An Atypical Network: Noninterpenetrating (10,3)-d Nets Using an

Aug 14, 2007 - The network structure was invariant to the choice of anion. ... A feature of the three structures was the invariance of the networks ...
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An Atypical Network: Noninterpenetrating (10,3)-d Nets Using an Unsymmetrical Flexible Ligand and Ag(I) as Three-Connected Nodes Cory A. Black and Lyall R. Hanton*

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 9 1868-1871

Department of Chemistry, UniVersity of Otago, P.O. Box 56, Dunedin, New Zealand ReceiVed May 13, 2007; ReVised Manuscript ReceiVed June 25, 2007

ABSTRACT: The flexible unsymmetrical thioether ligand 4-pyridylsulfanylmethyl-pyrazine (L) was prepared and reacted with AgX (X ) BF4, ClO4, PF6) to form three isomorphous coordination polymers {[Ag(L)](X)}∞ that were characterized by X-ray crystallography. Topological analysis revealed that the compounds formed rare noninterpenetrated (10,3)-d (or utp) nets. The ligand was designed to act as a three-connected node and provided three nonplanar topological links to form an extended three-dimensional structure. A feature of the three structures was the invariance of the networks to the choice of anion. The indicators for distinguishing between (10,3)-a and (10,3)-d nets are discussed. Introduction The design and synthesis of new functional solid materials will benefit from the understanding and insight gained from the development of extended networks with unusual topologies. An excellent and versatile means of generating such networks is through the use of coordination polymer chemistry. In particular, understanding how to incorporate chirality into coordination polymers1 is a current synthetic challenge because such systems are likely to find uses in chiral separation and catalysis.2 While there are many possible chiral network types,3 one of the more common that offers promise in the area of chiral synthesis is the three-connected enantiomorphic (10,3)-a network.4 The number of examples of coordination polymers with this topology continues to grow.5 One obvious requirement for the formation of (10,3) nets is the inclusion of three-connected metal and/or ligand nodes.6 Our strategy in this regard has been to design a multimodal ligand L (Chart 1) containing different types of binding sites arranged in an unsymmetrical fashion. This will allow L to act as a three-connected node by providing one chelating and two monodentate binding sites. Importantly, flexibility has been incorporated by way of a thioether moiety to permit the three topological links to be nonplanar and therefore extend into three dimensions. Reaction of L with AgX (X ) BF4, ClO4, PF6) produced three isomorphous threedimensional networks with the anticipated (10,3) topology. However, after careful analysis the topology was found to be that of a noninterpenetrated (10,3)-d net, which is a rare form of the (10,3) topology and represented only the second genuine noninterpenetrated example. The (10,3)-d net differs from the (10,3)-a net by having chiral channels with alternating handedness resulting in an overall racemic network. However, it is worth noting that the (10,3)-a net is enantiomorphic and that, unless homochiral ligands are used or homochiral crystallization occurs, the bulk sample will be a racemic mixture requiring separation. From the point of view of understanding the design and synthesis of new chiral materials, the (10,3)-d net offers considerable insight and the same intellectual curiosity value as the seemingly more practically useful (10,3)-a net. Further, care must be taken in assigning the topology of these (10,3) networks. Herein we report the structurally consistent formation * To whom correspondence should be addressed. Fax: (+64) 3-4797906. Phone: (+64) 3-479-7918. E-mail: [email protected].

Chart 1. Conventional Numbering Scheme for Ligand La

a

Arrows show L acting as a three-connected node.

