Direct determination of the self-exchange electron-transfer rate constant for a copper(II/I) macrocyclic pentathiaether complex

Macrocyclic [CuI/II(bite)]+/2+ (bite = biphenyldiimino dithioether): An Example of Fully-Gated Electron Transfer and Its Biological Relevance1

“…On the basis of the large enthalpic changes expected for a bond breaking/bond forming process, one might predict the self-exchange rate for this small-molecule system to be quite slow. In fact, a measurement has shown that the self-exchange rate constant of k = 2.2(1.1) × 10 5 M -1 s -1 (25 °C, D 2 O) for [Cu I/II ([15]aneS 5 )] +/2+ exceeds those of all other small-molecule copper systems published to date . Van de Linde et al have suggested that, in fact, the enthalpic contribution to the electron transfer may be much smaller than expected for a bond breaking/forming process.…”

“…The enthalpy of activation of Δ H ⧧ = 23.3(0.7) kJ/mol is fairly high and may be indicative of a Cu−ligand bond breaking/forming process that could occur before electron transfer. One system in which such a coordination number change is known to occur ([Cu I/II ([15]aneS 5 )] +/2+ ) was determined to have an enthalpy of activation of only Δ H ⧧ = 14(4) kJ/mol. The complex is 5-coordinate and square pyramidal in the Cu(II) state and tetracoordinate and pseudotetrahedral in the Cu(I) state …”

Electron Transfer by Copper Centers

“…One, designated as [15]aneS 5 (Figure 9), contains five thiaether sulfur donors and has yielded a k 11 value of 1 × 10 5 M -1 s -1 as determined by NMR. 168 An identical value was determined in 80% methanol (w/w). This value was corroborated from two cross-reactions involving the reduction of the Cu II L complex, while a somewhat smaller value was calculated from a single oxidation study, 132 but the latter value was not sufficiently different to provide definitive evidence for a change in reaction pathway.…”

Preparations for 1,4,7,11,14,17-hexa-thiacycloeicosane, 1,4,7-trithiecan-9-ol, 1,11 -dioxa-4,8,14,18-tetrathia-cycloeicosane, 1 -oxa-4,8-dithiacyclo-decane, and some complexes of iron, cobalt, nickel, and palladium

“…For example, they stabilize less common oxidation states such as Pd(III), Au(II), or Rh(I1) (9,11,12), unusual coordination geometries (13,14), and they have a large nephelauxetic effect (15,16) that leads to low-spin complexes in coordination geometries or with specific metal ions where low-spin cases are rarely encountered. Likewise, rapid electron exchange kinetics have been measured for these complexes and have been attributed to the unique structural and electronic features of the ligands (17)(18)(19). Many important future applications of chemistry in areas such as materials science, catalysis, communications, and data storage devices will require the ability to adjust properties of existing substances (20).…”