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

Tetrapodal imidazolium ligands L(1)-L(3) 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 L(1)·4PF6, L(2)·4PF6, and L(3)·4PF6 show the chair conformation of the tetrapodal imidazoliums (L(1)-L(3)), where 1,5- and 2,4-imidazolium moieties are oriented in opposite directions of the benzene plane. The PF6(-) salts of L(1)-L(3) 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 L(1)·4PF6 containing methyl as a wingtip substituent, and Ag2O shows the formation of a bimetallic silver NHC (NHC-Ag) complex, [(L(1)-4H)·2Ag]·2PF6, which is rotationally disordered over an inversion of symmetry of the space group P2(1)/c. Elemental analysis and solution-state (1)H and (13)C NMR studies confirm the above molecular formula of complex 1. When L(2)·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 [(L(2)-4H)·2Ag]·2PF6, as observed in the case of 1. Interestingly, the reaction of L(3)·4PF6 containing ethyl as the wingtip substituent and Ag2O shows the formation of a silver-ion-assisted tetranuclear molecular box of [Ag4(L(3)-4H)2](4+) (3).