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The Dual Behavior of Bromine Atoms in Supramolecular Chemistry; The Crystal Structure and Magnetic Properties of Two Copper(II) Coordination Polymers. Firas F. Awwadi, Roger D. Willett, Brendan Twamley, Mark M. Turnbull, and Christopher P. Landee Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b00393 • Publication Date (Web): 06 Jul 2015 Downloaded from http://pubs.acs.org on July 7, 2015

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Crystal Growth & Design

The Dual Behavior of Bromine Atoms in Supramolecular Chemistry; The Crystal Structure and Magnetic Properties of Two Copper(II) Coordination Polymers. Firas F. Awwadi,a, * Roger D. Willett,b Brendan Twamley, c Mark M. Turnbulld and Christopher P. Landeee a

Department of Chemistry, The University of Jordan , Amman11942, Jordan.

b

Department of Chemistry, Washington State University, Pullman, WA 99164, USA

c

School of Chemistry, Trinity College Dublin, Dublin 2, Ireland.dCarlson School of Chemistry and Biochemistry, Clark University, 950 Main St., Worcester, MA 01610, USA. e Department of Physics, Clark University, Worcester, MA 01610, USA Abstract. The crystal structures of two copper(II) chloride coordination polymers, (Cu2(2bp)2Cl4)n, henceforth “polydimer” and (Cu3(2bp)2Cl6)n, henceforth “polytrimer”, where 2bp = 2bromopyridine, and the temperature dependence of their magnetic susceptibilities are reported. Also, the temperature dependence of the magnetic susceptibility of the monomeric complex, Cu(2bp)2Cl2, is reported. The magnetic data were measured in the range 2-310 k. The organic bromine atom of the 2bp ligand shows electrophilic and nucleophilic character: an electrophilic nature via C-Br···Cl interactions and a nucleophilic nature via Cu···(π)Br interactions. The Cu···(π)Br interactions, previously underestimated in the literature, turn the 2bp ligand to a bidentate rather than monodentate ligand via the N-Cu coordination bond and the Cu···(π)Br non-covalent interactions. Thus, the 2bp ligand behaves as a molecular bender; the Cu···(π)Br interactions result in the bending of the (CuCl2)n infinite linear extended chains at specific locations. The magnetic data of the monomer were fit to the two-dimensional S=1/2 Heisenberg model (Curie constant = 0.43(1) emu-K/mol-Oe and an exchange constant J/k = 0.41(4) K ). The magnetic data of the polydimer were best fit to an alternating antiferromagnetic chain rather than a dimer model (Curie constant = 0.415(3), J1/k = -11.4(1) K, J2 = -2.2(3) K). The magnetic data of the polytrimer may be approximated by a model including both nearest neighbor (J1) and next nearest neighbor (J2) interactions within the polytrimer as well as a Curie-Weiss correction for inter-trimer exchange within a single chain [J1 = -53(1) K, J2 = -6.0(5) K, θ = -1.5(5) K]. Viewing the reported structures as bent coordination polymers is supported by observations that

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(a) the Cu···Cl coordination distance is quite short (avg. = 2.672Å) and (b) the magnetic data of the polydimer are best fit to the alternating antiferromagnetic chain model.

Introduction Carbon-bound halogen atoms (Y) play a significant and unique role in supramolecular chemistry; they can act as both nucleophilic and electrophilic supramolecular synthons (Scheme 1). This makes them of special interest in supramolecular synthesis. Nucleophiles tend to approach the halogen atom of the C-Y bond at an angle near 180° (Scheme 1, left).1-3 This is considered a type of halogen bonding interaction.2 When the nucleophile is a halide ion, these are known as halogen···halide interactions. They are characterized by Y···X- distances significantly less than the sum of the van der Waals radius, rvdW, of the halogen atom and the ionic radius, ri, of the halide anion.4-16 In contrast, electrophiles tend to approach the halogen atom at an angle near 90° (Scheme 1, right).1,3 These arrangements have been attributed to the anisotropy of electrostatic potential values and the deformation of electron density around the halogen atom; the halogen atom shows a positive electrostatic potential end cap and a negative electrostatic potential ring in the π-region of the halogen atom. The nature of halogen···halide interactions have been investigated theoretically and experimentally. The geometrical aspects are generally controlled mainly by electrostatic forces, but other factors such as charge transfer interactions and dispersion forces cannot be ruled out.11,17-23 Scheme 1. Interaction of covalently bound halogen atoms with nucleophiles (left) and with electrophiles (right). R R

Y

Y

Nu.

E.

R-Y = covalently halogen bound group or atom. Nu. = Nucleophile. E. = Electrophile.


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The role of halogen···halide interactions in the arrangement of species within the crystalline lattice has been investigated by many researchers.4,7,10,15,24-26 They are found to complement the role of hydrogen bonding in developing the crystal structure of simple halopyrdinium salts and halometallate salts.7-11,13,14,19,23,27

It was found that halogen···halide interactions are more

directional than the corresponding hydrogen bonds.28 The IUPAC provisional recommendation of the definition of halogen bonding limits the halogen bonding interactions to the interactions between the electrophilic part of the halogen atom and nucleophiles (Scheme 1, left). Hence, it ignores the interactions between the halogen atom and electrophiles (Scheme 1, right). However, some researchers, but by no means all, consider the interactions of the carbon bound halogen atoms with electrophiles as halogen bonding interactions.2,3,29 In this paper, we will show that the underestimated C-Y···E. interactions play a crucial role in the stabilizationing of two copper (II) chloride coordination polymers. We report the crystal structures of two copper chlorides coordination polymers with 2bromopyridine (2bp); (1) (Cu2(2bp)2Cl4)n, henceforth “polydimer”; (2) (Cu3(2bp)2Cl6)n, henceforth “polytrimer.” The magnetic properties of the polydimer and the polytrimer along with the related monomeric complex will also be discussed. The crystal structure of the monomeric complex, Cu(2bp)2Cl2, has been reported previously.6 The analysis of the crystal structures of the two polymers will show clearly that the dual behavior of the organic bromine atoms, that is their nucleophilicity and electrophilicity, can be used as a tool in supramolecular synthesis. The interaction of the π-region of the organic bromine atom with Cu(II) ion (Cu···(π)Br interaction) makes the 2bp to play the role of molecular bender.

