Dinuclear formation, oxygen atom transfer and intramolecular coordination isomerization in oxotechnetium complexes

Abstract

Dinuclear, asymmetric and dissymmetric, mixed-valent (III-IV), {mu}-oxo complexes of technetium spontaneously form in both neat pyridine and pyridine solutions in dichlorobenzene from higher valent, mononuclear species, with which they reach an equilibrium. When (OX{sub 4}Tc{sup V}){sup {minus}} is used as the starting material and only the neat pyridine ligand is present, the formation of the dinuclear species is second-order in Tc{sup V} and occurs with concomitant production of pyridine N-oxide, indicating that oxygen atom transfer occurs in the technetium reduction process. K{sub eq} for the formation of the dinuclear species in picoline solution is 193 {plus minus} 8 M{sup {minus}1} with k = (1.46 {plus minus} 0.1) {times} 10{sup {minus}2} M{sup {minus}1} s{sup {minus}1}. The asymmetric and dissymmetric dinuclear species spontaneously interconvert. In o-dichlorobenzene, this process is first-order in chloride ion and requires a minimum concentration ({approximately}0.1 M) of the pyridine ligand to prevent decomposition of the dinuclear starting material. With picoline as the ligand, at 140{degree}C the reaction proceeds with a specific rate of (3.12 {plus minus} 0.08) {times} 10{sup {minus}3} M{sup {minus}1} s{sup {minus}1} to approach an equilibrium between the asymmetric and dissymmetric complexes with K{sub eq} = 2.83 {plus minus} 0.12. Activation parameters are {Delta}H* = 144more » {plus minus} 2 kJ/mol and {Delta}S* = 53.5 {plus minus} 1 J/(mol K). Minor changes in the ligands generate marked differences in K{sub eq} and K{sub iso}.« less