NMR Investigations of Dinuclear, Single-Anion Bridged
Copper(II) Metallacycles: Structure and Antiferromagnetic Behavior
in Solution
- Publication date
- Publisher
Abstract
The nuclear magnetic resonance (NMR)
spectra of single-anion bridged, dinuclear copper(II) metallacycles
[Cu<sub>2</sub>(μ-X)(μ-<b>L</b>)<sub>2</sub>](A)<sub>3</sub> (<b>L</b><sub><i><b>m</b></i></sub> = <i>m</i>-bis[bis(1-pyrazolyl)methyl]benzene: X = F<sup>–</sup>, A = BF<sub>4</sub><sup>–</sup>; X = Cl<sup>–</sup>, OH<sup>–</sup>, A = ClO<sub>4</sub><sup>–</sup>; <b>L</b><sub><i><b>m</b></i></sub><b>*</b> = <i>m</i>-bis[bis(3,5-dimethyl-1-pyrazolyl)methyl]benzene:
X = CN<sup>–</sup>, F<sup>–</sup>, Cl<sup>–</sup>, OH<sup>–</sup>, Br<sup>–</sup>, A = ClO<sub>4</sub><sup>–</sup>) have relatively sharp <sup>1</sup>H and <sup>13</sup>C NMR resonances with small hyperfine shifts due to the strong
antiferromagnetic superexchange interactions between the two <i>S</i> = <sup>1</sup>/<sub>2</sub> metal centers. The complete
assignments of these spectra, except X = CN<sup>–</sup>, have
been made through a series of NMR experiments: <sup>1</sup>H–<sup>1</sup>H COSY, <sup>1</sup>H–<sup>13</sup>C HSQC, <sup>1</sup>H–<sup>13</sup>C HMBC, <i>T</i><sub>1</sub> measurements
and variable-temperature <sup>1</sup>H NMR. The <i>T</i><sub>1</sub> measurements accurately determine the Cu···H
distances in these molecules. In solution, the temperature dependence
of the chemical shifts correlate with the population of the paramagnetic
triplet (<i>S</i> = 1) and diamagnetic singlet (<i>S</i> = 0) states. This correlation allows the determination
of antiferromagnetic exchange coupling constants, −<i>J</i> (<b>Ĥ</b> = −<i>J</i><b>Ŝ</b><sub>1</sub><b>Ŝ</b><sub>2</sub>), in
solution for the <b>L</b><sub><i><b>m</b></i></sub> compounds 338(F<sup>–</sup>), 460(Cl<sup>–</sup>), 542(OH<sup>–</sup>), for the <b>L</b><sub><i><b>m</b></i></sub>* compounds 128(CN<sup>–</sup>), 329(F<sup>–</sup>), 717(Cl<sup>–</sup>), 823(OH<sup>–</sup>), and 944(Br<sup>–</sup>) cm<sup>–1</sup>, respectively. These values are of similar magnitudes to those previously
measured in the solid state (−<i>J</i><sub>solid</sub> = 365, 536, 555, 160, 340, 720, 808, and 945 cm<sup>–1</sup>, respectively). This method of using NMR to determine −<i>J</i> values in solution is an accurate and convenient method
for complexes with strong antiferromagnetic superexchange interactions.
In addition, the similarity between the solution and solid-state −<i>J</i> values of these complexes confirms the information gained
from the <i>T</i><sub>1</sub> measurements: the structures
are similar in the two states
