Effect of intermolecular interactions on the molecular structure; theoreticalstudy and crystal structures of 4-bromopyridinium tetrafluoroborate anddiaqua(3-bromopyridine)difluorocopper(II){

Firas F. Awwadi,*a Salim F. Haddad,a Brendan Twamleyb and Roger D. Willettc

Received 25th March 2012, Accepted 10th July 2012

DOI: 10.1039/c2ce25433f

The role of C–Br…F interactions in two crystal structures (4BP)BF4 (I) and Cu(H2O)2(3bp)F2 (II),

(where 4BP is the 4-bromopyridinium cation and 3bp is 3-bromopyridine) is investigated. Crystal

structure analysis indicates that the supramolecular assembly of I is based on symmetrical bifurcated

C–Br…F halogen bonding and the bifurcated N–H…F hydrogen bonding, while that of II is based

on O–H…F hydrogen bonding interactions. The Br…F distance in I is 0.13 A less than the sum of

van der Waals radii. In contrast, the Br…F distance in II is 0.04 A longer than the sum of van der

Waals radii, indicating that the C–Br…F interaction plays a minor role in developing the

supramolecular structure of II. The structure of I is the first reported with perfect symmetrical

bifurcated C–Br…F halogen bonding. II is the first reported crystal structure with C–Br…F–tM

interactions, tM = transition metal. Theoretical calculations have shown that a charge assisted

symmetrical bifurcated C–Br…F interaction is stronger than the corresponding linear one, whereas in

the normal (not charge assisted) C–Br…F halogen bonding both linear and bifurcated interactions

have comparable strength. This conclusion is supported by structure analysis of reported structures in

this work and the published data in Cambridge Structural Database (CSD).

Introduction

Interest in intermolecular interactions has grown tremendously

in the recent decades due to their applications in many chemical

aspects e.g. crystal engineering, nanotechnology, drug design,

chemical separation, stabilizing three dimensional structures of

proteins and DNA etc.1–16 One important type of these

interactions is halogen bonding. A halogen bond is a non-

covalent interaction between a covalently bound halogen atom

and nucleophiles, D–Y…Nu (D–Y = halogen donor; Nu =

nucleophile).9 Several types of these interactions have been

studied extensively, both experimentally and theoretically.

Halogen bonding has been the subject of many reviews as well

as a book.3–5,7,9,17,18

One significant type of halogen bonding is the C–Y…X–A

interaction, where C–Y is the halogen donor and X–A is the

halogen acceptor. The halogen acceptor can be a wide range of

different species.9,19–24 Our interest, as well as that in several

other laboratories, has been focused on cases where the halogen

acceptors are covalently bound halogen atoms C–Y…X–A (vide

supra) or separate halide anions C–Y…X2. This interaction is

important due to its role in the self assembly of halogenated

organic compounds and mixed organic/inorganic materials.19–33

This type of interaction is divided into two main categories; (a)

the simple halogen…halogen interactions, C–Y…X–A (A =

carbon, metal cations).20,34,35 There are two preferred geometries

for the C–Y…X–C, (where X = Y), supramolecular synthons; (1)

the perpendicular geometry, where C–Y…X angles = 180u and

Y…X–C angle = 90u; (2) the symmetrical geometry occurs when

both angles are about 150u (where ?X = Y).20 The arrangement

of C–Y…X–M synthons is characterized by essentially linear C–

Y…X angles.33 (b) Charge assisted halogen…halide interactions,

where the halogen donor species is a part of positively charge

species and the halogen accepter is part of negatively charged

species, [C–Y]+…[X–A]2. This includes [C–Y]w+…[X–M]z2 and

[C–Y]+…X2 interactions, the arrangements of these supramole-

cular synthons are similar to that of C–Y…X–M synthons with

shorter inter-halogen distances.19,21–23,25,26,29–32 We have shown

that these interactions are better tools in predicting the solid

state structure of halopyridinium salts than the corresponding

[N–H]+…X2 hydrogen bonding; [C–Y]+…X2 is closer to the

linear arrangement than the corresponding [N–H]+…X2.

