Systematic study on anion–cation interactions via doubly ionic H-bonds in 1,3-dimethylimidazolium salts comprising chalcogenolate anions MMIm [ER] (E = S, Se; R = H, tBu, SiMe3)

Abstract

We present convenient syntheses of so far inaccessible, crystalline and highly pure 1,3-dialkylimidazolium salts with extremely nucleophilic thiolate and selenolate anions [ER]⁻ (R = H, tBu, SiMe₃). While non existent for E = O, the title compounds exist as slightly less basic E = S and Se homologues. The anion cation H-bond interactions in the crystalline state have been studied systematically in six related ionic compounds of varying anion basicity and steric bulk, namely MMIm [SH] (1), MMIm [StBu] (2), and MMIm [SSiMe₃] (3), as well as MMIm [SeH] (4), MMIm [SetBu] (5) and MMIm [SeSiMe₃] (6). The chalcogenolate title compounds 3, 5, and 6 are prepared by the newly introduced method of desilylation of Me₃Si-ER (E = S, Se) by the super-nucleophile MMIm [StBu] (2), which is easily accessible via deprotonation of HStBu with in situ generated NHC 1,3-dimethylimidazolidin-2-ylidene. Focus is put on the role of the most acidic imidazolium proton C2–H as structural director and as ¹H NMR shift indicator. These salts show an unusually high volatility and tendency to sublime under vacuum without irreversible decay.