Boron trihalide adducts

Studies of Equilibria and Shift Reagents.—N.m.r. studies of the exchange of halogen in boron trihalide adducts of trimethylphosphine, its oxide, and sulphide,49 and exchange of chloro- and methoxy-groups between methylphosphino and methyl-silyl or -germyl moieties,80 have been reported. The rates of ionization of phosphoranes... 

II. Preparation, Detection, and Properties of Mixed Boron Trihalide Adducts 149... 

The following methods have been used to prepare the mixed boron trihalide adducts D BX Y3 for in situ study by spectroscopic methods. 

If the donor-acceptor bond is weak, two different boron trihalide adducts of the same donor will exchange halogen to form the mixed-halogen adducts. The weaker the donor-acceptor bond, the more rapidly the mixed adducts form, indicating that free boron trihalide formed by dissociation is an active species in halogen exchange. 

The first neutral mixed boron trihalide adducts were detected by NMR with dimethyl ether as donor (27, 73). Near-statistical amounts of mixed- and unmixed-halogen species are formed in the Me20-BF C13 system. Equilibration is almost complete in an hour at 0°C when uncomplexed dimethyl ether is present, and is extremely rapid when excess boron trihalide is present. 

Dialkyl ether adducts are typical of many boron trihalide adduct systems in that the BF3 adducts tend to be stable, whereas the BC13, BBr3, and BI3 adducts become successively more reactive, decomposing to alkyl halide and alkoxyboron dihalide (60,120). This in-... 

Preliminary studies of mixed boron trihalide adducts of other ethers have been carried out 78), but many of these adducts are highly susceptible to decomposition. 

Mixed boron trihalide adducts of amines must have been present as intermediates in earlier halogen displacement studies 40,124), but the experimental methods used led to detection of only the final unmixed-halogen product. 

Boron trihalide adducts of 4-methylpyridine show halogen exchange behavior similar to that of the adducts of trimethylamine 10), and isolation of individual mixed-halogen species is feasible here too. The stability of nitrogen-donor adducts of the mixed boron trihalides parallels the stability of nitrogen-donor adducts of BH2X and BHX2 which have been more extensively studied 19, 153,156). 

Detection of BF C14 species by NMR has been used as evidence for the formation of ionic boron trihalide adducts D2BF2+BFnCl4 from simple covalent adducts (82, 83) and as evidence for the abstraction of chloride ion from (C6H5)3PC12 by BF3 (32). Boron trihalides are often used as halide abstraction reagents, and when two different halogens are involved the mixed-halogen species should be formed, but often only indirect evidence has been obtained (51). 

Although most known BH X3 adducts are stable and most known BX Y3 adducts are unstable to redistribution of substituents about boron, this need not reflect a major difference in behavior of the two types of adduct but may, instead, reflect the interests of the investigators. The BH X3 adduct studies have emphasized synthesis and, hence, systems that are stable to redistribution, whereas mixed boron trihalide adduct studies have emphasized redistribution reactions, which are easier to observe when the donor is weak and redistribution is rapid. 

Halogen exchange in the coordinatively saturated boron trihalide adducts requires either a preliminary dissociative step or an associative interaction different from the doubly halogen-bridged transition state available to three-coordinate boron compounds [Eq. (1)] (113,114). The reactions can be classified as follows. 

In boron trihalide adducts and tetrahaloborate ions, halogen redistribution equilibria are reasonably close to this ideal random case when chlorine, bromine, and iodine are involved (27, 28, 80, 100, 112), as are equilibria in the uncomplexed heavier boron trihalides (111). 

The near-random redistribution of fluorine and chlorine in oxygen-donor boron trihalide adducts (27, 29) might be rationalized using this principle since oxygen is between fluorine and chlorine on the hard-soft scale (140), and there is less discontinuity of ligands in terms of hard-soft properties. However, amine adducts of BHF2 do tend to disproportionate to give the BF3 and BH3 adducts (130). 

Identification of most mixed boron trihalide adducts, and all information on rates and equilibria of halogen redistribution in these systems, depends on correct NMR peak assignments. The reliability of these is assured by a combination of two or more of the following distinctive heteronuclear NMR coupling patterns systematic variation of peak areas as the ratio of two halogens is changed identification and consistent behavior of peaks due to the same species in spectra of two or more of 1H, 19F, UB, and 13C nuclei agreement of observed parameters with values calculated from "pairwise interaction parameters (76,123). Fine points of structure can also be determined by... 

Correlations of NMR parameters to electronic structure have been carried out for a wide range of three-coordinate boron compounds (12, 188). However, the extensive NMR data that exist for boron trihalide adducts (70) are less easily interpreted. Among the factors leading to difficulties in correlating NMR parameters with structure... 

The mixed boron trihalide adducts hold few surprises in terms of their donor-acceptor bond behavior, but provide striking examples of dependence of halogen redistribution behavior on the nonhalogen substituent. The simplicity and accessibility of these systems suggests their use as model compounds in the study of ligand redistribution reactions. Many of the features complicating ligand redistribution in, for example, metal carbonyl systems (46) are simplified or absent here. 

---