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 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

Synthetic Models for the Active Sites of Nickel‐Containing Enzymes

“…Similar results were also found by the group of Schröder with a bisthiolate-bisthioether ligand [82]. For the NiFe 2 complex 21 with this latter ligand, the resulting species obtained after one-electron reduction showed ESR hyperfi ne coupling when enriched with 61 Ni, and an average shift of 70 cm Ϫ1 for the CO bands in the IR spectrum. Another unusual structural motif with remote analogy to the [NiFe]-hydrogenase active site was reported by Schröder et al [83].…”

“…The cyclam-based complex 12 bearing a methylthio pendant arm has been proposed as a model for F 430 [61]. While participation of the thioether group in Ni coordination is observed neither in solution nor in the solid state, the Ni II species is reversibly reduced to Ni I around Ϫ0.7 V compared with SCE, followed by thioether bond cleavage and formation of a thiol group at more negative potentials.…”

“…In this complex, the copper center adopts a distorted tetrahedral geometry, wherein one of the four Cu-S bonds is elongated, similar to the asymmetric coordination observed for the central atom in the native system. As the active site of the natural enzyme exhibits reactivity towards CO, which is apparent from ESR line broadening upon isotopic perturbation with 57 Fe, 61 Ni or 13 CO as well as from an IR vibrational band for ν CO at 1995 cm Ϫ1 , the reactivity of 48 towards CO was tested, but no features for a terminal CO bound to this dinuclear species could be observed. Rather, decomposition of the product with formation of mononuclear Cu(CO)(PhTt tBu ) occurred, together with reformation of the Ni{N 2 S 2 } metalloligand fragment [121].…”

Methyl‐Coenzyme M Reductase and its Nickel Corphin Coenzyme F 430 in Methanogenic Archaea

“…ϩ0.8 V compared with SCE, when a glassy carbon electrode was used. No cleavage of the ether bond was observed under similar conditions [104,105].…”

Enhanced Stereoselectivity of a Cu(II) Complex Chiral Auxiliary in the Synthesis of Fmoc-l-γ-carboxyglutamic Acid

“…Based on the known propensity of thioethers to coordinate very weakly 41 or not at all 42 axially to Ni(II), we did not expect the thioether to coordinate to the Ni center, making it necessary to swap this metal with Cu. Lastly, the unsubstituted benzyl complexes were prepared to complete the comparative study.…”