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

The 14-membered macrocyclic Ni²⁺ 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 H₂O, 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 Ni²⁺ complexes with 1 and 2 , in which a tetrahedrally distorted square-planar geometry around the Ni²⁺ results by the coordination of the four N-atoms of the macrocycle. Cyclic voltammetry of these complexes in MeCN reveals that Ni²⁺ 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 Ni²⁺ 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.