Synthesis and thermolytic behavior of tin(IV) formates: In search of recyclable metal-hydride systems

The synthesis and characterization of the series of organotin formates together with their thermolytic behavior are described. The diformate Bu ⁿ₂Sn{OC(O)H}₂ (1) was synthesized by the reaction of Buⁿ₂SnH₂ with formic acid. The triorganotin monoformate compounds R₃SnOC(O)H (R = Cy (cyclohexyl)) 3, Mes, (mesityl, 2,4,6-trimethylphenyl) 4, and Dmp (2,6-dimethylphenyl 5) were obtained by the reaction of R₃SnOH with formic acid. Their X-ray crystal structures along with that of the previously reported formate (PhCH ₂)₃SnOC(O)H (2) were determined. The diformate 1 exhibits an extended two-dimensional polymeric structure in which six-coordinate tin centers are linked by formate bridges. The tribenzyltin formate (2) possesses a chain structure in which the five-coordinate Sn(CH₂Ph)₃ units are bridged by formate ions. The cyclohexyl derivative 3 was observed to have a similar structure. In contrast, the Mes and Dmp derivatives 4 and 5 support monomeric structures in which the four-coordinate tin atom is bound to an oxygen of the formate ligand. Heating the compounds in various high boiling solvents produced no decomposition up to 120 °C in the case of 1 and refluxing a solution of 2 or 3 in mesitylene or diglyme left the starting material mostly unchanged, although 3 decomposed to an insoluble orange solid in refluxing decalin. In contrast, the heating of 4 and 5 in refluxing mesitylene led to elimination of CO to give the tin hydroxides. The results are in contrast to the known thermolytic behavior of R₃SnOC(O)H (R = Prⁿ or Buⁿ) complexes, which eliminate CO₂ to generate R ₃SnH. Compounds 3-5 are rare examples of structurally characterized tin formates.