Substituent effects in ditetrel alkyne analogues: multiple vs. single bonded isomers

Abstract

The synthesis and characterization of a series of digermynes and distannynes stabilized by terphenyl ligands are described. The ligands are based on the Ar′ (Ar′ = C₆H₃-2,6(C₆H₃-2,6-ⁱPr₂)₂) or Ar* (Ar* = C₆H₃-2,6(C₆H₂-2,4,6-ⁱPr₃)₂) platforms which were modified at the meta or para positions of their central aryl rings to yield 4-X-Ar′ (4-X-Ar′ = 4-X-C₆H₂-2,6(C₆H₃-2,6-ⁱPr₂)₂, X = H, F, Cl, OMe, ^tBu, SiMe₃, GeMe₃) and 3,5-ⁱPr₂-Ar′ or Ar* and 3,5-ⁱPr₂-Ar*. The compounds were synthesized by reduction of the terphenyl germanium(II) or tin(II) halide precursors with a variety of reducing agents. The precursors were obtained by the reaction of one equivalent of the lithium terphenyl with GeCl₂ dioxane or SnCl₂. For germanium, their X-ray crystal structures showed them to be either Ge–Ge bonded dimers with trans-pyramidal geometries or V-shaped monomers. In contrast, the terphenyl tin halides had no tin–tin bonding but existed either as halide bridged dimers or V-shaped monomers. Reduction with a variety of reducing agents afforded the digermynes ArGeGeAr (Ar = 4-Cl-Ar′, 4-SiMe₃-Ar′ or 3,5-ⁱPr₂-Ar*) or the distannynes ArSnSnAr (Ar = 4-F-Ar′, 4-Cl-Ar′, 4-MeO-Ar′, 4-^tBu-Ar′, 4-SiMe₃-Ar′, 4-GeMe₃-Ar′, 3,5-ⁱPr₂-Ar′, 3,5-ⁱPr₂-Ar*), which were characterized structurally and spectroscopically. The digermynes display planar trans-bent core geometries with Ge–Ge distances near 2.26 Å and bending angles near 128° consistent with Ge–Ge multiple bonding. In contrast, the distannynes had either multiple bonded geometries with Sn–Sn distances that averaged 2.65 Å and an average bending angle near 123.8°, or single bonded geometries with a Sn–Sn bond length near 3.06 Å and a bending angle near 98°. The 3,5-ⁱPr₂-Ar*SnSnAr*-3,5-ⁱPr₂ species had an intermediate structure with a longer multiple bond near 2.73 Å and a variable torsion angle (14–28°) between the tin coordination planes. Mössbauer data for the multiple and single bonded species displayed similar isomer shifts but had different quadrupole splittings.