F430 MODEL CHEMISTRY - A REEXAMINATION OF THE CLOHEXADECANE-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 CH3SCH2CH2SO3- to CH4 and HSCH2CH2SO3- was reexamined. CH3SCH2CH2SO3-, or cofactor me thyl coenzyme-M, carries the methyl group in the final step of methano gensis in methanogenic bacteria. The cleavage of the cofactor was repr oduced when the nickel complex of L synthesized from unpurified, techn ical 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 t etraethylenepentamine crystallizes in the monoclinic space group P2(1) /c (Z = 4) with unit cell dimensions a = 8.234(1) Angstrom, b = 13.439 (2) Angstrom, c = 18.915(2) Angstrom, beta = 95.370(10)degrees, and V = 2083.9(5) Angstrom(3). The structure was refined to R = 0.037 on F-o (2) on the basis of 2754 reflections with I > 2 sigma(I). The nickel a tom is coordinated in a meridional fashion by the three secondary nitr ogen atoms of L with the remaining three coordination sites occupied b y one chelating eta(2)-acetate and one nonchelating eta(1)-acetate. Ne ither this N3O3 octahedral form, which prevails in neutral aqueous sol ution (presumably with waters replacing the acetates as the oxygen don ors), nor the five-coordinate, square pyramidal, amide coordinated for m, which prevails in basic solution, effect cleavage of methyl coenzym e-M. Upon reexamination, the ligand prepared from technical grade tetr aethylenepentamine was found to be contaminated with 1,4,7,10-tetraaza cyclotridecane-11,13-dione, L', which crystallizes in the triclinic sp ace group <P(1)over bar> (Z = 2) with unit cell dimensions a = 8.658(2 ) Angstrom, b = 8.663(2) Angstrom, c = 8.888(2) degrees, alpha = 69.11 (3)degrees, beta = 83.51(3)degrees, gamma = 62.49(3)degrees, and V = 5 51.3(2) Angstrom(3). The structure was refined to R = 0.045 on F-2 on the basis of 1277 reflections with I > 2 sigma(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 t he UV-vis spectra observed during cleavage of methyl coenzyme-M but ca nnot cleave methyl coenzyme-M. The compound(s) or cooperative interact ions between compounds in the impure material that are responsible for the cleavage reaction have not been isolated or identified. Nonethele ss, based upon our observations, the originally proposed mechanism tha t involves Nit as the active complex is incorrect and cannot be taken as a precedent for the cleavage of methyl coenzyme-M by the enzyme met hyl coenzyme-M reductase.