Cobalt(II) Coordination Networks Dependent upon the Spacer Length

Nov 28, 2007 - Cobalt(II) Coordination Networks Dependent upon the Spacer. Length of Flexible Bis(tetrazole) Ligands. Pei-Pei Liu,† Ai-Ling Cheng,â€...
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CRYSTAL GROWTH & DESIGN

Cobalt(II) Coordination Networks Dependent upon the Spacer Length of Flexible Bis(tetrazole) Ligands Pei-Pei Liu,† Ai-Ling Cheng,† Qi Yue,† Na Liu,† Wei-Wei Sun,† and En-Qing Gao*,†,‡ Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal UniVersity, Shanghai 200062, China, and State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 35000, China

2008 VOL. 8, NO. 5 1668–1674

ReceiVed NoVember 28, 2007; ReVised Manuscript ReceiVed February 3, 2008

ABSTRACT: Four new coordination polymers with flexible bis(tetrazole)alkane ligands as bridges and thiocyanate ions as terminal ligands, [Co(btze)2(SCN)2]n (1), [Co(btze)3(SCN)2]n (2), [Co(btzb)2(SCN)2]n (3), and [Co(btzh)2(SCN)2]n (4), have been synthesized and characterized, where btze ) 1,2-bis(tetrazol-1-yl)ethane, btzb ) 1,4-bis-(tetrazol-1-yl)butane, and btzh ) 1,6-bis(tetrazol-1yl)hexane. The short btze ligand gives singly bridged 2D square grid layers with 2-fold parallel interpenetration (1) or 1D linear chains with deep interdigitation (2), dependent upon the metal-to-ligand ratio used for the synthesis. The btzb ligand links the metal ions into doubly bridged chains composed of bimetallacycles (3). Most interestingly, the long btzh ligand leads to 2D layers with both single and double bridges between Co(II) ions, and the inclined interpenetration of layers generates a 3D polycatenated structure with a high degree of catenation. The results show that the dimensionality and topology of the structures depend strongly upon the length and conformation of the flexible alkyl spacers between the tetrazole rings. Introduction The rational design and syntheses of novel coordination polymers have evoked considerable attention in the past two decades, the motivation arising not only from their intriguing structures but also from their potential applications such as catalysis, gas absorption, magnetic materials, and luminescence.1–3 The final structures of coordination polymers are not only dependent upon the geometrical and electronic properties of the metal ions and ligands but also may be influenced by other factors such as the counterion, the solvent system, and the metal-to-ligand ratio. Crystal engineering has attained such a relatively mature level that some coordination networks with specific topologies can be “designed” by the judicious selection of metal ions and organic ligands with definite coordination preferences.2 However, it is still a great challenge to predict the exact structure for most systems, and systematic research is highly desirable for understanding the roles of different factors in the assembly. This is the case with the flexible bridging ligands that can assume different conformation upon coordination.3–5 These ligands can lead to new coordination networks with interesting topologies or properties, such as dynamic structures3 and supramolecular isomerism.4 The bis(azole) ligands in which N-donor azole rings (imidazole, triazole, or tetrazole) are separated by alkyl (CH2)n spacers are a good choice of flexible bridging ligands. The conformational flexibility of the spacers makes the ligands adaptable to various coordination networks with one- (1D), two- (2D), and threedimensional (3D) structures. The ligands of this class that have been used are mainly bis(imidazol-1-yl)alkanes (n ) 1, 2, 4)6 and bis(1,2,4-triazol-1-yl)alkanes (n ) 2-4).7 For these ligands, it has been demonstrated that the coordination networks with a specific metal ion are strongly dependent upon the nature of the counterions used.6a–c,7a,b By contrast, the bis(tetrazol-1-yl) analogues is much less investigated, and previous studies on such ligands mainly * To whom correspondence should be addressed. E-mail: eqgao@ chem.ecnu.edu.cn. † East China Normal University. ‡ Chinese Academy of Sciences.

