Tunable Interaction Strength and Nature of the SÂ·Â·Â·Br Halogen Bonds

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Tunable Interaction Strength and Nature of the S···Br Halogen Bonds in [(thione)Br2] Systems Laura Koskinen, Sirpa Jääskeläinen, Pipsa Hirva, and Matti Haukka Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/cg501482u • Publication Date (Web): 11 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015

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Crystal Growth & Design

Tunable Interaction Strength and Nature of the S···Br Halogen Bonds in [(thione)Br2] Systems Laura Koskinen,† Sirpa Jääskeläinen,† Pipsa Hirva,†* and Matti Haukka‡* † ‡

Department of Chemistry, University of Eastern Finland, Finland. Department of Chemistry, University of Jyväskylä, Finland.

The strength and the nature of the S···Br and Br···Br interactions were systematically tuned by altering the electron donor properties of the thione group. Three new halogen-bonded compounds, [(N-methylbenzothiazole-2-thione)Br2]·0.5 CH2Cl2 (1), [(2(3H)benzothiazolethione)Br2] (2) and [(2-benzimidazolethione)Br]·[Br3] (3), were synthesized and studied structurally by using X-ray crystallography and computationally by using charge density analysis based on QTAIM calculations. Analysis of the interaction strength indicated a formation of surprisingly strong S···Br halogen bonds in 1 (-104 kJmol-1 and RBrS: 0.64) and 2 (-116 kJmol1 and RBrS: 0.63) with a substantial covalent contribution. The strong electron donor character of the thione ligand in 3 induced a heterolytic cleavage of the dibromine molecule and a change in the S···Br interaction nature to form a covalent bond with high interaction energy (-147 kJmol-1 and RBrS: 0.60).

*Pipsa Hirva Address: P.O. Box 111, FI-80101 Joensuu, Finland. Tel.: +358-503310829. E-mail: [email protected]. *Matti Haukka Address: P.O. Box 35, FI-40014 Jyväskylä, Finland. Tel.: +358-408054666. E-mail: [email protected].

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Tunable Interaction Strength and Nature of the S···Br Halogen Bonds in [(thione)Br2] Systems

Laura Koskinen,† Sirpa Jääskeläinen,† Pipsa Hirva,†* and Matti Haukka‡*

†

Department of Chemistry, University of Eastern Finland, P.O. Box 111, FI-80101 Joensuu, Finland

‡

Department of Chemistry, University of Jyväskylä, P.O. Box 35, FI-40014 Jyväskylä, Finland

Abstract

The strength and the nature of the S···Br and Br···Br interactions were systematically tuned by altering the electron donor properties of the thione group. Three new halogen-bonded compounds,

[(N-methylbenzothiazole-2-thione)Br2]·0.5

CH2Cl2

(1),

[(2(3H)-

benzothiazolethione)Br2] (2) and [(2-benzimidazolethione)Br]·[Br3] (3), were synthesized and studied structurally by using X-ray crystallography and computationally by using charge density analysis based on QTAIM calculations. Analysis of the interaction strength indicated a formation of surprisingly strong S···Br halogen bonds in 1 (-104 kJmol-1 and RBrS: 0.64) and 2 (-116 kJmol1

and RBrS: 0.63) with a substantial covalent contribution. The strong electron donor character of


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the thione ligand in 3 induced a heterolytic cleavage of the dibromine molecule and a change in the S···Br interaction nature to form a covalent bond with high interaction energy (-147 kJmol-1 and RBrS: 0.60).

1. Introduction

A halogen bond (XB) is conventionally defined as a net attractive interaction between an electrophilic region associated with a halogen atom (XB donor, X) and a nucleophilic Lewis base (XB acceptor, B). Halogen bonds are mainly electrostatically driven, but other type of noncovalent electron donor–electron acceptor contacts, such as charge-transfer (CT) and dispersion forces can contribute to the formation of the halogen bond.1 In recent years, it has been even proposed based on both theoretical2-4 and experimental5-6 findings, that in the case of extremely strong XB donors the halogen bond can possess substantial degree of covalency. Symmetric N···I+···N and S···I+···S motifs with strong 3-center-4-electron halogen bonds provide good examples of this type of halogen bonding.7-15 Halogen bonding has recently roused interest as a promising tool for crystal engineering and for the design of new materials induced by its strong directionality, specificity, and tunable interaction strength. These properties enable to design predictable extended molecular structures by the self-assembly of discrete molecules, and to efficiently modify functionality of molecular materials.16-19 Because of the mainly electrostatic nature of the halogen bonding, the interaction strength of halogen bonds can be efficiently tuned by modifying the chemical surroundings of the halogen atom. A substitution of electronegative chemical species near the halogen results in a larger σ-hole and, respectively, in a stronger halogen bond.20-24 The substitution can have a


