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

The tin(II) hydride [Ar i Pr 6 Sn(μ-H)] 2 (Ar i Pr 6 = C 6 H 3 -2,6(C 6 H 2 -2,4,6- i Pr 3 ) 2 ) ( 1a ) reacts with 2 equiv of ethylene or t -butylethylene at ca. 25 °C to yield Sn 2 (Ar i Pr 6 ) 2 R 2 (R = ethyl or t -butylethyl), which exist either as a symmetric distannene Ar i Pr 6 (R)SnSn(R)Ar i Pr 6 ( 2a or 5a ) or an unsymmetric stannylstannylene Ar i Pr 6 SnSnR 2 Ar i Pr 6 ( 3a ). In contrast, the less crowded Sn(II) hydride [Ar i Pr 4 Sn(μ-H)] 2 (Ar i Pr 4 = C 6 H 3 -2,6(C 6 H 3 -2,6- i Pr 2 ) 2 ) ( 1b ) reacts with excess ethylene to give Ar i Pr 4 (CH 2 CH 3 ) 2 Sn(CH 2 CH 2 )Sn(CH 2 CH 3 )(CHCH 2 )Ar i Pr 4 ( 4 ) featuring five ethylene equivalents, one of which is dehydrogenated to an vinyl, −CH═CH 2, group. The Ar i Pr 4 isomers of 2a and 3a, i.e., [Ar i Pr 4 Sn(C 2 H 5 )] 2 ( 2b ) and Ar i Pr 4 SnSn(C 2 H 5 ) 2 Ar i Pr 4 ( 3b ) are obtained by reaction of [Ar i Pr 4 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(C 2 H 5 )SnSn(C 2 H 5 )Ar (Ar = Ar i Pr 6 ( 2a ) or Ar i Pr 4 ( 2b )) and ArSnSn(C 2 H 5 ) 2 Ar (Ar = Ar i Pr 6 ( 3a ) or Ar i Pr 4 ( 3b )) with Δ G ⧧ = 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(CH 2 CH 2 t Bu)] 2 (Ar = Ar i Pr 6 ( 5a ) or Ar i Pr 4 ( 5b )), obtained from 1a or 1b and t -butylethylene, dissociate to ArSnCH 2 CH 2 t Bu monomers in solution. At lower temperature, they interconvert with their stannylstannylene isomers with parameters K eq = 4.09 ± 0.16 for 5a and 6.38 ± 0.41 for 5b and Δ G eq = −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 Sn 2 RHAr 2 which has the structure Ar i Pr 6 Sn–Sn(H)(CH 2 CH 2 t Bu)Ar i Pr 6 ( 6a ) or the monohydrido bridged Ar i Pr 4 S n(μ-H)S n(CH 2 CH 2 t Bu)Ar i Pr 4 ( 6b ). The latter represents the first structural characterization of a monohydrido bridged isomer of a ditetrelene.