Recently, M–X…X–M (M = Fe, Au) contacts have been

reported in the literature.26,36

The energy of interactions of halogen…halogen and halo-

gen…halide and their nature have been the subject of several

studies both theoretical and experimental. These interactions are

aDepartment of Chemistry, The University of Jordan, Amman, 11 942,Jordan. E-mail: f.awwadi@ju.edu.jobUniversity Research Office, University of Idaho, Moscow, ID, 83844,USAcDepartment of Chemistry, Washington State University, Pullman, WA,99164, USA{ CCDC reference numbers 873517 and 873518. For crystallographicdata in CIF or other electronic format see DOI: 10.1039/c2ce25433f

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controlled mainly by electrostatics, even though dispersion and

charge transfer cannot be ignored.5,7,19–21,37–40 Theoretical

calculations have shown that the strength of these interactions

span from weak interactions, ca. 1 KJ mol21 for C–Cl…Cl–C

interactions, to very strong interactions of ca. 150 kJ mol21 for

H–CMC–I…F2.19,20,39 The thermodynamic parameters of for-

mation (DHfu and DSfu) were determined experimentally for

some halogen bonding complexes.38,41 The formation of these

interactions is more energetically favored as the halogen atom of

the halogen donor becomes softer, and the halogen acceptor gets

harder. Therefore, the best halogen donor is C–I and the best

halogen acceptor is the fluoride anion. This hierarchy of halogen

bond strength has been used to rationalize the crystal structures

of a series of isomolecular organic–inorganic hybrid salts

(3YP)2MX4 (3YP = 3-halopyridinium cation; M = Co, Zn; Y

= F, Cl, Br, I; X = Cl, Br, I).21

In a previously published work we have shown that halogen

bonding of the type C–Y…X2 and C–Y…X–M affects the

molecular structure of halopyridinium cations in the solid state

series (nYP)X, (nYP)2CuX4 and Cu(nyp)2X2 (nYP and nyp are

n-halopyridinium cation and n-halopyridine, respectively; n = 2, 3,

4; Y = Cl, Br; X = Cl, Br, I).33 The role of the fluorine atom as a

halogen bond donor, in general, and halogen bond acceptor in C–

Y…X–C interactions has also been investigated.42,43 Metrangolo

et al. have shown that fluorine atom can act as halogen bond donor

in certain systems, especially if it is attached to electron with-

drawing group.42 Brammer et al. have investigated the role of the

fluorine atom as a halogen bond donor in mixed organic–inorganic

materials; their conclusions is that the C–F…X–M halogen

bonding interactions are absent.21 Whereas, to our knowledge,

the role of the fluoride ligand as a halogen bond accepter has not

been addressed. In this report, we present the structure of two

fluorine containing compounds; (1) (4BP)BF4 (4BP = 4-bromo-

pyridinium cation), henceforth I and (2) Cu(H2O)2(3bp)F2 (3bp =

3-bromopyridine), henceforth II. The structure analyses will be

supported with theoretical investigations of two types of halogen

bonding; C–Br…OH2 and [C–Br+]…[F–B2]. The latter includes

linear charge assisted [C–Br+]…[F–B2] interactions (Chart 1A)

and bifurcated charge assisted [C–Br+]…[F–B2] interactions

(Chart 1B). The structural data will be compared to the theoretical

data. The data will indicate that (a) the strong charge assisted

bifurcated [C–Br+]…[F–B2] halogen bond is preferred over, and is

more energetically favorable to, the analogous linear charge

assisted one (b) bifurcated halogen bonding affects the molecular

structure; the [C–Br+]…[F–B2] halogen bonding affects the

molecular structure of the [BF42] anion (c) the fluoride anion is

a better proton acceptor (hydrogen bonding interaction) than a

halogen acceptor (halogen bonding interaction).

