X-ray structures and redox properties of Copper(II) Bis(pyrazole) complexes

The copper complexes of bis[2-(3,5-dimethyl-l-pyrazolyl)ethyl]amine (1) and its benzylated derivative (2) have been studied as single crystals by X-ray crystallography and as powders and solutions by a variety of techniques. [Cu(1)·2NO3] (C14H25CUN7O7 including 1 H2O, 3) crystallized in the monoclinic space group Pc (No. 7) with a = 8.1596(9) Å,b = 31.164(2) Å, c = 7.7966(9) Å, β = 94.75(2)°, V = 1975.7(3) Å3, and Z = 4 (structure refinement converged to RF = 0.066 and RwF2 = 0.227); [Cu(1)C1O4·C2H5OH]C1O4 (C16H29CI2CUN5O9, 4) crystallized in the orthorhombic space group Pna21 (No. 33) with a = 13.3463(7) Å, b = 10.3925(9) Å, c = 17.511(1) Å, V = 2432.1(3) Å3, and Z = 4 (refined to RF = 0.063 and RwF = 0.052); [Cu(2)·2NO3] (C21H29-CuN7O6, 5) crystallized in the monoclinic space group C2/c (No.15) with a = 28.719(2) Å, b = 8.6043(6) Å, c = 23.034(2) Å, β = 124.767(7)°, V = 4675.7(7) Å3, and Z = 8 (refined to RF = 0.042 and RwF = 0.034); [Cu·(2)·C2H5OH·Y](C1O4)″ (C48H78Cl4Cu2N10O20, 6) (Y = H2O, n = 2, or C1O4−, n = 1) crystallized in the monoclinic space group P21/c (No. 14) with a = 10.9488(9) Å,b = 17.030(2) Å, c = 32.893(3) Å, β = 92.185-(7)°, V= 6129(1) Å3, and Z = 4 (refined to RF = 0.104 and RwF = 0.119). In complexes 3, 4, and 6, the ligand coordinated Cu(II) with a T-shaped arrangement of the nitrogen donor atoms. Ethanol coordinated in one of the apical positions in complexes 4 and 6. The amine nitrogen moved from the basal plane in 3 to the apical position in 5, where the copper had square pyramidal coordination geometry. The largest difference between 4 and 6 was a lengthening of 0.1 Å in the bond between the Cu(II) and the amine nitrogen. Conductivity measurements revealed that the nitrate complexes behave as 1:1 electrolytes in acetonitrile solution, whereas the Perchlorate complexes behaved as 2:1 electrolytes. The half-wave potentials, measured in acetonitrile by cyclic voltammetry, were −0.01, +0.15, +0.10, and +0.28 V for 3–6, respectively. The high redox potentials of the copper(II) bis(pyrazole) complexes, in particular for complex 6, are explained by the fact that the Cu(II) complexes have the correct ligand geometry for the stabilization of Cu(I).