Synthetic Models for Type II Copper

Type IIA copper is defined as a monomeric copper site with a tetragonal ligand environment, exhibiting the spectroscopic features of conventional synthetic Cu(II) complexes. The coordination structure of the type IIA site is not remarkable from an inorganic point of view. What is noteworthy is the interaction between the copper ion and a prosthetic group or substrate. The electron uptake or activation of a substrate that occurs through this interaction plays an essential role in the catalytic cycle, yet the structural and mechanistic details are not certain. Therefore, the synthetic model approach may provide useful information for understanding the catalytic processes. 

Copper-dependent amine oxidases contain a type II copper (83-85). The yellow-pink color of amine oxidase implies the existence of another organic cofactor whose identification has not yet been unambiguously 

In light of the accepted mechanism for cytochrome P-450 (97-100), a superoxo-Cu(II) intermediate is further reduced, leading to dioxygen activation. Accordingly, a monomeric peroxo or hydroperoxo copper(II) complex serves as a synthetic model for these intermediates of copper-containing monooxygenases. However, no well-characterized complexes of these types are available to date. Formation of a monomeric hydroperoxo or acylperoxo complex was reported to occur when a trans-/u-l,2-peroxo complex, [(Cu(TPA))2(02)]z+, was treated with H+ or RC(O)+, but no details of the structures and properties of the complexes were provided (101). A related complex, a monomeric acylperoxo cop-per(II) complex, was synthesized (102). Low-temperature reaction of a dimeric copper(II) hydroxide complex, [Cu(HB(3,5-iPr2pz)3)]2(OH)2, with 2 equivalents of m-CPBA (3-chloroperoxybenzoic acid) yielded a monomeric acylperoxo complex whose structure was characterized by 

Turning to the complexes of copper(II), copper(IV) is not stable and heterolysis of the 0—0 bond of the peroxide to form the copper(IV) oxo complex does not occur. In addition, the Lewis acidity of the copper(II) ion is not high enough to enhance the electrophilicity of the coordinated alkyl- or acylperoxide to promote direct oxo-incorporating reactions. With these points in mind, the inert activity of alkyl- and acylperoxo copper(II) complexes, experimentally observed, is understandable, and it is quite unlikely that the mechanism of copper monooxygenase parallels that of cytochrome P-450. 

The crystal structure of nitrite reductase (NiR) from Achromobacter cycloclastes was recently reported by Godden et al. (46). The protein contains both a type I and a type II copper site. The type I center has a distorted tetrahedral structure typical of type I copper, whereas the 

---