Models for conjugated metal acetylide polymers: Ruthenium oligothienylacetylide complexes

The series of ruthenium(II) mono(oligothienylacetylide) complexes trans-Ru(dppm)₂(Cl)(C≡CR) (dppm = Ph₂PCH₂PPh₂; R = 2-thienyl (1a), 5-(2,2′-bithienyl) (1b), and 5-(2,2′:5′,2″-terthienyl) (1c)) and bis(oligothienylacetylide) complexes trans-Ru(dppm)₂(C≡CR)₂ (R = 2-thienyl (2a), 5-(2,2′-bithienyl) (2b), and 5-(2,2′:5′,2″-terthienyl) (2c)) were synthesized. Complex 2c was crystallographically characterized. The cyclic voltammograms of complexes 1a-c all contain two oxidation waves, a Ru(II/III) wave and a ligand-based oxidation wave. As the length of the conjugated oligothienyl ligand increases, the thiophene-based oxidation wave becomes more chemically reversible. Complexes 2a-c all have a Ru(II/III) wave in their cyclic voltammograms, as well as multiple ligand-based oxidation waves. Complexes 2b and 2c both form films on the electrode surfaces upon repeated cycling in the range 0-1.4 V vs SCE. The UV-vis spectra of complexes 1a-c and 2a-c all contain intense absorptions due to the π-π* transition in the oligothienyl ligand, and these appear at lower energy than the π-π* transitions in the corresponding oligothiophenes. The monocations 1c⁺ and 2c⁺ were synthesized in solution at -20°C and were characterized by visible and near-IR spectroscopy. The π-π* transitions of the terthienyl ligand in 1c⁺ and 2c⁺ shift to higher energy compared with the analogous transitions in 1c and 2c, and a series of LMCT absorption bands of high intensity appear between 500 and 700 nm and between 900 and 1700 nm, respectively. These results support the conclusion that the π system of the conjugated oligothienyl ligands interacts strongly with the Ru(III) center.