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 inacetonitrile and N,N'-dimethylformamide solutions. The complexes have the general formula TcOCl(LB)Z or TcO(L,)(L& whereLB represents a bidentate-N,O Schiff base ligand or a bidentate-N,O 8-quinolinol ligand and LT represents a tridentate-O,N,OSchiff base ligand. Cyclic voltammetry at a platinum-disk electrode, controlled-potential coulometry, and thin-layer spectroelectrochemistrywere used to probe both the oxidation and the reduction of these complexes. The results of these studies, andpreviously reported results on the analogous Re(V) complexes, can be understood within a single general reaction scheme. Thesalient features of this scheme are (i) one-electron reduction of Tc(V) to Tc(IV), (ii) subsequent loss of a ligand situated cis tothe Tc=O linkage, and (iii) subsequent isomerization of this unstable Tc(IV) product to a more stable complex in which the sitetrans to the Tc=O linkage is vacant. The Tc(IV) complexes can also be reduced to analogous Tc(II1) species, which appear toundergo the same ligand loss and isomerization reactions. The technetium complexes are 400-500 mV easier to reduce than aretheir rhenium analogues. The 8-quinolinol ligands, and especially the 5-nitro derivative, both thermodynamically and kineticallystabilize the Tc(IV) and Tc(II1) oxidation states. These electrogenerated species are unusual in that they constitute the bulk of the known examples of monomeric Tc(IV) and Tc(II1) complexes containing only N- and 0-donating ligands.