DOI: 10.1021/cg900762s
2010, Vol. 10 158–164
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The Analogy of C-Br 3 3 3 Br-C, C-Br 3 3 3 Br-Fe, and Fe-Br 3 3 3 Br-Fe Contacts: Crystal Structures of (26DAPH)FeBr4 and (26DA35DBPH)2FeBr4 3 Br Firas Awwadi,*,† Salim F. Haddad,‡ Roger D. Willett,# and Brendan Twamley
Department of Chemistry, Tafila Technical University, Tafila 66110 Jordan, ‡Department of Chemistry, The University of Jordan, Amman11 942, Jordan, #Department of Chemistry, Washington State University, Pullman, Washington 99164, and University Research Office, University of Idaho, Moscow, Idaho 83844 )
†
Received July 5, 2009; Revised Manuscript Received October 19, 2009
ABSTRACT: The role of C-Br1 3 3 3 Br2-Fe and Fe-Br1 3 3 3 Br2-Fe interactions in the iron(III) structures (26DAPH)FeBr4 (I) and (26DA35DBPH)2FeBr4 3 Br (II) (where 26DAPH is 2,6-diaminopyridinium and 26DA35DBPH is 2,6-diamino-3,5dibromopyridinium) is investigated. In (I), the ions are arranged inside the crystalline lattice based on the N-H 3 3 3 Br-Fe hydrogen bond, including other types of interaction. The [FeBr4-] anions form a ladder structure with extremely short interbromide distances, 3.618 A˚ within the rungs and 3.814 A˚ within the rails. There are two Fe-Br1 3 3 3 Br2-Fe contact types; the first type has a Fe-Br1 3 3 3 Br2 angle = Br1 3 3 3 Br2-Fe angle which is equal to 166.6°. In the second, the Fe-Br1 3 3 3 Br2 angle = 100.5° and Br1 3 3 3 Br2-Fe angle = 159.1°. In contrast, in (II), the supramolecular assembly of cations and anions is dominated by the C-Br1 3 3 3 Br2-Fe interaction and the more traditional N-H 3 3 3 Br- hydrogen bond. Here the [FeBr4-] anions form linear chains with interbromide distances of ca. 3.83 A˚. Most interestingly, the geometrical characteristics of the C-Br1 3 3 3 Br2-Fe contacts and the Fe-Br1 3 3 3 Br2-Fe contacts, present in (I) and (II), are similar to the well characterized C-Br1 3 3 3 Br2-C contacts; the C-Br1 3 3 3 Br2 angles are essentially linear (avg = 165.5°, range 160.0-176.3°) and the Br1 3 3 3 Br2-Fe angles are essentially normal to the axis containing C-Br1 3 3 3 Br2 atoms (avg = 90.4°, range 82.1-113.8°).
Introduction The self-assembly of crystalline species inside crystalline lattices has received a lot of interest in recent years, in both theoretical and experimental chemistry.1-8 This is one of the main foci of crystal engineering. This science studies the arrangement of crystalline species inside crystalline lattices to predict the crystal structures, and hence designing new solid-state materials with desired properties such as magnetic, electrical, and nonlinear optical properties.9 One of the main factors that determine the self-assembly of molecules and ions in crystals is the intermolecular forces, such as hydrogenbonding, halogen-bonding, and others.5,10-12 The halogen bond is a noncovalent intermolecular interaction that leads to the arrangement A-Y 3 3 3 B, where Y is a halogen atom.6,11-13 Many types of halogen bonds have been studied in detail. This includes (a) the complexes between dihalogens XY and Lewis bases nucleophiles (Nu), XY 3 3 3 Nu. These complexes have been investigated using theoretical calculation and rotational spectroscopy.14,15 (b) Complexes between carbon-halogen atoms (except fluorine) and nucleophiles (Nu), C-Y 3 3 3 Nu.16 Nucleophiles tend to approach the halogen atom of the C-Y bond at an angle of around 180°.16 (c) Halogen-halogen contacts of the type (R-Y1 3 3 3 Y2-R); characterized by the fact that the interhalogen distance is less than the sum of van der Waals radii (rvdW).17,18 There are two preferred arrangements for these contacts; the first arrangement occurs when R-Y1 3 3 3 Y2 angle = Y1 3 3 3 Y2-R angle, henceforth, type I (Scheme 1). The second arrangement occurs when the R-Y1 3 3 3 Y2 angle = 180° and the Y1 3 3 3 Y2-R *To whom correspondence should be addressed. E-mail: fawwadi@ yahoo.com. pubs.acs.org/crystal
Published on Web 11/24/2009
