Solvated Square-Planar Ternary Copper(II) Complexes: Solvent

Two ternary copper(II) complexes having the general formula [Cu(pyp)X] with the ..... The third quarter saw specialties trend higher amid strong deman...
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CRYSTAL GROWTH & DESIGN

Solvated Square-Planar Ternary Copper(II) Complexes: Solvent-Dependent Zipper and Columnar Structures

2006 VOL. 6, NO. 9 2103-2108

Sunirban Das, Simi Alathady Maloor, Satyanarayan Pal, and Samudranil Pal* School of Chemistry, UniVersity of Hyderabad, Hyderabad 500 046, India ReceiVed May 25, 2006; ReVised Manuscript ReceiVed June 13, 2006

ABSTRACT: Two ternary copper(II) complexes having the general formula [Cu(pyp)X] with the tridentate Schiff base 2-N-(picolinylidene)phenol (Hpyp) and halide (X- ) Cl-, Br-) as the ancillary ligand have been synthesized and characterized. Both complexes exhibit solvatomorphism due to cocrystallization with different solvent molecules. Two cases have been investigated: (i) the dihydrated species [Cu(pyp)Cl]‚2H2O (1) and [Cu(pyp)Br]‚2H2O (2), isolated by slow evaporation of aqueous methanol solution of the complexes, and (ii) the corresponding monomethanolic forms [Cu(pyp)Cl]‚CH3OH (3) and [Cu(pyp)Br]‚ CH3OH (4), obtained by crystallization of 1 and 2 from dry methanol. To investigate the crystal compositions and packing features, single-crystal X-ray diffraction measurements as well as thermogravimetric and differential scanning calorimetric measurements have been carried out. In the context of structural features, the hydrated forms and also the monomethanolic forms are isomorphic. The dihydrated form shows a zipperlike infinite chain structure through hydrogen bonding with the water molecules and π‚‚‚π interactions. The parallel zippers are again connected to each other through hydrogen bonding between the water molecules to give a two-dimensional sheet structure. In contrast, the methanol-containing species form cyclic hydrogen-bonded dimeric host-guest units which are π-stacked to one-dimensional columnar structures. Introduction The formation of multistranded structural motifs by selfassembly of comparatively lower strands is very common in biological systems. Among these, the zipper motif is of fundamental importance for self-replication, fiber behavior, and formation of functional multicomponent complexes.1-5 In the past decade, chemists have prepared synthetic zipper systems from oligomeric nucleic acid derivatives,6 polypeptides or amides,7,8 steroid derivatives,9 metalloporphyrins,10 and coordination polymers.11-16 Such zippers are nevertheless architecturally conventional. More unconventional are the elegant nonoligomeric supramolecular zipper systems of small organic molecules or metal-organic building blocks. The self-assembly of nonoligomeric building blocks into a double-stranded zipperlike structure is thus more akin to the closure or the interlocking of the teeth of a man-made zipper (Figure 1a). Such supramolecular zippers are undoubtedly harder to design than the oligomeric ones. In particular, self-organization through hydrogen bonding poses the problem of lack of interpenetration of the molecular teeth, which results in the formation of a ladder structure instead of a zipper structure (Figure 1b). On the other hand, molecular teeth which are stacking motifs can be well interpenetrating (Figure 1c) to form the closed-zipper structure. Metal coordination complexes containing 2,2′-bipyridine, 1,10-phenanthroline, and terpyridine as ligands are well-known to form zipper structures through “aryl embrace” or multiple π-stacking interactions.12-14 The extended zipper structures of nonoligomeric components with noncoordinating backbones are very rare in the literature. In the following account, we will present a pair of isomorphous hydrated platelike copper(II) complexes (Figure 1d) which form a zipper architecture via hydrogen bonding and π-stacking. The same pair of complexes containing methanol instead of water are again isomorphous. However, they show only an asymmetric columnar packing due to π-π interaction and fail to form the zipper structure due to * To whom correspondence should be addressed. Tel: (+91) 40-23134756. Fax: (+91) 40-2301-2460. E-mail: [email protected].

