The BF3 Affinity Scale

Since the discovery of the complex H3N-BF3 by Gay-Lussac in 1809 [1], a great number of complexes of boron trifluoride with Lewis bases have been smdied. Indeed, BF3 is an archetypical Lewis acid. It is clear that the central boron is electron deficient since it has only six electrons in its outer shell and the highly electronegative fluorine atoms further decrease its o electron density. 

The thermodynamics of formation of BF3 complexes has been extensively investigated. However, the thermodynamic data collected in reviews prior to 1970 [2-4] do not allow the construction of homogeneous Lewis basicity or affinity scales because they do not refer to the same reaction and/or to the same medium. In fact, the types of reactions investigated 

Lewis Basicity and Affinity Scales Data and Measurement Christian Laurence and Jean-Fran9ois Gal 2010 John Wiley Sons, Ltd 

In 1993, at the end of these calorimetric studies, the enthalpies of complexation of BF3 had been obtained for 348 Lewis bases. The construction of the BF3 affinity scale from this set of homogeneous data is presented below. For a better understanding of stmcture-affinity relationships, a brief summary of the stmcture of BF3 complexes is given first. 

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An extensive compilation of values is available for a variety of main group Lewis bases, with most determined in heptane." There is interest in developing an I2 affinity scale defined similarly as the BF3 affinity scale (based on A//° of adduct formation). Many A f ° values have been determined for reactions of I2 with Lewis bases, but reconciliation of data obtained via different experimental conditions for reliable I2 affinity comparisons remains a challenge. 

In the context of the BF3 affinity scale, a further interesting structural feature is the importance of steric effects in determining the structure of BF3 complexes. For example, the complexes of benzaldehydes with BF3 [27, 35] have the phenyl group anti to the BF3. In other words, the B—O bond points towards the less hindered sp lone pair of the carbonyl oxygen. In three BFs-ketimine complexes [36], the BF3 group orients towards the less sterically demanding side of the imine function. The steric strain between BF3 and Lewis bases has been explained by orbital interaction theory [37]. 

The hard character of the BF3 affinity scale has been confirmed theoretically. The theoretical absolute hardness of BF3, p = 9.7 eV, is fairly high compared with other molecular Lewis acids (see Table 1.17). The hardness of the interaction of BF3 with NH3, NMc3 and CO has been studied [70] through the local Hard-Soft Acid-Base principle. 

The BF3 affinity scale represents an improved version of the SbCls affinity scale (DN scale). The concept is similar but the methodology produces cleaner complexation reactions and consequently more accurate complexation enthalpies. Additionally, the BF3 affinity database is more comprehensive and more varied than the SbCls affinity database. Finally, compared with SbCls the electronic structure of BF3 is much simpler. 

This chapter is intended to provide detailed examples of the spectroscopic and thermodynamic determination of most Lewis basicity scales presented in the previous chapters, namely the BF3 affinity scale, 4-fluorophenol basicity and affinity scales, the methanol infrared (IR) shift scale, the 4-nitrophenol solvatochromic shift scale, diiodine basicity and affinity scales, the iodine cyanide IR shift scale, the diiodine blue shift scale and the lithium cation basicity scale. With these examples, it is hoped that professional chemists, and also students of physicochemical sciences, will be able to supplement the scales for the molecules in which they are interested. 

Basicity in the gas phase is measured by the proton affinity (PA) of the electron donor and in solution by the pAj,. A solution basicity scale for aldehydes and ketones based on hydrogen bond acceptor ability has also been established [186]. Nucleophilicity could be measured in a similar manner, in the gas phase by the affinity for a particular Lewis acid (e.g., BF3) and in solution by the equilibrium constant for the complexation reaction. In Table 8.1 are collected the available data for a number of oxygen systems. It is clear from the data in Table 8.1 that the basicities of ethers and carbonyl compounds, as measured by PA and p , are similar. However, the nucleophilicity, as measured by the BF3 affinity, of ethers is greater than that of carbonyl compounds, the latter values being depressed by steric interactions. 

