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-C[triple bond]C-Fc-tpy; Fc = ferrocenyl; tpy = terpyridyl) and 1,4-bis(1'-[4'-(2,2':6',2''-terpyridinyl)]ferrocenyl)-1,3-butadiyne (3b; tpy-Fc-C[triple bond]C-C[triple bond]C-Fc-tpy) and their Ru(2+) 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 Ru(2+)/Ru(3+) redox couple (E(1/2) from 1.33 to 1.34 V), the Fe(2+)/Fe(3+) redox couples (E(1/2) from 0.46 to 0.80 V), and the tpy/tpy(-)/tpy(2-) 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 and are luminescent in H(2)O-CH(3)CN (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.