pubs.acs.org/crystal Published on Web 11/24/2009 r 2009 American Chemical Society

DOI: 10.1021/cg900762s

2010, Vol. 10158–164

The Analogy of C-Br 3 3 3Br-C, C-Br 3 3 3Br-Fe, and Fe-Br 3 3 3Br-Fe

Contacts: Crystal Structures of (26DAPH)FeBr4 and(26DA35DBPH)2FeBr4 3Br

Firas Awwadi,*,† Salim F. Haddad,‡ Roger D. Willett,# and Brendan Twamley )

†Department of Chemistry, Tafila Technical University, Tafila 66110 Jordan, ‡Department ofChemistry, 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 3Br2-Fe and Fe-Br1 3 3 3Br2-Fe interactions in the iron(III) structures (26DAPH)FeBr4(I) and (26DA35DBPH)2FeBr4 3Br (II) (where 26DAPH is 2,6-diaminopyridinium and 26DA35DBPH is 2,6-diamino-3,5-dibromopyridinium) is investigated. In (I), the ions are arranged inside the crystalline lattice based on the N-H 3 3 3Br-Fehydrogen bond, including other types of interaction. The [FeBr4

-] anions form a ladder structure with extremely shortinterbromide distances, 3.618 A within the rungs and 3.814 A within the rails. There are two Fe-Br1 3 3 3Br2-Fe contact types;the first type has a Fe-Br1 3 3 3Br2 angle = Br1 3 3 3Br2-Fe angle which is equal to 166.6�. In the second, the Fe-Br1 3 3 3Br2angle = 100.5� and Br1 3 3 3Br2-Fe angle = 159.1�. In contrast, in (II), the supramolecular assembly of cations and anions isdominated by the C-Br1 3 3 3Br2-Fe interaction and the more traditional N-H 3 3 3Br

- hydrogen bond. Here the [FeBr4-]

anions form linear chains with interbromide distances of ca. 3.83 A. Most interestingly, the geometrical characteristics of theC-Br1 3 3 3Br2-Fe contacts and the Fe-Br1 3 3 3Br2-Fe contacts, present in (I) and (II), are similar to the well characterizedC-Br1 3 3 3Br2-C contacts; the C-Br1 3 3 3Br2 angles are essentially linear (avg = 165.5�, range 160.0-176.3�) and theBr1 3 3 3Br2-Fe angles are essentially normal to the axis containing C-Br1 3 3 3Br2 atoms (avg = 90.4�, range 82.1-113.8�).

Introduction

The self-assembly of crystalline species inside crystallinelattices has received a lot of interest in recent years, in boththeoretical and experimental chemistry.1-8 This is one of themain foci of crystal engineering. This science studies thearrangement of crystalline species inside crystalline latticesto predict the crystal structures, and hence designing newsolid-state materials with desired properties such asmagnetic,electrical, and nonlinear optical properties.9 One of the mainfactors that determine the self-assembly ofmolecules and ionsin crystals is the intermolecular forces, such as hydrogen-bonding, halogen-bonding, and others.5,10-12

The halogen bond is a noncovalent intermolecular interac-tion that leads to the arrangement A-Y 3 3 3B, where Y is ahalogen atom.6,11-13 Many types of halogen bonds have beenstudied in detail. This includes (a) the complexes betweendihalogensXYandLewis bases nucleophiles (Nu), XY 3 3 3Nu.These complexes have been investigated using theoreticalcalculation and rotational spectroscopy.14,15 (b) Complexesbetween carbon-halogen atoms (except fluorine) and nucleo-philes (Nu), C-Y 3 3 3Nu.16 Nucleophiles tend to approach thehalogen atom of the C-Y bond at an angle of around 180�.16(c) Halogen-halogen contacts of the type (R-Y1 3 3 3Y2-R);characterized by the fact that the interhalogen distance is lessthan the sum of van der Waals radii (rvdW).17,18 There are twopreferred arrangements for these contacts; the first arrange-ment occurs when R-Y1 3 3 3Y2 angle = Y1 3 3 3Y2-R angle,henceforth, type I (Scheme 1). The second arrangement occurswhen the R-Y1 3 3 3Y2 angle = 180� and the Y1 3 3 3Y2-R

angle = 90�, henceforth, type II (Scheme 1). (d) Halogen--halide interactions of the type C-Y 3 3 3X-M (M=Cu(II),Co(II), Pd(II), Pt(II); X = F, Cl-, Br-, or I-). The C-Y 3 3 3X-M interactions are invariably characterized by essentiallylinear C-Y 3 3 3X angles with an Y 3 3 3X contact distance lessthan the sum of the rvdW.10,19-25 (e) Halogen-halide interac-tions of the type (R-Y 3 3 3X

