Structural and Electrochemical Investigations of the High Fluoride Affinity of Sterically Hindered 1,8-Bis(boryl)naphthalenes

The reaction of 10-bromo-9-oxa-10-boraanthracene with the tetrakis(tetrahydrofuran)lithium salt of dimesityl-1,8-naphthalenediylborate in diethyl ether affords 1-(dimesitylboryl)-8-(10‘-bora-9‘-oxaanthryl)naphthalene (2). This diborane
reacts with [Me3SiF2][S(NMe2)3)] to afford the anionic complex [2-μ2-F]-, which has been isolated as a [S(NMe2)3]+
salt. The cyclic voltammograms of diborane 2 as well as 1-(dimesitylboryl)-8-(10‘-bora-9‘-thiaanthryl)naphthalene
(1) exhibit two reversible reductions at E1/2 = −2.200 and −2.566 V (vs FcH/FcH+) for 1 and E1/2 = −2.248 and
−2.620 V (vs FcH/FcH+) for 2 corresponding to the sequential reduction of the two boron centers. These two
waves simultaneously disappear upon fluoride addition, thus indicating the formation of fluoride chelate complexes
[1-μ2-F]- and [2-μ2-F]-. To identify the origin of the high fluoride affinity displayed by these diboranes, the structures
of 2 and [2-μ2-F]- have been studied experimentally and computationally. The crystallographic studies show that
the structure of 2 is distorted, thus indicating the presence of important steric repulsions between the neighboring
boryl moieties. By contrast, the structure of the anionic complex [2-μ2-F]- is much more sterically relaxed than that
of 2, as indicated by a reduction of the B−B distance from 3.279(4) Å in 2 to 2.922(7) Å in [2-μ2-F]-. The structural
results suggest that the high fluoride affinity displayed by 2 results, at least in part, from the relief of steric repulsions
induced by fluoride binding. Finally, the nature of the bonding as well as the strength of the interactions involved
in the B−F−B bridge of [2-μ2-F]- has been studied using density functional theory calculations and Atoms-In-Molecules analyses. These calculations indicate that the enthalpic gain associated with the formation of two B−F
bonds in [2-μ2-F]- only amounts to a fraction of the energy of a terminal B−F bond. These calculations also
suggest that the relief of steric repulsions induced by fluoride binding in 2 may contribute to the high fluoride
affinity of these types of molecules.