Article pubs.acs.org/crystal
Exploring Supramolecular Self-Assembly of Metalloporphyrin Tectons by Halogen Bonding. 2 Goutam Nandi,* Hatem M. Titi, and Israel Goldberg* School of Chemistry, Sackler Faculty of Exact Sciences, Tel-Aviv University, Ramat-Aviv, 69978 Tel-Aviv, Israel S Supporting Information *
ABSTRACT: In expansion of earlier observations, one SnIV(L)2, one oxo− VIV, and nine oxo−MoV(L) new porphyrin compounds (L = anionic axial ligand) have been synthesized and structurally characterized by single crystal X-ray diffraction analysis to probe the occurrence of halogen bonds in crystalline metalloporphyrin assemblies. The [Sn(TIPP)(p-NO2-phenolate)2] six-coordinate system (TIPP = dianion of 5,10,15,20-meso-tetrakis(4-iodophenyl)porphyrin) (1) exhibits weak I···O interactions, stimulating further studies with the oxo−Mo/V porphyrin scaffolds. The domed complex [V(O)(TIPP)] (2) crystallizes in a chiral space group and shows short O···π contacts along the polar axis, but no distinct halogen bonds. In [Mo(O)(TIPP)(L)] (L = 1-hydroxybenzotriazolate (3), 5-bromonicotinate (4), pyrimidine-5-carboxylate (5), 4-iodobenzoate (6), 2-chlorobenzoate (7)) diverse (O···I, N···I, I···I) halogen bonds and halogen-bonding-type contacts have been observed, depending upon the nature of the axial ligand attached to the Mo center. In analogous compounds with TBrPP, [Mo(O)(TBrPP)(L)] (TBrPP = dianion of 5,10,15,20-meso-tetrakis(4-bromophenyl)porphyrin, L = 5bromonicotinate (8), isonicotinate (9)), directional halogen interactions involving bromine have not been detected. Intermolecular N···I halogen bonding has also been observed when the N-donor sites were positioned on the porphyrin periphery and the iodine acceptor on the axial ligand as in [Mo(O)(T4pyP)(4-iodobenzoate)] (10) (T4pyP = dianion of 5,10,15,20-meso-tetrakis(4-pyridyl)porphyrin). An attempt to induce the formation of halogen bonded chain assemblies through I···O interaction between the axial oxo and iodine sites of adjacent species in [Mo(O)(TTP)(4-iodobenzoate)] (11) (TTP = dianion of 5,10,15,20-meso-tetrakis(p-tolyl)porphyrin) has not been successful.
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INTRODUCTION Noncovalent interactions, such as hydrogen bonding and metal coordination, represent a fundamental set of tools for the construction of supramolecular architectures in the chemistry of organic or metal−organic compounds.1 In the past few years chemists have paid much interest toward the development of new types of intermolecular interactions. In particular, halogen bonding has grown from a scientific curiosity to one of the most interesting noncovalent interactions as it showed real promise in applications such as catalysis, medicinal chemistry, and molecular recognition processes.2 Halogen bonding is a specific and directional noncovalent interaction in which halogen atom(s) function as electrophilic species.3 Usually, halogen atoms are located at the periphery of organic molecules, and are thus ideally positioned to be involved in intermolecular interactions, owing to the presence of a region of positive electrostatic potential, the σ-hole, on their outermost cap/surface.4 This allows their effective interaction with an electron-rich atom (N, O, I, or Br) that acts as a Lewis base. In optimal situations the halogen interactions may be strong enough to control the aggregation of certain molecules in gas, liquid, and solid phases.5 In the above context our attention has been drawn recently to crystal engineering of supramolecular self-assemblies of © 2014 American Chemical Society
tetraarylporphyrins by halogen bonding, successfully demonstrating that halogen atoms (−Br, −I) can be readily involved in the construction of extended multiporphyrin architectures.6 To this end the porphyrin scaffolds were designed to contain both the halogen-bonding “donors” and “acceptors” substituted in a complementary manner on their periphery. In one approach, the porphyrin framework was decorated by mixed disposition of iodo/bromophenyl and N-pyridyl/triazole substituents. In a different strategy, metalloporphyrins were used with one potential halogen-bonding partner placed on the porphyrin aryls and the second partner on the axial ligands coordinated to the metal center.6 Remarkably, the expression of halogen bonds in the porphyrin assemblies has been manifested without further activation of the halophenyl donor groups by electron withdrawing substituents. As the largest contribution to halogen bonding comes from Coulombic plus first-order polarization terms,7 the optimal halogen-bonding synthon in our earlier studies referred to above was found to be a linear I··· N halogen bond with an interatomic distance of ≅ 3.0 Å.6 The latter is considerably shorter than the sum of the corresponding Received: April 10, 2014 Revised: May 8, 2014 Published: May 16, 2014 3557
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Scheme 1. Schematic Illustration of the Metalloporphyrin Scaffolds Involved in This Studya
a
(a) Six-coordinate tin−porphyrin (1), (b) vanadium−oxo−porphyrin (2), and (c) molybdenum−oxo porphyrins (3−11) with an axial ligand. Ar(i) is 4-iodophenyl in compounds 1−7, 4-bromophenyl in 8 and 9, 4-pyridyl in 10, and 4-tolyl in 11. L(i) is p-nitrophenolate in 1, 1hydroxybenzotriazole in 3, 5-bromonicotinate in 4, 8, and 11, pyrimidine 5-carboxylate in 5, 4-iodobenzoate in 6 and 10, 2-chlorobenzoate in 7, and isonicotinate in 9.
porphyrin compounds used in this study are [Sn(TIPP)(pNO2-phenolate)2] (TIPP = dianion of 5,10,15,20-meso-tetrakis(4-iodophenyl)porphyrin) (1); [V(O)(TIPP)] (2); [Mo(O)(TIPP)(L)], L = 1-hydroxybenzotriazolate (3), 5-bromonicotinate (4), pyrimidine-5-carboxylate (5), 4-iodobenzoate (6), and 2-chlorobenzoate (7); [Mo(O)(TBrPP)(L)] (TBrPP = dianion of 5,10,15,20-meso-tetrakis(4-bromophenyl)porphyrin), L = 5-bromonicotinate (8) and isonicotinate (9); [Mo(O)(T4pyP)(4-iodobenzoate)] (T4pyP = dianion of 5,10,15,20meso-tetrakis(4-pyridyl)porphyrin) (10); and [Mo(O)(TTP)(4-iodobenzoate)] (TTP = dianion of 5,10,15,20-meso-tetrakis(p-tolyl)porphyrin) (11). Most of these structures (except for 11) exhibit halogen-bonding-type intermolecular contacts of various types as discussed below. This diversity reflects on the delicate balance between competing halogen bonding and other noncovalent interactions during the self-assembly of the porphyrin complexes.
