Long-distance electronic interaction in a molecular wire consisting of a ferrocenyl–ethynyl unit bridging two [(η5-C5H5)(dppe)M] metal centers

Several multinuclear ferrocenyl–ethynyl complexes of formula [(η5-C5H5)(dppe)MII−Ctriple bond; length of mdashC–(fc)n–Ctriple bond; length of mdashC–MII(dppe)(η5-C5H5)] (fc = ferrocenyl; dppe = Ph2PCH2CH2PPh2; 1: MII = Ru2+, n = 1; 2: MII = Ru2+, n = 2; 3: MII = Ru2+, n = 3; 4: MII = Fe2+, n = 2; 5: MII = Fe2+, n = 3) were studied. Structural determinations of 2 and 4 confirm the ferrocenyl group directly linked to the ethynyl linkage which is linked to the pseudo-octahedral [(η5-C5H5)(dppe)M] metal center. Complexes of 1–5 undergo sequential reversible oxidation events from 0.0 V to 1.0 V referred to the Ag/AgCl electrode in anhydrous CH2Cl2 solution and the low-potential waves have been assigned to the end-capped metallic centers. The solid-state and solution-state electronic configurations in the resulting oxidation products of [1]+ and [2]2+ were characterized by IR, X-band EPR spectroscopy, and UV–Vis at room temperature and 77 K. In [1]+ and [2]2+, broad intervalence transition band near 1600 nm is assigned to the intervalence transition involving photo-induced electron transfer between the Ru3+ and Fe2+ metal centers, indicating the existence of strong metal-to-metal interaction. Application of Hush’s theoretical analysis of intervalence transition band to determine the nature and magnitude of the electronic coupling between the metal sites in complexes [1]+ and [2]2+ is also reported. Computational calculations reveal that the ferrocenyl–ethynyl-based orbitals do mix significantly with the (η5-C5H5)(dppe)Ru metallic orbitals. It clearly appears from this work that the ferrocenyl–ethynyl spacers strongly contribute in propagating electron delocalization.