of three coordination polymers with a noninterpenetrated (10,3)-d topology and contrast them with some recently reported examples,7,8 one of which is not correctly assigned.8 Experimental Section General. The precursor 2-(chloromethyl)pyrazine was prepared according to a literature method,9 while 4-picolylchloride‚HCl was purchased from Aldrich and used as received. All 1H, 13C, and twodimensional NMR spectra were measured with a 300 or 500 MHz Varian UNITYINOVA spectrometer at 298 K. IR spectra were measured with a Perkin-Elmer Spectrum BX FT-IR system (samples were prepared as KBr disks in all cases). Elemental analyses were performed by the Campbell Microanalytical Laboratory at the University of Otago, New Zealand. Samples were predried under vacuum. Caution: Although no problems were encountered in this work, transition metal perchlorates are potentially explosive. They should only be prepared in small amounts and handled with care. Synthesis of 4-Pyridylsulfanylmethyl-pyrazine (L). 2-(Chloromethyl)pyrazine (5.8 g, 45 mmol) was dissolved in ethanol (100 mL). Thiourea (3.5 g, 46 mmol) was added, and the mixture was refluxed for 1 h. The solvent was removed, leaving the isothiuronium salt. The salt was dissolved in methanol (150 mL). KOH (7.5 g, 0.13 mol) was dissolved in H2O (50 mL) and added via cannula to the methanol solution. This mixture was refluxed for 1.5 h under argon. A saturated solution of Na2CO3 (100 mL) was added via cannula to 4-picolylchloride‚ HCl (7.3 g, 44 mmol). The thiolate solution was added to this yelloworange solution via cannula, and the sample was refluxed for 19 h under argon. The reaction was allowed to cool and was reduced in volume to a predominantly aqueous solution. This solution was washed with CH2Cl2 (15 × 75 mL), dried (MgSO4), and the solvent removed again to give a crude black oil (11 g). Purification on a silica gel column (5% hydrated v/v) eluted with CHCl3 gave L as a yellow oil (2.5 g, 26%). NMR: δH (CDCl3, 500 MHz) 8.45 (1H, d, H(3), J ) 1.5 Hz), 8.41 (2H, br d, H(11), J ) 4.5 Hz), 8.36 (1H, dd, H(6), J ) 2.5, 1.5 Hz), 8.33 (1H, d, H(5), J ) 2.5 Hz), 7.15 (2H, br d, H(10), J ) 4.5 Hz), 3.60 (2H, s, H(7)), 3.55 (2H, s, H(8)); δC (CDCl3, 500 MHz) 153.92 (C2), 149.63 (C11), 146.60 (C9), 144.61 (C3), 143.53 (C6),

10.1021/cg070437j CCC: $37.00 © 2007 American Chemical Society Published on Web 08/14/2007

An Atypical Network: Noninterpenetrating (10,3)-d Nets

Crystal Growth & Design, Vol. 7, No. 9, 2007 1869

Table 1. Crystallographic Data for Complexes 1-3 empirical formula M crystal system space group a/ Å b/ Å c/ Å V/Å3 Z T/K µ/mm-1 reflections collected unique reflections (Rint) R1 indices [I > 2σ(I)] wR2 (all data)

1

2

3

C11H11AgBN3F4S 411.97 orthorhombic Pna21 13.173(5) 8.493(5) 12.555(5) 1404.6(11) 4 89(2) 1.622 10365 4433 (0.0452)

C11H11AgN3O4SCl 424.61 orthorhombic Pna21 13.080(4) 8.564(3) 12.607(4) 1412.2(7) 4 90(2) 1.782 20280 6568 (0.0279)

C11H11AgN3F6PS 470.13 orthorhombic Pna21 13.939(5) 8.334(5) 12.856(5) 1493.5(12) 4 123(2) 1.661 34865 14301 (0.0187)

