Metal−Organic Coordination Architectures with Thiazole-Spaced

CRYSTAL GROWTH & DESIGN

Metal-Organic Coordination Architectures with Thiazole-Spaced Pyridinecarboxylates: Conformational Polymorphism, Structural Adjustment, and Ligand Flexibility

2007 VOL. 7, NO. 1 124-131

Xu-Dong Chen, Hui-Fang Wu, Xiao-Hu Zhao, Xiao-Jun Zhao, and Miao Du* College of Chemistry and Life Science, Tianjin Normal UniVersity, Tianjin 300074, P. R. China ReceiVed July 31, 2006; ReVised Manuscript ReceiVed October 14, 2006

ABSTRACT: Two derivative ligands of nicotinic or isonicotinic acid, namely, 2-(3-pyridyl)-4-methylthiazole-5-carboxylic acid (Hpmtca) and 2-(4-pyridyl)thiazole-4-carboxylic acid (Hptca), have been employed to assemble with a variety of divalent metal ions, leading to the generation of diverse metal-organic supramolecular architectures in different dimensions. Among them, complexes 1a and 1b, [Ni(pmtca)2(H2O)4]n, present a pair of conformational polymorphs with different neutral layered arrays, which are obtained from similar reaction conditions with NiII salts of different sources. [Cu(pmtca)2(H2O)]n (2) and [Zn(pmtca)2(H2O)]n (3) are isostructural two-dimensional (2-D) corrugated coordination networks. These layered patterns adopt interdigitated three-dimensional (3-D) crystalline packing and are further consolidated by hydrogen bonding, while [Co(pmtca)2(CH3CH2OH)2]n (4) shows a distinct one-dimensional (1-D) double chain structure. As for the CdII species, [Cd(pmtca)2(H2O)2]n (5) comprises a similar layered framework as that in 1a, whereas the mononuclar complex [Cd(ptca)2(H2O)4] (6) displays a 3-D supramolecular network via multiple intermolecular hydrogen bonds. The ligand conformation is systematically investigated to further explore its relationship with the resultant crystalline architectures. Introduction The rational design and synthesis of metal-organic supramolecular architectures is of great interest and importance in producing new functional materials with desired magnetic, catalytic, electric, and fluorescent properties as well as potential applications in ion exchange and gas storage.1-3 To direct the coordination assemblies, three important aspects should be taken into account: (i) the elaboration of organic ligands with proposed binding capability and interaction sites; (ii) the choice of metal ions with well-defined coordination geometries; (iii) the use of weak interactions such as hydrogen bonding and π-π stacking that may stabilize the crystalline lattices. Among the present contributions, much effort has been devoted to the construction of coordination polymers or metal-organic frameworks (MOFs), in which the metal ions are interlinked by organic bridging ligands containing functional groups such as the familiar pyridyl and/or carboxylate groups to form infinite network structures.4-7 In this context, besides the symmetric ligands such as types of 4,4′-bipyridine and terephthalate,5-7 isonicotinate and its derivatives3e,8 have also been proven to be a series of promising organic connectors in crystal engineering of coordination polymers due to the coexistence of reliable pyridyl groups as well as carboxylate moieties with rich coordination modes.9 In this regard, we have initiated investigations of derivatives of isonicotinic acid dominated by different spacers between the two terminal groups, including 4-pyridylacetic acid (Hpya) and 4-pyridylthioacetic acid (Hpyta) (Scheme 1), which possess adjustable backbone lengths and binding modes. These studies10 suggest that such flexible ligands may exhibit a versatile nature in the direction of inorganic-organic supramolecular systems with almost all familiar metal ions, and a variety of novel solidstate arrays with dimensions from one-dimensional to threedimensional (1-D to 3-D) have been achieved. To further extend * Corresponding author. Fax: 86-22-23540315. Tel: 86-22-23538221. E-mail: [email protected].

Scheme 1

this attractive research, we chose two analogous ligands with a thiazole spacer in this work, that is, 2-(3-pyridyl)-4-methylthiazole-5-carboxylic acid (Hpmtca) and 2-(4-pyridyl)thiazole-4carboxylic acid (Hptca), anticipating that the rigidity of thiazole may introduce additional structural constraint in controlling the assembly of metal-organic networks. Furthermore, we also hope to reveal some structural factors of the ligands for dominating the self-assembly, and the result may provide new information of the spacer effect that such ligands exert on their coordination preferences. In this context, a series of divalent metal ions CoII, NiII, CuII, ZnII, and CdII are used, which usually prefer to bind to water to satisfy their preferred octahedral geometries.10 This feature may give rise to the potential for stabilizing and extending coordination arrays via secondary hydrogen-bonding contacts, as demonstrated in our previous work.10c,11 Herein we report seven new coordination complexes with the two ligands, which display different solid structures as well as ligand features. Among them, 1a and 1b, [Ni(pmtca)2(H2O)4]n, represent two unique conformational polymorphs; [Cu(pmtca)2(H2O)]n (2) and [Zn(pmtca)2(H2O)]n (3) are isostructural layered coordination polymers, whereas [Co(pmtca)2(CH3CH2OH)2]n (4), [Cd(pmtca)2(H2O)2]n (5), and [Cd(ptca)2(H2O)4] (6) are distinct 1-D, two-dimensional (2-D), and monomeric species. A systematic investigation of the role of ligand conformation has also been carried out based on this series of complexes.

