Contribution of Intramolecular NH···O Hydrogen Bonds to Magnesium–Carboxylate Bonds

A series
of magnesium carboxylate complexes containing intramolecular
NH···O hydrogen bonds were synthesized. Their molecular
structures were determined by X-ray analysis. A direct NH···O
hydrogen bond to the coordinated oxygen atom elongated the Mg–O
bond, while a hydrogen bond to the carbonyl group shortened the Mg–O
bond. Double NH···O hydrogen bonds significantly lowered
the basicity of the carboxylate anion and prevented coordination to
the Mg ion in a trans configuration; however, a cis-dicarboxylate complex was successfully obtained. Strong
coordination of water to the Mg2+ ion stabilizes the weak
Mg–carboxylate bond at the trans position.
In contrast, a weak Mg–carboxylate bond strengthens the Mg–O­(water)
bond, probably increasing the acidity. Based on the experimental results
and theoretical calculations, a new switching mechanism is proposed.
In the proposed mechanism, the acidity of the coordinated water on
magnesium is controlled during catalytic hydrolysis in endonuclease.