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

Abstract

Demethylation of the methoxy groups of 4-methyl-4'-{1-(3,4-dimethoxyphenyl)-ethen-2-yl}-2,2'-bipyridine (Me₂L¹) affords the new bridging ligand H₂L¹ [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)₂(Me₂L¹)][PF₆]₂ (1) and [Ru(bipy)₂(H₂L¹)][PF₆]₂ (2), were prepared and attachment of a second metal fragment to the pendant catechol site of 2 afforded the dinuclear complexes [(bipy)₂Ru(μ-L¹)PdL][PF₆]₂ (3, L = bipy; 4, L = 4,4'-^tBu₂-bipy), [(bipy)₂Ru(μ-L¹)Pt(PPh₃)₂][PF₆]₂ (5), [(bipy)₂Ru(μ-L¹)Ru(NO)(Cp^*)][PF₆]₂ (6) and [(bipy)₂Ru(μ-L¹)Ru(bipy)₂][PF₆]₃ (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)₃}²⁺ 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)₃}²⁺ core, in all of the dinuclear complexes the luminescence is quenched.