H. Sanaullah,"hungerbuhler et al., ELECTRON-TRANSFER-COUPLED LIGAND DYNAMICS IN CU-I II(TTCN)(2) COMPLEXES IN AQUEOUS-SOLUTION/, Journal of the American Chemical Society, 119(9), 1997, pp. 2134-2145
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riety of related Cu-I complexes in which the monodentate TTCN has been
replaced by other sulfur-containing ligands and which have been struc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.