Role of wingtip substituents on benzene-platform-based tetrapodal ligands toward the formation of a self-assembled silver carbene cage

Abstract

Tetrapodal imidazolium ligands L¹–L³ as their PF₆^– salts are synthesized in good yields by reacting 1,2,4,5-tetrakis(bromomethyl)benzene with N-methylimidazole, N-benzylimidazole, and N-ethylimidazole, respectively. Single-crystal X-ray diffraction studies of L¹·4PF₆, L²·4PF₆, and L³·4PF₆ show the chair conformation of the tetrapodal imidazoliums (L¹–L³), where 1,5- and 2,4-imidazolium moieties are oriented in opposite directions of the benzene plane. The PF₆^– salts of L¹–L³ are reacted with Ag₂O to synthesize different silver complexes of N-heterocyclic carbene (NHC), 1–3, respectively, in good yields. Crystals of all three complexes suitable for single-crystal X-ray diffraction study are also isolated. Structural analysis of 1, i.e., the complex of L¹·4PF₆ containing methyl as a wingtip substituent, and Ag₂O shows the formation of a bimetallic silver NHC (NHC-Ag) complex, [(L¹-4H)·2Ag]·2PF₆, which is rotationally disordered over an inversion of symmetry of the space group P2₁/c. Elemental analysis and solution-state ¹H and ¹³C NMR studies confirm the above molecular formula of complex 1. When L²·4PF₆ functionalized with the benzyl wingtip moiety is explored for similar complexation with Ag₂O, the isolated complex 2 shows the formation of a simple NHC-Ag complex with molecular formula [(L²-4H)·2Ag]·2PF₆, as observed in the case of 1. Interestingly, the reaction of L³·4PF₆ containing ethyl as the wingtip substituent and Ag₂O shows the formation of a silver-ion-assisted tetranuclear molecular box of [Ag₄(L³-4H)₂]⁴⁺ (3).