Ligand Concentration Controlled Supramolecular Isomerism in Two

Crystal Growth & Design , 2007, 7 (1), pp 64–68 ... Reactions of CuCl2, 4-pyridylthiol, and NH4SCN in mixed acetonitrile and ... For a more comprehe...
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Ligand Concentration Controlled Supramolecular Isomerism in Two CuSCN Based Coordination Polymers with in Situ Synthesized 4,4′-Dipyridylsulfide as a Co-Ligand Zheng-Ming Hao and Xian-Ming Zhang*

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 1 64-68

School of Chemistry & Material Science, Shanxi Normal UniVersity, Linfen, Shanxi 041004, China ReceiVed June 18, 2006; ReVised Manuscript ReceiVed October 9, 2006

ABSTRACT: Reactions of CuCl2, 4-pyridylthiol, and NH4SCN in mixed acetonitrile and water solutions at 140 °C and 96 h led to two polymorphs of CuSCN coordination polymers with stoichiometry [Cu2(SCN)2(dps)]∞ (dps ) 4,4′-dipyridyl sulfide). The dps ligand is generated in situ via desulfation coupling of 4-pyridinethiol during the hydrothermal treatment. Polymorph 1 is a twodimensional (2D) tubular sheet constructed by an unprecedented [CuSCN]∞ column and dps ligand. The [CuSCN]∞ column can be described as two zigzag chains arranged with approximate C2v symmetry, one chain being connected to the other by Cu-S contacts. An alternative description of the [CuSCN]∞ column is the stacking of [CuSCN]2 dimers, alternately rotated by 90° and linked by Cu-N contacts. Polymorph 2 is a 2D planar sheet constructed by [Cu(SCN)]∞ staircase chains and dps ligands. The [CuSCN]∞ staircase can be viewed as an alternating fusion of four-membered Cu-S-Cu-S rings and eight-membered Cu-S-C-N-CuS-C-N rings. The two polymorphs are very rare examples of coordination polymers that exhibit a similar local coordination geometry of metal and the same topology but different CuSCN structural motifs. Interestingly, 1 and 2 show quite different photoluminescent properties: the former has emission maxima at 538 nm assigned to a ligand-centered excited state, while the latter shows emission maxima at 636 nm tentatively attributed to both ligand-to-metal and metal-centered transitions. Introduction The term supramolecular isomerism was coined by Zaworotko to describe the existence of more than one type of network superstructure formed from the same building blocks.1 “In some instances, supramolecular isomerism can be a consequence of the effect of the same molecular components generating different supramolecular synthons and could be synonymous with polymorphism. However, in other situations, supramolecular isomerism is the existence of different architectures or superstructures. In this context, the presence of guest or solvent molecules that do not directly participate in the network itself, especially in open framework structures, is important to note as it means that polymorphism represents an inappropriate term to describe the superstructural differences between network structures.”1 Polymorphism in coordination polymers can be regarded as being a type of supramolecular isomerism but not necessarily vice versa. While some supramolecular isomers have been reported recently, many of them are not polymorphs for their different chemical compositions caused by the coexistence of different guest components.2-5 Thus, polymorphism is particularly rare for coordination polymers,6-8 in spite of its paramount importance in the pharmaceutical industries.9 In the continuing studies of coordination polymers of Cu(I) halides or pseudo-halides, we and others have found that in situ a desulfation reaction sometimes occurs via hydro(solvo)thermal cleavage of S-C bonds.10,11 In particular, we synthesized a 12-connected Cu6S4 cluster based coordination framework [Cu3(4-pyridinethiolate)2(CN)] by this method, during which the starting materials (4pyridylthio)acetic acid and thiocyanate have been converted into 4-pyridylthiolate and cyanide via cleavage of S-C bonds.10a As an extension of our coordination polymers of Cu(I) pseudohalides and hydro(solvo)thermal in situ desulfation reaction, we report herein two polymorphs of CuSCN-based coordination polymers formulated as [Cu2(SCN)2(dps)]∞ 1 and 2 (dps ) 4,4′-dipyridyl sulfide). * To whom correspondence should be addressed. Fax & Tel : Int. code +86 357 2051402; e-mail: [email protected].