angle = 90°, henceforth, type II (Scheme 1). (d) Halogen-halide interactions of the type C-Y 3 3 3 X-M (M = Cu(II), Co(II), Pd(II), Pt(II); X = F, Cl-, Br-, or I-). The C-Y 3 3 3 X-M interactions are invariably characterized by essentially linear C-Y 3 3 3 X angles with an Y 3 3 3 X contact distance less than the sum of the rvdW.10,19-25 (e) Halogen-halide interactions of the type (R-Y 3 3 3 X-); these interactions are characterized by linear C-Y 3 3 3 X- angles and separation distances less than the sum of rvdW of the halogen atom and the ionic radii of the halide anion.10,26-34 Halogen-halide interactions have been utilized in the synthesis of new solid-state materials, for example, organic based conducting materials.9,35 These interactions not only helped in shaping the internal architecture of the lattice, but also participated in the conducting properties. Yamamoto and co-workers have shown that the halogen bonded supramolecular networks can be used to insulate between conducting nanowires.36 Halogen-halide interactions have been found to play a dominant role in determining the crystal structure of several mixed organic-inorganic materials.19-23,37 Moreover, these interactions have been found to compete with classical hydrogen or complement its role in determining crystal structures.23,26,38-40 Recently, we have shown that the C-Br 3 3 3 X- angles are closer to a linear arrangement in comparison to the corresponding N-H 3 3 3 X- angles (X = Cl and Br).34 Also, halogen-halide interactions have been used in the separation of racemic mixtures.41 Together with the hydrogen bond, halogen-halide interactions have been used to chop CuBr2 infinite chains into decameric units; the longest known copper oligomers.42 The role of halogen bonding in biological molecules and as a potential tool to design enzyme inhibitors and thus drugs has been investigated.43,44 r 2009 American Chemical Society
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Scheme 1. The Two Preferred Geometries for Halogen 3 3 3 Halogen Contacts; Type I, θ1 = θ2; Type II, θ1 = 180°, θ2 = 90°
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Scheme 2. Hydrogen Bond Types
Table 1. Summary of Data Collection and Refinement Parameters for (26DAPH)FeBr4 and (26DA35DBPH)2FeBr4 3 Br crystal
Synthesis and Crystal Growth. (a). (26DAPH)FeBr4. Two mmoles of 2,6-diaminopyridine and FeBr2 3 4H2O were dissolved in 20 mL of absolute ethanol acidified with 5 mL of concentrated HBr. The solution was heated for 5 min then left to evaporate slowly. By the next day, large needle type dark brown crystals had developed, 0.68 g (70%). A section, of dimensions 0.5 0.4 0.08 mm3, was cut from a larger crystal and used for data collection. (b). (26DA35PH)2FeBr4 3 Br. Two mmoles of 2,6-diaminopyridine and FeBr2 3 4H2O were dissolved in 20 mL of absolute ethanol acidified with 5 mL of concentrated HBr. Three milliliters of Br2(l) were added. The solution was stirred and left to evaporate slowly. Red-brown crystals formed after two days, 0.62 g (63%). A crystal, of dimensions 0.26 0.11 0.08 mm3, was used for data collection. Crystal Structure Determination. The crystal structure of (26DAPH)FeBr4, (I), was determined at room temperature. The data collection was carried out on a Syntex P21 diffractometer upgraded to Bruker P4 specifications. Lattice dimensions were obtained from 47 accurately centered high angle reflections. Data were corrected for absorption utilizing Ψ-scan data assuming an ellipsoidal shaped crystal. For (26DA35DBPH)2FeBr4 3 Br, (II), the diffraction data were collected at 81 K on a Bruker 3-circle platform diffractometer equipped with SMART APEX CCD detector. Frame data were acquired with the SMART software, and the frames were processed using SAINT software to give an hkl file corrected for Lp/decay.46,47 Absorption corrections were performed using SADABS.48 For both structures, the SHELXTL package was used for the structure solution and refinement.49 The structures were refined by the least-squares method on F2. Carbon bound hydrogen atoms were placed at the calculated positions using a riding model. Nitrogen bound hydrogen atoms were refined isotropically with restraints; N(amino)-H bond distances were restrained to 0.88 A˚, while no restraints were added on N(aromatic)-H bond distances (Scheme 2). In the structure of II the high residuals are all 2σ] wR2b [I > 2σ] μ, mm-1 ΔFmin and max (e/A˚3) )
Recently, they have been used in surface chemistry to prepare layer-by-layer assembly.45 In this report, we examine the structures of two tetrabromoferrate(III) salts: (I), (26DAPH)FeBr4, where 26DAPH is the 2,6-diaminopyrdinium cation and (II), (26DA35DBPH)2FeBr4 3 Br, where 26DA35DBPH is the 2,6-diamino-3,5-dibromopyridinium cation. The analysis of these two structures will demonstrate that (1) Fe-Br1 3 3 3 Br2-Fe contacts and C-Br1 3 3 3 Br2-Fe interactions show similar geometrical characteristics to that of the well studied C-Br1 3 3 3 Br2-C interactions; (2) Fe-Br1 3 3 3 Br2-Fe contacts form spin 5/2 ladders systems and linear chains in (I) and (II), respectively.
