Ruthenium(II) bis(terpyridine) electron transfer complexes with alkynyl–ferrocenyl bridges: synthesis, structures, and electrochemical and spectroscopic studies
Two novel alkynyl-bridged symmetric bis-tridentate ligands 1,2-bis(1′-[4′-(2,2′:6′,2′′-terpyridinyl)]ferrocenyl)ethyne (3a; tpy–Fc–CC–Fc–tpy; Fc = ferrocenyl; tpy = terpyridyl) and 1,4-bis(1′-[4′-(2,2′:6′,2′′-terpyridinyl)]ferrocenyl)-1,3-butadiyne (3b; tpy–Fc–CC–CC–Fc–tpy) and their Ru2+ complexes 6a and 6b have been synthesized and characterized by cyclic voltammetry, UV-vis and luminescence spectroscopy, and in the case of 3b by single-crystal X-ray diffraction. Cyclic voltammograms of both compounds, 3a and 3b, display two severely overlapping ferrocene-based oxidative peaks with only one reductive peak. The redox behavior of 6a and 6b is dominated by the Ru2+/Ru3+ redox couple (E1/2 from 1.33 to 1.34 V), the Fe2+/Fe3+ redox couples (E1/2 from 0.46 to 0.80 V), and the tpy/tpy−/tpy2− redox couples (E1/2 from −1.19 to −1.48 V). The UV-vis spectra of 6a and 6b show absorption bands assigned to the 1[(d(π)Fe)6] → 1[(d(π)Fe)5(π*tpyRu)1] MMLCT transition at ∼555 nm. Complexes 6a and 6b are luminescent in H2O–CH3CN (4:1, v/v) solution at room temperature, and 6b exhibits the strongest luminescence intensity (λmaxem: 710 nm, Φem: 2.28 × 10−4, τ: 358 ns) relative to analogous ferrocene-based bis(terpyridine) Ru(II) complexes reported so far.