Reports: B6 46291-B6: Condensed Phase Effects on the Structural Properties of Friedel-Crafts Intermediates: RF-BF3

James A. Phillips, University of Wisconsin (Eau Claire)

Context: The main objective of this project is to characterize the structural properties of organofluoride – boron trifluoride complexes (RF'–BF3) in the gas-phase and in bulk, condensed-phase environments via computations and low-temperature infrared spectroscopy. These species are key intermediates in Friedel-Crafts reactions [1] - an important class of carbon-carbon forming processes that facilitate the conversion of petroleum feedstocks to commercially-viable compounds. One common example is the alkylation of benzene, viz.

It has been presumed for 50 years that the first step is the formation of a 1:1 RF–BF3 intermediate complex [1], but it is also known that the methyl complex is quite weak in the gas phase with an experimental B-F' distance of 2.42 [2]. Thus, its solution-phase reactivity of these 1:1 complexes can only be rationalized by invoking a substantial structural re-arrangement that results from interactions with the solvent.

Gas-Phase Structures and Binding Energies

(CH3)3C-BF3

Previously, we reported the calculated (X3LYP/aug-cc-pVTZ) gas-phase structures of CH3F–BF3, (CH3)2CHF–BF3, (CH3)3CF–BF3, which exhibited long B-F' distances (2.35 - 2.41), and a MP2/aug-cc-pVTZ binding energies of 3.6 to 6.4 kcal/mol. The structure of the t-butyl complex is shown at the left. 

B-F' potentials: Gas phase and dielectric media

(CH3)3CF–BF3

Calculated gas-phase (X3LYP/aug-cc-pVTZ) B-F' potential curves for CH3F–BF3, (CH3)2CHF–BF3 and (CH3)3CF–BF3, are shown in the figures below. The gas-phase potentials (top traces in each curve) show no major peculiarities, but are rather soft long the inner wall, such that the energy only rises by about 2.5 kcal/mol over several tenths of an between the minima and the inner wall. The curves computed for dielectric media (via PCM [5]), do differ notably between the methyl complex and the two larger systems. As the dielectric increases, the potential of CH3CF–BF3 responds uniformly across all bond lengths, such that there is no change in shape, and no signs that a distinct structure with a shortened B-F' bond length is stabilized by the medium. For the larger complex however, the inner portion of the curve is preferentially stabilized. For (CH3)2HCF–BF3 and the potential at 1.7 is only about 1 kcal/mol above the minimum.

Experimental and computed IR spectra for (CH3)2CH–BF3

The figure at the left shows the C-F stretching region of the IR spectrum of (CH3)2CHF–BF3 in solid Ne, for which the peaks assigned to the 1:1 complex are marked with asterisks. The top trace is the sample containing both isopropyl fluoride and BF3, the bottom two traces are controls that contain only BF3 or (CH3)2CHF. These peaks, both assigned to modes that involve some degree of C-F stretching motion, agree with our DFT predictions (X3LYP/aug-cc-pvTZ) for the gas-phase complex to within 3 cm-1. This provides a great deal of validation for our characterization of the gas-phase complex, and the at best meager effects of low-dielectric media.

Molecular Cooperativity: the Influence of a Second RF Subunit

At this point in the project, we are faced with a striking discrepancy. Conductivity measurements indicate that some sort of ionization occurs when BF3 is added directly to alkylfluorides (with R > ethyl), yet our theoretical characterization of the 1:1 complexes suggests that spontaneous ionization should not take place – even in fairly polar solvents. Moreover, our experimental frequencies indicate that our theoretical characterizations of the 1:1 complexes are reasonable, at least for the gas-phase and low dielectric environments. This paradox led us to consider the role of a second alkyl fluoride molecule, which may act mediate atom transfer to form ions. The graphic at the left displays an initial result from our attempt to characterize a series of 2:1, i.e. (RF2)–BF3 complexes via density functional calculations. This shows that the addition of a second CH3F subunit causes the B-F' distance to contract by about 0.66. This is a cooperative, molecular effect that is not predicted using continuum solvation models.

1. Olah, G.A. Friedel-Crafts and Related Reactions; Wiley/Interscience: New York, 1963.

2. Leopold, K.R.; Canagaratna, M.; Phillips, J.A. Accts. Chem. Res. 1997, 30, 57.

3. Reed, A.E.; Curtiss, L.A.; Weinhold, F. Chem. Rev. 1988, 88, 899.

4. Bader, R.F.W. Atoms in Molecules - A Quantum Theory; Oxford University Press: Oxford, 1990.

5. Miertus, S.; Tomasi, J. Chem. Phys, 1982, 65, 239.

6. van der Veken, B.J.; Sluyts, E.J. J. Phys. Chem. A. 1997, 107, 9070.

 
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