focused on the Fe(II) spin-transition coordination polymers with noncoordinated anions ClO4-, BF4-, or PF6-.8,9 In this paper, we report a systematic investigation on the assembly of Co(II) and bis(tetrazol-1-yl)alkanes of different lengths in the presence of thiocyanate ions. The resulting coordination polymers are [Co(btze)2(SCN)2]n (1), [Co(btze)3(SCN)2]n (2), [Co(btzb)2(SCN)2]n (3), and [Co(btzh)2(SCN)2]n (4) (btze ) 1,2-bis(tetrazol-1-yl)ethane, btzb ) 1,4-bis-(tetrazol-1-yl)butane, btzh ) 1,6-bis(tetrazol-1yl)hexane). As shown in Scheme 1, increasing the spacer length of the bis(tetrazole) ligand does not lead to simple network expansion but causes dramatic changes in network topologies. The short btze ligand gives 2D square-grid layers with single bridges or 1D linear chains with double bridges, dependent upon the metalto-ligand ratio used for the synthesis. The btzb ligand links the metal ions into doubly bridged chains, and the long btzh ligand leads to 2D layers with both single and double bridges. The layers in 4 exhibit a high degree of 3D polycatenation. Experimental Section Materials and Measurements. All chemicals were obtained from commercial sources and were used without further purification. The organic ligands btze, btzb, and btzh were prepared according to the literature.8a,9b,10 Elemental analyses (C, H, N) were determined on an Elementar Vario ELIII analyzer. IR spectra were measured as KBr pellets on a Nicolet Magna 750 FT-IR spectrometer in the range of 400-4000 cm-1, and solid-state UV-vis reflectance spectra were measured on a Shimadu UV-2550 spectrometer. Synthesis. [Co(btze)2(SCN)2]n (1). A mixture of cobalt(II) chloride hexahydrate (0.10 mmol, 0.017 g), btze (0.1 mmol, 0.033 g), and ammonium thiocyanate (0.20 mmol, 0.015 g) in H2O (5 mL) and ethanol (5 mL) was stirred for 10 min at room temperature. Slow evaporation of the solution at room temperature yielded red platelike crystals of 1 within two weeks. Yield: 46%. Elemental analysis calcd (%) for C10H12CoN18S2: C, 23.67, H, 2.38; N, 49.69. Found (%): C, 23.97; H, 2.63; N, 50.01. IR (KBr, cm-1): 3161m, 3140m, 2984m, 2956w, 2889w, 2097s, 1495s, 1442s, 1373m, 1193s, 1165s, 1103s, 998m, 882m, 672m, 647m, 511w, 470w. UV-visible (λmax, nm): 225, 325, 485. [Co(btze)3(SCN)2]n (2). Cobalt(II) chloride hexahydrate (0.10 mmol, 0.017 g), btze (0.3 mmol, 0.050 g), and ammonium thiocyanate (0.20 mmol, 0.015 g) were mixed under stirring in H2O (5 mL) and ethanol (5 mL) at room temperature. Slow evaporation of the solution at room temperature yielded red prismlike crystals of 1 within two weeks. Yield:

10.1021/cg701167e CCC: $40.75  2008 American Chemical Society Published on Web 04/19/2008

