Coenzyme F430 from Methanogenic Bacteria: Detection of a Paramagnetic Methylnickel(II) Derivative of the Pentamethyl Ester by ²H‐NMR Spectroscopy

A methylnickel(II) derivative of coenzyme F430 ( 1 ) was proposed as an intermediate in the enzymic process catalyzed by methyl‐CoM reductasc. Indirect evidence points to formation of CH 3 –F430M II in the reaction of F30M 1 (obtained from F430M II ( 2 )) with eleclrophilic methyl donors. The results presented here show, that such a compound does exist. A paramagnetic CD 3 –Ni II derivative 5b of the pentamethyl ester 2 (F430M) of coenzyme F430 was prepared by in situ methylation with (CD 3 ) 2 Mg and characterized by its isotropically shifted 2 H‐NMR spectrum. At −40°, the very broad D‐signal of the axially coordinated CD 3 group is found at −490 ppm. Comparison with the 2 H‐ and 1 H‐NMR spectra of mcthyl(tetramethylcyclam)nickel(II) derivatives 4 ([Ni II (CH 3 ))(tmc)]CF 3 SO 3 ( 4a ) is the only isolated CH 3 –Ni derivative of a N 4 macrocyclic Ni II complex' shows that the large isotropic shift to high field is characteristic for a Me group axially bound to the Ni center. The temperature dependence of the isotropic shift of the C D 3 –Ni group in both 4b and 5b follows Curie's law and yields 2 H hyperfine coupling constants of −0.65 ( 4b ) and −0.85 MHz ( 5b ), respectively. The 1 H‐NMR spectrum indicates that, in contrast to the five‐coordinate monochloro complex [Ni II Cl(tmc)] + , intermolecular exchange of the axial ligand in [Ni II (CH 3 )(tmc)] + 4a is either slow at the NMR time scale or does not occur at all.