Experimental

Theoretical method

Gaussian 03 was used for geometry optimization of the structures

BF42 anion, 4BP cation, I and II and energy calculations;44

Becke’s three-parameters formulation B3LYP was used with the

cc-pvdz basis set on all atoms except the bromine atom (aug-cc-

pvdz; aug = presence of diffuse function on the bromine atom).

The structure of I was optimized with applying constraints; the

plane that contains the two fluorine atoms and boron atom is

perpendicular to the plane of the 4BP cation ring. The energies of

interaction are calculated using the formula:

Eint = Ecomplex 2 EA 2 E4BP

where A = [BF42] or H2O. The calculated energies of interaction

were corrected for basis set superposition errors (BSSE) using the

counterpoise method.45

Synthesis and crystal growth

(a) 4-bromopyridnium tetrafluroborate, I. In an attempt to

formulate compounds with a Br…F bond, 1 mmole of

4-bromopyridinium chloride was dissolved in 25 mL of water

in a Pyrex beaker. 10 mL of 20% HF was added and the solution

left in the fume hood covered with a watch glass. In two days

beautiful parallelepiped colorless transparent crystals developed.

They were collected by decanting them on to cardboard. A

suitable crystal of dimensions 0.4 6 0.3 6 0.1 mm3 was

mounted on a glass fiber for structure determination. After

careful analysis of refinement parameters and bond lengths, the

anion was eventually identified as BF42. The boron source was

the borosilicate glass of the beaker.

(b) diaqua(3-bromopyridine)difluoro copper(II), II. 2 mmol (0.28 g)

of CuF2?2H2O were dissolved in a few millilitres of water, resulting

in a suspension. To this suspension, 2 mmol (0.32 g) of

3-bromopyridine were added, and a blue precipitate formed. The

solution was heated gently and 3 drops of conc. HF were added

to dissolve the precipitate. The solution was allowed to evaporate

to dryness and a few, very small, light blue crystals were present

Chart 1 Types of modelled halogen bonds; (A) Linear charge assisted

halogen bonding. (B) Charge assisted bifurcated halogen bonding. (C)

normal C–Br…OH2 halogen bonding. R = inter-halogen bonding

distance.

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in the otherwise amorphous powder. One of these crystals was

selected for X-ray analysis.

Crystal structure determination

The crystal structure of I was determined at room temperature.

The data collection was carried out on a Syntex P21 diffract-

ometer upgraded to Bruker P4 specifications. Lattice dimensions

were obtained from 25 accurately centered reflections. Data were

corrected for absorption utilizing Y-scan data assuming an

ellipsoidal shaped crystal.46 Data for II were collected at 89(2) K

using a Bruker/Siemens SMART APEX instrument (Mo-Ka

radiation, l = 0.71073 A) equipped with a Cryocool NeverIce

low temperature device. Data were measured using omega scans

of 0.3u per frame for 20 s, and a full sphere of data was collected.

A total of 2400 frames were collected with a final resolution of

0.83 A. Cell parameters were retrieved using SMART software

and refined using SAINTPlus on all observed reflections.47,48

Data reduction and correction for Lp and decay were performed

using the SAINTPlus software.47 Absorption corrections were

applied using TWINABS.49 The structure was solved by direct

methods and refined by least squares method on F2 using the

SHELXTL program package.50 The structure was solved in the

space group P21/m (#11) by analysis of systematic absences. The

structure was refined as a rotational twin with the second

component rotated from first domain by 179.7 degrees about

reciprocal axis 1.000 20.005 20.291 and real axis 1.000 0.001

0.001. The twin law obtained from Cell_Now is 1.001 0.001 0.002

20.011 21.000 20.004 20.583 0.006 21.001. All non-hydrogen

atoms were refined anisotropically. No decomposition was

observed during data collection. Details of the data collection

and refinement are given in Table 1.

