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The structures of the three title monosubstituted ferrocenes, namely 1-chloroferrocene, [Fe(C5H5)(C5H4Cl)], (I), 1-bromoferrocene, [Fe(C5H5)(C5H4Br)], (II), and 1-iodoferrocene, [Fe(C5H5)(C5H4I)], (III), were determined at 100 K. The chloro- and bromoferrocenes are isomorphous crystals. The new triclinic polymorph [space group P , Z = 4, T = 100 K, V = 943.8 (4) Å3] of iodoferrocene, (III), and the previously reported monoclinic polymorph of (III) [Laus, Wurst & Schottenberger (2005 ). Z. Kristallogr. New Cryst. Struct. 220, 229–230; space group Pc, Z = 4, T = 100 K, V = 924.9 Å3] were obtained by crystallization from ethanolic solutions at 253 and 303 K, respectively. All four phases contain two independent molecules in the unit cell. The relative orientations of the cyclopentadienyl (Cp) rings are eclipsed and staggered in the independent molecules of (I) and (II), while (III) demonstrates only an eclipsed conformation. The triclinic and monoclinic polymorphs of (III) contain nonbonded intermolecular II contacts, causing different packing modes. In the triclinic form of (III), the molecules are arranged in zigzag tetramers, while in the monoclinic form the molecules are arranged in zigzag chains along the a axis. Crystallographic data for (III), along with the computed lattice energies of the two polymorphs, suggest that the monoclinic form is more stable.
Once ferrocene had been synthesized, numerous applications were found for the compound and its derivatives. Many ferrocene-based materials were used in the development of bioorganometallic chemistry (Staveren & Metzler-Nolte, 2004 ), catalysis (Togni & Hayashi, 1995 ), dendrimers (Astruc et al., 2008 ), nonlinear optical materials (Kinnibrugh et al., 2009 ), anticancer agents (Jaouen, 2008 ), etc. For example, ferroquine has been perceived to be extremely active against a chloroquine-resistant strain CQ(−) of Plasmodium falciparum (Dubar et al., 2008 ). In this work, we report the first structural study of the monohalogen-substituted ferrocenes 1-chloroferrocene, (I), and 1-bromoferrocene, (II), and a triclinic form of 1-iodoferrocene, (III). It is surprising that the elucidation of the structures of the substituted ferrocenes presented here had not been carried out before, although this is probably due to experimental difficulties related to the low melting points of these compounds. All the title compounds contain two crystallographically independent molecules, denoted A and B, in the unit cell.
Disorder of the Cp rings in ferrocene is a well known phenomenon (Seiler & Dunitz, 1979 ). Previous workers have found a dynamic type of disorder for the metallocenes Cp2Co and Cp2V (Cp is cyclopentadienyl; Antipin et al., 1993 ; Antipin & Boese, 1996 ). Usually, monosubstituted ferrocenes do not show disorder, due to higher rotational barriers compared with unsubstituted Cp rings (Sato, Iwai et al., 1984 ). Nevertheless, we found that compound (I) has disordered Cp rings for molecule B with equal occupancies over the two orientations at 100 K. A disorder model for the C5H5 and C5H4Cl rings of molecule B was proposed, with the two orientations of each ring differing by rotations in the ring plane of about 20 and 16°, respectively.
The mean values of the Fe—C, C—C, C—X (X = Cl, Br or I) and FeCg (Cg is a ring centroid) bond lengths, and the η5-C5H4 X/η5-C5H5 angles for molecules (I), (II) and (III) are presented in Table 1 . The Fe—C and FeCg distances to the substituted η5-C5H4 X ring are slightly shorter than those for the η5-C5H5 ring, which is attributed to the substituent in the η5-C5H4 X ring. The shortening of these distances in (I)--(III) is statistically not significant but this trend was observed for all other monosubstituted ferrocenes, whether the substituent is an electron-donating or an electron-withdrawing group (Kaluski & Struchkov, 1966 ; Sato, Iwai et al., 1984 ; Sato, Katada et al., 1984 ; Drouin et al., 1997 ; Foucher et al., 1999 ; Lin et al., 1998 ; Alley & Henderson, 2001 ; Hnetinka et al., 2004 ; Nemykin et al., 2007 ; Gasser et al., 2007 ).