Materials and Methods All the starting materials were purchased commercially as reagent grade and used without further purification. Elemental analyses were performed on a Perkin-Elmer 240 elemental analyzer. The FTIR spectra were recorded from KBr pellets in range 400-4000 cm-1 on a Nicolet 5DX spectrometer. XRPD data were recorded in a Bruker D8 ADVANCE diffractometer. Thermal analysis (TG) was carried out in a nitrogen stream using SETARAM LABSYS equipment with a heating rate of 10 °C/min. Photoluminescence analyses were performed on an Edinburgh FLS920 luminescence spectrometer. [Cu2(SCN)2(dps)]∞ (1). A mixture of CuCl2‚2H2O (0.068 g, 0.4 mmol), 4-pyridylthiol (0.022 g, 0.2 mmol), NH4SCN (0.015 g, 0.2 mmol), CH3CN (3 mL), and water (2 mL) in a molar ratio of 2:1:1: 288:555 was sealed in a 15 mL Teflon-lined stainless container, which was heated to 140 °C and held for 96 h. After cooling of the sample to room temperature, yellowish rod-like crystals of 1 were obtained with the yield of 35%. Anal. calcd for 1, C12H8Cu2N4S3: C, 33.40; H, 1.87; N, 12.98. Found: C, 33.27; H, 1.91; N, 12.92. IR (KBr): 3010w, 2113m, 1624m, 1566m, 1403s, 1124m, 809w, 727w, 611w. [Cu2(SCN)2(dps)]∞ (2). A mixture of CuCl2‚2H2O (0.068 g, 0.4 mmol), 4-pyridylthiol (0.033 g, 0.3mmol), NH4SCN (0.023 g, 0.3 mmol), CH3CN (3 mL), and water (2 mL) in a molar ratio of 2:1.5: 1.5:288:555 was sealed in a 15 mL Teflon-lined stainless container, which is heated to 140 °C and held for 96 h. After cooling of the sample to room temperature, yellowish block crystals of 2 were obtained with a yield of 30%. Anal. Calcd for 2 C12H8Cu2N4S3: C, 33.40; H, 1.87; N, 12.98. Found: C, 33.44; H, 1.85; N, 12.93. IR (KBr): 3007w, 2109s, 1632m, 1577m, 1403s, 1104m, 813w, 722w. X-ray Crystallographic Study. Data were collected at 298 K on a Bruker Apex diffractometer (Mo-KR, λ ) 0.71073 Å). Lorentzpolarization and absorption corrections were applied. The structures were solved with direct methods and refined with full-matrix leastsquares technique (SHELX-97).12 Analytical expressions of neutralatom scattering factors were employed, and anomalous dispersion corrections were incorporated. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of organic ligands were geometrically placed and refined with isotropic temperature factors. The crystallographic data are listed in Table 1; selected bond lengths and bond angles are given in Table 2.

10.1021/cg060371c CCC: $37.00 © 2007 American Chemical Society Published on Web 12/02/2006

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Table 1. Crystallographic Data for Compound 1 formula FW crystal system space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalc (g cm-3) µ, (mm-1) F(000) size (mm) reflections Tmax/Tmin data/parameters S R1a wR2b ∆Fmax/ ∆Fmin(e A-3) a

C12H8Cu2N4S3 431.48 orthorhombic Pcca 17.999(10) 8.518(5) 10.203(6) 90.000 90.000 90.000 1564.3(16) 4 1.832 3.114 856 0.26 0.18 0.09 6703/1682 0.7699/0.4982 1682/0/96 1.080 0.0635 0.1413 0.652 and -0.448

C12H8Cu2N4S3 431.48 monoclinic P2/c 9.2011(17) 5.7246(10) 14.876(3) 90 104.855(3) 90 757.4(2) 2 1.892 3.216 428 0.21 0.14 0.04 3489/1475 0.8821/0.5516 1475/0/96 0.950 0.0716 0.1961 0.739 and -0.641

R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑w(Fo2 - Fc2)2/∑w(Fo2)2]1/2.