Experimental Section. 2-Bromopyridine (2bp) was purchased from Aldrich Chemical and copper(II) chloride dihydrate was purchased from Mallinckrodt Pharmaceuticals. Acetonitrile was purchased from VWR Scientific. All were used as received. IR spectra were recorded as KBr pellets on a PE Spectrum 100. X-Ray powder diffraction measurements were made on a Bruker AXS-D8 X-ray powder diffractometer. Synthesis and Crystal Growth. (a)Monomer. Trans-bis(2-bromopyridine)dichlorocopper(II)



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The monomer was prepared according to the published procedure.6 The identity and purity of the product was verified using powder x-ray diffraction. The experimental X-ray diffraction pattern agrees with that calculated based on the single crystal data (Figure 1S). IR (KBr): 3080m/3050m (υ C-H), 1558m/1459s (υ C-C, C-N), 777s (δ C-H). (b) Polydimer. catena-poly [di-µ-chloro-bis[(2-bromopyridine)chlorocopper(II)]] 1 mmol (0.17 g) of CuCl2.2H2O was dissolved in 50 mL of acetonitrile. To this solution 1mmol (0.16 g) of 2bp was added. The solution was gently heated while it was being stirred for 20 minutes. The solution was filtered and left for slow evaporation. After one day, parallelepiped yellowish green crystals formed (moles = 0.19 mmol; mass = 0.12 g; %yield = 38%). A crystal of suitable size was selected for collecting X-ray data. IR (KBr): 3086m (υ C-H), 1589s/1459m (υ C-C, C-N), 774s (δ C-H).

(c) Polytrimer, catena-poly[tetra-µ-chlorodichlorobis(2-bromopyridine)tricopper(II)] Two different procedures were followed to prepare the polytrimer; (a) one to prepare good quality crystals suitable for x-ray analysis and second one (b) to prepare pure crystalline product (lower quality crystals but pure samples) for collecting the magnetic data. (a) 1.5 mmols (0.26 g) of CuCl2.2H2O were dissolved in 100 mL of acetonitrile. To this solution, 1mmol (0.16 g) of 2bp was added. The solution was heated while it was being stirred for 20 minutes. The mixture was filtered and left for slow evaporation. After two days, two types of crystals formed, yellowish green and green crystals. The yellowish green crystals were identified as the polydimer (vide supra). One of the green crystals with a suitable size was picked for the X-ray data collection. (b) 4.5 mmol (0.70 g) of CuCl2 and 2.0 mmol (0.32 g) of 2bp was dissolved in 20 mL of 1-propanol. This solution was heated gently with stirring until the volume of solution was reduced to about 5 mL. Green small crystals formed. These crystals were collected by filtration in a desiccator. The product was dried in the oven for 1 hour at 80°C (moles = 0.17 mmol; mass = 0.13 g), the %yield based on 2bp is 17% The purity of the product was verified by measuring the powder x-ray diffraction pattern; the calculated pattern based on the single crystal


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structure agrees with the experimental one (Figure S2); no impurities were observed. IR (KBr): 1589m/1458m (υ C-C, C-N), 769s (δ C-H)

Crystal Structure Determination. Both crystal structures were determined at room temperature. The X-ray data for the polydimer was collected using a Syntex P21 diffractometer upgraded to Bruker P4 specifications. Lattice dimensions were obtained from 35 accurately centered reflections30. Data were corrected for absorption utilizing ψ-scan data assuming an ellipsoidal shaped crystal using SHELXTL XPREP.31 The X-ray data for the polytrimer were collected using a Bruker/Siemens SMART APEX instrument. Data were measured using ω-scans of 0.3 ° per frame for 5 seconds, and a hemisphere of data was collected. The first 50 frames were recollected at the end of data collection to monitor for decay. Cell parameters were retrieved using SMART software and refined using SAINTPlus on all observed reflections.32,33 Data reduction and correction for Lp and decay were performed using the SAINTPlus software. Absorption corrections were applied using SADABS.34 The structure of both compounds was solved by direct methods and refined by least squares method on F2 using the SHELXTL program package. Hydrogen atoms for all structures were included in calculated positions. Data collection parameters and refinement results are given in Table 1.

Magnetic Susceptibility Study Magnetic data for the three compounds were collected on crushed crystals using a Quantum Design MPMS SQUID Magnetometer. The magnetization of the sample was first measured as a function of field at 1.8 K from 0 Oe to 50 kOe; the data were linear up to 20 kOe. Several data points were collected as the field was returned to zero; no hysteresis was detected. Susceptibility data were collected between 1.8 and 300 K in an applied field of 1.0 kOe. Corrections to the molar susceptibility have been made for the temperature independent magnetization of the Cu(II) ion and the diamagnetic contributions from the constituent atoms were calculated from Pascal’s constants. All data were corrected for contributions from the sample holder. Data interpretation has been done using the Hamiltonian H = -J Σ Si · Sj. Results 5 ACS Paragon Plus Environment


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Two copper(II) coordination polymers and a monomer complex were prepared (Scheme 2). The monomer was prepared according to the published procedure,6 whereas the two polymers are newly reported. The yield of the polydimer is relatively good (38%), while that of the trimer is low (17%), no trials were made to improve the yield of the trimer to avoid precipitation of impurities which seriously affect the magnetic susceptibility data. Scheme 2. Synthesis of the monomer, polydimer and polytrimer.