van der Waals radii of N and I atoms (approximately 3.5−3.8 Å).8 In the next stage the scope of our studies has been expanded to probe the occurrence of halogen-to-oxygen interactions among supramolecular porphyrin arrays. Such halogen bonds in chemical as well as biological systems have been widely documented in the literature in recent years.9 Yet, multiporphyrin assembly with the aid of X(halogen)−O bonds has received little attention until now. In the present work, which is a direct expansion of our earlier studies of halogen bonding among porphyrin tectons,6 we thus focus on investigations of halogen bonding among halogenated oxo− V(IV) and oxo−Mo(V)−tetraarylporphyrins (with a single exception) as the model porphyrin platform with inherently incorporated O-site (Scheme 1). In the molybdenyl derivative, the electron density on the porphyrin ring as well as on the MoO oxygen can be tuned further by adding, or changing the nature of, a monoanionic extra axial ligand/counterion on the opposite side of the Mo center. A similar option is less relevant in the case of vanadyl(VO) porphyrins, which tend to stay 5-coordinate, rarely accommodating an additional axial moiety.10 We also refer to a single example of a tin−porphyrin derivative [Sn(TIPP)(p-NO2-phenolate)2] which reveals an apparent I···O halogen-bonding interaction between the phenolate oxygen of the tin-coordinated axial ligand and the iodine atom of an adjacent porphyrin unit. Crystallographic characterization of the assembly patterns formed by the various complexes throws light on the optimal supramolecular interaction of these metalloporphyrin units and utilization of their halogen-bonding molecular recognition features. One should keep in mind that the expression of directional but relatively weak interactions of this type in the crystalline self-assembly of organic species in common reaction environments is not straightforward, in view of competing intermolecular binding forces of similar strength. The latter, along with molecular-shape considerations, are of particular significance in systems with extended aromatic cores. The
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EXPERIMENTAL SECTION
Materials. Pyrrole, 4-iodobenzaldehyde, 4-bromobenzaldehyde, 4pyridinecarboxaldehyde, p-tolualdehyde, 1-hydroxybenzotriazole, 4iodobenzoic acid, 5-bromonicotinic acid, isonicotinic acid, and 2chlorobenzoic acid were obtained from Sigma-Aldrich. Solvents like propionic acid, chloroform, dichloromethane (DCM), methanol, acetone, dimethylformamide (DMF), and hexane were obtained from Bio-Lab Ltd., Israel. Petroleum ether (60−80 °C) was obtained from Frutarom Ltd., Israel. Solvents were distilled prior to use by standard procedures. VO(SO4)·xH2O was obtained from Aldrich. [Mo(NO)2(py)2Cl2] was prepared following the procedure reported earlier.11 Free-base porphyrins were prepared following the Adler method.12 All the molybdenum porphyrin complexes had been prepared from the corresponding μ-oxo bridged Mo(V)-dimeric complex, [{MoO(por)}2O] [por = TIPP (tetra(4-iodophenyl)porphyrin, TBPP (tetra(4-bromophenyl)porphyrinP, T4pyP (tetra(4pyridyl)porphyrin, TTP (tetratolylporphyrin)], which in turn were prepared following a methodology similar to that employed for mesotetraphenyl complex.13 3558
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Table 1. Crystal Data and Structure Refinement Summary for Complexes 1−11 complex
1
2
3
4
5
6
chem formula formula wt cryst syst space group T/K a/Å b/Å c/Å α/deg β/deg γ/deg Z V/ Å3 Dcalc/g cm−3 μ /mm−1 reflns collected unique reflns R1 [I > 2σ(I)] wR2 (all data) goodness-of-fit complex
C56H32N6O6I4Sn 1511.17 triclinic P1̅ 110 10.9162(7) 12.1614(8) 13.2689(9) 63.262(2) 83.086(2) 69.742(2) 1 1474.55(17) 1.702 2.578 18 215 5115 0.044 0.128 1.052 7
C44H24N4OI4V 1183.21 monoclinic P21 110 15.1179(4) 8.6311(2) 16.2660(4) 90 95.1330(10) 90 2 2113.94(9) 1.859 3.193 17 929 8308 0.049 0.146 1.039
C53H34N7O3I4Mo 1420.21 monoclinic C2/c 110 56.3440(18) 8.5570(2) 21.3490(7) 90 104.526(2) 90 8 9964.1(5) 1.894 2.793 32 178 8784 0.047 0.206 1.629
C50H27N5O3BrI4Mo 1429.22 monoclinic C2/c 110 55.469(3) 8.6711(5) 21.0353(13) 90 95.435(3) 90 8 10072.0(11) 1.885 3.551 35 455 9883 0.052 0.140 1.326 9
C49H27N6O3I4Mo 1351.30 triclinic P1̅ 110 15.8194(5) 16.4413(5) 22.4957(8) 75.395(2) 71.085(2) 87.063(2) 4 5353.2(3) 1.677 2.594 81 370 21 809 0.051 0.183 1.053 10
C51H28N4O3I5Mo 1475.22 monoclinic C2/c 110 37.5438(14) 16.2519(5) 16.3223(4) 90 104.076(2) 90 8 9660.1(5) 2.029 3.514 25 154 8589 0.063 0.215 0.997 11
chem formula formula wt cryst syst space group T/K a/Å b/Å c/Å α/deg β/deg γ/deg Z V/ Å3 Dcalc/g cm−3 μ /mm−1 reflns collected unique reflns R1 [I > 2σ(I)] wR2 (all data) goodness-of-fit
C51H28N4O3ClI4 Mo 1383.76 monoclinic C2/c 110 38.4438(16) 16.3170(7) 15.9435(6) 90 104.165(2) 90 8 9697.1(7) 1.896 2.919 34 130 8746 0.060 0.167 0.962
8 C50H27N5O3Br5 Mo 1241.21 triclinic P1̅ 110 9.8873(4) 16.2858(6) 16.7259(7) 109.494(2) 105.496(2) 105.315(2) 2 2253.97(16) 1.829 4.773 30 751 8279 0.037 0.127 0.965
C50H28N5O3Br4 Mo 1164.35 monoclinic C2/c 110 29.6929(10) 14.6788(4) 22.6360(8) 90 110.2220(10) 90 8 9257.9(5) 1.671 3.785 29 482 8175 0.050 0.122 1.035