Following the use of boron acids by Brown et al. (1953-1955) [91] in his classical work on steric effects in the complexation of amines with trimethylboron, an extensive set of calorimetric measurements was provided by the studies of BF3 complexes by Gal, Maria et al. (1970-1992) [80]. Boron trifluoride is the archetype of Lewis acids in the original Lewis definition. The promising BF3 affinity scale is presented in Chapter 3. 

This is confirmed by the quantitative BF3 affinity scale, which shows that trimethylamine and tetrahydrofuran have a higher BF3 affinity than trimethylphosphine and tetrahydroth-iophene, respectively. Moreover, the thiocarbonyl and thiophosphoryl sulfur bases show lower BF3 affinities than the corresponding carbonyl and phosphoryl oxygen bases. Indeed, the decrease in basicity (affinity) in descending groups 15 and 16 of the periodic table is characteristic of hard basicity scales. 

The question now is In spite of significant solvent effects, are the solution Lewis basicity scales closely related to the intrinsic gas-phase Lewis basicity scales This is an important question for computational chemists who need to identify the computational methods that yield reliable basicities. A relative comparison of gas-phase computed basicities with solution experimental basicities would avoid the difficult and approximate modelling of the solvent effect [105]. However, this comparison requires that experimental gas-phase and solution basicities (affinities) be strongly correlated. This correlation appears to exist for BF3 affinities and hydrogen-bond basicities. Equation 1.96... 

The discovery of charge-transfer bands in the UV spectra of diiodine complexes (Benesi and Hildebrand, 1949) [202] and the development of the underlying theory (Mulliken, 1952) [31] initiated a wealth of thermodynamic and spectroscopic measurements on diiodine complexes, mainly in the period 1949-1980. Complementary measurements by Berthelot, Guiheneuf, Laurence et al. (1970-2002) and Abboud et al. (1973-2004) enabled a homogeneous scale of diiodine basicity to be constructed. In addition, recommended values of diiodine affinity have been compiled from the literature (Laurence, 2006), for comparison with the SbCls, BF3 and 4-FC6H4OH affinity scales. UV and/or IR shifts upon complexation of the acids h, ICl and ICN have also been systematically measured by Berthelot, Laurence, Nicolet et al. (1981-1985). These thermodynamic and spectroscopic scales will allow the recent concept of a halogen bond to be treated quantitatively. They can be found in Chapter 5. 

For the above reasons, we discourage the use of the DN scale when precise and robust scales are available. In particular, the affinity scale constructed with the reference Lewis acid BF3 is an alternative for measuring the strength of dative bonds. This scale is presented... 

By far the most important body of data on the thermodynamics of this reaction has been provided by the group of Gal and Maria at the University of Nice-Sophia Antipolis. For technical reasons, these authors chose to study reaction 3.5 between gaseous BF3 and the Lewis base in a dilute solution of dichloromethane at 298 K. The affinity scale can be defined as the negative enthalpy of reaction 3.6 ... 

The existence of numerous theoretical studies on the Lewis affinity of BF3 and on the nature of the dative bond in BF3 complexes, generally performed at high theoretical levels, illustrates that the choice of BF3 as a reference Lewis acid for constructing an affinity scale of Lewis bases is well founded, not only from the experimental but also from the computational point of view. Indeed, the electronic structure of BF3 is very simple, since this trigonal planar molecule contains only four first-row atoms. Calculations on the thermodynamics of BF3 complexes can significantly improve our knowledge of Lewis affinity, as illustrated below. 

MCA and MCB scales provide a simple demonstration of the strength and specificity of the cation/ interaction. Compare the affinity of benzene, a n ligand, to that of pyridine, an n ligand of similar structure, for the class of cationic Lewis acids and that of non-cationic ones, such as BF3, IBr, I2 and 4-FC6H4OH. In the comparison plot in Figure 6.14, it appears that different comparison lines are required for each class. Their slopes show that the interaction of cations with a n ligand becomes fairly large compared with that of neutral Lewis acids. 

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