Nanoscale control of polyoxometalate assembly: A {Mn8W 4} cluster within a {W36Si4Mn10} cluster showing a new type of isomerism

Abstract

Two near isomeric clusters containing a novel {Mn8W4} Keggin cluster within a [W36Mn10Si4O 136(OH)4(H2O)8]24- cluster are reported: K10Li14 [W36Si4O 136MnII10(OH)4(H2O) 8] (1) and K10Li13.5Mn0.25[W 36Si4O136MnII10(OH) 4(H2O)8] (1′). Bulk characterization of the clusters has been carried out by single crystal X-ray structure analysis, ICP-MS, TGA, ESI-MS, CV and SQUID-magnetometer analysis. X-ray analysis revealed that 1′ has eight positions within the central Keggin core that were disordered W/Mn whereas 1 contained no such disorder. This subtle difference is due to a differences is how the two clusters assemble and recrystallize from the same mother liquor and represents a new type of isomerism. The rapid recrystallization process was captured via digital microscopy and this uncovered two "intermediate" types of crystal which formed temporarily and provided nucleation sites for the final clusters to assemble. The intermediates were investigated by single crystal X-ray analysis and revealed to be novel clusters K4Li22[W36Si4Mn 7O136(H2O)8]×56 H2O (2) and Mn2K8Li14[W36Si 4Mn7O136(H2O)8]×45 H2O (3). The intermediate clusters contained different yet related building blocks to the final clusters which allowed for the postulation of a mechanism of assembly. This demonstrates a rare example where the use X-ray crystallography directly facilitated understanding the means by which a POM assembled. Nanoassembly: Assembly isomers of a {Mn8W4} Keggin cluster within a [W36Mn10Si4O 136(OH)4(H2O)8]24- cluster are reported. The clusters are formed through the aggregation of new {γ-SiMnW9} fragments in a tetrahedral fashion. Investigation of the crystal growth of these clusters revealed two intermediate clusters, which unveiled a mechanism of formation (see scheme). Physical analysis of the magnetic and solution behaviour of the isomers is also reported. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.