NiII(dioxo[16]aneN5)-induced methane formation from methyl coenzyme M

Abstract

The nickel tetrapyrrole containing factor, F430, is implicated in the final methane evolution step in methanogenic bacteria1"4 and has attracted considerable attention.5"8 The essential role of F430 in methane formation was demonstrated by Ankel-Fuchs and Thauer, who reported the in vitro catalysis of H3CSCH2C-H2S03", methyl coenzyme M (methyl-CoM), to methane and CoM by purified methyl-CoM reductase under reducing conditions.9 Since F430 exists in both the Ni(I) (or Ni(III)) and the Ni(II) states in Methanobacterium thermoautotrophicum,10,11 it is of interest to examine the role of the nickel ion oxidation state12•13 in methyl-CoM catalysis. We have found both the mono-and the divalent oxidation states of the water-soluble Ni(dioxo[16]-aneN5), NiL, complex14 catalyze methyl-CoM to methane and CoM.In a typical reaction NinL (0.267 mM) in deoxygenated distilled

F430 Model Chemistry. An Investigation of Nickel Complexes as Catalysts for the Reduction of Alkyl Halides and Methyl Coenzyme-M by Sodium Borohydride

“…Our results differ from those described in footnote 23 of ref . This report stated that Raney nickel desulfurizes methyl CoM to afford a mixture of nickel sulfide, methane, ethane, methanol, ethanol, and an uncharacterized oil.…”

“…The Ni I complex [Ni(tmc)](OTf) showed no evidence of reaction, aside from precipitation, with tetrabutylammonium methyl CoM in organic solvents . The nickel complex of the ligand 1,4,7,10,13-pentaazacyclohexadecane-14,15-dione was reported to cleave methyl CoM to methane and H−S−CoM in a slow, stoichiometric process . However, we recently showed that the complex is unreactive toward methyl CoM and that the observed cleavage is caused by a yet unidentified impurity …”

F430 Model Chemistry. A Reexamination of the [1,4,7,10,13-Pentaazacyclohexadecane-14,16-dionato(2−)]nickel(II)-Induced Formation of Methane from Methyl Coenzyme-M

“…The crystalline L that we obtained had physical and spectroscopic properties that agreed with literature reports. − , Anaerobic reaction of L with Ni(OAc) 2 ·4H 2 O in absolute ethanol resulted in a green solution that afforded a green, hygroscopic solid upon evaporation. Attempts to purify the solid by recrystallization were unsuccessful.…”

“…We were struck by several inconsistencies between the reports of Drain et al , and those of other workers. − The Ni(II) complex of L that cleaves methyl CoM is reported to be a green hygroscopic solid that affords green solutions in both water and ethanol. The mechanism proposed for this reaction involves steps that are imbalanced by charge and ignores the protonation equilibria of the ligand amide groups and the concomitant structural changes of the complex.…”

“…In striking contrast to the apparent inability of Ni I complexes to effect C−S bond cleavage is the single existing report of a Ni II coordination complex that reacts directly with methyl CoM. The nickel complex of the ligand 1,4,7,10,13-pentaazacyclohexadecane-14,15-dione, L, was reported to cleave methyl CoM to methane and H−S−CoM in a slow, stoichiometric process. , The mechanism advanced for this reaction invokes a Ni(III)/Ni(II) redox couple rather than the Ni(II)/Ni(I) couple that is implied by the biochemical evidence. Cleavage of methyl CoM is proposed to cause concurrent oxidation of Ni(II) to Ni(III).…”