Figure 1. (a) A zipper. (b) Hydrogen bonding and ladder structure. (c) Zipper structure via π-stacking. (c) Chemical diagram of [Cu(pyp)X].

the lesser number of hydrogen bond donor sites in methanol compared to that in water. Experimental Section Materials. The Schiff base Hpyp was prepared in ∼80% yield by the condensation of 1 mol equiv each of 2-pyridinecarboxaldehyde and 2-aminophenol in boiling methanol. All other chemicals and solvents used in this work were of analytical grade available commercially and were used without further purification. Physical Measurements. Microanalytical (C, H, N) data were obtained with a Thermo Finnigan Flash EA1112 elemental analyzer. The infrared spectra were recorded by using KBr pellets on a Jasco5300 FT-IR spectrophotometer. Solution electrical conductivities were measured with a Digisun DI-909 conductivity meter. A Shimadzu 3101PC UV/vis/near-IR spectrophotometer was used to record the electronic spectra. Differential scanning calorimetric measurements were per-

10.1021/cg060305a CCC: $33.50 © 2006 American Chemical Society Published on Web 07/19/2006

2104 Crystal Growth & Design, Vol. 6, No. 9, 2006

Das et al.

Table 1. Crystallographic Data for [Cu(pyp)Cl]‚2H2O (1), [Cu(pyp)Br]‚2H2O (2), [Cu(pyp)Cl]‚CH3OH (3), and [Cu(pyp)Br]‚CH3OH (4) chem formula formula wt temp (K) cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z µ (mm-1) Fcalcd (g cm-3) no. of rflns collected no. of unique rflns no. of rflns (I g 2σ(I)) no. of params R1, wR2 (I g 2σ(I)) R1, wR2 (all data) goodness of fit (F2) largest peak/hole (e Å-3)

1

2

3

4

CuC12H13N2O3Cl 332.23 100 monoclinic P21/m 9.5269(7) 6.4027(5) 10.5813(8) 90 99.913(1) 90 635.80(8) 2 1.932 1.735 7385 1627 1533 121 0.0340, 0.0824 0.0367, 0.0840 1.099 0.738/-0.351

CuC12H13N2O3Br 378.87 100 monoclinic P21/m 9.5793(8) 6.4040(5) 10.7646(9) 90 100.534(1) 90 649.23(9) 2 4.763 1.927 7574 1706 1528 121 0.0285, 0.0695 0.0334, 0.0722 1.061 0.905/-0.399

CuC13H13N2O2Cl 328.24 298 triclinic P1h 6.9205(17) 9.2257(9) 10.4996(14) 100.529(10) 97.798(17) 92.593(13) 651.29(19) 2 1.879 1.674 3781 3781 2975 176 0.0315, 0.0754 0.0482, 0.0820 1.028 0.313/-0.365

CuC13H13N2O2Br 372.70 298 triclinic P1h 6.8991(17) 9.3473(9) 10.6527(15) 100.277(10) 98.699(15) 91.912(13) 666.9(2) 2 4.631 1.856 3045 3045 2658 176 0.0313, 0.0883 0.0385, 0.1004 1.164 0.577/-0.849

formed on a Mettler Toledo DSC 822e module, and thermogravimetric measurements were performed on a Mettler Toledo TGA/SDTA 851e module. The typical sample size was 4-6 mg for DSC and 9-12 mg for TGA. A SQUID magnetometer was used for the magnetic susceptibility measurements in the temperature range 2-300 K. Diamagnetic corrections, calculated from Pascal’s constants,17 were used to obtain the molar paramagnetic susceptibilities. [Cu(pyp)Cl]‚2H2O (1). A methanol solution (15 mL) of CuCl2‚ 2H2O (17.0 mg, 0.1 mmol) was added to a methanol solution (30 mL) of KOH (6.0 mg, 0.1 mmol) and Hpyp (20.0 mg, 0.1 mmol). The mixture was stirred in air at room temperature for 30 min. The red solution was then refluxed for 10 min and left at room temperature for slow evaporation. Red-brown crystals that formed after 2-3 days were collected by filtration. The yield was 24 mg (74%). Anal. Calcd for CuC12H13N2O3Cl: C, 43.38; H, 3.94; N, 8.43. Found: C, 43.12; H, 3.89; N, 8.01. Electronic spectrum in CHCl3 (λmax (nm) ( (M-1 cm-1))): 547 (7840), 363 (8830), 306 sh (9240), 285 (9960). [Cu(pyp)Br]‚2H2O (2). This complex was prepared in 77% yield from CuBr2, Hpyp, and KOH (1:1:1 mole ratio) by following the same procedure as described for 1. Anal. Calcd for CuC12H13N2O3Br: C, 38.26; H, 3.48; N, 7.44. Found: C, 38.14; H, 3.37; N, 7.27. Electronic spectrum in CHCl3 (λmax (nm) ( (M-1 cm-1))): 551 (7570), 363 (8770), 289 (13 290). [Cu(pyp)Cl]‚CH3OH (3). 1 was dissolved in dry methanol and kept at 35 °C under a moisture-free atmosphere overnight. The brown needleshaped crystals of 3 that deposited were collected by filtration. The yield was 63%. Electronic spectrum in CHCl3 (λmax (nm) ( (M-1 cm-1))): 547 (7910), 363 (8950), 306 sh (9370), 285 (10 060). [Cu(pyp)Br]‚CH3OH (4). Crystals of 4 were obtained from 2 by following the same procedure as described for 3. The yield was 69%. Electronic spectrum in CHCl3 (λmax (nm) ( (M-1 cm-1))): 551 (7640), 363 (8870), 289 (13 410). X-ray Crystallography. Unit cell parameters and the intensity data for 1 and 2 were obtained on a Bruker-Nonius SMART APEX CCD single-crystal diffractometer, equipped with a graphite monochromator and a Mo KR fine-focus sealed tube (λ ) 0.710 73 Å) operated at 2.0 kW. Data were collected at 100 K with a scan width of 0.3° in ω and an exposure time of 10 s/frame. The SMART software was used for data acquisition, and the SAINT-Plus software was used for data extraction.18 In each case, an absorption correction was performed with the help of the SADABS program.19 X-ray data for 3 and 4 were collected on an Enraf-Nonius Mach-3 single-crystal diffractometer using graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å) by the ω-scan method at 298 K. In each case, the ψ scans20 of selected reflections were used for an empirical absorption correction. The programs of the WinGX21 package were used for data reduction and absorption correction. The structures were solved by direct methods and refined on F2 by full-matrix least-squares procedures. All nonhydrogen atoms were refined anisotropically. In all of the cases, the

