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 Å

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. [Cu2(biib)2(BF4)2](BF4)2(H2O)4: triclinic, space group P1̄, a = 10.178(1) Å, b = 9.4881(9) Å, c = 11.037(1) Å, α = 95.130(10)°, β = 112.20(1)°, γ = 92.142(9)°, and Z = 1. [Cu2(biib)2(NO3)2](NO3)2(H2O)4: monoclinic, space group P̄21 /n, a = 9.207(6) Å, b = 17.0516(6) Å, c = 12.6107(7) Å, β = 109.82(1)°, and Z = 2. [Cu2(biib)2(CuBr3)2]: monoclinic, space group P21 /c, a = 11.583(2) Å, b = 11.864(2) Å, c = 16.070(2) Å, β = 112.459(12)°, 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 [Cu2(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.