Organic and organometallic derivatives of pentaphenylbenzene, C6Ph5X: correlation of peripheral phenyl ring orientations with the steric bulk of “X”

A series of C 6 Ph 5 X compounds, including X = H, Br, CO 2 H, CO 2 R, C[Formula: see text]CPh, cis-BrC=C(Br)Ph, 2-bornenyl, and ferrocenyl, have been characterized by use of X-ray crystallography. Also, the first organometallic complexes of pentaphenylbenzene have been prepared by reaction with chromium hexacarbonyl to yield (η 6 -C 6 Ph 5 H)Cr(CO) 3 complexes in which the metal tripod is attached either to an ortho peripheral ring or to the central ring. Crystalline pentaphenylbenzoic acid exists as a hydrogen-bonded dimer; however, the steric bulk of the substituents does not allow the carboxylic acid moieties to be linked directly but instead via two bridging methanol molecules. In the solid state, the orientation of the peripheral rings in bulky C 6 Ph 5 X systems is very sensitive to the size of the “X” substituents, such that the twist angle of the para ring responds inversely with increasing bulk of “X”, which drives the ortho rings farther out of the plane of the central ring. The relevance of these observations to correlated motion in molecular propellers, and ultimately to molecular machines, is discussed.