Understanding the Structure and Reactivity of Mixed Titanium(IV) Alkoxide and Tin(II)/(IV) Carboxylates as Esterification Catalysts

A set of new compounds are formed upon mixing of titanium­(IV) alkoxides with tin­(II) dicarboxylates or tin­(IV) dialkyl dicarboxylates. These mixed Ti/Sn catalysts outperform titanium alkoxides or tin complexes alone as polyesterification catalysts. However, the tin complexes employed are toxic, and efforts are underway to remove them from environmental circulation. This study elucidates the structures generated of mixed Ti/Sn complexes and how they lead to improved reactivity. A suite of characterization techniques was utilized in structural elucidation including ¹H, ¹³C­{¹H}, and ¹¹⁹Sn NMR, as well as ¹³C–¹H HSQC, ¹H–¹H COSY, ¹¹⁹Sn–¹H HMQC (heteronuclear multiple quantum coherence), DOSY (diffusion-ordered spectroscopy) NMR, and ASAP-MS (atmospheric solids analysis probe–mass spectrometry). These characterization techniques led to the identification of Sn–Ti heterobimetallic dimers, regardless of the tin source (viz., Sn­(II) or Sn­(IV)). The greater stability of the catalysts to agglomeration was identified ex situ by UV–vis spectroscopy by observing colloid formation. Probe reactions of Fischer esterification and transesterification were used to characterize catalyst robustness under reaction conditions and catalyst activity relative to pure Ti or Sn complexes. This set of techniques allows for characterization of nontrivial mixed esterification catalysts and will be able to be applied to nontoxic mixtures in the future as a step toward improving sustainable catalysis.