Physical Measurements. Elemental analyses for carbon, hydrogen, and nitrogen were obtained with a PerkinElmer 2400 microanalyzer. IR spectra were recorded on a Bruker Tensor 27 system spectrophotometer in ATR mode. Synthesis of 1. In a 1 mL DMF solution of [Sn(TIPP)(OH)2]6b (5 mg, 0.004 mmol), 4-nitrophenol (3 mg, 0.018 mmol) was added. The solution was heated at 100 °C for an hour and was then cooled to room temperature. 1 mL of CHCl3 and 2 drops of nitrobenzene were added to it. The solution was then left for slow evaporation for few days without disturbance, which yielded purple crystals of 1. Yield: 87%. Molecular formula: C56H32N6O6I4Sn. Molecular mass: 1511.76. Elem anal. Calcd (found) for C56H32N6O6I4Sn: C, 44.51 (44.59); H, 2.13 (2.32); N, 5.56 (5.63). FTIR (cm−1): 1668, 1578, 1484, 1385, 1330, 1276, 1157, 1109, 1057, 1027, 1001, 861, 798, 758, 702, 648. Synthesis of 2. H2TIPP (112 mg, 0.1 mmol) was taken in 50 mL of DMF in a 100 mL round-bottom flask and the mixture heated to reflux under N2 atmosphere. After half an hour V(O)(SO4)·xH2O (25 mg, 0.15 mmol) was added to it, and the solution was heated at reflux for another 3 h. The reaction mixture was cooled, and DMF was evaporated in a rotary evaporator under reduce pressure. The residue
C50H35N8O3I Mo 1018.70 monoclinic P21/n 110 11.9734(4) 27.3052(11) 14.9182(5) 90 96.621(2) 90 4 4844.8(3) 1.397 0.956 27 343 7609 0.062 0.166 1.081
C55H40N4O3I Mo 1027.75 monoclinic C2/c 110 36.181(2) 15.7826(9) 16.3625(9) 90 97.903(3) 90 8 9254.9(10) 1.475 1.000 46 811 8574 0.061 0.167 1.011
was then dissolved in a minimum amount of CHCl3 and subjected to column chromatography using neutral alumina. The major fraction was collected using CHCl3/ethyl acetate (5%). Yield: 86%. Molecular formula: C44H24N4OI4V. Molecular mass: 1182.76. Elem anal. Calcd (found) for C44H24N4OI4V: C, 44.66 (44.33); H, 2.04 (2.11); N, 4.74 (4.43). FTIR (cm−1): 1001 (νVO). Synthesis of 3. To 25 mL of a CHCl3/MeOH (1:1) solution of [{Mo(O)(TIPP)}2O] (25 mg, 0.01 mmol) was added 1-hydroxybenzotriazole (3.4 mg, 0.025 mmol), and the solution was stirred vigorously for half an hour. The solvent was evaporated in a rotary evaporator to give a dark green solid. The solid was recrystallized from acetone and petroleum ether to give dark green diffraction quality crystals. Yield: 93%. Molecular formula: C50H28N7O2I4Mo. Molecular mass: 1363.75. Elem anal. Calcd (found) for C50H28N7O2I4Mo: C, 44.08 (43.96); H, 2.07 (2.16); N, 7.20 (7.13). FTIR (cm−1): 948 (νMoO). Synthesis of 4−7. These complexes were prepared by following a methodology similar to that described for 2 using 5-bromonicotinic acid, pyrimidine-5-carboxylic acid, 4-iodobenzoic acid, and 2chlorobenzoic acid respectively instead of 1-hydroxybenzotriazole in 3559
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a similar stoichiometric ratio. All the complexes were isolated in more than 90% yield. Single crystals of 4, 6, and 7 were grown by slow evaporation of DCM/petroleum ether solution of each complex. Single crystals of 5 were obtained by slow evaporation of CHCl3/DMF solution. Data for 4. Molecular formula: C50H27N5O3BrI4Mo. Molecular mass: 1429.65. Elem anal. Calcd (found) for C50H27N5O3BrI4Mo: C, 42.02 (42.26); H, 1.90 (1.94); N, 4.90 (5.06). FTIR (cm−1): 943 (νMoO). Data for 5. Molecular formula: C49H27N6O3I4Mo. Molecular mass: 1352.74. Elem anal. Calcd (found) for C49H27N6O3I4Mo: C, 43.55 (44.33); H, 2.01 (2.17); N, 6.22 (6.28). FTIR (cm−1): 952 (νMoO). Data for 6. Molecular formula: C51H28N4O3I5Mo. Molecular mass: 1476.64. Elem anal. Calcd (found) for C51H28N4O3I5Mo: C, 41.52 (41.48); H, 1.91 (1.93); N, 3.80 (3.85). FTIR (cm−1): 940 (νMoO). Data for 7. Molecular formula: C51H28N4O3ClI4Mo. Molecular mass: 1510.60. Elem anal. Calcd (found) for C51H28N4O3ClI4Mo: C, 44.27 (41.08); H, 2.04 (1.81); N, 4.05 (3.81). FTIR (cm−1): 965 (νMoO). Synthesis of 8. To 25 mL of a CHCl3/MeOH (1:1) solution of [{Mo(O)(TBPP)}2O] (21 mg, 0.01 mmol) was added 5-bromonicotinic acid (5 mg, 0.025 mmol), and the solution was stirred vigorously for half an hour. The solvent was evaporated in a rotary evaporator to give a dark green solid. The solid was recrystallized from DCM and petroleum ether to give dark green diffraction quality crystals. Yield: 94%. Molecular formula: C50H27N5O3Br5Mo. Molecular mass: 1237.71. Elem anal. Calcd (found) for C50H27N5O3Br5Mo: C, 48.38 (48.51); H, 3.19 (3.56); N, 5.64 (5.62). FTIR (cm−1): 947 (νMoO). Synthesis of 9. This complex was synthesized by following a procedure similar to that described for 8 using isonicotinic acid instead of 5-bromonicotinic acid in a similar stoichiometric ratio. Yield: 95%. Molecular formula: C50H28N5O3Br4Mo. Molecular mass: 1162.35. Elem anal. Calcd (found) for C50H28N5O3Br4Mo: C, 51.67 (51.45); H, 2.43 (2.53); N, 6.03 (6.12). FTIR (cm−1): 946 (νMoO). Synthesis of 10. To 15 mL of a CHCl3/MeOH (1:1) solution of [{Mo(O)(T4pyP)}2O] (15 mg, 0.01 mmol) was added 4-iodobenzoic acid (6 mg, 0.025 mmol), and the solution was stirred vigorously for half an hour. The solvent was evaporated in a rotary evaporator to give a dark green solid. Solid was recrystallized from DCM and petroleum ether to give dark green diffraction quality crystals. Yield: 91%. Molecular formula: C47H28N8O3IMo. Molecular mass: 977.04. Elem anal. Calcd (found) for C47H28N8O3IMo: C, 57.86 (57.91); H, 2.89 (2.92); N, 11.49 (11.52). FTIR (cm−1): 939 (νMoO). Synthesis of 11. This complex was prepared by a methodology similar to that described for 10 using [{Mo(O)(TTP)}2O] instead of [{Mo(O)(T 4 pyP)} 2 O]. Yield: 95%. Molecular formula: C55H40N4O3IMo. Molecular mass: 1029.12. Elem anal. Calcd (found) for C55H40N4O3IMo: C, 64.27 (64.24); H, 3.92 (3.96); N, 5.45 (5.33). FTIR (cm−1): 947 (νMoO). Crystal Structure Determinations. The X-ray measurements [Bruker-ApexDuo diffractometer, Mo Kα radiation] were carried out at ca. 110(2) K on