Water-coordinated neutral silane complex: a frozen intermediate of hydrolysis of alkoxysilanes

1-Aquo- and 1-ethyloxonio-5-carbasilatranes 2 and 3 were synthesized. X-ray crystallographic analysis revealed that 2 has a nearly ideal trigonal bipyramidal structure, in which the water molecule is located at the apical position. This water molecule is exchangeable with the other Lewis bases, such as H218O and HMPA. Theoretical calculations suggested that 2 would acquire the stabilization energy (20 kcal mol-1) by complexation. Deprotonation of 2 by pyridine and not by m-nitroaniline showed that the pKa value is smaller than that of free water, and that the water molecule is activated by complexation.