Reduction reactions of Cu(dmp)(2)(2+) (dmp = 2,9-dimethyl-1,10-phenanthroline) by ferrocene (Fe(Cp)(2) = bis(cyclopentadienyl)iron(II)), decamethylferrocene (Fe(PMCp)(2) = bis(pentamethylcyclopentadienyl)iron(II)), and Co(bpy)(3)(2+) (bpy = 2,2'-bipyridine) and oxidation reactions of Cu(dmp)(2)(+) by Ni(tacn)(2)(3+) (tacn = 1,4,7-triazacyclononane) and Mn(bpyO(2))(3)(3+) (bpyO(2) = N,N'-dioxo-2,2'-bipyridine) were studied in acetonitrile for the purpose of interpreting the gated behavior involving copper(II) and -(I) species. It was shown that the electron self-exchange rate constants estimated for the Cu(dmp)(2)(2+/+) couple from the oxidation reaction of Cu(dmp)(2)(+) by Ni(tacn)(2)(3+) (5.9 x 10(2) kg mol(-)(1) s(-)(1)) and Mn(bpyO(2))(3)(3+) (2.9 x 10(4) kg mol(-)(1) s(-)(1)) were consistent with the directly measured value by NMR (5 x 10(3) kg mol(-)(1) s(-)(1)). In contrast, we obtained the electron self-exchange rate constant of Cu(dmp)(2)(2+/+) as 1.6 kg mol(-)(1) s(-)(1) from the reduction of Cu(dmp)(2)(2+) by Co(bpy)(3)(2+). The pseudo-first-order rate constant for the reduction reaction of Cu(dmp)(2)(2+) by Fe(Cp)(2) was not linear against the concentration of excess amounts of Fe(Cp)(2). A detailed analysis of the reaction revealed that the reduction of Cu(dmp)(2)(2+) involved the slow path related to the deformation of Cu(dmp)(2)(2+) (path B in Scheme 1). By using Fe(PMCp)(2) (the E degrees value is 500 mV more negative than that of Fe(Cp)(2)(+/0)) as the reductant, the mixing with another pathway involving deformation of Cu(dmp)(2)(+) (path A in Scheme 1) became more evident. The origin of the "Gated Behavior" is discussed by means of the energy differences between the "normal" and deformed Cu(II) and Cu(I) species: the difference in the crystal field activation energies corresponding to the formation of pseudo-tetrahedral Cu(II) from tetragonally distorted Cu(II) and the difference in the stabilization energies of the tetrahedral and tetragonal Cu(I) for the activation of Cu(I) species. The reduction reaction of Cu(dmp)(2)(2+) by Fe(PMCp)(2) confirmed that the mixing of the two pathways takes place by lowering the energy level corresponding to the less favorable conformational change of Cu(I) species.
The electron self-exchange rate constant for the Cu(dmbp)22+/+ couple (dmbp = 6,6'-dimethyl-2,2'-bipyridine) was measured in acetonitrile by the NMR method (kex = 5.5 × 103 kg mol-1 s-1, ΔH* = 35.0 ± 0.3 kJ mol-1 and ΔS* = -56 ± 1 J mol-1 K-1). Reduction reactions of Cu(bcp)22+ (bcp = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline) and Cu(dmbp)22+ with Co(bipy)32+ (bipy = 2,2'-bipyridine) and ferrocene (Fe(Cp)2 = bis(cyclopentadienyl)iron(II)), and oxidation reactions of Cu(bcp)2+ and Cu(dmbp)2+ by Ni(tacn)23+ (tacn = 1,4,7-triazacyclononane) and Mn(bipyO2)33+ (bipyO2 = N,N'-dioxo-2,2'-bipyridine) were also studied in acetonitrile. The electron self-exchange rate constants, kex, estimated for the Cu(bcp)22+/+ and Cu(dmbp)22+/+ couples from the oxidation reactions of Cu(bcp)2+ and Cu(dmbp)2+ by Ni(tacn)23+ and Mn(bipyO2)33+ were consistent with the directly measured values by NMR, while kex estimated from the reduction reactions of Cu(bcp)22+ and Cu(dmbp)22+ by Co(bipy)32+ ([Cu(bcp)22+]0, [Cu(dmbp)22+]0 >> [Co(bipy)32+]0) were 103 times smaller than those directly measured by the NMR method. The pseudo-first-order rate constant for the reduction reaction of Cu(bcp)22+ and Cu(dmbp)22+ by Fe(Cp)2 was not linear against the concentration of excess amounts of Fe(Cp)2. Analyses of the reactions revealed that the reductions of Cu(bcp)22+ and Cu(dmbp)22+ involve slow paths related to the deformation of Cu(II)N4 center from tetragonal to tetrahedral coordination. The energetic preference for the deformation of Cu(II) species rather than that of Cu(I) was discussed on the basis of the ligand field activation energy (LFAE).Key words: electron transfer, copper(II) and copper(I) complexes, gated behavior.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.