crystals coated with a thin layer of amorphous oil to minimize crystal deterioration, possible structural disorder, and related thermal motion effects and to optimize the precision of the structural results. These structures were solved by direct and Fourier methods and refined by full-matrix least-squares (using standard crystallographic software (SIR97, SHELXTL-2013, SHELXL-97).14,15 All non-hydrogen atoms were refined anisotropically. The hydrogen atoms were located in idealized/calculated positions and were refined using a riding model. Most compounds (except for 3 and 8) were found to contain molecules of severely disordered crystallization solvent, which could not be reliably modeled by discrete atoms. Their contribution was subtracted from the diffraction pattern by the SQUEEZE procedure and PLATON software.16 In some structures the iodophenyl groups (whether involved in halogen-bonding contacts or not) reveal partial orientational disorder as well, best manifested by split (major and minor) positions of the terminal I atoms. This had little effect, however, on the precision of the structure determination of the porphyrin assembly modes and of the intermolecular interaction distances relevant to the present discussion. In some cases the halogen-bonding-type contact involved only one of the disordered sites. This type of the observed disorder is mostly due to loose
intermolecular crystal packing of the bulky porphyrin entities and the weak nature of the halogen-bonding interactions. The crystallographic and experimental data for 1−11 are given in Table 1; CCDC 996345− 995355.
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RESULTS AND DISCUSSION Crystal structures of tetraaryl-metalloporphyrins are dominated to a major extent by dispersion forces between aromatic fragments and related molecular-shape effects,17 and halogen bonding provides only a secondary cohesive contribution to the stabilization of the corresponding crystalline solids. Nevertheless it has been shown earlier that formulation of crystalline multiporphyrin architectures affected by halogen bonding is also a feasible task.6 Different strategies can be applied to this end. The first one is to introduce the potential electrophilic “donor” (halogen atom) and “acceptor” (lone-pair possessing electron-rich partner) sites on a single porphyrin macrocycle. Indeed, it has been shown that the assembly of porphyrins with mixed iodophenyl or even bromophenyl and pyridyl mesosubstituents, via either I···N or Br···N halogen bonds, can be materialized successfully with both freebase and metalated entities.6a,c Manifestation of the halogen-bonding interactions occurred even without activating the halophenyl rings by electron-attracting substituents. Extension of that work used six-coordinate metalloporphyrin scaffolds, by placing the electrophilic (halogen) and electron-rich (N-sites) partners one on the tetraarylporphyrin macrocycle and the other on the axial ligand (or vice versa), respectively. Six-coordinate tin− porphyrin complexes with suitably activated axial ligands were found perfect model compounds for demonstrating their assembly in 1D and 2D arrays via the directional N···I and I···I halogen bonds between the interacting species.6b In this paper we provide additional examples of the occurrence of halogen bonds among porphyrin tectons, focusing this time on suitably functionalized V(O)/Mo(O)−porphyrin systems where the bare electron-rich O atom may engage also in halogen bonds. Possible indicators of the effective presence of such interactions are usually a donor−acceptor distance that is shorter than the sum of the corresponding van der Waals radii of the interacting partners and specific geometries of the binding synthon.2,3,5,9,18 Thus, e.g., in compounds containing peripheral Caryl−I or Caryl−Br polarizable residues (as it is in our case), there is a region of positive electrostatic potential at the cap of this bond, known as the σ-hole,2b,4,9 which is prone to attractively bind to electron-rich sites (Lewis bases). Hence there is a preferential formation of C−I/Br···Npyridyl or C−I/ Br···π halogen bonds of linear or perpendicular orientations, respectively. Direct halogen−halogen contacts fall in the latter category, where the σ-hole of one halogen approaches the electron-rich perimeter of the other halogen. For optimal halogen−O interactions the lone electron pair of the O atom should be directed at the electron-deficient cap of the Chalogen bond. Numerous theoretical investigations of a number of halogen-bonded systems have been reported as well.6b,9 Structure determination of compound 1 provides a connecting link between the earlier report on halogen bonding in Sn(L)2−tetraarylporphyrin complexes6b and the present work. All the anionic R−COO− or R−O− axial ligands discussed previously contained either a halogen electrophile or an N-Lewis-base-type functionality, the potential halogenbonding partners being incorporated into the porphyrin moiety. In complex 1, based on the tetra(iodophenyl)porphyrin scaffold, the nitrophenolate residue was used intentionally to 3560
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Figure 1. Supramolecular self-assembly in 1 into 1D halogen-bonding-type chains with short and linear C−I(purple)···O(red) contacts (3.3 Å). The H atoms are omitted for clarity. The porphyrin framework of the six-coordinate complex is essentially planar, and the Sn−O− coordination distances are 2.082(4) Å. The metal center and the interacting atoms are shown as spheres.
