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Dalton Transactions

The international journal for inorganic, organometallic and bioinorganic chemistry



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Dalton Trans., 2008, 4442 - 4450, DOI: 10.1039/b801040d

Fluoride ion complexation by a B2/Hg heteronuclear tridentate lewis acid

Christopher L. Dorsey, Pawel Jewula, Todd W. Hudnall, James D. Hoefelmeyer, Thomas J. Taylor, Nicole R. Honesty, Ching-Wen Chiu, Marcus Schulte and François P. Gabbaï

The reaction of [Li(THF)4][1,8--(Mes2B)C10H6] with HgCl2 affords [1,1-(Hg)-[8-(Mes2B)C10H6]2] ( 3) or [1-(ClHg)-8-(Mes2B)C10H6] ( 4), depending on the stoichiometry of the reagents. These two new compounds have been characterized by 1H, 13C, 11B and 199Hg NMR, elemental analysis and X-ray crystallography. The cyclic voltammogram of 3 in THF shows two distinct waves observed at E1/2 -2.31 V and -2.61 V, corresponding to the sequential reductions of the two boron centers. Fluoride titration experiments monitored by electrochemistry suggest that 3 binds tightly to one fluoride anion and more loosely to a second one. Theses conclusions have been confirmed by a UV-vis titration experiment which indicates that the first fluoride binding constant (K1) is greater than 108 M-1 while the second (K2) equals 5.2 (± 0.4) × 103 M-1. The fluoride binding properties of 3 have been compared to those of [1-(Me2B)-8-(Mes2B)C10H6] ( 1) and [1-((2,6-Me2-4-Me2NC6H2)Hg)-8-(Mes2B)C10H6] ( 2). Both experimental and computational results indicate that its affinity for fluoride anions is comparable to that of 2 but significantly lower than that of the diborane 1. In particular, the fluoride binding constants of 1, 2 and 3 in chloroform are respectively equal to 5.0 (± 0.2) × 105 M-1, 1.0 (± 0.2) × 103 M-1 and 1.7 (± 0.1) × 103 M-1. Determination of the crystal structures of the fluoride adducts [S(NMe2)3][ 1–2-F] and [S(NMe2)3][ 3–2-F] along with computational results indicate that the higher fluoride binding constant of 1 arises from a strong chelate effect involving two fluorophilic boron centers.

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