10.1021/cg060512a CCC: $37.00 © 2007 American Chemical Society Published on Web 12/14/2006

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Crystal Growth & Design, Vol. 7, No. 1, 2007 125

Table 1. Crystallographic Data and Structure Refinement Summary complex empirical formula formula weight crystal size/mm3 crystal system space group a/Å b/Å c/Å R / deg β / deg γ / deg V / Å3 Z Dcalc/ g‚cm-3 µ / mm-1 F(000) total/ independent reflections Rint parameters goodnessof-fit R1 [I > 2σ(I)] wR2 (all data)

1a

1b

2

3

4

5

Hptca

6

C9H6N2O2S2

C20H18N4O6S2Ni 533.21

C20H18N4O6S2Ni 533.21

C20H16N4O5S2Cu 520.03

C20H16N4O5S2Zn 521.86

C24H26N4O6S2Co 589.54

C20H18N4O6S2Cd 586.90

206.22

C18H18N4O8S2Cd 594.88

0.28 × 0.16 × 0.14 monoclinic

0.22 × 0.10 × 0.08 monoclinic

0.32 × 0.30 × 0.18 monoclinic

0.23 × 0.22 × 0.18 monoclinic

0.42 × 0.21 × 0.20 monoclinic

0.52 × 0.26 × 0.23 monoclinic

0.26 × 0.23 × 0.18 orthorhombic

0.45 × 0.36 × 0.20 triclinic

P21/c

P21/c

P21/c

P21/c

C2/c

P21/c

Pna21

P1h

7.887(7) 8.989(8) 16.96(2) 90 115.12(1) 90 1089(2) 2 1.626

9.332(8) 8.348(7) 14.65(1) 90 106.66(1) 90 1093(2) 2 1.620

11.252(3) 11.441(3) 16.478(4) 90 97.713(3) 90 2102.1(8) 4 1.643

11.219(2) 11.383(2) 16.613(2) 90 96.598(2) 90 2107.6(5) 4 1.645

21.696(5) 7.757(2) 19.197(4) 90 123.795(2) 90 2684(1) 4 1.458

8.975(4) 9.432(4) 14.366(8) 90 115.733(6) 90 1095.5(9) 2 1.779

10.978(3) 14.901(4) 5.297(1) 90 90 90 866.5(4) 4 1.581

6.265(4) 7.002(4) 12.870(8) 103.270(7) 95.834(7) 105.301(8) 522.0(6) 1 1.892

1.129 548 5620/1910

1.125 548 5726/1921

1.280 1060 10928/3594

1.405 1064 11249/3712

0.840 1220 6967/2364

1.234 588 5701/1934

0.343 424 4454/1469

1.303 298 2855/1819

0.0547 151 0.981

0.0301 152 1.048

0.0580 289 1.039

0.0348 291 1.031

0.0482 170 1.008

0.0251 151 1.062

0.0332 128 1.040

0.0122 151 1.082

0.0413 0.1174

0.0278 0.0692

0.0431 0.1074

0.0312 0.0798

0.0440 0.1193

0.0236 0.0645

0.0282 0.0655

0.0197 0.0503

Experimental Section General Materials and Methods. All chemicals were obtained commercially and used without further purification. Fourier transform infrared (FTIR) spectra were measured using a Nicolet AVATAR-370 spectrometer, and the samples were prepared as KBr pellets. Absorptions are described as follows: strong (s), medium (m), weak (w), and broad (b). Elemental analyses of carbon, hydrogen, and nitrogen were carried out on a CE-440 (Leemanlabs) analyzer. Powder X-ray diffraction (PXRD) patterns were recorded on a Rigaku RU200 diffractometer for Cu KR radiation (λ ) 1.5406 Å), with a scan speed of 2°/min and a step size of 0.02° in 2θ. Syntheses of the Complexes. [Ni(pmtca)2(H2O)4]n (1a). A mixture of Hpmtca (8.8 mg, 0.04 mmol) and Ni(NO3)2‚6H2O (10.6 mg, 0.04 mmol) in ethanol/water (1:1, 10 mL) was sealed in a parr Teflon-lined stainless steel vessel (20 mL). It was heated to 100 °C in 24 h and kept there for another 24 h under autogenous pressure, and then cooled to room temperature at a rate of -1 °C h-1. Blue strip-shaped crystals suitable for X-ray analysis were obtained in a yield of 41% (4.4 mg, based on Hpmtca). Anal. Calcd. for C20H18N4NiO6S2: C, 45.05; H, 3.40; N, 10.51%. Found: C, 44.74; H, 3.12; N, 10.27%. IR (cm-1): 3388b, 3039m, 1609m, 1587s, 1569m, 1530w, 1484m, 1410m, 1378s, 1361s, 1325m, 1191w, 1033w, 811m, 788m, 779m, 696m, 643w. [Ni(pmtca)2(H2O)4]n (1b). The same synthetic procedure as that for 1a was used except that the metal salt was replaced by Ni(OAc)2‚2H2O, generating