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Stepwise Crystallization: Illustrative Examples of the Use of Metalloligands [Cu6(mna)6]6– and [Ag6(Hmna)2(mna)4]4– (H2mna = 2‑Mercapto Nicotinic Acid) in the Formation of Heterometallic Two- and Three-Dimensional Assemblies with brucite, pcu, and sql Topologies

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journal contribution
posted on 03.09.2014, 00:00 by Tanaya Kundu, Ajay Kumar Jana, Srinivasan Natarajan
Three new inorganic coordination polymers, {[Mn­(H2O)6]­[Mn2(H2O)6]­[(Cu6(mna)6]­·6H2O}, 1, {[Mn4(OH)2(H2O)10]­[(Cu6(mna)6]­·8H2O}, 2, and {[Mn2(H2O)5]­[Ag6(Hmna)2(mna)4]­·20H2O}, 3, have been synthesized at room temperature through a sequential crystallization route. In addition, we have also prepared and characterized the molecular precursor [Cu6(Hmna)6]. Compounds 1 and 3 have a two-dimensional structure, whereas 2 has a three-dimensional structure. The formation of 2 has been achieved by minor modification in the synthetic composition, suggesting the subtle relationship between the reactant composition and the structure. The hexanuclear copper and silver cluster cores have Cu···Cu and Ag···Ag distances close to the sum of the van der Waals radii of Cu1+ and Ag1+, respectively. The connectivity between [Cu6(mna)6]6– cluster units and Mn2+ ions gives rise to a brucite related layer in 1 and a pcu-net in 2. The [Ag6(Hmna)2­(mna)4]4– cluster in 3, on the other hand, forms a sql-net with Mn2+. Compound 1 exhibits an interesting and reversible hydrochromic behavior, changing from pale yellow to red, on heating at 70 °C or treatment under a vacuum. Electron paramagnetic resonance studies indicate no change in the valence states, suggesting the color change could be due to changes in the coordination environment only. The magnetic studies indicate weak antiferromagnetic behavior. Proton conductivity studies indicate moderate proton migrations in 1 and 3. The present study clearly establishes sequential crystallization as an important pathway for the synthesis of heterometallic coordination polymers.