Synthesis and characterization of magnesium and zinc complexes of 1,4,7-triazacyclononane-N-acetate. Potential models for the active site of RuBisCo

Reaction of the tetradentate pendant-arm macrocycle 1,4,7-triazacyclononane-N-acetate (L) with Zn(O3SCF3)2 or Mg(O3SCF3)2 in alcohol produces the six-coordinate complexes [ZnL(H2O)]O3SCF3·C2H5OH (1) and [MgL(H2O)2]O3SCF3 (2). Compounds 1 and 2 were characterized by single-crystal X-ray crystallography. The complex cation of 1 forms a polymeric chain with zinc centers bridged in a syn, anti fashion by acetate pendant arms. The complex cation of 2 is mononuclear, with two adjacent coordination sites on magnesium occupied by water molecules. These labile coordination sites may provide a binding site for activation of substrates in functional modeling of magnesium enzymes such as RuBisCo. 1H and 13C NMR spectroscopy demonstrates that L remains bound to magnesium in methanol solution. Crystal data: 1, orthorhombic, space group P212121, a=9.0536(7), b=14.3750(11), c=14.4741(11) Å, V=1883.7(2) Å3, Z=4, R=0.041; 2, orthorhombic, space group P212121, a=8.5949(3), b=13.9694(4), c=14.0076(5) Å, V=1681.83(10) Å3, Z=4, R=0.0248.