protons of the solvent molecules were located in the corresponding difference maps and refined with geometric and thermal restraints. For 1 and 2 the water molecule found in the asymmetric unit is disordered. A pseudo-2-fold axis passes through the O atom and the single fully occupied H atom, while the other H atom is disordered in two positions around this 2-fold axis. The rest of the H atoms in all the structures were included at idealized positions by using a riding model. The SHELX-9722 programs were used for structure solution and refinement. The ORTEX6a23 and Platon24 packages were used for molecular graphics. Significant crystallographic data are summarized in Table 1.

Results and Discussion Synthesis and Characterization. The dihydrated complexes [Cu(pyp)Cl]‚2H2O (1) and [Cu(pyp)Br]‚2H2O (2) are prepared in moderate yields by reacting 1 mol equiv each of the corresponding copper(II) halide, Hpyp, and KOH in methanol. The elemental analysis data are consistent with their empirical formulas. The methanol-containing species [Cu(pyp)Cl]‚CH3OH (3) and [Cu(pyp)Br]‚CH3OH (4) are obtained by fast crystallization of 1 and 2 from dry methanol. None of the solvated complexes are electrically conducting in methanol solutions. The room temperature (300 K) effective magnetic moments of 1-4 are in the range 1.73-1.80 µB. These values are consistent with an S ) 1/2 spin ground state expected for d9 systems. Thus, the copper is in a +2 oxidation state in each complex. In the infrared spectrum, the free Schiff base Hpyp displays a medium-intensity band near 3360 cm-1 due to the phenolic OH. The absence of this band in the spectra of the solvated complexes indicates deprotonation and coordination of the phenolate O. However, the solvated species display a broad band centered at ∼3460 cm-1, possibly due to solvent -OH group stretching.25 The CdN stretch for the complexes is observed near 1585 cm-1. The low-energy shift (by ∼39 cm-1) of this band compared to that of Hpyp indicates metal coordination by the N atom of the -CHdN- fragment of pyp-. The electronic spectra were recorded using the chloroform solutions of all the solvated species. The spectral profiles are very similar. A strong absorption is observed within 547-552 nm and several closely spaced intense absorptions appear in the range 370-280 nm. The lower energy absorption is most likely due to the ligand-to-metal charge transfer, and the higher energy bands are due to ligand-based transitions.26

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Crystal Growth & Design, Vol. 6, No. 9, 2006 2105

Figure 2. Thermogravimetric plots for (a) [Cu(pyp)Br]‚2H2O (2) and (b) [Cu(pyp)Cl]‚CH3OH (3).

