DFT study of the hydrolysis reaction in atranes and ocanes: the influence of transannular bonding.

Abstract

Thermochemical kinetics of hydrolysis reactions of compounds with transannular intramolecular M…N bonds, i.e., atranes RM(OCH2CH2)3N and ocanes R2M(OCH2CH2)2NH (M = Si, Ge; R = F, Cl, Me), is studied at the B3LYP/aug-cc-pVDZ theoretical level. Several DFT methods are assessed for the reproduction of the experimental activation barrier for the Si-O bond cleavage of 1-methylsilatrane. Activation barriers for atranes and ocanes show the tendency for their growth with the decrease of the electronegativity of a substituent R on going from F to Me and their decrease from Si to Ge. Hydrolysis activation barriers of atranes and ocanes are compared with those of their acyclic analogs RM(OCH3)3 and R2M(OCH2)2NH in order to study the role of transannular M…N bonds in the stability of these molecules to hydrolysis. Substantially larger barriers for atranes support the opinion that stability of atranes may be explained by the formation of intramolecular bonds; however, the strengthening of transannular M…N bonds results in lower M-O cleavage barriers. It was proposed that the M-O cleavage barrier height is determined not by a weak M…N bonding itself, but rather by the contribution of a nitrogen lone pair to the antibonding orbitals of M-O bonds. The NBO analysis show that this interaction increases with the decrease of the electronegativity of a substituent R and decreases on going from atranes to ocanes. In ocanes, the presence of M…N bonds does not kinetically hinder the hydrolytic process; M-O cleavage activation barriers for acyclic analogs are higher. M-Hal cleavage barriers are substantially higher than those for M-O cleavage for R = F, but lower for R = Cl. Graphical Abstract The experimental barrier height of the Si-O bond cleavage in 1-methylsilatrane is well reproduced when three explicit water molecules are included in the B3LYP/aug-cc-pVDZ theoretical model.