Phase-transformation-induced twinning of an iron(III) calix[4]pyrrolidine complex.

The title compound, tetrachlorido-1κCl;2κ(3)Cl-(2,2,7,7,12,12,17,17-octamethyl-21,22,23,24-tetraazapentacyclo[16.2.1.1(3,6).1(8,11).1(13,16)]tetracosane-1κ(4)N,N',N'',N''')-μ2-oxido-diiron(III), [Fe2Cl4O(C28H52N4)], undergoes a slow phase transformation at ca 173 K from monoclinic space group P2(1)/n, denoted form (I), to the maximal non-isomorphic subgroup, triclinic space group P1, denoted form (II), which is accompanied by nonmerohedral twinning [twin fractions of 0.693 (4) and 0.307 (4)]. The transformation was found to be reversible, as on raising the temperature the crystal reverted to monoclinic form (I). In the asymmetric unit of form (I), Z' = 1, while in form (II), Z' = 2, with a very small reduction (ca 1.8%) in the unit-cell volume. The two independent molecules (A and B) in form (II) are related by a pseudo-twofold screw axis along the b axis. The molecular overlay of molecule A on molecule B has an r.m.s. deviation of 0.353 Å, with the largest distance between two equivalent atoms being 1.202 Å. The reaction of calix[4]pyrrolidine, the fully reduced form of meso-octamethylporphyrinogen, with FeCl3 gave a red-brown solid that was recrystallized from ethanol in air, resulting in the formation of the title compound. In both forms, (I) and (II), the Fe(III) atoms are coordinated to the macrocyclic ligand and have distorted octahedral FeN4OCl coordination spheres. These Fe(III) atoms lie out of the mean plane of the four N atoms, displaced towards the O atom of the [OFeCl3] unit by 0.2265 (5) Å in form (I), and by 0.2210 (14) and 0.2089 (14) Å, respectively, in the two independent molecules (A and B) of form (II). The geometry of the [OFeCl3] units are similar, with each Fe(III) atom having a tetrahedral coordination sphere. The NH H atoms are directed below the planes of the macrocycles and are hydrogen bonded to the coordinated Cl(-) ions. There are also intramolecular C-H···Cl hydrogen bonds present in both (I) and (II). In form (I), there are no significant intermolecular interactions present. In form (II), the individual molecules are arranged in alternate layers parallel to the ac plane. The B molecules are linked by a C-H···Cl hydrogen bond, forming chains along [100].