Reversible Coordination of H_2 by a Distannyne

摘要

The terphenyl tin(II) hydride [Ar~(iPr_(4))Sn(μ-H)]_(2) ( 1 ) (Ar~(iPr_(4)) = C_(6)H_(3)-2,6(C_(6)H_(3)-2,6-~( i )Pr_(2))_(2)) was shown to form an equilibrium with the distannyne Ar~(iPr_(4))SnSnAr~(iPr_(4)) ( 2 ) and H_(2) in toluene at 80 °C. The equilibrium constant and Gibbs free energy for the dissociation of H_(2) are 2.23 × 10~(–4) ± 4.9% and 5.89 kcal/mol ± 0.68%, respectively, by ~(1)H 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 Sn_(7) cluster Sn_(5)(SnAr~(iPr_(4)))_(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 p K _(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 p K _(a) and reduction potential. Studies with deuterium indicate ready exchange of D_(2) with the hydrides in 1 .