AsMo₇O₂₇-Bridged Dinuclear Sandwich-Type Heteropolymolybdates of Cr(III) and Fe(III): Magnetism of [MM′(AsMo₇O₂₇)₂]¹²⁻ with MM′ = FeFe, CrFe, and CrCr

Two new dinuclear sandwich-type heteropolymolybdates based on the mulitidendate inorganic fragment [AsMo₇O₂₇] and Cr(III) and Fe(III) ions, namely, the homometallic sandwich polyoxometalate (POM) (NH₄)₁₂[Fe₂(AsMo₇O₂₇)₂]·12H₂O (1) and the first example of the “symmetrical” heterometallic Cr(III)−Fe(III) sandwich POM, (NH₄)₁₂[FeCr(AsMo₇O₂₇)₂]·13H₂O (2), were simultaneously synthesized in high yield. Their magnetic properties are thoroughly investigated together with the homometallic sandwich POM (NH₄)₁₂[Cr₂(AsMo₇O₂₇)₂]·11H₂O (3). The χ_MT values for compounds 1−3 at 300 K correspond well to the calculated spin-only values for Fe^III (S = 5/2) and Cr^III (S = 3/2) with g_Fe = g_Cr = 2. Upon cooling, the χ_MT values decline monotonously and reach 0.14, 1.00, and 0.11 cm³ K mol⁻¹ at 2.0 K for 1, 2, and 3, respectively, indicating a significant antiferromagnetic exchange between the magnetic centers with J = −2.09, −4.09, and −6.26 cm⁻¹, respectively, for 1, 2, and 3. The magnetic results clearly establish that compound 2 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.