Highly Asymmetric Coordination in Alkenes:  Gas-Phase Structures of trans-1,2-Dichloro-1,2-disilylethene and 1-Bromo-1-silylethene

Abstract

The molecular structures of trans-1,2-dichloro-1,2-disilylethene and 1-bromo-1-silylethene have been determined by gas-phase electron diffraction (GED) and ab initio molecular orbital calculations (MP2/6-311G). Both compounds were found to have highly asymmetric coordination around the carbon atoms with [ab initio (r(e))/GED (r(a))] C=C-Cl [117.0/117.0(2) degrees] and C=C-Si [126.2/128.1(1) degrees] in the C(2)(h) structure of trans-1,2-dichloro-1,2-disilylethene and C=C-Br [119.2/120.7(4) degrees] and C=C-Si [125.0/125.0(4) degrees] in the C(s) structure of 1-bromo-1-silylethene. Other important structural parameters for trans-1,2-dichloro-1,2-disilylethene are C=C [135.2/134.5(3) pm], C-Si [189.4/187.9(2) pm], and C-Cl [175.1/174.9(1) pm], and C=C [134.2/133.4(2) pm], C-Si [187.8/187.2(3) pm], and C-Br [191.3/191.0(3) pm] for 1-bromo-1-silylethene. Further ab initio calculations were carried out on CH(2)CRX and trans-(CRX)(2) (R = SiH(3), CH(3), or H; X = H, F, Cl, or Br) to gauge the effects of electron-withdrawing and electron-donating groups on the structures. They reveal some even more distorted structures. The asymmetric appearance of these molecules can largely be accounted for by valence shell electron pair repulsion theory.

Why are trimethylsilyl groups asymmetrically coordinated? Gas-phase molecular structures of 1-trimethylsilyl-1,2,3-benzotriazole and 2-trimethylsilyl-1,3-thiazole

“…For example, the Si-C=C angle in trans-1,2-dichloro-1,2-disilylethene is 128.1(1) • . 10 The observed angles in 4, with SiCN about 4 • smaller than SiCS, suggests that there is a displacement of the trimethylsilyl group towards the ring nitrogen atom by an amount broadly comparable to that in the benzotriazole.…”

“…∠Si(1)N(2)C(6) syn 134.2(10) 132.3 131.1 132.1 ∠Si(1)N(2)C(6) anti 138.2(10) 137.0 136.7 136.8 ∠N(2)N(3)N(4) 109.8(4) 111.7 109.8 110.5 ∠N(3)N(2)C(6) 109.2(3) 107.7 109.0 108.2 ∠N(3)N(4)C(5) 107.3(3) 108.3 107.9 108.4 ∠N(4)C(5)C(6) 110.0(2) 107.8 108.8 108.3 ∠N(2)C(6)C(5) 103.7(2) 104.6 104.5 104.6 ∠C(5)C(6)C(7) 121.6(5) 121.1 121.8 121.2 ∠C(…”