Synthetic Models for Type I Copper

The unusual spectroscopic properties of type I copper have been particularly fascinating to many inorganic chemists. The most striking 

A considerable number of crystal structures of type I copper sites in proteins are now available, so there may be no particular advantage in the synthetic model approach to prove the coordination structure of type I. Yet, inorganic chemists still have an opportunity to utilize the spectroscopic and structural bases established by model studies to understand the precise electronic structure of type I copper. One should keep in mind that the generally accepted interpretation derived from spectroscopic and theoretical studies on the proteins (47-49) has not been definitely proved experimentally. A systematic comparison of a series of copper(II) thiolate complexes having an unusual distorted coordination structure is required for a conclusive description of the electronic structure of the type I copper. The synthetic approach is ultimately the most adequate way to clarify how the ligand donors and geometry affect the electronic property and function of type I copper as an electron transfer center. 

The main difficulty in synthesizing an accurate model for type I copper comes from the instability of the copper(II)-thiolate bond. This bond easily undergoes homolysis, causing reduction of the copper(II) ion with formation of disulfide as follows  

In order to mimic the spectroscopic characteristics of type I copper, both the stable Cu—S bond and the tetrahedral distortion should be 

These experimental results prompted reconsideration of the inscrutable result reported for Cu(SC5H4-p-N02)(HB(3,5-Me2pz)3) (36). The absorption spectrum of the complex was reasonably similar to that of type I copper, whereas the EPR spectrum was typical for a tetragonal complex. Because the EPR was recorded in THF at 77 K, our observa- 

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