Influence of the central diamagnetic cyanidometal on the distant magnetic interaction in cyanide-bridged Fe(iii)–M(ii)–Fe(iii) complexes

Abstract

To investigate how the central diamagnetic cyanidometal influences the distant magnetic interaction of cyanide-bridged Fe(III)–M(II)–Fe(III) complexes, cis-[Cp(dppe)Fe^II(NC)M^II(L)₂(CN)Fe^II(dppe) Cp][PF₆]₂ (M = Os, L = bpy 1; M = Os, L = phen 2; M = Fe, L = bpy 3; M = Fe, L = phen 4), and their one-electron oxidation products 5–7 and two-electron oxidation products 8–11 were synthesized and fully characterized. The cyclic voltammetry of complexes 1–4 suggests that both NC–Os^II(L)₂–CN and NC–Fe^II(L)₂–CN have electronic communication ability. The electronic absorption spectroscopy suggests the presence of the central M^II to the terminal Fe^III and the terminal Fe^II to the terminal Fe^III metal to metal charge transfers (MMCTs) in 5–7 and the central M^II to the terminal Fe^III MMCTs in 8–11. Moreover, for the two-electron oxidation products the MMCT energy increases with the central metal in the order Fe < Os < Ru. The two-electron oxidation complexes 8 and 9 exhibit a strong antiferromagnetic coupling (J ≈ −26 cm⁻¹) between the two distant Fe^III ions although separated by the diamagnetic cyanidometal NC–Os^II(L)₂–CN bridge. To the best of our knowledge, this is the strongest magnetic coupling between the distant paramagnetic metal ions across a diamagnetic cyanidometal bridge reported by far. For the two-electron oxidation complexes 10 and 11 with the diamagnetic NC–Fe^II(L)₂–CN bridge, however, the distant two Fe^III ions possess only very weak antiferromagnetic coupling (J = −0.15 and −0.19 cm⁻¹). Combined with our previous reported results, it could be found that the magnetic coupling strength between the distant Fe^III ions increases with the diamagnetic cyanidometal bridge in the order of Fe < Ru < Os.