-); these interactions are char-acterized by linear C-Y 3 3 3X

- angles and separationdistances less than the sum of rvdW of the halogen atom andthe ionic radii of the halide anion.10,26-34

Halogen-halide interactions have been utilized in thesynthesis of new solid-state materials, for example, organicbased conducting materials.9,35 These interactions not onlyhelped in shaping the internal architecture of the lattice, butalso participated in the conducting properties. Yamamotoand co-workers have shown that the halogen bonded supra-molecular networks can be used to insulate between conduct-ing nanowires.36 Halogen-halide interactions have beenfound to play a dominant role in determining the crystal struc-ture of several mixed organic-inorganic materials.19-23,37

Moreover, these interactions have been found to competewith classical hydrogen or complement its role in determiningcrystal structures.23,26,38-40 Recently, we have shown that theC-Br 3 3 3X

- angles are closer to a linear arrangement incomparison to the corresponding N-H 3 3 3X

- angles (X =Cl and Br).34 Also, halogen-halide interactions have beenused in the separation of racemic mixtures.41 Together withthe hydrogen bond, halogen-halide interactions have beenused to chop CuBr2 infinite chains into decameric units; thelongest known copper oligomers.42 The role of halogen bond-ing in biological molecules and as a potential tool to designenzyme inhibitors and thus drugs has been investigated.43,44

*To whom correspondence should be addressed. E-mail: fawwadi@yahoo.com.

Article Crystal Growth & Design, Vol. 10, No. 1, 2010 159

Recently, they have been used in surface chemistry to preparelayer-by-layer assembly.45

In this report, we examine the structures of two tetrabromo-ferrate(III) salts: (I), (26DAPH)FeBr4, where 26DAPH is the2,6-diaminopyrdinium cation and (II), (26DA35DBPH)2-FeBr4 3Br, where 26DA35DBPH is the 2,6-diamino-3,5-di-bromopyridinium cation. The analysis of these two structureswill demonstrate that (1) Fe-Br1 3 3 3Br2-Fe contacts andC-Br1 3 3 3Br2-Fe interactions show similar geometricalcharacteristics to that of the well studied C-Br1 3 3 3Br2-Cinteractions; (2) Fe-Br1 3 3 3Br2-Fe contacts form spin 5/2ladders systems and linear chains in (I) and (II), respectively.

Experimental Section

Synthesis and Crystal Growth. (a). (26DAPH)FeBr4. Twommoles of 2,6-diaminopyridine and FeBr2 3 4H2O were dissolvedin 20 mL of absolute ethanol acidified with 5 mL of concentratedHBr. The solution was heated for 5 min then left to evaporateslowly. By the next day, large needle type dark brown crystals haddeveloped, 0.68 g (70%). A section, of dimensions 0.5 � 0.4 � 0.08mm3, was cut from a larger crystal and used for data collection.

(b). (26DA35PH)2FeBr4 3Br. Two mmoles of 2,6-diaminopyri-dine and FeBr2 3 4H2O were dissolved in 20 mL of absoluteethanol acidified with 5 mL of concentrated HBr. Three millilitersof Br2(l) were added. The solution was stirred and left to evaporateslowly. 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 datacollection.

Crystal Structure Determination. The crystal structure of(26DAPH)FeBr4, (I), was determined at room temperature. Thedata collection was carried out on a Syntex P21 diffractometerupgraded to Bruker P4 specifications. Lattice dimensions wereobtained from 47 accurately centered high angle reflections. Datawere corrected for absorption utilizing Ψ-scan data assuming anellipsoidal shaped crystal. For (26DA35DBPH)2FeBr4 3Br, (II), thediffraction data were collected at 81K on a Bruker 3-circle platformdiffractometer equipped with SMART APEX CCD detector.Frame data were acquired with the SMART software, and theframes were processed using SAINT software to give an hkl filecorrected forLp/decay.46,47Absorption correctionswere performedusing SADABS.48 For both structures, the SHELXTL package wasused for the structure solution and refinement.49 The structureswere refined by the least-squares method on F2. Carbon boundhydrogen atoms were placed at the calculated positions using ariding model. Nitrogen bound hydrogen atoms were refined iso-tropically with restraints; N(amino)-H bond distances were re-strained to 0.88 A, while no restraints were added onN(aromatic)-H bond distances (Scheme 2). In the structure of IIthe high residuals are all <1 A to Br atoms. The highest residual of1.8 e/A3 is ca. 0.8 A from Br7. Data collection parameters andrefinement results are given in Table 1.