Results

Crystal structure

The crystal structure of I consists of planar 4-bromopyridinium

cation and distorted tetrahedral [BF42] anion. The F–B–F angles

are listed in Table 2. The B–F1 and B–F2 distances are 1.371(5)

and 1.359(5) A, respectively; F1 and F2 are the two crystal-

lographic independent fluorine atoms (Fig. 1A). The molecular

geometry of II is based on a distorted trigonal bipyramidal

around the copper center, as seen in Fig. 1B, with Cs symmetry.

The 3bp ring and the two fluoride ligands are located on the

mirror plane. This structure is stabilized by intramolecular

hydrogen bonding (C2–H…F2 and C6–H…F1). The two

fluoride ligands occupy the two axial positions, and the two

water molecules and 3bp occupy the equatorial positions. The

angle N–Cu–OH2 is 130.53u, which is larger than the equatorial

trigonal bipyramidal angle of 120u. In contrast, the H2O–Cu–

OH2 angle is 98.9u.Hydrogen and halogen bonding are involved in connecting the

molecular species within the crystalline lattices. The geometry of

these interactions and the arrangement of these supramolecular

synthons are shown in Fig. 2. Two types of hydrogen bonding

are present in the two structures; N–H…F–B and O–H…F–Cu

(ignoring the non classical C–H…A hydrogen bonding, where A

is the proton acceptor). Data summarizing the contacts are listed

in Tables 3 and 4. N–H…F–B hydrogen bonding is present in I

and O–H…F–Cu hydrogen bonding is present in II.

Examination of these hydrogen bonding interactions reveals

that the pattern of N–H…F interaction is symmetrically

bifurcated with N–H…F angles equal to 133.39u. C–Br…F

halogen bonding is present in I and II. The pattern of the

Table 1 Summary of data collection and refinement parameters for I and II

Crystal (I) (II)

Formula C5H5BrNBF4 C5H8BrCuF2NO2

Mr 245.82 295.57rc/Mg m23 1.931 2.281T/K 273 89(2)Crystal system Monoclinic MonoclinicSpace group C2/c P21/ma/A 5.2238(6) 7.7096(14)b/A 22.950(3) 6.7687(12)c/A 7.4020(8) 8.5459(15)b (u) 107.640(9) 105.241(4)V/A3 845.68(17) 430.27(13)ind. reflections 1129 1111Data/restraints/parameters 800/0/58 1111/0/7R(int) 0.0332 0.0348Z 4 2Goodness of fit 1.051 1.152R1

a [I . 2s] 0.0453 0.0224wR2

b [I . 2s] 0.0995 0.0615m/mm21 4.868 7.172Drmin and max (e/A3) 0.391 and 20.278 0.452 and 20.428a R1 = S||Fo| 2 |Fc||/| S |Fo|. b wR2 = {S w(Fo

2 2 Fc2)2/Sw(Fo

2)2}1/2.

Table 2 Selected bond angles (u)

I II

F1–B–F1Aa 106.8(6) F1–Cu–F2 177.1(1)F1–B–F2 110.0(2) O1–Cu–O1A 98.9(1)F1–B–F2Aa 108.8(2) N1–Cu–F1 92.0(1)F2–B–F2Aa 112.3(6) N1–Cu–F2 90.9(1)

N1–Cu–O1 130.5(1)F1–Cu–O1 89.5(1)F2–Cu–O1 88.6(7)

a F1A and F2A are symmetry equivalent to F1 and F2, respectively.

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C–Br…F halogen bond in I is symmetrically bifurcated as shown

in Fig. 2 with a Br…F contact distance 0.130 A less than the sum

of the van der Waals radii of the bromine and fluorine atoms

(Table 2). In contrast, the role of the C–Br…F halogen bonding

interaction is less significant in II with the Br…F distance 0.04 A

longer the sum of van der Waals radii. This reveals that C–Br…F

interactions compete and complement the role of N–H…F

interactions in I, whereas, the supramolecular structure of II is

dominated by O–H…F hydrogen bonding interactions and the

role of C–Br…F interactions is less significant.

