A structural strategy for generating rapid electron-transfer kinetics in copper(II/I) systems

Electron-transfer in low molecular weight copper(II/I) systems is generally accompanied by a large reorganization of the inner-coordination sphere. On the basis of recent kinetic studies involving Cu(II/I)-macrocyclic polythi aether complexes, it was hypothesized that forcing Cu(II) out of the macroc yclic cavity (i) decreases the changes in bond angles upon reduction and (i i) obviates any need for donor atom inversion. This should diminish the reo rganizational barrier and, thereby, increase the electron self-exchange rat e. This hypothesis has now been tested utilizing a somewhat soluble 12-memb ered macrocyclic tetrathiaether, oxathiane[12]aneS(4) (L). Crystal structur es of the (CuL)-L-II and (CuL)-L-I complexes confirm that, whereas one Cu-S bond dissociates upon reduction, the remaining bond lengths and angles cha nge only minimally. The free ligand, oxathiane[12]aneS(4), C10H18OS4, cryst allizes in the orthorhombic space group Pbca with Z = 8, a = 15.211(2) Angs trom, b = 8.5113(9) Angstrom, c = 20.548(3) Angstrom. The (CuL)-L-II comple x crystallizes as a 5-coordinate monomer with water as the apical ligand: [ CuL(OH2)]-(ClO4)(2). H2O, C10H22O11S4Cl2Cu, monoclinic P2(1)/c, Z = 4, a = 15.774(2) Angstrom, b = 8.485(5) Angstrom, c 16.508(9)Angstrom, beta = 112. 11(6)degrees. The (CuL)-L-I complex crystallizes as a binuclear species: [( CuL)(2)NCCH3](ClO4)(2). NCCH3, C24H42N2O10S8Cl2Cu2, in the triclinic space group P (1) over bar with Z = 4, a = 12.5917(2) Angstrom, b = 13.0020(3) An gstrom, c = 14.9285(3) Angstrom, alpha = 68.356(1)degrees, beta = 84.298(1) degrees, gamma = 61.129(1)degrees. The kinetics of Cu-II/I(oxathiane[12]ane S4) reacting with four selected counter reagents-two oxidants and two reduc tants-yield exceptionally large cross-reaction rate constants. Application of the Marcus cross relation yields calculated self-exchange rate constants ranging from 4 x 10(5) to 8 x 10(5) M-1 s(-1) (median: 6 x 10(5) M-1 s(-1) ) for this (CuL)-L-II/I redox system at 25 degrees C, mu = 0.10. A comparab le result of k(11) = (8.4 +/- 0.8) x 10(5) M-1 s(-1) has been obtained by N MR line-broadening measurements (at 25 degrees C, corrected to mu = 0.10). This is the largest self-exchange rate constant ever reported for a low mol ecular weight Cu(II/I) system. Thus, elimination of donor atom inversion co upled with a constrained inner sphere appears to represent a feasible appro ach for accelerating electron transfer in Cu(II/I) macrocyclic systems.