Thermal Studies. The thermogravimetric analysis and differential scanning calorimetric measurements of all the solvated species under flowing nitrogen gas were performed in the temperature range 30-400 °C. In the cases of 3 and 4, the endothermic desolvation starts at ∼51-55 °C and is completed at ∼96-99 °C, whereas for 1 and 2 it starts at ∼54-57 °C and is completed at ∼99-112 °C (Figure 2). The observed weight losses of 10.69, 9.33, 9.61, and 8.32% correspond well to the calculated values of 10.84, 9.56, 9.76, and 8.60% for the two water molecules and single methanol molecule per formula unit of 1-4, respectively. Upon further heating beyond ∼180 °C, the solvent-free residue starts decomposing, which continues beyond 400 °C. The differential scanning calorimetric measurements also correspond well with the thermogravimetric analysis data. Magnetic and EPR Spectral Properties. The magnetic susceptibilities of 1 and 2 in the powdered form were measured in the temperature range 2-300 K at a constant magnetic field of 5 kG. On cooling, there is no significant change in the moment values. At 2 K the moments are ∼1.76 µB. In each case, a linear plot is obtained when the inverse molar susceptibilities are plotted against temperature with a small negative intercept on the temperature axis (Figure 3). The data were fit using the expression for the Curie-Weiss law. The Curie (C) and Weiss (Θ in K) constants are very similar for both of the species (0.41 and -0.1 for 1 and 0.40 and -0.1 for 2). The values of the Weiss constants clearly indicate essentially the Curie-paramagnetic nature of both 1 and 2. The room-temperature (300 K) X-band EPR spectral profiles of 1-4 in the powder phase are very similar. The spectra display a strong axial signal (g| ≈ 2.2 and g⊥ ≈ 2.0) typical of squareplanar copper(II) species (Figure 4). Interestingly a very weak signal at g ≈ 4.2 is observed in all the spectra (Figure 4). The high-field strong signal is assigned to the ∆MS ) (1 transition. The weak signal at the low-field region is perhaps due to the ∆MS ) (2 transition commonly observed for weakly spin coupled copper(II) species.27,28 The phenomenon of metal ion spin exchange via intermolecular noncovalent interactions is not

Figure 3. Inverse molar magnetic susceptibility as a function of temperature for (a) [Cu(pyp)Cl]‚2H2O (1) and (b) [Cu(pyp)Br]‚2H2O (2).

Figure 4. EPR spectrum of [Cu(pyp)Cl]‚2H2O (1) in powder form at 300 K. Inset: the magnified ∆MS ) (2 region.

unknown.29 Recently we have observed similar EPR spectral features for some copper(II) systems which are involved in intermolecular noncovalent interactions in the solid state.30 Description of Molecular Structures. The dihydrated species 1 and 2 crystallize in the space group P21/m. In each case, the asymmetric unit contains half of [Cu(pyp)X)] (X- ) Cl- (1), Br- (2)) molecule and a water molecule. The complex molecule in each of 1 and 2 is perfectly planar as it resides on the crystallographic mirror plane. In contrast, the methanolinterspersed isomorphous pairs 3 and 4 crystallize in the space group P1h. In both cases, the asymmetric units contain a full molecule of [Cu(pyp)X] and a methanol molecule. The overall molecular structures of [Cu(pyp)X] and the bond parameters for the chloride-ligated complex and those of the bromidecoordinated complex in all of the solvated species are very similar. The representative molecular structure of [Cu(pyp)Cl] in 1 is depicted in Figure 5, and the selected bond parameters

2106 Crystal Growth & Design, Vol. 6, No. 9, 2006

Das et al. Table 3. Geometrical Parameters for Intermolecular Hydrogen Bonds compd

interaction

d (Å)

D (Å)

θ (deg)

1

O(2)-H‚‚‚O(1)a O(2)-H‚‚‚O(2)b O(2)-H‚‚‚O(2)c O(2)-H‚‚‚O(1)a O(2)-H‚‚‚O(2)b O(2)-H‚‚‚O(2)d O(2)-H‚‚‚O(1) C(7)-H‚‚‚O(2)e O(2)-H‚‚‚O(1) C(7)-H‚‚‚O(2)e

1.87(1) 1.80(2) 1.89(4) 1.92(2) 1.81(2) 1.91(6) 2.06(3) 2.48(2) 2.11(5) 2.34(3)

2.814(3) 2.736(5) 2.738(6) 2.847(3) 2.748(7) 2.736(7) 2.810(2) 3.313(3) 2.824(3) 3.303(4)

169(4) 165(7) 147(7) 162(4) 166(8) 143(8) 169(4) 155(2) 163(6) 157(3)

