Significant Role of Supramolecular Interactions on Conformational

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Crystal Growth & Design

Significant Role of Supramolecular Interactions on Conformational Modulation of Flexible Organic Cation Receptors in a Metal-bis(dithiolate) Coordination Complex Matrix Ravada Kishore, Bharat Kumar Tripuramallu and Samar K. Das* School of Chemistry, University of Hyderabad, P.O. Central University, Hyderabad, 500046, India *E-mail: [email protected]; [email protected]; fax: +91-40-2301-2460; tel: +91-40-2301-1007.

1 ACS Paragon Plus Environment


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Abstract A series of new ion-pair complexes [Bz,R-BzBimy]2[M(mnt)2] {[Bz,R-BzBimy]+ = 1-benzyl-3(4-R-benzyl)benzimidazolium; M = Cu, R = H (1a), NO2 (1b) and Br (1c); M = Ni, R = H (2a), NO2 (2b) and Br (2c) and mnt2- = maleonitriledithiolate} have been prepared and characterized by routine spectral analyses including single crystal X-ray crystallography.

Due to flexible

nature of aryl groups (–CH2–Ar) in benzimidazolium cations, [Bz,H-BzBimy]1+ and [Bz,NO2BzBimy]1+ of compounds [Bz,H-BzBimy]BF4 (a) and [Bz,NO2-BzBimy]BF4 (b) respectively, the conformational change of the aryl groups have been observed in their respective metaldithiolate compounds 1a, 1b, 2a and 2b. However, no change in orientation of the associated phenyl groups is observed between the cationic organic receptor [Bz,Br-BzBimy]1+ of compound [Bz,Br-BzBimy]2BF4 (c)

and that in resulting ion pair compounds

1c and 2c. Fluxional

behavior of the aryl groups in the cationic organic receptor (benzimidazolium moiety, [Bz,RBzBimy]+), when it is ion-paired with different counter anions e.g., tetrafluoroborate (BF4−) and [M(mnt)2]2−, is mainly dependent on the supramolecular interactions (for example, S···H, N···H, O···H, Br···Br etc. weak contacts) between the relevant cation and anion. The psubstituents (H, NO2 and Br) of one of the phenyl rings in benzimidazolium moiety (cationic part) are found to be responsible for the structural diversities, observed in the crystal structures of metal-dithiolate ion pair compounds 1a, 1b, 1c, 2a, 2b and 2c. In this context, it is worth mentioning that the nickel containing ion pair compounds 2a, 2b and 2c are isomorphous with corresponding copper analogues 1a, 1b and 1c. The near-IR absorbance bands at around 1210 nm, observed for the copper compounds (1a-1c), have been attributed to the charge transfer from the copper dithiolate anion [Cu(mnt)2]2– to the benzimidazolium cation [Bz,R-BzBimy]+. The absorption bands, observed at around 862 nm for nickel compounds (2a-2c) can be assigned to combined transitions consisting of d–d, MLCT, π→π* electronic transitions. DFT calculations have been carried out to have the knowledge of stability of bare organic molecules, used in this study, in the perspective of their apparent stability (energy consideration) in the matrix of metal dithiolate coordination complex.

Hirshfeld surface analyses have been performed to assess

additional supramolecular perceptions into crystal structure features. The relevant fingerprint plot areas portray the percentages of different intermolecular interactions in the crystal structures. Copper compounds 1a-1c are additionally characterized by electron spin resonance (ESR) studies. 2 ACS Paragon Plus Environment

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Introduction The molecular arrangement in a crystal directly leads to solid state physical properties of the pertinent compound / material. Hence the introduction of non-covalent interactions in the molecular crystals is an important strategy to create materials with novel and desired solid state physical properties.1-3 Non-covalent interactions in a crystal include intermolecular hydrogen bonding interactions, such as S···H, N···H, H···Br, O···H etc. weak contacts and π···π stacking interactions, that have attracted enormous attention during two decades in the context of constructing functional molecular materials.4-11 These non-covalent interactions play an important role in deciding the conformation / orientation of the molecules in molecular selfassembly leading to diverse architectures, that are observed in biological- as well as metal organic framework (MOF)- / coordination polymer (CP)-systems.12-14 In the domain of metal organic frameworks (MOFs) and porous coordination polymers (PCPs), linker flexibility plays a vital role to display the promising potential.15-17 The flexibility of a substituent or of an integral part of the linker can influence the adsorption properties of the concerned material upon removal of pertinent guest molecule. The major disadvantage in the area of coordination networks, is to predict the final architectures, based on the geometry of the flexible linker.

Hence, the

conformation control of flexible molecules in a supramolecular assembly or in coordination networks is an important issue to contemporary researchers. We have reported important factors, that affect the conformational modulation of flexible ligands in the self-assembly processes leading to coordination polymers to address the mechanistic aspects of self-assembly of metal carboxylates.18-21 We have also emphasized the importance of supramolecular interactions, exerted by metal bis(1,2-dithiolat) complexes (that have received enormous attention because of their importance as conducting and magnetic materials,22-24 nonlinear optics,25,26 NIR dyes,27-29 catalysts30-32 and red-ox active materials33), in assembling a pseudorotaxane into diverse supramolecular architectures having well-defined void spaces, the shape and size of which depend on the particular metal bis(1,2-dithiolate) complex, used in the relevant synthesis.34 In an another instance, we have described of how supramolecular interactions between an alkyl imidazolium derivative as a cation and a metal bis(1,2- dithiolate) complex anion play a key role to control the geometry in terms of degree of deviation (whether it will square planar or distorted square planar) of the copper metal ion in the concerned copper bis(1,2- dithiolate) complex anion.35 To extend our work in this direction, we have recently undertaken a project of 3 ACS Paragon Plus Environment


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conformational changes of organic cation receptors influenced by a metal bis(1,2- dithiolate) complex anion. In this article, we have focused on the conformational change of organic cation receptors with respect to two different extents of anions: tetrafluoroborate (BF4−), a conventional inorganic anion and the metal-bis(dithiolate) anion, a classical coordination complex anion. Herein, we have reported six new ion-pair dithiolene complexes [Bz,R-BzBimy]2[M(mnt)2] (M = Cu (1), Ni (2); R = H (1a, 2a), NO2 (1b, 2b) and R = Br (1c, 2c)) along with [Bz,RBzBimy]2BF4 (R = H (a), NO2 (b) and R = Br (c)). In the organic cation receptors [Bz,HBzBimy]1+

and [Bz,NO2-BzBimy]1+ of compounds [Bz,H-BzBimy]BF4 (a) and [Bz,NO2-

BzBimy]BF4 (b), two benzyl groups, that are attached to the nitrogen atoms of the central benzimidazole, are in syn and anti positions respectively with respect to the mean plane of the central benzimidazole ring . When these salts a and b react with Na2[M(mnt)2] (M = Cu (1), Ni (2)), the orientation/conformation of these phenyl groups change to anti and syn respectively in their corresponding ion-pair complexes 1a / 2a and 1b / 2b as shown in Scheme 1. However, there is no change in conformation of these two phenyl rings in compound c (BF4 salt) and its dithiolate complexes 1c / 2c (in both cases, this conformation remains anti). The Hirshfeld surface36-38 analysis of the crystal structures has become a valuable tool to elucidate quantitative intermolecular interactions and the relevant fingerprint plot gives a precious information on these interactions.39-40 The title compounds exhibit near-IR absorption bands in their electronic absorption spectra, characteristics of

metal-dithiolate complexes. These near-IR bands are

important in the context of Q-switching lasers applications.41,42 All the compounds 1a-1c, 2a-2c and a-c are unambiguously characterized by single crystal X-ray structure determinations and are further characterized by IR, 1H NMR, ESR, LCMS, UV-vis-NIR spectral studies and cyclic voltammetry. The phase purity of the compounds has been done by powder X-ray diffraction (PXRD) studies.


