913059-08-2Relevant academic research and scientific papers
Structural and electrochemical investigations of the high fluoride affinity of sterically hindered 1,8-bis(boryl)naphthalenes
Melaimi, Mohand,Sole, Stephane,Chiu, Ching-Wen,Wang, Huadong,Gabbai, Francois P.
, p. 8136 - 8143 (2006)
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 [Me3SiF 2][S(NMe2)3)] to afford the anionic complex [2-μ2-F]-, which has been isolated as a [S(NMe 2)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) A in 2 to 2.922(7) A 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-ln-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.
