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-

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Variable-temperature slow- and rapid-scan cyclic voltammetry has been applied in a solvent system of 80% methanol-20% water (w/w) to both the Cu(II) and Cu(I) complexes formed with a series of five ligands in which both of the ethylene bridges in the cyclic tetrathiaether [14]aneS(4) (i.e., 1,4,8,11-tetrathiacyclotetradecane) have been replaced by trans- and/or cis-cyclohexane. All five substituted complexes exhibit electrochemical behavior which is consistent with the type of dual-pathway electron-transfer mechanism previously observed for the parent Cu(II/I)([14]aneS(4)) system in which a conformational change is proposed to occur sequentially to the electron-transfer step. The kinetic parameters associated with the formation of the metastable Cu(II)L intermediate cannot be accurately established under the experimental conditions used. However, for the formation of the corresponding metastable Cu(I)L intermediate, both the equilibrium constant and rate constants for the presumed conformational interconversion have been determined with reasonable accuracy. Of the five systems studied, the meso-trans,trans- and dl-trans,trans-dicyclohexanediyl-substituted ligands show the extremes of behavior in terms of the relative stabilities of the Cu(I)L and Cu(II)L intermediate species. This behavior is shown to be consistent with molecular mechanical calculations for the possible metastable intermediates with these two systems. On the basis of the data obtained in this work, the two electron-transfer pathways are expected to be reasonably competitive for the dl-trans,trans derivative but extremely divergent for the meso-trans,trans derivative, the relative differences in behavior being attributed to the tendency of the cyclohexane moieties to predispose the four sulfur donor atoms toward the various planar or tetrahedral conformations which can exist for these species. Consideration of the differences to be expected in the internal strains of the various possible conformations of the two oxidation states leads to the hypothesis that these Cu(II/I) systems may actually involve a three-rung ladder mechanism rather than a simple square scheme, although it is doubtful that more than two rungs will ever be experimentally observable.

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The quinquedentate macrocyclic ligand cyclo-6,6'-[1,9-(2,5,8-trithianonane)]-2,2'-bipyridine ([15]aneS3bpy = L), containing two pyridyl nitrogens and three thiaether sulfurs as donor atoms, has been synthesized and complexed with copper. The CuII/IL redox potential, the stabilities of the oxidized and reduced complex, and the oxidation and reduction electron-transfer kinetics of the complex reacting with a series of six counter reagents have been studied in acetonitrile at 25 degrees C, mu = 0.10 M (NaClO4). The Marcus cross relationship has been applied to the rate constants obtained for the reactions with each of the six counter reagents to permit the evaluation of the electron self-exchange rate constant, k11. The latter value has also been determined independently from NMR line-broadening experiments. The cumulative data are consistent with a value of k11 = 1 x 10(5) M(-1) s(-1), ranking this among the fastest-reacting CuII/I systems, on a par with the blue copper proteins known as cupredoxins. The resolved crystal structures show that the geometry of the CuIIL and CuIL complexes are nearly identical, both exhibiting a five-coordinate square pyramidal geometry with the central sulfur donor atom occupying the apical site. The most notable geometric difference is a puckering of an ethylene bridge between two sulfur donor atoms in the CuIL complex. Theoretical calculations suggest that the reorganizational energy is relatively small, with the transition-state geometry more closely approximating the geometry of the CuIIL ground state. The combination of a nearly constant geometry and a large self-exchange rate constant implies that this CuII/I redox system represents a true geometric "entatic state."

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