The Chemistry of Perfluoroaryl Boranes

Warren E. Piers    University of Calgary, Department of Chemistry, 2500 University Dr. N. W., Calgary, Alta., Canada T2N 1N4

I Introduction and Scope

It has been just over 40 years since the first pentafluorophenyl substituted boranes were prepared using transmetallation reactions of C6F5SnMe3 and BCl3.1 Although investigated fairly thoroughly at the time, the rise to prominence of this family of boranes by virtue of their efficacy as activators for olefin polymerization pre-catalysts has occurred only in the past 15 years and is well documented.2 Indeed, the propensity of the parent tris-(pentafluorophenyl)borane,3 B(C6F5)3, to abstract anionic moieties from transition metals has opened up the chemistry of a wide variety of electrophilic organotransition metal cations and led to important commercial advances in the production of high quality polyolefin resins with superior properties when compared with plastics prepared traditionally via Ziegler–Natta technology.4 The properties of B(C6F5)3 that make it an excellent activator and its ready availability have led to a renewed interest in the chemistry of perfluoroaryl boranes, not only as catalyst activators but also as strong Lewis acids for other purposes.

Already in the early days of B(C6F5)3 chemistry, Massey and Park demarked its remarkable thermal stability and high affinity for even weak Lewis bases.5,6 It was noted at the time that, generally, perfluoroalkyl boranes were not particularly stable due to the strong thermodynamic driving force for the formation of B–F bonds and (relatively) stable fluorinated carbenes. The pentafluorophenyl group, however, resisted this pathway and B(C6F5)3 was observed to be stable up to temperatures of 270 °C with only minor decomposition. Furthermore, by virtue of the strongly electron withdrawing perfluoroaryl groups (the C6F5 group is estimated to have σp and σ− parameters of 0.47 and 0.99,8 respectively), B(C6F5)3 and related boranes are very strong Lewis acids, of comparable strength to BF3 and BCl3. Unlike the haloboranes, however, the B–C bonds in the perfluoroaryl compounds are resistant to cleavage by protic acids, giving these Lewis acids more chemical integrity. The perfluoroaryl substituents also provide steric protection to the boron center and raise the potential for –C6F5/–C6H5 π–π stacking interactions with incoming Lewis bases, which augment the primary Lewis acid/Lewis base dative interaction. The aryl groups also impart greater crystallinity to the adducts formed and many complexes have been studied crystallographically as a result, allowing for detailed analysis of the solid-state structures of the adducts of a variety of Lewis bases. Together, these attractive features have contributed to the increased use of this borane. The one inhibitor is the cost of the reagent; at the time of this writing it is available for 15–80 USD/g, depending on the quantities purchased, which is something of a deterrent for routine use in place of, for example, BF3·Et2O.

Because the use of B(C6F5)3 and related boranes in olefin polymerization applications has been reviewed extensively,2 this aspect of their chemistry will not serve as the focus of this chapter. The aspects of perfluoroaryl borane chemistry that fall outside the domain of olefin polymerization by single site catalysts will be covered, drawing parallels only where necessary. The article will begin with a brief discussion of methods for quantifying Lewis acidity followed by a survey of the chemistry of X2BArF, XB(ArF)2 and B(ArF)3 (ArF=perfluoroaryl group) compounds and their adducts, a section on polyfunctional perfluoroaryl boranes and conclude with applications that do not involve α-olefin polymerization by single site catalysts via a coordination polymerization mechanism. The role of perfluoroaryl borates as weakly coordinating anions (WCAs) will be touched on, but since this topic has also been adequately covered in the recent literature, it will not be emphasized here.

II Lewis Acid Strength Measurement

Unlike Bronsted acidity, which can be quantitatively assessed accurately using the pKa scale, quantitative measurement of Lewis acid strength is a more nebulous undertaking. Steric factors encompassing the structural features of the LA (Lewis acid) and the LB (Lewis base) play a much more significant role in the effective LA strength of a given acid and establishing an absolute scale of Lewis acid strength is difficult, since it is situation dependent. Therefore, in general, such methods are of diminished utility compared to pKa measurements.

Nonetheless, several attempts to quantify Lewis acidity have been made and a few methods are useful in the context of perfluoroaryl borane chemistry. The Childs method9 involves measurement of the perturbation of the 1H NMR signal for the β-proton in crotonaldehyde upon complexation to a given LA via the carbonyl oxygen (I).

The assumption here is that, because this proton is remote from the locus of coordination, it will be more or less immune to steric factors engendered upon complexation, and the chemical shift will be influenced primarily by the electronic effects caused by the electron withdrawing LA. The stronger the LA, the greater the perturbation from the chemical shift of this proton in free crotonaldehyde. Technical challenges include the need to exclude water completely and to use an excess of LA in measuring the complex's chemical shift, but aside from these potential pitfalls, the method provides a useful scale of Lewis acidity by which to judge main group Lewis acids relative to BBr3.

This empirical method has been validated thermochemically and computationally. Childs observed a moderate correlation between the observed heats of formation of the crotonaldehyde·LA adducts and the scale of acidity developed by the NMR method.10 Laszlo and Teston11 found a strong correlation between the Lewis acidities determined via the Childs' method and the change in the computed energy of the π∗ orbital of the carbonyl function upon complexation by the LA, suggesting a viable non-experimental method for assessing LA strength given the ready availability of the necessary computational resources in most modern laboratories.

Recently, Beckett et al. has used the Gutmann acceptor number12 (GAN) scale to assess the Lewis acidity of B(C6F5)3 in particular.13 This scale uses the perturbation in the 31P chemical shift of Et3P=O in hexane observed when the phosphine oxide is immersed in a Lewis acidic medium. Excellent correlation is observed between the GAN and the Childs' Lewis acidity for a variety of LAs, perhaps not too surprising given that they are related NMR methods.

Where these methods tend to falter is in sterically more demanding LAs—such as perfluoroaryl boranes with bulkier groups than C6F5—in that they tend to overestimate the strength of an LA. Thus, Marks et al. have observed less distinct correlations between Childs' acidities and enthalpic data for larger perfluoroaryl boranes,14 reflecting the steric back-strain that arises as a boron center is pyramidalized upon interaction with an LB. It is thus important to realize that, in assessing a pair of Lewis acids for comparative Lewis acid strength towards a given Lewis base, a competition experiment provides the most accurate information. Indeed, the relative LA strengths may flip depending on the base used. Thus, while the Childs' scale and the Laszlo/Teston methods described are qualitatively useful and will be referred to throughout the review, they may not reflect an absolute assessment of a given borane's LA strength, particularly in the more sterically significant members of the perfluoroaryl family of compounds.

III Synthesis and Chemistry of X2BArF Compounds

Because they serve as versatile starting materials for a variety of other boranes, the dihalo boranes, particularly the fluoro and chloro derivatives, are the most important members of this class of compounds. Their chemistry, along with other pentafluorophenyl boron halides, has been reviewed recently from a personal perspective by Chivers,15 whose PhD thesis described early explorations into the...