Macrocyclic dinuclear silver(i) complexes based on bis(N-heterocyclic carbene) ligands: synthesis and structural studies

Abstract

A series of new (CH₂)_n-bridged diimidazolium (or dibenzimidazolium) salts (n = 3, 4 and 5) were prepared. Reaction of the diimidazolium (or dibenzimidazolium) salts with Ag₂O or Ag₂CO₃ leads to eight macrocyclic dinuclear N-heterocyclic carbene (NHC) silver(I) complexes, [{1,1′-dialkyl-3,3′-(CH₂)_n-diimidazol-2-ylidene}₂Ag₂]·2Z (2a: alkyl = Et, n = 3, Z = 1/2[Ag₂Br₄], 2b: alkyl = PhCH₂, n = 3, Z = PF₆; 2c: alkyl = Et, n = 4, Z = PF₆; 2d: alkyl = PhCH₂, n = 4, Z = PF₆; 2e: alkyl = Et, n = 5, Z = PF₆; 2f: alkyl = PhCH₂, n = 5, Z = PF₆) and [{1,1′-dialkyl-3,3′-(CH₂)₃-dibenzimidazol-2-ylidene}₂Ag₂]·2PF₆ (2g: alkyl = ⁿBu; 2h: alkyl = PhCH₂). In these complexes, each cationic macrocycle is composed of two silver(I) atoms and two (CH₂)_n-bridged dicarbene ligands. The 2+ charge of each cation is balanced by different anions ([Ag₂Br₄]²⁻ for 2a and 2PF₆⁻ for 2b–2h). Interestingly, these cationic macrocycles of 2a–2h adopt four diverse conformations (i.e., trans, zigzag, helical and calix-like conformations) due to the different arrangements of bridging (CH₂)_n-chains in the solid state. Additionally, analyses of the crystal packings of 2a–2h reveal that 2D supramolecular layers of 2a, 2c, 2d and 2f–2h and 1D supramolecular chains of 2b and 2e are formed via intermolecular weak interactions, including π–π stacking interactions, hydrogen bonds and weak Ag⋯F bonds.