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 report that the nickel(II) complex of the macrocyclic ligand 1,4,7,10,13-pentaazacyclohexadecane-14,16-dione, L, cleaves CH 3 SCH 2 CH 2 SO 3 - to CH 4 and HSCH 2 CH 2 SO 3 - was reexamined. CH 3 SCH 2 CH 2 SO 3 -, or cofactor methyl coenzyme-M, carries the methyl group in the final step of methanogensis in methanogenic bacteria. The cleavage of the cofactor was reproduced when the nickel complex of L synthesized from unpurified, technical grade tetraethylenepentamine was used. However, authentic samples of the nickel complex were found to be incapable of carrying out the cleavage reaction. NiL(OAc) 2 prepared from L synthesized from pure tetraethylenepentamine crystallizes in the monoclinic space group P 2 1 / c ( Z = 4) with unit cell dimensions a = 8.234(1) Å, b = 13.439(2) Å, c = 18.915(2) Å, β = 95.370(10)°, and V = 2083.9(5) Å 3 . The structure was refined to R = 0.037 on F o 2 on the basis of 2754 reflections with I >2σ( I ). The nickel atom is coordinated in a meridional fashion by the three secondary nitrogen atoms of L with the remaining three coordination sites occupied by one chelating η 2 -acetate and one nonchelating η 1 -acetate. Neither this N 3 O 3 octahedral form, which prevails in neutral aqueous solution (presumably with waters replacing the acetates as the oxygen donors), nor the five-coordinate, square pyramidal, amide coordinated form, which prevails in basic solution, effect cleavage of methyl coenzyme-M. Upon reexamination, the ligand prepared from technical grade tetraethylenepentamine was found to be contaminated with 1,4,7,10-tetraazacyclotridecane-11,13-dione, L‘, which crystallizes in the triclinic space group P 1̄ ( Z = 2) with unit cell dimensions a = 8.658(2) Å, b = 8.663(2) Å, c = 8.888(2) Å, α = 69.11(3)°, β = 83.51(3)°, γ = 62.49(3)°, and V = 551.3(2) Å 3 . The structure was refined to R = 0.045 on F 2 on the basis of 1277 reflections with I > 2σ( I ). The Ni complex of L‘, NiH - 2 L‘, does not cleave methyl coenzyme-M. Solutions of physical mixtures of NiL(OAc) 2 and NiH - 2 L‘ can reproduce the features of the UV−vis spectra observed during cleavage of methyl coenzyme-M but cannot cleave methyl coenzyme-M. The compound(s) or cooperative interactions between compounds in the impure material that are responsible for the cleavage reaction have not been isolated or identified. Nonetheless, based upon our observations, the originally proposed mechanism that involves NiL as the active complex is incorrect and cannot be taken as a precedent for the cleavage of methyl coenzyme-M by the enzyme methyl coenzyme-M reductase.