Results and Discussion Compound 1 crystallizes in the orthorhombic space group Pcca, and the asymmetric unit consists of one copper(I) ion, one SCN, and half dps as shown in Figure 1a. The dps ligand came from in situ desulfation coupling of 4-pyridylthiol molecules during the reaction. The S(1) atom is localized on the special crystallographic position (1/2, -y, 3/4) with the site occupancy of 0.5. The Cu(1) adopts a distorted tetrahedral geometry, coordinated by one nitrogen and two sulfur atoms from three µ3-thiocyanate groups, and one nitrogen atom from dps. The Cu(1)-N distances are 1.915(7) and 2.021(5) Å, and Cu(1)-S distances are 2.433(2) and 2.455(2) Å. The L-Cu(1)-L (L ) S, N) angles are in the range of 103.29(6)-123.8(2)°. The bending angle of dps [C(3c)-S(1)-C(3)] is 105.6(4)° and the torsion angle [C(2)-C(3)-S(1)-C(3c)] is 148.1°. The N(1)-N(1c) distance is 7.57 Å. The overall structure of 1 is a two-dimensional (2D) tubular sheet constructed by [CuSCN]∞ column and dps ligands. The [CuSCN]∞ columns can be described as two zigzag chains arranged with approximate C2v symmetry, one chain being connected to the other by Cu-S contacts (Figure 1b). An alternative description of [CuSCN]∞ column is the stacking of [CuSCN]2 dimers, alternately rotated by 90° and linked by Cu-N contacts. Expressed in another way, the [CuSCN]∞ column is generated by the fusion of 4-membered Cu-S-Cu-S rings and 10-membered Cu-S-C-N-Cu-S-Cu-N-C-S rings. The adjacent [CuSCN]∞ columns are linked by bifunctional dps ligands to finish the tubular sheet (Figure 1c,d). The sheet can be described as a (4,4) topological net, in which the nodes are Cu2S2 rings. There are two types of connectors: one is dps and the other is double CN.13 Each Cu2S2 node is connected to adjacent four nodes via two dps and four CN groups. Compound 2 crystallizes in monoclinic space group P2/c, and the atoms in the asymmetric unit are the same as those in 1 (Figure 2a). The Cu(1) also adopts a distorted tetrahedral coordination geometry, coordinated by one nitrogen and two sulfur atoms from three µ3-thiocyanate groups, and one nitrogen atom from dps. The Cu(1)-N distances are 2.014(7) and 1.947(7) Å, and the Cu(1)-S distances are 2.420(2) and 2.493(3) Å. The L-Cu(1)-L (L ) S, N) angles are in the range of 102.6(2)-127.7(3)°. The bending angle of 103.0(5)° and torsion angle

Table 2. Bond Lengths (Å) and Angles (°) for 1 and 2a Cu(1)-N(2a) Cu(1)-N(1) Cu(1)-S(2) Cu(1)-S(2b) Cu(1)‚‚‚Cu(1b) C(3c)-S(1)-C(3)

Compound 1 1.915(7) N(2a)-Cu(1)-S(2) 2.021(5) N(2a)-Cu(1)-S(2b) 2.433(2) S(2)-Cu(1)-S(2b) 2.455(2) N(2a)-Cu(1)-N(1) 2.908(2) N(1)-Cu(1)-S(2) 105.6(4) N(1)-Cu(1)-S(2b)

108.3(2) 105.85(19) 103.29(6) 123.8(2) 108.01(16) 105.72(17)

Cu(1)-N(2) Cu(1)-N(1) Cu(1)-S(2a) Cu(1)-S(2b) Cu(1)-Cu(1c)3 C(3d)-S(1)-C(3)

Compound 2 1.947(7) N(2)-Cu(1)-N(1) 2.014(7) N(2)-Cu(1)-S(2a) 2.420(2) N(1)-Cu(1)-S(2a) 2.493(3) N(2)-Cu(1)-S(2b) 2.850(2) N(1)-Cu(1)-S(2b) 103.0(5) S(2a)-Cu(1)-S(2b)

127.7(3) 103.7(2) 107.7(2) 105.2(2) 102.6(2) 109.10(7)

a Symmetry codes: (a) x, -y + 1, z + 1/2; (b) -x + 3/2, -y + 1, z; (c) -x + 1, y, -z - 1/2. Symmetry codes: (a) x, y + 1, z; (b) -x + 1, -y, -z + 1; (c) -x + 1, -y + 1, -z + 1; (d) -x + 2, y, -z + 1/2.