0.0439 (3759) 0.1278

0.0260 (5931) 0.0606

0.0216 (12448) 0.0531

142.78 (C5), 123.81 (C10), 34.35 (C8), 34.16 (C7). Selected IR (neat)/ cm-1: 3030 (m), 2922 (m), 1599 (s), 1473 (m), 1413 (s), 1219 (m), 1057 (m), 1017 (s), 836 (m), 567 (m). Found: C, 60.59; H, 5.37; N, 19.64; S, 14.81. Calc. for C11H11N3S: C, 60.80; H, 5.10; N, 19.34; S, 14.76%. Synthesis of Complexes. {[Ag(L)](X)}∞ [X ) BF4 (1), ClO4 (2), PF6 (3)]. In a typical synthesis, AgX (0.25 mmol) dissolved in MeCN (25 mL) was added to L (0.23 mmol) dissolved in MeCN (25 mL). The solution was stirred overnight and concentrated to 10 mL. 1-Butanol (10 mL) was added, and the solution was further concentrated resulting in the formation of a tan precipitate. X-ray quality crystals were grown from the slow diffusion of a CHCl3 solution of L layered with benzene into a MeCN solution of AgX. {[Ag(L)](BF4)}∞, 1. Yield: 72%. Anal. Calc. for C11H11N3SAgBF4‚ H2O: C, 30.73; H, 3.05; N, 9.77; S, 7.46. Found: C, 30.64; H, 2.64; N, 10.15; S, 7.06%. Selected IR (KBr)/cm-1: 3030 (w) (L), 2927 (w) (L), 1604 (m) (L), 1417 (m) (L), 1058 (s, br) (BF4-). {[Ag(L)](ClO4)}∞, 2. Yield: 65%. Anal. Calc. for C11H11N3SAgClO4: C, 31.12; H, 2.61; N, 9.90; S, 7.55. Found: C, 30.77; H, 2.61; N, 9.87; S, 7.23%. Selected IR (KBr)/cm-1: 3029 (w) (L), 2927 (w) (L), 1603 (m) (L), 1417 (m) (L), 1109 (s, br) (ClO4-), 1017 (m) (L), 627 (s). {[Ag(L)](PF6)}∞, 3. Yield: 72%. Anal. Calc. for C11H11N3SAgPF6‚ H2O: C, 27.07; H, 2.68; N, 8.61; S, 6.57. Found: C, 27.28; H, 2.35; N, 8.93; S, 6.38%. Selected IR (KBr)/cm-1: 3036 (w) (L), 2926 (w) (L), 1606 (m) (L), 1419 (m) (L), 834 (s, br) (PF6-). X-ray Crystallography. X-ray diffraction data for 1-3 were collected on a Bruker APEX II CCD diffractometer, with graphite monochromated Mo-KR (λ ) 0.71073 Å) radiation. Intensities were corrected for Lorentz polarization effects,10 and a multiscan absorption correction11 was applied. The structures were solved by direct methods (SHELXS12 or SIR-9713) and refined on F2 using all data by fullmatrix least-squares procedures (SHELXL 9714). All calculations were performed using the WinGX interface.15 Detailed analyses of the extended structure were carried out using PLATON16 and MERCURY17 (Version 1.4.1). Topological analyses were conducted using OLEX (Version 2.55).18 Crystal data and refinement details for the three structures are summarized in Table 1.

Results and Discussion Ligand L was prepared by the reaction of 2-(chloromethyl)pyrazine9 with thiourea to generate the isothiuronium salt. Addition of base formed the thiolate, which was coupled with 4-picolylchloride to form L. Reaction of L with AgX (X ) BF4, ClO4, PF6) in a 1:1 molar ratio afforded tan colored precipitates that gave analyses consistent with 1:1 metal-toligand ratios. X-ray quality crystals were grown by the slow diffusion of L in CHCl3 into a solution of AgX in MeCN. X-ray structural analyses of 1-3 revealed the complexes to be isostructural three-dimensional arrays that crystallized in the orthorhombic space group Pna21. The asymmetric unit contained one Ag(I) ion, one ligand, and one BF4-, ClO4-, or PF6counterion for 1-3, respectively. The Ag(I) ion was in a

Figure 1. View (ac plane) of 1 showing a representative coordination environment for 1-3 (crystallographic numbering) with BF4- anions omitted for clarity (50% probability ellipsoids). Selected bond lengths (Å) and angles (°): Ag(1)-N(1) 2.562(3), Ag(1)-N(2A) 2.250(3), Ag(1)-N(3B) 2.298(4), Ag(1)-S(1) 2.4786(13); N(1)-Ag(1)-N(2A) 115.07(12), N(1)-Ag(1)-N(3B) 84.97(12), N(1)-Ag(1)-S(1) 77.39(8), N(2A)-Ag(1)-N(3B) 109.49(13), N(2A)-Ag(1)-S(1) 125.90(8), N(3B)-Ag(1)-S(1) 124.33(11). (symmetry codes: A x + 1/2, 1/2 y, z; B 1/2 - x, y - 1/2, z + 1/2).

