Structure-reactivity relationships in copper(II)/copper(I) electron-transfer kinetics: evaluation of self-exchange rate constants for copper polythia ether complexes

Abstract

Kinetic parameters are reported for electron-transfer cross-reactions of eight Cu(II)/Cu(I) systems involving closely related open-chain and macrocyclic polythia ether ligands. Both the oxidation kinetics of the Cu(I) species and the reduction kinetics of the Cu(II) species are included by using tris(4,7-dimethyl-l,10-phenanthroline)iron(III) and diaquo(2,3,9,10-tetramethyl-1,4,8,1 l-tetraazacyclotetradeca-l,3,8,10-tetraene)cobalt(II) as the principal cross-reagents. Reactions with other cross-reagents are also reported for selected Cu(II)/Cu(I) systems. The applicability of the Marcus cross-relation to these reactions is examined and is shown to yield calculated Cu(II)/Cu(I) self-exchange rate constants that differ significantly for the corresponding oxidation and reduction reactions. To account for these apparent discrepancies, a dual-pathway mechanism is proposed in which a major part of the conformational reorganization at the copper center occurs sequentially, rather than concertedly, with the electron-transfer step. Differences in the relative kinetic behavior of the various copper-polythia ether complexes are discussed In terms of the influence of ligand constraints upon the bond-making and bond-breaking sequences that accompany the conversion of Cu(II) to Cu(I) (and vice versa).(1) (a) Wayne State University, (b) University of Wisconsin at Eau Claire.(2) Copper Coordination Chemistry: Biochemical and Inorganic Per-

Electron Transfer by Copper Centers

“…For the Cu II/I ([14]aneS 4 ) system, the corresponding k 11(Red) and k 11(Ox) values were 1.6 × 10 3 and 2.2 M -1 s -1 , respectively, and all but one of these systems yielded k 11(Ox) values that were smaller than the corresponding k 11(Red) values by 3-7 orders of magnitude. 132 To corroborate the existence of Scheme 1 for the macrocyclic polythiaether complexes, Bernardo 49 investigated the cyclic voltammetric (CV) behavior of these systems as a function of sweep rate and temperature in 80% methanol-20% water (by weight), a solvent mixture that permitted the solution temperature to be lowered to -77°C. For slow scan rates at ambient temperature, completely reversible CVs were observed with the cathodic and anodic peaks representing equilibrated oxidant and reductant species.…”

Influence of Functionalized Substituents on the Electron‐Transfer Abilities of Copper Guanidinoquinoline Complexes

“…The electron self‐exchange rate depends on the temperature and on the reorganization energy of the ligand and solvent sphere, which consists of a vibrational and a solvent part . k 11 can be calculated based on the Marcus theory using the Marcus cross relation , , , . This method has been used for several copper and other transition metal complexes .…”

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).…”