High-Valent Imido Complexes of Manganese and Chromium Corroles

• Published in: Inorganic chemistry 2005-05, Vol.44 (10), p.3700-3708
• Main Authors: Edwards, Nicola Y, Eikey, Rebecca A, Loring, Megan I, Abu-Omar, Mahdi M
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic0484506

The oxidation reaction of M(tpfc) [M = Mn or Cr and tpfc = tris(pentafluorophenyl)corrole] with aryl azides under photolytic or thermal conditions gives the first examples of mononuclear imido complexes of manganese(V) and chromium(V). These complexes have been characterized by NMR, mass spectrometry, UV−vis, EPR, elemental analysis, and cyclic voltammetry. Two X-ray structures have been obtained for Mn(tpfc)(NMes) and Cr(tpfc)(NMes) [Mes = 2,4,6-(CH3)3C6H2]. Short metal−imido bonds (1.610 and 1.635 Å) as well as nearly linear M−N−C angles are consistent with triple M⋮NR bond formation. The kinetics of nitrene [NR] group transfer from manganese(V) corroles to various organic phosphines have been defined. Reduction of the manganese(V) corrolato complex affords phosphine imine and MnIII with reaction rates that are sensitive to steric and electronic elements of the phosphine substrate. An analogous manganese complex with a variant corrole ligand containing bromine atoms in the β-pyrrole positions, Mn(Br8tpfc)(NAr), has been prepared and studied. Its reaction with PEt3 is 250× faster than that of the parent tpfc complex, and its MnV/IV couple is shifted by 370 mV to a more positive potential. The EPR spectra of chromium(V) imido corroles reveal a rich signal at ambient temperature consistent with CrV⋮NR (d1, S = 1/2) containing a localized spin density in the d xy orbital, and an anisotropic signal at liquid nitrogen temperature. Our results demonstrate the synthetic utility of organic aryl azides in the preparation of mononuclear metal imido complexes previously considered elusive, and suggest strong σ-donation as the underlying factor in stabilizing high-valent metals by corrole ligands.