Br

Cl

N Cu

N

Cl

Br Monomer 0.5 mmol CuCl2.2H2O

Br N 1 mmol 1 mmol CuCl2.2H2O

1.5 mmol CuCl2.2H2O

Br

N Cu Cl

Cl

Cl

Cu

Cu Cl

Polytrimer

Cl

Cl

Cl

Cl Br

N

Cu

Cu

Br N

n

Cl

Cl

Br N

Polydimer

n

Description of the molecular structure of the monomeric units. The molecular structure of the monomer is shown in Figure 1A and was described previously.6 The structure of the dimeric unit (repeat unit) of the polydimer is illustrated in Figure 1B. The Cu1 ions show distorted 4+2 geometry with a small distortion as indicated by the values of Cl1Cu1-Cl3 and N-Cu1-Cl2 trans angles (Table 2). The copper ion completes its octahedral coordination via the interaction with the π-region of an organic bromine atom (Figure 1) from one side and with a chloride anion on the opposite side via a Cu···Cl semi-coordinate bond.1 The interactions of carbon-bound halogen atoms, if they are in suitable positions, with copper 6 ACS Paragon Plus Environment

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coordination centers have been shown to play a crucial role in determining the molecular structures of many copper complexes.5,7,27,35 The dimeric unit consists of two asymmetric units, which are related by a two-fold axis (parallel to b) along the Cl2 and Cl3 vector (Figure 1B). Hence, the 2bp ligands occupy syn positions in the dimeric unit and the two organic bromine atoms are located in anti positions. It is noteworthy that the coordination plane of the dimeric units is not exactly planar; the average deviation of the coordination atoms from the coordination plane is 0.098 Å. The structure of the trimeric unit (repeat unit) of the polytrimer also consists of two asymmetric units (Figure 1C). The central copper ion, Cu2, is located on an inversion center. In contrast to the polydimer, this results in that the 2bp ligands being located in anti positions. The two terminal copper centers, Cu1 and its symmetry equivalent Cu1A, complete their octahedral coordination in a similar manner to the polydimer (vide supra), whereas Cu2 shows approximately square planar coordination with a Cl2-Cu2-Cl3 angle of 87.45(4)° and all transangles 180° as required by symmetry. The result in both cases is that the plane of the pyridine ring is nearly perpendicular to the coordination plane. The angles between the two planes are 88.65 and 89.15º for the polydimer and the polytrimer, respectively. This is similar to the structure of the monomer where the angle between the plane of the pyridine ring and the coordination plane is 89.45°. This perpendicular arrangement in the polydimer and polytrimer is reinforced by the presence of C6-H6···Cl1 hydrogen bonding, in which the C6-H6 hydrogen atom of a repeat unit, in both polydimer or polytrimer, serves as proton donor to Cl1 of a neighboring unit within the chain (Figure 2). Also, the mean deviation of the coordination atoms of the trimeric unit (repeat unit) from the coordination plane of the polytrimer is 0.204Å which is significantly larger than the corresponding value in the polydimer (vide supra).

Comparison of the bond distances and angles within the coordination spheres indicate that; (a) within the polydimer and the polytrimer, the trans angle N-Cu1-Cl2 (avg. = 160.32º) is smaller than the Cl1-Cu1-Cl3 trans angle (avg. = 174.8º). This is due to the interaction between the πregion of the bromine atom and the cupric ion (Figure 1). The N-Cu-N trans angle in the monomer equals 180° due to the presence of the copper ion on an inversion center. (b) The Cu7 ACS Paragon Plus Environment


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Cl distances within the repeat units average 2.298 Å (range 2.274-2.316Å) and 2.299 Å (range 2.248-2.339 Å) in the polydimer and polytrimer, respectively. The length of the Cu-Cl bond in the monomer (2.230(3)Å)6 is shorter than the average Cu-Cl bond lengths in the dimeric unit and trimeric unit. In fact, it is shorter by ~ 0.02Å than the shortest Cu-Cl bond in the dimeric unit and trimeric unit, which is Cu-Cl1A in the trimeric unit. For each bridging chloride ligand within the repeat units, as one Cu-Cl bond length gets shorter, the other gets longer and the average length of the two bonds stays constant, with an average distance of 2.30(4)Å.


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Table 1: Summary of data collection and refinement parameters. Crystal polydimer polytrimer Formula C10H8Br2N2 Cu2Cl4 C10H8Br2N2Cu3Cl6 Mr 584.88 719.32 3 ρcalc (Mg/m ) 2.284 2.444 T(K) 295(2) 297(2) K Crystal system Monoclinic Monoclinic Space group C2/c P21/c a (Å) 15.749(2) 7.4469(15) b (Å) 9.2252(15) 15.272(3) c (Å) 12.0140(18 9.1109(18) 102.969(13) 109.39(3) β (°) 1701.0(4) 977.5(4) V(Å3) ind. Reflections 1489 2246 R(int) Z Goodness of fit R1a [I> 2σ] wR2b [I> 2σ] µ, mm-1 a

0.0549 4 1.040 0.0532 0.1383 7.815

0.0301 2 1.051 0.0299 0.0660 8.137

R1 = Σ||Fo| - |Fc||/| Σ |Fo|. wR2 = { Σ w(Fo2 - Fc2)2/ Σ w(Fo2)2}1/2.

