The Cyclooctatriene-η2-ynyl Potassium Zwitterionic Radical: Evidence for a Potassium Organometallic

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Abstract
Low-temperature (−120 °C) dehydrohalogenation of bromocyclooctatetraene (BrC8H7) with either sodium or potassium tert-butoxide followed by alkali metal reduction was used to generate the anion radical of [8]annulyne (C8H6•-) in tetrahydrofuran. EPR analysis at −120 °C reveals an extraordinarily large metal splitting when K or Cs (aK of 0.214 G and aCs of 3.26 G) serves as the reducing agent. The large aM is due to the metal cation interacting with the p-orbitals, within the alkyne moiety, that are in the plane of the ring system. The ionic radius of K+ is 1.33 Å, which is larger than the B3LYP predicted distance between carbons 1 and 2 (1.23 Å). However, the ionic radius of Na+ is only 0.95 Å, and it is too small to simultaneously interact with both p-orbitals. Hence, no aM is observed when Na (ordinarily aNa ≫ aK) or Li serves as the reducing agent. After the addition of 18-crown-6 to either the K or the Cs reduced system, two anion radicals are present. One is the system where the 18-crown-6 encapsulated metal complex is normally ion paired over the face of the ring system and aM = 0. The other is the cyclooctatriene-η2-ynyl 18-crown-6 encapsulated metal zwitterion radical exhibiting a large aM. The ion pair to organometallic equilibrium constant is 1.6 ± 0.1 and 3.5 ± 0.1 for the K and Cs systems, respectively.