green prismatic single crystals suitable for X-ray analysis in a yield of 50%. Anal. Calcd. for C20H18N4NiO6S2: C, 45.05; H, 3.40; N, 10.51%. Found: C, 44.61; H, 3.33; N, 10.77%. IR (cm-1): 3296b, 3039s, 1586s, 1486w, 1450m, 1361s, 1325s, 1260w, 1191m, 1121m, 1096w, 1056w, 1031m, 960w, 863w, 812m, 786s, 696m, 644m. [Cu(pmtca)2(H2O)]n (2). A CH3OH (10 mL) solution of Hpmtca (6.6 mg, 0.03 mmol) was carefully layered over an aqueous solution (5 mL) of CuSO4‚5H2O (12.5 mg, 0.06 mmol) in a straight glass tube. Blue block single crystals suitable for X-ray analysis were collected after a period of 2 weeks in a 57% yield (4.5 mg, based on Hpmtca). Anal. Calcd. for C20H16CuN4O5S2: C, 46.19; H, 3.10; N, 10.77%. Found: C, 45.82; H, 3.03; N, 10.51%. IR (cm-1): 3181b, 1599vs, 1517w, 1426m, 1394vs, 1331m, 1190w, 1128m, 1031w, 951w, 792m, 696m, 668w. [Zn(pmtca)2(H2O)]n (3). An ethanol (5 mL) solution of Hpmtca (4.4 mg, 0.02 mmol) was carefully layered over an aqueous solution

(5 mL) of Zn(OAc)2‚2H2O (4.4 mg, 0.02 mmol) in a straight glass tube. Colorless block single crystals suitable for X-ray analysis were obtained after several days in a 38% yield (4.0 mg, based on Hpmtca). Anal. Calcd. for C20H16N4O5S2Zn: C, 46.03; H, 3.09; N, 10.74%. Found: C, 45.77; H, 2.81; N, 10.39%. IR (cm-1): 3219b, 1601s, 1518w, 1485w, 1429m, 1375s, 1334m, 1302w, 1192w, 1123w, 1057w, 1031w, 790m, 696m, 669w, 643w. [Co(pmtca)2(CH3CH2OH)2]n (4). Co(OAc)2‚4H2O (5.0 mg, 0.02 mmol) in CH3OH (5 mL) was added into an ethanol solution (5 mL) of Hpmtca (4.4 mg, 0.02 mmol). After the sample was stirred for 20 min at ca. 50 °C, the clear solution was cooled to room temperature and filtered. Pink prism single crystals suitable for X-ray diffraction were obtained after several days in a yield of 59% (3.5 mg, based on Hpmtca). Anal. Calcd. for C24H26CoN4O6S2: C, 48.90; H, 4.45; N, 9.50%. Found: C, 48.56; H, 4.24; N, 9.09%. IR (cm-1): 3204b, 1592vs, 1431s, 1373s, 1345vs, 1257w, 1201w, 1127m, 1025m, 821m, 789s, 748m, 697m. [Cd(pmtca)2(H2O)2]n (5). An ethanol (5 mL) solution of Hpmtca (4.4 mg, 0.02 mmol) was carefully layered over an aqueous solution (5 mL) of CdII salt (0.02 mmol) such as Cd(NO3)2, CdCl2, or Cd(OAc)2 in a straight glass tube. Colorless strip crystals suitable for X-ray analysis were collected after several days in a 34% yield (2.0 mg, based on Hpmtca). Anal. Calcd. for C20H18CdN4O6S2: C, 40.93; H, 3.09; N, 9.55%. Found: C, 40.82; H, 2.84; N, 9.51%. IR (cm-1): 3214b, 3039b, 1531w, 1583s, 1448w, 1410m, 1356vs, 1330m, 1190m, 1119m, 1053w, 1026w, 812w, 784m, 693m, 640w. [Cd(ptca)2(H2O)4] (6). Hptca (5.1 mg, 0.025 mmol) was dissolved in a methanol solution of NaOH (5 mL, pH ) 8.5), to which was added a water solution (10 mL) of Cd(OAc)2‚2H2O (6.7 mg, 0.025 mmol) with stirring at ca. 50 °C for 30 min. Then the resultant colorless solution was cooled to room temperature and filtered. Colorless block single-crystals suitable for X-ray diffraction were obtained upon slow evaporation of the filtrate after a period of 1 week in a 60% yield (4.2 mg, based on Hptca). Anal. Calcd. for C18H18CdN4O8S2: C, 36.34; H, 3.05; N, 9.42%. Found: C, 36.32; H, 3.00; N, 9.50%. IR (cm-1): 3243b, 3110w, 1613s, 1458m, 1417w, 1378s, 1285w, 1253w, 1007m, 932w, 823m, 784m, 719w, 638m. X-ray Crystallography. Single-crystal X-ray diffraction data for complexes 1a-6 and the ligand Hptca were collected on a Bruker Apex II CCD diffractometer at the ambient temperature with Mo KR radiation

