The Role of Water in the Oligomerization Equilibria Involving Bis(pentafluorophenyl)borinic Acid in Dichloromethane Solution

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]3·OH2 (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)···HO(H)B hydrogen bond (δ 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 hydrogen-bond 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.