of 147.0° for dps in 2 are only slightly smaller than those in 1. Correspondingly, the N‚‚‚N distance of 7.39 Å is also slightly shorter than that in 1. The structure of 2 is a planar 2D sheet constructed by [Cu(SCN)]∞ staircase chains and bridging dps ligands. The [CuSCN]∞ staircase chain can be described as two trans zigzag chains fused together via Cu-S contacts (Figure 2b). The [CuSCN]∞ staircase also can be generated by alternating fusion of fourmembered Cu-S-Cu-S rings and eight-membered Cu-SC-N-Cu-S-C-N rings. Similar to 1, the topology of 2 is a (4,4) net, in which the nodes are Cu2S2 rings and the connectors are dps and double CN. Each Cu2S2 node is connected to adjacent four nodes via two dps and four CN groups. For CuSCN itself, only several structural motifs are known, including the three-dimensional R14 and β15 forms, as well as the {[Cu2(SCN)3-] ∞ anionic lattice.16 For complexes of CuSCN, a range of different structural architectures such as chain, staircase, and sheet motifs have been observed.17,18 The [CuSCN]∞ staircase chain observed in 2 has been reported before for related copper thiocyanate compounds with pyridyl donor ligands, such as [Cu(SCN)(2-rnethylpyridine)],18a [Cu(SCN)(quinoline)],18b and [Cu2(SCN)2(1,2-trans-bis(4-pyridyl)ethene)].17a However, to the best of our knowledge, the observed [CuSCN]∞ column in 1 is unprecedented. Several metal complexes of dps have been documented, and most of them are synthesized by the direct reaction of metal ions and dps ligand.19 Only one example of 4,4′-thiodipyridinium perhalometallate complex is obtained via the desulfation and coupling of pyridine-4-thiol to date.20 Polymorphism, which can be considered a specific subset of supramolecular isomerism, is particularly rare for coordination polymers.6-8 In 1 and 2, the building blocks are the same, and the local coordination geometries of the Cu(I) cations are similar. Ligand dps is an angular spacer ligand with a non-rigid backbone, and its conformation can be described by parameters such as bending angle, torsion angle, and N‚‚‚N distance. It should be noted that the minor differences of bending angle, torsion angle, and N‚‚‚N distance in 1 and 2 also contribute to the structural difference of 1 and 2. To check the bulk purity, XRPD of polymorphs 1 and 2 were recorded in a Bruker D8 X-ray powdered diffractometer. As can be seen from Figure 3, recorded and simulated XRPD patterns in both 1 and 2 are quite similar, confirming the bulk purity of polymorphs. Thus, 1 and 2 represent two polymorphs of a fixed stoichiometry in coordination polymers.1 The formation of polymorphs 1 and 2 may involve many factors including small differences of coordination geometries of the metal, the metal-to-ligand ratio, and various subtle supramolecular forces. However, syntheti-

66 Crystal Growth & Design, Vol. 7, No. 1, 2007

Hao and Zhang

Figure 1. (a) The coordination environments of copper site with 35% thermal ellipsoid probability; (b) the [CuSCN]∞ column; (c) top-view; and (d) side-view of the tubular layer in 1.

Figure 2. (a) The coordination environments of copper site with 35% thermal ellipsoid probability; (b) the staircase [CuSCN]∞ chain; (c) top-view; and (d) side-view of the planar layer in 2.

cally, 1 and 2 are two polymorphs controlled by ligand concentration because the types of starting materials, the amount of CuCl2, CH3CN, and water, reaction time, and reaction temperature in the synthesis of 1 and 2 are all the same. The only differences are the amount of 4-pyridylthiol and NH4SCN, which is responsible for the formation of the two polymorphs. In the synthesis of 2, the amount of 4-pyridylthiol and NH4SCN is larger than in 1. The calculated density for 2 (1.892 g cm-3) is slightly larger than that for 1 (1.832 g cm-3), which is luckily in agreement with the ligand concentration of starting materials. It should be noted that from a topological point of

view both 1 and 2 are (4,4) topological layers constructed by Cu2S2 nodes and dps and double CN connectors, although the layer in 1 and 2 is tubular and planar, respectively. Thermogravimetry and Photoluminescence. Thermogravimetric analysis for 1 and 2 in nitrogen atmosphere and under 1 atm pressure at the heating rate of 15 °C min-1 was performed on a polycrystalline samples, which showed similar thermal stabilities of 1 and 2. This may be attributed to their similar (4,4) topological layered structures. As shown in Figure 4, almost no weight loss occurred until 200 °C, indicating good thermal stability of 1 and 2. An abrupt weight loss of ca. 44.2