distorted trigonal pyramidal arrangement with one chelating (Npz, S) and two monodentate (Npz, Npy) donors (pz ) pyrazine; py ) pyridine) (Figure 1). The trigonal plane was formed from the S and the two monodentate (Npz, Npy) donors with the Ag(I) ion 0.072, 0.058, and 0.277 Å above the plane in 1-3, respectively. The flattening of the geometry about the Ag(I) ion was probably caused by the ligand rather than any effect of the counteranions, which were quite distant from the Ag(I) ion at about 3.0 Å. The lack of anion influence was further supported by the observation that 3 which had the closest anion (PF6-) to Ag(I) distance of 2.9553(19) Å also had the greatest deviation of the Ag(I) ion from the trigonal plane and that deviation was on the side of the plane opposite to the anion. The ligand used all available donor atoms to bind to three Ag(I) ions through one bidentate and two monodentate coordinate donors. The ligand was arranged in an endo-anti configuration19 with angles between ring planes of 20.7°, 20.3°, and 17.4° for 1-3, respectively. Orthogonal fit calculations were carried out in a pairwise fashion comparing the non-hydrogen atoms of the ligands in each structure and they showed the close similarity of ligand conformations. All bond distances to the Ag(I) center were within expected ranges (Ag-SC2 2.39-3.01 Å; Ag-Npy 2.12-2.82 Å), as determined by a search of the Cambridge Structural Database (CSD) Version 5.27 for four-coordinate Ag complexes, with the exception of the chelated Npz donor, N(1), which in all structures was near the limit of observed values in the CSD (Ag-Npz 2.18-2.53 Å).20,21 Furthermore, N(1) and the Npy donor [N(3B)] were coordinated to the Ag(I) center in a bent fashion similar to that found with related thioetherpyrazine-pyridine containing ligands.22 The Ag(I)-N(1)-centpz angles were 146.5°, 147.6°, and 151.0° with Ag(I)-N(3B)centpy angles of 158.4°, 157.7°, and 156.1° for 1-3 respectively. Interestingly, the Ag(I)-N(1)-centpz bond angles were all much smaller than the smallest bond angle of 154.0° found in 243 recorded Ag(I)-Npz-centpz bond angles in the CSD. Also, the Ag(I)-N(3B)-centpy angles were all well below the lower quartile of 163.1° for 1973 recorded Ag(I)-Npy-centpy bond angles in the CSD. The angles in the CSD ranged from most

1870 Crystal Growth & Design, Vol. 7, No. 9, 2007

Black and Hanton

Figure 2. View (left) of 1 in the ab plane with an overlay of the (10,3)-d net and view (right) along the c-axis showing the BF4- anions in one of the helical channels. Figure 4. 4. View (ab plane) of the (10,3)-d topological net formed in 1-3 with a distorted 4‚82 projection. The alternating handedness (orange and black) of the helices along the a-axis is shown.

Figure 3. View (left) of the ac plane of the (10,3)-d topological net formed in 1-3, which projects as a (6,3) net. The alternating handedness of the helices along the a-axis is shown. View (right) of the bc plane. The Ag(I) and ligand nodes are shown in purple and black, respectively.

acute at 129.7° to ideal at 180.0°. The other Npz donor [N(2A)] was coordinated with more conventional Ag(I)-Npz-centpz angles of 177.4°, 176.6°, and 178.6° for 1-3 respectively. Anions in each structure were located within helical cavities (Figure 2) and were held in place by hydrogen bonding interactions with C-H‚‚‚F-B, C-H‚‚‚O-Cl, and C-H‚‚‚F-P distances in the range 2.38-2.51 Å, 2.39-2.59 Å, and 2.412.55 Å, respectively (corresponding C‚‚‚F-B separations in the range 3.13-3.45 Å, C‚‚‚O-Cl separations in the range 3.113.51 Å, C‚‚‚F-P separations in the range 3.20-3.49 Å). The structures were found to be uninodal nets with a Schla¨fli topological term of 103 (Figure 2). The net had two threeconnected topologically equivalent nodes representing either the Ag(I) center or an averaged ligand position. Interestingly, the pseudo-tetrahedral Ag(I) center became three-connected because the chelating pocket of the multimodal ligand was reduced to a single topological link. Closer examination indicated the network had an extended Schla¨fli symbol of 102‚104‚104 and a c10 number of 621 and was therefore a (10,3)-d net according to Wells’ classification4 or a utp net.23 The c10 number allows for the distinction between nets with the same extended Schla¨fli symbol by describing the number of neighbors on expanding the net up to the 10th vertex.6 Wells notes the (10,3)-d topology is related to the chiral (10,3)-a net in that it has pseudo fourfold helices. The (10,3)-a net is the most symmetrical threeconnected three-dimensional net.24 In contrast to the (10,3)-a net, the (10,3)-d net has both left- and right-handed helices, which in 1-3 alternate along the a-axis resulting in an overall racemic network. The ac plane projects as a (6,3) net with each helix surrounded by six others that run through the structure creating cavities (Figure 3). However, the ab plane projects as a 4‚82 net with helices of alternating hand again running along the a-axis (Figure 4). One of the interesting features of this net

