Abstract
The [BX3(EMe2)] (X = Cl, Br, I; E = Se or Te) have been prepared by reaction of BX3 with the EMe2 in hexane under anhydrous conditions. The X-ray crystal structures of [BX3(TeMe2)] (X = Cl, Br, I) and [BX3(SeMe2)] (X = Cl, Br) have been determined; all are pseudo-tetrahedral monomers and show d(B−E) decreases with halogen, Cl > Br > I. Multinuclear NMR data (1H, 11B, 77Se and 125Te) are reported and compared with data on the corresponding [BX3(SMe2)], and the trends discussed. The unstable [BF3(SeMe2)], prepared from BF3 and SeMe2 in the absence of a solvent, has been similarly characterised by multinuclear NMR spectroscopy, and evidence for the existence of unstable [BF3(TeMe2)] obtained for the first time, although it could not be obtained pure. The results are discussed in the light of recent theoretical modelling of boron halide adducts.
Highlights
The study of boron Lewis acids remains a very active area of research and has been discussed in several recent reviews [1e6]
The multinuclear NMR data (Table 3) show some consistent trends, for example the 11B chemical shifts for a fixed chalcogenoether move to low frequency Cl / Br / I, whilst for a fixed halide the trend to low frequency is S / Se / Te. For complexes of these three boron halides with SeMe2 the 77Se coordination shifts (D 1⁄4 dcomplex e dligand) are large and positive with D I > Br > Cl
In low valent organometallic or carbonyl complexes the ratio of the coordination shifts are much larger, d(Te)/d(Se) > 2.3, rationalised as due to greater R2Te/M donation in the soft metal centres and with expanded metal d-orbitals resulting from the low oxidation states providing good orbital overlap with Te centres [33,42e44]
Summary
The study of boron Lewis acids remains a very active area of research and has been discussed in several recent reviews [1e6]. It is important to note that the properties of both the Lewis acid and Lewis base in a complex must be taken in consideration, and that calculations deal with gas phase species, meaning solvation or solid state effects may mask Lewis acidity trends in solution or in the solid state Due to their innate Lewis acidity, boron(III) halides have found widespread use in organic syntheses and catalysis [6,10]. The studies revealed that whilst flexible ligands produced [X3B(m-LÀL)BX3] complexes, with o-phenylene linked diphosphines and diarsines, very rare dihaloboronium cations, [BX2{o-C6H4(EMe2)2}]þ (E 1⁄4 P, As) were obtained [13] In these complexes and in the corresponding [BX3(PMe3)] [14] and [BX3(AsMe3)] [15] the d(BÀP) and d(BÀAs) bond lengths follow the expected order with X (F > Cl > Br > I), data on fluoride complexes are rather limited. Comparisons between the boron complexes and chalcogenoether complexes with other Group 13 halides are described
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