Evaluation of Electronics, Electrostatics and Hydrogen Bond Cooperativity 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 parts 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 FcBMes(2), 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 [BMes(2)/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.