b



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Figure 1. The repeating units of (A) monomer (B) polydimer and (C) polytrimer. The interaction between the pi region of the bromine atom and the cupric ion is represented 10 ACS Paragon Plus Environment

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by a dotted line. Thermal ellipsoids are shown at 50%. Hydrogen atoms have been excluded for clarity. Symmetry codes used to generate equivalent atoms; (A) monomer; x, -y,-z (B) polydimer; 1-x, y,-½-z and (C) polytrimer; -x, 1-y,-z. Although the structure of the monomer was discussed previously,6 we include it here for sake of comparison. Table 2. Selected Angles (°) and Distances (Å) in monomer, polydimer and the polytrimer. Polydimer Polytrimer Compound Monomer6 Cu1-Cl1 2.230(3) 2.274(2) 2.248(1) Cu1-Cl2 2.305(2) 2.339(1) Cu1-Cl3 2.316(2) 2.312(1) Cu1-N1 1.982(7) 2.021(6) 1.999(3) Cu2-Cl2 2.258(1) Cu2-Cl3 2.244(1) Br1···Cu1 3.221 3.124 3.198 Br···Cl 3.358 3.406 C-Br···Cl 167.9 157.8 Cl1-Cu1-Cl3 172.75(6) 176.85(3) Cl2-Cu1-N1 89.8 (2) 164.28(17) 156.36(8) Cu1-Cl2-Cuc 90.2(2) 95.90(10) 93.59(4) c Cu1-Cl3-Cu 95.32(10) 94.69(4) Cl2-Cu2-Cl2A 180.0 Cl3-Cu2-Cl3A 180.0 d Cu1···Cl1 2.646 2.697 a N1-Cu-Cl1 b N1-Cu-Cl1A c This is Cu1A in the polydimer and Cu2 in the polytrimer. d This is the semi-coordinate bond. Scheme 3. Stacking diagrams for the repeating units of polydimer (top) and polytrimer (bottom). The positions of the coordinated nitrogen atoms are indicated by the black dots36.



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2(3/2, 1/2)(3/2, -1/2)

3(5/2, 1/2)


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Description of the supramolecular structure of the dimer and trimer. In both the polydimer and polytrimer structures, the repeat units are linked via pairs of Cu···Cl semi-coordinate bonds, as well as C-H···Cl- hydrogen bonding to form chain structures (Figure 2). These chains extend parallel to the c-axis in both compounds. The presence of the Cu···(π)Br interactions plays a pivotal role in the way the repeat units link together in that they prevent the formation of multiple pairs of semi-coordinate bonds between the repeat units, as are observed in the structures of, for example, KCuCl3 or Cu2Br4(pyridine)2.36,37 These chains of pseudo-planar repeat units can be illustrated diagrammatically using the stacking patterns developed for such systems, as shown in Scheme 3. The stacking arrangement for the polydimer structure has not been observed before, while a arrangement similar to the polytrimer chains is found in (1,2-dimethylpyridinium)2Cu3Br8.36 Both the polydimer and polytrimer form layer structures in the bc plane for the former and in the ac plane for the later (Figure 3 and 4). The chains of the polydimer are connected only via the CH···Cl- hydrogen bond (C5-H5···Cl3) to form a layer structure, while the chains of the polytrimer are linked via two types of interactions to form a layer structure; (a) C-Br···Cl- synthons. The Br···Cl distance = 3.406Å and the C-Br···Cl angle =157.6°. The bromine···chloride distance is 0.2 Å less than the sum of the van der Waal radii rvdW (rvdW in Å, Cl = 1.75 and Br = 1.85).38 (b) C3H···Cl hydrogen bonding. This interaction is reinforced by close Cl···π contacts as shown in Figure 4. It is noteworthy that the C-Br···Cl- halogen bonding interactions are absent in the polydimer structure. The polydimer layers aggregate via C4-H4···Cl1 hydrogen bonds to form the three dimensional structure. The interaction between layers of the polytrimer is minimal.



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Figure 2. The chain network structure in the polydimer (above) the polytrimer (below). The chain structure runs parallel to the c axis. Cu···(π)Br interactions and C-H···Cl- hydrogen bond are represented by red and black dotted lines, respectively. Cu···Cl semi-coordinate bonds are represented by grey thin lines.


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Figure 3. The layer structure of the polydimer, where the layers lie parallel to the bc plane. The C-H···Cl- hydrogen bond is represented by dotted lines.

Figure 4. The layer structure of the polytrimer, where the layers lie parallel to the ac plane. The Cu···Br and Br···Cl- interactions are represented by blue and red dotted lines, respectively.

Susceptibility data for the three compounds were collected as a function of temperature and are shown in Figures 5-7. In the case of the monomer complex, the most likely superexchange lattice occurs via the two-halide pathway, where overlap between neighboring chloride ions provides the magnetic exchange mechanism. The shortest Cl···Cl contacts between molecules are 4.605Å and each molecule has four identical nearest neighbors (as a result of the crystallographic symmetry). The shortest Cl···Cl contacts between layers is greater than 7.5Å suggesting that the layers are very well isolated. The relatively large distance between the Cl ions within the layers suggests very weak interactions and such is observed. The data were fit to 15 ACS Paragon Plus Environment


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the two-dimensional S=1/2 Heisenberg model resulting in a Curie constant of 0.43(1) emuK/mol-Oe and an exchange constant J/k = -0.41(4) K (Figure 5). The polydimer presents a distinctly different case where superexchange likely occurs via the bihalide bridges. The dimers are bridged internally by a pair of chloride ions, both of which occupy equatorial positions on both Cu(II) ions. The dimers (repeat units) are then linked into an alternating chain via pairs of chloride ions which occupy an equatorial position on one Cu(II) ion and an axial position on the other Cu(II) ion. This latter interaction is expected to be significantly smaller than the equatorial-equatorial exchange. The data were fit according to two different models: the dimer model with a Curie-Weiss correction to account for the inter-dimer interactions, and the alternating antiferromagnetic chain model. Attempts to fit the data with the modified dimer model resulted in a Curie constant of 0.472(3) emu-K/mol-Oe, J/k = -9.7(1) K, θ = -2.5(2) K and a paramagnetic impurity of 12.4(3)%. The absence of a paramagnetic tail in the susceptibility data precludes this result.


