Hybridization affects the binding affinity and isomer resolution of cyanostar complexes with organo-trifluoroborate anions

The development of new classes of designer anions has opened up the potential to generate novel architectures by self-assembly. However, each new class of anion requires exploration of its structure-property relationships to understand their practical use as synthons for large-scale assemblies. We do this here with organo-trifluoroborate anions, RBF₃^–, that bear different substituents. We investigate how the hybridization (sp³, sp², sp) of the a-carbon atom on the substituent affects their binding to cyanostar macrocycles as 2:1 complesxes. For this purpose, a set of anions was designed and composed of phenylethyl-BF₃^– (sp³), phenylethylene-BF₃^– (sp²), and phenylethynyl-BF₃^– (sp). Electrostatic potential maps suggest that the order of affinity is phenylethyl-BF₃^– (sp³) > phenylethylene-BF₃^– (sp²) > phenylethynyl-BF₃^– (sp). However, the experimental result shows the affinity order is inverted. In addition, the use of a phenyl group creates a steric block that simultaneously lower the symmetry of the anionic guest. Surprisingly, the ¹H-NMR splitting pattern of phenylethynyl-BF₃^– resolves both the handedness of the macrocycles when they come together as a dimer as well as their inward or outward location on the non-symmetric rod. This resolution suggests that the ethynyl triple bond contributes to the molecular recognition unit. More complex architectures will be designed and presented based on this idea.