(26DAPH)FeBr4 (I)
Table 2. Selected Bond Distances (A˚) and Angles (°) crystal
(I)
(II)
Fe-Br1 Fe-Br2 Fe-Br3 Fe-Br4 Br1-Fe-Br2 Br1-Fe-Br3 Br1-Fe-Br4 Br2-Fe-Br3 Br2-Fe-Br4 Br3-Fe-Br4
2.331(2) 2.332(2) 2.318(2) 2.343(2) 109.07(7) 109.14(8) 107.44(8) 111.65(8) 107.37(8) 112.05(8)
2.336(1) 2.328(1) 2.359(1) 2.336(1) 110.41(4) 108.50(4) 107.08(4) 111.16(4) 109.48(4) 110.12(4)
Results Molecular Structure. The two analyzed structures contain an approximately tetrahedral [FeBr4-] anion. The Br-FeBr angles avg = 109.45° and 109.44° (range 107.44°-112.03° and 107.8°-111.16°) for (I) and (II), respectively (Table 2). The Fe-Br bond length range is 2.32-2.34 A˚. (II) is a mixedanion hybrid organic inorganic salt, containing a separate bromide anion as well as the [FeBr4-] anion. Electrical neutrality in the two structures is achieved by the isolated planar 2,6-diaminopyridinium cation in (I) and 2,6-diamino3,5-dibromopyrdinium cation in (II). The presence of the bromide anion within the crystalline lattice and bromination of the 2,6-diaminopyrdinium cation during the preparation of (II) has a great influence on the supramolecular chemistry of this compound as will be seen later. The displacement of the non-hydrogen atoms from the mean plane of 26DAPH cation is within (0.01 A˚ in crystal (I). Similarly, in the two crystallographically different 26DA35DBPH cations
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Table 3. C-Br1 3 3 3 Br2 Synthons Distances (A˚) and Angles (°) in (II) Br1 3 3 3 Br2 3.522(1) 3.634(1) 3.531(1) 3.433(1) C-Br1 3 3 3 Br2 160.43(19) 159.97(17) 165.36(18) 176.29(18) Br1 3 3 3 Br2-Fe 113.82(3) 91.48(3) 97.77(3) 82.06(3) C-Br1 3 3 3 Br2-Fe 33.64(52) 28.93(53) 102.85(66) 64.54(2.71) 0.109 0.065 0.002 0.007 organic bromine deviationa 1.246 1.435 0.108 0.019 inorganic bromine deviationa,b a Deviation from the plane of the aromatic ring. b Inorganic bromine is the iron bound bromine atom that is involved in the C-Br1 3 3 3 Br2-Fe interactions.
