X-ray Absorption Edge Spectroscopy of Copper(I) Complexes. Coordination Geometry of Copper(I) in the Reduced Forms of Copper Proteins and Their Derivatives with Carbon Monoxide

X-ray absorption edges have been measured for a variety of Cu(I) complexes of known structure. The structures of these complexes vary in a systematic way from linear two-coordinate through distorted trigonal and trigonal-pyramidal to tetrahedral stereochemistry. The structure of one of the model complexes, [Cu₂(N30R)](PF₆)2’CH₂Cl2 (3-(PF₆)2'CH₂Cl₂), is described. 3-(PF₆)₂-CH₂C1₂, C₄5H4₈Cl2Cu₂F₁₂N₆0₂P₂, crystallizes in the triclinic space group P1 with Z = 2 and a = 12.430 (4) Å, b = 13.456 (3) Å, c = 15.497 (4) Å, a = 81.75 (2)°, β= 83.43 (2)°, and γ= 79.68 (2)°. The structure consists of one discreet complex dication and two well-separated anions per asymmetric unit. The two crystallographically independent cuprous ions are each three-coordinate with ligation from two pyridine and one tertiary amino donor groups. The Cu-N(py) distances (Cu-N(py)_av= 1.90 A) are among the shortest observed within the tripodal tridentate PY2 group of ligands. The intensity of the 8983-eV absorption edge feature in the X-ray absorption spectra of these complexes is shown to correlate with the degree of distortion of the copper center from the N₃ plane, and a qualitative ligand field analysis is presented that invokes mixing of 4s and 4p_zorbitals in the final state wave function. The data indicate that absorption edge spectroscopy is a useful technique for the detection of ligand binding to Cu(I) in protein and enzyme systems and, in favorable circumstances, for indicating the probable coordination geometry. The absorption edges of the reduced forms of superoxide dismutase (SOD), dopamine β-hydroxylase (DBH), and deoxyhemocyanin (deoxy-Hc) have been investigated. The absorption edges of reduced SOD and DBH are consistent with three-coordinate Cu(I) centers with only minor distortions from planarity. The edge of deoxy-Hc on the other hand is more consistent with distorted trigonal or tetrahedral geometry. Comparison with the model compounds suggests that a fourth ligand per copper may be present in deoxy-Hc, additional to the three protein-derived histidine ligands, although the data are also consistent with highly distorted trigonal geometry. On binding of CO, the edge profiles indicate a change toward tetrahedral geometry at one or both coppers, and a model for CO binding is presented. DBH is also shown to bind CO.