Figure 2. Illustration of intermolecular interactions in 2 along the b-axis of the crystal, showing the shortest O(red)···π contacts (dotted lines). Only the major sites of the disordered iodine atoms are shown (hydrogen atoms are omitted for clarity).
cyaninato triiodide provided further encouragement for pursuing this promising avenue.19 To this end compounds 2−11 have been successfully synthesized in single crystalline form, and their structures analyzed by X-ray diffraction. Compound 2 is the only oxo−vanadium complex that yielded diffraction quality crystals. The difficulty in crystallizing additional derivatives is most probably due to the fact that the five-coordinate oxo−vanadium−porphyrin complex tends to exhibit orientational disorder of the V(O) moiety with respect to the porphyrin framework of square-planar shape. The domed molecular structure of 2 is depicted in Figure 2. The central vanadium atom is displaced by 0.414 Å from the N4 plane of the four pyrrole N atoms toward the oxygen it binds to at V O = 1.546(10) Å. The observed VO distance is among the shortest bonds reported so far in oxo−vanadyl porphyrins except for a recent low-precision report of 1.50(3) Å in a compound where the vanadyl moiety is disordered above and below the porphyrin N4 plane.20 The relatively short VO distance in relation to other vanadyl porphyrins indicates that the electron density of the oxygen atom is drained away by the VIV−tetra(iodophenyl)porphyrin moiety and makes the former electron deficient. This may explain the observation that no typical (unlike in 1) I···O halogen bonds are formed in this
probe whether in the absence of strong accessible halogenbonding donors an I-to-O interaction will be formed instead. Indeed, the two phenolate O atoms of the nitrophenolate ligands were found to be involved in short contacts (3.298 Å) with the iodine atoms of adjacent porphyrins (four linear C−I··· O contacts per molecule), forming 1D halogen bonded chains along the a−b axis of the crystal (Figure 1). The C−O···I angle is 105.7°, indicating that one of the oxygen lone electron pairs is directed at the electron-deficient cap of the I atom, being engaged in an attractive interaction. The observed I···O interaction distance is shorter by only about 0.2 Å than the sum of the corresponding van der Waals radii8 (a more pronounced shortening is required for classification of these contacts as typical halogen bonds). The above observation inspired us to explore the possible occurrence of halogen-to-oxygen interactions and probe some oxo−metalloporphyrins with halogenated backbone to this end as suitable model compounds for such an investigation. The oxo−metal frameworks contain a terminal oxygen atom in the system available for a potential interaction in a condensed crystalline phase with the electron deficient halogen atom(s) of adjacent species. A recent observation of the presence of I···O interactions in the crystals of oxomolybdenum(V)−phthalo3561
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Figure 3. Illustration of the O···I (red dotted lines) and N···I (blue dotted lines) halogen bonds in 3. Note that each porphyrin unit is engaged in two contacts of each type, in addition to intermolecular I···I interactions between the peripheral iodine substituents (only the major orientations of the disordered iodophenyl fragments are shown).
structure. Instead, the O atom is directed at the π-electron cloud of the meso-iodophenyl group of an adjacent porphyrin available in the close vicinity to overcome that deficiency. The shortest contact of O···π interaction is observed with the para carbon, the distance being 2.730 Å. This interaction propagates in a helical manner along the b-axis of the crystal (Figure 2). Along such helical chains the O atom is also involved in a relatively short intermolecular contact distance of 3.309 Å with an electron-rich perimeter surface of a nearby iodine atom, which is consistent with the electron deficient nature of the former. The outer surface of the chains is covered with the iodophenyl groups and C−H bonds, and their side by side packing gives rise to I···I van der Waals contacts and weak C− H···O interactions. The structural analysis of 2 suggests that oxo−vanadium− porphyrins are not optimal model systems for probing halogen bonding of the I-to-O type, mainly due to the orientational disorder of this scaffold in crystalline environments and the diminished electron density on the O-site. Correspondingly, we continued our further experiments with six-coordinate oxo− molybdenum building blocks. With MoV in the porphyrin center another anionic electron-rich ligand can coordinate readily to the metal from the opposite side of the porphyrin ring. Such a ligand (e.g., carboxylate or phenolate aromatic moieties with one or more nitrogens incorporated into their skeleton) can donate electron density to the metalloporphyrin ring, and thereby preserve or even increase the electron density on the metal-bound oxygen atom. Along this line of reasoning we synthesized complexes 3−5 of the oxo−molybdenum tetra(4-iodophenyl)porphyrin with the 1-hydroxybenzotriazolate (HBT), 5-bromonicotinate, and pyrimidine-5-carboxylate electron-rich ligands, respectively. Indeed, the structural analysis of complex 3 reveals the anticipated halogen-bonding-type interactions (Figure 3). 3 is a six-coordinate complex with corresponding MoO and Mo− O(1‑hydroxybenzotriazolate) distances of 1.684(5) and 2.144(5) Å.