in crystal (II), the displacement of carbon and nitrogen atoms is within (0.02 A˚. In contrast, the displacement of organic bromines is much larger (Table 3). It can be seen that two of the four crystallographically different organic bromine atoms are displaced significantly from the plane of the cation toward the inorganic bromine that is involved in the C-Br1 3 3 3 Br2-Fe interactions; when the deviation of the inorganic bromine is large (1.246 and 1.435 A˚), the deviation of organic bromine is large (0.109 and 0.065 A˚) and vice versa (Table 3); these interactions will be described later in detail. This distortion of the organic cation is due to the C-Br1 3 3 3 Br2-Fe interactions and reflects the effect of supramolecular structure on the molecular structure.19 Supramolecular Structures. The supramolecular structure of the analyzed structures can be viewed based on three types of interactions: (a) N-H 3 3 3 Br hydrogen bond, (b) C-Br1 3 3 3 Br2-Fe interactions, and (c) Fe-Br1 3 3 3 Br2-Fe contacts. N-H 3 3 3 Br- interactions and Fe-Br1 3 3 3 Br2 interactions are present in the two structures, while C-Br1 3 3 3 Br2-Fe exists only in (II) because the C-Br bond is absent in (I). The geometric nature of the three interactions is shown in Figure 1. Two types of Br 3 3 3 Br contacts are observed in (II) and the data are summarized in Tables 3 and 4; (a) C-Br1 3 3 3 Br2-Fe, where the Br1 3 3 3 Br2 distances range from 3.433 to 3.634 A˚ considerably less by 0.17 A˚ than the sum of van der Waals radii, with a C-Br1 3 3 3 Br2 angle range from 160° to 176.3°, essentially a linear arrangement. In contrast, the Br1 3 3 3 Br2-Fe angles are avg = 90.4° range from 82.07° to 113.82°, essentially a perpendicular arrangement (Type II, Scheme 1). (b) Fe-Br1 3 3 3 Br2-Fe interactions; Br1 3 3 3 Br2 distances range from 3.618 to 3.825 A˚, longer than the Br1 3 3 3 Br2 distance in the C-Br1 3 3 3 Br2-Fe interactions, and angles range from 100.5° to 166.33° (see Tables 3 and 4). Comparison of the interbromide distance in the Fe-Br1 3 3 3 Br2-Fe interactions in the two analyzed structures is not possible due to several reasons; the angles of contacts are different and the two structures are determined at two different temperatures. Most significantly, it will be seen that both C-Br1 3 3 3 Br2-Fe and Fe-Br1 3 3 3 Br2-Fe interactions show similar characteristics to that of well documented C-Br1 3 3 3 Br2-C interactions.4,8,17,51 Four different types of hydrogen bonding are observed in both structures: N(aromatic)-H 3 3 3 Br- and N(amino)H 3 3 3 Br- in (II) (Scheme 2A,B), N(aromatic)-H 3 3 3 Br-Fe in (I) (Scheme 1C), and N(amino)-H 3 3 3 Br-Fe (in both structures, Scheme 2D); the data are summarized in Table 5. Examination of hydrogen bonds of the type N(amino)H 3 3 3 Br-Fe reveals the presence of two patterns of hydrogen bonding: (a) linear (in both crystal structures) and (b) asymmetrical bifurcated hydrogen bonding in (II). In (II),
Figure 1. Synthon interactions in (a) (I), (b) (II). Thermal ellipsoids shown at 50%. N-H 3 3 3 Br, C-Br1 3 3 3 Br2-Fe, and Fe-Br1 3 3 3 Br2 interactions are represented by blue, red, and black dotted lines, respectively.50 Table 4. Fe-Br1 3 3 3 Br2-Fe Contacts Distances (A˚) and Angles (°) in (I) and (II) compound
(I)
(I)
(II)
Br1 3 3 3 Br2 Fe-Br1 3 3 3 Br2 Br1 3 3 3 Br2-Fe Fe-Br1 3 3 3 Br2-Fe
3. 618(2) 166.33(8) 166.33 (8) 180.0 (0)
3.813(2) 159.0(7) 100.5(6) 111.4(2)
3.825(1) 134.81(3) 154.15(3) 77.17(10)
