Ring-Opening Isomerization Based on the 3-Connecting Node

DOI: 10.1021/cg100742c

Ring-Opening Isomerization Based on the 3-Connecting Node: Formation of a 0-D M2L3 Cage, 1-D Loop-and-Chain, and 2-D (6, 3) Network

2010, Vol. 10 4076–4084

Qian Zhang,† Jianyong Zhang,† Qiong-Yan Yu,† Mei Pan,† and Cheng-Yong Su*,†,‡ †

MOE Laboratory of Bioinorganic and Synthetic Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China, and ‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China Received June 3, 2010; Revised Manuscript Received July 10, 2010

ABSTRACT: Two semirigid ditopic ligands, 1,4-bis(benzimidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (L1) and 1,3-bis(benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene (L2), reacted with Agþ salts to result in two series of complexes, namely [Ag2(L1)2](CF3SO3)2 (1-R), [Ag2(L1)3](CF3SO3)2 (1-C), {[Ag2(L1)3](CF3SO3)2 3 CH3CN}n (1-P), and [Ag2(L2)2](ClO4)2 3 1.5CH3CN (2-R), {[Ag3(L2)2](ClO4)3}n (2-P). All complexes have been structurally characterized by single-crystal X-ray diffraction with the phase purity of bulk samples attested by powder X-ray diffraction (PXRD). Four types of structures are formed: (1) a discrete M2L2 ring with two Agþ ions and two cis-L ligands comprising a molecular rectangle (1-R and 2-R), (2) a discrete M2L3 cage with two Agþ ions and three cis-L ligands comprising a trigonal cage (1-C), (3) a one-dimensional [M2L3]n loop-and-chain with 3-connecting Agþ ions bridged by both cis- and trans-L ligands (1-P), and (4) a two-dimensional [M2L3]n network of (6,3) topology with 3-connecting Agþ ions bridged by trans-L ligands (2-P). The M2L3 cage 2-C was not obtained as a solid-state complex but observable in solution by ESI mass spectrometry. The complexes 1-C, 1-P and 2-C, 2-P contain comparable 3-connecting M2L3 building blocks, constituting two pairs of ring-opening isomers corresponding to single ringopening (1-C to 1-P) and double ring-opening (2-C to 2-P) polymerization processes via cis-L to trans-L ligand conformation change, respectively. Investigations on solution behaviors by 1H NMR and ESI-MS and structural conversions monitored by PXRD disclose that the thermodynamically favored M2L2 ring can be converted to a thermodynamically disfavored M2L3 cage in solution through an L addition mechanism, which causes crystallization of isomeric structures of an M2L3 cage or [M2L3]n polymer due to ring-opening isomerization. Formation of an M2L3 cage or [M2L3]n polymer is influenced by kinetic or thermodynamic effects as well as the solubility-product constant (Ksp), implying predictable syntheses by controlling the crystallization conditions.

Introduction Design and synthesis of metal-organic materials (MOMs)1 with topological prototypes ranging from discrete metallacycles/cages (e.g., metal-organic polygons/polyhedra, MOPs, or metal-organic containers/cages, MOCs) to polymeric structures (e.g., metal-organic frameworks, MOFs, or coordination polymers, CPs) has triggered vigorous investigations in recent decades due to various promising applications.2,3 However, predicting assembly of a target structure on the basis of a metal-ligand coordination bond has often been found to go astray because of the structural diversification. Supramolecular isomerism represents a common phenomenon in the process of coordination assembly, which leads to formation of different structures even though the isomers contain the same building blocks.4 Since polymeric structures (one-, two-, or threedimensional: 1-D, 2-D, or 3-D) are usually believed not to be present in solution but may be crystallized from soluble cyclic precursors or oligomers, it is of interest to know how the closed metallacycles/cages are related to their extended isomeric structures of CPs.5 Study on such an intrinsic relationship between the discrete (zero-dimensional, 0-D) and polymeric (1-D to 3-D) structures may be helpful to understand the mechanism by which the CPs form and finally to approach the goal of synthetic control. We have previously used the term *To whom correspondence should be addressed. E-mail: cesscy@mail. sysu.edu.cn. pubs.acs.org/crystal

