Isolable Tin(II) Hydrides Featuring Sterically Undemanding Pincer-Type Ligands and Their Dehydrogenative Sn–Sn Bond Forming Reactions to Distannynes Promoted by Lewis Acidic Tri-<i>sec</i>-butylborane

Abstract

Unlike isolable tin(II) hydrides supported by bulky ligands reported in the literature, this research describes the synthesis and characterization of thermally stable tin(II) hydrides <b>L</b>^<b>Ph</b><b>SnH</b> (<b>1-H</b>) and ^<b>Me</b><b>LSnH</b> (<b>2-H</b>) stabilized by sterically undemanding N,N,N-coordinating pincer-type ligands (<b>L</b>^<b>Ph</b> = 2,5-dipyridyl-3,4-diphenylpyrrolato; ^<b>Me</b><b>L</b> = 2,5-bis(6-methylpyridyl)pyrrolato). The results from previous reports reveal that attempts to access tin(II) hydrides containing less-bulky ligands have had limited success, and decomposition to tin(I) distannynes often occurs. The key to the successful isolation of <b>1-H</b> and <b>2-H</b> is the identification of the role of Lewis acidic B^<i>s</i>Bu₃, generated upon delivering hydride from commonly used hydride reagents M[B^<i>s</i>Bu₃H] ("selectrides", M = Li or K). This study details compelling experimental evidence and theoretical results of the role played by B^<i>s</i>Bu₃, which catalyzes the dehydrocoupling reactions of <b>1-H</b> and <b>2-H</b> to yield tin(I) distannynes <b>L</b>^<b>Ph</b><b>Sn-SnL</b>^<b>Ph</b> (<b>1</b>_<b>2</b>) and ^<b>Me</b><b>LSn-Sn</b>^<b>Me</b><b>L</b> (<b>2</b>_<b>2</b>) with the liberation of H₂. To avoid the interference of B^<i>s</i>Bu₃, <b>1-H</b> and <b>2-H</b> can be isolated in pure forms using pinacolborane as the hydride donor with <b>L</b>^<b>Ph</b><b>SnOMe</b> (<b>1-OMe</b>) and ^<b>Me</b><b>LSnOMe</b> (<b>2-OMe</b>) as reactants, respectively. DFT calculations and experimental observations indicate that the coordination of the Sn-H bond of <b>1-H</b> to B^<i>s</i>Bu₃ leaves an electrophilic tin center, rendering the nucleophilic attack by the second equivalent of <b>1-H</b> forming a Sn-Sn bond.