Dinuclear complexes of a new bridging ligand containing 2,2'-bipyridyl and dioxolene binding sites: syntheses, electrochemical and electronic spectroscopic properties

Demethylation of the methoxy groups of 4-methyl-4'-{1-(3,4-dimethoxyphenyl)-ethen-2-yl}-2,2'-bipyridine (Me<SUB>2</SUB>L<SUP>1</SUP>) affords the new bridging ligand H<SUB>2</SUB>L<SUP>1</SUP> [4-methyl-4'-{1-(3,4-dihydroxyphenyl)-ethen-2-yl}-2,2'-bipyridine] which contains a 2,2'-bipyridyl binding site linked to a catechol binding site. The mononuclear complexes [Ru(bipy)<SUB>2</SUB>(Me<SUB>2</SUB>L<SUP>1</SUP>)][PF<SUB>6</SUB>]<SUB>2</SUB> (1) and [Ru(bipy)<SUB>2</SUB>(H<SUB>2</SUB>L<SUP>1</SUP>)][PF<SUB>6</SUB>]<SUB>2</SUB> (2), were prepared and attachment of a second metal fragment to the pendant catechol site of 2 afforded the dinuclear complexes [(bipy)<SUB>2</SUB>Ru(μ-L<SUP>1</SUP>)PdL][PF<SUB>6</SUB>]<SUB>2</SUB> (3, L = bipy; 4, L = 4,4'-<SUP>t</SUP>Bu<SUB>2</SUB>-bipy), [(bipy)<SUB>2</SUB>Ru(μ-L<SUP>1</SUP>)Pt(PPh<SUB>3</SUB>)<SUB>2</SUB>][PF<SUB>6</SUB>]<SUB>2</SUB> (5), [(bipy)<SUB>2</SUB>Ru(μ-L<SUP>1</SUP>)Ru(NO)(Cp<SUP>*</SUP>)][PF<SUB>6</SUB>]<SUB>2</SUB> (6) and [(bipy)<SUB>2</SUB>Ru(μ-L<SUP>1</SUP>)Ru(bipy)<SUB>2</SUB>][PF<SUB>6</SUB>]<SUB>3</SUB> (7). In 3-6 the coordinated dioxolene fragment is in the catechol oxidation state in each case; in 7 it has become oxidised to the semiquinone oxidation state. The dinuclear complexes show electrochemical and UV/VIS spectroscopic properties that are the sum of the component parts, with some perturbation: for example (i) 7 shows three reversible redox couples, associated with catecholate/semiquinone and semiquinone/quinone couples of the dioxolene fragment, and the Ru(II)/Ru(III) couple of the {Ru(bipy)<SUB>3</SUB>}<SUP>2+</SUP> core; (ii) the catecholate → bipy llct transitions of 3 and 4 are an order of magnitude more intense than that of the mononuclear Pd(II) complex [Pd(bipy)(cat)] (cat = catecholate dianion) alone. Whereas 1 and 2 show the characteristic luminescence of the {Ru(bipy)<SUB>3</SUB>}<SUP>2+</SUP> core, in all of the dinuclear complexes the luminescence is quenched