Dynamic Behavior And Isomerization Equilibria Of Distannenes Synthesized By Tin Hydride/Olefin Insertions: Characterization Of The Elusive Monohydrido Bridged Isomer

The tin(II) hydride [(ArSn)-Sn-iPr6(mu-H)](2)(Ar-iPr6 = C6H3-2,6(C6H2-2,4,6-Pr-i(3))(2)) (1a) reacts with 2,equiv of ethylene or t-butylethylene at ca. 25 degrees C to yield Sn-2(Ar-iPr6)(2)R-2(R = ethyl or t-butylethyl), which exist either as a symmetric distannene Ar-iPr6(R)SnSn(R)Ar-iPr6 (2a or 5a) or an unsymmetric stannylstannylene (ArSnSnR2AriPr6)-Sn-iPr6 (3a). In contrast, the less crowded Sn(II) hydride [(ArSn)-Sn-iPr4(mu-H)](2) (Ar-iPr4 = C6H3-2,6 (C6H3-2,6-(iPr)(2))(2)) (1b) reacts with excess ethylene to give Ar-iPr4(CH2CH3)(2)Sn(CH2CH2)Sn(CH2CH3)(CHCH2)Ar-iPr4 (4) featuring five ethylene equivalents, one of which is dehydrogenated to an vinyl, -CH=CH2, group. The Ar-iPr4 isomers of 2a and 3a, i.e., [(ArSn)-Sn-iPr4(C2H5)](2) (2b) and (ArSnSn)-Sn-iPr4(C2H5)(2)Ar-iPr4 (3b) are obtained by reaction of [(ArSn)-Sn-iPr4(mu-Cl)](2) with EtLi or EtIVIgBr. The isomeric pairs 2a and 3a are separated by crystallization at different temperatures. Variable-temperature H-1 NMR spectroscopy indicates fast ethyl group exchange between Ar(C2H5)SnSn(C2H5)Ar (Ar = Ar-iPr6 (2a) or Ar-iPr4 (2b)) and ArSnSn(C2H5)(2)Ar (Ar = Ar-iPr6 (3a) or Ar-iPr4 (3b)) with Delta G(double dagger) = 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((CH2CH2Bu)-Bu-t)](2) (Ar = Ar-iPr6 (5a) or Ar-iPr4 (5b)), obtained from 1a or 1b and t-butylethylene, dissociate to (ArSnCH2CH2Bu)-Bu-t 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 Delta 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 Sn2RHAr2 which has the structure (ArSn)-Sn-iPr6 Sn(H)((CH2CH2Bu)-Bu-t)Ar-iPr6 (6a) or the monohydrido bridged (ArSn)-Sn-iPr4(mu-H)Sn((CH2CH2Bu)-Bu-t)Ar-iPr4 (6b). The latter represents the first structural characterization of a monohydrido bridged isomer of a ditetrelene.