Synthesis,
Detailed Characterization, and Theoretical
Understanding of Mononuclear Chromium(III)-Containing Polyoxotungstates
[Cr<sup>III</sup>(HX<sup>V</sup>W<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]<sup>13–</sup> (X = P, As) with Exceptionally Large Magnetic
Anisotropy
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Abstract
Two monochromium(III)-containing
heteropolytungstates, [Cr<sup>III</sup>(HP<sup>V</sup>W<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]<sup>13‑</sup> (<b>1a</b>) and [Cr<sup>III</sup>(HAs<sup>V</sup>W<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]<sup>13‑</sup> (<b>2a</b>), were prepared
via simple, one-pot reactions in aqueous, basic medium, by reaction
of the composing elements, and then isolated as hydrated sodium salts,
Na<sub>13</sub>[Cr<sup>III</sup>(HP<sup>V</sup>W<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]·47H<sub>2</sub>O (<b>1</b>) and Na<sub>13</sub>[Cr<sup>III</sup>(HAs<sup>V</sup>W<sub>7</sub>O<sub>28</sub>)<sub>2</sub>]·52H<sub>2</sub>O (<b>2</b>). Polyanions <b>1a</b> and <b>2a</b> comprise
an octahedrally coordinated Cr<sup>III</sup> ion, sandwiched by two
{PW<sub>7</sub>} or {AsW<sub>7</sub>} units. Both compounds <b>1</b> and <b>2</b> were fully characterized in the solid
state by single-crystal XRD, IR spectroscopy, thermogravimetric and
elemental analyses, magnetic susceptibility, and EPR measurements.
Magnetic studies on <b>1</b> and <b>2</b> demonstrated
that both compounds exhibit appreciable deviation from typical paramagnetic
behavior, and have a ground state S = <sup>3</sup>/<sub>2</sub>, as
expected for a Cr<sup>III</sup> ion, but with an exceptionally large
zero-field uniaxial anisotropy parameter (<i>D</i>). EPR
measurements on powder and single-crystal samples of <b>1</b> and <b>2</b> using 9.5, 34.5, and 239.2 GHz frequencies and
over 4–295 K temperature fully support the magnetization results
and show that <i>D</i> = +2.4 cm<sup>–1</sup>, the
largest and sign-assigned <i>D</i>-value so far reported
for an octahedral Cr<sup>III</sup>-containing, molecular compound.
Ligand field analysis of results from CASSCF and NEVPT2-correlated
electronic structure calculations on Cr(OH)<sub>6</sub><sup>3–</sup> model complexes allowed to unravel the crucial role of the second
coordination sphere of Cr<sup>III</sup> for the unusually large magnetic
anisotropy reflected by the experimental value of <i>D</i>. The newly developed theoretical modeling, combined with the synthetic
procedure for producing such unusual magnetic molecules in a well-defined
and essentially magnetically isolated environment, appears to be a
versatile new research area