A study of high-valent manganese in aqueous H3PO4: Disproportionation of [MnIIIMnIV(μ-O)2(bpy)4] 3+

Joseph E. Sarneski, Linda J. Brzezinski, Brandon Anderson, Mary Didiuk, Rajesh Marchanda, Robert H. Crabtree, Gary W Brudvig, Gayle K. Schulte

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Abstract

The mixed-valence dimer [MnIIIMnIV(μ-O)2(bpy))4] 3+ (1; bpy = 2,2′-dipyridyl) has been shown to disproportionate in aqueous H3PO4. One product, [MnIV 2(μ-O)22-HPO 4)(bpy)2(H2PO4)2] (2), was isolated and characterized earlier (Inorg. Chem. 1991, 30, 2833). Here a manganese(III) product has been characterized crystallographically as [MnIII(bpy)(HPO4)(H2PO4)] x (3). 3 is a polymeric material which crystallizes in the space group P1 (No. 2) with a = 8.069(1) Å, b = 8.672(2) Å, c = 10.751(1) Å, α = 109.91(2)°, β = 102.04(1)°, and γ = 90.20(9)°. A total of 1569 unique data with I ≥ 3(σ)I were refined to values of R and Rw of 3.9 and 4.3%, respectively. The Mn(III) ions, bridged by two μ2-phosphate ligands, are 4.93 Å apart; each Mn(III) atom is bridged in this manner to two other [MnIII(bpy)(HPO4)(H2PO4)] formula units leading to a polymeric array. The aqueous chemistry of 2 and 3 has been characterized by UV-vis and EPR spectroscopy. The electrochemistry of the (IV,IV) dimer 2 in phosphate buffer shows that the complex decomposes on reduction to give the mixed-valence bis(μ-oxo) manganese dimer 1. Support for the electrochemical formation of 1 from 2 in phosphate buffer was obtained from the zero current potential curve. The Mn(III)- bpy phosphate (3) is found to disproportionate spontaneously to 1 and Mn(II) upon dissolution in phosphate buffer; for solutions of 2 or 3 in 0.1 M H3PO4 electrochemistry typical of 1 is observed. The temperature-dependent magnetic susceptibility shows strong antiferromagnetic coupling for the (IV,IV) dimer 2 with J = 79 cm-1 (ℋ = JS1·S2). The Mn(III) complex 3 shows weak antiferromagnetic coupling, consistent with the long Mn-Mn distance of 4.93 Å.

Original languageEnglish
Pages (from-to)3265-3269
Number of pages5
JournalInorganic Chemistry
Volume32
Issue number15
Publication statusPublished - 1993

ASJC Scopus subject areas

  • Inorganic Chemistry

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    Sarneski, J. E., Brzezinski, L. J., Anderson, B., Didiuk, M., Marchanda, R., Crabtree, R. H., Brudvig, G. W., & Schulte, G. K. (1993). A study of high-valent manganese in aqueous H3PO4: Disproportionation of [MnIIIMnIV(μ-O)2(bpy)4] 3+. Inorganic Chemistry, 32(15), 3265-3269.