Role of Wingtip Substituents on Benzene-Platform-Based Tetrapodal Ligands toward the Formation of a Self-Assembled Silver Carbene Cage

Abstract

Tetrapodal imidazolium ligands <b>L</b>^<b>1</b>–<b>L</b>^<b>3</b> as their PF₆^– salts are synthesized in good yields by reacting 1,2,4,5-tetrakis­(bromomethyl)­benzene with <i>N</i>-methylimidazole, <i>N</i>-benzylimidazole, and <i>N</i>-ethylimidazole, respectively. Single-crystal X-ray diffraction studies of <b>L</b>^<b>1</b>·4PF₆, <b>L</b>^<b>2</b>·4PF₆, and <b>L</b>^<b>3</b>·4PF₆ show the chair conformation of the tetrapodal imidazoliums (<b>L</b>^<b>1</b>–<b>L</b>^<b>3</b>), where 1,5- and 2,4-imidazolium moieties are oriented in opposite directions of the benzene plane. The PF₆^– salts of <b>L</b>^<b>1</b>–<b>L</b>^<b>3</b> are reacted with Ag₂O to synthesize different silver complexes of N-heterocyclic carbene (NHC), <b>1</b>–<b>3</b>, respectively, in good yields. Crystals of all three complexes suitable for single-crystal X-ray diffraction study are also isolated. Structural analysis of <b>1</b>, i.e., the complex of <b>L</b>^<b>1</b>·4PF₆ containing methyl as a wingtip substituent, and Ag₂O shows the formation of a bimetallic silver NHC (NHC-Ag) complex, [(<b>L</b>^<b>1</b>-4H)·2Ag]·2PF₆, which is rotationally disordered over an inversion of symmetry of the space group <i>P</i>2₁/<i>c</i>. Elemental analysis and solution-state ¹H and ¹³C NMR studies confirm the above molecular formula of complex <b>1</b>. When <b>L</b>^<b>2</b>·4PF₆ functionalized with the benzyl wingtip moiety is explored for similar complexation with Ag₂O, the isolated complex <b>2</b> shows the formation of a simple NHC-Ag complex with molecular formula [(<b>L</b>^<b>2</b>-4H)·2Ag]·2PF₆, as observed in the case of <b>1</b>. Interestingly, the reaction of <b>L</b>^<b>3</b>·4PF₆ containing ethyl as the wingtip substituent and Ag₂O shows the formation of a silver-ion-assisted tetranuclear molecular box of [Ag₄(<b>L</b>^<b>3</b>-4H)₂]⁴⁺ (<b>3</b>)