Influence of ring methylation in group 15 tetramethylcyclopentadienyl complexes, M(C₅Me₄H)_nI_3-n (M = As, Sb)

Arsenic and antimony triiodide react with 3 equiv of Na[C ₅Me₄H] in THF to form the yellow tris(tetramethylcyclopentadienyl) derivatives M(C₅Me ₄H)₃ in 61% and 76% yields, respectively. When equimolar amounts of SbI₃ and Na[C₅Me₄H] react in THF, the orange diiodo species Sb(C₅Me₄H)I₂ is produced in 87% yield. The effect on the complexes of the methylated cyclopentadienyl rings varies from minimal to substantial. For example, the properties of the antimony compound Sb(C₅Me₄H)₃ are similar to those of the unsubstituted complex Sb(C₅H ₅)₃; in contrast, As(C₅Me₄H) ₃ and Sb(C₅Me₄H)I₂ are considerably more thermally stable than their parent versions. The complexes are fluxional on the ¹H NMR time scale at room temperature. A single-crystal X-ray structure of As(C₅Me₄H)₃ reveals three σ-bound rings with strongly localized bonding. The As-C distances are nearly identical, averaging 2.033(3) Å, and the C-As-C angles range from 96.4° to 109.3°. The antimony structure is similar, with average Sb-C bonds of 2.238(6) Å and C-Sb-C angles from 93.6° to 107.7°. The diiodo complex Sb(C₅Me₄H)I₂ displays an η³-bound ring and forms a coordination polymer in the solid state, with ordering driven by intermolecular I⋯I′ and Me⋯Me′ van der Waals attractions. The structures of As(C ₅H₅)₃, As(C₅Me₄H) ₃, and Sb(C₅H₅)₃ were examined with density functional theory calculations; these indicate that the energy barrier for haptotropic rearrangements is roughly the same in the arsenic compounds, 12-15 kcal mol^-1, but is about half that for the antimony complex.