Amine-Boron N-B Interactions

The energy of the N B interaction has been calculated from the stepwise formation constants of potentiometric titrations. Based on the relative stabilities of ternary phosphate complexes, it was calculated that the upper and lower limits of the N-B interaction must be bound between 15 and 25 kJ mol in A-methyl-o-(phenylboronic acid)-A-benzylamine. This value is in good agreement with computational data, which estimated the N-B interaction to be 13 kJ mol or less in the absence of solvent. To qualify this in terms of familiar 

As we have discussed, the strength of this interaction is a central feature in many fluorescent PET sensors, where the N-B interaction plays a pivotal role in signalling the binding event. If the interaction were much stronger, then the binding of a diol would not be able to disrupt the N B interaction sufficiently so as to modulate a change in fluorescence. By the same token, if the interaction were much weaker, then there would be no significant intramolecular N-B interaction to disrupt in the first place. 

For some time, the formation of a direct bond between nitrogen and boron was assumed to be responsible for the fluorescence enhancement seen when boronic acids bound diols. This interpretation does, however, raise certain questions. 

The recent evaluation of sensor 93, the first fluorescent diboronic acid sensor to be reported as a single-crystal X-ray structure both in its bound and unbound state, has recently been published. In the case of the unbound receptor, the geometry at boron is trigonal planar. This is important as the absence of deviation from planarity implies that there is no direct N-B Lewis base-Lewis acid bond at boron. 

While speculative, this structural interpretation of the interaction between boronic acid and the proximal tertiary amine through a bound protic solvent molecule (solvent insertion into the N-B bond) corresponds well with contemporary computational and potentiometric titration data, in which the formation of intramolecular seven-membered rings should not be ignored. 

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