Jason S. Overby

SYNTHESIS AND CHARACTERIZATION OF CHROMOCENES CONTAINING BULKY CYCLOPENTADIENYL LIGANDS

Two equivalents of K[Cp′] (Cp′=C 5 ( i -Pr) 3 H 2 , C 5 ( i -Pr) 4 H, C 5 ( t -Bu) 2 H 3 ) react with CrCl 2 in THF to give the corresponding chromocenes, Cp′ 2 Cr, in good yield. Despite the presence of bulky substituents on the rings that could affect their properties, the complexes are extremely air- and moisture-sensitive, and possess a low-spin ground state. The low-spin paramagnetic nature of each chromocene was confirmed by magnetic susceptibility measurements. The solid-state structure of [C 5 ( i -Pr) 3 H 2 ] 2 Cr was obtained using single crystal X-ray analysis. It displays rigorously parallel rings, with an average Cr–C(ring) distance of 2.17(1)A; the chromium resides on a crystallographically imposed inversion center. The orientation of the isopropyl groups minimizes unfavorable steric interactions between the rings.

Steric influences on the stability and synthetic accessibility of tetraalkylcyclopentadienes

Abstract The phase transfer-catalyzed addition of cyclohexyl bromide to cyclopentadiene under basic conditions yields 2,3,5,5′-tetracyclohexylcyclopentadiene as the sole tetra-substituted isomer. An X-ray crystal structure reveals that some angle distortion exists around the quaternary carbon, including a compressed C 5 ring angle (99.4(5)°). The reason for the production of only the 2,3,5,5′ isomer from the seven possible for tetracyclohexylcyclopentadiene was probed with molecular mechanical and semiempirical MO calculations. The 2,3,5,5′ form was determined to be the lowest in strain energy, although not in the heat of formation. Close intramolecular H…H contacts are present in the cyclohexyl-substituted cyclopentadienes that are absent in isopropyl analogs.