. In this study, the redox chemistry of iron complexes with a dianionic tris-skatylmethylphosphonium (TSMP2-) scorpionate ligand was investigated. The results show that the complexes can undergo one-electron oxidation to form octahedral Fe(III) and Fe(IV) compounds, and the Fe(IV) compound has a triplet ground state. Extensive spectroscopic and computational methods were used to analyze its electronic structure.