Interactions of the ruthenium(III)-ruthenium(II) couple with malononitrile and related ligands

Article abstract of DOI:10.1021/ic00141a025

When dicyanamide and tricyanomethane act as bridging groups in binuclear pentaammineruthenium species, the bridging groups remain anionic over wide pH ranges. When malononitrile, fert-butylmalononitrile, and ethylmalononitrile are the bridging groups, the behavior is complicated by proton dissociation from the bridging ligand and instability of the mixed-valence form after proton dissociation. Electrochemical studies have led to estimates of the pK_a values for [3,L,3] and [3,L,2] for the first two ligands. These are -4.8 and -0.7 for L = malononitrile and -3.0 and 1.6 for L = butylmalononitrile. In the [2,L,2] states only lower limits on pK_a, were set, and these are 13 and 15 for malononitrile and tert-butylmalononitrile, respectively. The pK_a values of the free ligand are 11.2 and 13.1. The enormous enhancement of acidity when the nitrile is coordinated to (NH₃)₅RuIII is shown also in the mononuclear tert-butylmalononitrile complex, where pK_a was observed to be 3.85, here with use of a spectrophotometric method. The most striking feature of the UV-visible spectra is the strong absorption at high wavelengths shown by the anion-Ru(III) unit. For the [3,L^-,3] species the values of λ_max(ε) are (units of nm and M^-1 cm^-1, respectively) 784 (2.8 × 10⁴), 695 (1.7 × 10⁴), 562 (4.5 × 10³), and 400 (2 × 10³) for the tert-butylmalononitrile, malononitrile, tricyanomethanide, and dicyanamide anions, respectively. For the same bridging ligands, the characteristics of the near-IR absorption of the mixed-valence species are 1170(1.6 × 10⁴), 1280 (>10⁴), 1550 (6.2 × 10³), and 1100 (2.8 × 10³). The quotients governing the comproportionation equilibrium [3,L^-,3] + [2,L^-,2] = 2[3,L^-,2] for the same four bridging ligands are >10¹⁰°, >10¹¹, 2.3 × 10³, and 2 × 10², respectively. These data and other properties such as the width of the near-IR band at half-height suggest that the mixed-valence species based on the anions of tert-butylmalononitrile and malononitrile are strongly delocalized but that with the dicyanamide anion as bridging group the systems are valence trapped. When the lone electron pair of the central atom in the bridging group, which mainly appears to account for the coupling in the malononitrile anions, is engaged, as for example when dibenzylmalononitrile is the bridging group, coupling is very weak. This is evidenced by the small value (1.6 × 10²* M^-1cm^-1) of ε for the near-IR band in the corresponding mixed-valence species as well as the small value (-10) for the comproportionation constant.