Biomimetic hydrolytic activation by Fe(III) aggregates: structures, reactivity and properties of novel oxo-bridged iron complexes

The tetranuclear aggregate (enH(2))[Fe-4(mu(3)-O)(heidi)(4)(mu-O,O'-O2CNHC2H4NH3)].4H(2)O contains a novel bidentate zwitterionic carbamic acid ligand. Magnetic studies indicate that the unsymmetrical Fe4 core is ferrimagnetic with an S=4 ground state. Similar ligands have been obtained on rectangular tetranuclear aggregates [M-4(mu-O)(mu-OH)(hpdta)(2)(mu-X)(2)](n-) (M=Fe, Al, Ga). The carbamic acid ligands are considered to result from the hydrolytic activation (fixation) of atmospheric CO2 by the aggregate precursor to give a carbonato intermediate, which then reacts with the organic diamine used as base in the synthesis. Similar aggregates with acetate ligands result from hydrolytic activation of the DMA used as cosolvent. Closely related mechanisms for these two activation processes are proposed, which are also related to the accepted mechanisms for carbonic anhydrase and urease.