Crystal structure of a water oxidation catalyst solvate with composition (NH₄)₂[Fe^IV(L-6H)]?3CH_{3COOH (L = clathrochelate ligand)}

The synthetic availability of molecular water oxidation catalysts containing high-valent ions of 3d metals in the active site is a prerequisite to enabling photo- and electrochemical water splitting on a large scale. Herein, the synthesis and crystal structure of diammonium {mu-1,3,4,7,8,10,12,13,16,17,19,22-dodecaazatetracyclo[8.8.4.1(3,17).1(8,12)]tetracosane-5,6,14,15,20,21-hexaonato}ferrate(IV) acetic acid trisolvate, (NH4)(2)[Fe-IV(C12H12N12O6)]center dot 3CH(3)COOH or (NH4)(2)[Fe-IV(L-6H)]center dot 3CH(3)COOH is reported. The Fe-IV ion is encapsulated by the macropolycyclic ligand, which can be described as a dodeca-aza-quadricyclic cage with two capping triazacyclohexane fragments making three five- and six six-membered alternating chelate rings with the central Fe-IV ion. The local coordination environment of Fe-IV is formed by six deprotonated hydrazide nitrogen atoms, which stabilize the unusual oxidation state. The Fe-IV ion lies on a twofold rotation axis (multiplicity 4, Wyckoff letter e) of the space group C2/c. Its coordination geometry is intermediate between a trigonal prism (distortion angle phi = 0 degrees) and an antiprism (phi = 60 degrees) with phi = 31.1 degrees. The Fe-N bond lengths lie in the range 1.9376 (13)-1.9617 (13) angstrom, as expected for tetravalent iron. Structure analysis revealed that three acetic acid molecules additionally co-crystallize per one iron(IV) complex, and one of them is positionally disordered over four positions. In the crystal structure, the ammonium cations, complex dianions and acetic acid molecules are interconnected by an intricate system of hydrogen bonds, mainly via the oxamide oxygen atoms acting as acceptors.