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Scheme 1. (i) Conformation of the benzimidazolium salts (from left to right: a–c), anion (tetrafluoroborate) moiety is omitted for clarity, (ii) conformation of the benzimdazolium salts in metal-bis(dithiolene) ion-pair complexes, ([M(mnt)2]2– anion is omitted for clarity) (iii) packing diagrams through N···H; N···H, O···H and S···H, H···Br interactions of the copper-bis(dithiolene) ion-pair complexes of 1a, 1b and 1c respectively (from left to right).



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R N

1. BzBr, CH3CN, Base

N

N

2. p-R-BzBr, Toluene

N

H

R H (a)

BF4

NO2 (b)

3. NH4BF4

Br (c)

M

X

Cu

2

Ni

6

NC

S Na+

NC

S Na+

MCl2 XH2O MeOH

NC

R

S

N

S

CN

M

N 2

S

NC

S

M

CN

R

Cu (1a), Ni (2a)

H

Cu (1b), Ni (2b)

NO2

Cu (1c), Ni (2c)

Br

Scheme 2. Synthetic route for the preparation of organic cation receptors (a−c) and metal-dithiolene complexes (1a−1c and 2a−2c).

Experimental Details Materials and physical methods. All chemicals were purchased from commercial sources and used without further purification.


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Micro analytical (C, H, N) data was obtained with a FLASH EA 1112 Series CHN Analyzer. The IR spectra (with KBr pellets) were recorded in the range of 400-4000 cm-1 on a JASCO FT/ IR5300 spectrometer. Diffuse reflectance and near-IR absorption spectra were recorded on a UV-3600 Shimadzu UV-Vis-NIR spectrophotometer. The electron spin resonance (ESR) spectra were recorded on a (JEOL) JESFA200 ESR spectrometer. 1H NMR spectra were recorded on Bruker DRX- 400 spectrometer using Si(CH3)4 (TMS) as an internal standard. Mass spectra were recorded on a LCMS-2010A Shimadzu. A Cypress model CS-1090/CS-1087 electroanalytical system was used for cyclic voltammetic experiments. The electrochemical experiments were measured in acetonitrile containing [Bu4N][ClO4] as a supporting electrolyte, using a conventional cell consisting of two platinum wires as working and counter electrodes.

General preparation of benzimidazolium derivatives. Benzimidazolium compounds were prepared according to literature procedure.43 Benzimidazole (10 mmol) was suspended in acetonitrile (20 mL); to this, NaOH (10 mmol, 6.25 M) was added and stirred at room temperature for 30 min. Subsequently the reaction mixture was treated with benzyl bromide (10 mmol) and the mixture was stirred overnight at 80 °C. The solvent was removed and the residue was dissolved in water and extracted with CH2Cl2. Then the dichlomethane extract was reduced to dryness. The crude product was then dissolved in toluene (30 mL) and another portion of p-R-benzyl bromide (R=H, NO2, Br) (10 mmol) was added. The reaction mixture was stirred overnight at 90 °C. The white precipitate was filtered, washed with toluene, diethyl ether and air-dried.

General procedure for counter ion exchange (bromide salts to tetrafluoroborate salts). 1-Benzyl-3-(4-R-benzyl)benzimidazolium bromide (2 mmol) was dissolved in hot water and ammoniumtetrafluoro borate (>2 mmol) was added until no precipitation was observed. The product was extracted with ethyl acetate (2 × 20 mL) and dried by using Na2SO4; the white crude product was obtained after removal of the solvent by rotary evaporator. Single crystals, suitable for X-ray structure analysis, were obtained by recrystallization from hot methanol solution of compounds a–c (see also Scheme 2).



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1,3-Dibenzylbenzimidazolium tetrafluoroborate (a). Yield: 78% (based on Bz-bromide). Anal. Calc. for C21H19BF4N2: C, 65.31; H, 4.95; N, 7.25%. Found: C, 65.68; H, 4.89; N: 7.58%. IR spectrum (KBr pellet, ν/cm–1): 3133, 3084, 1599, 1566, 1451, 1374, 1193, 744, 706. LCMS (m/z) = 300 (M+H)+. 1H NMR (400 MHz, δ ppm) (DMSO-d6): 10.02 (s, 1H), 7.98-7.96 (m, 2H), 7.65-7.63 (m, 2 H), 7.52 (m, 4H), 7.46-7.37 (m, 6 H), 5.79 (s, 4H). 1-Benzyl-3-(4-nitro-benzyl)benzimidazolium tetrafluoroborate (b). Yield: 86% (based on Bzbromide). Anal. Calc. for C21H18BF4N3O2: C, 58.50; H, 4.20; N, 9.74%. Found: C, 58.82; H, 4.27; N: 9.38%. IR spectrum (KBr pellet, ν/cm–1): 3139, 3073, 1610, 1561, 1533, 1347, 1193, 1062, 755. LCMS (m/z): 345 (M+H)+. 1H NMR (400 MHz, δ ppm) (DMSO-d6): 10.01 (s, 1H), 8.29-7.43 (m, 13H), 5.96 (s, 2H), 5.80 (s, 2H). 1-Benzyl-3-(4-bromo-benzyl)benzimidazolium tetrafluoroborate (c). Yield: 82% (based on Bz-bromide). Anal. Calc. for C21H18BF4N2Br: C, 54.23; H, 3.90; N, 6.02%. Found: C, 54.62; H, 3.95; N: 6.48%. IR spectrum (KBr pellet, ν/cm–1): 3137, 3068, 1624, 1548, 1352, 1179, 765. LCMS (m/z): 377, 379 (M–H)+, (M+H)+. 1H NMR (400 MHz, δ ppm) (DMSO-d6): 9.98 (s, 1H), 7.96-7.45 (m, 13H), 5.78-5.76 (m, 4H).

General synthetic procedure for the ion pair compounds 1a-1c and 2a-2c. To a stirred solution of Na2mnt44 (2 mmol) in methanol (10 mL), MCl2•XH2O (M = Cu, Ni; X = 2, 6) (1 mmol) in MeOH (5 mL) was added and stirred for 30 min at room temperature. To this solution, [Bz,R-BzBimy][BF4] (R = H, NO2, Br; 2 mmol) in MeOH (15 mL) was added and stirred for 4 hours at room temperature (Scheme 2). A dark brown precipitate (appearance of which is black!) was obtained which was filtered oﬀ and crystallized from CH3CN/ether to afford dark-red crystal (2a–2c). Single crystals of compounds 1a–1c were grown by slow evaporation of respective methanol solutions.