In I, the tetrafluoroborate anion and 4BP cation are connected

via symmetrically bifurcated halogen bonds from one side and

symmetrically bifurcated hydrogen bonds from the opposite side

to form a layer structure which lies parallel to the ab plane

(Fig. 3A). These layers are linked via C–H…F hydrogen bonds,

as well as N(p)…F interactions, to form the 3D structure

(Fig. 4A). The O–H…F hydrogen bonding connects the

molecular units of II to produce a layer structure which lies

parallel to the ab plane (Fig. 3B). The 3bp ligands protrude out

of the layer structures, partially interdigitating between each

other on adjacent layers, as illustrated in Fig. 5. This

interdigitating is strengthened by the C–Br…F halogen bonding.

The formation of the 3D structure is facilitated by p–p stacking

and the C–H…F non-classical hydrogen bonding. The distance

between the 3bp planes is 3.384 A and the centroid to centroid

distance between two adjacent 3bp rings is 3.642 A. This

generates an angle of 23.2u between the line connecting the

centroids and the normal to the plane of the 3bp ring.

Theoretical results

The interaction energies of both linear and bifurcated halogen

bonding are listed in Table 5. It should be noticed that the energy

of charge assisted interactions includes both due to the direct

positive–negative electrostatic forces and either halogen and

hydrogen bonding interactions, henceforth for simplicity, halo-

gen and hydrogen bonding interactions. The normal bifurcated

C–Br…F–B halogen bonding is slightly stronger than or has the

same strength as the linear mode for the 4bpn [BF42] complex

(Table 5), whereas, the bifurcated interaction is clearly stronger

in the case of the charge assisted one; the energy difference is

20 kJ mol21. The structure of the [BF42] anion, I and II are

optimized. Selected distances and angles of the optimized

structures are tabulated in Table 6. The optimized structure of

the [BF42] anion is a perfect tetrahedral geometry; with all F–B–

F angles and B–F distances equal (i.e. y109.5u and 1.418 A,

respectively). In contrast, the F1–B–F1A angle that is made by

the two legs of the bifurcated halogen bond in the optimized

Fig. 1 The molecular structure of I (A) and II (B). Thermal ellipsoids

are shown at 50% probability.

Fig. 2 Synthons interactions in (A) I and (B) II. Hydrogen and halogen

bonding interactions are represented by blue and black dotted lines.

Table 3 C–Br…F and N–H…F synthons distances (A) and angles (u) inI

C–Br1…F N–H…F

Br1…F1 3.194 H…F 2.158C–Br…F1 159.83 N…F 133.39Br…F1–B 106.41 N–H…F 2.187

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structure of complex I is compressed to 104.8u (Table 6 and

Fig. 6). This forces F2–B–F2A to further open (Table 6). The B–

F bond distances that are involved in the halogen bonding

increase and the other two decrease. The optimized structure of

II indicates that the two Cu–F distances are not equal with a

difference of ca. 0.11 A, (see Table 6) due to intramolecular O–

H…F2 hydrogen bonding. The structure of the dimer of II (two

molecular units of II connected by halogen bonding) could not

be optimized due to the presence of a shallow energy minimum.

Discussion

Halogen bond interactions affect the molecular structure of

crystalline species. C–Br…F interactions force the angle that is

made by the two legs of the bifurcated halogen bond in I to be

more acute; the optimized F1–B–F1A (Fig. 6) angle is smaller

than the perfect tetrahedral angle (109.5u) by ca. 4.5u. This is in

complete agreement with the experimental results; the F1–B–

F1A angle (Fig. 2) is observed to be 106.8(6)u. Without including

the halogen bonding interaction in the calculation, the optimized

structure of the separated [BF42] anion is perfectly tetrahedral.

