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

The reactions of Cu­(ClO₄)₂ with NaCN and the ditopic ligands <i>m</i>-bis­[bis­(1-pyrazolyl)­methyl]­benzene (<b>L</b>_{<i><b>m</b></i>}) or <i>m</i>-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene (<b>L</b>_{<i><b>m</b></i>}*) yield [Cu₂(μ-CN)­(μ-<b>L</b>_{<i><b>m</b></i>})₂]­(ClO₄)₃ (<b>1</b>) and [Cu₂(μ-CN)­(μ-<b>L</b>_{<i><b>m</b></i>}<b>*</b>)₂]­(ClO₄)₃ (<b>3</b>). In both, the cyanide ligand is linearly bridged (μ-1,2) leading to a separation of the two copper­(II) ions of ca. 5 Å. The geometry around copper­(II) in these complexes is distorted trigonal bipyramidal with the cyanide group in an equatorial position. The reaction of [Cu₂(μ-F)­(μ-<b>L</b>_{<i><b>m</b></i>})₂]­(ClO₄)₃ and (CH₃)₃SiN₃ yields [Cu₂(<i>μ-</i>N₃)­(<i>μ-</i><b>L</b>_{<i><b>m</b></i>})₂]­(ClO₄)₃ (<b>2</b>), where the azide adopts end-on (μ-1,1) coordination with a Cu–N–Cu angle of 138.0° and a distorted square pyramidal geometry about the copper­(II) ions. Similar chemistry in the more sterically hindered <b>L</b>_{<i><b>m</b></i>}* system yielded only the coordination polymer [Cu₂(<i>μ-</i><b>L</b>_{<i><b>m</b></i>}*)­(<i>μ-</i>N₃)₂­(N₃)₂]. Attempts to prepare a dinuclear complex with a bridging iodide yield the copper­(I) complex [Cu₅(<i>μ-</i>I₄)­(μ-<b>L</b>_{<i><b>m</b></i>}*)₂]­I₃. The complexes <b>1</b> and <b>3</b> show strong antiferromagnetic coupling, −<i>J</i> = 135 and 161 cm^–1, respectively. Electron paramagnetic resonance (EPR) studies coupled with density functional theory (DFT) calculations show that the exchange interaction is transmitted through the d_<i>z</i>² and the bridging ligand s and p_<i>x</i> orbitals. High field EPR studies confirmed the d_<i>z</i>² ground state of the copper­(II) ions. Single-crystal high-field EPR has been able to definitively show that the signs of <i>D</i> and <i>E</i> are positive. The zero-field splitting is dominated by the anisotropic exchange interactions. Complex <b>2</b> has −<i>J</i> = 223 cm^–1 and DFT calculations indicate a predominantly d_{<i>x</i>²–y²} ground state