Published on Web 08/05/2010

ring-opening isomerism (ROI)6 to systematize the isomeric structures between the discrete and polymeric coordination compounds, and Puddephatt, James, et al. have proposed ring-opening polymerization (ROP) to explain the possible crystallization process of CPs from discrete ring/cage precursors.7 More generally, ROI can represent a common type of supramolecular isomerism with the isomers structurally related to each other by at least one ring-opening. In another words, if two or more structures have the same building blocks and stoichiometry, but interconversion between them accompanies at least one ring-opening, these structures can be considered as the ring-opening isomers. Therefore, an ROI structural relationship can exist not only between the discrete and polymeric supramolecular isomers (previously also termed by James as an ROP relationship,8 which represents a specific case of the more general ROI term used here) but also among the discrete isomers or polymeric isomers, respectively. For example, 0-D M2L3 cages can be transformed into 1-D polymeric [M2L3]n loops-and-chains accompanying one ring-opening9a,b or 2-D polymeric (6, 3) networks accompanying two ring-opening.9c,d Two ring-opening polymerization of 0-D M2L3 cages may also lead to formation of a 3-D polymeric (10, 3) framework, although such examples are waiting for discovery. All these structures contain the same 3-connecting nodes as building blocks, and their ROI structural relationship can be depicted by Scheme 1. Both a 3-connecting 2-D (6, 3) network and a 3-D (10, 3) framework can be regarded as one ring opened isomers r 2010 American Chemical Society

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Crystal Growth & Design, Vol. 10, No. 9, 2010

Scheme 1. Formation of a 1-D Loop-and-Chain, a 2-D (6, 3) Network, and a 3-D (10, 3) Framework via ROP Processes from 0-D M2L3 Cages, Showing the ROI Structural Relationship between These Isomers

of the 1-D loop-and-chain, and all these polymeric structures can be theoretically formed through ROP processes from 0-D M2L3 cages. So far, most of the reported ring-opening isomers, which have been observed for Ag(I), Au(I), Hg(II), and Cu(II) coordination complexes in a number of cases,10 are limited to the ring-closed 0-D metallacycles and the corresponding ringopened 1-D polymers. That is, ROP of the 0-D cyclic species normally results in a 1-D polymer with single ring-opening, while very few examples have been observed for double ringopening of a 0-D cage to a 2-D network.9 The ROP mechanism implies that, as precursors, the discrete ring or cage species form first in solution, and then ring-opening takes place during crystallization to form a polymer.7 Since the simple ring species, for example an M2L2 ring, are often more preferable in solution than the cage species, for example an M2L3 cage of a larger molecule, it is understandable that the single ROP processes can be more easily observed than the multiple ROP processes for the formation of the ring-opened polymers from the ring-closed precursors. We previously found that reaction of Agþ salts with the cliplike semirigid ditopic ligands afforded both a Ag2L2 dinuclear metallacycle and a Ag2L3 trigonal prismatic box.11 Herein, we report a systematic study on Agþ complexes of the ligands 1,4-bis(benzimidazol-1-ylmethyl)-2,3,5,6-tetramethylbenzene (L1) and 1,3-bis(benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene (L2). The structural relationship and potential assembly process of two series of complexes, namely [Ag2(L1)2](CF3SO3)2 (M2L2 ring, 1-R),11 [Ag2(L1)3](CF3SO3)2 (M2L3 cage, 1-C),11 {[Ag2(L1)3](CF3SO3)2 3 CH3CN}n ([M2L3]n polymer, 1-P), and [Ag2(L2)2](ClO4)2 3 1.5CH3CN (M2L2 ring, 2-R), {[Ag3(L2)2](ClO4)3}n ([M2L3]n polymer, 2-P) have been investigated and discussed (in the labeling scheme, P = polymer, C = cage, R = ring). A one L addition mechanism is proposed for the formation of an M2L3 0-D cage from the thermodynamically favored M2L2 0-D ring and further related to its single ring-opened 1-D polymer (1-P) and double ring-opened 2-D polymer (2-P). Results and Discussion Crystal Structures. The single crystal structures of [Ag2(L1)2](CF3SO3)2 (1-R) and [Ag2(L1)3](CF3SO3)2 (1-C) have been previously described,11 the former consisting of an [Ag2(L1)2]2þ ring and two CF3SO3- anions while the latter consists of an [Ag2(L1)3]2þ cage and two CF3SO3- anions. As seen from Figure 1, one of the common structural features in these two complexes is that the L1 ligands take the

