Copper(II) Complexes of Inverted Porphyrin and Its Methylated Derivatives

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Abstract
The inverted porphyrins 2-aza-5,10,15,20-tetraphenyl-21-carbaporphyrin (CTPPH2) and its methylated derivatives 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-carbaporphyrin (2-NCH3CTPPH) and 2-aza-2-methyl-5,10,15,20-tetraphenyl-21-methyl-21-carbaporphyrin (2-NCH3-21-CH3CTPPH) stabilize the rare organocopper(II) complexes (CTPP)CuII (1), (2-NCH3CTPP)CuII (2), (CTPPH)CuIIX (3-X), (2-NCH3CTPPH)CuIIX (4-X) (X = Cl-, TFA), and (2-NCH3-21-CH3CTPP)CuIICl (5). The EPR spectra recorded for 1, 2, 4, and 5 revealed typical features diagnostic of the copper(II) electronic structure. The superhyperfine coupling pattern indicates a presence of three nitrogen donors in the first coordination sphere. An addition of HX acid to 1 and 2 to yield the species 3-X and 4-X. The reaction mechanism includes protonation of the inner C(21) carbon accompanied by an axial coordination of anion. Methylation of (2-NCH3CTPP)CuII (2) with methyl iodide resulted in formation of (2-NCH3-21-CH3CTPP)CuIICl (5) which implies an existence of a σ-carbanion−copper(II) bond in 2. The 2H NMR investigations carried out for the pyrrole deuterated derivatives (CTPP-d7)CuII, (2-NCH3-21-CH3CTPP-d7)CuIICl, and the methyl deuterated (2-NCH3-21-CD3CTPP)CuIICl one confirmed independently the copper(II) electronic structure with the considerable dx2-y2 metal orbital contribution to the SOMO. The redox properties of copper(II) inverted porphyrins were studied by the cyclic and differential pulse voltammetry. The halfwave potentials indicate a metal-centered oxidation of 1 (390 mV) and 2 (343 mV). The dimethylated homologue 5 reveals the reduction process at −224 mV attributed to the CuII/CuI transformation.