Cobalt(II) Coordination Networks Scheme 1

60%. Elemental analysis calcd (%) for C14H18N26S2Co: C, 24.97; H, 2.69; N, 54.07. Found (%): C, 25.01; H, 2.63; N, 54.30. IR (KBr, cm-1): 3176m, 3133s, 3015m, 2970w, 2885w, 2084s, 1497s, 1430s, 1370m, 1180s, 1143s, 1102 s, 997m, 874m, 687m, 658m, 474w. UV-visible (λmax, nm): 223, 330, 485. [Co(btzb)2(SCN)2]n (3). A mixture of cobalt(II) chloride hexahydrate (0.10 mmol, 0.017 g), btzb (0.2 mmol, 0.040 g), and ammonium thiocyanate (0.20 mmol, 0.015 g) in H2O (5 mL) and ethanol (5 mL) was stirred for 10 min at room temperature. Slow evaporation of the solution at room temperature yielded red platelike crystals of 3 within 1 month. Yield: 50%. Elemental analysis calcd (%) for C14H20CoN18S2: C, 29.84, H, 3.58; N, 44.76. Found (%): C, 30.13; H, 3.87; N, 44.77. IR (KBr, cm-1): 3144m, 2098s, 1648m, 1497m, 1461m, 1434m, 1180m, 1131w, 1106m, 1004m, 871w, 744w, 657m. UV-visible (λmax, nm): 223, 336, 490. [Co(btzh)2(SCN)2]n (4). A mixture of cobalt(II) chloride hexahydrate (0.10 mmol, 0.017 g), btzh (0.2 mmol, 0.044 g), and ammonium thiocyanate (0.20 mmol, 0.015 g) in H2O (10 mL) and ethanol (8 mL) was stirred for 10 min at room temperature. Slow evaporation of the solution at room temperature yielded red platelike crystals of 4 within 3 month. Yield: 80%. Elemental analysis calcd (%) for C18H28CoN18S2: C, 34.89, H, 4.55; N, 40.69. Found (%): C, 35.10; H, 4.78; N, 40.84. IR (KBr, cm-1): 3125m, 2941m, 2860m, 2069s, 1498m, 1442m, 1399m, 1170s, 1104s, 1002s, 885w, 756w, 662w, 479w. UV-visible (λmax, nm): 223, 335, 495. Crystal Data Collection and Refinement. Diffraction intensity data were collected at 293 K on a Bruker APEX II diffractometer equipped with a CCD area detector and graphite-monochromated Mo KR radiation (λ ) 0.71073 Å). Empirical absorption corrections were applied using the SADABS program.11 The structures were solved by the direct method and refined by the full-matrix least-squares method on F2, with all nonhydrogen atoms refined with anisotropic displacement parameters.12 All the hydrogen atoms attached to carbon atoms were placed in calculated positions and refined using the riding model. Crystallographic data are summarized in Table 1.

Results and Discussion Synthesis and Spectral Characterization. All the compounds are synthesized by room-temperature evaporation of the aqueous ethanol solutions containing CoCl2, NH4SCN, and the appropriate ligands. The products from btze are dependent upon the metal-to-ligand ratio. As confirmed by powder X-ray diffraction analyses, with the starting ratio being 1:1 or 1:1.5, the reaction gave compound 1, which has a metal-to-ligand ratio of 1:2; when a starting ratio of 1:2 was used, the product was compound 2, which a metal-to-ligand ratio of 1:3. Further increasing the amount of the ligand also led to 2. By contrast, with the starting metal-to-ligand ratio ranging from 1:1 to 1:3, the reactions involving btzb or btzh always yield compound 3 or 4, respectively. The infrared spectra of all the compounds exhibit very strong absorption in the region of 2070-2098 cm-1 because of the

