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-κ³)(TTCN-κ¹)]⁺, 1, a coordination complex with a tetrahedral CuS₄ core, to [Cu(II)(TTCN-κ³)₂]²⁺, 2, with an octahedral CuS₆ core, has been studied by pulse radiolysis and electrochemistry in aqueous solution at various pH values. In addition to the geometry change about the metal ion in this oxidation, the nonchelating 1,4,7-trithiacyclononane (TTCN) ligand in 1 changes conformation on becoming chelated in 2. However, pulse radiolysis reveals that this process does not occur intramolecularly but affords a bimolecular reaction in which the oxidized copper incorporates an external TTCN. Evidence for this mechanism is drawn from corresponding experiments with a variety of related Cu(I) complexes in which the monodentate TTCN has been replaced by other sulfur-containing ligands and which have been structurally characterized by X-ray crystallography.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 generating [Cu(II)(TTCN-κ³)(H₂O)₃]²⁺, 3. Transient 3 is characterized by an optical absorption with λ(max) = 370 nm and ε ~ 2000 M^-1 cm^-1 which depends on pH because this transient participates in three acid/base equilibria. Deprotonation of the three water ligands associated with Cu(II) results in increasingly blue-shifted absorptions. Undeprotonated transient 3 prevails at pH ≤ 6, and converts directly into the stable Cu(II) complex 2 via reaction with an unoxidized molecule of 1 or free TTCN. The corresponding bimolecular rate constants are 5.2 (± 0.5) x 10⁵ and 8.4 (± 1.0) x 10⁵ M^-1 s^-1, respectively. For the deprotonated forms of 3 this process is increasingly slowed down and at higher pH (≤ 9) the formation of 2 is completely prevented. The formation 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 concentration 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 ¹H NMR spectroscopic studies on 1.