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 ‐[MO 2 (py) 4 ] + have been followed by 1 H‐NMR in CD 3 NO 2 for M = Re, Tc: k 298 S −1 = (5.5 ± 0.1) × 10 −6 , 0.04 ± 0.02; Δ H ≠ /kJmol −1 = 111 ± 3, 101 ± 9; Δ S ≠ /JK −1 mol −1 = +28 ± 10, +68 ± 35. For the Re v complex, pyridine and oxygen exchanges have been measured simultaneously by 1 H‐ and 17 O‐NMR in deuterated water: k 298 /s −1 = (8.6 ± 0.2) × 10 −6 (py), (14.5 ± 0.3) × 10 −6 (oxygen); Δ H ≠ /kJmol −1 = 111 ± 1, 91 ± 1; Δ S /JK −1 mol −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.