Bis(pyridine)difluoroboron, tris(pyridine)fluoroboron, and other (pyridine)haloboron cations. A systematic NMR study

Abstract

Abstract:The adducts pyrBF,Br and pyrBFBr, (pyr = pyridine) form fluoroboron cations by displacement of Br-by excess pyridine, the ease of cation formation being pyr,BFc >>pyr,BFBr+ > > p y r 3~~2 + .C1-can be displaced from pyrBF,Cl and pyr.BFCl,, but much less readily, to form pyr,BFc, pyr2BFC1+, and, under forcing conditions, a few percent of p y r ,~~' + .Nonfluorine-containing mixed boron trihalide adducts of pyridine also form haloboron cations by heaviest-halide-ion displacement, for example pyrBC11, giving pyr,BClI+, the ease of displacement always being I-> Br-> C1-, and displacement always occurring more readily from mixed boron trihalide adducts than from unmixed-halogen adducts. The mechanistic implications of this are discussed. ortho Substituents greatly reduce the ability of pyridine to displace heavy halide ion, so 2-methylpyridine gives 2-Mepyr,BF: and 2-Mepyr,BFBr+ but not 2-Mepyr,BFCl+ or 2 -~e~~r ,~~' + , while 2,6-dimethylpyridine does not form any haloboron cations. '9 spin-lattice relaxation times of the fluoroboron cations are much shorter than those of neutral boron trihalide adducts in the same solution, and provide a further diagnostic test for their presence.Key words: fluoroboron cations, pyridines, mixed boron trihalide adducts, fluorine-19 NMR, boron-11 NMR.RCsumC : Les adduits pyrBF,Br et pyr.BFBr, (pyr = pyridine) forment des cations du fluorobore par dkplacement de B r par l'excks de pyridine; la facilitk de formation du cation est la suivante : pyr,BF,+ >>pyr,BFBr+ > > p y r 3~~2 + . I1 est possible de dkplacer le C1-des adduits pyr.BF,Cl et pyrBFCl,, pour former les pyr,BFc, pyr,BFCl+, et en for~ant les conditions quelques pour cents du p y r 3~~' + , mais la rkaction est beaucoup plus lente. Les adduits de la pyridine avec des trihalogknures mixtes du bore ne contenant pas de fluor forment aussi des cations halobores par le dkplacement de l'halogene le plus lourd; par exemple, le pyr.BC11, conduit au pyr,BClI+, la facilitk de dkplacement est I-> B r > C1-et le dkplacement se fait toujours plus facilement 8 partir d'adduits trihalogknures mixtes du bore qu'8 partir d'adduits non melanges d'halogknures de bore. On discute des implications mkcanistiques de ces rksultats. Les substituants ortho ralentissent beaucoup la facilitk de la pyridine 8 dkplacer les ions halogknures lourds; ainsi, la 2-mkthylpyridine conduit aux 2-Mepyr,BF,+ et 2-Mepyr,BFBr+, mais pas aux 2-Mepyr,BFClf ou 2 -~e~~r ,~~' + alors que la 2,6-dimkthylpyridine ne forme aucun cations halobore. Les temps de relaxation spin-rkseau du ' 9~ dans les cations du fluorobore sont beaucoup plus courts que ceux des adduits trihalogknures de bore neutres dans la m&me solution et ils fournissent une mkthode supplkmentaire pour diagnostiquer leur presence.Mots clks : cations du fluorobore, pyridines, adduits des trihalogknures mixtes du bore, RMN du 1 9~, RMN du "B.[Traduit par la rkdaction]

Bis- and tris(amidine)fluoroboron cations and mixed tetrahaloborate anions: NMR studies of mixed boron trihalide adduct redistribution reactions involving amidines as strong nitrogen bases

“…The 1 9~ and "B NMR parameters of the amidine -mixed boron trihalide adducts and fluoroboron cations follow the patterns seen in other adduct systems (2,3,19,38), with the number and type of heavy halogen atoms having greater effects than the donor or the fluorines. ' 9~ chemical shifts are also affected by donor steric hindrance (3,19).…”

“…For example, days at 50°C are required to attain equilibrium in the sealed-tube reaction of Table 3. A wide range of reaction rates is not surprising in complex exchanging systems, and has been noted in other D.BF,,X,-, adduct systems (3,32).…”

“…However, we cannot completely exclude the possibility that the large low-field "B NMR peak of Fig. 5 arises from BC1,-rather than from DBU-BCl,, since BCl, and D-BC1, adducts give "B singlets of very similar chemical shift (3). Even if BC1,-were present in place of DBUeBCl,, the same argument would apply: it would be equally noteworthy that no C1-is displaced from BC1; while all of the available C1-is displaced from the mixed chlorofluoroboron species, and our conclusion, that chloride displacement occurs much more easily from mixed-than from unmixed-halogen species, would remain valid.…”

“…Both of these solutions were kept at -63°C in a slush bath. After 1&20 min, during which time D.BFnC13-, formed (2,3,19), a solution of 0.2-0. For personal use only.…”

“…9~ chemical shifts are also affected by donor steric hindrance (3,19). DBU has greater steric hindrance than DBN since the five-membered ring of DBN holds back the amidine substituents from the imino nitrogen donor site more effectively than the seven-membered ring of DBU.…”