Crystal structure of a water oxidation catalyst solvate with composition (NH 4 ) 2 [Fe IV ( L -6H)]·3CH 3 COOH ( L = clathrochelate ligand).

The synthetic availability of mol-ecular water oxidation catalysts containing high-valent ions of 3 d 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 di-ammonium {μ-1,3,4,7,8,10,12,13,16,17,19,22-dodeca-aza-tetra-cyclo-[8.8.4.1 3,17 .1 8,12 ]tetra-cosane-5,6,14,15,20,21-hexa-onato}ferrate(IV) acetic acid tris-olvate, (NH 4 ) 2 [Fe IV (C 12 H 12 N 12 O 6 )]·3CH 3 COOH or (NH 4 ) 2 [Fe IV ( L -6H)]·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 tri-aza-cyclo-hexane fragments making three five- and six six-membered alternating chelate rings with the central Fe IV ion. The local coord-ination environment of Fe IV is formed by six deprotonated hydrazide nitro-gen 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 C 2/ c . Its coordination geometry is inter-mediate between a trigonal prism (distortion angle φ = 0°) and an anti-prism (φ = 60°) with φ = 31.1°. The Fe-N bond lengths lie in the range 1.9376 (13)-1.9617 (13) Å, as expected for tetra-valent iron. Structure analysis revealed that three acetic acid mol-ecules 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 mol-ecules are inter-connected by an intricate system of hydrogen bonds, mainly via the oxamide oxygen atoms acting as acceptors.