Hydroxide-Bridged
Cubane Complexes of Nickel(II) and Cadmium(II): Magnetic, EPR, and
Unusual Dynamic Properties
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Abstract
The
reactions of M(ClO<sub>4</sub>)<sub>2</sub>·<i>x</i>H<sub>2</sub>O (M = Ni(II) or Cd(II)) and <i>m</i>-bis[bis(1-pyrazolyl)methyl]benzene
(<b>L</b><sub><b>m</b></sub>) in the presence of triethylamine
lead to the formation of hydroxide-bridged cubane compounds of the
formula [M<sub>4</sub>(μ<sub>3</sub>-OH)<sub>4</sub>(μ-<b>L</b><sub><b>m</b></sub>)<sub>2</sub>(solvent)<sub>4</sub>](ClO<sub>4</sub>)<sub>4</sub>, where solvent = dimethylformamide,
water, acetone. In the solid state the metal centers are in an octahedral
coordination environment, two sites are occupied by pyrazolyl nitrogens
from <b>L</b><sub><b>m</b></sub>, three sites are occupied
by bridging hydroxides, and one site contains a weakly coordinated
solvent molecule. A series of multinuclear, two-dimensional and variable-temperature
NMR experiments showed that the cadmium(II) compound in acetonitrile-<i>d</i><sub>3</sub> has <i>C</i><sub>2</sub> symmetry
and undergoes an unusual dynamic process at higher temperatures (Δ<i>G</i><sub>Lm</sub><sup>‡</sup> = 15.8 ± 0.8 kcal/mol at 25 °C) that equilibrates the
pyrazolyl rings, the hydroxide hydrogens, and cadmium(II) centers.
The proposed mechanism for this process combines two motions in the
semirigid <b>L</b><sub><b>m</b></sub> ligand termed the
“Columbia Twist and Flip:” twisting of the pyrazolyl
rings along the C<sub>pz</sub>–C<sub>methine</sub> bond and
180° ring flip of the phenylene spacer along the C<sub>Ph</sub>–C<sub>methine</sub> bond. This dynamic process was also followed
using the spin saturation method, as was the exchange of the hydroxide
hydrogens with the trace water present in acetonitrile-<i>d</i><sub>3</sub>. The nickel(II) analogue, as shown by magnetic susceptibility
and electron paramagnetic resonance measurements, has an <i>S</i> = 4 ground state, and the nickel(II) centers are ferromagnetically
coupled with strongly nonaxial zero-field splitting parameters. Depending
on the Ni–O–Ni angles two types of interactions are
observed: <i>J</i><sub>1</sub> = 9.1 cm<sup>–1</sup> (97.9 to 99.5°) and <i>J</i><sub>2</sub> = 2.1 cm<sup>–1</sup> (from 100.3 to 101.5°). “Broken symmetry”
density functional theory calculations performed on a model of the
nickel(II) compound support these observations