Typically in all the oxo−molybdenum studied here, the central Mo ion slightly deviates from the porphyrin plane toward the O site, and there is a marked disparity between its distances to the covalently bound O atom (short) and to the coordinated anionic ligand (long). It appears in 3 that, due to the presence of the triazole ring on the axial ligand, the oxo site gained adequate electron density to interact preferentially with the iodine electrophile of another porphyrin entity. The relevant O···I distance is 3.227 Å (considerably shorter than the ≥3.5 Å sum of the van der Waals radii of the two atoms), the corresponding MoO···I angle being 121.4°. In our previously reported structure of Sn(IV)−porphyrin with HBT ligand, [Sn(TIPP)(HBT)2], the nitrogen atom was involved in a very short 2.991 Å contact with the iodine atom of an adjacent porphyrin species, where no I···O interactions were observed. However, the latter dominate in the present case, along with the other nearly linear N···I (3.339 Å) contacts between electron-rich (O/N) and electron-deficient sites (I). The halogen-bonded supramolecular arrays (corrugated 2D ensembles) thus formed are aligned parallel to the (−1,0,1) plane of the crystal. Secondary I···I halogen−halogen interactions of type I as well as type II (between perpendicularly oriented C−I groups) are also present,21 operating in a perpendicular direction to the above-mentioned arrays (Figure 3). The channel-type voids created in between extend along the b-axis of the crystal and are accommodated by molecules of the acetone solvent. An even more pronounced manifestation of the intermolecular O···I interaction is in structure 4 decorated with the 5bromonicotinate ligand. The six-coordinate complex is characterized by MoO and M−O5‑bromonicotinate of 1.698(4) Å and 2.136(4) Å, respectively, along with a slight distortion of the Mo ions from the porphyrin plane toward the oxo O-site. The latter interacts with the iodine atom of an adjacent iodophenyl group of neighboring porphyrin unit with a remarkably short O···I distance of 3.127 Å (and MoO···I 3562
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Figure 4. Illustration of the O···I ladder-type halogen-bonded arrays (red dotted lines) in 4. Note that each porphyrin unit is involved in two halogen bonds (only the major orientations of the disordered iodophenyl fragments are shown). The three other I-sites on every porphyrin moiety lie on the surface of the 1D chains and take part in I···I interactions between them.
Figure 5. Ladder-type I···N (blue dotted lines) and I···π (red dotted lines) halogen-bonded arrays in 5. Every porphyrin unit is involved in four halogen bonds.
angle of 123.3°). Every porphyrin complex takes part in two such interactions through one of its I-electrophiles and the oxo atom. This leads to the formation of one-dimensional ladderlike halogen-bonded chains, which propagate along the b-axis of the crystal (Figure 4). I···I halogen−halogen interactions operate between such 1D arrays with iodine atoms disposed on their periphery, with shortest iodine−iodine distance of 3.746 Å. The nitrogen atom on the nicotinic axial ligand is not involved in halogen bonds in this structure. Attempts to crystallize the oxo−Mo−TIPP analogue with isonicotinate as the axial ligand were unsuccessful. Given the favorable geometric features of the O···I interaction (C−I···O and MoO···I of 159 and 123°, respectively) and a ratio between the contact distance of the donor and acceptor atoms and the
sum of the VDW radii of these atoms smaller than 0.9, this interaction can safely defined as a halogen bond. The halogen-bonding pattern changes dramatically in 5 when the axial ligand associated with the oxo−Mo TIPP is pyrimidine 5-carboxylate. The pyrimidine residue is a powerful Lewis base, where the negative charge on one N-site is increased by the presence of another N atom in the ring, thus increasing their potential to engage in halogen bonds with the iodine electrophiles. In the crystal structure of 5 there are two molecules of the six-coordinate porphyrin complex in the asymmetric unit. The two N-sites of the pyrimidine ring of one of the molecules are involved in strong halogen-bonding interaction with the iodine atoms of two different neighboring porphyrins. One of these interactions represents a rather strong halogen bond with an N···I contact of 2.995 Å (which is shorter 3563
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Figure 6. (a) Intermolecular halogen−halogen interactions observed in structure 6 (blue dotted lines). (b) I···I (blue dotted lines) contacts of types I and II, and I···π (red dotted lines) interactions in 7. Note that in panel b the axial ligand is not involved in halogen bonds.
Figure 7. Partial view of the intermolecular organization in 8, showing the Br···Br interactions (of type I; dotted lines). Note the inverted orientation of the MoO bonds of adjacent species in the central part of this figure arranged in van der Waals distance from each other (the O atom pointing at the center of one of the pyrrole rings).
ligand has not resulted in their interaction with the oxo nucleophile. Instead, the intermolecular organization reveals an extensive interporphyrin halogen−halogen interaction (of type I as well as type II)21 through the terminal I atoms (Figure 6a). Each porphyrin unit is involved in four such contacts through three I-substituents on the porphyrin and one I-substituent on the axial ligand. The corresponding I···I interaction distances are 3.741 Å between two iodines of neighboring mesoiodophenyl groups, and 3.744 Å when the contact is between the axial-ligand iodine of one species and one of the iodines on the porphyrin framework of another species. A similar halogen−halogen intermolecular interaction scheme characterizes the crystal structure of complex 7 (Figure 6b). The I···I contacts of type I are 3.942 Å, the type II I···I interactions being considerably shorter, 3.734 Å. In the typical T-shape orientation of the iodophenyl fragments I-to-π interactions could also be observed with the C−I bond of one species pointing at the aryl ring of an adjacent porphyrin with mean Ito-aryl distance of 3.4 Å (Figure 6b). Evidently, common dispersive interactions of various types (e.g., C−H···O and C− H···Cl contacts) could be observed in the two structures.
by more than 0.5 Å from the sum of the corresponding van der Waals radii), with the C−I bond being aligned roughly in the plane of the pyrimidine ring (the corresponding C−I··· Npyrimidine angle is 173.3°). The other N-site interacts with an I atom of another porphyrin at 3.125 Å in another direction, with the C−I bond being oriented nearly perpendicular to the pyrimidine ring, which could be perhaps better classified as a I···π interaction. The structure of the resulting supramolecular halogen bonded assembly resembles a 1D ladder, as shown in Figure 5. The second porphyrin molecule of the asymmetric unit is not involved in significant halogen bonding of similar type. The electron-rich O and Npyrimidine sites take part instead in rather weak C−H···O and C−H···N hydrogenbonding interactions to the neighboring halogen-bonded “ladders” (shown in Figure 5), filling at the same time the space between them. The following two examples of the Mo(O)−TIPP complexes with 4-iodobenzoate (6) and 2-chlorobenzoate (7) as axial ligands also failed to demonstrate the presence of I···O halogen bonds. The increased number of iodine electrophiles in 6 positioned both on the porphyrin framework and on the axial 3564
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Figure 8. Perspective view of 10 showing the N(blue)···I(purple) interactions within the weakly halogen-bonded pairs of porphyrin units (hydrogen atoms are omitted for clarity). In the crystal, n-hexane solvent is accommodated between the two porphyrins.
