η⁵-Cyclohexadienyltricarbonylchromium(0) Complexes from Addition of Carbon Nucleophiles to η⁶-Benzenetricarbonylchromium(0). Formation, Chemical and Spectroscopic Features, and X-ray Diffraction Analysis

The addition of reactive carbanions to η⁶-benzenetricarbonylchromium(0) produced η⁵-(6-alkylcyclohexadienyl)tri-carbonylchromium(O) anion complexes, as the lithium salts. Reaction with a variety of oxidizing agents (iodine, cerium(lV), oxygen) removes the hydrogen from C-6 and detaches the Cr(CO)₃ unit, to produce a substituted arene. Reaction with electrophiles (potential hydride acceptors) leads to cleavage of the carbon-carbon bond at C-6 with regeneration of η⁶-benzenetricar-bonylchromium(O). Solution spectral (¹H NMR, IR) data are consistent with an η⁵-cyclohexadienyl structure. X-ray diffraction analysis on the crystalline adduct of η⁶-benzenetricarbonylchroinium(0) and 2-lithio-1,3-dithane defines the mode of addition (exo), the deformation (38.6°) of C-6 from the plane of carbons 1-5 in the cyclohexadienyl Iigand. and the preferred conformation of the Cr(CO)₃ unit.