Syntheses, structure, and electrochemical properties of homo-metallic binuclear complexes containing ferrocenyl–ethynyl spacers

Structural determinations and electrochemical properties in the series of multinuclear ferrocenyl–ethynyl complexes with formula [(η5-C5R5)(P2)MII–CC–(fc)n–CC–MII(P2)(η5-C5R5)] (fc = ferrocenyl; M = Fe(II), Ru(II), Os(II); R = H, CH3; P2 = Ph2PCH2CH2PPh2 (dppe), (C2H5)2PCH2CH2P(C2H5)2 (depe)) are reported. Complexes with more electron-rich ligand environment, such as [M(η5-C5R5)P2] (R = CH3 and P2 = dppe, depe), were also prepared with regard to the understanding of electronic coupling mechanism. Structural determinations confirm that the ferrocenyl group is directly linked to the ethynyl linkage which is linked to the pseudo-octahedral [(η5-C5R5)(P2)M] metal center. These complexes undergo sequential reversible oxidation events from 0.0 to 1.0 V referred to the Ag/AgCl electrode in anhydrous CH2Cl2 solution and the low-potential waves have been assigned to the two end-capped metallic centers. The magnitude of the electronic coupling between the two terminal metallic centers in the series of complexes was estimated by the electrochemical technique. Based on the correlation between the ΔE1/2 values and the second redox potentials of the end-capping metallic centers in the series of complexes, a qualitative explanation for the difference of the electronic coupling is given.