AsMo<sub>7</sub>O<sub>27</sub>-Bridged Dinuclear Sandwich-Type Heteropolymolybdates of Cr(III) and Fe(III): Magnetism of [MM′(AsMo<sub>7</sub>O<sub>27</sub>)<sub>2</sub>]<sup>12−</sup> with MM′ = FeFe, CrFe, and CrCr

Two new dinuclear sandwich-type heteropolymolybdates based on the mulitidendate inorganic fragment [AsMo<sub>7</sub>O<sub>27</sub>] and Cr(III) and Fe(III) ions, namely, the homometallic sandwich polyoxometalate (POM) (NH<sub>4</sub>)<sub>12</sub>[Fe<sub>2</sub>(AsMo<sub>7</sub>O<sub>27</sub>)<sub>2</sub>]·12H<sub>2</sub>O (<b>1</b>) and the first example of the “symmetrical” heterometallic Cr(III)−Fe(III) sandwich POM, (NH<sub>4</sub>)<sub>12</sub>[FeCr(AsMo<sub>7</sub>O<sub>27</sub>)<sub>2</sub>]·13H<sub>2</sub>O (<b>2</b>), were simultaneously synthesized in high yield. Their magnetic properties are thoroughly investigated together with the homometallic sandwich POM (NH<sub>4</sub>)<sub>12</sub>[Cr<sub>2</sub>(AsMo<sub>7</sub>O<sub>27</sub>)<sub>2</sub>]·11H<sub>2</sub>O (<b>3</b>). The χ<sub>M</sub><i>T</i> values for compounds <b>1</b>−<b>3</b> at 300 K correspond well to the calculated spin-only values for Fe<sup>III</sup> (<i>S =</i> 5/2) and Cr<sup>III</sup> (<i>S</i> = 3/2) with <i>g</i><sub>Fe</sub> = <i>g</i><sub>Cr</sub> = 2. Upon cooling, the χ<sub>M</sub><i>T</i> values decline monotonously and reach 0.14, 1.00, and 0.11 cm<sup>3</sup> K mol<sup>−1</sup> at 2.0 K for <b>1</b>, <b>2</b>, and <b>3</b>, respectively, indicating a significant antiferromagnetic exchange between the magnetic centers with <i>J</i> = −2.09, −4.09, and −6.26 cm<sup>−1</sup>, respectively, for <b>1</b>, <b>2</b>, and <b>3</b>. The magnetic results clearly establish that compound <b>2</b> is formed by bimetallic Cr(III)−Fe(III) units and not by a mixture of the two antiferromagnetically coupled homometallic species. Their thermal properties are also characterized.