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44832-AC4
ortho-Bis(methylium)phenylene and Related Dications: Synthesis, Characterization, and Anion Complexation

François P. Gabbaï, Texas A&M University

In this funding period, the funds provided by this PRF grant have been used to support our research in three distinct areas.  The results will therefore be presented in three separate sections.

1) Borenium and boronium cations – Development of a turn-on fluoride indicator

Hoping to discover new methods to sense anions, we have investigated the synthesis of novel cationic boron compounds including the borenium cations [1]⁺ and the boronium cation [2-DMAP]⁺ (DMAP = p-dimethylaminopyridine).  These compounds have been prepared from the corresponding fluoride derivatives by treatment with trimethylsilyl triflate and DMAP.  In the presence of iodide ions, [2–DMAP]⁺ behaves as a turn–on fluoride indicator and reacts with fluoride ions to afford the corresponding brightly fluorescent difluoride 2–F.  This fluorescence increase results from the greater sensitivity of cationic [2–DMAP]⁺ (when compared to neutral 2–F) to the external heavy atom effects imparted by I^-.

2) Polyfunctional Lewis acidic boranes as receptors for fluoride and cyanide anions

We have synthesized a series of novel naphthalene-based multidentate boranes and evaluated their affinity for fluoride anions.  In the course of these studies, we have discovered that the trinuclear Lewis acid 3 is able to bind two fluoride anions.  However, binding of the second fluoride anion is much less favorable than that of the first because of unfavorable Coulombic and steric effects. Comparative studies indicates that the fluoride binding constant of 3 is similar to that of simple B/Hg bidentate Lewis acids such as 4 but significantly lower than that of bidentate diboranes such as 5.

In an effort to strengthen the host guest interaction, we have more recently considered cationic multidentate boranes such as [o-6]⁺.  Remarkably, the reaction of equimolar amounts of [o-6]I and p-6-F in CDCl₃ leads to [p-6]⁺ and p-6-F in a quantitative yield, indicating that the fluoride affinity of [o-6]⁺ is far superior than that of [p-6]⁺.  More quantitative information could be gained from fluoride titration experiments carried out in MeOH.  In this solvent, the fluoride binding constant of [o-6]⁺ (K > 10⁶ M^-1) exceeds the measurable range and is at least 4 orders of magnitude higher than that measured for [p-6]⁺ (K = 400 (±50) M^-1).  Clues to the higher fluoride affinity of [o-6]⁺ were derived from crystallographic measurements which pointed to the presence of a bonding interaction between the fluorine and phosphorus atoms of o-6-F.  The crystal structure of this derivative confirmed this proposal.  Specifically, the boron bound fluorine atom F(1) (B(1)-F(1) = 1.482(3) Å) is located only 2.666(2) Å away from the P(1) atom, which is well within the sum of the van der Waals radii of the two elements (ca. 3.45 Å) (Figure 1).  Another conspicuous feature concerns the F(1)-P(1)-C(31) angle of 176.36(9)°.

Figure 1: Crystal structure of o-6-F

These results indicate the presence of a B-F→P interaction, which contributes to the increased fluorophilicity of [o-6]⁺.  While the F→P interaction must bear a large electrostatic component, an Atoms-in-Molecules (AIM) analysis carried out at the DFT optimized geometry indicates the presence of a bond path connecting the two atoms (Figure 2).  Furthermore, an Natural Bond Orbital (NBO) analysis identifies a donor-acceptor interaction involving a fluorine lone-pair as a donor and the phosphorus-carbon σ*-orbital as the acceptor.  Thus, [o-6]⁺ can be regarded as a cationic bidentate Lewis acid, whose high fluoride affinity arises from both fluoride ion chelation and Coulombic attractions.  In turn, these results further demonstrate that Coulombic and chelate effects are additive and can be combined to boost the anion affinity of Lewis acidic hosts.

Figure 2: AIM and NBO analyses of the B-F→P interaction in o-6-F. Top: AIM electron density map with relevant bond paths and bond critical points. Bottom: NBO contour plot showing the lp_(F)→σ^*_(P-C) interaction.

3) Unusual agostic interactions

We have synthesized the cationic fluorosilane 1-(dimethylfluorosilane)-8-(9-xanthylium)naphthalenediyl ([7]⁺) as a tetrafluoroborate salt and converted it into 1-(dimethylfluorosilane)-8-(9H-xanthene)naphthalenediyl (8) by reaction with NaBH₄.  The most interesting aspect of this chemistry pertains to the presence of an agostic C-H → Si interaction in 8. This interaction, which is characterized by a Si-H separation of 2.32(2) Å and a F-Si-H angle of 177.0(5)°, leads the silicon atom to adopt a distorted trigonal-bipyramidal geometry.

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