Syntheses, Structural, Magnetic, and Electron Paramagnetic
Resonance Studies of Monobridged Cyanide and Azide Dinuclear Copper(II)
Complexes: Antiferromagnetic Superexchange Interactions
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Abstract
The
reactions of Cu(ClO<sub>4</sub>)<sub>2</sub> with NaCN and
the ditopic ligands <i>m</i>-bis[bis(1-pyrazolyl)methyl]benzene
(<b>L</b><sub><i><b>m</b></i></sub>) or <i>m</i>-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene
(<b>L</b><sub><i><b>m</b></i></sub>*) yield
[Cu<sub>2</sub>(μ-CN)(μ-<b>L</b><sub><i><b>m</b></i></sub>)<sub>2</sub>](ClO<sub>4</sub>)<sub>3</sub> (<b>1</b>) and [Cu<sub>2</sub>(μ-CN)(μ-<b>L</b><sub><i><b>m</b></i></sub><b>*</b>)<sub>2</sub>](ClO<sub>4</sub>)<sub>3</sub> (<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 [Cu<sub>2</sub>(μ-F)(μ-<b>L</b><sub><i><b>m</b></i></sub>)<sub>2</sub>](ClO<sub>4</sub>)<sub>3</sub> and (CH<sub>3</sub>)<sub>3</sub>SiN<sub>3</sub> yields [Cu<sub>2</sub>(<i>μ-</i>N<sub>3</sub>)(<i>μ-</i><b>L</b><sub><i><b>m</b></i></sub>)<sub>2</sub>](ClO<sub>4</sub>)<sub>3</sub> (<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><sub><i><b>m</b></i></sub>* system yielded only the coordination polymer [Cu<sub>2</sub>(<i>μ-</i><b>L</b><sub><i><b>m</b></i></sub>*)(<i>μ-</i>N<sub>3</sub>)<sub>2</sub>(N<sub>3</sub>)<sub>2</sub>]. Attempts to prepare
a dinuclear complex with a bridging iodide yield the copper(I) complex
[Cu<sub>5</sub>(<i>μ-</i>I<sub>4</sub>)(μ-<b>L</b><sub><i><b>m</b></i></sub>*)<sub>2</sub>]I<sub>3</sub>. The complexes <b>1</b> and <b>3</b> show strong
antiferromagnetic coupling, −<i>J</i> = 135 and 161
cm<sup>–1</sup>, respectively. Electron paramagnetic resonance
(EPR) studies coupled with density functional theory (DFT) calculations
show that the exchange interaction is transmitted through the d<sub><i>z</i><sup>2</sup></sub> and the bridging ligand s and
p<sub><i>x</i></sub> orbitals. High field EPR studies confirmed
the d<sub><i>z</i><sup>2</sup></sub> 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<sup>–1</sup> and DFT calculations indicate
a predominantly d<sub><i>x</i><sup>2</sup>–y<sup>2</sup></sub> ground state