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dramatic effect on the interaction strength, and for example the binding energy of an unsubstituted chlorobenzene-acetone complex has been observed to increase by 102% when five hydrogen atoms of the benzene ring are replaced by electronegative fluorine atoms.25 The strength of the halogen bond can be further tuned by the modification of the electron donation strength of the XB acceptor. Halide anions and sp3- and sp2-hybridized nitrogen atoms often give stronger halogen bonding interactions than substantially less reported sulfur, selenium, oxygen, and covalently bound halogen atoms (in metallates).26-28 The charge density of the XB acceptor is sensitive to its chemical surroundings, and even small changes, such as a substitution of a single atom, affect strongly.29-30 In the present study, the S···Br-Br halogen bonds are modified by subtle variations in the thione structure by using N-methylbenzothiazole-2-thione (mbtt), 2(3H)-benzothiazolethione (btt), and 2-benzimidazolethione (bit) as the XB acceptors (Scheme 1). Thiones were selected as halogen bond acceptors for their excellent electron donor properties, which are based on their ability to support the partial positive charge upon the formation of the complex by delocalizing the charge over the rest of the molecule.30-31 Thiones have been commonly used in chargetransfer complexes with diiodine. The typical spoke structure S···I2 of these complexes is formed as the lone pair orbitals of the sulfur atom and antibonding orbitals of the halogen atom are mixed.32 In the case of strong charge transfer between the electron donor and acceptor, this n→σ* nature of the S···I interaction causes the I-I bond to be significantly elongated and in the extreme case to brake heterolytically.30-34 Although the iodine adducts of thiones have been most extensively studied, bromine shows similar behavior. The structures of dibromine with thione sulfur donor atoms typically form T-shaped Br-S-Br and ionic S-Br+-Br- motifs, in which the


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electron donor strength of the thione sulfur is supported by the nitrogen or sulfur atoms of the conjugated ring.35-39

Scheme 1. Schematic structures of the thiones.

2. Experimental 2.1. Materials and Methods

N-methylbenzothiazole-2-thione (mbtt) (Sigma-Aldrich, 99%), 2(3H)-benzothiazolethione (btt) (Alfa Aesar, 97%), 2-benzimidazolethione (bit) (Sigma-Aldrich, 98%), and Br2 (Merck, p.a.) were commercially available, and used as received. All solvents used were dried with molecular sieves. The elemental analysis was determined by VarioMICRO V1.7 instrument. 1H NMR spectra were measured with a Bruker Avance 400 MHz spectrometer. UV-VIS spectra were measured in THF with a Perkin Elmer Lambda 900 UV/VIS/NIR spectrometer.


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2.2. Synthesis and Analysis 2.2.1. Synthesis and Analysis of [N-methylbenzothiazole-2-thioneBr2]·0.5 CH2Cl2 (1) N-methylbenzothiazole-2-thione (125.0 mg, 0.690 mmol) was dissolved in CH2Cl2 (3.0 mL) and cooled at 0°C. Br2 (two drops, in excess) was added leading to an immediate formation of a yellow precipitate. The mixture was stirred at 0°C for 1 h. Yield 134.3 mg, 56.9% vs. Nmethylbenzothiazole-2-thione. Recrystallization from CH2Cl2 at +4°C gave yellow crystals. 1H NMR (d-THF): δ 7.72 (Ar, 1H); 7.67 (Ar, 2H); 7.39 (Ar, 1H); 3.89 (CH3, 3H) ppm. UV-VIS (THF): λmax 237; 318 nm. Calc. for Br2S2C8NH7 C% 28.17; H% 2.07; N% 4.11; S% 18.80. Found. C% 28.39; H% 2.37; N% 4.18; S% 18.90.

2.2.2. Synthesis and Analysis of [2(3H)-benzothiazolethioneBr2] (2) A solution of 2(3H)-benzothiazolethione (125.0 mg, 0.747 mmol) in CH2Cl2 (3.0 mL) was cooled at 0°C and Br2 (five drops, in excess) was added leading to an immediate formation of a yellow precipitate. The mixture was stirred at 0°C for 1 h. Yield 199.1 mg, 81.2% vs. 2(3H)benzothiazolethione. Recrystallization from CH2Cl2 at -20°C gave yellow crystals in few days. 1

H NMR (d-THF): δ 8.08 (NH, 1H); 7.90 (Ar, 1H); 7.80 (Ar, 1H); 7.51 (Ar, 1H); 7.42 (Ar, 1H)

ppm. UV-VIS (THF): λmax 232; 262; 318 nm. Calc. for Br2S2C7NH5 C% 25.71; H% 1.54; N% 4.28; S% 19.61. Found. C% 25.57; H% 1.69; N% 4.32; S% 19.46.

2.2.3.