8). The observed N···I distance is 3.313 Å, somewhat shorter than the sum of the corresponding van der Waals radii of N and I atoms (approximately 3.5−3.8 Å).8 The n-hexane moiety of the crystallization solvent occupies the interporphyrin void between the interacting porphyrin units. The three other pyridyl substituents as well as the Mo-bound O atom are not involved in halogen bonds. Instead, they take part in weak intermolecular C−H···O/N type attractive contacts between positively and negatively charge molecular surfaces. In a different variation, the tetrapyridyl platform was substituted by the tetra(4-tolyl)porphyrin macrocycle in 11, preserving the Mo(O)(4-iodobenzoate) composition around the metal center. The tolyl residue represents an “innocent” meso substituent with respect to its involvement in directional intermolecular interactions. This leaves the iodine atom on the axial ligand as the only halogen electrophile in the porphyrin building unit. It was anticipated that in the absence of competing halogen sites on the porphyrin macrocycle the odds of invoking O···Iaxial‑ligand interaction may increase. Disappointingly, however, the targeted (Mo)O···I halogen bonding was not observed in 11, and more condensed dispersion-driven crystal packing occurred instead.17
In compounds 8 and 9 the 4-iodophenyl functional groups on the porphyrin macrocycle are replaced by the analogous 4bromophenyl residue. On the one hand the bromine electrophile is characterized by lower polarizability than iodine and is less prone to engage in halogen bonds. Yet, on the other hand the Br···O and Br···N interactions have been observed before.6,9 Expression of the latter has been documented both in crystals of porphyrins with mixed bromophenyl and pyridyl meso-substituents and in the six-coordinate tin−tetra(4bromophenyl)porphyrin with isonicotinic acid as axial ligands,6 this even without enhancing the reactivity of the bromine (by means of increasing the positive electrostatic potential along the Caryl−Br bond and the depth of the relatively shallow σ-hole at its cap) by substitution of the hydrocarbon by electron withdrawing groups. However, contrary to the earlier findings crystal structures of the Mo(O)TBrPP complexes with 5bromonicotinate (8) and isonicotinate (9) as axial ligands did not show any pronounced interporphyrin halogen bonding. Common dispersion forces appear to provide the cohesive energy in these two structures, without any marked expression of specific halogen bonding of one type or another. While in 8 one can still observe intermolecular bromine−bromine contacts of 3.65 Å (a distance closely related to the 3.7 Å sum of the van der Waals radii of Br; Figure 7), the bromophenyl arms and the N-nucleophile of the axial ligand in 9 are directed at C−H surfaces of neighboring entities. It has been shown in the first part of this study6b that, in systems where the disposition of the iodine electrophile and Npyridyl site is reversed, the latter being incorporated into the porphyrin framework and the former on the axial ligand, the interporphyrin halogen bonding is less effective. In such a case, the molecular electrostatic potential surface exhibits less pronounced positive charge density at the axially oriented iodine atom than when it is part of the equatorially oriented iodophenyl arms in TIPP. Indeed calculations of Mulliken charge density confirmed that the atomic positive charge density on the iodine atom when attached to the para position of the axial ligand is smaller than that of an iodine atom substituted at the equatorial positions of the tetraarylporphyrin macrocycle.6b In relation to the earlier studied system of Sn(4iodobenzoate)2−tetra(4-pyridyl)porphyrin, we synthesized in this work an analogous complex Mo(O)(4-iodobenzoate)− tetra(4-pyridyl)porphyrin and characterized its crystal structure (10). In the present case the molecular entities are organized in pairs, the monomeric units being interconnected to one another by a pair of weak C−I···Npyridyl interactions (Figure
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CONCLUDING REMARKS Our efforts to formulate supramolecular porphyrin assemblies in which halogen bonds would play an important role met only with partial success. This work focused on metalloporphyrin systems diversely functionalized with halogen electrophiles (I or Br) and with complementary electron-rich partners of Lewisbase nature, with an emphasis to probe in particular the occurrence of intermolecular I···O halogen bonds. In the targeted synthesis of suitable building blocks in the above context (as rationalized in the Introduction), the halogen electrophiles were placed either on the equatorial aryl groups of the porphyrin units and/or on the metal-coordinated axial ligand, while the potential halogen-bond donors were either the metal-bound/coordinated O atoms, the halogen substituents themselves, or the pyridyl-type N-sites incorporated into the porphyrin framework or the axial ligand. Somewhat disappointingly, the I···O halogen-bonding-type interactions (between the electron deficient cap of the Caryl−I bond and one of the lone electron pairs of the oxygen site; the C/Mo−O···I angle being close to 120°) have been observed only in systems 1, 3, and 4. In 1 and 3 the I···O contact distances are not slightly shorter than the sum of the van der Waals radii of I and O and their formal classification as typical halogen bonds may be somewhat 3565