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cis-conformation to act as “molecular clips”, thus wrapping around two Agþ ions to form M2L2 type or M2L3 type discrete metallacycles. On the contrary, in complex {[Ag2(L1)3](CF3SO3)2 3 CH3CN}n, the L1 ligands display both cis- and trans-conformations to form a 1-D coordination polymer which is assigned as 1-P (Figure 1c). The asymmetric unit consists of one Agþ ion, one cis-L1 ligand, a half of a trans-L1 ligand which is located on an inversion center, two halves of CF3SO3- anions, and half of a CH3CN molecule. The Agþ ion adopts a “T-shaped” AgN3 coordination geometry, binding two cis-L1 ligands and one trans-L1 ligand. Therefore, the 1-D polymer has a repeating M2L3 building unit, which is the same as the 0-D M2L3 cage complex 1-C. From Figure 1c we can see that the 1-D polymer shows a loop-andchain structural feature. That is, two cliplike cis-L1 ligands connect two Agþ ions to form an M2L2 ring similar to 1-R, and the trans-L1 ligands bridge such M2L2 rings to extend infinitely along the [100] direction. If the CH3CN solvent molecules and the CF3SO3- counteranions are ignored, 1-P and 1-C contain the same 3-connecting nodes, forming a pair of isomers: 0-D M2L3 vs 1-D [M2L3]n. The structural relationship between 1-C and 1-P is obviously inherent in ring-opening isomerization. As illustrated in Scheme 1, one ring-opening of 1-C can lead to formation of 1-P through polymerization. Different from the ligand L1, reaction of L2 with AgClO4 only afforded the discrete M2L2 ring complex [Ag2(L2)2](ClO4)2 3 1.5CH3CN (2-R) and the polymeric [M2L3]n complex {[Ag3(L2)2](ClO4)3}n (2-P). Efforts in preparing a [Ag2(L2)3]2þ cage complex (2-C) failed, although various reaction and crystallization conditions had been tried with different Agþ salts. As shown in Figure 2, the [Ag2(L2)2]2þ ring in 2-R looks similar to the [Ag2(L1)2]2þ ring in 1-R, with the ligand L2 displaying the same cis-conformation. Two Agþ ions join two L2 ligands in almost a linear fashion to form a molecular rectangle. In contrast, all the L2 ligands in polymeric complex 2-P display a trans-conformation. The Agþ ion is in an AgN3 trigonal coordination geometry, coordinating to three different L2 ligands, while every L2 ligand bridges two different Agþ ions, thus generating a 2-D (6, 3) network (Figure 2c). It is worthy of note that the 3-connecting building unit in 2-P looks like an M2L3 opencage. As detailed by Figure 2b, three C3-symmetry related L2 ligands wrap around an Agþ center, rather similar to that in the trigonal cage 1-C. However, the other end of the cage is opened by rotating the L2 ligand from the cis-conformation to the trans-conformation. In other words, the 2-P polymer is apparently related to a visualized 2-C through double ringopening polymerization, constituting a pair of ring-opening isomers. The solid-state thermal stability of these complexes has been estimated by thermogravimetric analysis (TG), as shown in Figure S1 of the Supporting Information. Complexes 1-P and 2-R display a small weight loss in the temperature range 35-180 C, which corresponds to escape of acetontrile solvent molecules. Except this, all complexes show similar thermal behavior with a major weight loss in between 250 and 700 C, corresponding to decomposition of the coordination structures. Synthesis Control and Structural Conversion. The X-ray single-crystal analyses suggest that structural diversification did occur in the process of coordination assembly; namely, different or isomeric structures were obtained from the

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Figure 1. (a) 0-D M2L2 ring in 1-R. (b) 0-D M2L3 cage in 1-C. (c) 1-D [M2L3]n loop-and-ring in 1-P. All solvent molecules, CF3SO3- anions, and H atoms are omitted for clarity.

