Evaluation of electronics, electrostatics and hydrogen bond cooperative in the binding of cyanide and fluoride by lewis acidic ferrocenylboranes

Synthetic approaches based on the direct borylation of ferrocene by BBr₃, followed by boryl substituent modification, or on the lithiation of ferrocene derivatives and subsequent quenching with the electrophile FBMeS₂, have given access to a range of ferrocene derivatized Lewis acids with which to conduct a systematic study of fluoride and cyanide binding. In particular, the effects of borane electrophilicity, net charge, and ancillary ligand electronics/cooperativity on the binding affinities for these anions have been probed by a combination of NMR, IR, mass spectrometric, electrochemical, crystallographic, and UV-vis titration measurements. In this respect, modifications made at the para position of the boron-bound aromatic substituents exert a relatively minor influence on the binding constants for both fluoride and cyanide, as do the electronic properties of peripheral substituents at the 1 ′- position (even for cationic groups). By contrast, the influence of a CH₂NMe₃ ⁺ substituent in the 2- position is found to be much more pronounced (by >3 orders of magnitude), reflecting, at least in part, the possibility in solution for an additional binding component utilizing the hydrogen bond donor capabilities of the methylene CH₂ group. While none of the systems examined in the current study display any great differentiation between the binding of F^- and CN^- (and indeed some, such as FcBMeS₂, bind both anions with equal affinity within experimental error), much weaker boronic ester Lewis acids will bind fluoride (but give a negative response for cyanide). Thus, by the incorporation of an irreversible redox-matched organic dye, a two-component [BMes₂/B(OR)₂] dosimeter system can be developed capable of colorimetrically signaling the presence of fluoride and cyanide in organic solution by Boolean AND/NOT logic.