Synthesis, Detailed Characterization, and Theoretical Understanding of Mononuclear Chromium(III)-Containing Polyoxotungstates [Cr^III(HX^VW₇O₂₈)₂]^13– (X = P, As) with Exceptionally Large Magnetic Anisotropy

Two monochromium­(III)-containing heteropolytungstates, [Cr^III(HP^V­W₇O₂₈)₂]^13‑ (<b>1a</b>) and [Cr^III(HAs^V­W₇O₂₈)₂]^13‑ (<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, Na₁₃[Cr^III(HP^V­W₇O₂₈)₂]·47H₂O (<b>1</b>) and Na₁₃[Cr^III(HAs^V­W₇O₂₈)₂]·52H₂O (<b>2</b>). Polyanions <b>1a</b> and <b>2a</b> comprise an octahedrally coordinated Cr^III ion, sandwiched by two {PW₇} or {AsW₇} 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 = ³/₂, as expected for a Cr^III 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^–1, the largest and sign-assigned <i>D</i>-value so far reported for an octahedral Cr^III-containing, molecular compound. Ligand field analysis of results from CASSCF and NEVPT2-correlated electronic structure calculations on Cr­(OH)₆^3– model complexes allowed to unravel the crucial role of the second coordination sphere of Cr^III 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