Previous kinetic and electrochemical studies of copper complexes with macrocyclic tetrathiaethers-such as 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4)-have indicated that electron transfer and the accompanying conformational change occur sequentially to give rise to a dual-pathway mechanism. Under appropriate conditions, the conformational change itself may become rate-limiting, a condition known as "gated" electron transfer. We have recently hypothesized that the controlling conformational change involves inversion of two donor atoms, which suggests that "gated" behavior should be affected by appropriate steric constraints. In the current work, two derivatives of [14]aneS4 have been synthesized in which one of the ethylene bridges has been replaced by either cis- or trans-1,2-cyclopentane. The resulting copper systems have been characterized in terms of their Cu(II/I)L potentials, the stabilities of their oxidized and reduced complexes, and their crystal structures. The electron self-exchange rate constants have been determined both by NMR line-broadening and by kinetic measurements of their rates of reduction and oxidation with six or seven counter reagents. All studies have been carried out at 25 degrees C, mu = 0.10 M (NaClO4 and/or Cu(ClO4)2), in aqueous solution. Both Cu(II/I) systems show evidence of a dual-pathway mechanism, and the electron self-exchange rate constants representative of both mechanistic pathways have been determined. The first-order rate constant for gated behavior has also been resolved for the Cu(I)(trans-cyclopentane-[14]aneS4) complex, but only a limiting value can be established for the corresponding cis-cyclopentane system. The rate constants for both systems investigated in this work are compared to values previously determined for the Cu(II/I) systems with the parent [14]aneS4 macrocycle and its derivatives involving phenylene and cis- or trans-cyclohexane substituents. The results are discussed in terms of the influence of the fused rings on the probable conformational changes accompanying the electron-transfer process.

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.

Complex formation and dissociation rate constants have been independently determined for solvated nickel(II) ion reacting with eight macrocyclic tetrathiaether ligands and one acyclic analogue in acetonitrile at 25 degrees C, mu = 0.15 M. The macrocyclic ligands include 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4) and seven derivatives in which one or both ethylene bridges have been substituted by cis- or trans-1,2-cyclohexane, while the acyclic ligand is 2,5,9,12-tetrathiatridecane (Me2-2,3,2-S4). In contrast to similar complex formation kinetic studies on Ni(II) reacting with corresponding macrocyclic tetramines in acetonitrile and N,N-dimethylformamide (DMF), the kinetics of complex formation with the macrocyclic tetrathiaethers show no evidence of slow conformational changes following the initial coordination process. The differing behavior is ascribed to the fact that such conformational changes require donor atom inversion, which is readily accommodated by thiaether sulfurs but requires abstraction of a hydrogen from a nitrogen (to form a temporary amide). The latter process is not facilitated in solvents of low protophilicity. The rate-determining step in the formation reactions appears to be at the point of first-bond formation for the acyclic tetrathiaether but shifts to the point of chelate ring closure (i.e., second-bond formation) for the macrocyclic tetrathiaether complexes. The formation rate constants for Ni(II) with the macrocyclic tetrathiaethers parallel those previously obtained for Cu(II) reacting with the same ligands in 80% methanol-20% water (w/w). By contrast, the Ni(II) dissociation rate constants show significant variations from the trends in the Cu(II) behavior. Crystal structures are reported for the Ni(II) complexes formed with all five dicyclohexanediyl-substituted macrocyclic tetrathiaethers. All but one are low-spin species.