Characterization of Porphyrin-Co(III)-‘Nitrene Radical’ Species Relevant in Catalytic Nitrene Transfer Reactions

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To fully characterize the CoIII–‘nitrene radical’ species that are proposed as intermediates in nitrene transfer reactions mediated by cobalt(II) porphyrins, different combinations of cobalt(II) complexes of porphyrins and nitrene transfer reagents were combined, and the generated species were studied using EPR, UV–vis, IR, VCD, UHR-ESI-MS, and XANES/XAFS measurements. Reactions of cobalt(II) porphyrins 1P1 (P1 = meso-tetraphenylporphyrin (TPP)) and 1P2 (P2 = 3,5-DitBu-ChenPhyrin) with organic azides 2Ns (NsN3), 2Ts (TsN3), and 2Troc (TrocN3) led to the formation of mono-nitrene species 3P1Ns, 3P2Ts, and 3P2Troc, respectively, which are best described as [CoIII(por)(NR″•–)] nitrene radicals (imidyl radicals) resulting from single electron transfer from the cobalt(II) porphyrin to the ‘nitrene’ moiety (Ns: R″ = −SO2-p-C6H5NO2; Ts: R″ = −SO2C6H6; Troc: R″ = −C(O)OCH2CCl3). Remarkably, the reaction of 1P1 with N-nosyl iminoiodane (PhI═NNs) 4Ns led to the formation of a bis-nitrene species 5P1Ns. This species is best described as a triple-radical complex [(por•–)CoIII(NR″•–)2] containing three ligand-centered unpaired electrons: two nitrene radicals (NR″•–) and one oxidized porphyrin radical (por•–). Thus, the formation of the second nitrene radical involves another intramolecular one-electron transfer to the “nitrene” moiety, but now from the porphyrin ring instead of the metal center. Interestingly, this bis-nitrene species is observed only on reacting 4Ns with 1P1. Reaction of the more bulky 1P2 with 4Ns results again in formation of mainly mono-nitrene species 3P2Ns according to EPR and ESI-MS spectroscopic studies. The mono- and bis-nitrene species were initially expected to be five- and six-coordinate species, respectively, but XANES data revealed that both mono- and bis-nitrene species are six-coordinate Oh species. The nature of the sixth ligand bound to cobalt(III) in the mono-nitrene case remains elusive, but some plausible candidates are NH3, NH2–, NsNH–, and OH–; NsNH– being the most plausible. Conversion of mono-nitrene species 3P1Ns into bis-nitrene species 5P1Ns upon reaction with 4Ns was demonstrated. Solutions containing 3P1Ns and 5P1Ns proved to be still active in catalytic aziridination of styrene, consistent with their proposed key involvement in nitrene transfer reactions mediated by cobalt(II) porphyrins.

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Journal of the American Chemical Society, v. 137, issue 16, p. 5468-5479