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presence of the thiocyanate ion. The stretching vibration of the C-H bond on the tetrazole ring appears in the region of 3125-3140 cm-1 as strong sharp bands.8b The absorption at about 1497 cm-1 is attributable to the CdN vibrations of the tetrazole ring. The UV-vis reflectance spectra of the four compounds are similar and exhibit three bands in the 190-800 nm region. The two strong bands centered at about 230 and 330 nm are attributable to intraligand and ligand-to-metal charge transfer (LMCT) transitions, respectively, and the broad and relatively weak band around 490 nm is caused by the 4T1g(F) f 4T1g(P) transition of Co(II) in the octahedral ligand field. The other spinallowed ligand-field transitions were not observed and should appear in the near-infrared region.13 Crystal Structures. Compound 1. X-ray crystallographic analyses revealed that 1 is a 2D gridlike coordination polymer (Figure 1). Selected bond distances and angles are given in Table 2. Each Co(II) ion in the complex resides at the inversion center and assumes the axially compressed octahedral coordination geometry. The equatorial plane is defined by four tetrazole nitrogen atoms (N1, N1A, N8B, N8C) from different ligands with the Co-N distances being 2.178(3) Å, and the axial positions are occupied by two thiocyanate nitrogen atoms (N9, N9A) with Co-N ) 2.067(3) Å. Each btze ligand assumes the gauche conformation with a N4-C2-C3-N5 torsion angle of 80.5(4)° and links two Co(II) ions through the nitrogen atoms at the 4-positions of the two tetrazole rings. Consequently, each Co(II) ion is linked to four neighboring ones through four btze ligands to form a 2D grid layer along the bc plane, which contains squares (36-membered tetrametallacycles) with the Co(II) ions as corners and the btze ligands as edges. The Co · · · Co distance spanned by the ligand is 10.498(2) Å. All the Co(II) ions in each layer are strictly coplanar. The layer motif in 1 is highly undulated because of the gauche conformation of the ligand. To fill the large void meshes in a single layer, two neighboring layers are entangled together in a parallel fashion, with the Co(II) ions of one layer reside at the mesh centers of the other layer, to form an interpenetrated double layer (Figure 2a and b). The 2-fold parallel interpenetration may be reinforced by interlayer C-H · · · N hydrogen bonds involving the tetrazole CH groups (C1, C4) of one layer and some tetrazole nitrogen atoms (N7, N3) from the other layer (Figure 2a, Table 3). The double layers are stacked in parallel, with the sulfur atoms and the CH2 groups being on the surface of the layer, and weak C-H · · · S hydrogen bonds are operative between the double layers (Figure 2c). Compound 2. The compound contains 1D chains with single btze bridges (Figure 3). Selected bond distances and angles are given in Table 4. Each Co(II) ion also resides on an inversion center and assumes the axially compressed octahedral geometry. The equatorial positions are occupied by four nitrogen atoms (N1, N1A, N9, and N9A) from four ligands btze, with the Co-N distances ranging from 2.165 (2) to 2.195(2) Å. The axial positions are occupied by two thiocyanate nitrogen atoms (N13 and N13A), with the Co-N distances [2.060(2) Å] being significantly shorter than the equatorial ones. In contrast to those in 1, only two of the four btze ligands around each Co(II) ion act as bridging ligands, with the other two acting as monodentate terminal ligands. While the terminal ligand assumes the G conformation with a torsion angle of N-C-C-N ) 64.58(1)°, the bridging ligand assumes a centrosymmetric T conformation (N-C-C-N ) 180°). This leads to an infinite linear-chain motif equipped with twisted arms (the terminal ligands that project toward two opposite sides from the chain). The Co · · · Co

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Table 1. Crystal Data and Structure Refinement for Complexes 1-4 compound

1

2

3

4

formula fw cryst syst space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z D (g cm-3) V ( Å3) µ (mm-1) F(000) Rint GOF on F2 R1 [I > 2σ(I)] wR2 [I > 2σ(I)] R1 (all data) wR2 (all data)

C10H12N18S2Co 507.43 monoclinic C2/c 19.927(6) 7.337(2) 15.016(4) 90 116.619(4) 90 1962.7(10) 4 1.717 1.131 1028 0.0640 0.985 0.0541 0.1270 0.0830 0.1430

C14H18N26S2Co 673.59 monoclinic P21/n 6.8262(14) 10.183(2) 19.735(4) 90 92.97(3) 90 1369.9(5) 2 1.633 0.840 686 0.0229 1.052 0.0382 0.0912 0.0479 0.0975

C14H20N18S2Co 563.53 triclinic P1j 6.5628(13) 9.2726(19) 11.037(2) 66.93(3) 83.14(3) 78.35(3) 604.6(2) 1 1.548 0.926 289 0.0243 1.189 0.0698 0.1958 0.0816 0.2033

