Multi-Ion Complexes and Competition from Bulky BArF^- Anions for Chloride Binding in Ion Pairing Conditions

Cationic charges have long been used to enhance anion binding. Embedding charge introduces strong ion pairing for target anions but also for off-target ions, ultimately generating a mixture of multi-ion species that are hard to identify and quantify. While many sidestep this problem using polar solvents and weakly coordinating ions, these approaches exclude a substantial cross-section of conditions found in applications spanning recognition, assembly, separations, templation, and catalysis. To confront this complexity, we study the binding of an anion to a cationic receptor featuring low shape complementarity in a low-polarity solvent to maximize ionic interactions. We prepared the receptor as a salt of the weakly coordinating tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BArF^-) anion and studied the binding of small chloride (Cl^-) and medium-sized iodide (I^-) anions. Surprisingly, the use of the bulky BArF^- anion does not suppress ion pairing interactions, with 65% of the receptor being paired at 0.5 mM in dichloromethane. We observe multi-ion receptor-Cl^- complexes (2:1, 1:1, 1:2), reinforcing the complexity that emerges when working in low-polarity media. We reveal the dependence of affinity on anion charge density and size and that bulky BArF^- counteranions compete for chloride binding. These studies reveal the noninnocence of BArF^- anions and strategies to quantify multi-ion species.