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 m-bis­[bis­(1-pyrazolyl)­methyl]­benzene
(Lm) or m-bis­[bis­(3,5-dimethyl-1-pyrazolyl)­methyl]­benzene
(Lm*) yield
[Cu2(μ-CN)­(μ-Lm)2]­(ClO4)3 (1) and [Cu2(μ-CN)­(μ-Lm*)2]­(ClO4)3 (3). 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-Lm)2]­(ClO4)3 and (CH3)3SiN3 yields [Cu2(μ-N3)­(μ-Lm)2]­(ClO4)3 (2), 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 Lm* system yielded only the coordination polymer [Cu2(μ-Lm*)­(μ-N3)2­(N3)2]. Attempts to prepare
a dinuclear complex with a bridging iodide yield the copper­(I) complex
[Cu5(μ-I4)­(μ-Lm*)2]­I3. The complexes 1 and 3 show strong
antiferromagnetic coupling, −J = 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 dz2 and the bridging ligand s and
px orbitals. High field EPR studies confirmed
the dz2 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 dx2–y2 ground state.