Synthesis

and

Analysis

of

[2-benzimidazolethioneBr]·[Br3]

(3)

and

[2-

benzimidazolethioneBr2] (3b) A solution of 2-benzimidazolethione (125 mg, 0.832 mmol) in CH2Cl2 (10.0 mL) was cooled at 0°C and Br2 (twenty drops, in excess) was added leading to an immediate formation of a


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yellow precipitate. The mixture was stirred at 0°C for 1h. The precipitate was filtered and the clear solution was crystallized at -20°C to give orange yellow crystals in few days. Only a few crystals of 3 were obtained. Compound 3 was characterized by using X-ray crystallography as the low yield prevented the bulk material analysis of the compound. The further analyses were conducted for the precipitate (3b). Yield 171.7 mg, 66.6 % vs. 2-benzimidazolethione. 1H NMR (d-THF): δ 14.22 (NH, 2H); 7.70 (Ar, 2H); 7.54 (Ar, 2H) ppm. UV-VIS (THF): λmax 231; 250; 307 nm. Calc. for Br2SC7N2H6 C% 27.12; H% 1.95; N% 9.04; S% 10.34. Found. C% 27.07; H% 2.02; N% 9.03; S% 10.38.

2.3. Crystal Structure Determination

The crystals of 1-3 were immersed in perfluoropolyether cryo-oil, mounted in a Nylon loop and measured at a temperature of 100 K (1) or 120 K (2 and 3). The X-ray diffraction data was collected on a Bruker Kappa Apex II Duo diffractometer using Mo Kα radiation (λ = 0.71073 Å) (1) or using an Agilent Technologies SuperNova diffractometer using Cu radiation (λ = 1.54184 Å) (2 and 3). The Apex240 or CrysAlisPro41 program packages were used for the cell refinement and data reduction. The structure was solved by direct methods or by charge flipping using the SHELXS-9742 or SUPERFLIP43 programs. A semi-empirical absorption correction (SADABS)44 was applied to the data. Structural refinement was carried out using the SHELXL-2013 and SHELXL-2014 refinement programs42. The NH hydrogen atoms were located from the difference Fourier map and refined isotropically. Other hydrogen atoms were positioned geometrically and constrained to ride on their parent atoms, with C-H = 0.99 Å, and Uiso = 1.2 Ueq(parent atom). The crystallographic details for the structures 1-3 are summarized in Table 1. The selected bond lengths and angles for 1-3 are summarized in Tables 2 and S1.


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2.4. Computational Details

All models were calculated with the Gaussian09 program package45 at the DFT level of theory with a hybrid density functional PBE046. X-ray crystal structures were used to cut the geometries of the adducts, and were analyzed without optimization. The selected basis set included the standard all-electron basis sets 6-31G(d) for C and H atoms, 6-311+G(d) for S and N atoms, and basis set def2-TZVPPD for Br.47 The DFT wavefunction was used in the topological charge density analysis with the QTAIM (Quantum Theory of Atoms in Molecules)48, which was performed with the AIMALL program49. It has been suggested that the description of the halogen bonding interaction would need dispersion corrected functionals.50 The optimization of the models was tested with several other density functionals, including M062X, as well as B2PLYPD and mPW2PLYPD double hybrid functionals with dispersion corrections. Although the functionals gave reasonable results in the case of “conventional” halogen-bonding interactions, the tests showed that when the halogen bond is contributed by substantial covalent nature, none of the functionals performed very well in optimization. Similar observations on the weak performance of the dispersion corrected functionals in description of halogen bonds including chalcogens have been previously reported.51 Therefore, the previously utilized functional PBE0 with models cut directly from the experimental structures was used, and rather than giving definite conclusions about the absolute values, the relative differences between the three structures are relied on.


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Table 1. Crystallographic Data for Compounds 1-3. 1

2

3

Empirical formula

C17H16Br4Cl2N2S4

C7H5Br2NS2

C7H6Br4N2S

Fw (g/mol)

767.10

327.06

469.84

Temp (K)

100(2)

120(2)

120(2)

λ (Å)

0.71073

1.54184

1.54184

Crystal system

Triclinic

Orthorhombic

Triclinic

Space group

P1

P 212121

P1

a (Å)

8.5309(5)

3.98635(9)

8.2516(3)

b (Å)

11.7926(7)

9.2709(2)

8.6933(2)

c (Å)

13.1939(8)

25.0901(6)

8.9277(3)

α (deg)

69.389(2)

90

98.400(3)

β (deg)

81.045(2)

90

113.166(3)

γ (deg)

83.723(2)

90

92.397(2)

V (Å3)

1225.09(13)

927.25(4)

578.98(3)

Z

2

4

2

ρcalc (Mg/m3)

2.080

2.343

2.695

µ (Mo Kα) (mm-1)

7.139

14.816

18.360

No. reflns.

20067

5937

11790

No. param.

264

110

135

Unique reflns.

5498

1914

2427

GOOF (F )

1.034

1.050

1.160

Rint

0.0362

0.0402

0.0270

R1a (I ≥ 2σ)

0.0282

0.0237

0.0221

wR2b (I ≥ 2σ)

0.0717

0.0588

0.0561

2

a

R1 = Σ||Fo| – |Fc||/Σ|Fo|.

b

wR2 = [Σ[w(Fo2 – Fc2)2]/ Σ[w(Fo2)2]]1/2.


