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Three new pyridyl thienopyridines 5-pyridin-2-yl-thieno[3,2-b]pyridine (1), 6-pyridin-2-yl-thieno[2,3-b]pyridine (2) and 6-pyridin-2-yl-thieno[3,2-c]pyridine (3) have been synthesized. 1 was synthesized via a chalcone-like precursor derived from (2-formyl-thiophen-3-yl)-carbamic acid ethyl ester (9). Removal of the carbamate group with HBr led to an immediate ring-closure of the intermediate amine to give 1 in an overall yield of 32% after 5 steps. 2 was synthesized by a Beckmann rearrangement of E-1-(5-bromo-thiophen-2-yl)-ethanone oxime (16) to yield N-(5-bromo-thiophen-2-yl)-acetamide (18). Reaction of 18 under modified Vilsmeier conditions led to the ring-closed product, 2-bromo-6-chloro-thieno[2,3-b]pyridine (19). Debromination followed by a Negishi cross-coupling with 2-pyridyl zinc bromide gave 2 in an overall yield of 8%. 3 was synthesized via formylation of 2-thiophen-3-yl-[1,3]dioxolane (22). A Wittig reaction between 22 and triphenyl-pyridin-2-yl-phosphonium bromide (25) gave the alkene, 2-[2-(3-[1,3]dioxolan-2-yl-thiophen-2-yl)-vinyl]­pyridine (26) in moderate yield.   Deprotection and ring closure of the oxime derivative of the resulting aldehyde gave 3 in an overall yield of 7.5% for this six step reaction.

The electrochemical and photophysical properties for a range of ruthenium(II) tris-2,2’-bipyridine complexes in which a thiophene substituent is attached to one of the bipyridine ligands via either a pendant or a fused mode have been determined. The fused mode of attachment eliminates torsional movement between the thiophene unit and the chelating bipyridine, thereby offering optimal overlap between the p systems of the chelating unit and the attached thiophene unit. The electrochemical properties of these complexes were found to be similar, however the luminescence lifetimes and intensities (in CH₃CN at room temperature) were found to be correlated to the mode of attachment. The longest luminescence lifetime was

observed for the complex [Ru(bpy)₂(4-thiophen-2-yl-[2,2']-bipyridine)]²⁺ (3000 ns), as compared to the prototypic [Ru(bpy)₃]²⁺ (1745 ns). This complex also had the highest quantum yield (0.045). In the four isomeric complexes where the thiophene ring was fused to the b or c face of the pyridine the lifetimes fell in the interval 275-1510 ns and the quantum yield ranged between 0.0047-0.014.

(© WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

The luminescence lifetimes (in CH 3CN at room temperature) and electrochemical potentials (in CH 3CN) of a range of mono- and bis(bidentate) 2,2′-bipyridine-capped oligothiophene-bridged Ru II complexes based on the 6-(2-thienyl)-2,2′-bipyridine and 4-(2-thienyl)-2,2′-bipyridine motifs have been measured. The redox potentials occurred in a very narrow range and showed only small shifts from that of [Ru(bpy) 3] 2+, which indicates that the inductive effects of the substituents on the 2,2′-bipyridine ligands are very similar across this series. In the complexes that incorporated a bithiophene moiety the oxidation of the bithienyl group occurred at higher potentials than the metal-centered Ru III/II oxidation. No or very weak interaction between the metal cores in the dinuclear complexes was observed. It was found that the luminescence lifetimes of the complexes where the attachment point of the oligothiophene bridge is in the 4-position of a 2,2′-bipyridine ligand were extended compared to [Ru(bpy) 3] 2+, whereas the luminescence was very short-lived (<30 ns) or completely quenched in the complexes where the oligothiophene bridge was attached in the 6-position. The difference in lifetimes is probably due to steric interactions between the thienyl bridge and the auxiliary bipyridyl ligands, resulting in disturbances in coordination symmetry of the metal core.

The tridentate thienopyridines 5-phenyl-7-(pyridin-2-yl)thieno[2,3-c]pyridine (L1), 7-(pyridin-2-yl)-5-(thiophen-2-yl)thieno[2,3-c]pyridine (L2) and 5,7-di(pyridin-2-yl)thieno[2,3-c]pyridine (L3) have been synthesized via the Hurtley reaction. L1 and L2 were synthesized by condensing 3-bromothiophene-2-carboxylic acid with phenyl-1,3-butanedione and 1-thienyl-1,3-butanedione respectively. L3 was synthesized by condensing 3-bromothiophene-2- carboxylic acid with benzoylacetonitrile. Ring closure and a subsequent Negishi or Stille cross-coupling afforded L1, L2 and L3 in an overall yield of 20%, 3% and 6% respectively. The corresponding [Ru(tpy)L]ⁿ⁺ complexes were prepared and their electrochemical and structural properties were examined.

Two new bis-terdentate ligands,  3,8-bis-(6-thiophen-2-yl-pyridin-2-yl)-[4,7]phenanthroline (L1) and 2,6-bis(6-(thiophen-2-yl)pyridin-2-yl)-1,5-naphthyridine (L2), capable of reversible double cyclometallation have been synthesized. Both syntheses proceeded via a Skraup synthesis followed by Stille cross-coupling with 2-thiophen-2-yl-6-tributylstannylpyridine to yield the final product. The Ru₂(tpy)₂-complex of L1 was shown to be capable of reversible cyclometallation, where the bis-cyclometallated (C,C) isomer of the complex had an oxidation potential of +249 mV while the oxidation potential of the doubly S-coordinated (S,S) complex was +987 mV. No ground state interaction between the Ru cores was however found. In theory the complex should display a third, mixed, state. An S,C-coordinated isomer would open the way for molecular electronic applications based on ternary logic. However, despite attempts the S,C-coordinated complex could not be generated.

A number of [Ru(tpy)L] complexes (where L is a ligand based on the 6-thien-2-yl-2,2’-bipyridine motif) have been generated to study the ability of this terdentate motif to switch between a N,N,C and a N,N,S binding mode as a function of pH and irradiation. Whilst the parent system 6-thien-2-yl-2,2’-bipyridine (1) displayed facile switching between the orange and purple forms, heavier analogues displayed sluggish switching – most likely resulting from a large moment of inertia about the bond between the S/C-bonded thiophene ring and the chelating bipyridine unit. Prototypic unsymmetrical bimetallic complexes (C7S and C7C) have been generated in order to test if the binding mode of the switching unit can affect the oxidation potential of the remote [Ru(bpy)₂(L)] unit. The results of cyclic voltammetry studies suggest that only weak electronic coupling between the metal cores exists, and that the coordination mode (N,N,C or N,N,S) of the thiophene moiety only has a small effect on the oxidation potential of the [Ru(bpy)₂(L)] unit.