Aminoguanidinium Hydrolysis Effected by a Hydroxo-Bridged Dicobalt(II) Complex as a Functional Model for Arginase and Catalyzed by Mononuclear Cobalt(II) Complexes

The dinuclear complex
[Co2(μ-OH)(μ-XDK)(bpy)2(EtOH)](NO3),
where XDK is the dinucleating
dicarboxylate ligand m-xylylenediamine bis(Kemp's
triacid imide) and bpy = 2,2‘-bipyridine, was prepared
as a functional model for arginase. The substrate aminoguanidinium
nitrate was hydrolyzed to urea in ethanol
by the complex but not by free hydroxide ion under the same conditions.
The amino group of the substrate
binds to cobalt, as demonstrated by UV−vis spectroscopic studies.
The syntheses of related dinuclear cobalt(II)
complexes
[Co2(μ-XDK)(NO3)2(CH3OH)2(H2O)],
[Co2(μ-Cl)(μ-XDK)(bpy)2(EtOH)2](NO3),
and [Co2(μ-XDK)(py)3(NO3)2] are described.
Mononuclear complexes
[Co(XDK)(bpy)(H2O)] and
[Zn(XDK)(bpy)(H2O)] were
also prepared and characterized. The former catalytically
hydrolyzes aminoguanidinium nitrate to urea in
basic 1:1 methanol/water solutions, whereas the latter does not promote
this reaction. Hydrolysis of
aminoguanidinium ion is effected by
[Co(CH3COO)2] and
[Cu(CH3COO)2] in the presence of
bpy, but not by
[Zn(CH3COO)2],
[Ni(CH3COO)2], or
[Mn(CH3COO)2] in the presence of
bpy in 1:1 methanol/water solution.
In all cases, coordination of the amino group of the substrate to
the metal center under the reaction conditions
may activate the leaving group and orient the guanidinium moiety close
to the attacking nucleophile, metal-bound hydroxide ion, to promote the hydrolysis reaction.