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4.6: General Aspects of the Chemistry of Copper

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    The chemistry of copper in biological systems is limited to oxidation states I and II. The CuI state has electronic configuration d10. Unless there are ligand bands or strong ligand-to-copper charge-transfer bands, diamagnetic CuI species are colorless. Complexes of CuII (d9) are often blue in color. The single unpaired electron makes CuII amenable to electron paramagnetic resonance (EPR) techniques, at least if the electron spins of CuII centers are independent of one another. In oxyhemocyanin the spins are so strongly coupled (-J > 600 cm-1) that at room temperature and below the system is effectively diamagnetic and the pair of CuII ions is EPR silent.14

    In aqueous solutions the CuI ion is unstable with respect to disproportionation to Cu metal and CuII ion:62

    \[Cu^{II} + 2e^{-} \rightleftharpoons Cu \qquad E^{o} = 0.3402 V\]

    \[Cu^{+} + e^{-} \rightleftharpoons Cu \qquad E^{o} = 0.522 V \tag{4.35}\]

    \[2 Cu^{+} \rightleftharpoons Cu + Cu^{II-} \qquad E^{o} = 0.182 V\]

    The CuI state may be stabilized by ligands, especially sulfur-containing ones, or by immobilization as afforded by a protein matrix, or in nonaqueous solvents, such as acetonitrile, in the absence of dioxygen. Whereas CuI thiolate species are stable, CuII thiolate species usually are unstable with respect to the disproportionation:101

    \[2 Cu^{II}—SR \rightarrow 2 Cu^{I} + R—S—S—R \tag{4.36}\]

    Again, immobilization may give kinetic stability to CuII thiolate species, as occurs in the blue-copper family of electron-transport proteins.

    Copper(l) complexes are often two-coordinate with a linear arrangement of ligands. Three-, four-, and possibly five-coordinate complexes are known.

    In the presence of O2, nonbiological copper(l) [and iron(II)] complexes are often susceptible to ligand degradation, which may give the illusion of O2 binding.102 The mechanisms by which this reaction occurs remain essentially unknown. Iron-porphyrin systems are rather more robust. Nonetheless, there are now several well-characterized copper(l) systems that reversibly bind dioxygen,15b,103 at least at low temperature. One that has been structurally characterized features a clipboard_e690176447803ff9ed0a6947c4c70cf76dicopper(II)-peroxo moiety,103f while a second, with more properties in common with oxyhemocyanin, features a clipboard_eb4f885d23c1cbd71a4faeb7c589de1demoiety.103g

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