We have shown in a previous study that bromine halide

interactions distort bromopyridinium cation molecular struc-

tures.20 The effect of halogen bonding on the molecular structure

of II cannot be investigated, since the structure of the dimer of II

cannot be optimized and also because of the presence of the O–

H…F intramolecular hydrogen bond (Fig. 6). The effect of the

non-classical C–H…X hydrogen bonding interactions have been

shown to play a major role in determining the molecular

structures of some inorganic complexes.36

The effect of halogen bonding on the molecular geometry can

be explained using the calculated electrostatic potential.

Theoretical calculations have shown the presence of a positive

electrostatic potential end cap on bromine atoms (Fig. 7).20 This

electrostatic potential end cap plays a major role in the

arrangements of halogen–halide synthons in the crystalline

lattices. The two fluoride anions compete to approach the

electrostatic potential end cap; hence, the molecular geometry of

the [BF42] anion deviates from the perfect tetrahedral geometry.

Structures and theoretical calculations presented in this paper

and previously carried out indicate that the bifurcated pattern

halogen bonding is energetically favored in strong C–Br…SP (SP

= F or O) halogen bonding interactions.51 In contrast, the linear

C–Br…SP pattern is favored in weak C–Br…SP interactions,

whereas, there is no energy preference in C–Br…SP interactions

of medium strength. Theoretical calculations have shown that

the charge assisted bifurcated C–Br…F halogen bond is

energetically favorable over the corresponding linear one.

While, in the case of the normal C–Br…F halogen bond, both

the bifurcated and linear interactions have similar strength. In

contrast, theoretical calculations carried out by other research

groups on C–Br…O halogen bonds show that the linear C–

Br…O interaction is stronger than the corresponding bifurcated

example.51 They studied C–Br…O interactions between hydro-

gen bromide and nitromethane. These contradicting results can

Fig. 3 Layer structure of I (A) and II (B) view along a-axis and (C) view

along c-axis. (C–Br…F interactions are shown in black dotted lines, N–

H…F hydrogen bonding in I and O–H…F in II are shown light blue

dotted lines. Hydrogen atoms are omitted for clarity.

Table 4 C–Br…F and O–H…F synthons distances (A) and angles (u) inII

C–Br1…F1 O–H…F1 O–H…F2

Br1…F1 3.369 H…F 1.812 H…F 1.786C–Br…F1 126.71 O…F 2.665 O…F 2.683Br…F1–Cu 99.13 O–H…F 168. 38 O–H…F 175.86

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be reconciled by taking into account the relative strength of

halogen bonding interactions; they investigated weak halogen

bonding interactions while we are investigating medium to

strong ones. Halogen bonding can be classified according to its

strength as strong, medium and weak depending on the charge

on the halogen donor and halogen acceptor since these

interactions are mainly controlled by electrostatics. Strong

halogen bonds occur when the halogen donor is positively

charged and halogen acceptor is negatively charged as in I, and

the calculated energy of interaction is 2257.6 kJ mol21 (Table 5).

This agrees with experimental results; the bifurcated pattern is

observed in I. If only the donor or the acceptor is charged, it is

expected to be of medium strength, the energy of the bifurcated

interaction in 4bp?BF4 is 218.2 kJ mol21 (Table 5), which is

much weaker than that in I (vide supra). Also, if a negative

charge is concentrated on the halogen acceptor atom or a

positive charge is concentrated on the halogen donor, the

interaction is expected to be of medium strength as in II, since

the fluorine atom is the most electronegative atom in the periodic

table and it is attached to an electropositive copper atom. This

explains why the C–Br…F bifurcated pattern is absent in II. In

fact, C–Br…F interactions play a minor role in developing the

supramolecular structure of II, due to the presence of strong O–

H…F hydrogen bonding interactions. If both the halogen donor

Fig. 6 The optimized structure of (A) I and (B) II.

