The Doubly Oxidized, Antiaromatic Tetraphenylporphyrin Complex [Li(TPP)][BF 4 ] Julie A. Cissell, ‡ Thomas P. Vaid,* ,‡ and Glenn P. A. Yap § Center for Materials InnoVation and Department of Chemistry, Washington UniVersity in St. Louis, St. Louis, Missouri 63130, and Department of Chemistry and Biochemistry, UniVersity of Delaware, Newark, Delaware 19716 Vaid@wustl.edu Received March 30, 2006 ABSTRACT Oxidation of dilithium tetraphenylporphyrin, Li 2 (TPP), yields the doubly oxidized porphyrin complex [Li(TPP)][BF 4 ]. Free TPP appears to be unstable. The crystal structure of [Li(TPP)][BF 4 ] reveals that the 16-atom C-N inner ring of the porphyrin consists of alternating single and double bonds. NMR spectroscopy and nucleus-independent chemical shift (NICS) calculations, in addition to the distortion from planarity observed in the crystal structure, indicate that the 16 π-electron inner C-N ring of the porphyrin is antiaromatic. In the vast majority of known porphyrin complexes the oxidation state of the porphyrin ring system is 2-. We recently reported the first crystallographic characterization of a porphyrin complex with a ring system in the 3- oxidation state, the aluminum tetraphenylporphyrin radical, Al(TPP)(THF) 2 . 1 In another recent communication we de- scribed the antiaromatic Si(TPP)(THF) 2 , a porphyrin complex with a ring system in the 4- oxidation state. 2 The one reported example of a neutral porphyrin complex with a ring system in the 1- oxidation state is Li(TPP), 3 and there are several examples of porphyrin radical cations with ring systems in the 1- oxidation state. 4 The existence of stable porphyrin complexes with ring systems in the 4-,3-,2-, and 1- oxidation states led us to explore the possibility of isolating a porphyrin with a ring system in the zero oxidation state. We were encouraged by the fact that many porphyrin complexes can be reversibly oxidized by two electrons in cyclic voltammetry experiments. 5 Our interest has been in neutrally charged, oxidized or reduced molecules as isos- tructural dopants for molecular semiconductors, 6 and in the case of tetraphenylporphyrin the doubly oxidized, neutral species would be simply “TPP”, which contains no central metal atom or hydrogens. Li(TPP) was synthesized by the oxidation of Li 2 (TPP) with ferrocenium hexafluorophosphate. 3 Cyclic voltammetry of Li(TPP) in CH 2 Cl 2 showed that it is reversibly oxidized at ‡ Washington University in St. Louis. § University of Delaware. (1) Cissell, J. A.; Vaid, T. P.; Rheingold, A. L. Inorg. Chem. 2006, 45, 2367-2369. (2) Cissell, J. A.; Vaid, T. P.; Rheingold, A. L. J. Am. Chem. Soc. 2005, 127, 12212-12213. (3) Gebauer, A.; Dawson, D. Y.; Arnold, J. J. Chem. Soc., Dalton Trans. 2000, 111-112. (4) Vangberg, T.; Lie, R.; Ghosh, A. J. Am. Chem. Soc. 2002, 124, 8122- 8130 and references therein. (5) Kadish, K. M.; Smith, K. M.; Guilard, R., Eds. The Porphyrin Handbook; Academic Press: San Diego, CA, 2000; Vol. 9. (6) Vaid, T. P.; Lytton-Jean, A. K.; Barnes, B. C. Chem. Mater. 2003, 15, 4292-4299. ORGANIC LETTERS 2006 Vol. 8, No. 11 2401-2404 10.1021/ol060772l CCC: $33.50 © 2006 American Chemical Society Published on Web 05/06/2006