Complexes of tris(pentafluorophenyl)boron with nitrogen-containing compounds : Synthesis, reactivity and metallocene activation

The strong Lewis acid tris(pentafluorophenyl)boron, B(C6F5)(3), reacts with several nitrogen-containing Lewis bases (nitriles, amines, imines, pyridines, etc.) and also with non-basic substrates (such as pyrroles and indoles) producing in both cases the B-N coordination adduct. With particular substrates (some tertiary amines, the imine tBu(Me)C=NBn, N-methyl-pyrrole and -indole) the 1:1 borane/N-compound reaction produces zwitterions where a new B-C bond is generated. Some of the borane-N-compound adducts present Bronsted acidity and can be reacted with dimethyl group 4 complexes with generation of weakly associated ion pairs, which are active catalysts for the polymerization of olefins

Conformational behavior of tris(pentafluorophenyl)borane-benzotriazole adducts

The stereomutations in a class of tris(pentafluorophenyl)borane-benzotriazole adducts are investigated by means of variable-temperature F-19 NMR and X-ray crystallography. At low temperature the presence of a pair of conformational enantiomers is confirmed by NMR spectra obtained in a chiral medium. Two different energy barriers, corresponding to an enantiomerization process and a B-N rotation, were observed and their values measured (Delta G double dagger = 10.7 and 12.8 kcal mol(-1) in the case of 1). In the case of the bis-adduct 3, two conformational diastereoisomers with different populations were detected at low temperature, while only one diastereoisomer is present in the crystalline state

Synthesis and reactivity of N-heterocycle-B(C6F5)(3) complexes. 4. Competition between pyridine- and pyrrole-type substrates toward B(C6F5)(3): Structure and dynamics of 7-B(C6F5)(3)-7-azaindole and [7-azaindolium](+)[HOB(C6F5)(3)](-)

Reaction between 7-azaindole and B(C6F5)(3) quantitatively yields 7-(C6F5)(3)B-7-azaindole (4), in which B(C6F5)(3) coordinates to the pyridine nitrogen of 7-azaindole, leaving the pyrrole ring unreacted even in the presence of a second equivalent of B(C6F5)(3). Reaction of 7-azaindole with H2O-B(C6F5)(3) initially produces [7-azaindolium](+)[HOB(C6F5)(3)]- (5) which slowly converts to 4 releasing a H2O molecule. Pyridine removes the borane from the known complexes (C6F5)(3)B-pyrrole (1) and (C6F5)(3)B-indole (2), with formation of free pyrrole or indole, giving the more stable adduct (C6F5)(3)B-pyridine (3). The competition between pyridine and 7-azaindole for the coordination with B(C6F5)3 again yields 3. The molecular structures of compounds 4 and 5 have been determined both in the solid state and in solution and compared to the structures of other (C6F5)(3)B-N-heterocycle complexes. Two dynamic processes have been found in compound 4. Their activation parameters (Delta H-double dagger= 66 (3) kJ/mol, Delta S-double dagger= -18 (10) J/mol K and Delta H-double dagger= 76 (5) kJ/mol, Delta S-double dagger= -5 (18) J/mol K) are comparable with those of other (C6F5)(3)B-based adducts. The nature of the intramolecular interactions that result in such energetic barriers is discussed

Oxygen-bridged borate anions from tris(pentafluorophenyl)borane: Synthesis, NMR characterization, and reactivity

The previously known anion [(C6F5)(3)B(mu-OH)B(C6F5)(3)](-) (2) has been prepared by a two-step procedure, involving deprotonation of (C6F5)(3)BOH2 to give [B(C6F5)(3)OH](-) (1), followed by addition of B(C6F5)(3). The solution structure and the dynamics of 2 have been investigated by H-1 and F-19 NMR spectroscopy. The reaction of [NHEt3]2 with NEt3 resulted in the formation of [NHEt3](+)[(C6F5)(3)BOH](-), [NHEt3](+)(C6F5)(3)[BH](-), and (C6F5)(3)B-(CH2CH=N+Et2). This indicates that in the presence of a nucleophile anion 2 can dissociate to B(C6F5)(3) and 1. The reaction of [HDMAN]2 with 1,8-bis(dimethylamino) naphthalene (DMAN) confirmed this trend. In the presence of water, 2 transformed into the adduct [(C6F5)(3)BO(H)(HO)-O-...(H)B(C6F5)(3)](-) (3), containing the borate 1 hydrogen-bonded to a water molecule coordinated to B(C6F5)(3). The same compound is formed by treating (C6F5)(3)BOH2 with 0.5 equiv of a base. A competition study established that for 1 the Lewis acid-base interaction with B(C6F5)3 is about 5 times preferred over H-bonding to (C6F5)(3)BOH2. The X-ray single-crystal analysis of [2-methyl-3H-indolium]3 provided the first experimental observation of an asymmetric H-bond in the [H3O2](-) moiety, the measured O-H and (HO)-O-... bond distances being significantly different [1.14(2) vs 1.26(2) angstrom]. The reaction of NEt3 with an equimolar mixture of B(C6F5)(3) and bis(pentafluorophenyl)borinic acid, (C2F5)(5)BOH, afforded the novel borinatoborate salt [NHEt3](+)[(C6F5)(3)BOB(C6F5)(2)](-) ([NHEt3]4). X-ray diffraction showed that the B-O bond distances are significantly shorter than in [(C6F5)(3)B(mu-OH)B(C6F5)(3)](-). Variable-temperature F-19 NMR revealed high mobility of the five aryl rings, at variance with the more crowded anion 2. 2D NMR correlation experiments showed that in CD2Cl2 the two anions [(C6F5)(3)BOH](-) and [(C6F5)(3)BH](-) form tight ion pairs with [NHEt3](+), in which the NH proton establishes a conventional ((BOHN)-H-...) or an unconventional ((BHHN)-H-...), respectively, hydrogen bond with the anion. The diborate anions 2-4, on the contrary, gave loose ion pairs with the ammonium cation, due both to the delocalized anionic charge and to the more sterically encumbered position of the oxygen atoms that should act as H-bond acceptors

Synthesis and reactivity of (C6F5)3B-N-heterocycle complexes. 1. Generation of highly acidic sp3 carbons in pyrroles and indoles

The reaction of pyrroles and indoles with B(C6F5)3 and BCl3 produces 1:1 B-N complexes containing highly acidic sp3 carbons, for example, N-[tris(pentafluorophenyl)borane]-5H-pyrrole (1) and N-[tris(pentafluorophenyl)borane]-3H-indole (2), that are formed by a new formal N-to-C hydrogen shift, the mechanism of which is discussed. With some derivatives, restricted rotation around the B-N bond and/or the B-C bonds was observed by NMR techniques, and some rotational barriers were calculated from experimental data. The acidity of the sp3 carbons in these complexes is shown by their ability to protonate NEt3, with formation of pyrrolyl- and indolyl-borate ammonium salts. The driving force for this reaction is given by the restoration of the aromaticity of the heterocycle