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Figure 5: χ(T) (○) and χT(T) (□) for the monomer. The solid lines represent the best fit to the two-dimensional S = ½ Heisenberg model.

Figure 6. : χ(T) (○) and χT(T) (□) for the polydimer. The solid lines represent the best fit to the S = ½ alternating chain model. Fitting the data to the S = ½ alternating antiferromagnetic chain model provided a much more reasonable result (Figure 6) with a Curie constant of 0.415(3), J1/k = -11.4(1) K, J2 = -2.2(3) K and a paramagnetic impurity fraction of only 1.5(1) %. The susceptibility (○) and inverse susceptibility (x) data per mole of polytrimer of S = ½ moments are shown in Figure 7; the temperature scale is common but the susceptibility data appear on a log scale (left axis) while the inverse susceptibility uses a linear scale (right axis). Qualitatively, the susceptibility data present the expected picture for three S = ½ units forming an antiferromagnetic trimer. Starting at 300 K and with decreasing temperature, the susceptibility rises with a gradually decreasing slope until a maximum is suggested near 40 K, before the 17 ACS Paragon Plus Environment


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susceptibility resumes its increase at lower temperatures. No maximum is observed above 1.8 K. Qualitatively, at high temperatures (T > J) the polytrimer approaches the behavior of three independent S = ½ units. Upon cooling, (T ≈ J) the antiferromagnetic interaction converts the trimer (repeat unit) into an S = ½ ground state with a thermally activated S = 3/2 quartet excited state. At low temperatures (T < J), the susceptibility of the collective S = ½ state diverges as T-1.

Figure 7. The susceptibility (○, left axis) and inverse susceptibility (X, right axis) data per mole of polytrimer of S = ½ moments are shown. The χ-1 data (140 – 310 K) were compared to the Curie-Weiss equation, with best fit parameters of C = 1.40(2) emu-K/mol-Oe and θCW = 25.4(0.6) K (red line). Similarly the low-temperature (1.8 – 12 K) data were fit to the same equation with parameters C = 0.441(5) emu-K/mol-Oe and θCW = -0.82(2) K (green line). This crossover can also be seen in the temperature dependence of the inverse susceptibility data in Figure 7. A comparison of the data above 140 K to the Curie-Weiss equation reveals a Curie constant (per mole of moments) of 1.40(2) emu-K/mol-Oe, corresponding to a Curie constant per copper of 0.47(1) units, as would be expected for three independent S = ½ moments with a gvalue of ~2.2 (slightly higher than expected, but not excessively so). The Weiss intercept of 25.4(0.6) K, indicates significant antiferromagnetic interactions within the trimer (repeat unit) in 18 ACS Paragon Plus Environment

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agreement with the observed inflection in χ near 40 K. A second linear region of the χ-1 versus T data is found for temperatures below 12 K; this region corresponds to the collective S = ½ ground state of the trimer. The Curie-Weiss parameters in this region are C = 0.44(1) emu•K/mol•Oe and a Weiss intercept of θ = -0.82(2) K.

Figure 8. The χT data () for polytrimer are plotted as a function of temperature. The colored lines correspond to the best-fit results for a variety of models as described in the text, with fitting parameters given in Table 3. Simulated data (see text) are represented by stars. The crossover in effective moment can be seen more clearly in Figure 8, a plot of the χT product versus temperature which shows a clear plateau near 0.42 as expected for the S = ½ ground state of an antiferromagnetic trimer. Initial attempts to fit the susceptibility per individual moment made using the isolated linear trimer model (Eqn. 1, θ = 0)39 with, or without a Curie-Weiss correction for inter-trimer interactions over different temperature ranges were unsuccessful (the resulting fitting parameters are listed in Table 3; see the Supplementary Information for full details).



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+ 10e ). C (1+ e − J /T −1.5 J /T (T − θ ) (1+ e + 2e ) − J /T

χ mol =

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−1.5 J /T

Eqn. 1

The most relevant results were obtained by introduction of an interaction J2 between the two terminal moments, in addition to the nearest neighbor interaction (J1). To explore this possibility, the data was modeled again using the MAGMUN program40 as a trimer with an exchange J1 between the central and terminal moments, an exchange J2 between the terminal moments, and a Curie-Weiss correction to account for exchange between trimer units. Table 3: Fitting parameters to the χT product data presented in Figure 8. Range (K) Color, style C(emu-k/mol-Oe) g(calc. from C) J/k (K) θ or J2 (K) 12 - 310 Red, solid 0.48(2) 2.26 -78(2) (not applicable) 12 – 310 Blue dot-dash 0.49(2) 2.28 -75(2) -2.6(2) 1.8 - 310 Black, solid 0.48(2) 2.26 -76(1) -1.3(1) 0.48(1) 2.26 -53(1) J2 = -6.0(5), 1.8 – 310 Black, stars Θ = -1.5(5) Fit number – 1, Linear trimer alone; 2, 3 linear trimer with Curie-Weiss term for inter-trimer interactions; 4, Linear trimer with J(1…3) interaction and Curie-Weiss term for inter-trimer interactions. C = Curie constant.

