The Role of Water in the Oligomerization Equilibria Involving Bis(pentafluorophenyl)borinic Acid in Dichloromethane Solution
Articolo
Data di Pubblicazione:
2004
Citazione:
The Role of Water in the Oligomerization Equilibria Involving Bis(pentafluorophenyl)borinic Acid in Dichloromethane Solution / T. Beringhelli, G. D’Alfonso, D. Donghi, D. Maggioni, P. Mercandelli, A.A.D. Sironi. - In: ORGANOMETALLICS. - ISSN 0276-7333. - 23:23(2004 Nov 08), pp. 5493-5502.
Abstract:
The 1H, 19F, and 11B NMR data indicated that in CD2Cl2 solution monomeric bis(pentafluorophenyl)
borinic acid, (C6F5)2BOH (1m), is in equilibrium with the cyclic trimer (1t)
observed in the solid state. The position of the association equilibrium shifted to the right
on increasing the concentration, on decreasing the temperature, and on decreasing solvent
polarity, in the series CD2Cl2, CDCl3, CCl4, in agreement with the higher polarity of the
monomer (2.38 D for 1m and 0.65 D for 1t, according to PM3 computations). At temperatures
lower than 210 K the 1H and 19F NMR spectra revealed the simultaneous (reversible)
formation of two novel compounds, which have been formulated as the (C6F5)2BOB(C6F5)2
anhydride (2) and the trimeric species [(C6F5)2BOH]3âOH2 (3), of C2 symmetry, with a water
molecule formally inserted into a B-O(H)-B bridge of 1t, to give a very strong BO(H)âââHO(H)B
hydrogen bond (ä 18.6). 1H and 19F EXSY experiments at 184 K revealed exchange between
3 and 1m, and not 1t. The data showed that the formation of 3, observed at temperatures
where the monomer-trimer equilibrium is frozen, occurs by aggregation of monomeric units
and not by cycle opening from 1t. The stabilization of the water molecule in 3 is strong
enough to promote the dehydration of 1 to give the anhydride 2; for entropic reasons, the
reaction occurs only at very low temperatures and is reversed on raising the temperature.
At higher temperatures, the position of the monomer-trimer equilibrium is affected by the
amount of water, which stabilizes the trimeric form, owing to the formation of a hydrogenbond
adduct 4 containing exocyclic water. At low temperatures, in the presence of the
monomer, this species progressively dehydrated, due to the formation of 3. The amount of
water present in solution also affected the rate of attainment of the 1m/1t equilibrium, the
oligomerization being exceedingly slow in anhydrous conditions. The catalytic role of water
can be attributed to the increased nucleophilicity of the BOH group upon water coordination,
which allows alternative aggregation pathways. Semiempirical computations, at the PM3
level, provided a picture of the oligomerization in the presence and in the absence of water
that well agrees with the experimental findings.
Tipologia IRIS:
01 - Articolo su periodico
Elenco autori:
T. Beringhelli, G. D’Alfonso, D. Donghi, D. Maggioni, P. Mercandelli, A.A.D. Sironi
Link alla scheda completa: