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

Abstract

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 [ⁿ BuSn(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.