The rings of (I) are eclipsed in molecule A, with the torsion angle C1A(Cl)Cg1Cg2C6A = −2.90 (11)°. Molecule B exists in two different conformations. The Cp rings of compound (II) are eclipsed in molecule A and staggered for molecule B; the torsion angles C1A(Br1A)Cg1Cg2C6A and C1B(Br1B)Cg3Cg4C6B are −2.6 (11) and −29.2 (11)°, respectively. The rings of compound (III) are in an eclipsed conformation in both independent molecules; the torsion angles C1A(I1A)Cg1Cg2C6A and C1B(I1B)Cg3Cg4<C6B are −2.2 (11) and −1.9 (11)°, respectively. The η5-C5H4 X and η5-C5H5 rings are almost parallel in the molecules of (I), (II) and (III) (Figs. 1 , 2 and 3 , and Table 1 ).
Crystals of (I) and (II) obtained from ethanolic solutions are monoclinic and isomorphous. In these crystal structures, four molecules form tetramers via intermolecular C—HX (X = Cl or Br) hydrogen bonds between the C—H groups of molecules with eclipsed conformations and the X atoms of molecules with staggered conformations, and also C—HX hydrogen bonds between molecules with eclipsed conformations (Fig. 4 and Table 2 ). These tetramers are, in turn, linked to each other by weak C—Hπ interactions along the a axis.
The new triclinic polymorph of (III) [space group P , Z = 4, T = 100 K, V = 943.8 (4) Å3] and the previously reported monoclinic polymorph (space group Pc, Z = 4, T = 100 K, V = 924.9 Å3) (Laus et al., 2005 ) were obtained upon crystallization of ethanol solutions at 253 and 303 K, respectively. Crystals of another previously reported monoclinic polymorph (space group Pc, Z = 4, T = 228 K, V = 953.7 Å3) were grown by vacuum sublimation (Laus et al., 2005 ). Since this previously reported structure was studied at 228 K, we obtained X-ray diffraction data for both polymorphs of (III) at 100 K and their comparison is based on these data. Both forms contain two crystallographically independent molecules (A and B). The bond lengths and angles in both polymorphs are very similar. The molecular conformations are eclipsed for the triclinic polymorph of (III), and deviate slightly from an eclipsed conformation in the monoclinic polymorph; the torsion angles C1A(I1A)Cg1Cg2C6A and C1B(I1B)Cg3Cg4C6B are −4.8 (11) and 7.0 (11)°, respectively.
The triclinic and monoclinic polymorphs of (III) both contain short nonbonded intermolecular II contacts but have different molecular packing modes. The two pairs of independent molecules A and B in triclinic (III) form zigzag tetramers via II contacts [I1AI1B = 4.129 (1) Å and C1A—I1AI1B = 150.78 (10)°; I1B I1B iii = 4.123 (1) Å, C1B—I1BI1B iii = 136.71 (9)° and I1A—I1B I1B iii = 71.07 (10)°; symmetry code: (iii) 1 − x, 2 − y, 1 − z] (Fig. 5 ). These II contacts are longer than the sum of spherical van der Waals radii proposed by Bondi (3.96 Å; Bondi, 1964 ; Rowland & Taylor, 1996 ), but shorter than the sum of spheroidal van der Waals radii for I (4.26 Å; Nyburg & Faerman, 1985 ). The I atoms of molecules B demonstrate fork-type II interactions, while the I atoms of molecules A possess only one II contact. All four II contacts form an almost planar zigzag tetramer.
Molecules in the monoclinic form of (III) are arranged in chains along the a axis connected by zigzag II contacts [I1AI1B = 4.183 (1) Å and C1A—I1AI1B = 155.3 (8)°; I1B I1A ii = 3.913 (1) Å, C1B—I1BI1A ii = 93.7 (1)° and I1A—I1BI1A ii = 101.9 (1)°; symmetry code: (ii) −1 + x, y, z] (Fig. 6 ). The II contacts between independent molecules A and B are shorter than the sum of the van der Waals radii proposed by Bondi, while the II contacts which connect pairs of molecules B and A# (Fig. 6 ) are somewhat longer than the sum of van der Waals radii proposed for spherical and somewhat shorter than for spheroidal I atoms. The lengths of the II contacts vary for the monoclinic polymorph from those of the triclinic by ca 0.2 Å, while the angles differ significantly.