Results

Molecular Structure. The two analyzed structures containan approximately tetrahedral [FeBr4

-] anion. The Br-Fe-Br 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 amixed-anion hybrid organic inorganic salt, containing a separatebromide anion as well as the [FeBr4

-] anion. Electricalneutrality in the two structures is achieved by the isolatedplanar 2,6-diaminopyridinium cation in (I) and 2,6-diamino-3,5-dibromopyrdinium cation in (II). The presence of thebromide anion within the crystalline lattice and brominationof the 2,6-diaminopyrdinium cation during the preparationof (II) has a great influence on the supramolecular chemistryof this compound as will be seen later. The displacement ofthe non-hydrogen atoms from the mean plane of 26DAPHcation is within (0.01 A in crystal (I). Similarly, in thetwo crystallographically different 26DA35DBPH cations

Table 1. Summary of Data Collection and Refinement Parameters for

(26DAPH)FeBr4 and (26DA35DBPH)2FeBr4 3Br

crystal (26DAPH)FeBr4 (I)(26DA35DBPH)2-

FeBr4 3Br (II)

formula C5H8Br4FeN3 C10H12Br9FeN6

Mr 485.63 991.30Fcalc (Mg/m3) 2.514 2.736T (K) 295 81crystal system monoclinic orthorhombicspace group P2(1)/c Pbcaa (A) 6.971(1) 15.794(1)b (A) 9.046(1) 15.194(1)c (A) 21.165(2) 20.058(1)β (�) 106.01(1) 90V (A3) 1282.9(3) 4813.2(6)ind reflections 2251 5990data/restraints/parameters 2251/4/139 5990/8/267R (int) 0.0731 0.0957Z 4 8goodness of fit 0.996 0.961R1

a [I > 2σ] 0.0580 0.0426wR2

b [I > 2σ] 0.1323 0.0922μ, mm-1 13.598 15.579ΔFmin and max (e/A

3) 0.644 and -0.873 1.794 and -0.987

a R1 = Σ )Fo| - |Fc )/|Σ|Fo|.b wR2 = {Σw(Fo

2 - Fc2)2/Σ w(Fo

2)2}1/2.

Scheme 1. The Two Preferred Geometries for

Halogen 3 3 3Halogen Contacts; Type I, θ1 = θ2; Type II,θ1 = 180�, θ2 = 90�

Scheme 2. Hydrogen Bond Types

Table 2. Selected Bond Distances (A) and Angles (�)

crystal (I) (II)

Fe-Br1 2.331(2) 2.336(1)Fe-Br2 2.332(2) 2.328(1)Fe-Br3 2.318(2) 2.359(1)Fe-Br4 2.343(2) 2.336(1)Br1-Fe-Br2 109.07(7) 110.41(4)Br1-Fe-Br3 109.14(8) 108.50(4)Br1-Fe-Br4 107.44(8) 107.08(4)Br2-Fe-Br3 111.65(8) 111.16(4)Br2-Fe-Br4 107.37(8) 109.48(4)Br3-Fe-Br4 112.05(8) 110.12(4)

160 Crystal Growth & Design, Vol. 10, No. 1, 2010 Awwadi et al.

in crystal (II), the displacement of carbon and nitrogenatoms is within (0.02 A. In contrast, the displacement oforganic bromines is much larger (Table 3). It can be seen thattwo of the four crystallographically different organic bro-mine atoms are displaced significantly from the plane of thecation toward the inorganic bromine that is involved in theC-Br1 3 3 3Br2-Fe interactions; when the deviation ofthe inorganic bromine is large (1.246 and 1.435 A),the deviation of organic bromine is large (0.109 and0.065 A) and vice versa (Table 3); these interactions willbe described later in detail. This distortion of the organiccation is due to the C-Br1 3 3 3Br2-Fe interactions andreflects the effect of supramolecular structure on the mole-cular structure.19

Supramolecular Structures. The supramolecular structureof the analyzed structures can be viewed based on three typesof interactions: (a) N-H 3 3 3Br hydrogen bond, (b)C-Br1 3 3 3Br2-Fe interactions, and (c) Fe-Br1 3 3 3Br2-Fecontacts. N-H 3 3 3Br

- interactions and Fe-Br1 3 3 3Br2 in-teractions are present in the two structures, whileC-Br1 3 3 3Br2-Fe exists only in (II) because the C-Br bondis absent in (I). The geometric nature of the three interactionsis shown in Figure 1.