2

3 4

Symmetry transformation: x, 1 + y, z. b Symmetry transformation: x, - y, z. c Symmetry transformation: -x, 2 - y, -z. d Symmetry transformation: -x, 2 - y, 1 - z. e Symmetry transformation: 1 - x, -y, -z. a

Figure 5. Structure of [Cu(pyp)Cl] with the atom-labeling scheme. All non-hydrogen atoms are represented by their 40% probability thermal ellipsoids. Table 2. Selected Bond Lengths (Å) and Angles (deg) 1

3

Cu-O(1) Cu-N(1) Cu-N(2) Cu-Cl Cu-Br

1.945(2) 1.946(2) 2.019(3) 2.1973(8)

1.9538(14) 1.9481(16) 2.0258(17) 2.1928(7)

O(1)-Cu-N(1) O(1)-Cu-N(2) N(1)-Cu-N(2) O(1)-Cu-Cl N(1)-Cu-Cl N(2)-Cu-Cl O(1)-Cu-Br N(1)-Cu-Br N(2)-Cu-Br

83.23(9) 164.07(10) 80.84(10) 95.18(7) 178.41(7) 100.74(8)

83.39(6) 163.65(6) 80.30(7) 97.24(5) 177.89(5) 99.10(5)

2

4

1.947(2) 1.958(3) 2.016(3)

1.950(2) 1.945(2) 2.028(2)

2.3355(5)

2.3333(5)

83.31(10) 164.39(10) 81.09(11)

83.29(9) 163.91(9) 80.67(9)

94.57(7) 177.87(7) 101.04(8)

96.66(6) 178.25(6) 99.41(7)

found in all the structures are given in Table 2. In each case, the tridentate pyp- coordinates to the metal ion via the phenolate O, the imine N, and the pyridine N atoms, forming two fivemembered chelate rings. The halide ion occupies the fourth coordination site and completes an ON2X square plane around the metal center. The chelate bite angles for the five-membered rings formed by pyp- are in the range 80.29-83.39°. The CuO1(phenolate), the Cu-N1(imine), and the Cu-N2(pyridine) bond lengths (Table 2) in all of the structures are very similar. The Cu-N2(pyridine) bond lengths are slightly longer than the Cu-N1(imine) bond length. This difference is possibly due to the combined effect of π-back-bonding in the Cu-N(imine) bond being better than that in the Cu-N(pyridine) bond, the rigidity of the tridentate ligand, and the trans effect of the phenolate O.31,32 The Cu-Cl and Cu-Br bond lengths are comparable to those reported for chloride- and bromidecoordinated copper(II) species.32,33 Self-Assembly to Zipper and Columnar Motifs. Both of the complex molecules [Cu(pyp)X] (X- ) Cl-, Br-) have perfect or near-perfect square-planar geometry. Such species with sufficient π-electrons are expected to form stacked supramolecular structures.34 Since stacking interactions result in slipped assembly of the molecules, formation of either staircase type or zigzag columnar type structural motifs is very common.35 Inclusion of other functionalities on such a molecule which can participate in additional intermolecular noncovalent interactions with themselves or with the incorporated solvent molecules can lead to interesting architectural motifs. The molecules of [Cu(pyp)X] possess the hydrogen bond acceptor site phenolate O, which can be involved in hydrogen-bonding interactions if some donor groups are available in close proximity. The solvatomorphs 1-4 described in this work contain water and methanol. It may be noted that, in principle, water and methanol can