The tetramers in triclinic (III) and the zigzag chains in monoclinic (III) are linked to each other by weak C—Hπ interactions (Table 3 ). The intermolecular C—Hπ(C5H5) contacts for the monoclinic polymorph of (III) are approximately the same as for the triclinic polymorph. In the case of the monoclinic polymorph of (III), there are C—HI hydrogen bonds between neighbouring molecules in the zigzag chains (Table 2 ), while the I atoms of the triclinic polymorph of (III) do not participate in hydrogen bonding.
We evaluated the crystal energies of the two polymorphs of (III) using the Cerius2 program (Molecular Simulations, 1999 ). Crystal energies were calculated using the Dreiding force field (Mayo et al., 1990 ). The initial crystal energies were −16.8 and −18.4 kcal mol−1 (1 kcal mol−1 = 4.184 kJ mol−1) and the energies after minimization were −17.9 and −18.9 kcal mol−1 for the triclinic and monoclinic polymorphs, respectively. These results, along with data on the densities of the polymorphs and their unit-cell volumes, lead us to suggest that the noncentrosymmetric monoclinic polymorph is more stable than the triclinic one.
Compounds (I), (II) and (III) were prepared according to standard literature procedures (Fish & Rosenblum, 1965 ; Perevalova, 1972 ). Slow evaporation from ethanol solutions produced yellow crystals of (I) and brown crystals of (II). The triclinic and monoclinic polymorphs of (III) were obtained as yellow and orange crystals, respectively, upon crystallization from ethanol solutions at 253 and 303 K, respectively. During crystal selection on the stage of a polarizing microscope, crystals of (I) and (II) melted rapidly due to their low melting points and the heat produced by the microscope lamp. To avoid this problem we used a microscope cooling stage (INSTEC) for crystal selection.
All H atoms were positioned geometrically, with C—H = 1.00 Å, and refined in riding mode, with U iso(H) = 1.2U eq(C). The crystals of (I) were found to be twinned. The structure of (I) was refined by the method of Pratt et al. (1971 ) and Jameson (1982 ), with a TWIN matrix defined as (100/00/00), which is the default for a monoclinic twinning type with β close to 90° and a twin fraction of 0.380 (1). A disorder model for the C5H5 and C5H4Cl rings was found with two orientations of the rings with equal occupancies for the two positions, differing by rotations in the ring plane of about 20 and 16°, respectively. The C atoms of the disordered C5H5 and C5H4Cl rings of molecule B of (I) were restrained to be planar within 0.001 Å. The distances between C atoms were fixed in a pentagon fashion at 1.425 (1) and 2.300 (1) Å for 1,2- and 1,3-distances, respectively. The 33 reflections which did not agree with the ideal model of the disordered molecule were omitted from the refinement.
For all compounds, data collection: APEX2 (Bruker, 2005 ); cell refinement: SAINT-Plus (Bruker, 2001 ); data reduction: SAINT-Plus; program(s) used to solve structure: SHELXTL (Sheldrick, 2008 ); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.
Crystal structure: contains datablocks global, I, II, III, IV. DOI: 10.1107/S0108270109034763/sf3110sup1.cif
Structure factors: contains datablocks I. DOI: 10.1107/S0108270109034763/sf3110Isup2.hkl
Structure factors: contains datablocks II. DOI: 10.1107/S0108270109034763/sf3110IIsup3.hkl
Structure factors: contains datablocks III. DOI: 10.1107/S0108270109034763/sf3110IIIsup4.hkl
The authors are grateful to the NIH for support via the RIMI program (grant No. P20MD001104) and to the NSF for support via grant Nos. CHE-0832622 and DMR-0120967. We thank A. A. Yakovenko for assistance with the X-ray diffraction analysis of compound (I).
Supplementary data for this paper are available from the IUCr electronic archives (Reference: SF3110). Services for accessing these data are described at the back of the journal.