Boron trihalide adducts donor

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. 

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). 

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. 

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). 

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. 

An i.r. study of the mixed boron trihalide adducts of carbonyl donors (ethyl acetate and benzophenone) shows that the mixed adducts do indeed possess Lewis acidities intermediate between those of the corresponding BX3 systems.267... 

All 20 of the possible boron trihalide adducts of NMcs have been detected (halogen exchange to give the mixed halide species only occurs when an excess of BX3 is present). In contrast to the free boron trihalides and their O-donor adducts, the iV-donor adducts allow isolation of the individual... 

Boron trihalides are strong Lewis acids that react with a wide collection of Lewis bases. Many adducts form with donor atoms from Group 15 (N, P, As) or Group 16 (O, S). Metal fluorides transfer F ion to BF3 to give tetrafluoroborate salts LiF + BF3 LiBF4 Tetrafluoroborate anion is an important derivative of BF3 because it is nonreactive. With four <7 bonds, [BF4 ] anion has no tendency to coordinate further ligands. Tetrafluoroborate salts are used in synthesis when a bulky inert anion is necessary. 

Reaction of a donor molecule with a previously equilibrated mixture of free boron trihalides gives an initial adduct mixture corresponding to the equilibrium mixture of the free boron trihalides. This method should be suitable for all mixtures except BF3/BI3 (as noted previously). However, equilibria in the adducts can be quite different from the corresponding equilibria in the free boron trihalides. If halogen redistribution is fast and if the mixed adducts are discriminated against, then this method does not succeed (28). 

In the uncomplexed boron trihalides, mixtures of fluorine with chlorine, bromine, and iodine are successively less favored 111), in accord with the symbiotic principle. This principle can rationalize halogen redistribution equilibria in adducts of the mixed boron trihalides as well. Incompatibility of fluorine with the heavier halogens is greatest with the softest donors. In the extreme case of PH3 as... 

Use of the complexation shift as a measure of donor-acceptor interaction is especially treacherous with nuclei other than protons, because chemical shifts of these nuclei are more dependent on the paramagnetic than on the diamagnetic term of the screening tensor (52, 53). Both 19F and 11B resonances of the boron trihalides do shift to high field on complexation, as expected if the complexation shift were due to the increase in electron density on the boron trihalide, and early work indicated that 19F complexation shifts of BF3 could be correlated with enthalpies of formation of the complexes. Although this is true for BF3 adducts of some series of closely related donors (42, 91, 151), such correlations do not occur in other series (169). Table II illustrates that, although there is a tendency for the strongest... 

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. ... 

If the donor molecule contains reactive hydrogen, as H2O, NH3, PH3, AsHs, R2NH, RNH2, and lower alcohols, hydrogen halides are produced (equations 17 -19). In these reactions the initial adducts formed with boron trifluoride are much more stable to protonolysis than with the other trihalides, and these reactions proceed slowly or not at all. 

---