Tetra-Me-III-Containing 30-Tungsto-4-phosphate, [Mn-4(III)(H2O)(2)(P2W15O56)(2)](12-): Synthesis, Structure, XPS, Magnetism, and Electrochemical Study

Reaction of the mixed-valent Mn-12-acetato complex [(Mn8Mn4O12)-Mn-III-O-IV(CH3COO)(16)(H2O)(4)] with the trilacunary Wells-Dawson-type heteropolytungstate [P2W15O56](12-) in acidic acetate solution (pH 1.1) resulted in the tetra-Mn-III-containing polyanion [Mn-4(III)(H2O)(2)(P2W15O56)(2)](12-) (1). Single-crystal XRD on Na-12[Mn-4(III)(H2O)(2)(P2W15O56)(2)]center dot 84H(2)O (1a) revealed that four MnIII ions form a rhombic Mn4O16 core encapsulated by two [P2W15O56](12-) units. X-ray photoelectron spectroscopy (XPS) data confirm the +3 oxidation state of the four manganese ions in 1. Magnetic measurements from 1.8-300 K in a 100 Oe magnetic field allowed for the extraction of full fitting parameters from the susceptibility data for 1. The negative J(a) value (J(a) = -2.16 +/- 0.08 K, J(b) = 3.24 +/- 1.73 K, g = 2.35 +/- 0.040, and rho = 0.34 +/- 0.03) suggests a dominant antiferromagnetic spin exchange interaction between the four Mn-III ions, with the positive J(b) being an accompanying result of J(a). Electrochemical studies revealed a reversible Mn-IV/Mn-III redox couple in 1 at the +0.80 to +1.1 V potential region with E-1/2 = +0.907 V.