Models for Met-hemocyanin Derivatives: Structural and Spectroscopic Comparisons of Analogous Phenolate and X (X = OH-, OMe-, N3 -, Cl-, OAc-, OBz-) Doubly Bridged Dinuclear Copper(II) Complexes


The extensive use of spectroscopic and chemical studies of the binding of small molecules such as acetate, chloride, azide, etc. in chemically modified hemocyanin derivatives has prompted us to develop the chemistry of the dinucleating ligand L-OH, which can be utilized to form the compounds [Cu2(L-O-)X]2+, where two copper(II) ions are bridged by a phenolate oxygen atom from L-O- and the exogenous X- bridging atom. Here, we report structural, magnetic, and spectroscopic comparisons for a series of complexes, X = OH- (I2+), OMe- (IV2+), N3 - (II2+), Cl- (III2+), Br- (V2+), OBz- (VI2+), OAc- (VII2+), including complete X-ray structural characterization of complexes III, IV, and V. Compound III crystallizes in the triclinic space group P1 with Z = 2 and a = 10.986 (3) Å, b = 15.138 (3) Å, c = 23.292 Å, α = 91.11 (2)°, β = 99.77 (2)°, and γ = 91.18 (2)°. Complex IV crystallizes in the triclinic space group P1 with Z = 2 and a = 10.843 (2) Å, b = 15.045 (3) Å, c = 23.170 (4) Å, α = 90.14 (2)°, β = 100.29 (2)°, and γ = 96.27 (2)°. Compound V crystallizes in the monoclinic space group C2/c with Z = 4 and a = 15.740 (4) Å, b = 32.033 (4) Å, c = 13.033 (4) Å, and β = 134.03 (2)°. In these complexes, each copper atom is coordinated in a square-based pyramidal (SP) or distorted SP geometry to three nitrogen atoms provided by L-O-, the bridging phenolate oxygen atom, and X. Structural comparisons are made among compounds I-V; the presence of a larger X atom results in a distortion away from pure SP geometry with an opening of the Cul-Ol-Cu2 bridging angle resulting in a greater Cu⋯Cu separation. UV-vis spectroscopic comparisons indicate that the OPh- → Cu(II) LMCT transition (λmax = 378-475 nm) and the d-d envelope (λmax = 625-680 nm) vary systematically with structure; a shift to lower energy is observed for both types of electronic absorption as X becomes larger. On the basis of these and other observations, complexes VI and VII are assigned a monoatomic μ-carboxylato-O(O′) structure in solution, but a μ-carboxylato-O,O′ coordination in the solid state. Temperature-dependent magnetic measurements on the structurally characterized complexes I-III and V reveal that the halide-bridged complexes are the least strongly coupled with singlet-triplet separations, -2J, of 335 cm-1 for each. The OH- complex is the most strongly coupled of the series (-2J = 600 cm-1), while the azide complex falls in between (-2J = 440 cm-1). On the basis of only the Cu-Ophenolate-Cu angles, the coupling should increase in the direction OH- < μ-1,1-N3 - < Br-, but the opposite trend is actually observed and is attributed to the modulating effect of the exogenous bridge. © 1987 American Chemical Society.



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© 1987 American Chemical Society (ACS), All rights reserved.