Dynamic Behavior and Isomerization Equilibria of Distannenes Synthesized by Tin Hydride/Olefin Insertions: Characterization of the Elusive Monohydrido Bridged Isomer.

The tin(II) hydride [Ari Pr6 Sn(μ-H)]2 (Ari Pr6 = C6 H3 -2,6(C6 H2 -2,4,6-i Pr3 )2 ) (1a) reacts with 2 equiv of ethylene or t-butylethylene at ca. 25 °C to yield Sn2 (Ari Pr6 )2 R2 (R = ethyl or t-butylethyl), which exist either as a symmetric distannene Ari Pr6 (R)SnSn(R)Ari Pr6 (2a or 5a) or an unsymmetric stannylstannylene Ari Pr6 SnSnR2 Ari Pr6 (3a). In contrast, the less crowded Sn(II) hydride [Ari Pr4 Sn(μ-H)]2 (Ari Pr4 = C6 H3 -2,6(C6 H3 -2,6-i Pr2 )2 ) (1b) reacts with excess ethylene to give Ari Pr4 (CH2 CH3 )2 Sn(CH2 CH2 )Sn(CH2 CH3 )(CHCH2 )Ari Pr4 (4) featuring five ethylene equivalents, one of which is dehydrogenated to an vinyl, -CH═CH2 , group. The Ari Pr4 isomers of 2a and 3a, i.e., [Ari Pr4 Sn(C2 H5 )]2 (2b) and Ari Pr4 SnSn(C2 H5 )2 Ari Pr4 (3b) are obtained by reaction of [Ari Pr4 Sn(μ-Cl)]2 with EtLi or EtMgBr. The isomeric pairs 2a and 3a are separated by crystallization at different temperatures. Variable-temperature 1 H NMR spectroscopy indicates fast ethyl group exchange between Ar(C2 H5 )SnSn(C2 H5 )Ar (Ar = Ari Pr6 (2a) or Ari Pr4 (2b)) and ArSnSn(C2 H5 )2 Ar (Ar = Ari Pr6 (3a) or Ari Pr4 (3b)) with ΔG⧧ = 14.2 ± 0.65 kcal mol-1 for 2a/3a and 14.8 ± 0.36 kcal mol-1 for 2b/3b. The bulkier distannenes [ArSn(CH2 CH2 t Bu)]2 (Ar = Ari Pr6 (5a) or Ari Pr4 (5b)), obtained from 1a or 1b and t-butylethylene, dissociate to ArSnCH2 CH2 t Bu monomers in solution. At lower temperature, they interconvert with their stannylstannylene isomers with parameters Keq = 4.09 ± 0.16 for 5a and 6.38 ± 0.41 for 5b and ΔGeq = -1.81 ± 0.19 kcal mol-1 for 5a and -1.0 ± 0.03 kcal mol-1 for 5b at 298 K. The 1:1 reaction of 1a or 1b with 5a or 5b yields the unknown monohydrido species Sn2 RHAr2 which has the structure Ari Pr6 Sn-Sn(H)(CH2 CH2 t Bu)Ari Pr6 (6a) or the monohydrido bridged Ari Pr4 S n(μ-H)S n(CH2 CH2 t Bu)Ari Pr4 (6b). The latter represents the first structural characterization of a monohydrido bridged isomer of a ditetrelene.