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

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-Pr-i(2))(2)) or Ar* (Ar* = C6H3-2,6(C6H2-2,4,6-Pr-i(3))(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-Pr-i(2))(2), X = H, F, Cl, OMe, Bu-t, SiMe3, GeMe3) and 3,5-Pr-i(2)-Ar' or Ar* and 3,5-Pr-i(2)-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-Pr-i(2)-Ar*) or the distannynes ArSnSnAr (Ar = 4-F-Ar', 4-Cl-Ar', 4-MeO-Ar', 4-Bu-t-Ar', 4-SiMe3-Ar', 4-GeMe3-Ar', 3,5-Pr-i(2)-Ar*), which were characterized structurally and spectroscopically. The digermynes display planar trans-bent core geometries with Ge-Ge distances near 2.26 angstrom and bending angles near 128 degrees consistent with Ge-Ge multiple bonding. In contrast, the distannynes had either multiple bonded geometries with Sn-Sn distances that averaged 2.65 angstrom and an average bending angle near 123.8 degrees, or single bonded geometries with a Sn-Sn bond length near 3.06 angstrom and a bending angle near 98 degrees. The 3,5-Pr-i(2)-Ar*SnSnAr*-3,5-Pr-i(2) species had an intermediate structure with a longer multiple bond near 2.73 angstrom and a variable torsion angle (14-28 degrees) between the tin coordination planes. Mossbauer data for the multiple and single bonded species displayed similar isomer shifts but had different quadrupole splittings.