Anomalous Stoichiometry, 3-D Bridged Triangular/Pentagonal Layered Structured Artificial Antiferromagnet for the Prussian Blue Analogue A3MnII5(CN)13 (A = NMe4, NEtMe3). A Cation Adaptive Structure

Abstract

The size of the organic cation dictates both the composition and the extended 3-D structure for hybrid organic/inorganic Prussian blue analogues (PBAs) of A _aMn^II _b(CN) _{a+2 b} (A = cation) stoichiometry. Alkali PBAs are typically cubic with both MC₆ and M'N₆ octahedral coordination sites and the alkali cation content depends on the M and M' oxidation states. The reaction of Mn^II(O₂CCH₃)₂ and A⁺CN^- (A = NMe₄, NEtMe₃) forms a hydrated material of A₃Mn^II₅(CN)₁₃ composition. A₃Mn^II₅(CN)₁₃ forms a complex, 3-D extended structural motif with octahedral and rarely observed square pyramidal and trigonal bipyramidal Mn^II sites with a single layer motif of three pentagonal and one triangular fused rings. A complex pattern of Mn^IICN chains bridge the layers. (NMe₄)₃Mn^II₅(CN)₁₃ possesses one low-spin octahedral and four high-spin pentacoordinate Mn^II sites and orders as an antiferromagnet at 11 K due to the layers being bridged and antiferromagnetically coupled by the nonmagnetic cyanides. These are rare examples of intrinsic, chemically prepared and controlled artificial antiferromagnets and have the advantage of having controlled uniform spacing between the layers as they are not physically prepared via deposition methods. A₃Mn₅(CN)₁₃ (A = NMe₄, NEtMe₃) along with [NEt₄]₂Mn^II₃(CN)₈, [NEt₄]Mn^II₃(CN)₇, and Mn(CN)₂ form stoichiometrically related A _aMn^II _b(CN) _{a+2 b} ( a = 0, b = 1; a = 2, b = 3; a = 1, b = 3; and a = 3, b = 5) series possessing unprecedented stoichiometries and lattice motifs. These unusual structures and stoichiometries are attributed to the very ionic nature of the high-spin N-bonded Mn^II ion that enables the maximization of the attractive van der Waals interactions via minimization of void space via a reduced ∠MnNC. This A _aMn^II _b(CN) _{a+2 b} family of compounds are referred to as being cation adaptive in which size and shape dictate both the stoichiometry and structure.