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Crystal Growth & Design, Vol. 7, No. 1, 2007 67

Figure 5. Photoluminescent emissions of 1 and 2 upon excitation at 260 nm.

Figure 3. The simulated (A) and measured (B) XRPD patterns for 1 (a) and 2 (b).

with an intense and broad emission band ranging from 450 to 800 nm. LC transitions should be shifted by less than 1000 cm-1 compared to those of the free ligands and, therefore, the observed emission band in 2 is too low in energy to originate from dps ligands.22 For MLCT transitions, the antibonding π* orbitals of the ligands must be positioned at comparable energies to the metal ion orbitals to ensure a sufficient overlap for efficient transitions. The antibonding π* orbitals of dps in 2 are expected to be at too high energy; thus, MLCT emission can also be eliminated. Fortunately, we found quite similar luminescence properties including maxima and shape of emission bands for 1 and two related copper(I) thiocyanate coordination polymers [Cu2(SCN)2(2,5-dimethylpyrazine)]∞ and [Cu2(SCN)2(2,3-dimethylpyrazine)]∞.23 Similar to [Cu2(SCN)2(2,5dimethylpyrazine)]∞ and [Cu2(SCN)2(2,3-dimethylpyrazine)]∞, the transition in 2 most possibly originates from both MC and LMCT excited states. Conclusion

Figure 4. TGA curves of 1 and 2 in nitrogen atmosphere and at the heating rate of 15 °C/min.

and 44.6% occurs between the temperature range of 210-340 °C and 210-365 °C, in good agreement with that calculated for the removal of dps ligands (calc. 43.8%). The product of this reaction consists of CuSCN. As can be seen, the unstable intermediate CuSCN decomposes upon further heating, and a stable residue is not formed up to 700 °C. In the solid state, 1 and 2 show intense and broad emission bands centered at 538 and 636 nm upon photoexcitation at 260 nm (Figure 5). In general, possible assignments for the excited states which are responsible for emission phenomena of Cu(I)-complexes are ligand-centered π f π* transitions (LC), ligand-to-metal (LMCT) or metal-to-ligand (MLCT) chargetransfer transitions, or metal-centered d10 f d9s1 (MC) transitions.21 Solid-state emission properties of luminophore dps and its Ag(I) and Zn(II) complexes have been investigated by Vittal and Co-workers,19c which shows that free ligand dps displays a less intense, broad emission band centered at 536 nm compared with its Ag(I) and Zn(II) complexes. Owing to a similar energy of emission band for free dps ligand and 1, the emission band in 1 is tentatively assigned to intraligand transition. Compared with free dps and 1, the emission in 2 is red-shifted to 636 nm

We have synthesized two polymorphs of CuSCN coordination polymers via hydrothermal in situ desulfation coupling of 4-pyridinethiol. As both polymorphs of [(CuSCN)2(dps)] form under similar reaction conditions, but with a change in the amount of ligands, it is reasonable to conclude that there are only minor energy differences in the formation of the two polymers. The two polymorphs are very rare examples of coordination polymers that exhibit a similar local coordination geometry of metal and the same topology but different CuSCN structural motifs. The role played by ligand concentration has been exemplified in the present work. Acknowledgment. This work was financially supported by NSFC (20401011), A Foundation for the Author of National Excellent Doctoral Dissertation of P. R. China (200422), A Program for New Century Excellent Talents in University (NCET-05-0270), Youth Academic Leader of Shanxi and Returned Overseas Students of Shanxi. Supporting Information Available: Crystal structural data for 1 and 2 in CIF format. This material is available free of charge via http:// pubs.acs.org.

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