PENTAISOPROPYLCYCLOPENTADIENYL - SINGLET ANION, DOUBLET RADICAL, AND TRIPLET CATION OF A CARBOCYCLIC PI-SYSTEM

Chemical or electrochemical oxidation of pentaisopropylcyclopentadieny lsodium yields yellow-green crystals of the pentaisopropylcyclopentadi enyl radical (1), which is very air sensitive under formation of tetra isopropylcyclopentadienone (2). 1 has been characterized by EPR/ENDOR, proton NMR spectroscopy, and mass spectrometry. X-ray structural anal ysis of 1 reveals stacks of eclipsing, equally spaced, planar five-mem bered rings. Cyclovoltammetric measurements as well as a low-energy fi rst PES ionization band of 1 at 7.02 eV are in accordance with facile generation of the pentaisopropylcyclopentadienyl cation 5 from pentais opropylcyclopentadienyl chloride (4) or bromide (3) and antimony penta chloride. In accordance with MNDO/CI calculations cation 5 is a ground -state triplet. Its EPR spectrum at 115 K shows a zero field splitting parameter D = 0.1615(4) cm-1 with coincidence of the x and y lines, i ndicating no detectable distortions from axial symmetry and only weak electron-delocalizing properties of the isopropyl substituents.