[Bz,H-BzBimy]2[Cu(mnt)2] (1a). Yield: 52% (based on copper). Anal. Calc. for C50H38N8S4Cu: C, 63.70; H, 4.06; N, 11.88. Found: C, 63.55; H, 4.15; N: 11.65. IR spectrum (KBr pellet, ν/cm– 1

): 3119, 2191, 1556, 1464, 1145, 1016, 742, 702. LCMS (m/z): 300 (M+H)+. 1H NMR (400

MHz, δ ppm) (DMSO-d6): 10.17 (s, 1H), 8.02-7.40 (m, 14H), 5.84 (s, 4H). [Bz,H-BzBimy]2[Ni(mnt)2] (2a). Yield: 68% (based on nickel). Anal. Calc. for C50H38N8S4Ni: C, 64.03; H, 4.08; N, 11.94. Found: C, 64.21; H, 4.15; N, 12.16. IR spectrum (KBr, ν/cm–1): 8 ACS Paragon Plus Environment

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3119, 3059, 2191, 1604, 1556, 1485, 1182, 1145, 744, 702. LCMS (m/z): 300 (M+H)+. 1H NMR (400 MHz, δ ppm) (DMSO-d6): 10.02 (s, 1H), 7.98-7.96 (2H), 7.65-7.39 (m, 12H), 5.79 (s, 4H). [Bz,NO2-BzBimy]2[Cu(mnt)2] (1b). Yield: 46% (based on copper). Anal. Calc. for C50H36N10O4S4Cu: C, 58.15; H, 3.51; N, 13.56. Found: C, 58.31; H, 3.45; N, 13.68. IR spectrum (KBr, ν/cm–1): 3074, 2986, 2920, 2191, 1604, 1562, 1452, 1346, 1249, 1026, 966, 806, 746, 528. LCMS (m/z): 345 (M+H)+. 1H NMR (400 MHz, δ ppm) (DMSO-d6): 10.02 (s, 1H), 8.28-7.43 (m, 13H), 5.97 (s, 2H), 5.81 (s, 2H). [Bz,NO2-BzBimy]2[Ni(mnt)2] (2b). Yield: 64% (based on nickel). Anal. Calc. for C50H36N10O4S4Ni: C, 58.43; H, 3.53; N, 13.62. Found: C, 58.36; H, 3.61; N, 13.71. IR spectrum (KBr, ν/cm–1): 2924, 2293, 2193, 1604, 1562, 1521, 1479, 1346, 738. LCMS (m/z): 345 (M+H)+. 1

H NMR (400 MHz, δ ppm) (DMSO-d6): 10.01 (s, 1H), 8.30-7.43 (m, 13H), 5.96 (s, 2H), 5.80

(s, 2H). [Bz,Br-BzBimy]2[Cu(mnt)2] (1c). Yield: 48% (based on copper). Anal. Calc. for C50H36N8Br2S4Cu: C, 54.57; H, 3.29; N, 10.18. Found: C, 54.16; H, 3.34; N, 10.24. IR spectrum (KBr, cm–1): 2191, 1633, 1556, 1458, 1369, 1217, 1105, 1010, 758. LCMS (m/z): 377, 379 (MH)+, (M+H)+. 1H NMR (400 MHz, δ ppm) (DMSO-d6): 10.19 (s, 1H), 7.98-7.39 (m, 13H), 5.825.81 (br, 4H). [Bz,Br-BzBimy]2[Ni(mnt)2]

(2c).

Yield:

61%

(based

on

nickel).

Anal.

Calc.

for

C50H36N8Br2S4Ni: C, 54.81; H, 3.31, N, 10.22. Found: C, 54.76, H, 3.24, N, 10.29. IR spectrum (KBr, ν/cm-1): 2193, 1558, 1487, 1406, 1369, 1130, 1070, 806. LCMS (m/z): 377, 379 (M-H)+, (M+H)+. 1H NMR (400 MHz, δ ppm) (DMSO-d6): 9.93 (s, 1H), 7.91-7.27 (m, 13 H), 5.75 (br, 4 H).

X-ray Diffraction Data were measured at room temperature for compounds 1a–1c, 2a–2c and a–c on a Bruker SMART APEX CCD area detector system [λ (Mo Kα) = 0.7103 Å] with a graphite monochromator. 2400 frames were recorded with an ω scan width of 0.3°, each for 10 s and crystal-detector distance of 60 mm, collimator 0.5 mm.45-48 Data reduction was performed with the SAINTPLUS software.45 Absorption correction was made using an empirical method SADABS.46 Structure solution was performed using SHELXS-97 program47 and it was refined



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using SHELXL-97 program.48 Hydrogen atoms on the aromatic rings were introduced on calculated positions and included in the refinement riding on their respective parent atoms. Thermal ellipsoidal diagrams, hydrogen bonding tables for compounds a–c, 2a–2c and crystallographic data for compounds a–c are described in Section-1 (Supporting Information). CCDC-1404176, CCDC-1404167, CCDC-1404164, CCDC-1404165, CCDC-1404166, CCDC 1404168, CCDC-1404169, CCDC-1404170 and CCDC-1404171 contain the supplementary crystallographic data for compounds 1a, 1b, 1c, 2a, 2b, 2c, ‘a’, ‘b’ and ‘c’, respectively. Relevant

crystal

data

can

be

obtained

free

of

charge

via

http://www.ccdc.cam.atc.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223−336−033; or e-mail: [email protected].

Results and discussion Synthesis We have synthesized a series of ion-pair metal-bis(dithiolene) complexes by the treatment of metal chlorides (CuCl2·2H2O and NiCl2·6H2O) with 2 equivalents of Na2mnt (disodium maleonitriledithiolate) in methanol. To this, various substituted benzimidazolium derivatives (2 equiv.) were added at room temperature. Benzimidazolium derivatives of tetrafluoroborate compounds a-c, and corresponding ion-pair dithiolene complexes 1a-1c, 2a-2c have been characterized by elemental analyses, IR, NMR, UV-vis-NIR spectroscopic techniques, cyclic voltammetry and single crystal X-ray structure determinations.

Description of Crystal Structures Compounds [Bz,R-BzBimy][BF4] (a) and [Bz,R-BzBimy]2[M(mnt)2] [R= H, M= Cu (1a), Ni (2a)] Dark red single crystals of compounds 1a and 2a were chosen for X-ray structure analysis. Crystallographic analysis reveals that both the compounds 1a and 2a are isomorphous and crystallizes in triclinic space group P-1. The basic crystallographic data for compounds 1a and 2a are presented in supporting information (Section 1). Since compounds 1a and 2a are isomorphous, the crystal structure analysis of 1a is discussed in details. The selected bond


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lengths and bond angles in the crystal structure of compound 1a are given in Table S6 (Supporting Information). The relevant asymmetric unit contains half of the molecule, which has been represented by labeled atoms in ORTEP diagram as shown in Figure 1a. In [Cu(mnt)2]2– of compound 1a, four sulfur atoms define a plane resulting square-planar geometry around the Cu(II) with an S–Cu–S bond angles of 90.78(3)° within the five membered chelate ring. The average Cu–S, C–S, and C=C bond lengths are 2.262, 1.729 and 1.347 Å respectively in compound 1a, that are in good accordance with the literature reported values. The overall charge of the metal complex anion [Cu(mnt)2]2– in compound 1a is compensated by a two [Bz,HBzBimy]+ cations as observed in the crystal structure. In the crystal structure

(Figure 1b) of compound 1,3-dibenzylbenzimidazolium

tetrafluoroborate (a), the angles between benzimidazole mean plane and its adjacent two phenyl rings ({C2-C7} and {C16-C21}) are 86.11° and 78.01° respectively. The angle between mean planes of two phenyl rings ({C2-C7} and {C16-C21}) is 50.77°. This indicates that the two phenyl rings are not parallel to each other and one of these phenyl rings {C2-C7} is nearly perpendicular to the central benzimidazole mean plane. In the molecular structure of the 1,3dibenzylbenzimidazolium cation (Figure 1b), both the phenyl rings are syn to each other, with a synperiplanar torsion angle of 5.49° (viewed through C2-C1-C15-C16). The tetrafluoroborate anion acts as a counter anion and stabilizes the thermodynamically unfavorable syn conformation of the benzimidazolium cation of organic receptor. When this tetrafluoroborate anion in a is replaced by the [Cu(mnt)2]2− complex anion resulting in the formation of ion pair compound [Bz,H-BzBimy]2[Cu(mnt)2] (1a), the syn conformation of benzimidazolium cation (Figure 1b) in compound a changes to its anti conformation in 1a (Figure 1a). In the ion pair compound 1a, the angle between {C6-C11} phenyl ring and the central benzimidazole ring is 84.87° and the angle between other phenyl ring {C20-C25} and the central benzimidazole ring is 71.84°. And the angle between mean planes of two phenyl rings ({C6-C11} and {C20-C25}) is 85.55°, which clearly indicates that these two phenyl rings are nearly perpendicular to each other. The torsion angle between these two phenyl rings with respect to each other is 166.88° (viewed through C20C19-C5-C6), that confirms

the anti conformation of the cation in compound 1a. The

conformational modulation of the benzimidazolium cation from the syn (compound a) to anti (compound 1a) occurs by replacing the counter anion BF4− with [Cu(mnt)2]2−. Supramolecular