Figure 2. (a) 0-D M2L2 ring in 2-R. (b) M2L3 open-cage building unit in 2-P. (c) 2-D (6, 3) network in 2-P. All solvent molecules, ClO4- anions, and H atoms are omitted for clarity.

same L1/L2 ligands and Agþ ions. Now the questions arise: is it possible to control the structures by choosing appropriate reaction conditions? And can the isomeric structures be converted to each other? To explore these possibilities, assembly of Agþ complexes was carried out in an ethanol-acetonitrile (MeOH-MeCN) solvent system at room temperature by altering the metalto-ligand ratios and the crystallization rates, and the products were monitored by X-ray powder diffraction (XRPD) measurements. The results are summarized in Scheme 2. The crystallization rates were controlled by choosing two crystal growth methods: (a) rapid mixing of two solutions

containing L1/L2 ligand and Agþ salt; (b) slow diffusion between two solutions containing L1/L2 ligand and Agþ salt. The products turned out not only to correlate to metalto-ligand ratios and crystallization rates, but also to depend on the ligand natures. Rapid mixing of a solution of L1 in EtOH and a solution of AgCF3SO3 in MeCN led to immediate precipitation of white crystalline solids. The products were identified by comparing the XRPD patterns of the precipitates with the simulations from the single-crystal data. As depicted in Figure 3, the products were dependent on the molar ratio of Ag/L1. When the Ag/L1 ratio was 1:1, Ag2L2 ring complex 1-R was obtained, while Ag2L3 cage complex 1-C was obtained when

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Scheme 2. Formation of Different Complexes and Isomers under Different Conditions in an EtOH-MeCN Solvent System

the Ag/L1 ratio decreased to 2:3 or 1:2. On the other hand, slow diffusion of a solution of L1 in EtOH into a solution of AgCF3SO3 in MeCN also afforded Ag2L2 ring complex 1-R when the Ag/L1 ratio was 1:1; however, when the Ag/L1 ratio was decreased to 2:3 or 1:2, polymeric complex 1-P was obtained. These results indicated that formation of M2L2 or M2L3 structures is dictated by the metal-to-ligand ratio, while formation of M2L3 or [M2L3]n isomers is directed by the crystallization rate. Addition of L1 ligand from 1:1 to 2:3 ratios can cause conversion of M2L2 ring complex to an M2L3 cage or [M2L3]n polymer, but further addition of L1 ligand up to a 1:2 ratio cannot change M2L3 structures any longer. As a contrast, assembly of Agþ complexes with the ligand L2 proceeded in a different way. Fast crystallization often affords a mixture of different products. That is, rapid mixing of L2 in EtOH and AgClO4 in MeCN at Ag/L2 ratios of 1:1, 2:3, or 1:2 all afforded precipitates which contain more than one solid phase according to XRPD patterns. However, after recrystallization of these impure precipitates from DMSO or DMF, the polymeric complex {[Ag3(L2)2](ClO4)3}n (2-P) was inevitably obtained, which was confirmed by XRPD measurements as shown in Figure 4. Similar to that of ligand L1, pure product of polymeric 2-P can be obtained by slow diffusion of a solution of L2 in EtOH and a solution of AgClO4 in MeCN when the Ag/L1 ratio was 2:3 or 1:2 (Figure 4). Nevertheless, the Ag2L22 ring complex 2-R was difficult to crystallize as a single phase whenever the Ag/L2 ratios were kept at 1:1 or even 2:1. Instead, a mixture of 2-R and 2-P was obtained, as revealed by XRPD monitoring. It seems that the product contains more 2-R when excess Agþ salt was used, but 2-P always exists as a concomitant product. The cage complex 2-C, like 1-C, was not obtained in the solid-state under any circumstances. These results suggest that crystallization of polymeric 2-P is predominated over other possible products regardless of the metal-to-ligand ratios, which may be due to a rather low solubility-product constant (Ksp) and fast polymerization speed of 2-P. Formation of 2-R or 2-C complexes may be overwhelmed by fast crystallization of 2-P, which was hinted at by observation of 2-R at relatively slow crystallization rates. On the other hand, formation of pure 2-P from mixed products through recrystallization in DMSO or DMF implies that structural conversion takes place and polymerization of 2-P may not be only kinetically favored (vide infra).

