Synthesis, characterization, and single-crystal EPR studies of three dinuclear copper(II) complexes and one mixed-valence tetranuclear copper(I)-copper(II) cluster with an asymmetric imidazole-containing tripodal ligand with copper(II)-copper(II) distances between 3.35 and 3.63 angstrom

The synthesis and characterization of three dinuclear copper(II) complexes and one mixed-valence tetranuclear cluster with the asymmetric imidazole-containing ligand bis(1,1'-imidazole-2-yl)(4-imidazole-4(5)-yl)-2-azabutane (biib) are described. X-ray crystallographic parameters for the copper complexes are as follows. [Cu-2(biib)(2)-(BF4)(2)](BF4)(2)(H2O)(4): triclinic, space group <P(1)over bar>, a = 10.178(1) Angstrom, b = 9.4881(9) Angstrom, c = 11.037(1) Angstrom, alpha = 95.130(10)degrees, beta = 112.20(1)degrees gamma = 92.142(9)degrees, and Z = 1. [Cu-2(biib)(2)(NO3)(2)(NO3)(2)(H2O)(4): monoclinic, space group (P) over bar 2(1)/n, a = 9.207(6) Angstrom, b = 17.0516(6) Angstrom, c = 12.6107(7) Angstrom, beta = 109.82(1)degrees and Z = 2. [Cu-2(biib)(2)-(CuBr3)(2)]: monoclinic, space group P2(1)/c, a = 11.583(2) Angstrom, b = 11.864(2) Angstrom c = 16.070(2) Angstrom, beta = 112.459- (12)degrees, and Z = 2. The two Cu(II) ions in all four complexes are coordinated in a square-pyramidal geometry by three imidazole nitrogens and one amine nitrogen donor in the equatorial plane, and each copper ion is weakly coordinated at the axial position by respectively a tetrafluoroborate, a perchlorate, a nitrate, or a tribromocuprate-(I) anion. By comparison of the structural data of the four complexes a relationship has been established between the donor strength of the anion and some structural features, like the Cu(II)-Cu(II) distance, of the dinuclear Cu(II)-Cu(II) unit in the four complexes. Single-crystal EPR spectra of [Cu-2(biib)(2)(BF4)(2)](BF4)(2)(H2O)(4) were recorded at room temperature at X-band frequencies. The triplet spectra have been fit with nonparallel g and D tensors, whose principle values are as follows: g(xx) = 2.022(8), g(yy) = 2.060(7), g(zz) = 2.211(8), D-x'x' = -0.0182- (9) cm(-1), D-y'y' = -0.081(6) cm(-1), D-z'z' = 0.0264(7) cm(-1). The compounds were further characterized and studied by ligand field and by frozen-solution and polycrystalline powder EPR spectroscopy. EPR spectra recorded at 77 K of frozen solutions of the perchlorate complex show that upon dilution in methanol the dinuclear complex reacts to form a mononuclear species.