Trans-kinetic effects in ligand substitution processes of ruthenium polypyridyl complexes

Abstract

A concerted mechanism is disclosed when studying trans-kinetic effects in substitution processes of complexes of formula [Ru(trpy)(4,4′-(X) 2 -bpy)(H 2 O)] 2+ , with trpy = 2,2′:6′,2″-terpyridine, bpy = 2,2′-bipyridine and X = H, CH 3 , OCH 3 , NH 2 and N(CH 3 ) 2 . The second order rate constants k 2 for substitution of water by acetonitrile increase with increasing donor ability of the X group. The correlation between ln k 2 and Hammett substituent constants σ p of X gives a slope of near 1, indicating high sensitivity of the kinetic parameters for water replacement to the donor ability of the para-substituent attached to a bipyridine trans- to the water ligand and considerably separated from it. Besides, the activation enthalpies decrease with increasing pK a values of the 4,4′-(X) 2 -bpy ligands and the activation entropies have negative values almost one order of magnitude higher than those reported before in similar processes. These data indicate a concerted ligand interchange mechanism, infrequently found in substitution reactions of octahedral complexes which can be explained by the fact that H-bonds between the leaving ligand (H 2 O) and the entering ligand (CH 3 CN) will favor an intermediate transition state of increased order respect to the initial state. These results are pertinent for finding the best candidates in the quantitative detection of CH 3 CN, an important contaminant in radiopharmaceuticals used in PET (positron emission tomography) studies.