Two types of Br 3 3 3Br contacts are observed in (II) and thedata are summarized in Tables 3 and 4; (a) C-Br1 3 3 3Br2-Fe, where the Br1 3 3 3Br2 distances range from 3.433to 3.634 A considerably less by 0.17 A than the sum of vanderWaals radii, with a C-Br1 3 3 3Br2 angle range from 160�to 176.3�, essentially a linear arrangement. In contrast, theBr1 3 3 3Br2-Fe angles are avg = 90.4� range from 82.07� to113.82�, essentially a perpendicular arrangement (Type II,Scheme 1). (b) Fe-Br1 3 3 3Br2-Fe interactions; Br1 3 3 3Br2distances range from 3.618 to 3.825 A, longer than theBr1 3 3 3Br2 distance in the C-Br1 3 3 3Br2-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 3Br2-Fe interactions in the two analyzed structures is notpossible due to several reasons; the angles of contacts aredifferent and the two structures are determined at twodifferent temperatures. Most significantly, it will be seenthat both C-Br1 3 3 3Br2-Fe and Fe-Br1 3 3 3Br2-Fe inter-actions show similar characteristics to that of well documen-ted C-Br1 3 3 3Br2-C interactions.4,8,17,51

Four different types of hydrogen bonding are observed inboth structures: N(aromatic)-H 3 3 3Br

- and N(amino)-H 3 3 3Br

- in (II) (Scheme 2A,B), N(aromatic)-H 3 3 3Br-Fein (I) (Scheme 1C), and N(amino)-H 3 3 3Br-Fe (in bothstructures, Scheme 2D); the data are summarized in Table 5.Examination of hydrogen bonds of the type N(amino)-H 3 3 3Br-Fe reveals the presence of twopatterns of hydrogenbonding: (a) linear (in both crystal structures) and (b)asymmetrical bifurcated hydrogen bonding in (II). In (II),

it is noticed that the separate bromide anion is involved in theN(aromatic)-H 3 3 3Br

- hydrogen bonding, and the tetra-bromoferrate(III) anion is involved in both C-Br1 3 3 3Br2-Fe and N(amino)-H 3 3 3Br-Fe interactions.52 Thiscan be explained by the fact that N(aromatic)-H is a betterproton donor and Br- is a better proton acceptor. Incontrast, this competition is not observed in (I) because thereis no separate halide anion in it.

In the crystal structure of (I), Br 3 3 3Br interactions of thetype Fe-Br1 3 3 3Br2-Fe connect tetrabromoferrate(III) an-ions to form a ladder structure that runs parallel to the a axis.The Br 3 3 3Br contact distances along the rungs and rails are3.618 and 3.814 A, respectively. The contact distances withinthe rung are extremely short in comparison to all reportedcopper ladder structures.53 The ladder structure is stabilizedbyN(amino)-H 3 3 3Br-Fe hydrogen bonds; within the rails,the two Fe-Br 3 3 3Br-Fe synthons are effectively hydrogenbonded to the same cation, as shown in Figure 2. Further-more, the nonclassical C-H 3 3 3Br-Fe hydrogen bond par-ticipates in stabilizing these structures.

The organic cations tie, via hydrogen bonding, theseladders together to form a layer structure lying in the ab-plane (Figure 3). These layers are then aggregated into the

Table 3. C-Br1 3 3 3Br2 Synthons Distances (A) and Angles (�) in (II)

Br1 3 3 3Br2 3.522(1) 3.634(1) 3.531(1) 3.433(1)C-Br1 3 3 3Br2 160.43(19) 159.97(17) 165.36(18) 176.29(18)Br1 3 3 3Br2-Fe 113.82(3) 91.48(3) 97.77(3) 82.06(3)C-Br1 3 3 3Br2-Fe 33.64(52) 28.93(53) 102.85(66) 64.54(2.71)organic brominedeviationa

0.109 0.065 0.002 0.007

inorganicbromine deviationa,b

1.246 1.435 0.108 0.019

aDeviation from the plane of the aromatic ring. b Inorganic bromineis the iron bound bromine atom that is involved in the C-Br1 3 3 3Br2-Fe interactions.

