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 polythiaether complexes, it was hypothesized that forcing Cu(II) out of the macrocyclic cavity (i) decreases the changes in bond angles upon reduction and (ii) obviates any need for donor atom inversion. This should diminish the reorganizational barrier and, thereby, increase the electron self-exchange rate. This hypothesis has now been tested utilizing a somewhat soluble 12-membered macrocyclic tetrathiaether, oxathiane[12]aneS4 (L). Crystal structures of the CuIIL and CuIL complexes confirm that, whereas one Cu−S bond dissociates upon reduction, the remaining bond lengths and angles change only minimally. The free ligand, oxathiane[12]aneS4, C10H18OS4, crystallizes in the orthorhombic space group Pbca with Z = 8, a = 15.211(2) Å, b = 8.5113(9) Å, c = 20.548(3) Å. The CuIIL complex 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) Å, b = 8.485(5) Å, c = 16.508(9) Å, β = 112.11(6)°. The CuIL complex crystallizes as a binuclear species:  [(CuL)2NCCH3](ClO4)2·NCCH3, C24H42N2O10S8Cl2Cu2, in the triclinic space group P1̄ with Z = 4, a = 12.5917(2) Å, b = 13.0020(3) Å, c = 14.9285(3) Å, α = 68.356(1)°, β = 84.298(1)°, γ = 61.129(1)°. The kinetics of CuII/I(oxathiane[12]aneS4) reacting with four selected counter reagentstwo oxidants and two reductantsyield exceptionally large cross-reaction rate constants. Application of the Marcus cross relation yields calculated self-exchange rate constants ranging from 4 × 105 to 8 × 105 M-1 s-1 (median:  6 × 105 M-1 s-1) for this CuII/IL redox system at 25 °C, μ = 0.10. A comparable result of k 11 = (8.4 ± 0.8) × 105 M-1 s-1 has been obtained by NMR line-broadening measurements (at 25 °C, corrected to μ = 0.10). This is the largest self-exchange rate constant ever reported for a low molecular weight Cu(II/I) system. Thus, elimination of donor atom inversion coupled with a constrained inner sphere appears to represent a feasible approach for accelerating electron transfer in Cu(II/I) macrocyclic systems.

No abstract

The complete series of alcoholic derivatives for the 13- through 16-membered macrocyclic tetrathia ethers, in which a single −OH group is attached to the central carbon of a trimethylene bridge, has now been synthesized. The derivatized ligands show a marked improvement in aqueous solubility, permitting the measurement of both the Cu(II) complex stability constants (K Cu II L‘) and the corresponding formation (k f) and dissociation (k d) rate constants in aqueous solution at 25 °C. Insofar as possible, the K Cu II L‘, k f, and k d values were determined from independent measurements to assess the level of consistency between them. The CuII/IL formal potentials for complexes of the OH-substituted ligands have also been determined, permitting the calculation of the stability constants for the CuIL complexes. Except for the potential measurements, all values were determined in the presence of both 0.10 and 1.0 M perchlorate to assess the uniformity of anion adduct effects upon the various kinetic and thermodynamic values. All values for the alcoholic derivatives are compared to the corresponding parameters determined previously for the unsubstituted macrocyclesfor which, due to solubility limitations, the formation kinetic data had to be extrapolated to aqueous conditions from measurements made in methanol−water mixtures. In general, the substitution of an −OH group on the ligand backbone results in a 5−8-fold decrease in the k f values. This decrease in complex formation rate is attributed to the influence of hydrogen bonding between the −OH group and the surrounding solvent molecules upon the preferred ligand conformation in water. The k f values for both series of ligands are consistent with a mechanism in which the closure of the first chelate ring is the rate-determining step. The effect of −OH substitution upon the k d values is somewhat variable but tends to be less pronounced. The OH-substituted 14-membered macrocycle appears to be somewhat unique from the standpoint that the Cu(II) complex is twice as stable as the unsubstituted analogue, whereas all other −OH derivatives show a decrease in CuIIL stability. Also included in the current study are measurements on the Cu(II) complex formed with a water-soluble oxathiane derivative of the 12-membered macrocyclic tetrathia ether (oxathiane-[12]aneS4).