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3. Results and Discussion 3.1. Structure of [(mbtt)Br2]·0.5 CH2Cl2 (1)

Single crystal X-ray diffraction analysis (Table 1) of compound [(mbtt)Br2]·0.5 CH2Cl2 (1) indicated a presence of two [(mbtt)Br2] molecules and one dichloromethane molecule in the asymmetric unit. The structural parameters of the [(mbtt)Br2] molecules (1A and 1B) differ to some extent due to the interacting solvent molecule, which forms hydrogen bonds with 1B. The selected bond lengths and angles for 1A are presented in Table 2 and for 1B in Table S1. In compound [(mbtt)Br2]·0.5 CH2Cl2 (1) N-methylbenzothiazole-2-thione (mbtt) interacts through the thione sulfur with Br2 to form a linear S(1)-Br(1)-Br(2) moiety (174.54(2)°) as presented in Figure 1. The strong electron donation from the thione sulfur to the dibromine induces a formation of a short halogen bond with S(1)-Br(1) distance of 2.3244(8) Å. The n→σ* nature of the S(1)···Br(1) interaction causes the Br(1)-Br(2) contact (2.6673(4) Å) to be significantly elongated when compared to the distance in the free dibromide in the solid state (2.3 Å).52 In the case of 1B the interacting solvent causes elongation of the Br(1)-Br(2) contact to 2.7504(4) Å and, respectively, the S(1)-Br(1) interaction to get slightly shorter 2.2931(7) Å (Figure S1). In similar diiodine and interhalogen complexes the elongation of the halogenhalogen covalent bond is strongly affected by the electron donor strength of the sulfur atom and increases as the interaction between the thione sulfur atom and the halogen gets stronger.53-54 The compound 1 is notably similar with the previously reported [(mbtt)I2],55 and small differences are observed mainly on the supramolecular structures of the compounds. Most probably these small deviations are caused by the larger van der Waals radii and polarizability of iodine compared to bromine.


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The strength of the S···Br halogen bond in 1 was evaluated with halogen bonding ratio RXB (RXB=dXB/(Xvdw+Bvdw))56-58, in which the distance between the halogen bond donor and acceptor, dXB [Å], is divided by the sum of vdW radii [Å] of the donor (X) and the acceptor (B). The interaction ratio RXB is an efficient method to evaluate any non-covalent interaction, even though the distance between interacting atoms does not linearly correlate with the interaction strength. For halogen bonding the distance between the XB donor and XB acceptor is less than the sum of the van der Waals radii (RXB < 1), but only seldom the interaction is shortened more than 30% (RXB = 0.70).4, 59 The RBrS ratio of 1 is 0.64 indicating an extremely strong halogen bond. Similar strongly halogen-bonded complexes are encountered e.g. in some F-Cl···CN-R and F-Cl···SiN-R systems, in which the distance between the XB donor and acceptor may be as short as half of the sum of the van der Waals radii (RXB = 0.50).60-61 In addition to the estimation based on the geometry of the interactions, the nature and strength of the S···Br and Br···Br contacts in compounds 1-3 were analyzed further through the Quantum Theory of Atoms in Molecules (QTAIM) method48 using the non-optimized, experimental structures determined from X-ray diffraction studies (Table 3). In literature this method has been assessed to be one of the most reliable analytical methods to quantitatively assess halogen bonding interactions in the solid state.62 The models consisted of one thione unit and one dibromine molecule in the case of 1 and 2, and two [(btt)Br]+ units and two [Br3]- counter anions in the case of 3 (Figure S2). A molecular model consisting of two neighboring units was used to take both the intermolecular Br(1)···Br(2) halogen bond between the adjacent molecules, and the intramolecular Br(1)···H(1) and Br(4)···H(2) hydrogen bonds into account. The QTAIM analysis (Table 3) of the compound 1 confirmed a formation of a surprisingly strong halogen bonding interaction from S(1) to the Br(1) atom with interaction energy of -104


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kJmol-1. Typically, the strength of the halogen bonds ranges between very weak (-5 kJmol-1) and very strong (-180 kJmol-1) thus being comparable with the strength of hydrogen bonds.26-28 The strong S···Br halogen bond causes weakening of the Br···Br contact (-45 kJmol-1) and depletion of the electron density in the bonding bromine Br(1) as the dibromide molecule polarizes and the electron density concentrates to the non-bonding end of the dibromide molecule Br(2) (q(Br(1)): 0.001 and q(Br(2)): -0.457).

Figure 1. The structure and numbering of [(mbtt)Br2]·0.5 CH2Cl2 (1) (a) and the formation of a layered structure in the ab-plane (b). Solvent molecules are omitted for clarity.


