THE EFFECT OF 2ND-SPHERE COORDINATION .2. ADDUCT FORMATION BETWEEN [RU(NH3)5L](PF6)N (N = 2 AND 3) AND 18-CROWN-6 ETHER IN SOLUTION AND THE EFFECT ON REDOX BEHAVIOR

The adduct formation of [Ru(NH3)5L](PF6)n (n = 2 for L = isonicotinamide and pyrazine; n = 3 for L = 4-aminopyridine and 4-dimethylaminopyridine) with 18-crown-6 ether (18C6) was investigated spectrophotometrically and electrochemically in non-aqueous solution. The adduct formation gave a red shift of the MLCT band for [Ru(NH3)5L](PF6)2 and a blue shift of the LMCT band for [Ru(NH3)5L](PF6)3. The magnitude of the shifts is linearly dependent on the reciprocal of the dielectric constant of the solvents examined. The adduct formation was interpreted in terms of the hydrogen bonding between the ammine coordinating to ruthenium and the ether oxygen of 18C6. The stoichiometry of the adducts in non-aqueous solution was determined to be 1 : 1 (complex: 18C6) for [Ru(NH3)5L](PF6)2 and 1 : 2 for [Ru(NH3)5L](PF6)3. The potential of a ruthenium(III)/ruthenium(II) redox couple in acetonitrile solution was measured at various concentrations of 18C6 by cyclic voltammetry. The redox couples shifted to more negative potentials with an increasing 18C6 concentration, where no additional redox peak appeared. The shift of redox potentials was about - 130 mV at 0.10 mol dm-3 18C6 concentration, irrespective of the nature of L. Thus, it becomes possible to tune the redox potential of the ruthenium complexes by second-sphere coordination with 18C6.