Fit 1 2 3 4

The results are shown in Figure 8 (black stars). The best result was generated using C = 0.48(1) emu-K/mol-Oe, J1 = -53(1) K, J2 = -6.0(5) K, θ = -1.5(5) K and a fixed 1% impurity. The results suggest that a significant interaction is present between the terminal Cu(II) ions (next nearest neighbors). As is clear from the figure, these values provide a moderate fit over the entire temperature range. Although the models used are clearly insufficient to interpret the subtleties of the system, the overall behavior is unambiguous.

Discussion. Halogen atoms (except fluorine) that are bonded to a carbon atom show a dual behavior as a synthon in supramolecular chemistry. They form supramolecular bonds with both electrophiles and nucleophiles (Scheme 1), which makes them a significant synthon in supramolecular synthesis. The arrangement of these two interactions is completely different; the nucleophiles approach the halogen atom such that the C-X···Nu angle = 180º.2 In contrast, the electrophiles approach the halogen atom such that C-X···E. angle = 90º.3 This behavior agrees with the theoretically calculated electrostatic potential of carbon-bonded halogen atoms and determined experimentally as well as the anisotropic distribution of electron density around halogen atom 20 ACS Paragon Plus Environment

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(Figure 9). This dual behavior resulted in the preparation of the polydimer and polytrimer. This analysis is supported by molecular orbital analysis. The highest two occupied molecular orbitals (HOMO and HOMO-1) have been calculated (Figure 10). The molecular orbital distribution indicates that the 2bp pyridine and the copper center are bonded together via the interaction between HOMO-1 orbital of the 2bp ligand and the orbitals of the copper center. The HOMO is distributed over the π-region of the aromatic system and the p-orbital of the bromine atom normal to the plane of the pyridine ring, whereas, the lone electron pair of the pyridinic nitrogen and the p-orbital of the organic bromine in the plane of the ring are part of the HOMO-1.

Figure 9. Mapped electrostatic potential on total electron density for 2bp. Electron density contour isovalue is set to 0.01. The electrostatic potential is calculated at the B3LYP/cc-pvdz level. Units are in atomic units.41



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Figure 10. HOMO molecular orbital (top) and HOMO-1 molecular orbital (bottom) of the free 2bp ligand.41 The Cu···(π)Br interaction transforms 2bp from a monodentate to a bidentate ligand. In both reported compounds, the π-region of the bromine atom interacts with a cupric ion to collaborate in completing the octahedral coordination of the cupric ion (Cu1). Thus, within the chain, the 2bp ligand can be considered a bidentate ligand; one through the covalent Cu-N bond and the other through the Cu···(π)Br interaction; The negative electrostatic potential ring ( Figure 9) interacts with the copper(II) ion. This interaction affects the Cl-Cu-N trans angle, which averages 160º in the two oligomeric structures (repeat units) reflecting the strength of this interaction, while the trans angles in the monomer, where such an interaction is absent, equal 180°. Furthermore, Cu···(π)Br interactions along with C6-H···Cl hydrogen bonding resulted in the plane of aromatic rings being perpendicular to the Cu(II) coordination planes (Figures 1 and 2). Similarly the formation of the Cu···(π)Br interaction resulted in a perpendicular arrangement in the monomer.6 Also, this perpendicular arrangement was observed in similar complexes; the coordination plane in Cu(25dbp)2Br2 and Cu(2bp)2Br2 (where 25dbp = 2,5-dibromopyridine) is perpendicular to the aromatic planes.6,42 In contrast, the aromatic planes are not perpendicular to the coordination plane in Cu(3cp)2Br2 and Cu(3bp)2X2 (where 3cp =3-chloropyridine ; 3bp = 3bromopyridine), the angles between the coordination plane and the aromatic planes are 28, 61 and

61° for Cu(3cp)2Br2, Cu(3bp)2Cl2 and Cu(3bp)2Br2, respectively.5,6 This indicates that 22 ACS Paragon Plus Environment

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Cu···(π)Br interactions play a crucial role in determining the molecular structure of these copper complexes.

The effect of the bifunctional nature of the bromine substituents on the structure of the dimer (repeat unit) system can be shown dramatically by comparing it with the structure of Cu2Br4(pyridine)2.

In this latter compound, the pyridine ligands coordinate in a trans

configuration. The planes of the pyridine rings are now twisted only 54º from the plane of the Cu2Br4, rather than the ~90º observed in the 2bp systems. Thus, Cu2Br4(pyridine)2 dimers are able to stack as depicted in Scheme 4, and each Cu atom completes its 4+2 coordination sphere by forming semi-coordinate bonds to halide ions on adjacent dimers.37 Scheme 4. Stacking diagram for Cu2Br4(pyridine)2. The position of the coordinated nitrogen atoms are indicated by the black dots.

The 2bp ligand in the two newly reported structures behaved as a molecular bender. This explanation is supported by the structural and magnetic data. The two reported structures in this article can be considered as coordination polymers of dimeric and trimeric repeating units. The Cu···Cl semi-coordinate bonds in the two structures are relatively short (Cu···Cl ≈ 2.67 Å). In a previously reported copper trimer the length of the Cu···Cl interaction was ≈ 3 Å.43

The

shortness of the Cu···Cl semi-coordinate bonds may be attributed to the fact that the Cu···(π)Br interactions are weaker than Cu···Cl semi-coordinate bonds; it is known that as one of the bonds becomes weaker in a molecule the others may become stronger. This indicates that the observed semi-coordinate bonds in the two reported structures are fairly strong to be considered as a 23 ACS Paragon Plus Environment


