Synthesis, characterization and crystal structures of the boratranes: 2,10,11-trioxa-6-aza-1-boratricyclo[4.4.4.01,6] tetradecane (tri-n-propanolamine borate), and 3-(4-methoxy)phenoxymethyl-7,10-dimethyl-2,8,9-trioxa-5-aza-1-boratricyclo[3.3.3.01,5]-unde

The crystal structures of boratranes 2,10,11-trioxa-6-aza-1-boratricyclo[4.4.4.01,6] 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-boratricyclo[3.3.3.01,5]-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.

Graphical abstractThe synthesis, spectroscopic properties and structures of two boratranes with five- or six-membered rings are reported. X-ray crystal structure results are compared with those of theoretical calculations, showing the importance of solvation/intermolecular contacts in controlling the length of the N → B interaction.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Crystal structures of five- and six-membered ring boratranes. ► N → B transannular bond lengths and quantum chemical calculations. ► Hydrolysis of boratranes invoking N-protonation concomitant with B–O bond reaction with water. ► Five-membered ring boratranes for controlled-release of boric acid biocide for biomaterials.