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interactions play an important role in this conformation change (vide infra). The same structural features, as expected, have been observed in case of nickel analogue (compound 2a). According to topological representation of crystal structure of compound a (Figure 2a), the alignment of phenyl groups are in syn arrangement, in which all the tetrafluoroborate anions are arranged back and front sides of the benzimidazolium moieties and all the cations are arranged in a CC*CC*CC* (C* is the mirror image of C) pattern. When the tetrafluoroborate anion is replaced by [Cu(mnt)2]2– anion resulting in compound 1a, the conformation of cations changes to anti conformation (Figure 2b). As shown in Figure 2b, each anion (A) is situated in between two organic cations (C), giving ACCACCACC type arrangement.

Compounds [Bz,R-BzBimy][BF4] (b) and [Bz,R-BzBimy]2[M(mnt)2] [R= NO2, M= Cu(1b), Ni(2b)] In order to assess the substitution effect in the conformational modulation, the compound b was designed, in which the nitro (−NO2) group was introduced at the p-position of one phenyl ring of benzimidazolium cation (see Scheme 2). The colorless block-shpaed crystals of compound b with BF4− as an anion and the dark red crystals of the compounds 1b ([Cu(mnt)2]2as an anion) and 2b ([Ni(mnt)2]2- as an anion) were crystallized in the same triclinic space group P-1. In the crystal structure (Figure 3a) of compound b, in the cation, the torsion angle between two aryl groups ({C16-C21(NO2)} and {C2-C7}) is 177.86° with respect to each other (viewed through C1-C2-C15-C16). This torsion angle of 177.86° indicates a anti conformation of the phenyl rings with respect to the each other. The angles between the planes of the terminal phenyl rings {C16-C21-(NO2)} and {C2-C7} with respect to mean plane of the central benzimidazole ring are 75.23° and 70.79° respectively (see Figure 3a). The angle between the two mean planes of two phenyl rings {C16-C21-(NO2)} and {C2-C7} is 79.26°. The anti arrangement of phenyl rings in compound b is changed to syn arrangement when the surrounding environment (anion) of the cation in b is changed from BF4− anion to coordination complex anions [M(mnt)2]2– in compounds 1b and 2b. Compounds 1b and 2b are isomorphous. The crystal structure details of compound 1b is described in details. Thermal ellipsoidal diagram of the compound 1b is shown in Figure 3b, in which the relevant asymmetric unit contains half of the molecule, represented by labeled atoms. The four sulfur atoms surround the copper ion, providing the square planar coordination geometry with the average S–Cu–S


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angle of 90.47 Å. In the five membered chelate ring of the dithiolate moiety, the average S–Cu, C–S and C=C bond lengths are consistent with those, described in the crystal structure of compound 1a. As already mentioned, in the crystal structure of compound 1b, the two aryl groups {C6-C11-(NO2) and C20-C25} are in syn positions with synclinal torsion angle of 16.51° (viewed through C5-C6-C19-C20). When nitro group is introduced in the skeleton of the organic moiety (Scheme 2), the typical anti conformation of the organic receptor cation is favored in BF4− anion matrix and an unusual syn conformation of this organic receptor cation is observed in [M(mnt)2]2− anion matrix, in which the unusual syn conformation is stabilized by supramolecular π···π interactions (vide infra). A topological representation of the cations and anions for both b and 1b is presented in Figure 4. As shown in Figure 4a, if we name the first horizontal line by ‘A’ followed down by a line distinguished by naming B (because they are not identical), then the entire topological depiction can be descried by ABABAB representation. In the case of compound 1b, the anionic moieties are indicated by orange color, all the cations are represented by blue color and the substituent group (-NO2) is indicated with green color (Figure 4b). As shown in the relevant topological representation (compound 1b), all the complex anions are situated between the organic cation receptors and the cations are arranged in a chainlike arrangement (combination of two horizontal lines). The nickel analogue (compound 2b) exhibits identical structural features, as exhibited by the compound 1b.

Compounds [Bz,R-BzBimy][BF4] (c) and [Bz,R-BzBimy]2[M(mnt)2] (R= Br, M= Cu (1c), Ni (2c)) The substitutional effect was further studied by synthesizing compound c, where a bromo group was introduced at the skeleton of the linker at one of the phenyl rings in the benzimidazolium cation (see Scheme 2). The thermal ellipsoidal plot of compound c is shown in Figure 5a. In the cationic part of compound c, the phenyl groups are anti to each other, where one of the phenyl rings is p-bromo group substituted (Figure 5a). Using this bromo-substituted cation receptor, we synthesized compounds 1c and 2c where the anionic parts are [Cu(mnt)2]2− and [Ni(mnt)2]2− respectively. The dark red crystals of compounds 1c and 2c were crystallized in triclinic P-1 space group. Structural features of 1c have only been described here because compound 2c is isomorphous with compound 1c. The asymmetric unit in the crystal structure of compound 1c is 13 ACS Paragon Plus Environment


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given in Figure 5b, which represents half of the molecule. Accordingly the formula unit of compound 1c includes two cation receptor molecules and one copper bis(dithiolate) complex anion, as were found for compounds 1a and 1b. In cationic part of compound c, the two aryl rings ({C16-C21-Br} and {C9-C14}) are in thermodynamically favorable anti positions to each other with torsion angle of 177.64° (viewed through C8-C9-C15-C16). When this organic cationic receptor is isolated in the [M(mnt)2]2– matrix as compounds 1c and 2c, the anti conformation of the aryl groups is retained as shown in Figure 5b. Thus in the crystal structure of compound 1c, the aryl groups ({C13-C18-Br} and {C20-C25}) are in anti conformation with antiperiplanar torsion angle of 164.89° (viewed through C19-C20-C12-C13). This observation is in contrast to the observations during conversions of a to 1a/2a and b to 1b/2b. In the topological representation of compound c, the BF4− anions are observed in between cation moieties as shown in Figure 6a. If we designate the first horizontal plane including both cationic and anionic moieties by ‘C’ and the following one by D, then the entire topological description can be called CDCDCD representation. In the topological representation of the ion pair compound 1c, the complex anions are arranged in a similar fashion in the entire vertical plane and the cations are located in a anti arrangement when looking down to the crystallographic b-axis, as shown in Figure 6(b).

