Aromatic and Aliphatic CH Hydrogen Bonds Fight for Chloride while Competing Alongside Ion Pairing within Triazolophanes

Abstract

Triazolophanes are used as the venue to compete an aliphatic propylene CH hydrogen-bond donor against an aromatic phenylene one. Longer aliphatic C-H...Cl(-) hydrogen bonds were calculated from the location of the chloride within the propylene-based triazolophane. The gas-phase energetics of chloride binding (ΔG(bind) , ΔH(bind) , ΔS(bind) ) and the configurational entropy (ΔS(config) ) were computed by taking all low-energy conformations into account. Comparison between the phenylene- and propylene-based triazolophanes shows the computed gas-phase free energy of binding decreased from ΔG(bind) =-194 to -182 kJ mol(-1) , respectively, with a modest enthalpy-entropy compensation. These differences were investigated experimentally. An (1) H NMR spectroscopy study on the structure of the propylene triazolophane's 1:1 chloride complex is consistent with a weaker propylene CH hydrogen bond. To quantify the affinity differences between the two triazolophanes in dichloromethane, it was critical to obtain an accurate binding model. Four equilibria were identified. In addition to 1:1 complexation and 2:1 sandwich formation, ion pairing of the tetrabutylammonium chloride salt (TBA(+) ⋅Cl(-) ) and cation pairing of TBA(+) with the 1:1 triazolophane-chloride complex were observed and quantified. Each complex was independently verified by ESI-MS or diffusion NMR spectroscopy. With ion pairing deconvoluted from the chloride-receptor binding, equilibrium constants were determined by using (1) H NMR (500 μM) and UV/Vis (50 μM) spectroscopy titrations. The stabilities of the 1:1 complexes for the phenylene and propylene triazolophanes did not differ within experimental error, ΔG=(-38±2) and (-39±1) kJ mol(-1) , respectively, as verified by an NMR spectroscopy competition experiment. Thus, the aliphatic CH donor only revealed its weaker character when competing with aromatic CH donors within the propylene-based triazolophane.

Flexibility Coexists with Shape-Persistence in Cyanostar Macrocycles

“…Similarly, prior work with triazolophane macrocycles showed only four low-lying, semi-planar conformations. 14 These macrocycles clearly follow the structural definition of shape-persistence by displaying little conformational freedom. In these cases, shape persistence is self-evident.…”

Anion receptor chemistry: highlights from 2008 and 2009

“…In a separate manuscript, Flood and co-workers have used the triazolophane scaffold to compare the strength of aromatic and aliphatic C-H hydrogen bond donors by replacing one of the phenylene groups in receptor 62 with a propylene linker in receptor 65. 45 Gas phase computational studies showed that the length of the CHÁ Á ÁCl distance in the propylene based triazophane 65 was greater for the aliphatic (3.02 Å) than the aromatic (2.96 Å) CH donor, supporting its classification as a weaker interaction. The gas phase studies also suggested that the free energy of binding decreased from À194 to À182 kJ mol…”

Synthesis, structure, protein binding of Cu(II) complexes with a tridentate NNO Schiff-base ligand

“…In addition, the benzene ring of the Schiff base ligand and the pyridine ring of the Schiff base ligand were twisted, at an angle of 55.25°with respect to each other. Intermolecular C-HÁ Á ÁCl interaction also has been reported in several recent literature sources [35,36]. Whilst exploring the acting forces in the crystal lattice, we discovered that there exist C-HÁ Á ÁCl weak interactions: C11-H11Á Á ÁCl1 i = 2.876 Å; C2-H2Á Á ÁCl2 ii = 2.876 Å.…”