MULTINUCLEAR NMR-STUDIES OF LIGAND-EXCHANGE REACTIONS ON ANALOGOUS TECHNETIUM(V) AND RHENIUM(V) COMPLEXES - RELEVANCE TO NUCLEAR-MEDICINE

The kinetics of pyridine exchange on trans-[MO2(py)4]+ have been followed by H-1-NMR in CD3NO2 for M = Re, Tc: k298/s-1 = (5.5 +/- 0.1) x 10(-6), 0.04 +/- 0.02; DELTA-H not-equal/kJmol-1 = 111 +/- 3, 101 +/- 9; DELTA-S not-equal/JK-1mol-1 = +28 +/- 10, +68 +/- 35. For the Re(V) complex, pyridine and oxygen exchanges have been measured simultaneously by H-1- and O-17-NMR in deuterated water: k298/s-1 = (8.6 +/- 0.2) x 10(-6) (py), (14.5 +/- 0.3) x 10(-6) (oxygen); DELTA-H not-equal/kJmol-1 = 111 +/- 1, 91 +/- 1; DELTA-S not-equal/JK-1mol-1 = +32 +/- 3, -32 +/- 4. For both complexes, the rate law for pyridine exchange is first-order in complex and zero-order in pyridine; together with the activation parameter values, and the fact that the rate does not depend significantly on the nature of the solvent, this strongly implies the operation of a dissociative mechanism. The ratio of pyridine exchange rates for the Tc and Re complexes at room temperature is ca. 8000. The consequences of these observations for radiopharmaceutical synthesis are discussed.