Influential role of supramolecular interactions on conformational modulation of organic receptors: why in one case, conformation modulation is syn to anti, in other case anti to syn and in some case no change in conformation! The anomaly of conformational modulation of a cationic organic receptor (benzimidazolium moiety) by switching the anion from BF4− to [M(mnt)2]2− is explained in a logical manner in the following discussion. A molecule always prefers to stay in thermodynamically favorable anti conformation (energy consideration) unless it is compelled by an external force to drive to change its conformation. In order to explain the change in the conformation of the cationic receptor, we have attempted to correlate the conformation of the particular receptor molecule with the supramolecular interactions, exhibited by the flexible -CH2- groups of the receptor toward the anionic part of the ion pair compound. Let us consider the Scheme 3, in which X1 and X2 positions indicate the sp3 hybridized carbon atoms. As shown in Scheme 3, the orientation of


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hydrogen atoms and phenyl rings can result in either staggered form or eclipsed form; it is obvious from the scheme that eclipsed form gives syn-conformation of the attached phenyl

Scheme 3: Orientation of the hydrogen atoms at X1 and X2 sp3 hybridized carbon atom (hydrogen atoms are omitted for clarity in first picture).

groups and staggered form offers anti-conformation of the phenyl groups. We have demonstrated using the present system that whether, an eclipse form or a staggered form is stabilized, is dependent on the type of supramolecular interactions of the concerned cationic receptor with surrounding anionic counterpart as discussed below. In the crystal structure of compound ‘a’, eight C–H···F hydrogen bonds are present between cation and anion. Among these eight C– H···F hydrogen bonds, three hydrogen bonds are aligned with the hydrogen atoms, that are belonged to free rotating carbon atoms (—CH2— groups) at X1 and X2 positions, which are nearly in an eclipsed form as observed in the crystal structure of compound a (see Figure 7a and Scheme 3). This eclipsed form is imposed by the supramolecular interactions, offered by surround BF4‒ anions as shown in Figure 7a. Hence the two phenyl rings, which are attached to the sp3 hybridized carbons (X1, X2 positions of carbons, Scheme 3), are in a syn arrangement. On the other hand, in the crystal structure of metal complex 1a (obtained by replacement of BF4‒ anion in compound ‘a’ by complex anion [M(mnt)2]2–), C–H···N and C–H···S supramolecular interactions (less than the sum of the van der Waals radii, see Table 1) play a significant role. Due to these interactions, the hydrogens at X1 and X2 positions (see Scheme 3) are nearly in staggered alignment as shown in Figure 7b. The C–H···N hydrogen bond at X1 position is above the mean plane of the benzimidazole ring and the C–H···S hydrogen bond at X2 position is



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below to the mean plane of the benzimidazole ring. As a result of this staggered supramolecular hydrogen bonding interactions (Figure 7b), the associated phenyl rings are in anti arrangement in compound 1a. In the crystal structure of compound ‘b’, the aryl groups are in anti positions with respect to the mean plane of benzimidazole ring, whereas the two aryl groups are in the syn alignment in compound 1b. This conformational modulation from anti (with BF4– anion) to syn (with [M(mnt)2]2– anion) can be explained on the basis of weak supramolecular interactions between cation and anions, such as H···N, H···S, H···O and H···F contacts, π···π stacking interactions. In the crystal structure of compound ‘b’, thirteen C–H···F hydrogen bonding interactions are observed. In the crystal structure (compound ‘b’), the alignment of hydrogen atoms, that are attached to the carbon atoms at X1 and X2 positions, are in almost staggered form (Figure 8a), where two hydrogen bonding interactions (H1B···F1, H1B···F4) operate above the mean plane of the central benzimidazole ring and the remaining two hydrogen bonding interactions (H15B···F4, H15A···F3) operate below to the mean plane of benzimidazole ring. Hence the two phenyl rings are in anti orientation in compound b. In the crystal structure of compound 1b, C– H···N, C–H···S and C–H···O hydrogen bonding interactions between the cation and the [M(mnt)2]2– anion are observed. In this case, hydrogen bonding interactions at X2 position (see Scheme 3) is in eclipsed form (Figure 8b), and the remaining hydrogen bonding interactions (phenyl group attached to carbon at X1 position) operate above the mean plane of central benzimidazole ring as shown in Figure 8b. Thus the phenyl groups are in syn positions in the crystal structure of compound 1b. In the crystal structure of compound ‘c’, two aryl groups (which are attached to the sp3 carbon atoms) are in the anti alignment. Whereas, the two phenyl groups in compound 1c (formed by the replacement of BF4– anion in compound ‘c’ [M(mnt)2]2– anion) are also in anti alignment. So, in this case there is no conformational change of the organic receptor (benzimidazolium cationic moiety) in going from compound ‘c’ to compound 1c / 2c. There are nine C–H···F hydrogen bonding interactions, observed in the crystal structure of compound ‘c’, where the hydrogen atoms at X1 and X2 carbon positions (see Scheme 3) are found to be in staggered conformation (Figure 9a); the hydrogen bonding interactions at X1 position occur above the mean plane of central benzimidazole ring and those in X2 position are found below to the mean plane of benzimidazole ring. Due to these staggered supramolecular interactions, the 16 ACS Paragon Plus Environment

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two phenyl rings are in the anti orientation. In the crystal structure of compound 1c, there are six types of hydrogen bonding interactions as described in Table 1. The hydrogen atoms at sp3 carbon atoms (X1 and X2 positions, see Scheme 3) are found nearly in staggered form; in this case, there are only two hydrogen bonding interactions (H12A···Br1, H12B···S1) and both of them operate from X1 position (see Scheme 3), which are above the mean plane of benzimidazole ring as shown in Figure 9b. Thus carbon atom at X2 position (see Scheme 3) is free from hydrogen bonding interactions (Figure 9b). It is then logical to argue that the concerned C–C single bond (–CH2–Ph) can freely rotate to the anti/syn position with respect to the bromo substituted phenyl ring which is attached to the X1 position. In this case, the phenyl ring (C20-C25), which is attached to the sp3 carbon (at X2 position) atom is more preferably situated in a anti orientation with respect to the bromo substituted phenyl ring. This is because there are C–H21···π1 (H21···π1 = 3.263 Å) interactions with the chelate ring from [Cu(mnt)2]2– anion (π1 indicates the centroid of the Cu1S1C1C2S2) as shown in Figure 9c. In the crystal structure of compound 2c, strong Br···Br interactions are also evidenced with a distance of 3.63 Å (Figure S2, Supporting Information), which is shorter than sum of van-der Waals radii (1.85 Å for bromine),49 and there is weak Br···Br interactions of 3.90 Å in compound 1c; these interactions are consistent with the related literature values.50-52

Role of π···π interactions towards conformational modulation Apart from the above mentioned supramolecular weak interactions, the conformations are further stabilized by the π···π interactions as described below. In compound ‘a’, both the phenyl rings are in syn conformation. The hydrogen atoms in the phenyl rings ({C2-C7} and {C16-C21}) at p-position are labeled as H5, H19; among these, (C2-C7) phenyl ring has C–H····π interactions: C5–H5····Cg1 (Cg1 = C9-C14 phenyl ring of benzimidazole) with a distance of 3.226 Å.53,54 The central benzimidazole ring has C–H····π interactions: C12–H12····Cg2 (Cg2 = C16-C21, one of the terminal phenyl rings) within a distance of 3.303 Å as shown in Figure S3a (Supporting Information). When these terminal phenyl rings change their orientation to anti in compound 1a, there are no C–H····π interactions with the p-position hydrogen atoms from the phenyl rings. In compound 1a, one of the terminal phenyl rings {C20-C25} act as centroid (Cg3 = {C20-C25}) for the C14−H14····Cg3 interactions with a distance of 3.073 Å.

