ELECTRON-TRANSFER-COUPLED LIGAND DYNAMICS IN CU-I II(TTCN)(2) COMPLEXES IN AQUEOUS-SOLUTION/

One-electron oxidation of copper(I) bis(1,4,7-trithiacyclononane), [Cu -I(TTCN-kappa(3))(TTCN-kappa(1))](+), 1, a coordination complex with a tetrahedral CuS4 core, to [Cu-II(TTCN-kappa(3))(2)](2+), 2, with an o ctahedral CuS6 core, has been studied by pulse radiolysis and electroc hemistry in aqueous solution at various pH values. in addition to the geometry change about the metal ion in this oxidation, the nonchelatin g 1,4,7-trithiacyclononane (TTCN) ligand in 1 changes conformation on becoming chelated in 2. However, pulse radiolysis reveals that this pr ocess does not occur intramolecularly but affords a bimolecular reacti on in which the oxidized copper incorporates an external TTCN. Evidenc e for this mechanism is drawn from corresponding experiments with a va riety of related Cu-I complexes in which the monodentate TTCN has been replaced by other sulfur-containing ligands and which have been struc turally characterized by X-ray crystallography. From all these studies it is concluded that oxidation of 1 and all these other complexes of Cu-I is accompanied by immediate loss of the monodentate ligand genera ting [Cu-II(TTCN-kappa(3))(H2O)(3)](2+), 3. Transient 3 is characteriz ed by an optical absorption with lambda(max) = 370 nm and epsilon simi lar to 2000 M(-1) cm(-1) which depends on pH because this transient pa rticipates in three acid/base equilibria. Deprotonation of the three w ater ligands associated with Cu(II) results in increasingly blue-shift ed absorptions. Undeprotonated transient 3 prevails at pH less than or equal to 6, and converts directly into the stable Cu-II complex 2 via reaction with an unoxidized molecule of 1 or free TTCN. The correspon ding bimolecular rate constants are 5.2 (+/-0.5) x 10(5) and 8.4 (+/-1 .0) x 10(5) M(-1) s(-1), respectively. For the deprotonated forms of 3 this process is increasingly slowed down and at higher pH (greater th an or equal to 9) the formation of 2 is completely prevented. The form ation of transient 3 is also consistent with the pH dependence of the electrochemistry of 1. Under electrochemical conditions the conversion into 2 follows first-order kinetics due to a relatively high TTCN con centration available near the electrode surface after oxidation of 1. All the results require rapid Ligand exchange in 1 and a particularly labile monodentate TTCN ligand. This has been corroborated by H-1 NMR spectroscopic studies on 1.