Reversible Coordination of H2 by a Distannyne.

The terphenyl tin(II) hydride [AriPr4Sn(μ-H)]2 (1) (AriPr4 = C6H3-2,6(C6H3-2,6-iPr2)2) was shown to form an equilibrium with the distannyne AriPr4SnSnAriPr4 (2) and H2 in toluene at 80 °C. The equilibrium constant and Gibbs free energy for the dissociation of H2 are 2.23 × 10-4 ± 4.9% and 5.89 kcal/mol ± 0.68%, respectively, by 1H NMR spectroscopy and 2.33 × 10-4 ± 6.2% and 5.86 kcal/mol ± 0.73%, respectively, by UV-vis spectroscopy, indicating that the hydride 1 is strongly favored. Further heating of 2 at ca. 100 °C afforded the known pentagonal-bipyramidal Sn7 cluster Sn5(SnAriPr4)2 (3). Mechanistic studies show that 3 is formed from distannyne 2, which is generated from 1. The order of the reaction for the conversion of 2 into 3 was found to be zero, and the rate constant is 1.77 × 10-5 M s-1 at 100 °C. Hydride 1 was further characterized by cyclic voltammetry, and its pKa was found to be 18.8(2) via titration with 1,8-diazabicyclo[5.4.0]undec-7-ene. The bond dissociation free energy was estimated to be 51.1 kcal/mol ± 3.4% on the basis of its pKa and reduction potential. Studies with deuterium indicate ready exchange of D2 with the hydrides in 1.