Crystal structure of diaqua-monodimethylglycine-hexakis(μ2-dimethylglycine- O:O)-μ3-oxo-triiron(III) heptanitrate—water—dimethylgycine solvate (1:4:0.63), Fe3O(C4H9NO2)7(H2O)2(NO3)7 · 4H2O · 0.63C4H9NO2

Abstract

Source of materialThe title compound was obtained from an aqueous solution of dimethylglycine and iron nitrate in a 3:1 proportion. After many months of room temperature evaporation of the solvent small single crystals were grown. They showed hygroscopic behaviour. Experimental detailsThe crystals were inserted in capillary and then tested by Laue photographic methods. The H atoms of the organic moieties were placed at calculated positions and refined as riding using SHELXL-97 [1] defaults. H atoms of the water molecules were not located. DiscussionThe title compound contains a triiron core linked by a m 3-bridging oxide ion. Trinuclear iron compounds are of interest because oxo-bridged polynuclear Fe(III) centres perform important biological functions, such as oxygen storage and transport in a variety of proteins. It has been established that the iron atoms in the Fe 3 O core are coupled antiferromagnetically both for Fe(III) and mixed valence (Fe(II)/Fe(III)), and spin frustation occurs due to the triangular geometry [2]. The magnetic Jahn-Teller effect has been observed in a few such compounds causing deviation of the clusters from an equilateral configuration [3].In the title compound, the (Fe 3O(dmgly)7(H2O)2) 7+ cation has no crystallographic imposed symmetry. Two of the Fe(III) ions are coordinated by four dimethylglycine molecules, one water molecule and the bridging oxide ion, while the third metal ion has the water molecule trans to the central oxide ion replaced by another dimethylglycine molecule. Each of the Fe(III) ions has a distorted octahedral geometry with distances to the basal planes of 0.192(2) Å, 0.101(2) Å and 0.202(2) Å for Fe1, Fe2, Fe3 respectively. The shortest Fe···Fe distance is 3.277(2) Å, too large to allow direct metal-metal interactions. A magnetic superexchange mechanism mainly through the central oxide ion has been used to explain the observed magnetic properties in similar compounds [2], the Fe-O1 distances range from 1.902(3) Å to 1.951(3) Å. The dimethylglycine molecules are present in neutral zwitterionic form, with the carboxylic group deprotonated and the amino group positively charged. The nitrate cations are disordered in the structure which accounts for their high displacement factors. One of the ions was refined in two different orientations. There is an extra dimethyl-glycine molecule that does not coordinate the metal core and its occupation refines to less than the unity. The oxygen atoms of four solvent water molecules were also located. These high solvent content and disorder is usual in these type of compounds [2]