126 Crystal Growth & Design, Vol. 7, No. 1, 2007 (λ ) 0.71073 Å). A semiempirical absorption correction was applied using SADABS, and the program SAINT was used for integration of the diffraction profiles.12 The structures were solved by direct methods using SHELXS program of SHELXTL package.13 The final refinement was performed by full-matrix least-squares methods on F2 with SHELXL program.13 Hydrogen atoms bonded to carbon were placed geometrically using a riding mode with an isotropic displacement parameter fixed at 1.2 times Ueq of the parent atoms. Hydrogen atoms of aqua or ethanol solvates were first located in difference Fourier maps and then fixed in the given positions. All hydrogen atoms are included in the final refinement. Detailed crystallographic data and structural refinement parameters are summarized in Table 1.

Results and Discussion Syntheses of the Complexes. In most synthetic cases of these coordination complexes, direct solution reactions at room temperature give rise to microcrystalline precipitates. So the hydrothermal and layered synthetic methods were applied to obtain larger single crystals suitable for X-ray diffraction. All complexes have a metal/ligand ratio of 1:2, although most of them were tentatively prepared with a molar ratio of 1:1. As a matter of fact, the final products do not rely on this metal-toligand ratio (1:1 or 1:2) of the reactants, which have been validated by IR spectra and microanalysis. It is very interesting for 1a and 1b that although their compositions are confirmed to be independent of the inorganic counter anions, just as in all the other cases, the employment of different NiII sources leads to the generation of a pair of polymorphic crystalline materials. This can be attributed to the complicated mechanism and conditions of hydrothermal synthesis used in both cases, which can result in diversification of products. The anions (nitrate for 1a and acetate for 1b) may act as structure-directing reagents in this process. The compositions of 1a and 1b were confirmed by microanalytical technique, and their phase purities of the bulk samples were identified by powder X-ray diffraction. On the other hand, although great efforts have also been made for

Figure 1. A portion view of 1a (a) and 1b (b) with atom labeling of the asymmetric unit and NiII coordination environment. Symmetry codes: (a) N1A: 1 - x, 1/2 + y, 1/2 - z; N1B: x - 1, -1/2 - y, z - 1/2; O1A/O3A: -x, -y, -z. (b) N1A: 1 - x, y - 1/2, 1/2 - z; N1B: x - 1, 1/2 - y, z - 1/2; O2A/O3A: -x, -y, -z.

Chen et al. Table 2. Bond Distances (Å) of Metal Coordination in Complexes 1a-6a 1a Ni1-O1 Ni1-O3 Ni1-N1A

2.072(3) 2.103(3) 2.130(3)

Zn1-O4 Zn1-O2 Zn1-N4B Zn1-N2A

2.060(2) 2.095(2) 2.101(2)

Co1-O1A Co1-O3 Co1-N1

1b Ni1-O2 Ni1-O3 Ni1-N1A

2.037(2) 2.039(2) 2.118(2) 2.142(2) 4 2.069(2) 2.088(3) 2.176(3) 5

2 Cu1-O4 Cu1-O2 Cu1-O5 Cu1-N1A Cu1-N3B

1.964(3) 1.966(3) 1.968(3) 2.038(4) 2.335(4) 3

Zn1-O5

Cd1-O1 Cd1-O3 Cd1-N1A

2.277(2) 2.350(2) 2.347(2) 6

Cd1-O4 Cd1-N1 Cd1-O3

2.298(2) 2.308(2) 2.347(2)

2.034(2)

a

Symmetry transformations used to generate equivalent atoms: 1a: A: x -1, -y - 1/2, z - 1/2; 1b: A: -x + 1, y - 1/2, -z + 1/2; 2: A: -x + 2, y + 1/2, -z + 3/2, B: x + 1, y, z; 3: A: -x + 3, y + 1/2, -z + 1/2, B: x + 1, y, z; 4: A: -x + 2, y, -z + 3/2; 5: A: x + 1, -y - 1/2, z + 1/2.

