Metastable Se6 as a ligand for Ag+: from isolated molecular to polymeric 1D and 2D structures

Abstract

Attempts to prepare the hitherto unknown Se₆²⁺ 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(Se₆)Ag(OSO)][Sb(OTeF₅)₆]₂1 and [(OSO)₂Ag(Se₆)Ag(OSO)₂][Al(OC(CF₃)₃)₄]₂2a. 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/∞{[Ag(Se₆)]_∞[Ag₂(SbF₆)₃]_∞} 3 and {1/∞[Ag(Se₆)Ag]_∞}[AsF₆]₂4. Pure bulk 4 was best prepared by the reaction of Se₄[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 ¹⁰⁹Ag MAS NMR investigations of 4 as well as AgF, AgF₂, AgMF₆ and {1/∞[Ag(I₂)]_∞}[MF₆] (M = As, Sb). Compounds 1 and 2a/b, with the very large counter ions, contain isolated [Ag(Se₆)Ag]²⁺ heterocubane units consisting of a Se₆ molecule bicapped by two silver cations (local D_3d sym). 3 and 4, with the smaller anions, contain close packed stacked arrays of Se₆ rings with Ag⁺ residing in octahedral holes. Each Ag⁺ ion coordinates to three selenium atoms of each adjacent Se₆ ring. 4 contains [Ag(Se₆)⁺]_∞ stacks additionally linked by Ag(2)⁺ into a two dimensional network. 3 features a remarkable 3-dimensional [Ag₂(SbF₆)₃]⁻ anion held together by strong Sb–F⋯Ag contacts between the component Ag⁺ and [SbF₆]⁻ ions. The hexagonal channels formed by the [Ag₂(SbF₆)₃]⁻ anions are filled by stacks of [Ag(Se₆)⁺]_∞ 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 Se₆ 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 Se₆ molecule from grey selenium is thermodynamically driven by the coordination to the Ag⁺ ions.