Synthesis, Detailed Characterization, and Theoretical Understanding of Mononuclear Chromium(III)-Containing Polyoxotungstates [CrIII(HXVW7O28)2]13– (X = P, As) with Exceptionally Large Magnetic Anisotropy

Two monochromium(III)-containing heteropolytungstates, [CrIII(HPVW7O28)2]13- (1a) and [CrIII(HAsVW7O28)2]13- (2a), 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[CrIII(HPVW7O28)2]·47H2O (1) and Na13[CrIII(HAsVW7O28)2]·52H2O (2). Polyanions 1a and 2a comprise an octahedrally coordinated CrIII ion, sandwiched by two {PW7} or {AsW7} units. Both compounds 1 and 2 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 1 and 2 demonstrated that both compounds exhibit appreciable deviation from typical paramagnetic behavior, and have a ground state S = 3/2, as expected for a CrIII ion, but with an exceptionally large zero-field uniaxial anisotropy parameter (D). EPR measurements on powder and single-crystal samples of 1 and 2 using 9.5, 34.5, and 239.2 GHz frequencies...