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coordinate bond, which makes our assumption that these two coordination polymers are indeed bent polymers rather than separated dimeric and trimeric units justified. The 2bp ligand behaved as “a molecular bender”. This can be attributed to the nature of 2bp as a bidentate ligand (vide supra). It is covalently bound to cupric ion from one side, and through supramolecular interaction from the other side. This behavior is not expected in classical bidentate ligands due to the fact they are bonded to the metal ion center through two rigid covalent bonds. The magnetic data of the dimer is better fit as alternating antiferromagnatic chain rather than a dimer model. The electrophilic nature of the bromine atom is also observed in the trimer structure. The trimer chains are linked via bromine···chloride synthons to form a layer structure. Bromine···chloride synthons are characterized by Br···Cl distances less than the sum of the rvdw of bromine and chlorine atom and a nearly linear C-Br···Cl- angle; this agrees with the observed values in the polytrimer structure. The Br···Cl- distance is 0.19 Å less than sum of rvdw and the C-Br···Cl- angle equals 157.8°. In contrast, the Br···Cl- synthons are absent in the polydimer structure.

Conclusion. The analysis of the crystal structures of the two reported coordination polymers reported in this paper indicated that 2bp behaves as molecular bender. The analysis of the crystal structures of the two repeat units within the polydimer and polytrimer coordination polymers indicates that a carbon-bonded bromine atom can behave as a nucleophile and electrophile in the same molecule. This agrees with calculated electrostatic potential surface of the 2bp ligand and the molecular orbital analysis; the calculation indicated that the bonding between the 2bp lignad and Cu(II) ions is due the interaction of the HOMO-1 molecular orbital of the 2bp with orbitals of the Cu(II) ions, electrostatically, there is an interaction between the negative electrostatic potential ring around the π-region of organic bromine atom and the positively charged Cu(II) ion. This makes carbon bound halogen atoms a significant synthon in supramolecular synthesis. The interaction of an organic bromine atom with nucleophiles (C-Br···Cl ) is observed only in the polytrimer complex presented here (Br···Cl distance = 3.406Å and C-Br···Cl =157.6°), whereas, the interaction of the organic bromine atoms with the Cu(II) ions (Cu···(π)Br) is observed in both structures. The Cu···(π)Br interaction is a fairly strong one, and converts 2bp from a monodentate ligand to a bi-dentate one via the N-Cu bond and the weaker Cu···(π)Br interaction. The 24 ACS Paragon Plus Environment

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Cu···(π)Br interaction results in the plane of the aromatic 2bp being perpendicular to the copper ion coordination plane. Viewing of the reported structures in this paper as coordination polymers rather than separate dimeric and trimeric units is supported by (a) the semi-coordinate bond distances Cu···Cl is quite short (avg. 2.672Å). (b) Also, the magnetic data of polydimer is better fit as alternating antiferromagnetic chain rather than a separate dimer.

Supporting Information Available: Magnetic data analysis of the polytrimer, powder x-ray pattern of the monomer and,polytrimer, the crystal data for polydimer and polytrimer.This information is available free of charge via the Internet at http://pubs.acs.org/. References. (1)

Jagarlapudi, A. R.; Sarma, P.; Desiraju, G. R. Acc. Chem. Res. 1986, 19,

222. (2) Halogen Bonding Fundamentals and Applications Metrangolo, P.; Resnati, G., Eds.; Springer: Heidelberg, 2008; Vol. 126. (3) Pigge, F. C.; Vangala, V. R.; Swenson, D. C. Chem. Commun. 2006, 2123. (4) Awwadi, F. F.; Taher, D.; Haddad, S. F.; Turnbull, M. M. Cryst. Growth Des. 2014, 2014, 1961. (5) Awwadi, F. F.; Twamley, B.; Willett, R. D. Cryst. Growth Des. 2011, 11, 5316. (6) Awwadi, F. F.; Willett, R. D.; Haddad, S. F.; Twamley, B. Cryst. Growth Des., 2006, 6, 1833. (7) Awwadi, F. F.; Willett, R. D.; Twamley, B. Cryst. Growth Des. 2007, 7, 624. (8) Brammer, L. Chem. Soc. Rev. 2004, 33, 476. (9) Brammer, L.; Espallargas, G.; Libri, S. CrystEngComm 2008, 10, 1712. (10) Brammer, L.; Espallargas, G. M.; Adams, H. CrystEngComm 2003, 5, 343. (11) Espallargas, G.; Zordan, F.; Marin, L.; Adams, H.; Shankland, K.; Streek, J.; Brammer, L. Chem. Eur. J. 2009, 15, 7554. (12) Ormond-Prout, J. E.; Smart, P.; Brammer, L. Cryst. Growth Des. 2012, 12, 205. (13) Willett, R. D.; Awwadi, F. F.; Butcher, R.; Haddad, S. F.; Twamley, B. Cryst. Growth Des., 2003, 3, 301. (14) Zordan, F.; Brammer, L. Acta Crystallogr.,Sect. B:Struct.Sci. 2004, 60, 512. (15) Zordan, F.; Brammer, L.; Sherwood, P. J. Am. Chem. Soc. 2005, 127, 5979. 25 ACS Paragon Plus Environment