C18H28N18S2Co 619.63 monoclinic P21/n 13.467(2) 10.5968(17) 19.501(3) 90 95.560(2) 90 2769.8(8) 4 1.486 0.816 1284 0.0733 0.995 0.0597 0.1289 0.1410 0.1612

distance spanned by the bridging ligand is 12.259(2) Å, which is much longer than that in 1, consistent with the different conformations. In the lattice, the chains propagating along two different directions, (110) and (11j0), are separately arranged in alternate layers parallel to the ab plane, with the chains in the same layer being parallel. In each layer, as Figure 4a shows, neighboring chains are associated through three sets of nonclassical hydrogen bonds: two independent C-H · · · N hydrogen bonds involve CH groups (C1 and C3 from the terminal ligand) in one chain and tetrazole nitrogens (N11 and N10 from the bridging ligand) in a neighboring chain, and the C-H · · · S hydrogen bond is formed between the C6-H6B group from the bridging ligand of one chain and the thiocyanate sulfur atom (S1) from a neighboring chain. The terminal ligands sticking outward from the neighboring layers are deeply interdigitated to give the 3D architecture shown in Figure 4b and c. The

Figure 1. Top (top) and side (bottom) views of the 2D sheet in 1. Hydrogen atoms have been omitted for clarity.

Table 2. Bond Lengths (Å) and Angles (deg) for Complex 1a Co(1)-N(1) Co(1)-N(8)B N(9)-Co(1)-N(1) N(9)-Co(1)-N(8)C N(1)-Co(1)-N(8)C

2.178(3) 2.178(3) 91.80(12) 90.32(11) 88.58(11)

Co(1)-N(9) N(9)A-Co(1)-N(1) N(9)-Co(1)-N(8)B N(1)-Co(1)-N(8)B

2.067(3) 88.20(12) 89.68(11) 91.42(11)

a Symmetry codes: (A) -x, -y + 2, -z; (B) -x, y + 1, -z + 1/2; (C) x, -y + 1, z - 1/2.

interdigitation is reinforced by weak interlayer C-H · · · S hydrogen bonds, which involve the C2-H2A groups of the terminal ligands from a layer and the thiocyanate S1 atoms from a different layer. The relevant parameters for the hydrogen bonds are listed in Table 3. Compound 3. The structure of 3 comprises 1D chains in which Co(II) ions are bridged by double btzb ligands (Figure 5). Selected bond distances and angles are given in Table 5. The Co(II) ion is located at an inversion center, and the local metal environment is very similar to those in 1 and 2, with four equatorial tetrazole nitrogens and two axial thiocyanate nitrogens completing an axially compressed octahedron. However, with the (CH2)4 spacer between tetrazole rings in the GGT conformation, the btzb ligand in the structure adopts a twisted arclike shape. Two such ligands set up a double bridge between neighboring Co(II) atoms to generate a 22-membered cnetrosymmetric bimetallacycle of [Co2(btzb)2], with a Co · · · Co separation of 10.221(2) Å. The connection of such metallacycles by sharing metal centers leads to a polymeric chain extending along the (11j0) direction. The chains are stacked in parallel to give the 3D architecture shown in Figure 6, and there are two independent sets of interchain C-H · · · N hydrogen bonds (Table 3). The hydrogen bonds involving the tetrazole C6-H6A · groups and the N2 atoms from different chains lead to supramolecular layers along the ab plane, with the metallacycles from different chains being partially overlapped, and the hydrogen bonds between C5-H5A groups and · · N6 atoms from different chains lead to supramolecular layers along the (111) plane. Compound 4. The structure of 4 consists of 2D coordination layers with btzh as bridges between Co(II) ions (Figure 7). The local environment of Co(II) is similar to those in the previous compounds, with an axially compressed octahedral geometry furnished by four equatorial tetrazole nitrogens (N1, N5, N9, N16A) and two axial thiocyanate nitrogens (N17, N18). Selected

Cobalt(II) Coordination Networks

Crystal Growth & Design, Vol. 8, No. 5, 2008 1671

Figure 2. (a) Two-fold parallel interpenetration of the 2D grid sheets in 1. (b) Schematic presentation of the 2-fold interpenetration. (c) Packing of the double layers. Table 3. Hydrogen Bond Lengths (Å) and Angles (deg) for Complexes 1-4 compound