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Table 2. Selected bond lengths [Å] and angles [°] for 1-3.a 1

2

3

C(1)-S(1)

1.712(3)

1.710(5)

1.743(3)

S(1)-Br(1)

2.3244(8)

2.2827(13)

2.1824(7)

Br(1)-Br(2)#

2.6673(4)

2.8141(7)

3.2395(4)

C(1)-S(2)

1.717(3)

1.729(5)

C(1)-N(1)

1.324(3)

1.325(6)

C(1)-N(2)

1.335(3) 1.343(4)

C(1)-S(1)-Br(1)

97.12(10)

101.3(2)

99.08(9)

S(1)-Br(1)-Br(2)#

174.54(2)

175.33(4)

176.16(2)

a

Symmetry transformations used to generate equivalent atoms for compound 3: # -x, -y, -1-z.

The nature of the interactions in 1 was estimated using the ratio of the densities of the potential energy V and kinetic energy G at the bond critical points (BCPs) (|VBCP|/GBCP), which for noncovalent interactions shows values under 1 and for covalent bonds over 2.63 Values between 1 and 2 indicate an intermediate nature between a covalent and a non-covalent (ionic) interaction. The |VBCP|/GBCP ratio of the S(1)···Br(1) halogen bond is 1.69 suggesting that the interaction is not purely electrostatic, but interestingly it exhibits considerable covalent character. The coordinative nature of the interaction is further supported by the delocalization index Ω(S,Br), which for compound 1 is 0.88 (Table 3). The delocalization index describes the average number of shared electrons between (bonding) atoms at the bond critical point (BCP).64 The comparison of the S···Br and Br···Br contacts in compound 1 revealed that although the interaction energies of the contacts are distinct, the key parameters of the electron density at the bond critical points are rather similar. The |VBCP|/GBCP ratio (S···Br: 1.69 and Br···Br: 1.37) and delocalization index


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Ω (S···Br: 0.88 and Br···Br: 0.81) indicate similar degree of electron sharing between the interacting atoms. The structural differences of the two [(mbtt)Br2] molecules in the asymmetric unit of 1 caused by the interacting solvent affect slightly to the key parameters of electron density at the bond critical points. The QTAIM analysis of 1B indicates a formation of a stronger S···Br halogen bond in comparison to non-interacting 1A indicated by higher interaction energy Eint -113kJmol1

, |VBCP|/GBCP ratio of 1.74, and Ω(S,Br) of 0.93. The selected properties of the electron density at

the bond critical points for 1B are presented in Table S2. The final 3D network of 1 is formed when the two [(mbtt)Br2] molecules in the asymmetric unit interact via hydrogen bonds between the outer bromine Br(2) and the methyl hydrogen H(2B2) (Br(2)-H(2B2): 2.9415(3) Å) and the aromatic hydrogen H(4B) (Br(2)-H(4B): 2.8527(3) Å). The 1B unit interacts further with a dichloromethane molecule via hydrogen bonds Br(2B)H(9A) 2.8220(3) Å and Br(1B)-H(9A) 3.1476(3) Å as shown in Figure S1. The outer bromines Br(2) and Br(2B) interact additionally with the aromatic hydrogens of the adjacent molecules thus forming a planar structure (Br(2)-H(6): 2.8062(3) Å and Br(2B)-H(6B): 2.8775(3) Å). The formation of the layered structure is shown in Figure 1b. The final molecular packing of the planes of 1 to a 3D network is governed by several weak hydrogen bonds, π-π, and Br-C interactions.


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Table 3. Selected properties of the electron density at the bond critical points for 1-3.a BCP

d

ρ

Eint

(Å)

(eÅ-3)

(kJmol-1)

|V|/G

Ω (A,B)

q (S(1))

q (Br(1))

q (Br(2))

0.203

0.001

-0.457

0.263

-0.002

-0.497

0.267

0.062

-0.846b

[(mbtt)Br2] (1) …

S(1) Br(1)

2.325

0.6362 -104

1.69

0.88

Br(1)…Br(2)

2.667

0.3745 -45

1.37

0.81

C(1)=S(1)

1.712

1.4044 -498

2.66

1.34

[(btt)Br2] (2) S(1)…Br(1)

2.283

0.6863 -116

1.76

0.95

Br(1) Br(2)

2.814

0.2868 -29

1.24

0.71

C(1)=S(1)

1.710

1.4072 -515

2.59

1.35

…

[(bit)Br]·[Br3] (3) S(1)…Br(1)

2.182

0.8517 -147

2.19

1.24

Br(1)…Br(2)

3.240

0.1120 -11

0.90

0.21

C(1)=S(1)

1.743

1.3708 -347

3.39

1.19

a

d= interatomic distance; ρ= electron density; Eint= interaction energy; V= potential energy density; G= kinetic energy density; Ω= delocalization index; q= AIM atomic charge. b

Total charge of [Br3]-.

