Metal Complexes with Macrocyclic Ligands. Part XLII. Tetraazamacrocyclic nickel(II) complexes with a methylthio or a methoxy pendant chain as model for cofactor F430

Abstract The 14‐membered macrocyclic Ni 2+ complexes of 1 and 2 , with a methylthio pendant chain, and those of 3 and 4 , with a methoxy pendant chain, have been synthesized and their chemistry has been studied. Solution spectra in H 2 O, MeCN, and DMF indicate no participation of the side‐chain donor group in metal coordination. This is also the case in the solid state as shown by the X‐ray structures of the Ni 2+ complexes with 1 and 2 , in which a tetrahedrally distorted square‐planar geometry around the Ni 2+ results by the coordination of the four N‐atoms of the macrocycle. Cyclic voltammetry of these complexes in MeCN reveals that Ni 2+ is reversibly reduced to Ni + between −0.7 and −0.8 V vs. SCE. For the complexes with 1 and 2 , the thioether bond is cleaved at more negative potentials, whereby a thiol group is formed. This thiol group is then oxidized at ca. +0.7 V vs. SCE, when a glassy C electrode is used, or at ca. 0 V vs. SCE at a dropping Hg electrode. No cleavage of the ether bond in the complexes with 3 and 4 is observed under similar conditions. Reduction of the Ni 2+ complexes of 1 and 2 with Na‐amalgam in DMF produces small amounts of methane only in the case of 1 , indicating the importance of the proximity between the Ni I centre and the MeS group.