Structural conversion between the isomeric 1-C cage complex and coordination polymer 1-P has also been investigated. Since 1-C is the product of fast crystallization while 1-P is grown via slow diffusion, recrystallization of cage 1-C under different solvent systems was tested: (1) dissolving 1-C in DMSO and then reprecipitating quickly by addition of Et2O; (2) recrystallizing 1-C in a EtOH-DMF (v/v = 1:1) solvent system; (3) recrystallizing 1-C in a EtOH-DMSO (v/ v=1:1) solvent system. As depicted in Figure 5, the XRPD patterns recorded for the recrystallized products denoted that 1-C was converted into 1-P in all cases. That means, ring-opening isomerization between 1-C and 1-P did exist and polymerization from 1-C to 1-P may take place along with recrystallization. It is a surprise that 1-C was converted to 1-P even upon quick precipitation by Et2O, indicating that ring-opening isomerization is a fast procedure in solution. In this context, the polymeric isomer 1-P should be thermodynamically favored over the discrete isomer 1-C, which was further verified by the fact that recrystallization of 1-P under similar conditions could not lead to formation of 1-C or any other products. Solution Study. To further understand the processes of Ag-ligand coordination assembly and structural conversion, solution study was carried out by the means of 1H NMR spectra and electrospray ionization mass spectrometry (ESI-MS). As shown in Figure 6, all proton signals on benzimidazole rings were shifted downfield in comparison with those of the “free” ligands, indicative of coordination between Agþ ion and benzimidazole N donors due to the metal induction effect.5a,b,11a Because H1 atom is the closest proton to Agþ ion, the largest downfield movement should be observed for H1 proton. However, this seems not to be significant in both complexes of L1 and L2 ligands, suggesting that the solution species keep the ring- or cagelike structures as in the solid-state, thus rendering the H1 atom under a ring current shield, which causes a reverse upfield shift.12 Such speculation was further convinced by observation of less downfield shift of the H1 proton in L1 complexes than that in L2 complexes, because the H1 proton in L1 is completely shielded by the phenyl ring while that in L2 is only partially shielded. 1 H NMR spectra confirmed that the discrete Ag2L3 complex 1-C and its ring-opened polymer 1-P have the same signal profiles in DMSO solution, and similar spectra were also observed for the 2-P coordination polymers prepared by

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either fast mixing or slow diffusion (Figure 6). This means the precursors for crystallization of an M2L3 cage or polymer structures are the same in solution. In addition, the signals of Ag2L2 complexes 1-R or 2-R on the benzimidazole ring were further shifted downfield compared to corresponding Ag2L3 complexes but could not be moved downfield any longer upon addition of excess Agþ ions.11a This denotes that a final dynamic equilibrium is established at a metal-to-ligand ratio of 1:1, and the M2L2 species represents the thermodynamically stable structure in solution.11a Further information on solution structures was obtained from ESI-MS measurements. As shown in Figure 7, the ESI-MS spectra of 1-R and 2-R displayed dominant peaks corresponding to M2L2 species, namely [Ag2(L1)2(CF3SO3)]þ (m/z 1153.1) and [Ag2(L2)2(CF3SO3)]þ (m/z 1075.1). Other peaks assignable to [Ag(L1)2]þ and [AgL1]þ species were also observed, suggesting dissociation of M2L2 species due to the labile nature of the Ag-N bonds and fast metal-ligand exchange.11a,13 The spectrum of 1-P was quite similar to that of 1-C, which has been reported previously,11a where a small peak at m/z 1546.2 was detected for [Ag2(L1)3(CF3SO3)]þ besides the salient peaks of [Ag2(L1)2(CF3SO3)]þ, [Ag(L1)2]þ, and [AgL1]þ similar to those in 1-R. These findings confirm that the M2L3 species is not thermodynamically favored in solution, and dissociation of either 1-C or 1-P leads to the same dynamic equilibrium between M2L2, M2L3, and other soluble oligomeric