3/

2

participate in four and three hydrogen bonds, respectively. The self-assembly patterns and the resulting supramolecular architechtures of 1-4 are guided by π-stacking as well as the number of hydrogen bonds formed by the solvent molecule. In the isomorphous pair 1 and 2, the guest water molecule is connected to the phenolate O of the complex molecule and to another symmetry-related water molecule through strong hydrogen bonds. The phenolate O of each complex molecule is serving as a hydrogen bond acceptor for two water molecules on both sides. Consequently, a water dimer is trapped between two complex molecules. All of the hydrogen-bonding parameters are given in Table 3. Thus, the alternating water dimer and the planar complex molecule form a single-strand zipper structure, where the distances between the parallel teeth are 6.473 and 6.404 Å for 1 and 2, respectively. Due to the dimeric water spacer, there is enough space between the teeth for selfrecognition of a similar strand through π-stacking of the perfectly planar complex molecules. As a result, an infinite zipper structure has been formed along the crystallographic b axis (Figure 6). The interplanar distances between the two inversely stacked teeth are 3.236 and 3.202 Å for 1 and 2, respectively. The Cu‚‚‚Cu distances in these linear arrangements are 6.058 Å for 1 and 6.050 Å for 2. The shortest Cg‚‚‚Cg distance involves the phenolate benzene ring and the chelate ring formed by the imine N and the pyridine N. These distances are 3.327 and 3.319 Å for 1 and 2, respectively. The parallel zippers are again connected by hydrogen bonds involving the water molecules, and a two-dimensional sheet structure is formed which is parallel to the crystallographic ab plane (Figure 6). As a result, a zigzag infinite water chain has been trapped between the one-dimensional π-stacked columns of the complex molecules. As expected, the packing pattern of the isomorphous pair 3 and 4, having the less symmetric solvent molecule methanol, is not of the type observed for the more symmetric watercontaining 1 and 2. The reduction of the symmetry and the hydrogen bond donor arm of the guest methanol molecule results in dimerization of 3 and 4 through O-H‚‚‚O and C-H‚‚‚O hydrogen bonds (Table 3) instead of a continuous hydrogenbonded single-stranded zipper structure (1 and 2). Both hydrogenbonding interactions involve the methanol molecule. The metalcoordinated phenolate O and the azomethine C-H group act as the acceptor and donor in these O-H‚‚‚O and C-H‚‚‚O interactions, respectively. The Cu‚‚‚Cu distances in these hydrogen-bonded cyclic dimeric units are 6.082 and 6.069 Å for 3 and 4, respectively. These cyclic dimeric units form a onedimensional columnlike structure due to π-stacking (Figure 7). Mainly the five-membered chelate rings and the phenolate

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Crystal Growth & Design, Vol. 6, No. 9, 2006 2107

The isomorphous [Cu(pyp)X]‚2H2O species show zipperlike structural motifs. The zipper structure is formed by the hydrogen-bonded water chain and π-stacked flat complex molecules which are hydrogen-bonded to the water chain. It may be noted that the zipperlike structural motif from nonoligomeric building blocks is not reported in the literature. On the other hand, the methanol-containing species form simple π-stacked one-dimensional columnar structures of hydrogenbonded cyclic dimers of [Cu(pyp)X]‚CH3OH units. The contrast between the supramolecular structures of the two types of solvatomorphs is primarily due to the smaller number of hydrogen bond donor sites in the less symmetric methanol molecule compared to that in the more symmetric water molecule.

Figure 6. Zipper motif of 1: (a) double strand; (b) two single strands connected by hydrogen bonds; (c) two-dimensional sheet structure from the parallel zippers.

Acknowledgment. Financial support for this work was provided by the Council of Scientific and Industrial Research (CSIR), New Delhi (Grant No. 01(1880)/03/EMR-II). S.D. and S.P. thank the CSIR for research fellowships. Our sincere thanks are due to Prof. W. Fujita for providing the cryomagnetic data. We thank Prof. A. Nangia for allowing the use of the TGA/ DSC facility. X-ray crystallographic studies were performed at the National Single Crystal Diffractometer Facility, School of Chemistry, University of Hyderabad (funded by the Department of Science and Technology, New Delhi). We thank the University Grants Commission, New Delhi, for the facilities provided under the UPE and CAS programs. Supporting Information Available: CIF files giving X-ray crystallographic data for 1-4. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 7. Columnar packing of the cyclic dimers of (a) [Cu(pyp)Cl]‚ CH3OH (3) and (b) [Cu(pyp)Br]‚CH3OH (4) due to π-π interactions.

benzene ring are participating in the π-π interactions. The interplanar distances in the solvent-mediated dimeric units are 3.338 and 3.323 Å for 3 and 4, respectively, whereas the same distances for the π-stacked pair of complex molecules are 3.239 Å for 3 and 3.238 Å for 4. Interestingly, the Cu‚‚‚Cu distances (4.473 Å for 3 and 4.500 Å for 4) in the π-stacked side are much shorter than those in the hydrogen-bonded side. This difference indicates that in the hydrogen-bonded dimeric unit the complex molecules are laterally more slipped than the complex molecules that are involved in the π-π interaction. Conclusion The characterization and structural analyses of the solvatomorphs [Cu(pyp)X]‚2H2O and [Cu(pyp)X]‚CH3OH (X- ) Cl-, Br-), containing perfectly to near perfectly planar [Cu(pyp)X] molecules, have been described. In all of the cases, the planar complex molecules form one-dimensional π-stacked columnar structures. However, the final supramolecular architechture is decided by the solvent molecule present in the crystal lattice.

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