Synthesis, characterization and crystal structures of the boratranes: 2,10,11-trioxa-6-aza-1-boratricyclo[4.4.4.0] tetradecane (tri-n-propanolamine borate), and 3-(4-methoxy)phenoxymethyl-7,10-dimethyl-2,8,9-trioxa-5-aza-1-boratricyclo[3.3.3.0]-undecane

The crystal structures of boratranes 2,10,11-trioxa-6-aza-1-boratricyclo4.4.4.0^1,6] tetradecane (tri-n-propanolamine borate) 1 as the tri-hydrate, and 3-(4-methoxy)phenoxymethyl-7,10-dimethyl-2,8,9-trioxa-5-aza-1-boratricyclo3.3.3.0^1,5]-undecane 2 as the partial (0.2) hydrate have been determined. Compound 1 has a near-tetrahedral coordination of both the N (108.8°) and B atoms (111.2°), N → B bond length 1.656 Å and all-chair tricyclic conformation, whereas 2 has a slightly-longer N → B dative bond length (1.667 Å) and the O-B-O angle, 114.8°, was slightly distorted from near-tetrahedral to adopt a flatter conformation. Theoretical calculations on 1 and 2 showed that the B-N distance in each shortened markedly between isolated gas phase molecules and ‘solvated’ models. Neither structural results, nor calculated parameters, were able to explain the propensity towards slow hydrolysis of boratranes with five-membered rings compared with the relative hydrolytic stability of boratranes with six-membered rings.