Figure 3. XRPD patterns of the products prepared from ligand L1 in comparison with the single-crystal structural simulations. (Left) Rapid mixing in different Ag/L1 ratios: (a) simulated 1-R; (b) 1:1 (1-R); (c) simulated 1-C; (d) 2:3 (1-C); (e) 1:2 (1-C). (Right) Slow diffusion in different Ag/L1 ratios: (a) simulated 1-R; (b) 1:1 (1-R); (c) simulated 1-P; (d) 2:3 (1-P); (e) 1:2 (1-P).

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species, in agreement with the NMR results discussed above. Similarly, the ESI-MS spectra of 2-R and 2-P also displayed a series of peaks corresponding to [Ag2(L2)2(ClO4)]þ, [Ag(L2)2]þ, and [AgL2]þ species, indicative of similar solution behaviors to L1 complexes. It should not be neglected that a small peak assignable to the [M2L3(anion)]þ species is observable, indicating that the M2L3 cage species might exist in solution just as a short-lived intermediate which makes the solid product 2-C unobtainable. Assembly Mechanism and Ring-Opening Isomerization. On the basis of the above solid and solution study, we may be able to postulate assembly processes of Agþ complexes as depicted in Scheme 3. In solution, M2L2 species represent the most thermodynamically preferential structure,5,11 which is the smallest cyclic structure possible for coordination assembly of Agþ ions and L1/L2 ligands. This is also justified by the significantly short Ag-N bond distances in M2L2 rings (2.076-2.111 A˚) compared with those in an M2L3 cage (2.263 A˚). However, due to the lability of the Ag-N bond and fast metal-ligand exchange, M2L2 species may dissociate or reorganize in solution to form other species, such as ML, ML2, M2L3, and so on.11a These solution species establish a dynamic equilibrium which is dominated by the M2L2 structure. Upon addition of ligand L, formation of

Figure 5. XRPD patterns showing structural conversion of 1-C under different conditions: (a) 1-P, simulated; (b) dissolved in DMSO and then precipitated by addition of Et2O; (c) recrystallization in EtOH-DMF (v:v = 1:1); (d) recrystallization in EtOHDMSO (v:v=1:1); (e) simulation of 1-C.

Figure 4. XRPD patterns of the products prepared from L2 in comparison with the single-crystal structural simulations. (Left) Rapid mixing in different Ag/L2 ratios: (a) simulated 2-P; (b) 1:1 (2-P); (c) 2:3 (2-P); (d) 1:2 (2-P). (Right) Slow diffusion in different Ag/L2 ratios: (a) simulated 2-R; (b) 2:1 (2-R þ 2-P); (c) 1:1 (2-R þ 2-P); (d) 2:3 (2-P); (e) 1:2 (2-P); (f ) simulated 2-P.

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Figure 6. 1H NMR of ligands and their complexes in DMSO-d6 (300 MHz). (Left) (a) L1; (b) 1-C; (c) 1-P; (d) 1-R. (Right) (a) L2; (b) 2-P synthesized by rapid mixing; (c) 2-P synthesized by slow diffusion; (d) 2-R.

Figure 7. Partial ESI-MS spectra of complexes in DMSO: (a) 1-R; (b) 1-P; (c) 2-R; (d) 2-P.

