Electronic Spectra of the Tetraphenylcyclobutadienecyclopentadienylnickel(II) Cation and Radical

Abstract

Properties of the tetraphenylcyclobutadienecyclopentadienylnickel(II) cation 1 and its tetra-o-fluoro derivative 1a have been measured and calculated. The B3LYP/TZP optimized geometry of the free cation 1 agrees with a single-crystal X-ray diffraction structure except that in the crystal one of the phenyl substituents is strongly twisted to permit a close-packing interaction of two of its hydrogens with a nearby BF4(-) anion. The low-energy parts of the solution electronic absorption and magnetic circular dichroism (MCD) spectra of 1 and 1a have been interpreted by comparison with TD-DFT (B3LYP/TZP) results. Reduction or pulse radiolysis lead to a neutral 19-electron radical, whose visible absorption and MCD spectra have been recorded and interpreted as well. The reduction is facilitated by ∼0.1 V upon going from 1 to 1a. Unsuccessful attempts to prepare several other aryl substituted derivatives of 1 by the classical synthetic route are described in the Supporting Information .

Magnetic circular dichroism of an unaromatic planar [8]annulene

“…The ADF program package offers an opportunity to perform a TDDFT calculation of MCD spectra and successful results for many compounds have been published, but, as noted above, its single‐reference approach cannot be used at or very near the D 8 h ring geometries, where 1 and 2 are perfect or nearly perfect biradicals. The spectra of 2 calculated by the TD‐B3LYP/ATZP method are shown in Figure and listed in Table .…”