Inorganic Chemistry, Vol.52, No.8, 4269-4276, 2013
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 Xray diffraction studies of L-1 center dot 4PF(6), L-2 center dot 4PF(6), and L-3 center dot 4PF(6) 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-2 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 center dot 4PF(6) containing methyl as a wingtip substituent, and Ag2O shows the formation of a bimetallic silver NHC (NHC-Ag) complex, [(L-1-4H)center dot 2Ag]center dot 2PF(6), which is rotationally disordered over an inversion of symmetry of the space group P2(1)/c. Elemental analysis and solution-state H-1 and C-13 NMR studies confirm the above molecular formula of complex 1. When L-2 center dot 4PF(6) 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)center dot 2Ag]center dot 2PF(6), as observed in the case of 1. Interestingly, the reaction of L-3 center dot 4PF(6) containing ethyl as the wingtip substituent and Ag2O shows the formation of a silver-ion-assisted tetranuclear molecular box of [Ag-4(L-3-4H)(2)](4+) (3).