One of the SP3-hybridized carbons is involved in C– 17

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H····π interactions using its both hydrogen atoms: C19−H19A····Cg3 and C19−H19B····Cg3 interactions with distances of

3.327 and 3.172 Å respectively, as shown in Figure S3b

(Supporting Information). In addition to these intra-molecular C–H····π interactions, there are inter-molecular π ····π stacking interactions (Cg4···Cg4 = 3.749 Å), that are present between two terminal phenyl rings (Cg4 = {C12-C17}) of two adjacent cations (Figure S3c, Supporting Information). In the crystal of compound ‘b’, the nitro group substituted phenyl rings are arranged in anti fashion. In this compound, no C–H····π interactions are observed with the hydrogen atom at p-position (C5–H5). Two terminal phenyl rings (Cg5 = {C16-C21}, Cg6 = {C2-C7} act as centroids for the C–H····π (H7····Cg5, H9····Cg5 and H20····Cg6) interactions within the range of 3.100–3.795 Ǻ, as shown in Figure S4a (Supporting Information). In addition to these C– H····π interactions, intermolecular π····π stacking interactions are present between the centroids of phenyl rings Cg5····Cg5 with a distance of 3.747 Å. When the orientation of these two phenyl rings are converted to syn with an torsion angle 16.51° (Table 2) in compound 1b, we observe only C10–H10····Cg7 (Cg7 = {C20-C25}) interaction with a distance of 3.045 Å, as shown in Figure S4b (Supporting Information). There is no intermolecular and intramolecular π····π stacking interactions between the phenyl rings ({C6-C11} and {C20-C25}) in the crystal structure of compound 1b. However, when we change the metal ion from Cu2+ to Ni2+ resulting in compound 2b, we observe intermolelcular π····π (Cg7···Cg7 = 3.698 Å) stacking interactions between

{C20-C25} phenyl rings (Figure S4c, Supporting Information), even though

compounds 1b and 2b are isomorphous. This anomaly can be explained by the difference in angle between two intramolecular phenyl rings ({C6-C11} and {C20-C25}), which is 68.86° in the crystal structure of compound 1b (copper complex) and 82.67° in compound 2b (nickel analogue). When a bromo group is substituted instead of hydrogen atom at p-position at one of the phenyl rings in the compound ‘a’, it gives a new compound ‘c’. The angles between two terminal phenyl rings ({C9-C14} and {C16-C21-(Br)}) with respect to the mean plane of benzimidazole ring are 72.03° and 76.39° respectively. The two phenyl rings are situated in a anti alignment with an torsion angle 177.64° and the angle between these terminal rings is 80.73°. In compound ‘c’, p-position atoms are H12 and Br1 in the {C9-C14} and {C16-C21} phenyl rings respectivley. There is only one p-position C–Br····π interaction i.e. C19–Br1····Cg8 (Cg8 = 18 ACS Paragon Plus Environment

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{C1-C6} ring) with a distance of 3.597 Å. The two phenyl rings (Cg9 = {C9-C14}, Cg10 = {C16-C21-(Br)}) act as centroids for the C–H····π interactions, namely, H20····Cg9 and H10····Cg10 interactions with distances of 3.207 Å and 3.824 Å respectively. Apart from these, there are additional intermolecular π····π stacking interactions are observed between the {Cg10····Cg10} rings with a distance of 3.803 Å (Figure S5a, Supporting Information). When BF4− anion in compound ‘c’ is replaced by [Cu(mnt)2]2– anion, the resulting compound 1c retains two phenyl rings (Cg11 = {C20-C25} and Cg12 = {C13-C18-(Br)})

in the anti

alignment with a torsion angle of 164.89°. The angles of these terminal rings Cg11 and Cg12 with the mean plane of central benzimidazole ring are 88.36° and 74.08° respectively and the angle between these two terminal rings is 77.90°. When we consider the angles between the bromo-substituted phenyl ring and the mean plane of central benzimidazole ring in the compound ‘c’ and in corresponding metal complex 1c, the bromo-substituted phenyl ring Cg10 (compound ‘c’) that makes an angle of 76.39° with central benzimidazole ring, does not change its conformation to a greater extent when it is ion-paired with [M(mnt)2]2– forming 1c; the angle between bromo-substituted ring Cg12 and the benzimidazole ring in 1c remains almost same (74.08°). On the contrary, the other phenyl ring Cg11 in metal complex 1c has undergone conformational change with a difference of ~16°, in comparison to the concerned ring (Cg9) in compound ‘c’. This rotation facilitates an extra intermoleclular π····π stacking interaction in compound 1c (Cg11····Cg11), which do not exist in the crystal structure of the organic receptor compound ‘c’. In the crystal structure of compound 1c, no C–H···Br ‘head to tail’ intermolecular interactions among the p-position Br1 centre of bromo-substituted ring of one molecule and C23–H23 atoms (p-position) of the other terminal phenyl ring of other adjacent molecule are observed (see Figures 5a and 5b). But the terminal phenyl rings Cg11 and Cg12 act as centroids offering intermolecular C–H···π interactions C9–H9····Cg11 and C7–H7····Cg12 with distances of 3.321 and 3.023 Ǻ respectively, as shown in Figure S5b, section-2 (Supporting Information). A prominent feature, in this study, has been observed that the metal-bis(dithiolate) complex anions in compounds 1a–1c exhibit Z-shaped non-planar geometry with the ligand fragment bent away from the {CuS4} plane with dihedral angles of 1.94°, 16.63° and 3.92° respectively (Scheme S1, Supporting Information). These values are comparable to those of known copper bis(dithiolate) complexes.55 This indicates that non-planar geometry of metal bis(dithiolate)



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complexes depends on the nature of substituent group at p-position of the phenyl ring in the associated organic receptor cation. Is there any role of substituents (nitro and bromo groups) of the terminal phenyl moiety of the cation receptor on conformational modulation: a supramolecular analysis We have seen that the nitro-substituted organic receptor cation in compound [Bz,NO2BzBimy]BF4 (b) has anti conformation. When BF4‒ anion in compound ‘b’ is replaced by [Cu(mnt)2]2‒ / [Ni(mnt)2]2‒ anion resulting in compound [Bz,NO2-BzBimy]2[Cu(mnt)2] (1b) / [Bz,NO2-BzBimy]2[Ni(mnt)2] (2b), the anti conformation (compound ‘b’) goes to syn conformation (compound 1b / 2b). We have explained this conformational modulation by giving a supramolecular model (Scheme 3) of cationic receptor, on which two flexible ‒CH2‒ groups can be either eclipsed or staggered form with respect to their hydrogen positions. In the case of compound ‘b’, the hydrogen atoms of the flexible carbons are in staggered form giving rise to anti-conformation of the cation receptor in the crystal structure of compound ‘b’ (vide supra). On the other hand, in the crystal structure of compound 1b, the hydrogen atoms of the flexible carbons are in eclipsed form, that results in the formation of syn-conformation of cation receptor in compound 1b. We wanted to investigate of why it is eclipsed in one case and staggered in other case. As shown in Scheme 4(a), the hydrogen bonding situation around one flexible ‒CH2‒ group is quite different from that around other flexible ‒CH2‒ group. In this case, one of the flexible ‒CH2‒ groups is hydrogen bonded to one BF4‒ group through C‒H···F hydrogen bond and a nitro group of an adjacent receptor molecule (C‒H···O hydrogen bond). The other flexible

‒CH2‒ group is hydrogen bonded to only one BF4‒ group through two bifurcated

C‒H···F hydrogen bonds as shown in Scheme 4a. In other words, the hydrogen bonding environments around two flexible ‒CH2‒ groups of the cationic receptor in compound ‘b’ are not identical, that probably make these flexible

‒CH2‒ groups staggered with respect their

hydrogen positions, thereby results in the formation of anti-conformation of the cation receptor. Whereas in the crystal structure of compound [Bz,NO2-BzBimy]2[Cu(mnt)2] (1b), two cation receptors form a supramolecular dimer using their substituted nitro groups through the formation of C‒H···O hydrogen bonds (Scheme 4b). The formation of such supramolecular cation dimer is possible because of syn conformation of the cationic receptor through the participation of substituent nitro group. This supramolecular dimer is stabilized by surrounding [Cu(mnt)2]2-


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complex anions that are attached to the supramolecular dimer through C‒H···N and C‒H···S hydrogen bonds as shown in Scheme 4b. These hydrogen bonds make the flexible

‒CH2‒

groups of the cationic receptor eclipsed with respect to their hydrogen positions and thereby result in the formation of syn -conformation of the cationic receptor in compound 1b. Thus in both compounds ‘b’ and 1b, nitro group plays an important role on conformational modulation. In the crystal structures of compound [Bz,Br-BzBimy]BF4 (c) as well as in [Bz,BrBzBimy]2[Cu(mnt)2] (1c) / [Bz,Br-BzBimy]2[Ni(mnt)2] (2c), the bromo-substituted cation

Scheme 4

(a) Hydrogen bonding situation around flexible ‒CH2‒ groups of the cation receptor in compound ‘b’.

