Alkynyl-Bridged Ruthenium(II) 4′-Diferrocenyl-2,2′:6′,2′′-terpyridine Electron Transfer Complexes: Synthesis, Structures, and Electrochemical and Spectroscopic Studies

The novel ligand 4′-diferrocenylalkyne-2,2′:6′,2′′-terpyridine (7; Fc-CC-Fc-tpy; tpy = terpyridyl; Fc = ferrocenyl) and its Ru2+ complexes 8–10 have been synthesized and characterized by single-crystal X-ray diffraction, cyclic voltammetry, and UV–vis and luminescence spectroscopy. Electrochemical data and UV absorption and emission spectra indicate that the insertion of an ethynyl group causes delocalization of electrons in the extended π* orbitals. Cyclic voltammetric measurements of 7 show two successive reversible one-electron-oxidation processes with half-wave potentials of 0.53 and 0.78 V. The small variations of the E1/2 values for the Fe2+/Fe3+ redox couples after the coordination of the Ru2+ ion suggest a weak interaction between the Ru2+ and Fe2+ centers. After insertion of an ethynyl group, UV–vis absorption spectra show a red shift of the absorption peak of the 1[(d(π)Fe)6] → 1[(d(π)Fe)5(π*tpyRu)1] MMLCT of the Ru2+ complexes. The Ru2+ complex 8 exhibits the strongest luminescence intensity (λmaxem 712 nm, Φem = 2.63 × 10–4, τ = 323 ns) relative to analogous ferrocene-based terpyridine Ru(II) complexes in H2O/CH3CN (4/1 v/v) solution.