EFFECT OF CONFORMATIONAL CONSTRAINTS ON GATED ELECTRON-TRANSFER KINETICS - A MULTIFACETED STUDY ON COPPER(II/I) COMPLEXES WITH CIS-CYCLOHEXANEDIYL-[14]ANES(4) AND TRANS-CYCLOHEXANEDIYL-[14]ANES(4)

A multifaceted study has been conducted on the electron-transfer reactions of the copper(II/I) complexes formed with 2,3-cis- and 2,3-trans-cyclohexanediyl-1,4,8,11-tetrathiacyclotetradecane (designated as cis- and tmns-cyhx-[14]aneS(4)). Each system has been studied by (i) H-1-NMR line broadening in D2O to determine the electron self-exchange rate constants at zero driving force, (ii) rapid-scan cyclic voltammetry in 80% methanol-20% water (w/w) to determine the rate constants for conformational changes and heterogeneous electron transfer, and (iii) stopped-flow spectrophotometry using a total. of eight oxidizing and reducing counterreagents to determine the cross-reaction electron-transfer rate constants from which self-exchange rate constants can be calculated for various driving forces. The crystal structures of both Cu(II)L complexes and of Cu-I(trans-cyhx-[14]aneS(4)) have also been determined. From the NMR measurements, the electron self-exchange rate constants have been evaluated [at 25 degrees C, mu = 0.10 M (NO3-)] as k(11(ex)) = (5.0 +/- 0.5) x 10(4) and less than or equal to 10(3) M(-1) s(-1) for Cd-II/I(cis-) and Cu-II/I(trans-cyhx-[14]aneS(4)), respectively. Application of the Marcus relationship to the numerous cross-reaction rate constants yields variable behavior which is consistent with a dual-pathway mechanism for which the following self-exchange rate constants have been resolved [25 degrees C, mu = 0.10 M (ClO4-)]: for Cu-II/I(cis-cyhx-[14]aneS(4)), k(11(A)) = 5 x 10(4), k(11(B)) less than or equal to 10 M(-1) s(-1); for Cu-II/I(trans-cyhx-[14]aneS(4)), k(11(A)) = 2 x 10(3), k(11(B)) less than or equal to 10 M(-1) s(-1). The reduction reactions proceed by the most favorable pathway (pathway A) involving a metastable Cu(I)L intermediate (P) while the limiting oxidation reactions proceed by an alternate pathway (pathway B) involving a less stable Cu(II)L intermediate (Q). The change in pathway is mediated by the rate constant (k(RP)) for the formation of the Cu(I)L(P) intermediate from the stable Cu(I)L(R) complex. This latter rate constant has been estimated from both cyclic voltammetric measurements (CV, 80% methanol) and Cu(I)L homogeneous oxidation kinetics (Ox, H2O) as follows [25 degrees C]: for Cu-I(cis-cyhx-[14]aneS(4)), k(RP) = 4.4 x 10(2) (CV) and 1.1 x 10(2) s(-1) (Ox); for Cu-I(trans-cyhx-[14]aneS(4)), k(RP) = 1.5 x 10(2) (CV) and 32 s(-1) (Ox). The values obtained from homogeneous oxidations are believed to be the more reliable. The crystal structures reveal that both Cu(II)L complexes are square pyramidal with the four sulfur donor atoms occupying the basal plane and a coordinated water molecule (or anion) at the apex. The Cu-I(trans-cyhx-[14]aneS(4)) complex is in a flattened tetrahedral geometry in which all four sulfur donor atoms remain coordinated. These structures imply that, for each Cu(III)L system, two sulfur donor atoms must invert during the overall electron-transfer process. It is postulated that these donor atom inversions may represent the primary barrier for the conformational change represented in the R --> P step. The self-exchange rate constant representative of the electron-transfer step itself, corrected for the separate conformational change step, is estimated to be on the order of 10(6) M(-1) s(-1) for both systems, equivalent to the largest self-exchange rate constants known for rigid Cu(II/I)L systems. Crystal data [Mo K alpha radiation (lambda = 0.710 73 Angstrom)] are as follows. For [Cu-II(cis-cyhx-[14]aneS(4))(H2O)](ClO4)(2) (1): CuS4C14H28Cl2O9, triclinic system, space group P $(1) over bar$$, a = 9.734(4) Angstrom, b = 10.155(3) Angstrom, c = 13.058(4) Angstrom, alpha = 91.73(2)degrees, beta = 91.52(3)degrees, gamma = 117.75(3)degrees, V = 1140.6(7) Angstrom(3), Z = 2, R = 0.049, R(w) = 0.050, T = -110 degrees C. For [Cu-II(trans-cyhx-[14]aneS(4))(H2O)](ClO4)(2) (2a): CuS4Cl4H28Cl2O9, triclinic system, space group P $(1) over bar$$, a = 9.177(5) Angstrom, b = 10.641(5) Angstrom, c = 13.037(4) Angstrom, alpha = 87.26(3)degrees, beta = 88.13(4)degrees, gamma = 69.19(3)degrees, V = 1188.5(8) Angstrom(3), Z = 2, R = 0.050, R(w) = 0.056, T = -110 degrees C. For [Cu-II(trans-cyhx-[14]aneS(4))Cl]. 1/2CuCl(4) . H2O (2b): Cu1.5C14H28S4Cl3O, orthorhombic system, space group Pbcn, a = 28.206(7) Angstrom, b = 10.115(3) Angstrom, c = 14.707(2) Angstrom, V = 4196(2) Angstrom(3), Z = 8, R = 0.038, R(w) = 0.042, T = 22 degrees C. For [Cu-I(trans-cyhx-[14] aneS(4))]ClO4 . 1/4H(2)O (3): CuS4C14H26.5ClO4.25, monoclinic system, space group P2(1)/n, a = 10.135(2) Angstrom, b = 16.044(2) Angstrom, c = 12.675(2) Angstrom, beta = 105.10(1)degrees, V = 1989.9(5) Angstrom(3), Z = 4, R = 0.038, R(w) = 0.038, T = -110 degrees C.