Influence of the central diamagnetic cyanidometal on the distant magnetic interaction in cyanide-bridged Fe(III)-M(II)-Fe(III) complexes

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 approximate to -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.