Syntheses, spectroscopic and molecular quadratic nonlinear optical properties of dipolar ruthenium(ii) complexes of the ligand 1,2-phenylenebis(dimethylarsine)

Six new complex salts trans-[RuIICl(pdma)2L][PF6]n [pdma = 1,2-phenylenebis(dimethylarsine); L = (E,E,E)-1,6-bis(4-pyridyl)hexa-1,3,5-triene (bph), n = 1, 5; L = N-methyl-4-[(E)-2-(4-pyridyl)ethenyl]pyridinium (Mebpe+), n = 2, 7; L = N-methyl-4-[(E,E)-4-(4-pyridyl)buta-1,3-dienyl]pyridinium (Mebpb+), n = 2, 8; L = N-methyl-4-{(E,E,E)-6-(4-pyridyl)hexa-1,3,5-trienyl}pyridinium (Mebph+), n = 2, 9; L = bis(4-pyridyl)acetylene (bpa), n = 1, 10; L = N-methyl-4-[2-(4-pyridyl)ethynyl]pyridinium (Mebpa+), n = 2, 11] have been prepared. The electronic absorption spectra of 5 and 7–11 display intense, visible metal-to-ligand charge-transfer (MLCT) bands, with λmax values in the range 434–492 nm in acetonitrile. Cyclic voltammetric studies reveal reversible RuIII/II waves with E1/2 values in the range 1.06–1.15 V vs. Ag–AgCl, together with irreversible L-based reduction processes. Along with a number of previously reported related compounds (B. J. Coe et al., J. Chem. Soc., Dalton Trans., 1996, 3917; 1997, 591; 2000, 797), salts 5 and 7–11 have been investigated by using Stark (electroabsorption) spectroscopy in butyronitrile glasses at 77 K. These studies have afforded dipole moment changes Δμ12 for the MLCT transitions which have been used to calculate molecular static first hyperpolarizabilities β0 according to the two-state equation β0 = 3Δμ12(μ12)2/(Emax)2 (μ12 = transition dipole moment, Emax = MLCT energy). MLCT absorption and electrochemical data show that a trans-{RuIICl(pdma)2}+ centre is considerably less electron-rich than a {RuII(NH3)5}2+ unit. Although the β0 responses of the pdma complexes are only a little smaller than those of their {RuII(NH3)5}2+ analogues, this result is partly attributable to unexpected changes in the relative μ12 values on freezing. Thus, substantial increases in μ12 for the arsine compounds act to partially offset the β0-decreasing influence of their higher Emax values when compared with the analogous pentaammine species. Single crystal X-ray structures have been obtained for the salts 1·2.5MeCN, 4·2MeCN, 7 and 11, but only 1·2.5MeCN adopts a non-centrosymmetric space group (Fdd2) such as may show bulk NLO effects.