Dinuclear ruthenium(II) complexes [{(L)ClRuII}2(μ-tppz)]2+ (L = an arylazopyridine ligand) incorporating tetrakis(2-pyridyl) pyrazine (tppz) bridging ligand:: synthesis, structure and spectroelectrochemical properties

A series of dinuclear complexes [{(L1-4)C1Ru(II)}(2) (mu-tppz)][ClO4](2) {[1](ClO4)(2) to [4](ClO4)(2)} has been prepared, in which two {Ru-II(L1-4)Cl}(+) fragments [L = a 2-arylazopyridine ligand of the type 2-(C5H4N)-N=N-C6H4R; for L-1, R = H; L-2, R = p-Me; L-3, R = p-Cl; L-4, R = m-Me] are linked by the bridging ligand tppz [2,3,5,6-tetrakis(2-pyridyl) pyrazine]. A single isomer forms during the synthesis in each case, and the crystal structure of [4](ClO4)(2) shows it to be a twofold-symmetric isomer with each ligand L arranged such that its pyridine donor is on the long axis of the molecule (trans to the pyrazine ring of tppz) and the azo donor is trans to one of the pyridyl donors of tppz. This allows the peripheral aryl ring attached to the azo unit of each ligand L to be oriented over either face of the bridging ligand giving a three-layer pi-stacked (aryl-pyrazine-aryl) sandwich. Electrochemical studies revealed (i) separations of 190-250 mV (depending on the aryl substituent of L) between the successive Ru(II)/ Ru(III) couples, indicative of a significant inter-metallic electronic coupling, and (ii) several ligand-based reductions of the pi-acceptor pyrazine and arylazopyridine ligands. A UV/Vis/NIR spectroelectrochemical study showed the presence of an IVCT transition at ca. 1900 nm in MeCN for the Ru(II)-Ru(III) mixed-valence states, whose narrowness is indicative of borderline class III behaviour. Several reduced forms of the complexes were also spectroscopically characterised.