Syntheses, Structural, Magnetic, and Electron Paramagnetic Resonance Studies of Monobridged Cyanide and Azide Dinuclear Copper(II) Complexes: Antiferromagnetic Superexchange Interactions

摘要

The reactions of Cu(ClO4)(2) with NaCN and the ditopic ligands m-bis[bis(1-pyrazolyl)methyl]benzene (L-m) or m-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene (L-m*) yield [Cu-2(mu-CN)(mu-L-m)(2)](ClO4)(3) (1) and [Cu-2(mu-CN)(mu-L-m*)(2)](ClO4)(3) (3). In both, the cyanide ligand is linearly bridged (mu-1,2) leading to a separation of the two copper(II) ions of ca. 5 angstrom. The geometry around copper(II) in these complexes is distorted trigonal bipyramidal with the cyanide group in an equatorial position. The reaction of [Cu-2(mu-F)(mu-L-m)(2)](ClO4)(3) and (CH3)(3)SiN3 yields [Cu-2(mu-N-3)(mu-L-m)(2)](ClO4)(3) (2), where the azide adopts end-on (mu-1,1) coordination with a CuNCu angle of 138.0 degrees and a distorted square pyramidal geometry about the copper(II) ions. Similar chemistry in the more sterically hindered L-m* system yielded only the coordination polymer [Cu-2(mu-L-m*)(mu-N-3)(2)(N-3)(2)]. Attempts to prepare a dinuclear complex with a bridging iodide yield the copper(I) complex [Cu-5(mu-I-4)(mu-L-m*)(2)]I-3. The complexes 1 and 3 show strong antiferromagnetic coupling, -J = 135 and 161 cm1, respectively. Electron paramagnetic resonance (EPR) studies coupled with density functional theory (DFT) calculations show that the exchange interaction is transmitted through the d(z)(2) and the bridging ligand s and p(x) orbitals. High field EPR studies confirmed the dz(2) ground state of the copper(II) ions. Single-crystal high-field EPR has been able to definitively show that the signs of D and E are positive. The zero-field splitting is dominated by the anisotropic exchange interactions. Complex 2 has -J = 223 cm(-1) and DFT calculations indicate a predominantly d(x)(y)(2)(2) ground state.