The boron trifluoride – base system: application of the coordination model of reaction mechanisms

Abstract

. Can. J. Chem. 71, 51 (1993). The mechanism of reaction of boron trifluoride with amines, dialkyl ethers, and pyridine has been analyzed on the basis of the coordination model of reaction mechanisms. This model is tested mathematically by carrying out kinetic simulations of pathways P(X,C), accompanied by the calculation of structures of postulated intermediates by GAUSSIAN86 methods. Analyses of reaction mechanisms, including calculation of concentration vs. time curves, are reported for the following systems: NH3-BF,, MeNH2-BF,, Me20-Et20-BF,, Me20-Et20:BF3, pyridine-BF3, pyridine-Me3N-BF3, F3B:N-N:BF3, and (N-N)BF2+. XIAOBO Ou, ROBERT WALLACE et ALEXANDER F. JANZEN. Can. J . Chem. 71, 51 (1993). Faisant appel au modkle de coordination des mkcanismes rkactionnels, on a analysk le mecanisme reactionnel du trifluorure de bore avec les amines, les ethers dialkylks et la pyridine. On a kvaluk le modkle d'une faqon mathematique en effectuant des simulations cinktiques des diverses voies rkactionnelles P(X,C), tout en calculant les structures des intermkdiaires prksumks par des methodes gaussiennes-86. On rapporte des analyses des mkcanismes reactionnels, y compris des calculs de concentration vs. temps, pour les systkmes suivants : NH3-BF3, MeNH2-BF3, Me20-Et20-BF,, Me,OEt,0:BF3, pyridine-BF3, pyridine-Me3N-BF,, F3B:N-N:BF3 et (N-N)BF,+.[Traduit par la rkdaction]The coordination model of reaction mechanisms (1-3) describes in a formal way the discrete changes in coordination number and electron count of individual atoms and reDresents the connectivity of atoms in multistep pathways in graphical form as pathway P(x,c)., In this paper we wish to describe a mathematical test of the coordination model by comparing calculated results with extensive experimental data that are available for the reactions of boron trifluoride with Lewis bases (4-6). The model assumes that intermolecular steps, + C , are limited by the rate of diffusion and that steps which decrease the coordination number, -C and -Cc, are slower and depend on the strength of the bond that is cleaved, but cyclic intramolecular n-center steps, +C', are statistically favoured, particularly in the case of 3-, 4-, 5-, and 6-center steps. Kinetic simulation of pathways P(X,C) is carried out by associating elementary rate constants with the four coordination number operators +C, -C, +Cc, and -Cc, while molecular orbital calculations are helpful in estimating relative rate constants for intermediates where experimental data are not available.Any mechanistic description of the BF,-base system must accommodate the f o l l o w~n~ experimental facts: (a) the formation of base:BF3 is a bimolecular process with rate constants in the range 1 0~-1 0~~ M-' s-' (7); (b) rates of reaction 'Presented at the 203rd ACS National Meeting, San Francisco, April 5-10, 1992. '~u t h o r to whom correspondence may be addressed. 3~otation: A graph P(X,C) consists of a vertex set X of reactants, intermediates, and products, and an edge set C of coordination number operators +c, -C, ...

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

“…The relative B-X bond strengths, B-F >> B-Cl > B-Br > B-I, are also important in favouring the loss of the heaviest halogen but this cannot be the sole effect because the same heavy halogen is lost much less readily from the unmixed rather than the mixed boron trihalide adducts. Because free fluoride is never displaced, halogen redistribution involving fluorine requires the presence of at least traces of uncomplexed Lewis acid so that a transitory B-F-B bridge can form (21,(38)(39)(40).…”

“…No similar "B peaks were observed in the corresponding quinuclidine.BCl,,I,-,, system (2) in which .rr bonding from the donor is not possible. Any such stabilization of pyridine-containing three-coordinate boron cations would be another factor favouring ready formation of the four-coordinate boron cations, since they could easily add another pyridine molecule to Can (38), this would be a factor favouring the observed facile formation of pyr2BF2+ even when Lewis acid is present in excess.…”

Boron‐Based Functional Additives Enable Solid Electrolyte Interphase Engineering in Calcium Metal Battery

“…Then, the fluoride ligand can leave the boron center forming CaF + , a similar mechanism proposed before for ligand exchange of BF 3 complexes, in which fluoride ligand exchange is favored by the collaborative effect of Lewis acid species that can accept the leaving F À ligand. [26] The energy increase for this reaction is significantly lower than in the previous case (43 kJ mol À 1 ) which is compensated by the precipitation of solid CaF 2 . Indeed, the formation of crystalline CaF 2 and its precipitation is a highly exothermic process (the heat of sublimation for CaF 2 is 419 kJ mol À 1 ) leading to an overall negative enthalpy of reaction and indicating a thermodynamic preference for fluoride ligand exchange whenever Ca 2 + ions are present in the solution.…”

Silicon−Fluorine and Silicon−Carbon Bond Cleavage in Organofluorosilicates: A Molecular Orbital Study

“…A three-stage method of analyzing reaction mechanisms has been proposed recently. ,, This method requires, first, the construction of a reaction pathway P( X , C ) which defines each elementary step of a reaction mechanism and specifies the connectivity of atoms along that pathway, taking into account their coordination number and electron count. Second, the pathway P( X , C ) is tested by carrying out molecular orbital calculations of all postulated intermediates, and that is the subject of this paper with regard to the mechanisms of Schemes and .…”

“…Finally, kinetic simulations of the proposed pathways P( X , C ) may be carried out by associating elementary rate constants with the four elementary steps of a reaction, i.e. , +C, −C, +C c , −C c , as illustrated previously for the BF 3 -base system …”