M₃(P₂O₇)₂^2–-Type Open Frameworks Featuring [M₂O₈] and [M₃O₁₂] Multinuclear Transition-Metal Oxide Units. Serendipitous Synthesis of Six Polymorphic Salt-Inclusion Magnetic Solids: Na₂M₃(P₂O₇)₂·ACl (M = Mn, Fe; A = Rb, Cs) and K₂M₃(P₂O₇)₂·CsCl (M = Fe, Mn)

Abstract

Single crystals of six polymorphic salt-inclusion phosphates of the A(2)M(3)(P(2)O(7))(2)·A'Cl type, Na(2)Mn(3)(P(2)O(7))(2)·CsCl (1), Na(2)Mn(3)(P(2)O(7))(2)·RbCl (2), Na(2)Fe(3)(P(2)O(7))(2)·CsCl (3), Na(2)Fe(3)(P(2)O(7))(2)·RbCl (4), K(2)Mn(3)(P(2)O(7))(2)·CsCl (5), and K(2)Fe(3)(P(2)O(7))(2)·CsCl (6), were grown in reactive molten chloride flux media. Compounds 1-4 are isostructural and crystallize in the space group C2/c (No. 15), while 5 and 6 crystallize in P2/c (No. 13) and P1̅ (No. 2), respectively. The title compounds have demonstrated an unprecedented versatility, where the M(3)(P(2)O(7))(2)(2-) covalent open frameworks contain [M(3)O(12)] (M = Mn(2+), Fe(2+)) trimeric units in 1-4 and [M(2)O(8)] dimers in 5 and 6. These multinuclear, transition-metal oxide units are linked by Cl(-) ions through the M-Cl bonds to form one-dimensional (1D) chains. The 1D chains and [P(2)O(7)] groups share common O atoms to form the extended network. The M(3)(P(2)O(7))(2)(2-) open-framework structures exhibit channels where the respective Na(+)/K(+) ions and A'Cl salt (A' = Rb, Cs) reside. Magnetic susceptibility of 2 and 4 suggests bulk antiferromagnetic properties as expected. The local structure and thermal decomposition are examined by IR and differential scanning calorimetry of representative compounds. The factors that determine the reticular chemistry of the M(3)(P(2)O(7))(2)(2-) type are illustrated in terms of the inclusion of ionic lattices of different sizes and contents.