Catalytic oxidations using cobalt(II) complexes - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 317

The stereoisomerism and equilibrium properties of dioxygen carrying
cobalt(II) complexes of histamine and its derivatives have recently been
reinvestigated 183 .
The Co(II) - glycyl- L -histidine-imidazole - Co(II)- L -histidylglycine-
imidazole - and the corresponding L -histidine and glycylglycine systems
have been studied 184 . Potentiometric, gas-volumetric and spectroscopic
(UV/VIS, near IR, ESR) measurements indicated the presence of two types
of ternary species: with parent complexes containing a deprotonated amide
group (as for glycylglycine and similar dipeptides) and with parent histidine-
like active complexes. The overall stability constants of the ternary
complexes have been determined. An increase in reversibility of dioxygen
uptake was found in both cases relative to the systems without imidazole.
The oxygen uptake by Co(II) complexes with a group of diastereo-
isomeric dipeptides, consisting of alanine and leucine in various chiral
forms, has been studied in aqueous solution 185 .

15. CARBOXYLATOCOBALT COMPLEXES AND SALTS

Cobalt(II) ion has been found to catalyze the dioxygen driven oxidation
of N -(phosphonomethyl)iminodiacetic acid (PMIDA ) to N -(phosphono-
methyl)glycine (PMG ) in aqueous solution 186 . Additional products are
and formic acid.
This homogeneous catalytic conversion is novel and represents, in effect,
an oxidative dealkylation of one carboxymethyl moiety, yielding the N -
substituted glycine. The reaction is selective to the desired product PMG
when carried out at the natural pH of the free acid substrate (approximately
1-2) and when carried out at substrate loadings less than 5% by weight.
PMG is the active agent in the herbicide Roundup 187 .

Kinetic studies on dilute systems have been carried out. The reaction is
first order in substrate and The oxygen pressure dependence exhibits
saturation kinetics, while the selectivity increases as the oxygen pressure
increases.
The rate is also inversely proportional to The proposed mechanism
consists of the reaction sequence (a) - (d), involving complexation of Co(II)
and generation of the active oxidant via oxidation

by Its oxidative dealkylation to the product PMG is shown in Figure 65.