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

MECCHIA ORTIZ, JUAN H.; PEYROT, ANALÍA M.; FAGALDE, FLORENCIA; KATZ, NÉSTOR E.

INORGANIC CHEMISTRY COMMUNICATIONS

ELSEVIER SCIENCE BV

Lugar: Amsterdam; Año: 2018 vol. 98 p. 44 - 47

1387-7003

A concerted mechanism is disclosed when studying trans-kinetic effects in substitution process of complexes of formula [Ru(trpy)(4,4'-(X)2-bpy)(H2O)]2+, with trpy = 2,2´:6´,2´´-terpyridine, bpy = 2,2´-bipyridine and X = H, CH3, OCH3, NH2 and N(CH3)2. The second order rate constants k2 for substitution of water by acetonitrile increase with increasing donor ability of the X group. The correlation between ln k2 and Hammett substituent constants sigmap 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 pKa 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 (H2O) and the entering ligand (CH3CN) 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 CH3CN, an important contaminant in radiopharmaceuticals used in PET (positron emission tomography) studies.