is that it is noninterpenetrated. Interpenetration in networks is a capricious property.25 It is likely that in our system interpenetration is not possible because the building blocks deviate from ideal geometries producing a rather distorted and congested (10,3)-d net in which anions occupy the chiral channels (Figure 2). There are only five examples of coordination polymers with (10,3)-d topologies, three interpenetrated26 and two very recently reported noninterpenetrated examples.7,8 Our analysis of the topology of the most recent noninterpenetrated example8 revealed it to have a (10,3)-a rather than a (10,3)-d network as claimed by the authors. There are a number of indicators that point to the incorrect topological assignment of these lanthanide(III) carboxylate phosphonate complexes. Our analysis showed the networks had extended Schla¨fli symbols of 105‚105‚105 and not the reported 102‚104‚104. Furthermore unlike (10,3)-d nets, (10,3)-a nets project in all three planes as 4‚82 nets, and this is the case for the lanthanide(III) complexes (see Supporting Information). Such identical projections are possible in the space group P212121 found for the lanthanide(III) complexes but not in the space groups Pna21,7 Pbca,26a,b or Ccca26c in which (10,3)-d nets have been found to occur. The net of the lanthanide(III) complexes belongs to the space group P212121 and is clearly chiral. This is consistent with a noninterpenetrated (10,3)-a net, which would be expected to be chiral, but rather less consistent with a (10,3)-d topology, which would be expected to be racemic. Some circumspection is, however, required as it is possible for a net with a nonchiral topology to crystallize in either a chiral or a nonchiral space group.27 Related to this issue is the chirality of the helices. The authors correctly identify all tetragonal and all octagonal helices as being rightand left-handed, respectively.6 This is expected for a (10,3)-a net but is not the case for a (10,3)-d net. In determining whether the net had helices of alternating handedness, they incorrectly compare different types of helices rather than comparing the same type of helix. The three isostructural networks described here therefore constituted only the second example of a noninterpenetrated coordination polymer network with a (10,3)-d topology. Conclusion This work showed that a flexible unsymmetrical ligand can be used effectively as a three-connected node in conjuction with

An Atypical Network: Noninterpenetrating (10,3)-d Nets

Ag(I) salts and noncoordinating anions to produce a structurally consistent network of unusual topology. It seems that in our systems the combination of a number of factors has provided an environment favorable to the consistent formation of a noninterpenetrated (10,3)-d net. First, Ag(I) displays a lack of stereochemical preference and has an accommodating coordination sphere. Second, noncoordinating anions have a minimal influence on the coordination environment about the Ag(I) ion. By using this soft metal ion as a node together with noncoordinating anions, the conformation adopted by the ligand is allowed to dominate the structural outcome. Finally, the flexible nature of the ligand in concert with the endo-anti conformation provides three nonplanar topological links. Furthermore, we have discussed the indicators for distinguishing between (10,3)-a and (10,3)-d nets. A challenge for crystal engineering is the ability to produce compounds with topologies that have no precedence in inorganic compounds or minerals. The networks we have reported showed a remarkable degree of structural consistency considering that Ag-heterocyclic systems, particularly those with flexible ligands, are notoriously difficult to direct. Acknowledgment. We thank the University of Otago Research Committee and the Department of Chemistry, University of Otago for financial support. Supporting Information Available: X-ray data in CIF format and reinterpreted topological analysis for complexes in ref 18. This material is available free of charge via the Internet at http://pubs.acs.org.

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