3.2. Structure of [(btt)Br2] (2)

The structure of [(btt)Br2] (2) is effectively similar to 1, as the thione sulfur interacts with dibromine with an almost linear coordination mode S(1)-Br(1)-Br(2) 175.334° (Figure 2 and Table 2). 2(3H)-benzothiazolethione (btt) ligand compared to N-methylbenzothiazole-2-thione (mbtt) induces slightly shorter S(1)···Br(1) halogen bond (S(1)-Br(1): 2.3244(8) and 2.2827(13) Å in 1 and 2, respectively), thus indicating strengthening of the halogen bonding interaction in 2 (RBrS: 0.63). The stronger interaction has been explained by the generally higher pKBI2 values of


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the secondary thiones RC(S)NHR’ with diiodine as compared to the tertiary thiones RC(S)NMeR’.65 The difference is induced mainly by a formation of an intermolecular hydrogen bond I···H(NH), which supports the halogen bond.66-67 In the complex 2 the halogen bond is supported by a hydrogen bond between the dibromine and amino hydrogen (Br(2)-H(1): 2.4050(5) Å). The increasing strength of the S(1)···Br(1) interaction can be seen in the value of the interaction energy, Eint, which changes from -104 in 1 to -116 kJmol-1 in 2, and is supported by the increasing electron density ρ(S···Br) between the S(1) and Br(1) (1: 0.6362 and 2: 0.6863 eÅ-3). The stronger electron donation from the S(1) to the σ-hole of the dibromine introduces more coordinative nature to the S···Br interaction in 2 compared to 1, and the equilibrium of the S···Br and the Br···Br contacts changes. The S···Br halogen bond is strongly supported by a covalent contribution indicated by the |VBCP|/GBCP ratio (1.76) and delocalization index Ω(S, Br) (0.95). Notably, the stronger interaction formed between the thione sulfur and dibromine leads to a weaker Br-Br contact (-45 and -29 kJmol-1 in 1 and 2, respectively), and decreases both the electron density (0.3745 vs. 0.2868 eÅ-3) and the delocalization index (0.81 vs. 0.71) of the BrBr bond when compared to the equivalent bond in compound 1. Respectively, the Br(1)-Br(2) interaction elongates notably from 2.6673(4) Å in 1 to 2.8141(7) Å in 2. The supramolecular structure of 2 is mainly covered by a weak S···Br halogen bond (S(1)Br(2): 3.5162(13) Å and Br···H hydrogen bonds (Br(2)-H(1): 3.0320(5) Å and Br(2)-H(3): 3.0482(5) Å), which connect the adjacent molecules of 2 forming a zigzag chain in bc-plane with Br(1)-Br(2)-S(1) interaction angle of 69.91(3)° (Figure 2b). The structure is further expanded by a moderately strong Br(2)-H(1) 2.4050(5) Å and S(1)-S(2) 3.544(2) Å contacts, which layer the zigzag chains in the ab-plane.


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Figure 2. The structure of [(btt)Br2] (2) (a) and the formation of the S(1)-Br(1)-Br(2)-S(1) zigzag structure in the bc-plane (b).

3.3. Structure of [(bit)Br]·[Br3] (3)

The reaction of 2-benzimidazolethione (bit) with bromine in dichloromethane led to an instantaneous formation of the yellow precipitate, which was analyzed using elemental analysis and NMR spectroscopy indicating a formation of [(bit)Br2] (3b) similar to structures 1 and 2. However, upon crystallization of the solution or recrystallization of the precipitate exclusively single crystals of [(bit)Br]·[Br3] (3) suitable for X-ray analysis were obtained (Figure 3 and Table 2).


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Figure 3. The structure of [(bit)Br]·[Br3] (3) (a) and the formation of the linear S(1)-Br(1)-Br(2)Br(4)-S(1) double chain in the ac–plane (b).

In 3 the thione sulfur atom of the bit ligand interacts with the bromine atom (S(1)-Br(1): 2.1824(7) Å and C(1)-S(1)-Br(1): 99.084(2)°) with interaction energy as high as -147 kJmol-1. The |VBCP|/GBCP ratio (2.19) and delocalization index Ω(S, Br) (1.24) indicate the S(1) and Br(1) atoms to share significant degree of electron density and to form a conventional covalent bond. The formation of the strong interaction is promoted by the nitrogen atoms of the heterocycle of the thione ligand, as they support the partial positive charge formed upon the complexation. The


18

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strong interaction formed between S and Br strongly polarizes the dibromine species, in which the electron density concentrates to the exterior Br(2) (q(Br3-): -0.846) and leaves the Br(1) effectively neutral or slightly positive (q(Br(1)): 0.062). The polarization reaches the limit where the electron density difference causes a heterolytic cleavage of the Br(1)-Br(2) bond to form [(bit)Br]+ with [Br3]- as the counter ion. The Br(1) interacts with the [Br3]- counter anion of the adjacent molecule by a Br···Br interaction (Br(1)-Br(2): 3.2395(4) Å). The Br···Br interaction is a typical halogen bond with minimal amount of electron sharing (Ω(Br, Br): 0.21) and rather weak strength of -11 kJmol-1. The interaction is substantially weaker than the equivalent Br···Br contacts in 1 (-45 kJmol-1) and 2 (-29 kJmol-1), and has purely electrostatic nature indicated by the |V|/G ratio of 0.90.