Scheme 2

the coordination-driven assembly of Hptca with various metal ions, unfortunately, only two metal complexes were achieved, including a known 3-D CuII coordination polymer14 and a mononuclear CdII complex 6. Structural Description and Discussion. Conformational Polymorphs 1a and 1b. Complexes 1a and 1b have the same composition, and both crystallize in space group P21/c. However, the exact conformations of the anionic ligand pmtca are different in the two structures, resulting in distinct 2-D coordination networks. Thus, they represent an interesting pair of conformational polymorphs. Polymorphs refer to two or more different crystal structures of the same compound and conformational polymorphism is the existence of diverse conformers in polymorphic structural modifications.1m,15,16 Although conformational polymorphs have been well-known for organic crystalline solids, this phenomenon is quite rare in polymeric coordination complexes.17,18 With pmtca as the anionic bridging ligand, both 3-D lattices of 1a and 1b are constructed from the stacking of neutral 2-D layered metal-organic frameworks without any auxiliary anion. The NiII centers in both 1a and 1b adopt distorted octahedral geometry, coordinating to four pmtca (via two pyridyl N and two carboxylate O) and a pair of aqua ligands (see Figure 1). In each case, the NiO4 coordination plane holds a small acute angle to carboxylate (being 20.3° and 30.1° for 1a and 1b, respectively). This is due to the fixation of an intramolecular hydrogen bond between the aqua ligand and the uncoordinated carboxylate oxygen [O3-H3A‚‚‚O2 for 1a and O3A-H3AA‚ ‚‚O1 for 1b; H‚‚‚O/O‚‚‚O distance: 1.88/2.719(5) Å for 1a and 1.82/2.666(3) Å for 1b, angle: 168° for 1a and 171° for 1b]. Analysis of the NiII coordination spheres in both polymorphs reveals that the ligand binding ability in 1a is weaker than that in 1b, as indicated by the slightly longer bond distances in 1a compared with those in 1b (see Table 2 for details). This may be related to the conformational discrepancy of the ligands in 1a and 1b, which gives rise to different structural restraints.

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Crystal Growth & Design, Vol. 7, No. 1, 2007 127

Figure 2. (a) Two types of layered coordination networks for 1a (left) and 1b (right) viewed along the [001] direction. (b) A 3-D packing diagram of 1a along [010]. (c) A 3-D packing diagram of 1b along [010]. The adjacent layers are indicated by different colors for both figures.

Figure 3. (a) A local coordination environment of CuII in complex 2. (b) A side view of the 2-D network in 2 viewed along [100]. (c) A perspective view of the 2-D network viewed along [001]. (d) A schematic representation of the corrugated layer. The “inner” and “outer” ligands are shown in pink and green, respectively.

Further investigation on the crystal structures indicates that although pmtca acts as a bidentate bridging ligand in both cases,

its binding modes are different in 1a and 1b. As elucidated in Scheme 2, pmtca adopts a syn,syn mode I in 1a, while it takes

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

Chen et al.

Figure 6. A perspective view of the crystal structure of Hptca, showing the perpendicular relationship of the 1-D hydrogen-bonded chains. The dotted lines in purple and green refer to C-H‚‚‚O and O-H‚‚‚N interactions, respectively.

Figure 4. (a) The interdigitated stacking of the 2-D layers in 2, which are fixed by interlayer hydrogen bonding. (b) A local view of the hydrogen-bonded pattern in 2.

Figure 5. The 1-D double chain structure in the CoII complex 4.

a syn,anti mode II in 1b. In 1a, the central thiazole of pmtca is almost coplanar to the terminal pyridyl ring with a dihedral angle of 4.5°, and it holds a dihedral angle of 29.7° to carboxylate. However, the pmtca ligand in 1b is deviated more from planarity with the corresponding values of 30.5° and 34.8°, respectively. As a result, although the coordination layers in both cases show a simple (4,4) network topology, they are diverse and can be obviously differentiated (Figure 2). As for the repeated rhombus grid in each net, it has the dimension of 9.877 Å2 for 1a and 10.907 Å2 for 1b, and the diagonal distances are 8.989/17.590 Å for 1a and 8.348/20.154 Å for 1b, respectively. On the other hand, both structures contain interlayer O3-H3B‚‚‚N2thiazole hydrogen bonding [symmetry code: x - 1, -y + 1/2, z - 1/2 for 1a and -x + 1, -y, -z for 1b, H‚‚‚N/O‚‚‚N distance: 2.13/ 2.971(5) Å for 1a and 2.07/2.919(3) Å for 1b, angle: 176° for 1a and 175° for 1b], and they display different 3-D lattices (see Figure 2). In 1a, the 2-D layers are parallel and well isolated, while in 1b the parallel 2-D patterns are interdigitated, when viewing along [010]. Also from the [001] direction, we can see