Fig. 7 Calculated electrostatic potential: (A) 4-bromopyridinium

cation, bromine atom faces the viewers, (B) 4-bromopyridinium cation,

N–H faces the viewer and (C) tetrafluoroborate anion. Energy in Hartree

and charge in electronic charges.

Fig. 4 Illustration of 3D structure of I viewed along the a axis. N–H…F

hydrogen bonding, C–Br…F halogen bonding and N(p)…F interactions

are represented by blue, black and red dotted lines, respectively.

Fig. 5 Illustration of 3D structure of II. O–H…F hydrogen bonding

and C–Br…O halogen bonding are represented by blue and black dotted

lines, respectively.

Table 5 Energies of interaction of linear and bifurcated C–Br…F–Band C–Br…OH2 halogen bonding

Complex Energy (linear) (kJ mol21) Energy (bifurcated) (kJ mol21)

4bpn BF4 217.4 218.24BP BF4 2237.5 2257.64bpn?H2O 24.4134BP H2O 231.44

Table 6 Selected distances of the optimized I complex and II structures

I II

B–F1 1.449 Cu–F1 1.918B–F2 1.389 Cu–F2 1.811Br…F1 2.619 Cu–O 2.165F1–B–F1A 104.83 O–Cu–O 96.7F2–B–F2A 109.63 F1–Cu–F2 177.4F3–B–F4 113.16C–Br…F1 154.0

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and the halogen acceptor are neutral and the halogen acceptor

with low negative charge character, the strength of the halogen

bond is expected to be weak, as in the investigated system by Lu

et al. since the 4-bromopyridinium cation is a better halogen

donor than hydrogen bromide and the [BF42] anion is a better

halogen acceptor than nitromethane.51

The authentication of theoretical results (vide supra) was tested

by comparing it with the experimental data obtained from the

reported structures in this work and also extracted from the

Cambridge Structural Database, CSD. The CSD (version 5.33,

November 2011, updates February 2012) was searched for C–

Br…F–BF3 contacts within the sum of van der Waals radii.

Structures that contained disordered [BF42] anion were

excluded. Fourteen structures were found to contain C–Br…F–

B contacts; unsymmetrical bifurcated in one structure, tetra-

furcated in another one and linear in the others (Table 7 and

Scheme 1). The halogen bonding in two of the fourteen

structures is a normal (not charge assisted) C–Br…F–B

interaction and in twelve it is charge assisted. There are no

structures that contain perfectly symmetrical bifurcated C–

Br…F–B halogen bonding which indicates that structure I is to

date, the only structure that shows perfectly symmetrical

bifurcated C–Br…F–B interactions. The organic bromine in

the two structures with multi-furcated halogen bonding pattern

is divalent (except in I, see Scheme 1). In eleven of the fourteen

structures extracted from the data base, the halogen donor is a

large molecule (Table 7).

Structure analysis of the extracted data from the CSD

supports the conclusion (vide supra) that the charge assisted

bifurcated or multi-furcated halogen C–Br…F–B bond is

preferred over a linear charge assisted one in strong C–Br…F

halogen bonding interactions. The effect of ‘‘charge assisting’’ is

reduced as either one or both species that are involved in halogen

bonding get larger. Halogen bonding is charge assisted in twelve

of the fourteen structures. Except in one instance, the cation is a

large species, namely p-bromobenzenediazonium tetrafluorobo-

rate. Even though this structure is close in size to I, the halogen

bonding is linear rather than bifurcated. This is mainly due to the

fact that fluorine atoms of the [BF42] anion are involved in

several F…N interactions. In the two structures that contain

multi-furcated halogen bonding, the organic bromine atom is

divalent. This concentrates the positive charge on the bromine

atom, and hence, strengthens the halogen bonding which results

in the formation of multi-furcated halogen bonding.

