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 Cu^I state has electronic configuration d¹⁰. Unless there are ligand bands or strong ligand-to-copper charge-transfer bands, diamagnetic Cu^I species are colorless. Complexes of Cu^II (d⁹) are often blue in color. The single unpaired electron makes Cu^IIamenable to electron paramagnetic resonance (EPR) techniques, at least if the electron spins of Cu^IIcenters 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 Cu^II ions is EPR silent.¹⁴

In aqueous solutions the Cu^I ion is unstable with respect to disproportionation to Cu metal and Cu^II 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 Cu^Istate 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 Cu^Ithiolate species are stable, Cu^II 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 Cu^IIthiolate 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 O₂binding.¹⁰²The 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 dicopper(II)-peroxo moiety,¹^03f while a second, with more properties in common with oxyhemocyanin, features a clipboard_eb4f885d23c1cbd71a4faeb7c589de1de moiety.^103g