Methanol does not significantly affect the position and the rate of the equilibrium between monomeric (1m) and cyclic trimeric (1t) forms of bis(pentafluorophenyl)borinic acid Ar2BOH (1, Ar = C6F5) in CD2Cl2 solution. This contrasts with what previously observed with THF and it is mainly due to the stabilization of the 1m.MeOH covalent adduct through the formation of the hydrogen bonded [Ar2B(OH)(MeOH)]2 dimer (7), characterized in solution and by single crystal X-ray analysis. This dimer can be viewed as the intermediate in the conversion of 1 into its methyl ester 6, via fast proton transfer along the H-bond, that transforms the 1m.MeOH adduct into the 6.H2O adduct. Extrusion of H2O by water scavengers drives the equilibrium toward the ester 6. X-ray analysis showed that, at variance with 1, the ester 6 in solid is a monomer. Hindered rotation (H# = 38(1) kJ/mol, S# = 35(6) J mol-1) around the B-OMe bond has been observed, due to O-to-B -donation. This donation lowers the Lewis acidity of 6, as shown by the thermal instability of its adducts with MeOH and THF. Water addition to 6 affords the dimeric adduct 7, in fast equilibrium with both 1m and 6, even at 183 K, as revealed by 2D EXSY. Borinic acid itself can act as water scavenger at low temperature, so that 6 is formed also in the early stages of the titration of 1 with MeOH. The other main initial product was the [Ar2B(OH)]3.MeOH adduct (3) containing a MeOH molecule bound within an octaatomic –B O(H) B O(Me) H...O(H) B O(H) cycle. Fast proton transfer along the strong H-bond was revealed by 2D EXSY. At variance with THF, the adduct containing MeOH exocyclically hydrogen bonded to 1t could be obtained in high concentration at 183 K only when due allowance for the kinetic requirements of the slow trimerization equilibrium was made.

Hydrogen Bonding and Lewis Acid-Base Interactions in the System Bis(pentafluorophenyl)borinic Acid-Methanol / D. Donghi, D. Maggioni, T. Beringhelli, G. D’Alfonso, P. Mercandelli, A. Sironi. - In: EUROPEAN JOURNAL OF INORGANIC CHEMISTRY. - ISSN 1434-1948. - 10(2008), pp. 1645-1653. [10.1002/ejic.200701210]

Hydrogen Bonding and Lewis Acid-Base Interactions in the System Bis(pentafluorophenyl)borinic Acid-Methanol

D. Donghi
Primo
;
D. Maggioni
Secondo
;
T. Beringhelli;G. D’Alfonso;P. Mercandelli
Penultimo
;
A. Sironi
Ultimo
2008

Abstract

Methanol does not significantly affect the position and the rate of the equilibrium between monomeric (1m) and cyclic trimeric (1t) forms of bis(pentafluorophenyl)borinic acid Ar2BOH (1, Ar = C6F5) in CD2Cl2 solution. This contrasts with what previously observed with THF and it is mainly due to the stabilization of the 1m.MeOH covalent adduct through the formation of the hydrogen bonded [Ar2B(OH)(MeOH)]2 dimer (7), characterized in solution and by single crystal X-ray analysis. This dimer can be viewed as the intermediate in the conversion of 1 into its methyl ester 6, via fast proton transfer along the H-bond, that transforms the 1m.MeOH adduct into the 6.H2O adduct. Extrusion of H2O by water scavengers drives the equilibrium toward the ester 6. X-ray analysis showed that, at variance with 1, the ester 6 in solid is a monomer. Hindered rotation (H# = 38(1) kJ/mol, S# = 35(6) J mol-1) around the B-OMe bond has been observed, due to O-to-B -donation. This donation lowers the Lewis acidity of 6, as shown by the thermal instability of its adducts with MeOH and THF. Water addition to 6 affords the dimeric adduct 7, in fast equilibrium with both 1m and 6, even at 183 K, as revealed by 2D EXSY. Borinic acid itself can act as water scavenger at low temperature, so that 6 is formed also in the early stages of the titration of 1 with MeOH. The other main initial product was the [Ar2B(OH)]3.MeOH adduct (3) containing a MeOH molecule bound within an octaatomic –B O(H) B O(Me) H...O(H) B O(H) cycle. Fast proton transfer along the strong H-bond was revealed by 2D EXSY. At variance with THF, the adduct containing MeOH exocyclically hydrogen bonded to 1t could be obtained in high concentration at 183 K only when due allowance for the kinetic requirements of the slow trimerization equilibrium was made.
Fluoroarylboranes; Hydrogen bonds; Lewis acids; NMR spectroscopy; X-ray diffraction
Settore CHIM/03 - Chimica Generale e Inorganica
2008
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/47185
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