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The macrocyclic nickel complexes shown in Figure 1 are able to catalyze the electrochemical reduction of CO 2 to oxalate. In the case of the complexes with R 2 ) COOEt or COMe, the overall reaction can be interpreted in terms of an outer-sphere electron-transfer reaction (6) followed by a dimerization of the CO 2 •radical anions ( 7), but the variation of the electron-transfer rate constants with the standard potentials points to a coordinative interaction between the complexes and the CO 2 molecule. Complexes without COOEt or COMe substitution in the R 2 position undergo a fast deactivation reaction (first order with respect to [CO 2 ]) that is even visible in the time scale of the cyclic voltammetric experiments. The results of the cyclic voltammetric investigations could be confirmed in preparative-scale electrolyses where the complex Ni-Etn(Me/COOEt)-Etn proved to be the most active and persistent redox catalyst for the electrochemical reduction of CO 2 to oxalate that has been described so far.

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The complex (6,13-bis(ethoxycarbonyl)-5,14-dimethyl-1,4,8,11-tetraazacyclotetradeca-4,6,12,14-tetraenato(2−))iron(III) iodide, 3 (chemical formula C18H26O4N4FeI·0.5 C7H8), has been characterized by X-ray crystallography (a = 16.063(3) Å, b = 15.895(3) Å, c = 19.703(4) Å, β = 107.11(3)°; monocline; I2/a; Z = 8), EPR and Mössbauer spectroscopy, and temperature-dependent magnetic susceptibility as well as by MO calculations. The pentacoordinated iron atom is 0.34 Å outside of the [N4] plane. The equatorial Fe−N distances (1.92 Å for the N's in neighborhood of the methylated C-atoms and 1.88 Å for the others) are not significantly different from those found in octahedral low-spin iron(III) complexes with the same macrocyclic [N4 2-] ligand. The magnetic moment (μ eff = 4.19 μB up to 20 K; ϑ = −7.37 K), the EPR measurements (g y eff = 5.38, g x eff = 2.70, and g z eff = 1.74), and the Mössbauer data (δ = 0.18 mm/s, Δ E Q = 3.56 mm/s, T = 120 K) indicate a pure intermediate-spin state of S = 3/2. The MO calculations yield a complete picture of bonding modes, intramolecular interactions, and the electronic structure around the iron center, as indicated by the correct determination of the ground state and by the agreement between the measured and calculated quadrupole splitting.

Äthylendiamin‐ und ortho‐Phenylendiaminderivate von Oxymethylen‐β‐dicarbonylverbindungen bilden mit Nickel(II) und Kupfer(II) planare 1:1‐Chelate mit cis‐[N2O2]‐Koordination, die am Chelatring in Mesostellung eine freie Carbonylgruppe COR (R  CH3, C6H5, OC2H5) tragen. Die Esterderivate mit R  OC2H5 lassen sich zu Carbonsäuren mit unveränderter Koordination am Zentralatom verseifen. Die Struktur der Metallchelate wird diskutiert und der Einfluß freier Carbonylgruppen auf das magnetische und spektrophotometrische Verhalten NO‐koordinierender Sechsringchelate wird erläutert.