Electrochemistry of oxo-technetium(V) complexes containing Schiff base and 8-quinolinol ligands

The electrochemistry of six-coordinate, monooxo technetium(V) complexes containing Schiff base ligands has been studied in acetonitrile and N,N?-dimethylformamide solutions. The complexes have the general formula TcOCl(LB)2 or TcO(LT)(LB), where LB represents a bidentate-N,O Schiff base ligand or a bidentate-N,O, 8-quinolinol ligand and LT represents a tridentate-O,N,O Schiff base ligand. Cyclic voltammetry at a platinum-disk electrode, controlled-potential coulometry, and thin-layer spectroelectrochemistry were used to probe both the oxidation and the reduction of these complexes. The results of these studies, and previously reported results on the analogous Re(V) complexes, can be understood within a single general reaction scheme. The salient features of this scheme are (i) one-electron reduction of Tc(V) to Tc(IV), (ii) subsequent loss of a ligand situated cis to the Tc=O linkage, and (iii) subsequent isomerization of this unstable Tc(IV) product to a more stable complex in which the site trans to the Tc=O linkage is vacant. The Tc(IV) complexes can also be reduced to analogous Tc(III) species, which appear to undergo the same ligand loss and isomerization reactions. The technetium complexes are 400-500 mV easier to reduce than are their rhenium analogues. The 8-quinolinol ligands, and especially the 5-nitro derivative, both thermodynamically and kinetically stabilize the Tc(IV) and Tc(III) oxidation states. These electrogenerated species are unusual in that they constitute the bulk of the known examples of monomeric Tc(IV) and Tc(III) complexes containing only N- and O-donating ligands.