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questionable.18 On the other hand, the I···O halogen bond in 4 is characterized by a considerably shorter contact distance of 3.1 Å. Relatively strong linear Iporphyrin···Naxial‑ligand halogen bonds were found to direct the supramolecular organization in 5, while weaker Nporphyrin···Iaxial‑ligand interactions were observed in 10 with reversed disposition of the interacting partners on the porphyrin complex, in consistence with earlier observations in the first part of this study.6b In compounds 6 and 7 the interporphyrin organization incorporates halogen−halogen (I··· I) bonding of both types I and II.21 No halogen bonding with contact distances between the interacting components that are considerably shorter than the sum of the corresponding van der Waals radii has been detected in 2, 8, 9, and 11 (8 and 9 indicating, as expected from theoretical estimates, that the Brelectrophile is much less effective to this end). As indicated at the outset halogen bonds of the type described here (as are most of the hydrogen bonds and other weak interactions)22 provide only a secondary contribution to the cohesive free energy of molecular solids, and their expression in crystals should be accommodated favorably by the more dominant crystal packing forces (particularly of large molecular species) in the condensed crystalline phase. Thus, while it is relatively easy to design multidentate organic scaffolds with diverging halogen bond donors and acceptors, in the regime of such weak interactions it seems much more difficult to control the supramolecular connectivity scheme and architecture that may form in a given system. Yet, the considerable number of successfully formulated halogen-bonded porphyrin assemblies in this and the earlier report6b provide a promising perspective for further crystal engineering investigation in this rapidly advancing field of research.2b,5a,b,23
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(5) (a) Meazza, L.; Foster, J. A.; Fucke, K.; Metrangolo, P.; Resnati, G.; Steed, J. W. Nat. Chem. 2013, 5, 42−47. (b) Priimagi, A.; Cavallo, G.; Metrangolo, P.; Resnati, G. Acc. Chem. Res. 2013, 46, 2686−2695. (c) Farina, A.; Meille, S. V.; Messina, M. T.; Metrangolo, P.; Resnati, G.; Vecchio, G. Angew. Chem., Int. Ed. 1999, 38, 2433−2436. (d) Messinaa, M. T.; Metrangoloa, P.; Panzeria, W.; Raggb, E.; Resnati, G. Tetrahedron Lett. 1998, 39, 9069−9072. (e) Cheetham, N. F.; Pullin, A. D. E. Chem. Commun. 1967, 233−234. (f) Legon, A. C. Angew. Chem., Int. Ed. 1999, 38, 2686−2714. (g) Legon, A. C. Chem.Eur. J. 1998, 4, 1890−1897. (6) (a) Titi, H. M.; Karmakar, A.; Goldberg, I. J. Porphyrins Phthalocyanines 2011, 15, 1250−1257. (b) Titi, H. M.; Patra, R.; Goldberg, I. Chem.Eur. J. 2013, 19, 14941−14949. (c) Muniappan, S.; Lipstman, S.; Goldberg, I. Chem. Commun. 2008, 1777−1779. (d) Lipstman, S.; Muniappan, S.; Goldberg, I. Cryst. Growth Des. 2008, 8, 1682−1688. (7) Gavezzotti, A. Mol. Phys. 2008, 106, 1473−1485. (8) (a) Awwadi, F. F.; Wilett, R. D.; Peterson, K. A.; Twamley, B. Chem.Eur. J. 2006, 12, 8952−8960. (b) Gavezzotti, A. J. Am. Chem. Soc. 1983, 105, 5220−5225. (c) Bondi, A. J. Phys. Chem. 1964, 68, 441−451. (9) (a) Lo, R.; Ballabh, A.; Singh, A.; Dastidar, P.; Ganguly, B. CrystEngComm 2012, 4, 1833−1841. (b) Nelyubina, Y. V.; Antipin, M. Y.; Lyssenko, K. A. Mendeleev Commun. 2011, 21, 250−252. (c) Riley, K. E.; Murray, J. S.; Politzer, P.; Concha, M. C.; Hobza, P. I. J. Chem. Theory Comput. 2009, 5, 155−163. (d) Auffinger, P.; Hays, F. A.; Westhof, E.; Ho, P. S. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 16789− 16794. (10) Ghosh, S. K.; Patra, R.; Rath, S. P. Inorg. Chem. 2008, 47, 9848− 9856. (11) Subramaniam P. Stabilization of Lower Oxidation States of Molybdenum by Reductive Nitrosylation using Hydroxylamine in Aqueous Aerobic Medium, PhD Thesis, IIT Kanpur, Kanpur, India, 1981. (12) Adler, A. D.; Longo, F. R.; Finarelli, J. D.; Goldmacher, J.; Assour, J.; Korsakoff, L. J. Org. Chem. 1967, 32, 476. (13) Nandi, G.; Sarkar, S. J. Porphyrins Phthalocyanines 2014, 28, 282−289. (14) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435. (15) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (16) Spek, A. L. Acta Crystallogr., Sect. D: Biol. Crystallogr. 2009, D65, 148−155. (17) (a) Byrn, M. P.; Curtis, C. J.; Hsiou, Y.; Khan, S. I.; Sawin, P. A.; Tendick, S. K.; Terzis, A.; Strouse, S. E. J. Am. Chem. Soc. 1993, 115, 9480−9497 and references cited therein. (b) Krishna Kumar, R.; Balasubramanian, S.; Goldberg, I. Inorg. Chem. 1998, 37, 541−552. (18) Desiraju, R. G.; Ho, P. S.; Kloo, L.; Legon, A. C.; Marquardt, R.; Metrangolo, P.; Politzer, P.; Resnati, G.; Rissanen, K. Pure Appl. Chem. 2013, 85, 1711−1713. (19) Janczak, J.; Kubiak, R. Inorg. Chem. 1999, 38, 2429−2433. (20) Zou, C.; Xie, M.-H.; Kong, G.-Q.; Wu, C.-D. CrystEngComm. 2012, 14, 4850−4856. (21) Metrangolo, P.; Resnati, G. IUCrJ 2014, 1, 2−5. (22) Dunitz, J. D.; Gavezzotti, A. Cryst. Growth Des. 2012, 12, 5873− 5877. (23) Meyer, F.; Dubois, P. CrystEngComm 2013, 15, 3058−3071.
ASSOCIATED CONTENT
S Supporting Information *
X-ray crystallographic details (in CIF format; CCDC 996345− 996355). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*(I.G.) Tel: +972-3-6409965. Fax: +972-3-6409293. E-mail:
[email protected]. *(G.N.) Tel: +972-3-6409965. Fax: +972-3-6409293. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Israel Science Foundation: Grant No. 108/12.
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REFERENCES
(1) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives;VCH: Weinheim, 1995. (2) (a) Metrangolo, P.; Resnati, G. Halogen Bonding: Fundamentals and Applications (Structure and Bonding); Springer: Heidelberg, 2010. (b) Metrangolo, P.; Resnati, G. Cryst. Growth Des. 2012, 12, 5835− 5838. (3) Metrangolo, P.; Meyer, F.; Pilati, T.; Resnati, G.; Terraneo, G. Angew. Chem., Int. Ed. 2008, 47, 6114−6127. (4) Clark, T.; Hennemann, M.; Murray, J. S.; Politzer, P. Halogen bonding: the sigma hole. J. Mol. Model. 2007, 13, 291−296. 3566
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