that the layers in 1a are completely overlapped, while those in 1b are alternate and slip with an offset of 0.5(x + y) along the ab plane. The decisive physical principles governing polymorphism, even conformational polymorphism, remain poorly understood, although they are widely observed. The cases of 1a and 1b may reveal some intrinsic factors that dominate the metal-ligand coordination, further manage the crystal packing, and finally drive the generation of two polymorphs. It is notable that compounds 1a and 1b are obtained under similar hydrothermal reaction conditions except for the sources of NiII ions. Although no inorganic anions are included in the structures of 1a and 1b, it is suggested that they may act as a template and seem to play a dominant role here. Isostructural Polymeric Complexes 2 and 3. The CuII complex 2 and ZnII complex 3 constitute an isostructural pair and display similar 2-D corrugated frameworks. Structural analyses reveal that two types of pmtca ligands exist in the asymmetric unit of 2 or 3, and both of them act as bidentate bridges to interlink the CuII or ZnII centers into a 2-D network (see Figure 3). Concerning the two ligand entities, one adopts a syn,anti binding mode II as that in 1b, while the other takes an anti,syn mode III (see Scheme 2). Each metal center is pentacoordinated to four pmtca ligands via two pyridyl-N and two carboxylate-O atoms as well as one aqua ligand (see Figure 3a), taking a distorted square-pyramidal geometry with one pyridyl-N occupying the apex site. In 2, the three sets of Cu-O bonds are comparable (1.964, 1.966, and 1.968 Å), and the equatorial Cu-N length is 2.038 Å, being significantly shorter than that of the apical Cu-N (2.335 Å) bond due to the JahnTeller distortion, whereas in complex 3 the Zn-N bond distances are similar at 2.142 (equatorial) and 2.118 (apical) Å, which are significantly longer than the Zn-O bond lengths (2.034, 2.037, and 2.039 Å). As for the local coordination polyhedron, the τ value19 is 0.071 for CuII and 0.152 for ZnII; the CuII center in 2 is deviated from the basal plane of 0.098 Å, while the corresponding value for ZnII in 3 is 0.246 Å. These parameters clearly indicate a more distorted square-pyramidal geometry of ZnII. In both structures, the two opposite carboxylates that bind to the same metal ion make a small dihedral angle of ca. 1.9°.

Metal-Organic Coordination Architectures

Crystal Growth & Design, Vol. 7, No. 1, 2007 129 Table 3. Hydrogen-Bonding Geometries in the Structure of 6a D-H‚‚‚A

D‚‚‚A (Å)

H‚‚‚A (Å)

D-H‚‚‚A (deg)

O3-H3B‚‚‚O2a

2.810(3) 2.760(3) 2.671(3) 2.749(3)

2.07 1.93 1.82 1.89

147 165 179 172

O3-H3A‚‚‚O2b O4-H4B‚‚‚O1c O4-H4A‚‚‚O2b

a Symmetry code: (a) x, y - 1, z - 1; (b) 2 - x, -y, 1 - z; (c) x - 1, y - 1, z - 1.

Figure 7. (a) Molecular structure of the mononuclear complex 6. (b) 3-D hydrogen-bonded net of 6. The dotted lines in different colors refer to four sets of hydrogen bonds.

