Abstract
Tetraaza-macrocyclic pyridinophane L-Ts, decorated with a p-toluenesulfonyl (tosyl; Ts) group, appear to be a useful tool to provide evidence on how the interplay of various supramolecular forces can help stabilise exotic anionic species such as tribromide (Br3−) anions. Indeed, crystals of (H2L-Ts)(Br3)1.5(NO3)0.5 unexpectedly grew from an acidic (HNO3) aqueous solution of L-Ts in the presence of Br− anions. The crystal structure of this compound was determined by single crystal XRD analysis. Hydrogen bonds, salt-bridges, anion-π, π-π stacking, and van der Waals interactions contribute to stabilising the crystal lattice. The observation of two independent Br3− anions stuck over the π-electron densities of pyridine and tosyl ligand groups, one of them being sandwiched between two pyridine rings, corroborates the significance of anion-π interactions for N-containing heterocycles. We show herein the possibility of detecting anion-π contacts from fingerprint plots generated by Hirshfeld surface analysis, demonstrating the effective usage of this structural investigation technique to further dissect individual contributions of stabilising supramolecular forces.
Highlights
Since the early years of coordination chemistry, N-heterocyclic ligands have aroused a great deal of interest for their ability to form metal complexes with relevant properties [1]
The formation of Br3 − was appreciated by a change of colour of the mother liquor that turned from colourless to pale orange after several days of evaporation when the solution had reduced to a small volume
The anion itself is unusual, not to mention the bizarre Br3 − ···NO3 − interaction. This structure prompted further examination of our previous studies and literature material concerned with anion-π interactions, which were reported before the popularisation of Hirshfeld surface analysis and fingerprint plot as tools
Summary
Since the early years of coordination chemistry, N-heterocyclic ligands have aroused a great deal of interest for their ability to form metal complexes with relevant properties [1]. The basic character of heterocyclic nitrogen atoms, which can undergo protonation to form cationic species, and the polarisation of their aromatic π-electrons clouds, have more recently promoted the use of these ligands for the preparation of anion complexes [2,3], as well as for the construction of assemblies of neutral molecules [4]. Pyridinophane 3,6,9,15-tetraazabicyclo[9.3.1]pentadeca-1(15),11,13-triene (L, Figure 1) is a member of this family that was first synthesised in 1981 [5]. It was later in the spotlight for its ability to form metal complexes [6,7,8], but its wider success was achieved thanks to its utilisation as a scaffold for the preparation of new materials that have proven effective in a variety of applications.
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