Metastable Se-6 as a ligand for Ag+: from isolated molecular to polymeric 1D and 2D structures

Attempts to prepare the hitherto unknown Se-6(2+) cation by the reaction of elemental selenium and Ag[A] ([A](-) = [Sb(OTeF5)(6)](-), [Al(OC(CF3)(3))(4)](-)) in SO2 led to the formation of [(OSO)Ag(Se-6)Ag(OSO)][Sb(OTeF5)(6)](2) 1 and [(OSO)(2)Ag(Se-6)Ag(OSO)(2)][Al(OC(CF3)(3))(4)](2) 2a. 1 could only be prepared by using bromine as co-oxidant, however, bulk 2b (2a with loss of SO2) was accessible from Ag[Al(OC(CF3)(3))(4)] and grey Se in SO2 (chem. analysis). The reactions of Ag[MF6] (M = As, Sb) and elemental selenium led to crystals of 1/infinity{[Ag(Se-6)](infinity)[Ag-2(SbF6)(3)](infinity)}(3) and {1/infinity[Ag(Se-6)Ag](infinity)}[AsF6](2) 4. Pure bulk 4 was best prepared by the reaction of Se-4[AsF6](2), silver metal and elemental selenium. Attempts to prepare bulk 1 and 3 were unsuccessful. 1-4 were characterized by single-crystal X-ray structure determinations, 2b and 4 additionally by chemical analysis and 4 also by X-ray powder diffraction, FT-Raman and FT-IR spectroscopy. Application of the PRESTO III sequence allowed for the first time Ag-109 MAS NMR investigations of 4 as well as AgF, AgF2, AgMF6 and {1/infinity[Ag(I-2)](infinity)}[MF6] (M = As, Sb). Compounds 1 and 2a/b, with the very large counter ions, contain isolated [Ag(Se-6)Ag](2+) heterocubane units consisting of a Se-6 molecule bicapped by two silver cations (local D-3d sym). 3 and 4, with the smaller anions, contain close packed stacked arrays of Se-6 rings with Ag+ residing in octahedral holes. Each Ag+ ion coordinates to three selenium atoms of each adjacent Se-6 ring. 4 contains [Ag(Se-6)(+)](infinity) stacks additionally linked by Ag(2)(+) into a two dimensional network. 3 features a remarkable 3-dimensional [Ag-2(SbF6)(3)](-) anion held together by strong Sb-F center dot center dot center dot Ag contacts between the component Ag+ and [SbF6](-) ions. The hexagonal channels formed by the [Ag-2(SbF6)(3)](-) anions are filled by stacks of [Ag(Se-6)(+)](infinity) cations. Overall 1-4 are new members of the rare class of metal complexes of neutral main group elemental clusters, in which the main group element is positively polarized due to coordination to a metal ion. Notably, 1 to 4 include the commonly metastable Se-6 molecule as a ligand. The structure, bonding and thermodynamics of 1 to 4 were investigated with the help of quantum chemical calculations (PBE0/TZVPP and (RI-)MP2/TZVPP, in part including COSMO solvation) and Born-Fajans-Haber-cycle calculations. From an analysis of all the available data it appears that the formation of the usually metastable Se-6 molecule from grey selenium is thermodynamically driven by the coordination to the Ag+ ions.