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Issue 6, 2005
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Syntheses, structural analyses and redox kinetics of four-coordinate [CuL2]2+ and five-coordinate [CuL2(solvent)]2+ complexes (L = 6,6′-dimethyl-2,2′-bipyridine or 2,9-dimethyl-1,10-phenanthroline): completely gated reduction reaction of [Cu(dmp)2]2+ in nitromethane

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Abstract

[Cu(2,9-dimethyl-1,10-phenanthroline)2]2+ and [Cu(6,6′-dimethyl-2,2′-bipyridine)2]2+/+ complexes with no coordinated solvent molecule were synthesized and the crystal structures were analyzed: the coordination geometry around the Cu(I) center was in the D2d symmetry while a D2 structure was observed for the four-coordinate Cu(II) complexes. Coordination of a water or an acetonitrile molecule was found in the trigonal plane of the five-coordinate Cu(II) complex in the Tbp (trigonal bipyramidal) structure. Spectrophotometric analyses revealed that the D2 structure of the Cu(II) complex was retained in nitromethane, although a five-coordinate Tbp species (green in color), was readily formed upon dissolution of the solid (reddish brown) in acetonitrile. The electron self-exchange reaction between D2d-Cu(I) and D2-Cu(II), observed by the NMR method, was very rapid with kex = (1.1 ± 0.2) × 105 kg mol−1 s−1 at 25 °C (ΔH* = 15.6 ± 1.3 kJ mol−1 and ΔS* = −96 ± 4 J mol−1 K−1), which was more than 10 times larger than that reported for the self-exchange reaction between D2d-Cu(I) and Tbp-Cu(II) in acetonitrile. The cross reduction reactions of D2-Cu(II) by ferrocene and decamethylferrocene in nitromethane exhibited a completely gated behavior, while the oxidation reaction of D2d-Cu(I) by [Ni(1,4,7-triazacyclononane)2]3+ in nitromethane estimated an identically large self-exchange rate constant to that directly obtained by the NMR method. The electron self-exchange rate constant estimated from the oxidation cross reaction in 50% v/v acetonitrilenitromethane mixture was 10 times smaller than that observed in pure nitromethane. On the basis of the Principle of the Least Motion (PLM) and the Symmetry Rules, it was concluded that gated behaviors observed for the reduction reactions of the five-coordinate Cu(II)–polypyridine complexes are related to the high-energy C2vD2d conformational change around Cu(II), and that the electron self-exchange reactions of the Cu(II)/(I) couples are always adiabatic through the C2v structures for both Cu(II) and Cu(I) since the conformational changes between D2d, D2 and C2v structures for Cu(I) as well as the conformational change between Tbp and C2v structures for Cu(II) are symmetry-allowed. The completely gated behavior observed for the reduction reactions of D2-Cu(II) species in nitromethane was attributed to the very slow conformational change from the ground-state D2 to the entatic D2d structure that is symmetry-forbidden for d9 metal complexes: the very slow back reaction, the forbidden conformational change from entatic D2d to the ground-state D2 structure, ensures that the rate of the reduction reaction is independent of the concentration of the reducing reagent.

Graphical abstract: Syntheses, structural analyses and redox kinetics of four-coordinate [CuL2]2+ and five-coordinate [CuL2(solvent)]2+ complexes (L = 6,6′-dimethyl-2,2′-bipyridine or 2,9-dimethyl-1,10-phenanthroline): completely gated reduction reaction of [Cu(dmp)2]2+ in nitromethane

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Publication details

The article was received on 28 Sep 2004, accepted on 02 Feb 2005 and first published on 16 Feb 2005


Article type: Paper
DOI: 10.1039/B415057K
Citation: Dalton Trans., 2005, 1066-1078
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    Syntheses, structural analyses and redox kinetics of four-coordinate [CuL2]2+ and five-coordinate [CuL2(solvent)]2+ complexes (L = 6,6′-dimethyl-2,2′-bipyridine or 2,9-dimethyl-1,10-phenanthroline): completely gated reduction reaction of [Cu(dmp)2]2+ in nitromethane

    S. Itoh, N. Kishikawa, T. Suzuki and H. D. Takagi, Dalton Trans., 2005, 1066
    DOI: 10.1039/B415057K

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