Models for Methemocyanin Derivatives: Structural and Spectroscopic Comparisons of Related Azido-Coordinated (N_{3_}) Mono- and Dinuclear Copper(II) Complexes

The use of the azide (N_3-) ligand as a probe of the dinuclear copper active site in the dioxygen-transport protein hemocyanin has prompted us to synthesize model complexes that may shed light on the structural and spectroscopic features associated with N₃~ binding to Cu(II) in well-defined chemical systems. We report the synthesis and structural and spectroscopic properties of N_3-bound to a dinuclear Cu(II) complex and a mononuclear analogue. The dinucleating ligand, L-OH, forms a phenoxo-bridged dicopper(II) complex, 2b, where each Cu(II) ion is also coordinated in a square-based-pyramidal geometry to the nitrogen atoms of a tridentate py2 unit (py2 = bis(2-(2-pyridyl)ethyl)amine) and to a μ-1,1-N_3-ligand. A mononucleating analogue, [Cu₁₁-(L‘-0_-)(N_3-)] (3b), contains Cu(II) coordinated to the same N₃0 donor set provided by L’-OH and a terminally coordinated N_3-ligand in a coordination environment very similar to that found in 2b. Compound 2b crystallizes in the triclinic space group Pi with Z = 2 and a = 9.583 (1) A, b = 10.123 (2) A, c = 23.758 (4) A, a = 87.19 (1)°, β = 88.83 (1)°, and γ= 84.85 (1)°. Complex 3b crystallizes in the monoclinic space group P2₁/c, with Z = 4 and a = 9.529 (7) A, b = 18.950 (2) A, c = 13.829 (5) A, and β = 109.38 (5)°. Comparisons of the charge-transfer (CT) features observed in UV-vis spectra of these complexes and related chloride and/or nitrate ion containing derivatives of L-OH, L’-OH, and py2 allow assignment of the PhO_- —a- Cu(II) (450–460 nm) and/or N₃~ Cu(II) (370–405 nm) CT bands. Other structural and spectroscopic comparisons are also made.