Table 4. Fe-Br1 3 3 3Br2-Fe Contacts Distances (A) and Angles (�) in (I)and (II)

compound (I) (I) (II)

Br1 3 3 3Br2 3. 618(2) 3.813(2) 3.825(1)Fe-Br1 3 3 3Br2 166.33(8) 159.0(7) 134.81(3)Br1 3 3 3Br2-Fe 166.33 (8) 100.5(6) 154.15(3)Fe-Br1 3 3 3Br2-Fe 180.0 (0) 111.4(2) 77.17(10)

Figure 1. Synthon interactions in (a) (I), (b) (II). Thermal ellipsoidsshown at 50%. N-H 3 3 3Br, C-Br1 3 3 3Br2-Fe, and Fe-Br1 3 3 3Br2 interactions are represented by blue, red, and black dotted lines,respectively.50

Article Crystal Growth & Design, Vol. 10, No. 1, 2010 161

final three-dimensional lattice via hydrogen bonding. The[FeBr4

-] anions pack in such a way (Figure 4) so as to forman arrangement similar to the preferred arrangement inC-Br1 3 3 3Br2-C interactions,17 with a long Br 3 3 3Br dis-tance, 4.197 A, and Fe-Br1 3 3 3Br2 and Br1 3 3 3Br2-Feangles 177.98� and 104.61�, respectively. This packing pat-tern, henceforth long-range Fe-Br1 3 3 3Br2-Fe packingcontacts, is probably due to reduced repulsion forces ratherthan to attractive forces.

The brominationof the 2,6-dibromopyridinumcation trans-forms the cation from a trifunctional synthon (via hydrogenbonding) into a pentafunctional synthon (via both hydrogenbonding and C-Br1 3 3 3Br2-Fe interactions). Hydrogenbonding as previously described links the cationic and anionicspecies to form a chain structure, with the chains runningparallel to the c-axis (Figure 5). These chains are linked viaC-Br1 3 3 3Br2-Fe interactions to form layer structures lyingin the ac-plane as shown in Figure 6. Using N(amino)-H 3 3 3Br-Fe hydrogen bonding, the layers aggregate to form

a 3D lattice. This aggregation is facilitated by π-π stacking.There are two crystallographically different cations in thecrystal structure, and each of them is involved in π-π overlapwith its crystallographically equivalent cation (Figure 7). Theperpendicular distances between the planes and the cen-troid-centroid distance are 3.24 and 3.64 A for cation I, and3.25 and 3.77 A for cation II. Tetrabromoferrate(III) anionsform a spin 5/2 linear chain; the interbromide distance is 3.825A and the Fe-Br1 3 3 3Br2 and Br1 3 3 3Br2-Fe angles are154.2� and 134.8�, respectively (Figure 8).

Discussion

The geometrical characteristics ofC-Br1 3 3 3Br2-Fe inter-actions and Fe-Br1 3 3 3Br2-Fe contacts are similar to the

Table 5. Hydrogen Bond Parameters

crystal N 3 3 3X- H 3 3 3X

- N-H 3 3 3X- pattern of H-bond type of H-bond

(I) 3.651(9) 2.792(68) 149.0(4.7) linear N(aromatic)-H 3 3 3Br-Fe3.581(13) 2.808(53) 147.0(7.5) linear N(amino)-H 3 3 3Br-Fe3.617(11) 2.924(88) 136.6(10.2) linear N(amino)-H 3 3 3Br-Fe3.709(13) 3.006(62) 138.5(7.4) linear N(amino)-H 3 3 3Br-Fe

(II) 3.173(5) 2.347(59) 167.6(6.4) linear N(aromatic)-H 3 3 3Br-

3.182(5) 2.335(59) 162.5(6.1) linear N(aromatic)-H 3 3 3Br-

3.492(5) 3.062(63) 112.6(5.1) asym bifurcated N(amino)-H 3 3 3Br-Fe3.979(6) 3.163(35) 156.3(5.9) N(amino)-H 3 3 3Br

-

3.649(5) 2.836(34) 154.0(5.5) asym bifurcated N(amino)-H 3 3 3Br-

3.373(5) 2.974(62) 109.5(4.7) N(amino)-H 3 3 3Br-Fe3.948(5) 3.236(45) 138.9(5.1) asym bifurcated N(amino)-H 3 3 3Br

