Synthesis and Characterization of Prussian Blue Analogues Incorporating the Edge-Bridged Octahedral [Zr6BCl12]2+Cluster Core

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Abstract
In attempts to produce a microporous magnet, two approaches were explored for expanding the Prussian blue structure type via incorporation of edge-bridged octahedral [Zr6ZCl12]2+ (Z = B, Be) cluster cores. Dissolution of Rb5Zr6BCl18 and K5Zr6BeCl15 in an acetonitrile solution of Et4N(CN) led to the isolation of (Et4N)5[Zr6BCl12(CN)6] (1) and (Et4N)5[Zr6BeCl12(CN)6]·2MeCN·2THF (2), respectively. The crystal structure of 1·1.5MeCN revealed the expected cyano-terminated cluster complex with a trans-N···N span of 11.73(3) Å. Unfortunately, both [Zr6ZCl12(CN)6]5- clusters rapidly lose their cyanide ligands in aqueous solution making them ill-suited for solid-forming reactions with hydrated metal ions. Such outer-ligand exchange, however, allows the use of [Zr6BCl18]4- in the synthesis of expanded Prussian blue-type solids through reactions with [Cr(CN)6]3-. The use of 2.2 M aqueous LiCl to stabilize the cluster during the reaction gave (Et4N)2[Zr6BCl12][Cr(CN)6]Cl·3H2O (3), while the use of 1 M acetic acid yielded (Et4N)2[Zr6BCl12][Cr(CN)6]Cl·2H2O·CH3CO2H (4). A Rietveld refinement against X-ray powder diffraction data collected for 3 confirmed the presence of a cubic Prussian blue framework structure, featuring alternating [Zr6BCl12]2+ cores and [Cr(CN)6]3- anions. The temperature dependence of magnetization data obtained for 4 revealed activation of magnetic exchange interactions between the S = 1/2 cluster units and the S = 3/2 hexacyanochromate complexes below 10 K.