it is noticed that the separate bromide anion is involved in the N(aromatic)-H 3 3 3 Br- hydrogen bonding, and the tetrabromoferrate(III) anion is involved in both C-Br1 3 3 3 Br2-Fe and N(amino)-H 3 3 3 Br-Fe interactions.52 This can be explained by the fact that N(aromatic)-H is a better proton donor and Br- is a better proton acceptor. In contrast, this competition is not observed in (I) because there is no separate halide anion in it. In the crystal structure of (I), Br 3 3 3 Br interactions of the type Fe-Br1 3 3 3 Br2-Fe connect tetrabromoferrate(III) anions to form a ladder structure that runs parallel to the a axis. The Br 3 3 3 Br contact distances along the rungs and rails are 3.618 and 3.814 A˚, respectively. The contact distances within the rung are extremely short in comparison to all reported copper ladder structures.53 The ladder structure is stabilized by N(amino)-H 3 3 3 Br-Fe hydrogen bonds; within the rails, the two Fe-Br 3 3 3 Br-Fe synthons are effectively hydrogen bonded to the same cation, as shown in Figure 2. Furthermore, the nonclassical C-H 3 3 3 Br-Fe hydrogen bond participates in stabilizing these structures. The organic cations tie, via hydrogen bonding, these ladders together to form a layer structure lying in the abplane (Figure 3). These layers are then aggregated into the
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Table 5. Hydrogen Bond Parameters crystal (I)
(II)
-
N3 3 3X 3.651(9) 3.581(13) 3.617(11) 3.709(13) 3.173(5) 3.182(5) 3.492(5) 3.979(6) 3.649(5) 3.373(5) 3.948(5) 3.495(5) 3.404(5) 3.618(5) 3.649(6)
-
H3 3 3X 2.792(68) 2.808(53) 2.924(88) 3.006(62) 2.347(59) 2.335(59) 3.062(63) 3.163(35) 2.836(34) 2.974(62) 3.236(45) 2.829(47) 2.808(44) 2.932(45) 2.883(33)
N-H 3 3 3 X149.0(4.7) 147.0(7.5) 136.6(10.2) 138.5(7.4) 167.6(6.4) 162.5(6.1) 112.6(5.1) 156.3(5.9) 154.0(5.5) 109.5(4.7) 138.9(5.1) 133.0(4.9) 126.4(4.5) 136.0(5.0) 148.6(4.5)
pattern of H-bond linear linear linear linear linear linear asym bifurcated
type of H-bond N(aromatic)-H 3 3 3 Br-Fe N(amino)-H 3 3 3 Br-Fe N(amino)-H 3 3 3 Br-Fe N(amino)-H 3 3 3 Br-Fe N(aromatic)-H 3 3 3 BrN(aromatic)-H 3 3 3 BrN(amino)-H 3 3 3 Br-Fe N(amino)-H 3 3 3 BrN(amino)-H 3 3 3 BrN(amino)-H 3 3 3 Br-Fe N(amino)-H 3 3 3 BrN(amino)-H 3 3 3 Br-Fe N(amino)-H 3 3 3 Br-Fe N(amino)-H 3 3 3 BrN(amino)-H 3 3 3 Br-Fe
asym bifurcated asym bifurcated asym bifurcated linear
Figure 2. Ladder structure of (I). The ladder runs parallel to the a axis. The Fe-Br 3 3 3 Br-Fe contacts and N(amino)-H 3 3 3 Br-Fe hydrogen bond are represented by black and blue dotted lines, respectively. Figure 4. Three-dimensional structure of (I) showing two layers connected via hydrogen bonding. Fe-Br1 3 3 3 Br2-Fe interactions are shown in black, N-H 3 3 3 Br-Fe hydrogen bonding in blue, and long packing Fe-Br1 3 3 3 Br2-Fe interactions in red.
Figure 3. Layer structure of (I). The layer lies in the ab plane. Fe-Br 3 3 3 Br-Fe interactions, N(amino)-H 3 3 3 Br-Fe and N(aromatic)-H 3 3 3 Br-Fe hydrogen bonds are represented by black, blue, and red dotted lines, respectively.
final three-dimensional lattice via hydrogen bonding. The [FeBr4-] anions pack in such a way (Figure 4) so as to form an arrangement similar to the preferred arrangement in C-Br1 3 3 3 Br2-C interactions,17 with a long Br 3 3 3 Br distance, 4.197 A˚, and Fe-Br1 3 3 3 Br2 and Br1 3 3 3 Br2-Fe angles 177.98° and 104.61°, respectively. This packing pattern, henceforth long-range Fe-Br1 3 3 3 Br2-Fe packing contacts, is probably due to reduced repulsion forces rather than to attractive forces. The bromination of the 2,6-dibromopyridinum cation transforms the cation from a trifunctional synthon (via hydrogen bonding) into a pentafunctional synthon (via both hydrogen bonding and C-Br1 3 3 3 Br2-Fe interactions). Hydrogen bonding as previously described links the cationic and anionic species to form a chain structure, with the chains running parallel to the c-axis (Figure 5). These chains are linked via C-Br1 3 3 3 Br2-Fe interactions to form layer structures lying in the ac-plane as shown in Figure 6. Using N(amino)H 3 3 3 Br-Fe hydrogen bonding, the layers aggregate to form
Figure 5. Chain structure of (II). The chain runs parallel to the c-axis. Hydrogen bonds are shown in blue.