-

3.495(5) 2.829(47) 133.0(4.9) N(amino)-H 3 3 3Br-Fe3.404(5) 2.808(44) 126.4(4.5) asym bifurcated N(amino)-H 3 3 3Br-Fe3.618(5) 2.932(45) 136.0(5.0) N(amino)-H 3 3 3Br

-

3.649(6) 2.883(33) 148.6(4.5) linear N(amino)-H 3 3 3Br-Fe

Figure 2. Ladder structure of (I). The ladder runs parallel to the aaxis. The Fe-Br 3 3 3Br-Fe contacts and N(amino)-H 3 3 3Br-Fehydrogen bond are represented by black and blue dotted lines,respectively.

Figure 3. Layer structure of (I). The layer lies in the ab plane.Fe-Br 3 3 3Br-Fe interactions, N(amino)-H 3 3 3Br-Fe and N-(aromatic)-H 3 3 3Br-Fe hydrogen bonds are represented by black,blue, and red dotted lines, respectively.

Figure 4. Three-dimensional structure of (I) showing two layersconnected via hydrogen bonding. Fe-Br1 3 3 3Br2-Fe interactionsare shown in black, N-H 3 3 3Br-Fe hydrogen bonding in blue, andlong packing Fe-Br1 3 3 3Br2-Fe interactions in red.

Figure 5. Chain structure of (II). The chain runs parallel to thec-axis. Hydrogen bonds are shown in blue.

162 Crystal Growth & Design, Vol. 10, No. 1, 2010 Awwadi et al.

characteristics of the well studied C-Br1 3 3 3Br2-C interac-tions (Scheme 1).4,8,17,51 We extend definition of type I and IIinteractions to include the analogous C-Br1 3 3 3Br2-Fe andFe-Br1 3 3 3Br2-Fe contacts. Type I arrangement is observedin the rungs of the ladder in (I) with Fe-Br1 3 3 3Br2 =Br1 3 3 3Br2-Fe=166.3�. All C-Br1 3 3 3Br2-Fe interactionsin (II) follow the type II arrangement, θ1 avg=165.5� and θ2avg = 96.4� (Tables 3 and 4). Similarly, Fe-Br1 3 3 3Br2-Feinteractions within the rails of (I) and the long-range packingcontacts in it obey type II arrangement, with θ1 = 159� and

θ2 = 100.5� and θ1 = 179.0� and θ2 = 104.6� for normalFe-Br1 3 3 3Br2-Fe interactions and long-range packing in-teractions, respectively.

The electrostatic effects and the deformation of electriccharge around the bromine atom have been found to play acrucial role in the geometrical arrangements of the synthons inthe C-Br1 3 3 3Br2-C and C-Br 3 3 3Br

- interactions.17,26,54

Similarly, electrostatic interactions are expected to influencethe arrangements of synthons in the C-Br1 3 3 3Br2-Fe andFe-Br1 3 3 3Br2-Fe interactions. In this context, in (II), thenearly linear angle is always C-Br1 3 3 3Br2 and the perpendi-cular angle is the Br1 3 3 3Br2-Fe angle (Type II). This wouldbe because the iron attached bromine atom carries morenegative charge than the carbon attached bromine atom.Also, this would point to the fact that the anisotropic dis-tribution of the electronic charge idea can be extended to theiron attached bromide anion.55 The electrostatic potentialaround the bromine atom in [FeBr4

-] is calculated as shownin Figure 9.56 The electrostatic potential values in the π regionof the bromine atom are lower than that in the atom end-cap.This figure predicts the two observed geometries in the twoanalyzed structures; the higher the electrostatic potential inthe first synthon (either the positive electrostatic end-cap inC-Br bond or less negative electrostatic end-cap in Fe-Brbond) encounters the lower electrostatic potential in thesecond synthon (the π-region around the bromine atom inFe-Br bond) either to increase the attractive forces or toreduce the repulsive forces. The negative electrostatic poten-tial ring around the π region of bromine atom perpendicularto the Fe-Br bond should face either the positive electrostaticend-capalong theC-Brbond in 26DA35DBPHcationor lessnegative electrostatic potential end-cap along Fe-Br bond in[FeBr4

