Synthesis, structural features, absorption spectra, redox behaviour and luminescence properties of ruthenium(II) rack-type dinuclear complexes with ditopic, hydrazone-based ligands

The isomeric bis(tridentate) hydrazone ligand strands la-c react with [Ru(terpy)Cl-3] (terpy=2,2':6',2-terpyridine) to give dinuclear rack-type compounds 2a-c, which were characterised by several techniques, including X-ray crystallography and NMR methods. The absorption spectra, redox behaviour and luminescence properties (both in fluid solution at room temperature and in rigid matrix at 77 K) of the ligand strands 1a-c and of the metal complexes 2a-c have been studied. Compounds la-c exhibit absorption spectra dominated by intense pi-pi* bands, which, in the case of 1b and 1c, extend within the visible region, while the absorption spectra of the rack-type complexes 2a-c show intense bands both the in the UV region, due to spin-allowed ligand-centred (LC) transitions, and in the visible, due to spin-allowed metal-to-ligand charge-transfer (MLCT) transitions. The energy position of these bands strongly depends on the ligand strand: in the case of 2a, the lowest energy MLCT band is around 470 rim, while in 2b and 2c, it lies beyond 600 nm. Ligands 1a-c undergo oxidation processes that involve orbitals based mainly on the CH3-N-N= fragments. The complexes 2a-c undergo reversible metal-centred oxidation, while reductions involve the hydrazone-based ligands: in 2b and 2c, the bridging ligand is reduced twice and in 2a once before reduction of the peripheral terpy ligands takes place. Ligands la-c exhibit luminescence from the lowest-lying (1)pi-pi* level. Only for complex 2a does emission occur; this may be attributed to a (MLCT)-M-3 state involving the bridging ligand. Taken together, the results clearly indicate that the structural variations introduced translate into interesting differences in the spectroscopic, luminescence and redox properties of the ligand strands as well as of the rack-type metal complexes.