Sterically Crowded Organometallics. 2. Influence of Complexation and Intramolecular Edge-to-Face Aromatic-Aromatic Interactions upon the Solid-State Conformational Preference of Arboroles

The solid-state conformations of the prototypal doubly branched “arborole” molecule 1,2,4,5-C₆H₂{CH(CH₂Ph)₂}₄, 4, and its FeCp⁺ complex, 3a, have been determined via single-crystal X-ray crystallography. 4 adopts a conformation with two of the eight benzyl groups roughly in the plane of the central C₆ ring and opposite the unsubstituted C₆ ring carbon atoms. The other six benzyl groups are oriented away from the central ring in a manner that suggests edge-to-face aromatic–aromatic interactions between phenyl rings on branches attached to adjacent ring carbon atoms (CH⋯C₆ plane distances of 2.72 Å). 3a adopts a conformation with four benzyl groups distal to the FeCp⁺ moiety and four proximal. The proximal benzyl groups are roughly coplanar with each other and the central C₆ ring whereas the distal groups are oriented toward each other and away from the central C₆ plane. Once again edge-to-face aromatic-aromatic interactions appear to be occurring, between distal phenyl rings attached to branches on adjacent carbon atoms (CH⋯C₆ plane distances of 2.57 and 2.61 Å). Steric crowding and/or electronic effects in 3a appear to manifest themselves by causing a clear distortion in the C₆ ring toward a boat conformation. The conformations observed in 3a and 4 therefore appear to limit the ability of this particular arborole to act as a host via encapsulation of small molecules. Indeed, both 3a and 4 crystallize with acetone solvate molecules that do not interact with the arborole. Crystal data: 3a, C₇₁H₆₇FePF₆·3(acetone), monoclinic, P2₁, a = 13.100 (6) Å, b = 18.972 (10) Å, c = 15.542 (4) Å, β = 112.42 (3)°, Z = 2, R = 0.060; 4, C₆₆H₆₂·acetone, monoclinic, A2/a, a = 17.584 (5) Å, b = 12.3220 (18) Å, c = 24.313 (9) Å, Z = 4, R = 0.051.