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

Abstract

Six new complex salts trans-[Ru^IICl(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 λ_max values in the range 434–492 nm in acetonitrile. Cyclic voltammetric studies reveal reversible Ru^III/II waves with E_1/2 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/(E_max)² (μ₁₂ = transition dipole moment, E_max = MLCT energy). MLCT absorption and electrochemical data show that a trans-{Ru^IICl(pdma)₂}⁺ centre is considerably less electron-rich than a {Ru^II(NH₃)₅}²⁺ unit. Although the β₀ responses of the pdma complexes are only a little smaller than those of their {Ru^II(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 E_max 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.