Reversible Coordination of H2 by a Distannyne

The terphenyl tin(II) hydride [AriPr(4)Sn(mu-H)](2) (1) (Ar(iP)r(4) = C6H3-2,6(C6H3-2,6-Pr-i(2))(2)) was shown to form an equilibrium with the distannyne Ar(iPr4)SnSnArr Pr-4 (2) and H-2 in toluene at 80 degrees C. The equilibrium constant and Gibbs free energy for the dissociation of H-2 are 2.23 X 10(-4) +/- 4.9% and 5.89 kcal/mol +/- 0.68%, respectively, by H-1 NMR spectroscopy and 2.33 X 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 degrees C afforded the known pentagonal-bipyramidal Sn-7 cluster Sn-5(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 X 10(-5) M s(-1) at 100 degrees C. Hydride 1 was further characterized by cyclic voltammetry, and its pK(a) 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 pK(a) and reduction potential. Studies with deuterium indicate ready exchange of D-2 with the hydrides in 1.