a 3D lattice. This aggregation is facilitated by π-π stacking. There are two crystallographically different cations in the crystal structure, and each of them is involved in π-π overlap with its crystallographically equivalent cation (Figure 7). The perpendicular distances between the planes and the centroid-centroid distance are 3.24 and 3.64 A˚ for cation I, and 3.25 and 3.77 A˚ for cation II. Tetrabromoferrate(III) anions form a spin 5/2 linear chain; the interbromide distance is 3.825 A˚ and the Fe-Br1 3 3 3 Br2 and Br1 3 3 3 Br2-Fe angles are 154.2° and 134.8°, respectively (Figure 8). Discussion The geometrical characteristics of C-Br1 3 3 3 Br2-Fe interactions and Fe-Br1 3 3 3 Br2-Fe contacts are similar to the
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Figure 9. Calculated electrostatic potential surface for [FeBr4-] anion and 25DA35DBPH cation. The energy is expressed in Hartrees. The electron density contour isovalue is set to 0.005. The potential was calculated using b3lyp and aug-cc-pvtz basis sets on all atoms except on Iron (6-31 g).
Figure 6. Layer structure of (II). The layer lies parallel to ac plane. C-Br1 3 3 3 Br2-Fe interactions and hydrogen bonds are shown in blue and red dotted lines, respectively.
Figure 7. Illustration of π-π stacking in crystal II for (A) cation I and (B) cation. II Views are from the normal to the planes of the cations.
Figure 8. Illustration of the structure of the magnetic chains in (II). The Fe-Br1 3 3 3 Br2-Fe contacts are represented by black lines.
characteristics of the well studied C-Br1 3 3 3 Br2-C interactions (Scheme 1).4,8,17,51 We extend definition of type I and II interactions to include the analogous C-Br1 3 3 3 Br2-Fe and Fe-Br1 3 3 3 Br2-Fe contacts. Type I arrangement is observed in the rungs of the ladder in (I) with Fe-Br1 3 3 3 Br2 = Br1 3 3 3 Br2-Fe = 166.3°. All C-Br1 3 3 3 Br2-Fe interactions in (II) follow the type II arrangement, θ1 avg = 165.5° and θ2 avg = 96.4° (Tables 3 and 4). Similarly, Fe-Br1 3 3 3 Br2-Fe interactions within the rails of (I) and the long-range packing contacts in it obey type II arrangement, with θ1 = 159° and
θ2 = 100.5° and θ1 = 179.0° and θ2 = 104.6° for normal Fe-Br1 3 3 3 Br2-Fe interactions and long-range packing interactions, respectively. The electrostatic effects and the deformation of electric charge around the bromine atom have been found to play a crucial role in the geometrical arrangements of the synthons in the C-Br1 3 3 3 Br2-C and C-Br 3 3 3 Br- interactions.17,26,54 Similarly, electrostatic interactions are expected to influence the arrangements of synthons in the C-Br1 3 3 3 Br2-Fe and Fe-Br1 3 3 3 Br2-Fe interactions. In this context, in (II), the nearly linear angle is always C-Br1 3 3 3 Br2 and the perpendicular angle is the Br1 3 3 3 Br2-Fe angle (Type II). This would be because the iron attached bromine atom carries more negative charge than the carbon attached bromine atom. Also, this would point to the fact that the anisotropic distribution of the electronic charge idea can be extended to the iron attached bromide anion.55 The electrostatic potential around the bromine atom in [FeBr4-] is calculated as shown in Figure 9.56 The electrostatic potential values in the π region of the bromine atom are lower than that in the atom end-cap. This figure predicts the two observed geometries in the two analyzed structures; the higher the electrostatic potential in the first synthon (either the positive electrostatic end-cap in C-Br bond or less negative electrostatic end-cap in Fe-Br bond) encounters the lower electrostatic potential in the second synthon (the π-region around the bromine atom in Fe-Br bond) either to increase the attractive forces or to reduce the repulsive forces. The negative electrostatic potential ring around the π region of bromine atom perpendicular to the Fe-Br bond should face either