Interaction of Adenosine with[(H2O)- (tap)2RuORu(tap)2(H2O)]2+ Ion in Aqueous Medium: Kinetic and Mechanistic Studies

Reaction of [(H2O)(tap)2RuORu(tap)2(OH2)](1) with adenosine has been examined spectrophotometrically in aqueous medium as a function of [complex ion], [ligand], pH and temperature at constant ionic strength. At the physiological pH (7.4), (1) shows two consecutive steps; both of which are [ligand] dependent. The activation parameters from Eyring plots are: ∆H1 = 9.0 + _ 0.4 kJ mol-1, ∆S1 = -266 + _ 2 J K-1mol-1, ∆H2 = 35.6 + _ 2.2 kJ mol-1 and ∆S2 = -212 + _ 7 JK-1 mol-1. Available kinetic and activation parameters are in consonance with an associative interchange mechanism. From the temperature dependence of the outersphere association equilibrium constant the thermodynamic parameters calculated are: ∆H1 = 25.8 + _ 1.0 kJ mol-1, ∆S1 = 126 + _ 3 J K-1mol-1 and ∆H2 = 19.7 + _ 6.1 kJ mol-1, ∆S2 = 108 + _19 J K-1 mol-1, a negative ∆Go value at all temperatures studied supports spontaneous formation of an outersphere association complex.