Studies of CNI Copper Coordination Compounds: What Determines the Electron-Transfer Rate of the Blue-Copper Proteins?

Unlike heme iron and iron-sulfur electron-transfer proteins, cuproproteins have no extrudable coordination complex, since the active-site structure exists only through chelation of the copper ion with protein residues.1 Thus, the study of small-molecule copper complexes offers one of the few means to evaluate the active-site contribution to electron transfer for copper proteins. Small-molecule model compounds for copper protein electron-transfer dynamics should ideally demonstrate coordination number invariance (CNI) and an outer-sphere mechanism of electron transfer. Synthetic copper systems rarely meet these two criteria, and the literature documents only a few well-defined candidates.2–6 The high kinetic lability of copper and its tendency to adopt different coordination numbers and geometries in the +1 and +2 oxidation states pose formidable obstacles in the design and synthesis of appropriate small-molecule systems. Despite such difficulties, we have obtained data for several CNI five-coordinate complexes and for one four-coordinate complex by synthesizing ligands carefully tailored to help control such problems. The synthesis and characterization of the five-coordinate complexes have been described elsewhere,6 while the four-coordinate complex is presented here for the first time.