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7, 350. (17) Rosokha, S. V.; Stern, C. L.; Ritzert, J. T. Chem. Eur. J. 2013, 19, 8774. (18) Rosokha, S. V.; Vinakos, M. K. Phys. Chem. Chem.Phys. 2014, 16, 1809. (19) Awwadi, F. F.; Willett, R. D.; Peterson, K.; Twamley, B. J. Phys. Chem. A 2007, 111, 2319. (20) Brisdon, A. K. Annu. Rep. Prog. Chem., Sect. A 2007, 103, 126. (21) Bosch, E.; Barnes, C. L. Cryst. Growth Des. 2002, 2, 299. (22) Politzer, P.; Murray, J. S.; Clark, T. Phys. Chem. Chem. Phys. 2010, 12, 7748. (23) Espallargas, G.; Brammer, L.; Sherwood, P. Angew. Chem., Int. Ed. Engl. 2006, 45, 435. (24) Abate, A.; Brischetto, M.; Cavallo, G.; Lahtinen, M.; Metrangolo, P.; Pilati, T.; Radice, S.; Resnati, G.; Rissanen, K.; Terraneo, G. Chem. Commun. 2010, 46, 2724. (25) Meazza, L.; Marti-Rujas, J.; Terraneo, G.; Castiglioni, C.; Milani, A.; Pilati, T.; Metrangolo, P.; Resnati, G. CrystEngComm 2011, 13, 4427. (26) Marti-Rujas, J.; Meazza, L.; Lim, G. K.; Terraneo, G.; Pilati, T.; Harris, K. D. M.; Metrangolo, P.; Resnati, G. Angew. Chem., Int. Ed. Engl. 2014, 52, 134444. (27) Awwadi, F. F.; Willett, R. D.; Peterson, K. A.; Twamley, B. Chem. Eur. J. 2006, 12, 8952. (28) Awwadi, F. F.; Willett, R. D.; Twamley, B. J. of Mol. Struct. 2009, 918, 116. (29) Awwadi, F. F.; Haddad, S. F.; Willett, R. D.; Twamley, B. Cryst. Growth Des. 2010, 10 158. (30) XSCANS, Siemen Analytical X-ray Instrument, Inc., Version 2.00, Madison, WI, USA, 1993. (31) SHELXTL (XCIF, XL, XP, XPREP, XS), version 6.10, Bruker AXS Inc.: Madison, WI, 2002. (32) SMART, version 5.625, Bruker AXS Inc.: Madison, WI, 2002. . (33) SAINTPlus, v. 6.22, Bruker AXS, Inc.: Madison, WI, 2001. (34) SADABS 2.03; Bruker AXS Inc.: Madison, WI., 2001. (35) Awwadi, F. F.; Willett, R. D.; Peterson, K. A.; Twamley, B. Chem. Eur. J. 2006, 12, 8952. (36) Bond, M. R.; Place, H.; Wang, Z.; Willett, R. D.; Liu, V.; Grigereit, T.; Drumheller, J.; Tuthill, G. Inorg. Chem. 1995, 34, 3134. (37) Swank, D. D.; Willett, R. D. Inorg. Chem. 1980, 19, 2321. (38) Bondi, A. J. Phys. Chem. A 1964, 68, 441. (39) Kahn, O. Molecular Magnetism; VCH: NewYork, NY, 1993. (40) Thompson, L. K.; Waldmann, O.; Xu, Z. Coord. Chem. Rev. 2005, 249, 2677. (41) 26 ACS Paragon Plus Environment

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Frisch, M.J.: Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Montgomery, J.A. Jr.; Vreven, T.; Kudin, K.N.; Burant, J.C.; Millam, J.M.; Iyengar, S.S.; Tomasi , J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J.E.; Hratchian, H.P.; Cross, J.B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.E.; Yazyev, O.; Austin, A.J.; Cammi, R.; Pomelli, C.; Ochterski, J.W.; Ayala, P.Y.; Morokuma, K.; Voth, G.A.; Salvador, P.; Dannenberg, J.J.; Zakrzewski V.G.; Dapprich, S.; Daniels, A.D.; Strain, M.C.; Farkas, O.; D Malick,.K.; Rabuck, A.D.; Raghavachari, K.; Foresman, J.B.; Ortiz, J.V.; Cui, Q.; Baboul, A.G.; Clifford, S.; Cioslowski, J.; Stefanov, B.B.; Liu, G.; iashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R.L.; Fox, D.J.; Keith, T.; Al- Laham, M.A.; Peng, C.Y.; Nanayakkara, A.; Challacombe, M.; Gill, P.M.W.; Johnson, B.; Chen, W.; Wong, M.W.; Gonzalez, C.; Pople, J.A.; Gaussian 03, Revision D.01., Gaussian, Inc., Pittsburgh, PA, 2003. (42) Awwadi, F. F.; Haddad, S. F.; Turnbull, M. M.; Landee, C. P.; Willett, R. D. CrystEngComm 2013, 15, 3111. (43) Zordan, F.; Espallargas, G. M.; Brammer, L. CrystEngComm 2006, 8, 425. ··



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TOC Synopsis The Dual Behavior of Bromine Atoms in Supramolecular Chemistry; The Crystal Structure and Magnetic Properties of Two Copper(II) Coordination Polymers. Firas F. Awwadi,a, * Roger D. Willett,b Brendan Twamley, c Mark M. Turnbulld and Christopher P. Landeee a

Department of Chemistry, The University of Jordan , Amman11942, Jordan.

b

Department of Chemistry, Washington State University, Pullman, WA 99164, USA

c

School of Chemistry, Trinity College Dublin, Dublin 2, Ireland.dCarlson School of Chemistry and Biochemistry, Clark University, 950 Main St., Worcester, MA 01610, USA. e Department of Physics, Clark University, Worcester, MA 01610, USA Two Copper(II) chloride coordination polymers were prepared, the repeating units are Cu2Cl4(2bp)2 (dimer) and Cu3Cl6(2bp)2 (trimer), 2bp = 2-bromopyridine.The polymers (polydimer and poly trimer) are viewed as bent polymers. 2bp plays the role of a molecular bender; the π-region of bromine atom interacts with copper(II) center. This is supported by the crystal structure and magnetic data.


Dual Behavior of Bromine Atoms in ... - ACS Publications

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