Comparison of Twists in Isosteric Propellers: X-Ray Structures of Tris(2,6-dimethoxyphenyl)borane, Tris(2,6-dimethoxyphenyl)methyl Cation and Tris(2,6-dimethoxyphenyl)methyl Radical

Article abstract of DOI:10.1107/S0108768191014982

The X-ray structures of three per-o-methoxytriphenyl-X molecules were determined by direct methods.Tris(2,6-dimethoxyphenyl)borane (1), C24H27BO6, M_r = 422.3, monoclinic, C2/c, a = 11.076 (5), b = 20.839 (8), c = 9.944 (4) Angstroem, β = 98.40 (3) deg, V = 2271 (1) Angstroem³, Z = 4, D_x = 1.24 g cm^-3, λ(Mo Kα) = 0.71073 Angstroem, μ = 0.8 cm^-1, F(000) = 896, T = 295 K.Refinement based on 926 unique observed reflections and 170 parameters gave a final R(F_o) = 0.045.The tetrafluoroborate salt of the isoelectronic tris(2,6-dimethoxyphenyl)methyl cation (2), C25H27O6⁺*BF4^-, M_r = 510.3, triclinic, P<*>, a = 7.214 (5), b = 12.931 (4), c = 13.633 (3) Angstroem, α = 83.13 (2), β = 77.70 (3), γ = 80.56 (4) deg, V = 1221 (1) Angstroem³, Z = 2, D_x = 1.39 g cm^-3, λ(Cu Kα) = 1.5418 Angstroem, μ = 9.7 cm^-1, F(000) = 532, T = 110 K.Refinement based on 2393 unique observed reflections and 325 parameters gave a final R(F_o) = 0.056.Tris(2,6-dimethoxyphenyl)methyl radical (3), C25H27O6, M_r = 423.5, monoclinic, P2/n, a = 10.405 (1), b = 9.429 (2), c = 11.767 (2) Angstroem, β = 102.120 (9) deg, V = 1128.7 (6) Angstroem³, Z = 2, D_x = 1.25 g cm^-3, λ(Mo Kα) = 0.71073 Angstroem, μ = 0.8 cm^-1, F(000) = 450, T = 295 K.Refinement based on 1189 unique observed reflections and 142 parameters gave a final R(F_o) = 0.059.The borane (1) adopts an approximate D₃ conformation in the lattice at a site with crystallographic C₂ symmetry and aryl twist angles of 62.8 (2) and 64.18 (2) deg for the special and general rings.The cation (2) adopts a general position in the lattice with approximate C₂ symmetry.The aryl twist angles are 32.6 (2), 46.1 (2) and 48.9 (2) deg.The free radical (3) adopts a conformation that deviates markedly from the expected ground-state structure; the special ring on a crystallographic C₂ axis is twisted out of the plane of coordination by only 12.3 (5) deg while the general rings are twisted by 61.0 (1) deg.