(b) Supramolecular hydrogen bonded dimer, formed by C‒H···O hydrogen bonds from two syn-cation receptors in compound 1b.

receptor has anti conformation. This is because, in both cases, the flexible ‒CH2‒ groups of the cationic receptors are in staggered positions with respect to their hydrogen positions (vide supra).



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Scheme 5 (a)

(b)

(a) Hydrogen bonding situation around flexible ‒CH2‒ groups of the cation receptor in compound ‘c’.

(b) Hydrogen bonding situation around flexible ‒CH2‒ groups of the cation receptor in compound 1c.

The existence of staggered forms of the flexible ‒CH2‒ groups of the cationic receptors in both ‘c’ and 1c/2c, responsible for anti conformations of the cation, can be explained by hydrogen bonding environments around flexible

‒CH2‒ groups, that are dissimilar. For example, in

compound ‘c’, one flexible ‒CH2‒ group is hydrogen bonded to two BF4‒ groups through C‒H···F hydrogen bonds and other ‒CH2‒ group is hydrogen bonded to only one BF4‒ group through two bifurcated C‒H···F hydrogen bonds (Scheme 5a). Similarly, in compound 1c, one of the flexible

‒CH2‒ groups is hydrogen bonded to substituted bromo atom of an adjacent

cationic receptor molecule (C‒H···Br hydrogen bond) as well as to a surrounding [Cu(mnt)2]2complex anion through C‒H···S hydrogen bond. On the other hand, the other ‒CH2‒ group is not at all hydrogen bonded (Scheme 5b). This dissimilar mode hydrogen bonding environments between two flexible ‒CH2‒ groups make them in staggered positions. Thus the substituted bromo atom plays a vital role in giving anti-conformation of the cationic receptor.


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Hirshfeld Surface Analysis The Hirshfeld surfaces (HSs) mapped with dnorm (Figure 10a) and 2D fingerprint plots (Figure 10b), presented for the cationic receptors in this report, were generated using Crystal Explorer 3.156 based on results of single crystal X-ray diffraction studies. Directions and strengths of intermolecular interactions within the crystal are mapped onto the Hirshfeld surfaces (Figure 10a) using the descriptor ‘dnorm’, which is a ratio encompassing the distance of any surface point to the nearest interior (di) and exterior (de) atom to the van-der Waals radii (vdW) of the atoms.57-59 A plot of di versus de is a 2D fingerprint plot (Figure 10b) which identifies the occurrence of different kinds of intermolecular interactions.57 The intermolecular contacts, contained in Table 1, are effectively summarized in spots on Hirshfeld surfaces; the large circular depressions (deep red) which are visible on the dnorm surfaces are an indicator of hydrogen bonding contacts. The small extent of area and light color on the surface indicate weaker and longer contacts other than hydrogen bonds. Complementary regions are visible in the fingerprint plots, where one molecule acts as a donor (de > di) and the other as an acceptor (de < di). Hirshfeld surface with the respective dnorm and the 2D fingerprint plots of the compounds 1a−1c and a−c, have been shown in Figure 10. The N···H close contacts vary from 10.8% in 1c to 13.3% in 1a, and 0.4% in c to 1.7% in b (Figure S6, Supporting Information). Also the C···H contacts vary from 27.1% in 1c to 32.0% in 1a, and 19.9% in b to 35.2% in a (Figure S6, Supporting Information). A significant difference between the molecular interactions for compounds (a−c and 1a−1c) in terms of H···H interactions is reflected in the distribution of scattered points in the fingerprint plots, which spread only up to di = de = 1.11 Å in 1a, di = de = 1.32 Å in 1b, di = de = 1.20 Å in 1c.60,61 In compound 2c, 1.9% of the interactions are attributable to direct Br···Br attractions. The tips of the spikes on the middle part of the graph in Figure S7 (Supporting Information) are located at di = de = 1.82 Å, with di+de representing the shortest distance between atoms inside the molecular surface and outside the surface, correspondingly. Thus, the shortest calculated interaction of about 3.63 Å is in impeccable agreement with the shortest Br···Br bond observed in 2c. Similarly, in 1c, the Br···Br contacts contribute only 0.2%. The minimal di+de calculated distance between the tips of the spikes is near 3.92 Å, which is in covenant with the observed 3.90 Å value in the crystal structure.42-44 Relative contributions in percentage of various intermolecular contacts to the Hirshfeld surface areas of molecular crystals 1a−1c and a−c have been shown in Figure 11. 23 ACS Paragon Plus Environment


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Theoretical Calculations To determine the stability of the benzimidazolium cations in different conformations, present in compounds 1a−1c, 2a−2c and a−c, theoretical calculations have been performed and relevant values have been given in Table 3. The molecular geometries of only the organic cation receptors were taken from the CIF files of the compounds and the single point calculations have been carried out by using B3LYP with 6-311g** basis set employing the G03 suite of program.62,63 As shown in Table 3, the cations [Bz,H-BzBimy]+, [Bz,NO2-BzBimy]+ have similar energy values (in hartree units) in both syn and anti conformations for compounds a, b and 1a, 1b, 2a, 2b. In a similar way, the energy values of the cation [Bz,Br-BzBimy]+ are almost equivalent in the compounds c and 1c, 2c. The relevant computational data are descrbed in Figures S8 and S9 (Supporting Information). Spectroscopic Characterization Electronic Absorption Spectra UV-vis absorption spectra of all these compounds are measured in acetonitrile solutions and the obtained spectra are similar to those of reported other [M(mnt)2]n– complexes.64,65 As shown in Figure S10 (Supporting Information), all the ion-pair complexes 1a-1c and 2a-2c, exhibit five absorption bands in the region of 200-1300 nm. Comparing with relevant literature values, bands at 270 and 370 nm are assigned due to the L → M charge anti fer transitions of [M(mnt)2]2− (M = Cu(1), Ni(2)). Bands at 320 and 470 nm can be attributed as L → L* and M → L transitions respectively. The low energy region broad and strong absorption band for

metal-bis(dithiolate)

complexes is generally assigned to π→π* transition between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). Among the title Ion-pair compounds [Bz,R-BzBimy]2[M(mnt)2] [R= H, NO2, Br; M= Cu, Ni], the nickel compounds exhibit a strong absorption at around 860 nm (Figure S10, Supporting Information) and the copper analogues show a prominent absorption at around 1210 nm (Figure S10, Supporting Information). These are consistent with literature values: nickel bis(dithiolate) complexes absorb at 855 nm (ε = 30 M–1 cm–1) and copper bis(dithiolate) complexes absorb at 1205 nm (ε = 94 M–1 cm–1).64,66 The ion pair compound 2a, formed from H-substituted benzimidazolium cation, shows the absorption band at 860 nm in CH3CN solution with ε = 683 M–1 cm–1, whereas for nitro-substituted nickel compound 2b and bromo-substituted nickel compound 2c absorb at 860 24 ACS Paragon Plus Environment

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nm with molar extinction coefficients (ε) 308 and 670 M–1 cm–1 respectively. In a relevant report, Ren and co-workers have demonstrated that there is negligible substituent effect of the cation on the lowest unoccupied molecular orbital LUMO energy levels; thus the lower frequency absorbance broad band (at ~860 nm) can be assigned to combined transitions that can arise from forbidden d–d transition, allowed d–π* transition (MLCT in the [Ni(mnt)2]2– anion) and intraligand π–π* transition (in the mnt2– ligand) as well as the IPCT transition.67 In case of copper complexes, the broad and weak absorption bands appear at ~1210 nm; the intensities (extension coefficients) of the near-IR absorption band for copper complexes 1a, 1b and 1c are 91, 51, and 96 M–1 cm–1 respectively. In the solid state, compounds 1a−1c exhibit a broad band at ~1110 nm in their diffuse reflectance spectra (section-5, Figure S10, Supporting Information). This absorption band can be assigned to a mixture of d–d transition in [Cu(mnt)2]2– and IPCT (ion pair charge transfer) transition. The shift of 100 nm towards high energy region in going from solution state to solid state can be explained by the intermolecular interactions between anion and cation in the solid state.