D-H · · · A

D-H

H· · ·A

D· · ·A

∠(DHA)

symmetry codesa

1

C4-H4A · · · N3 C1-H1A · · · N7 C3-H3A · · · S1 C2-H2A · · · S1 C6-H6B · · · S1 C3-3A · · · N10 C1-1A · · · N11 C6-H6A · · · N2 C5-H5A · · · N6 C16-16A · · · N6 C6-H6A · · · N14 C10-10B · · · N3 C5-H5A · · · N10 C6-H6B · · · N10

0.93 0.93 0.97 0.97 0.97 0.97 0.93 0.93 0.97 0.93 0.97 0.97 0.93 0.97

2.45 2.55 2.83 2.79 2.82 2.62 2.51 2.52 2.64 2.42 2.53 2.60 2.51 2.48

3.093(5) 3.097(5) 3.687(4) 3.728(3) 3.621(3) 3.516(3) 3.183(3) 3.298(10) 3.399(9) 3.234(6) 3.491(6) 3.465(7) 3.253(6) 3.285(6)

125.9 118.2 147.5 161.9 140.2 153.0 129.6 140.9 135.7 146.5 169.8 149.2 136.9 139.9

x, y - 1, z -x, y, -z + 1/2 x - 1/2, y - 1/2, z -x + 3/2, y - 1/2, -z + 1/2 x, y - 1, z x - 1, y, z x - 1, y, z -x, -y + 1, -z -x - 1, -y + 2, -z - 1 x, y + 1, z -x + 2, -y + 1, -z + 1 -x + 5/2, y + 1/2, -z + 1/2 -x + 3/2, y - 1/2, -z + 1/2 -x + 3/2, y - 1/2, -z + 1/2

2

3 4

a

Symmetry codes to generate the acceptor atoms.

bond lengths and angles are shown in Table 6. There are three independent btzh bridging ligands in the structure. Two of them are centrosymmetric and assume the TTTTT conformation, and the other one assumes the GTGGG conformation. The former conformation defines the ligands as quasi-linear bridges, while the latter leads to an L-shaped bridge with the bent angle being close to 90°. The combination of these two conformations generates an interesting 2D structure, which bears some features of both 1 and 3. Similar to the btzb ligands in 3, two L-shaped ligands in 4 set up a double bridge between neighboring Co(II) atoms to produce a 26-membered centrosymmetric bimetallacycle of [Co2(btzb)2], which exhibits the rectangular shape, with the diagonal Co · · · Co separation being 12.484(3) Å. In contrast with that in 3, each metallacycle in 4 is connected to four identical motifs through the linear btzh ligands, and this leads to a 2D sheet, which contains two types of rings: the small rectangular bimetallacycle and the much larger ring formed by six Co(II) ions, two bent ligands, and four linear ligands. As expected, the Co · · · Co distances [17.075(9) and 17.183(4) Å] separated by the linear bridges are much longer than that in the metallacycle. In the lattice, there are two identical and mutually inclined sets of sheets that extend in parallel to different crystallographic planes, (121) and (12j1). They are interlocked to fill the large cavities in individual layers, generating a 3D polycatenated network, with each large ring of a layer being catenated by four identical rings from different layers (Figure 8). Ciani et al. have recently proposed that the “degree of catenation” (DOC) for infinite interlocking may be defined by the symbol (a/b/ · · · ), where a, b, and · · · are the numbers of “external” rings catenated

Figure 3. View of the 1D chain in 2. Hydrogen atoms have been omitted for clarity. Table 4. Bond Lengths (Å) and Angles (deg) for Complex 2a Co(1)-N(1) Co(1)-N(9) N(13)-Co(1)-N(1)A N(13)A-Co(1)-N(9) N(1)A-Co(1)-N(9) a

