Structural diversities in Ag(i) complexes of xylyl platform based isomeric bis-NHC ligands: effects of pyridine wingtip substituents

Abstract

Pyridine is employed as a wingtip substituent for the preparation of xylyl platform based bis-NHC ligands to investigate its effect on the formation of Ag(I)–NHC complexes. Hence, three new ligands L₁–L₃ are synthesized as their imidazolium salts H₂(L₁–L₃)(PF₆)₂ by 2 : 1 reaction of 2-(1H-imidazole-1-yl)pyridine with ortho-, meta- and para-bis(bromomethyl)benzene followed by anion exchange. Treatment of these imidazolium salts with 2 equivalents of Ag₂O generates Ag(I)–NHC complexes with diverse structural motifs. Ligand L₁ generates a metallo-macrocycle (1) via dimeric assembly of the ligand through Ag(I) coordination. Interestingly, these dimeric assemblies are further connected through C(sp³)–H⋯C(sp²) hydrogen bonding interactions which thereby construct an infinite 1D polymer of metallo-macrocycles. In the presence of Ag(I), L₂ generates metallo-macrocycle 2 that is connected via Ag(I)⋯Ag(I) interactions. Noticeably, in this polymeric arrangement, Ag(I) ions are arranged almost in a linear fashion with bond angles of 177.95° in an Ag₃ unit. In the presence of Ag(I), ligand L₃ generates an open chain 1D polymer (3), where the two substituents on the benzene ring are in a transoid position and the C–Ag–C bond is significantly bended (166.75°) towards the pyridyl substituent. Simultaneously, this 1D structure turns to a 2D polymeric structure via π–π stacking interaction between the imidazole and pyridyl units of two separate 1D channels. All three imidazolium salts and Ag(I)–NHC complexes 1–3 are completely characterized by using ESI-MS, IR spectroscopy and 1D/2D NMR studies. Molecular structure determination shows a significant effect of a wingtip pyridine group on the geometry of complexes 1–3.