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
CH3SCH2CH2SO3-
to CH4 and
HSCH2CH2SO3-
was reexamined.
CH3SCH2CH2SO3-,
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
P21/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)
Å3. The structure was refined to R
=
0.037 on Fo2 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 η2-acetate and one nonchelating
η1-acetate. Neither this
N3O3 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
P1̄ (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) Å3. The structure was refined to
R = 0.045 on F2 on the basis
of
1277 reflections with I > 2σ(I). The
Ni complex of L‘, NiH-2L‘, does not cleave
methyl coenzyme-M. Solutions
of physical mixtures of NiL(OAc)2 and
NiH-2L‘ 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.