The nearly linear S(1)-Br(1)-Br(2) arrangement (176.16(2)°)

corroborates the high directionality of the halogen bond. The change in the degree of electron donation of the thione ligands can be further well seen from the differences in the highest occupied molecular orbitals (HOMOs) (Figures 4 and S3). The HOMOs of the free thione ligands consist mainly of the aromatic π orbitals with a large contribution from the thione sulfur atomic orbitals (52-61%) (Figure S3). They greatly remind each other indicating the charge donation ability of the thione sulfur S(1) to be very similar in each thione. 2-benzimidazolethione (bit) shows slightly higher donation ability as the energy of HOMO of bit is destabilized compared to the other two thione ligands, and also the HOMO is slightly more concentrated on the S(1) sulfur atom. In compound 1, there is still a considerable contribution of S(1) and Br(1) in the molecular orbital, even if the main contribution comes from the exterior Br(2) (79%) (Figure 4). This indicates some degree of electron sharing between the three interacting atoms, thus signifying the relatively strong S···Br and Br···Br interactions. In 2, the contribution of S(1) and Br(1) diminishes, and the electron density is more concentrated at


19


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the exterior bromine (90%). Finally, in 3 the electron density is completely concentrated at the [Br3]- counter anion (99%) since the S(1) formally donates one electron to the [Br3]- moiety as indicated also by the AIM atomic charge analysis (q(Br3-): -0.846). This strong polarization reflects the change in the nature of the Br···Br interaction from a partially covalent to a purely ionic. The supramolecular structure of 3 is constructed by a non-covalent Br(1)···Br(2) interaction (3.2395(4) Å) between the adjacent [(bit)Br]+ and [Br3]- moieties, and a S(1)···Br(4) contact (3.4405(9) Å) between the thione sulfur and the end bromine Br(4) of the [Br3]- counter anion forming a linear chain structure. The chain contacts with the neighboring chain by a S(1)···Br(4) interaction (3.4543(7) Å) to construct a linear double chain in the ac-plane. The double chain is further supported by weak intra- and intermolecular hydrogen bonds Br(1)···H(1) 2.74(3) Å and Br(4)···H(2) 2.79(4) Å (Figure 3b). The chain formed in compound 3 reminds greatly the zigzag chain of 2 with similar S(1)-Br(1)-Br(2)-S(1) (in 2) and S(1)-Br(1)-Br(2)-Br(4)-S(1) (in 3) moieties. The chain structure of both complexes is supported by similar inter- and intramolecular Br···H hydrogen bonds.


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Figure 4. The composition of the highest occupied molecular orbital in 1-3. Additionally the energies of the HOMOs and the atomic orbital percentage contribution of the interacting atoms are presented (%(S1), %(Br1), %(Br2), and %(Br3-)).

3.4. Laplacian of the Electron Density

The change in the nature of the S···Br and Br···Br interactions in compounds 1-3 can be visualized using the 2D presentation of the Laplacian of electron density, which is the second derivative of electron density at the BCPs (Figure 5). In 1 the electron density of the S(1) atom is strongly directed towards the electron deficient σ-hole in the Br(1) thus forming an extremely strong halogen bonding interaction (Figure 5a). The electron density of Br(1) is accumulated around the S(1)-Br(1) axis being in agreement with the directional tendency of interactions between a halide and an electrophilic site (90-120°).20 The Br(1) and Br(2) atoms share electron density to form a Br-Br contact. In compound 2 the emerging electron donor strength of the


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sulfur atom S(1) strengthens the S(1)···Br(1) halogen bond as can be seen from the Laplacian of electron density, in which the electron density of S(1) bulges towards the bromine atom (Figure 5b). Respectively the electron density decreases between the interacting bromine atoms and the contact between them elongates. In compound 3 the nature of the halogen bond between S(1) and Br(1) changes to a covalent bond indicated by the strong bulge connecting the atoms as can be seen in Figure 5c. As for the nature of the Br···Br interaction, it changes as a σ-hole in Br(1) interacts with the electron density of Br(2) and forms a classical halogen bond.

Figure 5. Laplacian of the electron density along the S(1)-Br(1)-Br(2) plane in the models of compounds 1-3 (a-c, respectively).