The 2-D corrugated networks in 2 and 3 also have a simple (4,4) topology, however, in which the two types of pmtca ligands act as the “inner” and “outer” linkers, respectively (see Figure 3). These 2-D layers are embedded into each other and adopt an interdigitated packing fashion, as illustrated in Figure 4a, which are also fixed by interlayer hydrogen bonding between the aqua ligands and the uncoordinated carboxylate-O atoms. Details are as follows: O5-H5A‚‚‚O3 and O5-H5B‚‚‚O1 for 2 [symmetry code: 1 - x, 1 - y, 1 - z, H‚‚‚O/O‚‚‚O distance: 1.87/2.659(4) Å and 1.88/2.707(4) Å, angle: 154° and 164°]; O5-H5A‚‚‚O1 and O5-H5B‚‚‚O3 for 3 [symmetry code: -x + 2, -y + 1, -z, H‚‚‚O/O‚‚‚O distance: 1.89/2.702(3) Å and 1.87/2.661(3) Å, angle: 160° and 154°]. The adjacent metal ions are involved in two 10-membered hydrogen-bonded patterns, including two pairs of O-H‚‚‚O bonds as stated above to constitute a “box” (see Figure 4b). In our experience, the familiar divalent metal centers prefer to combine the aqua solvents within their coordination spheres, which are potential participators for strong hydrogen bonds that can extend the coordination architecture into higher dimensions. Conformations of the two types of pmtca ligands in each complex are distinct. Concerning the pair of dihedral angles between the central thiazole ring and the pyridyl or carboxylate group, they are 14.1/2.9° (2) and 12.8/3.5° (3) for the “inner” pmtca ligands and 9.4/2.3° (2) and 7.3/2.6° (3) for the “outer” ones. This indicates that the “outer” pmtca is more planar, which may be attributed to the steric requirements for the interdigitated stacking to embed the “outer” ones into the side grooves of the adjacent layers. 1-D CoII Complex 4. Among these metal complexes with Hpmtca, only complex 4 exhibits a 1-D structure with the double chain motif. Each CoII node takes a distorted octahedral geometry, coordinating to four pmtca ligands and a pair of ethanol solvates (see Figure 5). It is proposed that the involvement of ethanol in the CoII coordination sphere may be a dominant factor in generating this 1-D array, in which the adjacent CoII centers are interlinked by a pair of pmtca ligands in a syn,syn mode I. For each pair of pmtca linkers, they are in a head-to-tail arrangement with their thiazole-S atoms pointing

to the same direction. The adjacent intrachain Co‚‚‚Co separation is 9.70 Å. Within each pmtca ligand, the dihedral angles between the central thiazole and the terminal pyridyl/carboxylate are 31.2/ 2.6°. Notably, a similar intramolecular O3-H3A‚‚‚O2 hydrogen bonding is also observed between the ethanol solvate and the free carboxylate acceptor [symmetry code: 2 - x, y, 3/2 - z, H‚‚‚O/O‚‚‚O distance: 1.76/2.595(5) Å, angle: 166°], as reflected by a small dihedral angle of only 9.3° between the CoO4 basal plane and the carboxylate. No significant interchain weak interaction is found. 2-D Polymeric CdII Complex 5. The CdII complex 5 shows a similar 2-D layered structure to that of 1a but with significantly longer coordination bond distances due to the larger radius of CdII. The ligand exhibits the same syn,syn coordination mode I as that in 1a, which however exhibits a more planar conformation. The central thiazole ring holds the dihedral angles to the pyridyl plane and the carboxylate group at 2.8° and 16.3°, respectively. In the structure of 5, a similar intramolecular O-H‚ ‚‚O hydrogen bond also results in an acute dihedral angle (26.5°) between the CdO4 coordination plane and the carboxylate group, which is slightly larger than that in 1a. Another significant difference between complexes 1a and 5 is the interlayer distance. Despite the longer lengths of coordination bonds around CdII as well as the neighboring Cd‚‚‚Cd separation (9.920 Å) within the 2-D layer, the layer-to-layer separation in 5 (6.661 Å) is shorter than that in 1a (6.889 Å, calculated from any two metal coordination planes of the adjacent layers). The explanation could be the fact that the lower steric restriction of the ligands in 5 enables a more compact stacking. Crystal Structures of Hptca and the Mononuclear CdII Complex 6. Single crystals of the ligand Hptca were obtained accidentally during a failed attempt to prepare its Fe2(SO4)3 complex under a pH value of 6.5-7.0 (adjusted by NaOH) in water medium. The adjacent ligand molecules are interlinked in a head-to-tail fashion via a pair of strong O1-H1A‚‚‚N1 [symmetry code: 3/2 - x, 1/2 + y, -3/2 + z, H‚‚‚N/O‚‚‚N distance: 1.79/2.603(3) Å, angle: 171°] and weak C5-H5‚‚‚ O2 [symmetry code: 3/2 - x, -1/2 + y, 3/2 + z, H‚‚‚O/C‚‚‚O distance: 2.86/3.464(3) Å, angle: 124°] interactions to form single chains. This hydrogen-bonded pattern with a graph set R22(7) between carboxyl and pyridyl groups is well-known and thus is denoted as a classical heterosynthon.20 An interesting point for this structure resides in that each chain within an identical (011) plane is perpendicular to those at the adjacent (011) plane, which leads to the formation of a regular 3-D packing (see Figure 6). Regarding the mononuclear complex 6, each CdII center is coordinated to two ptca ligands using their pyridyl nitrogen and four aqua molecules, forming an octahedral sphere (Figure 7a). It is notable that ptca is monodentate with the uncoordinated carboxylate group. With aqua ligands as donors and carboxylates as acceptors, abundant hydrogen-bonding interactions are observed in this structure (see Table 3 for details), which assemble the mononuclear complex units to give a 3-D pillar-

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

Chen et al.

