Spin-state regulation of iron(III) centres by axial ligands with tetradentate bis(picolinamide) in-plane ligands

Abstract

Mononuclear iron(III) complexes trans-[Fe(bpb)X₂]^+/–[H₂bpb = 1,2-bis(pyridine-2-carboxamido)-benzene; X = pyridine (py)1, N₃^–2, MeCO₂^–3 or CN^–4] and trans-[Fe(bpc)X₂]^+/–[H₂bpc = 4,5-dichloro-1,2-bis(pyridine-2-carboxamido)benzene; X = py 5, Cl^–6 or MeCO₂^–7] have been synthesised and characterized. Complexes 1 and 4 are low-spin (µ_eff= 2.50 and 2.10 at 298 K, solution phase) whereas 2, 3, 6 and 7(µ_eff= 5.86–5.98 at 298 K, solution phase) are high spin. Solid-state magnetic susceptibility (40–300 K) and Mössbauer spectral measurements at 300 and 77 K of 1 and 7 confirmed the spin state. The complex [NEt₄][Fe(bpc)(MeCO₂)₂]·CHCl₃7 has been characterized by X-ray crystallography: space group P2₁/c, a= 12.283(3), b= 18.819(3), c= 16.437(3)Å, β= 101.02(2)°, Z= 4, R=R' = 0.041 for 5069 observed reflections. The solid-state EPR spectra (300 and 77 K) of 1, 4 and 5 are typical of low-spin iron(III) and have been analysed to determine the ligand-field parameters. All the complexes display ligand-to-metal charge-transfer transitions in the visible region (550–800 nm). Cyclic voltammetric measurements of 1 in pyridine and 4 in dimethylformamide revealed a quasi-reversible Fe^III–Fe^II reduction [E_½=–0.06 V, ΔE_p= 150 mV for 1 and E_½=–0.83 V, ΔE_p= 100 mV for 4vs. saturated calomel electrode (SCE)]. The high-spin complexes exhibit an electrochemically irreversible but chemically reversible Fe^III–Fe^II reduction (E_pc=–0.44 to –0.83 V) in acetonitrile solution. Complexes 2, 4 and 6 show a reversible one-electron oxidation (E_½=+0.66 to +1.05 V vs. SCE).