Toward charge-neutral ‘soft scorpionates’: Coordination chemistry and Lewis acid promoted isomerization of tris(1-organo-imidazol-2-ylthio)methanes

Two tris(1-organo-imidazol-2-ylthio)methanes, HC(S-tim^R)₃ (R = organo = methyl, tert-butyl), have been prepared by a triphasic reaction between chloroform, the appropriate heterocycle, and saturated aqueous solutions of Na₂CO₃, in the presence of a phase transfer agent, (NBu₄)(Br). These ligands have been characterized both spectroscopically and by single crystal X-ray diffraction. The reaction chemistry of these potentially N,N,N-tripodal ligands with AgBF₄ was also explored where simple 1:1 coordination complexes could be isolated from reactions performed in THF solution at room temperature. The derivative {Ag[HC(S-tim^Me)₃]}(BF₄) was structurally characterized which showed that the ligand binds in a μ-κ²N,κ¹N-mode to give a coordination polymer with an interesting layered supramolecular structure. Surprisingly, heating CH₃CN solutions of the silver complexes at reflux resulted in decomposition of the complex and concomitant isomerization of the ligands to give metal-free tris(3-organo-1-imidazole-2-thione)methane, HC(N-imt^R)₃; the heretofore elusive charge-neutral analogues of the well-studied ‘soft scorpionate’ Tm^R− anions. The solution isomerization of HC(S-tim^R)₃ to HC(N-imt^R)₃ was found to be general, occurring in a variety of solvents with any of a host of different Lewis acids [para-toluenesulfonic acid, KPF₆, and M(CO)₅Br (M = Mn, Re)] but did not occur by heating in the absence of Lewis acid. The compound HC(N-imt^Me)₃ exhibited unusually low solubility in common organic solvents. Single crystal X-ray diffraction of HC(N-imt^Me)₃ revealed a remarkable honeycomb supramolecular structure with ca. 5 Å channels filled with solvent. The robust nature of this solid is a result of strong dipolar stacking interactions of molecules into polymer chains bolstered by concerted π-π and CH-π interactions involving the heterocycles, holding the chains together in the remaining two dimensions.