The CSD Database was searched for C–Br…F–M interactions

within the sum of van der Waals radii plus 0.1 A. The search

returned five hits. In all of these five structures the halogen

bonding acceptor is the [SbF62] anion (Table 7). This indicates

that II is the first structure with C–Br…F–tM (tM = transition

metal) interactions. In three of these five structures (CEYNID,

Table 7 Ref code, type and pattern of the reported C–Br…F–B and C–Br…F–M halogen bonding in CSD

Ref code Type Pattern Ref code Type Pattern

AKOXEE Charge assisted Linear CEYNID Charge assisted BifurcatedBEZQUT Charge assisted Linear MOHLOK Charge assisted LinearBIBVENa Charge assisted Linear NAGTUL Normal LinearBPTHPY Charge assisted Linear VIVFUB Charge assisted LinearDABVEHa Normal Linear VIVGAI Charge assisted BifurcatedGAFZIW Charge assisted LinearHOHJUJ Charge assisted BifurcatedJEKBEH Charge assisted LinearKASDAK Charge assisted TetrafurcatedPEWQIR Charge assisted LinearSAXQAJ Charge assisted LinearSOCQUX Charge assisted LinearWAFZUZ Charge assisted LinearXAJHASa Normal Lineara Size of halogen donor is small. Size is considered large if it contains more than 12 atoms except hydrogen.

Scheme 1 Illustrations of C–Br…F–B interactions.

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VIVFUB and VIVGAI), the bromine atom is divalent. CEYNID

and VIVGAI form bifurcated C–Br…F halogen bonding

interactions which agrees with previous discussion (vide supra).

Both bifurcated halogen bonding and bifurcated hydrogen

bonding are present in the two analyzed structures. Investigating

two series of halopyridinium cation containing crystal structures

(nYP)CuX4 and Cu(nyp)2X2, (nYP and nyp are n-halopyrdinyum

cation and n-halopyridine, respectively; n = 2, 3, 4; Y = Cl, Br; X =

Cl, Br, I) indicates that the bifurcated hydrogen bond is present in

many of these structures, while the bifurcated halogen bond is

absent in all.19,25,26,31,33 Although the bifurcated C–Br…Cl halogen

bond is rare, it has been reported by Brammer et al.52 The fact that

C–Br…Cl interactions are absent in these structures, whereas N–

H…X is not, is easily explained using the calculated electrostatic

potential of the 4BP cation and the size of CuX422 moiety. The

CuX422 moiety occupies a large volume in comparison to the BF4

2

anion. Furthermore, the positive electrostatic potential decreases in

the p-region of the organic bromine atom, while such a sharp

decrease is not observed in the p-region of the hydrogen atom that

is attached to nitrogen (Fig. 7A and 7B).

Conclusions

The energetically favored pattern of halogen bonding depends on the

relative strength of the halogen bonding when the halogen acceptor is

oxygen or fluorine atoms. Bifurcated halogen bonding pattern is

energetically favored in the very strong halogen bonding. In contrast,

the linear pattern of halogen bonding is energetically favored in the

weak halogen bonding. No preference is found in the case of medium

strength halogen bonding interactions. This has been proved using

quantum mechanical calculations presented in this paper as well as

those previously published by other research groups.51 This

conclusion is supported by a crystal structure analysis of I, II and

those extracted from CSD. In I, the C–Br…F pattern is bifurcated

halogen bonding since it is a strong interaction. Also, the bifurcated

or the tetrafurcated pattern is observed in the cases where the organic

bromine atom is divalent, resulting in the formation of very strong

halogen bonding (Table 7). In medium strength halogen bonding,

both patterns have a similar strength as in the 4bp?BF4 complex

(Table 5). This conclusion is supported by the absence of the

bifurcated pattern in II.

Halogen bonding interactions affects the molecular structure.

F1–B–F1A in I compressed to 106.9u; a more acute angle than

expected for a separate [BF42] anion (Fig. 6 and Fig. 2A). Also,

this calculation is supported by a previous study; the bromi-

ne…halide interactions distort the molecular structure of the

halopyridinium cation.33

Acknowledgements

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.

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