Structure of BF3 Complexes

The molecular structure of BF3 complexes has been determined in both the gaseous and solid phases. Solid-state structures, retrieved mainly from the CSD [6], are listed in Table 3.1, while Table 3.2 corresponds to gas-phase structures. In both tables, the complexes are arranged according to the intermolecular distance d between the boron atom and the electron donor atomic site of the base. 

8-Crown-6 aqua-trifluoroborate clathrate toluene solvate. IS-Crown-b bis(aqua-trifluoroboron) dihydrate. 

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]. 

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Table 3.1 Solid-state structures of BF3 complexes references and intermolecular distances d(A).

Treatment of carbenes 4 (R = Me, Et, Pr, R = Me) with Et O BF3 resulted in the isolation of thermally stable BF3 complexes, 24 (22). No information regarding the structures of these complexes is available however, a related BH3 complex has been characterized crystal-lographically and is discussed later (23). 

The structure of the complex (96) between benzaldehyde (95) and boron trifluoride (equation 15) was investigated by X-ray crystallography169. In 96, BF3 is in the anti position to the phenyl ring and this geometry remains also in solutions, as tested by the 19F-NMR spectrum in CD2CI2. An ab-initio study170 on interactions between formaldehyde and boron trihalides showed that these complexes (mainly donor-acceptor complexes) affect spectroscopic properties and the reactivity of the carbonyl group the polarization of the C=0 bond favours the attack of nucleophiles. 

The nature of donor-acceptor complexes has been the subject of various NMR studies conducted as early as the 1960s. Early calorimetric studies showed that boron trihalides are capable of forming donor-acceptor complexes with a number of Lewis bases and the heats of adduct formation for some of these complexes were determined. Gaseous boron trifluoride, for example, was shown to form a ctxnplex with ethyl acetate in a highly exothermic reaction (-A// = 32.9 0.2 kcal mol ). IR and UV analysis of BF3 complexes of aromatic aldehydes indicated a o-complex with a lengthened CVO bond and a highly delocalized ir-system. More detailed structural information, however, was acquired only after closer inspection by low temperature H, B, C and F NMR studies. ... 

Figure 13 Enol resonance structure of BF3/enal complexes...

Corey and co-workers performed H NMR molecular dynamics and NOE studies of the 2-methylacrolein-BF3 complex in CD2CI2 and demonstrated that the 5-trans structure of the complex predominates in the solution at 185 K [18]. 

S4.1I The ether oxygen atom will behave as a Lewis base and donate one of its lone electron pairs forming a dative bond with the boron atom of BF3. The geometry at the boron atom will go from trigonal planar in BF3 to tetrahedral in F3B-OEt2. The structure of the complex is shown below. 

Figure 3.1 Structure of the complex of BF3 with pyridine (distances in A).

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. 

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. 

In the white solid product, the geometry around both the donor nitrogen atom and the acceptor boron atom is tetrahedral (Structure 9.3) the structure of the complex is analogous to that of a substituted ethane, C2H6. It is isoelectronic with (CH3)3C—CX3. The Lewis base may also be a halide ion thus when BF3 is heated with solid KF, the tetrahedral tetrafluoroborate ion (Structure 9.4) is formed ... 

By 0-acylation with 2-methyl-1,3-dioxolenium fluoroborate, which reacts as 0-acetyl ethylene oxide, 2,6-dimethyl-4-pyroiie is converted into 4-acetoxy-2,0-dimethylpyrylium fluoroborate (24, Y = 0, R = Ac, X=BT 4). The alleged compound with this structure which has been obtained from 8 and acetyl fluoroborate is, in fact, the BF3-complex of the pyrone. 

Summary of Facial Stereoselectivity in Aldol and Mukaiyama Reactions. The examples provided in this section show that there are several approaches to controlling the facial selectivity of aldol additions and related reactions. The E- or Z-configuration of the enolate and the open, cyclic, or chelated nature of the TS are the departure points for prediction and analysis of stereoselectivity. The Lewis acid catalyst and the donor strength of potentially chelating ligands affect the structure of the TS. Whereas dialkyl boron enolates and BF3 complexes are tetracoordinate, titanium and tin can be... 

Fig. 6.4. Structure of the BF3-2-methylpropenal complex. Reproduced from Tetrahedron Lett., 33, 6945 (1992), by permission of Elsevier. 

Fig. 6.7. Transition structures for the reaction between 1,3-butadiene and the methyl acrylate—BF3 complex calculated at die ab initio HF/6-31G level. Relative energies are in kcal/mol. Adapted from Tetrahedron, 53, 6057 (1997), by permission of Elsevier.

Such a bond, in which the donor molecule (or anion) provides both bonding electrons and the acceptor cation provides the empty orbital, is called a coordinate or dative bond. The resulting aggregation is called a complex. Actually, any molecule with an empty orbital in its valence shell, such as the gas boron trifluoride, can in principle act as an electron pair acceptor, and indeed BF3 reacts with ammonia (which has a lone pair, NH3) to form a complex H3N ->BF3. Our concern here, however, is with metal cations, and these usually form complexes with from 2 to 12 donor molecules at once, depending on the sizes and electronic structures of the cation and donor molecules. The bound donor molecules are called ligands (from the Latin ligare, to bind), and the acceptor and donor species may be regarded as Lewis acids and Lewis bases, respectively. 

In this Sect, we discuss 1H, 19F and nB NMR studies of BF3 NH2C2H5 and BF3 NHC5H10 complexes, with principal emphasis on the former. We present the chemical composition of commercial BF3 amine complexes, their thermal stability in the solid state and in solution, the effect of moisture and heat upon their composition, the nature of their interaction with the epoxide and amine components utilized in TGDDM-DDS commercial prepregs, the composition of BF3 amine complexes in commercial prepregs, their thermal stability in the prepregs, and the chemical structure of the predominant catalytic species of the cure reactions of the prepreg. 

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