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

摘要

The tin(Ⅱ) hydride [Ar~iPr6)Sn(μ-H)]_2(Ar~(iPr6) = C_6H_3-2,6(C_6H_2-2,4,6-~iPr_3)_2) (la) reacts with 2 equiv of ethylene or t-butylethylene at ca. 25 ℃ to yield Sn_2(Ar~(iPr6)_2R_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 stannyl- stannylene Ar~(iPr6)SnSnR_2Ar~(iPr6) (3a). In contrast, the less crowded Sn(Ⅱ) hydride [Ar~(iPr4)Sn(μ-H)]_2 (Ar~(iPr4) = C_6H_3-2,6(C_6H_3- 2,6-~iPr_2)_2) (1b) reacts with excess ethylene to give Ar~(iPr4)(CH_2- CH_3)_2Sn(CH_2CH_2)Sn(CH_2CH_3)(CHCH_2)Ar~(iPr4) (4) featuring five ethylene equivalents, one of which is dehydrogenated to an vinyl, -CH=CH_2, group. The Ar~(iPr4) isomers of 2a and 3a, i.e., [Ar~(iPr4)Sn(C_2H_5)]_2 (2b) and Ar~(iPr4)SnSn(C_2H_5)_2Ar~(iPr4) (3b) are obtained by reaction of [Ar~(iPr4)Sn(μ-Cl)]_2 with EtLi or EtMgBr. The isomeric pairs 2a and 3a are separated by crystallization at different temperatures. Variable-temperature ~1H NMR spectroscopy indicates fast ethyl group exchange between Ar(C_2H_5)SnSn(C_2H_5)Ar (Ar = Ar~(iPr6) (2a) or Ar~(iPr4) (2b)) and ArSnSn(C_2H_5)_2Ar (Ar = Ar~(iPr6) (3a) or Ar~(iPr4) (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(CH_2CH_2~tBu)]_2 (Ar = Ar~(iPr6) (5a) or Ar~(iPr4) (5b)), obtained from 1a or 1b and t-butylethylene, dissociate to ArSnCH_2CH_2~tBu 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 ΔG~(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 mono- hydrido species Sn_2RHAr_2 which has the structure Ar~(iPr6)Sn-Sn(H)(CH_2CH_2~tBu)Ar~(iPr6) (6a) or the monohydrido bridged Ar~(iPr4)Sn(μ-H)Sn(CH_2CH_2~tBu)Ar~(iPr4) (6b). The latter represents the first structural characterization of a monohydrido bridged isomer of a ditetrelene.