2.165(2) 2.195(2) 90.71(7) 87.87(7) 92.57(7)

Co(1)-N(13) N(13)-Co(1)-N(1) N(13)-Co(1)-N(9) N(1)-Co(1)-N(9)

2.060(2) 89.29(7) 92.13(7) 87.43(7)

Symmetry codes: (A) -x + 1, -y + 1, -z.

to a single ring in different motifs.1g The overwhelming majority of the known polycatenated species with two inclined 2D motifs have DOC ) (1/1) (2-fold inclined interpenetration), and several networks with DOC ) (2/2) or (3/3) have also been reported. The 3D polycatenated network in 4 has DOC ) (4/4). The high catenation is obviously related to the linear TTTTT conformation of the long btzh ligand. The 3D polycatenation is sustained by weak hydrogen bonds (Figure 8a and b). Between the adjacent parallel layers, there are two independent sets of C-H · · · N hydrogen bonds involving

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Figure 4. (a) The layer of parallel chains with interchain hydrogen bonds in 2. (b) The interdigitating packing of the layers, green dashes representing the interlayer hydrogen bonds. (c) A schematic presentation of the interdigitation. Different colors in (b) and (c) are used to distinguish the chains of different directions.

Figure 5. View of the 1D chain in 3. Hydrogen atoms have been omitted for clarity. Table 5. Bond Lengths (Å) and Angles (deg) for Complex 3a Co(1)-N(1) Co(1)-N(8)B N(9)-Co(1)-N(8)B N(9)-Co(1)-N(1)A N(8)C-Co(1)-N(1)

2.174(6) 2.159(6) 90.9(2) 93.1(2) 92.4(2)

Co(1)-N(9) N(9)-Co(1)-N(8)C N(9)-Co(1)-N(1) N(8)B-Co(1)-N(1)

2.083(6) 89.1(2) 86.9(2) 87.6(2)

a Symmetry codes: (A) -x, -y, -z; (B) x + 1, y - 1, z; (C) -x 1, -y + 1, -z.

Figure 7. View of the 2D sheet in 4. Hydrogen atoms have been omitted for clarity. Table 6. Bond Lengths (Å) and Angles (deg) for Complex 4a

Figure 6. View of the chain packing in 3. Interchain hydrogen bonds are highlighted by red dashes.

both L-shaped and linear btzh ligands, and between the interpenetrating layers, each btzh ligand threading through the large ring in a layer forms at least one C-H · · · N hydrogen bond with the ring. Discussion A few Fe(II) coordination polymers with btze, btzb, or similar bis(tetrazole) ligands have been investigated for their spin-

Co(1)-N(1) 2.180(3) Co(1)-N(5) 2.184(3) Co(1)-N(9) 2.204(4) Co(1)-N(16)C 2.143(3) Co(1)-N(18) 2.082(3) Co(1)-N(17) 2.074(4) N(17)-Co(1)-N(18) 177.37(14) N(17)-Co(1)-N(16)C 94.24(13) N(18)-Co(1)-N(16)C 88.39(13) N(17)-Co(1)-N(1) 86.56(13) N(18)-Co(1)-N(1) 90.82(13) N(16)C-Co(1)-N(1) 176.60(13) N(17)-Co(1)-N(5) 88.42(13) N(18)-Co(1)-N(5) 91.41(13) N(16)C-Co(1)-N(5) 92.29(13) N(1)-Co(1)-N(5) 91.03(13) N(17)-Co(1)-N(9) 95.15(14) N(18)-Co(1)-N(9) 84.94(13) N(16)C-Co(1)-N(9) 89.14(13) N(1)-Co(1)-N(9) 87.49(13) N(5)-Co(1)-N(9) 176.04(12) a Symmetry codes: (A) -x + 2, -y, -z; (B) -x + 1, -y, -z + 1; (C) -x + 2, -y + 2, -z + 1.

