Ruthenium(ii) bis(terpyridine) electron transfer complexes with alkynyl–ferrocenyl bridges: synthesis, structures, and electrochemical and spectroscopic studies

Abstract

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 (E_1/2 from 1.33 to 1.34 V), the Fe²⁺/Fe³⁺ redox couples (E_1/2 from 0.46 to 0.80 V), and the tpy/tpy⁻/tpy²⁻ redox couples (E_1/2 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*_tpy^Ru)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 (λ_max^em: 710 nm, Φ_em: 2.28 × 10⁻⁴, τ: 358 ns) relative to analogous ferrocene-based bis(terpyridine) Ru(II) complexes reported so far.