Sterically Crowded Organometallics. Influence of Complexation upon the Conformation of Hexakis(phenylethyl)benzene

In order to determine the relative importance of intraligand versus ligand-metal steric repulsions in sterically crowded arenes with flexible substituents, the effect of complexation upon the conformation of hexakis(phenylethyl)benzene (1) has been studied. X-ray crystallographic characterization of the free ligand and the hexafluorophosphate salt of its previously known complex with [Fe(C₆H₅)]⁺, 2, reveals that, whereas the ligand itself adopts the expected alternating three-up/three-down conformation, complexation forces an energetically unfavorable six-up (distal) conformation. To our knowledge, the latter represents the first example of such a conformation being exclusively adopted in the solid state. Furthermore, a VT-NMR study of 2 reveals no evidence of dynamic behavior, suggesting that the six-distal conformation also persists in solution. These results are discussed in the context of more widely studied hexaethylbenzene analogues. Crystal data: 1, C₅₄H₅₄, triclinic, P1, a = 11.567 (5) Å, b = 12.115 (6) Å, c = 15.887 (8) Å, α = 86.96 (5)°, β = 77.30 (5)°, γ = 76.01 (5)°, Z = 2, R = 0.070; 2, C₅₉H₅₉F₆FeP, orthorhombic, Pn2₁a, a = 26.192 (9) Å, b = 18.447 (6) Å, c = 10.066 (7) Å, Z = 4, R = 0.074.