Dinuclear Iron(III) and Cobalt(III) Complexes Featuring a Biradical Bridge: Their Molecular Structures and Magnetic,...

Bis-bidentate ligand H₄L^B featuring two o-amidophenol noninnocent units was used to synthesize novel binuclear complexes [(L^R)M^III(•L^B•)M^III(L^R)](ClO₄)₂, M = Fe (1) and Co (2, 3), with HL^R (R = CH₃, Cl) being the facially coordinating tetradentate coligands. Upon the synthesis, the fully reduced amidophenolate form of the ligand (L^B)^4- becomes oxidized, resulting in the formation of a rare example of a biradical (•L^B•)^2- bridge connecting two metal ions, as supported by X-ray crystallography. The electronic structures of the complexes have been probed by Mössbauer spectroscopy, magnetic susceptibility measurements, and electron paramagnetic resonance (EPR) spectroscopy. Species 1 contains two high-spin Fe(III) ions (S = 5/2) each coupled strongly antiferromagnetically (|J| > 150 cm^-1; Ĥ = -2JŜ₁Ŝ₂) with a semiquinone π-radical (S = 1/2) form of the bridging (•L^B•)^2- ligand. The effective S = 2 spins of each [Fe(III)+R^●] monomeric unit are then weakly ferromagnetically coupled with J = +0.22 cm^-1. Species 2 and 3 reveal very similar electronic structures: the low-spin Co(III) ion is diamagnetic, which leaves the two-spin carriers at the bridging (•L^B•)^2- biradical to display an isotropic EPR signal at g = 1.995 for 2 (1.993 for 3) in solution at room temperature and in the frozen state with no hyperfine structure. The weak half-field signal at g = 3.988 for 2 (3.978 for 3) was also observed at 17 K for the spin-forbidden |ΔM_S| = 2 transition due to ferromagnetically coupled S = 1/2 spins (J = +47 cm^-1) of the bridging biradical. The compounds show rich electrochemistry, displaying two (1) or four (2, 3) one-electron reversible processes. Normal and differential pulse voltammetry as well as constant potential coulometry, combined with EPR experiments, confirmed that the observed electron transfers are all localized at the bridging noninnocent (•L^B•)^2- ligand.