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 react ions of the copper(II/I) complexes formed with 2,3-cis- and cyclohexan ediyl-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 br oadening in D2O to determine the electron self-exchange rate constants at zero driving force, (ii) rapid-scan cyclic voltammetry in 80% meth anol-20% water (w/w) to determine the rate constants for conformationa l changes and heterogeneous electron transfer, and (iii) stopped-flow spectrophotometry using a total. of eight oxidizing and reducing count erreagents to determine the cross-reaction electron-transfer rate cons tants from which self-exchange rate constants can be calculated for va rious 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 t he NMR measurements, the electron self-exchange rate constants have be en 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-I I/I(cis-) and Cu-II/I(trans-cyhx-[14]aneS(4)), respectively. Applicati on of the Marcus relationship to the numerous cross-reaction rate cons tants yields variable behavior which is consistent with a dual-pathway mechanism for which the following self-exchange rate constants have b een 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 reacti ons proceed by the most favorable pathway (pathway A) involving a meta stable 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 estim ated from both cyclic voltammetric measurements (CV, 80% methanol) and Cu(I)L homogeneous oxidation kinetics (Ox, H2O) as follows [25 degree s 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 1 0(2) (CV) and 32 s(-1) (Ox). The values obtained from homogeneous oxid ations are believed to be the more reliable. The crystal structures re veal that both Cu(II)L complexes are square pyramidal with the four su lfur donor atoms occupying the basal plane and a coordinated water mol ecule (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(II I)L system, two sulfur donor atoms must invert during the overall elec tron-transfer process. It is postulated that these donor atom inversio ns may represent the primary barrier for the conformational change rep resented in the R --> P step. The self-exchange rate constant represen tative of the electron-transfer step itself, corrected for the separat e 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, triclini c system, space group P $(1) over bar$$, a = 9.734(4) Angstrom, b = 10 .155(3) Angstrom, c = 13.058(4) Angstrom, alpha = 91.73(2)degrees, bet a = 91.52(3)degrees, gamma = 117.75(3)degrees, V = 1140.6(7) Angstrom( 3), Z = 2, R = 0.049, R(w) = 0.050, T = -110 degrees C. For [Cu-II(tra ns-cyhx-[14]aneS(4))(H2O)](ClO4)(2) (2a): CuS4Cl4H28Cl2O9, triclinic s ystem, space group P $(1) over bar$$, a = 9.177(5) Angstrom, b = 10.64 1(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-c yhx-[14]aneS(4))Cl]. 1/2CuCl(4) . H2O (2b): Cu1.5C14H28S4Cl3O, orthorh ombic 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.