Electrochemistry The electrochemical behavior of the complexes 1a-1c and 2a-2c are recorded in acetonitrile solutions, each containing 0.10 M [Bu4N][ClO4] as supporting electrolyte for a platinum working electrode. The results of these complexes are summarized in Table 4. Representative cyclic voltammograms for the compounds 1a and 2a have been shown in Figure S11 (Supporting Information). All the copper and nickel compounds (1a–1c and 2a–2c) exhibit a reversible oxidative response at E1/2 = +0.44 V, +0.48 V, 0.49 V and +0.42 V, +0.30, +0.24 V vs Ag/AgCl respectively. The present electrochemical data can be explained on the basis of Scheme 6, proposed by McCleverty, Hoyer and others.68,69 According to this scheme, the oxidative response

for

compounds

1a–1c

and

2a–2c

are

ascribed

due

to

the

couple

[MIII(mnt)2]─1/[MII(mnt)2]─2 (M= Cu, Ni). All these oxidative response values of ion-pair dithiolene complexes are in accord with the reported electrochemical data of relevant compounds;70-72,35 from this, we can say that there is no change in oxidative response by changing the different substituents of counter cation in ion-pair dithiolene complexes. Cyclic voltammetry data for compounds 1b, 1c and 2b, 2c are presented in section-6 in Supporting Information (Figure S11). 25 ACS Paragon Plus Environment


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Scheme 6.

ML2

1 e

2 ML2

-e

e -e

3 ML2

ESR Spectroscopy Electron paramagnetic resonance (EPR) studies were performed at room temperature (as solid) and liquid nitrogen temperature as frozen state for all the copper complexes. Figure S12 (Supporting Information) illustrates the representative EPR spectra of complex 1b. All title copper complexes exhibit similar type of feature at room temperature. The ligand hyperfine structure provides straight information about the environment of the electronic ground state of the complex and the extent to electron spin delocalization over ligand orbitals.73 We have observed hyperfine splitting in the EPR spectra of compounds 1a-1c in frozen state at liquid nitrogen temperature (Figure S12, section-7, Supporting Information). HOMO level of the [Cu(mnt)2]2– consists of the 3dxy orbital of copper (S = 1/2, ICu = 3/2, natural abundance: 63Cu = 69.2 %, 65Cu = 30.8 %) and hybrids of 3s, 3px, and 3py orbitals of sulfur atoms (IS = 3/2, natural abundance 0.75 %).74 These atomic orbitals are overlapped with the pz orbitals of copper and sulfur, and such combination has a direct effect on the copper hyperfine splitting.75,76 The g values of copper complexes at room temperature are 2.034, 2.033 and 2.034 for 1a. 1b and 1c respectively. These g values are comparable (g = 2.045 and 2.03) for the known planar copperbis(dithiolate) complexes,77,78 [Bu4N]2[Cu(dcbdt)2] and [Bu4N]2[Cu(ecda)2]. From the EPR spectral studies, it can be concluded that there is an unpaired electron in the dx2-y2 orbital of copper (II) in the ground state.79,80

PXRD studies To ensure the phase purity of the products, X-ray powder diffraction data for all the compounds have been recorded. Similar diffraction patterns for the simulated data (calculated from single crystal data) and observed data confirm the bulk homogeneity of the crystalline solids (see Section-8, Supporting Information). Although the experimental patterns have few unindexed diffraction peaks and some are faintly broadened and shifted in comparison to those simulated from the single-crystal data, it can still be regarded that the bulk as-synthesized materials represent compounds 1a–1c and 2a–2c.


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Summary: concluding remarks Bis(dithiolate)M(II) coordination complex anions are belonged to a well-known class of inorganic compounds, that are important in the context of bio-inorganic chemistry, materials science, optics, laser applications and catalysis. We34,35 and others81 have recently introduced their importance in supramolecular chemistry. Three organic benzimidazolium cations, namely, [Bz,H-BzBimy]+, [Bz,NO2-BzBimy]+ and [Bz,Br-BzBimy]+, described in this work, can exist either in syn conformation or in anti conformation. We have demonstrated that the conformation of the benzimidazolium cations can be modulated by changing the anions from tetrahedral BF4– to the planar [M(mnt)2]2–. In order to explain this conformational modulation of the receptor cations, we have given a new supramolecular concept, mentioning that if the two flexible -CH2groups of the cation are in eclipsed conformation with respect to the positions of the hydrogen atoms of the -CH2- groups, then the cation receptor will have syn conformation with respect to the position of terminal phenyl groups. And if the two flexible -CH2- groups of the cation are in staggered form, then the organic cation will have anti modulation. This is possible because each of these flexible -CH2- groups is directly attached to one terminal phenyl group (Schemes 2 and 3). Whether these two -CH2- groups of the organic receptor cation are in eclipsed form or in staggered form is decided by their supramolecular hydrogen bonding interactions with their surrounding anions (BF4‒ and /or [M(mnt)2]2‒). We have also demonstrated that the substituent groups (Br and NO2) on the phenyl moiety play an important role in this conformation modulation of the cation receptor. The supramolecular interactions in the crystal structures of the title compounds have quantitatively been described by Hirshfeld surface analysis. The relevant fingerprint plots depict the percentage of different intermolecular interactions of close contacts in the crystal structures. We have performed theoretical calculations on the cation receptor molecules to investigate of why they are so easily interconvertable between syn and anti depending on the supramolecular situation. The metal dithiolate complex anions are consistent with electrochemical studies. This work has added new findings of conformational modulation in the area of supramolecular chemistry.



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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]; fax: +91-40-2301-2460; tel: +91-40-2301-1007. Acknowledgments

We thank CSIR, Government of India (Project No. 01 (2556)/12/EMR-II) for financial support. The National X-ray Diffractometer facility at University of Hyderabad by the Department of Science and Technology, Government of India, is gratefully acknowledged. We are grateful to UGC, New Delhi, for providing infrastructure facility at University of Hyderabad under UPE grant. We thank to Dr. K. Santhosh, Dr. S. N. Reddy and Dr. Naba Kamalnath for helpful discussion in preparing this manuscript. RK and TBK thank CSIR, Government of India, for thier fellowships. Supporting Information: Crystallographic data, 1H NMR, EPR, electrochemical and other related data of the compounds 1a-1c, 2a-2c have been entered in this section. This information is available free of charge via the Internet at http://pubs.acs.org/.

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Table 1. Hydrogen bonding parameters for compounds 1a–1c [Å and deg.]

D-H...A

d(D-H)

d(H...A) d(D...A)

Significant Role of Supramolecular Interactions on Conformational

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