Introduction. -The functionalization of polyazamacrocycles with pendant chains containing donor atoms, which can coordinate, is a powerful technique to modify the chemical and physical properties of the metal ion [2]. Whereas this aspect has been studied in great detail, much less has been done to investigate how the metal ion modifies the properties and reactivity of the side-chain functional group. Such systems are good models for metalloenzymes, since through the side chain, the reactive group is brought close to the metal center, which can activate it in a similar way as in an enzyme-substrate complex. Examples of enhanced hydrolytic reactions of esters [3] and nitriles [4] have been observed.In contrast, nearly no mimicking of redox processes has been reported in the litera-

The 14-membered macrocyclic Ni2+ complexes of 1 and 2, with a metbylthio 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,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 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 NiZ+ 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' centre and the MeS group.Introduction. -Methanogenic bacteria are able to reduce CO, to methane in a complex catabolic pathway [2]. The last step of this process is the reductive cleavage of the methylthio group in methyl-coenzyme-M by methyl-coenzyme-M-reductase, whereby F430, a hydrocorphinoid nickel macrocycle, is used as cofactor. The elucidation of the structure of cofactor F430 has revealed several interesting aspects [3]. Coenzyme F430 is the first Ni-containing tetrapyrrole with a biological function, and the coordination chemistry of this metal ion is, therefore, especially interesting. The Ni-atom is tetracoordinated by the pyrazol N-atoms, but easily binds additional axial ligands, thereby changing its electronic configuration from low-spin to high-spin [4]. The stability constants for the coordination of axial ligands have been measured and indicate that penta-as well as hexacoordinate species are formed.The .reduction of F430 to a Ni' species has been achieved either electrochemically or by chemical means [5] [6]. The EPR study of the so formed Ni+ complex reveals that the

No abstract