-] (Figure 9).C-Br1 3 3 3Br2-Fe interactions and Fe-Br1 3 3 3Br2-

Fe contacts were reported in several published crystal struc-tures.52,57-62 These structures and those reported in thispaper indicates that (a) for C-Br1 3 3 3Br2-Fe interactions,C-Br1 3 3 3Br2 angle is always greater than Br1 3 3 3Br2-Feangle.52,60-62 This agrees with our observation that C-Br1 3 3 3Br2 angle is nearly linear angle, while Br1 3 3 3Br2-Feis the perpendicular one (Type II interactions) (Scheme 1). (b)Fe-Br1 3 3 3Br2-Fe contacts compete with and complementthe role C-Br1 3 3 3Br2-Fe interactions in determining thesupramolecular structure of these crystalline materials. If theorganic part of these materials contains carbon bound bro-mine atom, the crystal structure is dominated C-Br1 3 3 3Br2-Fe interactions (crystal II and those previously pub-lished structures); there is no Fe-Br1 3 3 3Br2-Fe contactswithin the sum of rvdW of the bromine atoms.52,60-62 Even

Figure 6. Layer structure of (II). The layer lies parallel to ac plane.C-Br1 3 3 3Br2-Fe interactions and hydrogen bonds are shown inblue 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 thecations.

Figure 8. Illustration of the structure of the magnetic chains in (II).The Fe-Br1 3 3 3Br2-Fe contacts are represented by black lines.

Figure 9. Calculated electrostatic potential surface for [FeBr4-]

anion and 25DA35DBPH cation. The energy is expressed in Har-trees. The electron density contour isovalue is set to 0.005. Thepotential was calculated using b3lyp and aug-cc-pvtz basis sets onall atoms except on Iron (6-31 g).

Article Crystal Growth & Design, Vol. 10, No. 1, 2010 163

though there are Fe-Br1 3 3 3Br2-Fe contacts with inter-bromine distance little bit longer than the rvdW. The Br1 3 3 3Br2distance in crystal II is 3.825 A (Table 4). In contrast, if theorganic part does not contain carbon boundbromine atom, insome of these crystal structures, the inter-bromine distance inFe-Br1 3 3 3Br2-Fe contacts becomes less than the sum ofrvdW (crystal I and previously published structures).23,57-59

This indicated that C-Br1 3 3 3Br2-Fe interactions prevailover Fe-Br1 3 3 3Br2-Fe contacts.

The magneto-structural correlations of mixed organic-inorganic materials have received a lot of interest.53,63-66 Aspecial type of magneto-structural pathway is the two halidepathway. The crystal structure analysis of (I) reveals thatFe-Br1 3 3 3Br2-Fe interactions define a spin ladder struc-ture; the Br1 3 3 3Br2 distances within the rungs are extremelyshort, 3.618 A.27,53 The Br 3 3 3Br distances in (I) are shorterthan any Br 3 3 3Br distance in any reported copper spin ladderstructure.27 Also, (II) forms a linear chain structure based onBr 3 3 3Br interactions, also with a relatively short contactdistance, 3.825 A. This makes these two structures potentialmodels for the study of the two-halide pathway in spin 5/2systems; both in spin linear chains and spin ladder systems.These short contact distances are expected to influence themagnetic properties of these two compounds. Further studiesinto this area are ongoing.

Conclusions

The above analysis indicates that (a) the geometrical ar-rangement of C-Br and Fe-Br synthons in the C-Br1 3 3 3Br2-Fe interactions and Fe-Br1 3 3 3Br2-Fe contacts is si-milar to C-Br1 3 3 3Br2-C interactions; (b) Fe-Br1 3 3 3Br2-Fe contacts compete with C-Br1 3 3 3Br2-Fe interac-tions, though the later prevails. (c) C-Br1 3 3 3Br2-Feinteractions and Fe-Br1 3 3 3Br2-Fe contacts can be ex-plained using the calculated electrostatic potential of the[FeBr4

-] ion and the electrostatic potential around theC-bound bromine atom.17 The positive electrostatic end-capof the first synthon (C-Br) should encounter the morenegative potential ring in the second synthon (Fe-Br). Inthe Fe-Br1 3 3 3Br2-Fe contacts, the less negative electro-static potential end-cap should face more negative electro-static potential ring.

Acknowledgment. The Bruker (Siemens) SMART CCDdiffraction facility was established at the University of Idahowith the assistance of the NSF-EPSCoR program and theM.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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