3.5. Coordinative Nature of Halogen Bonding

Halogen bonding is defined to be a mainly electrostatically driven non-covalent contact between a halogen atom and an electron donating moiety.1,

20

In recent years it has been

suggested that in the case of extremely strong halogen bonds the interaction can additionally


22

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possess substantial degree of covalency.2-6 Symmetric [N···I+···N] and [S···I+···S] systems with equal distance and strength of the B···I+ halogen bonds between a Lewis base (B) and a iodonium cation provide good examples of halogen bonding with possible covalently contributed nature.715

Similar [N···X···N]+ motifs of the lighter halogens (Br, Cl, and F) have been observed to

exhibit symmetric systems in the solution, but in the solid state to form asymmetric ion-molecule complexes N···[X-N]+ with a covalent N-X bond and a longer and weaker N···X halogen bond.6, 68

Formation of symmetric halogen bonds with bromine and chlorine seems to be energetically

less favorable than with iodine.68 The studied S···Br···Br halogen bond systems in 1-2 with rather balanced S···Br and Br···Br interactions remind of the 3-center-4-electron (3c-4e) bond model of the previously reported [B···I+···B] systems as three atoms interact with some degree of dative sharing of electrons.69 The results indicate that halogen bonding with partially coordinative nature can also be obtained by using bromine as a halogen bond donor, and instead of using a halonium ion a strongly polarized dibromine can be used. The description “coordinative nature of XB” should be considered here as an emphasis of the greater degree of electron sharing and interaction energy rather than a claim of a fundamental change in the nature of the halogen bonding.4 In general, the nature of any contact, rather than being purely covalent or non-covalent, lies somewhere between these extremes and cannot be fully understood by the localized Lewis representations. The B···X2 halogen bonds are widely studied and their interaction length has been observed often to be notably short approaching that of covalent bonds.29 Although the strength of these interactions can be efficiently tuned by altering the electron donor strength of the XB acceptor,29-30 there are virtually no studies in which the tunable nature of these interactions would be exploited to systematically change the interaction nature. The series of the [(thione)Br2] systems (1-3) is thus


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an interesting and novel example with the gradation from a covalently contributed halogen bond to a pure covalent bond.

4. Conclusions

In summary, the strength of the S···Br and Br···Br interactions in [(thione)Br2] systems were systematically tuned by the variation of the electron donor properties of the heterocyclic thione ligands. Thiones were observed to be excellent halogen bond acceptors with variable electron donor strength, which can be modified by a single atom or by a single chemical group mutation. The computational calculations based on the QTAIM method indicated that the electrostatic nature of the S···Br halogen bonds in the compounds 1 and 2 was supported by a notable covalent contribution. The covalent character of the S···Br contact was further strengthened in 3 to produce a classical covalent bond with considerable degree of electron sharing between the atoms. In the current study we have demonstrated the possibility of systematic reinforcement of the halogen bonding, which in the extreme case leads to the change of the contact nature from a covalently contributed non-covalent interaction to a covalent bond. This allows for better understanding of the versatile nature of halogen bonding and may provide a useful tool for the design of functional materials based on halogen bonding.


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Associated Content Supporting Information

The structural parameters of 1B, selected properties of electron density for 1B, the model of 1B showing the hydrogen bonding with the solvent, the model of 3 used in the QTAIM analysis and the composition of the highest occupied molecular orbital in the thiones. X-ray crystallographic information files (CIF) are available for compounds 1-3. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic information files are also available from the Cambridge Crystallographic Data Centre (CCDC) upon request (http://www.ccdc.cam.ac.uk, CCDC deposition numbers 1012799 (1), 1012800 (2), and 1012801 (3)).

Author Information Corresponding Authors

*(P. H.): P. O. Box 111, FI-80101 Joensuu, Finland. Tel.: +358-503310829. E-mail: [email protected].

*(M. H.): P. O. Box 35, FI-40014 Jyväskylä, Finland. Tel.: +358-408054666. E-mail: [email protected].

Notes

The authors declare no competing financial interest.


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Acknowledgments

The authors gratefully acknowledge the Academy of Finland (M. H. project number 139571), the Inorganic Materials Chemistry Graduate Program (EMTKO) and the strategic funding of the University of Eastern Finland for the financial support. The computational work has been facilitated by access to the Finnish Grid Infrastructure resources. We thank Mrs Taina Nivajärvi for her technical support.

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For Table of Contents Use Only Tunable Interaction Strength and Nature of the S···Br Halogen Bonds in [(thione)Br2] Systems Laura Koskinen,† Sirpa Jääskeläinen,† Pipsa Hirva,†* and Matti Haukka‡*

Synopsis

Strong S···Br-Br halogen bonds with covalently contributed nature were synthesized and analyzed. The systematic reinforcement of the halogen bonding by the variation of the electron donor properties of the heterocyclic thiones strengthened the covalent character of the S···Br contact to form a classical covalent bond with considerable degree of electron sharing and induced a heterolytic cleavage of the dibromine molecule.


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Tunable Interaction Strength and Nature of the SÂ·Â·Â·Br Halogen Bonds

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