Journal of Chemical Crystallography, Vol. 30, No. 1, 2000 Note Molecular structure of [trans- FeCl 2 (imidazole) 4 ]ClTHFH 2 O Stephen J. Obrey, (1) Simon G. Bott, (2) * and Andrew R. Barron (1) * Received January 20, 2000 The structure of [trans-FeCl 2 (imidazole) 4 ]Cl has been determined as the THF/H 2 O solvate. The Fe-Cl and Fe-N distances in the cation are shorter than analogous trans-FeCl 2 (L) 4 compounds, concomitant with the decreased ionic radius of Fe 3+ versus Fe 2+ . Crystal data: orthorhombic, Pnma, a = 8.456(2), b = 20.991(4), c = 13.288(3) A ˚ , V = 2358.6(8), and Z = 4. KEY WORDS: Iron; imidazole; chloride. Introduction The ionic complex, [trans-FeCl 2 (imidazole) 4 ] ClTHFH 2 O was formed as a crystalline by-product in an attempt to synthesize an iron(III) complex of 2,2-dimethyl propane-1,3-diol (neol-H 2 ) using imid- azole as an auxiliary base. Neutral complexes of iron(III) chloride or iron(II) chloride with nitrogen bases [e.g., FeCl 3 (L) 3 and FeCl 2 (L) 4 ] are well known, 1–3 however, analogous cationic complexes of iron(III) have not been structurally characterized. In order to provide a comparison with neutral com- plexes, the structure of [trans-FeCl 2 (imidazole) 4 ] ClTHFH 2 O was therefore determined by x-ray crys- tallography. Experimental Synthesis of [trans-FeCl 2 (imidazole) 4 ]ClTHFH 2 O To a solution of FeCl 3 (1.0 g, 61 mmol) in THF (50 mL) was added imidazole (1.260 g, 18.49 mmol), (1) Department of Chemistry, Rice University, Houston, Texas 77005. (2) Department of Chemistry, University of Houston, Houston, Texas 77204. * To whom correspondence should be addressed. 61 1074-1542/00/0100-0061$18.00/0  2000 Plenum Publishing Corporation 2,2-dimethyl propane-1,3-diol (1.923 g, 18.49 mmol). The resulting solution was stirred for 24 h at room temperature in air. The resulting orange-red reaction mixture was concentrated to 20 mL and cooled to -40°C overnight during which time bright orange x- ray crystallographic quality crystals were formed. Yield: 2.4 g (75%). X-ray crystallography A crystal of [trans-FeCl 2 (imidazole) 4 ]ClTHF H 2 O was sealed in a glass capillary and mounted on the goniometer of a Bruker CCD SMART system, equipped with graphite monochromated Mo-K  radi- ation ( = 0.71073 A ˚ ) and corrected for Lorentz and polarization effects. Data collection and unit cell and space group determination were all carried out in the usual manner. 4 Pertinent details are given in Table 1. The structure was solved by direct methods and the model was refined using full-matrix least squares techniques. 5 The solvent was disordered over two sites, related by 180° rotation such that the carbon positions of the THF molecule are roughly coincident giving the appearance of a six membered ring. Each end atom is a half-occupancy oxygen atom. The water of solvation is also  occupancy. All hydrogen atoms were placed in calculated positions [U iso = 1.3U (C) ;