Table 4. Summary on the Geometrical Parameters of pmtca in the Complexesa 2 1a

1b

“inner”

“outer”

“inner”

“outer”

4

I

II

III

II

III

II

I

I

A-B

4.5

30.5

14.1

9.4

12.8

7.3

31.2

2.8

B-C

29.7

34.8

2.9

2.3

3.5

2.6

2.6

16.3

binding mode of pmtca dihedral angle (deg) a

3 5

A, B, and C refer to the pyridyl, thiazole, and carboxylate planes in pmtca, respectively.

layered supramolecular network (Figure 7b). Besides the good donor ability, the aqua ligands also bear the advantage in that their hydrogen atoms have rich spatial tropisms, which further facilitates and enriches the formation of hydrogen bonds. The application of such aquated metal ions as the basic building blocks may be useful for crystal engineering of inorganicorganic hybrid structures. Discussions on Ligand Conformation. A comparison of the backbones of Hpmtca/Hptca with the previously studied Hpya/ Hpyta suggests that the rigidity of the thiazole spacer is a key factor that dominates their conformational nature. The flexibility of -CH2- and -SCH2- spacers in Hpya/Hpyta allows them to adjust easily to afford complicated network structures upon metal complexation, whereas the thiazole group has only the flexibility of limited rotation. Our attempts to obtain crystals of Hpmtca for X-ray analysis were unsuccessful, so the discussion can be only on the basis of the metal complexes. Systematic analyses of this series of complexes reveal that pmtca prefers to act as a bidentate bridging ligand, in which carboxylate is in a monodentate mode, and the thiazole group is unlikely to participate in coordination. Furthermore, although several binding fashions and conformations of pmtca are observed as described above (see Scheme 2), rotation of the thiazole group is restricted to a limited range due to the conjugation with the pyridyl ring and the carboxylate moiety (see Table 4). It is inclined to the terminal groups to form the largest dihedral angle of 34.8°, and most of the values are less than 16.3° in all these structures. As for the pmtca ligand taking a syn,syn-binding fashion, it has a comparable ligand conformation in the cases of 1a and 5 that display similar 2-D coordination networks, while a distinct ligand distortion in 4 corresponds to its 1-D double chain array. In 1b, the ligand adopts a syn,anti-binding fashion, and the structural restraint for generating the 2-D layered framework corresponds to the most distorted ligand conformation in this series of complexes. Significantly, two types of ligands are observed in complexes 2 and 3. One has a similar binding mode to that in 1b but with the most planar conformation, while the other exhibits a unique anti,syn-binding fashion with an antiorientation of the 3-pyridyl nitrogen donor. In the molecular structure of Hptca, the central thiazole plane is inclined to the pyridyl and carboxylate terminal groups with dihedral angles of 12.8° and 4.4°, respectively. However, in complex 6, the corresponding dihedral angles within the anionic ligand ptca are 5.7 and 12.6°, indicating an evident backbone rotation upon metal coordination. In addition, the distinct hydrogen-bonded motions in both structures may also be responsible for this shift. Regarding the reason for different ligand conformations even with the same binding mode in these complexes, at this stage we can only conclude that the rotation of the single C-C bonds between the central thiazole and the pyridyl/carboxylate terminals can provide changeable bridging orientation. In our understanding, it is a structural requirement for forming an

individual coordination array in which the ligand adopts a steady conformation to meet the steric and bonding factors. More experimental results and/or computational studies are necessary to further realize the connection between ligand binding/ conformation and the resultant supramolecular arrays. Conclusions and Perspectives In this contribution, we have presented a series of new metalorganic supramolecular architectures with thiazole-spaced pyridinecarboxylate ligands. Among them, we discover a pair of unique conformational polymorphs generated from similar reactive conditions except the source of metal ion. The results dictate the fact that even if the inorganic counter anions are not involved in the final crystalline solids, they can also subtly affect these materials to result in polymorphism. The ligand conformation being a key factor in the direction of these coordination structures, hence in further designing new organic ligands for crystal engineering of such crystalline materials, a balance consideration between the flexibility and the rigidity of the ligand backbone is of significant importance. Also, on the basis of this work and previous research cases, the utilization of aquated metal ions as the building blocks is reliable in the construction of hydrogen-bonded hybrid systems. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (No. 20401012), the Key Project of Chinese Ministry of Education (No. 205008), the National Fundamental Research Project of China (No. 2005CCA01200) and Tianjin Normal University. Supporting Information Available: X-ray crystallographic files (CIF) for all compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

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