A high-spin Fe(II)/low-spin Fe(III) redox couple featuring the hydro[tris(4-chloro-3,5-dimethyl-pyrazolyl)]borate ligand: Synthesis, spectroscopic and X-ray crystallographic characterization

The white homoleptic high-spin iron(II) complexes Fe[Tp^Me2,4Cl]₂ (1) was isolated in quantitative yield from reaction mixtures containing 1 equiv of FeCl₂(THF)_1.5 and 2 equiv of K[Tp^Me2,4Cl] (Tp^Me2,4Cl = hydrotris[(4-chloro-3,5-dimethyl-pyrazolyl)]borate). Its purple low-spin iron(III) counterparts 1[O₃SCF₃] and 1[PF₆] were synthesized and isolated in 85% yields upon treatment of 1 with 1 equiv of silver triflate and silver hexafluorophosphate, respectively. The three paramagnetic compounds are air and thermally stable as solids and in solution; they were characterized by elemental analyses, IR, magnetic susceptibility measurements, ¹H NMR, and Mössbauer spectroscopy. In addition, 1[PF₆] was authenticated by a single-crystal X-ray diffraction. The two scorpionate ligands are κ³-N,N′,N′′ ligated to the central Fe^III ion, forming an almost perfect FeN₆ octahedron with an average Fe-N bond distance of 1.9551(18) Å. In addition, complex 1 which oxidizes reversibly at E_1/2 = 0.483 V/SCE (ΔE_p = 94 mV), remains high-spin (S = 2) when the temperature is lowered to 2 K.