Unprecedented Polycrystal Structure of a New Cadmium Thiolate Containing an Unusually Highly Charged [ClCd<sub>8</sub>{SCH(CH<sub>2</sub>CH<sub>2</sub>)<sub>2</sub>N(H)Me}<sub>16</sub>]<sup>15+</sup> Core
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A new cadmium thiolate with overall composition [ClCd8{SCH(CH2CH2)2N(H)Me}16](ClO4)15·16[SCH(CH2CH2)2N(H)Me]·32H2O has been obtained in a high ionic strength aqueous medium. The solid is a polycrystal consisting of equally oriented microcrystals of formula [ClCd8{SCH(CH2CH2)2N(H)Me}16](ClO4)15·16H2O held together by uncoordinated aminothiolate ligands and water molecules occupying the intercrystal spaces. The composition and structure of the microcrystals is shown by X-ray, XPS, and 113Cd NMR data. The uncoordinated aminothiolate ligands in zwitterionic form and water molecules are essential for the initial formation but not for the continued stability of the microcrystals as their chemical extraction leads to a new microcrystalline powder with essentially the same powder X-ray diffraction pattern. The existence of intercrystal spaces is confirmed by electron and atomic force microscopy techniques, SEM and AFM. The [ClCd8{SCH(CH2CH2)2N(H)Me}16]15+ cage can be described in terms of metal and ligand concentric polyhedra of increasing size as centro-Cl-tetrahedro-Cd4-cuboctahedro-(μ-SRH)12-tetrahedro-(CdSRH)4, where SRH denotes -SCH(CH2CH2)2N+(H)Me, and thus it constitutes the first example of a metal thiolate cluster where the topology of the bridging ligands is that of a cuboctahedron. Comparison with dimensions predicted by Dance for an idealized topology is made. The structural correspondences between a [ClCd4(μ-SRH)4(SRH)6] fragment in [ClCd8{SCH(CH2CH2)2N(H)Me}16]15+ and the Cd4(S-Cys)11 cluster in the α domain of metallothioneins, both formed by two fused six-membered cycles, are also discussed.
