Chelate ring sequence effects on thermodynamic, kinetic and electron-transfer properties of copper(II/I) systems involving macrocyclic ligands with S4 and NS3 donor sets

The kinetic behavior of electron-transfer reactions involving several copper(II/I) complexes has previously been attributed to a dual-pathway “square scheme” mechanism in which changes in the coordination geometry occur sequentially, rather than concertedly, with the electron-transfer step. In the case of 14-membered macrocyclic quadridentate ligand complexes studied to date, the major geometric change appears to be the inversion of two coordinated donor atoms during the overall electron-transfer process. However, the relative importance of these two inversions has been a matter of speculation. In the current investigation, a comparison is made of Cu(II/I) systems involving two pairs of ligands with S4 and NS3 donor sets: 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4-a); 1,4,7,11-tetrathiacyclotetradecane ([14]aneS4-b); 1,4,8-trithia-11-azacyclotetradecane ([14]aneNS3-a); and 1,7,11-trithia-4-azacyclotetradecane ([14]aneNS3-b). In each pair of ligands, isomer a has the common chelate ring size sequence 5,6,5,6 while isomer b has the sequence 5,5,6,6. A crystal structure for [CuII([14]aneNS3-b)(H2O)](ClO4)2 demonstrates that, when coordinated to Cu(II), the b isomers stabilize the relatively rare ligand conformation designated as conformer II in which one donor atom is oriented opposite to the other three relative to the plane of the macrocycle. This eliminates one of the donor atom inversion steps which normally occurs during Cu(II/I) electron transfer. The copper complexes formed with these a and b isomers are examined in terms of (i) their CuIIL and CuIL stability constants, (ii) their CuIIL formation and dissociation rate constants, (iii) their CuII/IL redox potentials and (iv) their apparent electron self-exchange rate constants. Of the two donor atom inversions which occur in the case of the a-isomer complexes, the specific donor atom inversion which is common to the b-isomer complexes is judged to exhibit the larger energy barrier. Thus, it is presumed to represent the rate-limiting process responsible for the onset of “gated” electron transfer in previous studies on a-isomer complexes.