Syntheses and Properties of Heterobimetallic Ligand-Bridged Ruthenium(II)/Rhenium(I) Complexes and Their Monometallic Congeners

We have prepared three new heterobimetallic complexes containing electron-donating trans-{Ru^(II)Cl(pdma)_2}^+ [pdma = 1,2-phenylenebis(dimethylarsine)] centers linked to electron-deficient fac-{Re^I(biq)(CO)_3}^+ (biq = 2,2′-biquinolinyl) units. The bridging units are 4,4′-bipyridyl (4,4′-bpy), E-1,2-bis(4-pyridyl)ethylene (bpe), or 1,4-bis[E-2-(4-pyridyl)ethenyl]benzene (bpvb) ligands. A number of new monometallic precursor complexes have also been synthesized and fully characterized, primarily for purposes of comparison. The electronic absorption spectra of the bimetallic species are dominated by intense, visible d(Ru^(II)) → π^*(4,4′-bpy/bpe/bpvb) metal-to-ligand charge-transfer (MLCT) bands and d(Re^I) → π^*(biq) absorptions in the near-UV region. Cyclic voltammetric studies reveal both Ru^(III/II) oxidation and ligand-based reduction processes and show no evidence for significant electronic communication between the two metal centers. Stark spectroscopic studies on the visible MLCT bands show that extending the conjugation leads to increases in the dipole moment change and the transition dipole moment, and these changes combine to afford increased static first hyperpolarizabilities, β_0, estimated by using the two-state model. Comparisons with monometallic RuII complexes reveal that methylation of the free pyridyl nitrogen leads to larger β_0 responses than does coordination of the fac-{Re^I(biq)(CO)_3}^+ center. Single-crystal X-ray structures have been determined for solvated adducts of the bimetallic complex salts trans,fac-[Ru^(II)Cl(pdma)_2(μ-L−L)Re^I(CO)_3(biq)](PF_6)_2 (L−L = 4,4′-bpy or bpe) and also for the monometallic compounds fac-[Re^I(biq)(CO)_3(L−L)]PF_6 (L−L = 4,4′-bpy or bpe) and trans-[Ru^(II)Cl(pdma)_2(L−L)]PF_6 [L−L = 2,7-diazapyrene or (E,E)-1,4-bis(4-pyridyl)-1,3-butadiene]