Microwave assisted solid-state synthesis of functional organotin carboxylates from sterically encumbered 3,5-di-tert-butylsalicylic acid

Microwave assisted solid-state reaction between equimolar quantities of sterically encumbered 3,5-di-tert-butylsalicylic acid (H2-DTBSA) and n-butylstannoic acid results in the formation of hexameric drum shaped stannoxane [nBuSn(O)(H-DTBSA)]6 (1). Synthesis of 1 could not be achieved under normal thermal conditions or mechanical grinding. However, the azeotropic removal of water produced in the reaction of nBu2SnO with 3,5-di-tert-butyl salicylic acid in benzene yielded the tetrameric ladder shaped stannoxane [{nBu2Sn(H-DTBSA)}2O]2 (2), which could also be synthesized in better yields by microwave irradiation as in the case of 1. Compounds 1 and 2 have been characterized by elemental analysis, IR, MALDI-MS and NMR (1H and 13C) spectroscopy. The structures of compound 1 and 2 are determined by single crystal X-ray diffraction techniques. Compound 1 is hexameric with a Sn6O6 drum core while compound 2 forms a ladder structure with three Sn2O2 rings, both decorated with –OH functionalities on the exterior of the polyhedral structure. While the formation of 1 from n-butylstannoic acid is straightforward, the formation of 2 from nBu2SnO (and not a cyclic structure similar to 3, where the phenolic oxygen also coordinates to tin) can be understood in terms of the increased steric hindrance in DTBSA for the phenolic protons to react with tin.