Formation of Stable NiIII N-Confused Porphyrins Aided by a 3-Ethoxy Group

Abstract

We report the synthesis, characterization, and reactivities of two stable Ni^III N-confused porphyrin (NCP) complexes. Metalation of 3-OEt NCP 1 with NiCl₂ ⋅ 6H₂ O in CHCl₃ /EtOH gave 3-OEt Ni^II NCP 3 initially, which was easily oxidized in air to form the Ni^III complex of NCP inner C-oxide 4. Bis-ethoxy-modified Ni^III complex 5 was synthesized by oxidation of 3 with PIFA in ethanol and CHCl₃ . The structures of 4 and 5 were determined by single-crystal X-ray diffraction analysis. An unusually long Ni^III -C bond (2.170(9) Å) was observed in 4. The g-factor (g>2.1) observed in the EPR spectra of 4 and 5 further confirmed that they are paramagnetic Ni^III complexes. Comparative experiments showed that the 3-ethoxy group plays an important role in the formation of 4 and 5. Reduction of 4 and 5 with NaBH₄ regenerated complex 3.

Keywords: EPR spectroscopy; high valent metal; nickel; porphyrinoids; reduction.