transition behaviors.8,9 In these compounds, the counterions used are usually the noncoordinated anions ClO4-, BF4-, or PF6-. Consequently, the octahedral metal coordination is exclusively completed by tetrazole groups, and the metal ions are linked into 1D chains with triple or double bis(tetrazole) bridges,8a–c 2D networks with both single and double bis(tetrazole) bridges,8d

Cobalt(II) Coordination Networks

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mainly determined in the supramolecular level: the coordination networks can be stabilized by appropriate interpenetration, interdigitation, or hydrogen bonding between the networks (chains or layers). For example, in 1 and 2, the dimension of the coordination moiety (1) or the nonbridging btze ligand (2) in a network matches very well with the space spanned by the bridging btze ligand in the neighboring network and, as a result, interpenetration or interdigitation occurs. This arranges the hydrogen bonding sites from different networks in appropriate positions to form intermolecular C-H · · · N and C-H · · · S hydrogen bonds (Figures 2 and 4). Likely, the replacement of btze by btzh or btzb in the structures would lead to space mismatch for interpenetration and interdigitation and would also make it impossible to form appropriate intermolecular hydrogen bonds. Conclusions

Figure 8. Views showing (a) the hydrogen bonds between parallel layers, (b) the hydrogen bonds between a large ring and the four bridging ligands threading through it, and (c) the 3D polycatenated network.

or 3D simple cubic frameworks with single btzb bridges.9 Our investigation has suggested the assembly of Co(ClO4)2 with btze also led to the 1D chain compound with triple btze bridges, [Co(btze)3](ClO4)2,14 which is isomorphous with a known Fe(II) compound, [Fe(btze)3](BF4)2.8b In the four compounds presented above, the thiocyanate counterions, which have a strong tendency to coordinate, occupy two trans positions around each Co(II), and thus leave four equatorial positions for the bis(tetrazole) groups. It is obvious that the assembly is strongly influenced by the presence of the thiocyanate ion, which induces the [Co(NCS)2(tetrazolyl)4] coordination spheres. The effects of other coordinative anions on the assembly need further investigations. In the present cases with thiocyanate couteraions, it seems worthwhile to consider the effects of the bis(tetrazole) ligands on the structures. There are several possible ways in which the ligands connect the coordination spheres into polymeric structures. Under our synthetic conditions, it is obvious that the final outcomes of the self-assembly are dependent upon the spacer length and conformation of the ligands (and the metal-to-ligand ratio in the case of btze). Increasing the spacer length in the bis(tetrazole) ligands does not lead to simple network expansion, but instead, it leads to dramatic changes in network topology: while btze links Co(II) into singly bridged 1D linear chains or 2D square grid layers, btzb and btzh lead to 1D chains with double bridges or 2D layers with both single and double bridges. Although it is impossible to rationalize the formation of each structure, it is conceivable that the btze and btzb ligands are too short for the layer topology of 4: the short ligands would lead to significant interligand steric hindrance within the layer. The preference of the ligands for the specific structures may be

We have presented systematic investigations on the coordination polymers assembled from cobalt(II) and flexible bis(tetrazole) of different lengths in the presence of thiocyanate. The short btze ligand gives singly bridged 2D square grid layers with 2-fold parallel interpenetration or 1D linear chains with deep interdigitation, dependent upon the metal-to-ligand ratio used for the synthesis. The btzb ligand links the metal ions into doubly bridged chains composed of bimetallacycles. Most interestingly, the long btzh ligand leads to 2D layers with both single and double bridges between Co(II) ions, and the inclined interpenetration of layers generates a 3D polycatenated structure with a high degree of catenation. The results show that the dimensionality and topology of the structures depend strongly upon the length and the conformation of the flexible alkyl spacer between the tetrazole rings. Acknowledgment. The authors thank NSFC (20571026, 20771038 and 20490210), MOE (NCET-05-0425), Shanghai Leading Academic Discipline Project (B409), and STCSM (06SR07101) for financial support. Supporting Information Available: X-ray crystallographic files in CIF format for complexes 1-4. This material is available free of charge via the Internet at http://pubs.acs.org.

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