the positive electrostatic end-cap along the C-Br bond in 26DA35DBPH cation or less negative electrostatic potential end-cap along Fe-Br bond in [FeBr4-] (Figure 9). C-Br1 3 3 3 Br2-Fe interactions and Fe-Br1 3 3 3 Br2Fe contacts were reported in several published crystal structures.52,57-62 These structures and those reported in this paper indicates that (a) for C-Br1 3 3 3 Br2-Fe interactions, C-Br1 3 3 3 Br2 angle is always greater than Br1 3 3 3 Br2-Fe angle.52,60-62 This agrees with our observation that CBr1 3 3 3 Br2 angle is nearly linear angle, while Br1 3 3 3 Br2-Fe is the perpendicular one (Type II interactions) (Scheme 1). (b) Fe-Br1 3 3 3 Br2-Fe contacts compete with and complement the role C-Br1 3 3 3 Br2-Fe interactions in determining the supramolecular structure of these crystalline materials. If the organic part of these materials contains carbon bound bromine atom, the crystal structure is dominated C-Br1 3 3 3 Br2-Fe interactions (crystal II and those previously published structures); there is no Fe-Br1 3 3 3 Br2-Fe contacts within the sum of rvdW of the bromine atoms.52,60-62 Even
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though there are Fe-Br1 3 3 3 Br2-Fe contacts with interbromine distance little bit longer than the rvdW. The Br1 3 3 3 Br2 distance in crystal II is 3.825 A˚ (Table 4). In contrast, if the organic part does not contain carbon bound bromine atom, in some of these crystal structures, the inter-bromine distance in Fe-Br1 3 3 3 Br2-Fe contacts becomes less than the sum of rvdW (crystal I and previously published structures).23,57-59 This indicated that C-Br1 3 3 3 Br2-Fe interactions prevail over Fe-Br1 3 3 3 Br2-Fe contacts. The magneto-structural correlations of mixed organicinorganic materials have received a lot of interest.53,63-66 A special type of magneto-structural pathway is the two halide pathway. The crystal structure analysis of (I) reveals that Fe-Br1 3 3 3 Br2-Fe interactions define a spin ladder structure; the Br1 3 3 3 Br2 distances within the rungs are extremely short, 3.618 A˚.27,53 The Br 3 3 3 Br distances in (I) are shorter than any Br 3 3 3 Br distance in any reported copper spin ladder structure.27 Also, (II) forms a linear chain structure based on Br 3 3 3 Br interactions, also with a relatively short contact distance, 3.825 A˚. This makes these two structures potential models for the study of the two-halide pathway in spin 5/2 systems; both in spin linear chains and spin ladder systems. These short contact distances are expected to influence the magnetic properties of these two compounds. Further studies into this area are ongoing. Conclusions The above analysis indicates that (a) the geometrical arrangement of C-Br and Fe-Br synthons in the C-Br1 3 3 3 Br2-Fe interactions and Fe-Br1 3 3 3 Br2-Fe contacts is similar to C-Br1 3 3 3 Br2-C interactions; (b) Fe-Br1 3 3 3 Br2Fe contacts compete with C-Br1 3 3 3 Br2-Fe interactions, though the later prevails. (c) C-Br1 3 3 3 Br2-Fe interactions and Fe-Br1 3 3 3 Br2-Fe contacts can be explained using the calculated electrostatic potential of the [FeBr4-] ion and the electrostatic potential around the C-bound bromine atom.17 The positive electrostatic end-cap of the first synthon (C-Br) should encounter the more negative potential ring in the second synthon (Fe-Br). In the Fe-Br1 3 3 3 Br2-Fe contacts, the less negative electrostatic potential end-cap should face more negative electrostatic potential ring. Acknowledgment. The Bruker (Siemens) SMART CCD diffraction facility was established at the University of Idaho with the assistance of the NSF-EPSCoR program and the M. J. Murdock Charitable Trust, Vancouver, WA, USA. Supporting Information Available: Crystal data in CIF format. This material is available free of charge via the Internet at http:// pubs.acs.org.
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