M2L3 cage species is forced. Nevertheless, the M2L3 cage structure is apparently thermodynamically disfavored in solution. This is clear from NMR and ESI-MS solution study results, and the X-ray single-crystal analyses of the M2L3 1-C cage did disclose a strained structure with Ag-N bonds longer by 0.16 A˚ than those in the M2L2 1-R ring. Formation of the M2L3 2-C cage may be even more difficult because of more ring strain arisen from arrangement of three unsymmetric L2 ligands around two Agþ ions than that of three symmetric L1 ligands. The polymeric structures 1-P and 2-P are not likely present in solution, but their solid-state structure analyses reveal that the average Ag-N bonds

(2.156-2.465 A˚ for 1-P and 2.247 A˚ for 2-P) are comparable with those of the M2L3 cage structure, indicative of a similar enthalpic contribution in the formation of a 3-connecting M2L3 cage or [M2L3]n polymer isomers. Crystallization of a solid-state product needs a perturbation on the above-mentioned solution dynamic equilibrium, which can be directed by kinetic or thermodynamic effects as well as the Ksp of a precipitate. For ligand L1, as shown in Schemes 2 and 3, the M2L2 ring complex 1-R is the sole product at a Ag/L ratio of 1:1, because 1-R is the thermodynamically stable product. At a Ag/L ratio of 2:3 or 1:2, M2L3 cage species are increased in solution due to addition of

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Scheme 3. Possible Assembly Processes of Complexes 1-R, 1-C, 1-P and 2-R, 2-P

one L ligand to an M2L2 ring species. Such an equilibrium shift will promote crystallization of M2L3 products, either M2L3 cage 1-C or [M2L3]n polymer 1-P, because M2L3 cage species are thermodynamically disfavored in solution. Fast crystallization gives rise to M2L3 cage 1-C, which can be considered the kinetically favored product, while slow crystallization leads to [M2L3]n polymer 1-P, which can be considered the thermodynamically favored product. Since formation of an M2L3 cage and an [M2L3]n polymer has a comparable enthalpic effect (vide supra), crystallization of [M2L3]n polymer 1-P may be more attributed to the entropic contribution due to increase of the M2L3 species concentration. This means single ring-opening isomerization takes place during crystallization; namely, 1-C cage species act as precursors in solution to open one ring to polymerize into the 1-P polymer upon crystallization, in agreement with the findings that recrystallization of 1-C results in 1-P but not vice versa. For ligand L2, formation of M2L3 cage 2-C is even thermodynamically disfavored, owing to steric interference among three L2 ligands; therefore, the [M2L3]n polymer 2-P becomes the crystallization product favored by both kinetic and thermodynamic effects. Probably due to the rather low Ksp value of 1-P, crystallization of thermodynamically stable M2L2 ring complex 2-R is always accompanied by precipitation of 2-P even at a Ag/L ratio of 2:1. At Ag/L ratios of 2:3 or 1:2, more 2C cage species might be produced as intermediates in solution to promote crystallization of 2-P, no matter in a fast or slow speed. This means that double ring-opening isomerization takes place during crystallization; namely, 2-C cage species open two rings to polymerize into 2-P polymer upon crystallization. Conclusion Coordination assembly of Agþ ion with two semirigid ditopic ligands has been systematically studied, affording four types of structures: discrete M2L2 0-D rings (1-R and 2-R) and an M2L3 0-D cage (1-C), and infinite [M2L3]n 1-D polymer (1-P) and 2-D polymer (2-P). The X-ray single-crystal analyses reveal that (1) in 0-D M2L2 rings and M2L3 cage structures (1-R, 2-R, and 1-C), all ligands adopt the cis-configuration; (2) in the 1-D loop-and-chain structure (1-P), the ligands in the loop adopt the cis-conformation while the ligands in the chain adopt the trans-conformation; and (3) in the 2-D (6, 3) network structure (2-P), all ligands adopt the trans-conformation. Solution and structural conversion studies indicate that M2L3 cage structures are formed through a ligand addition mechanism from M2L2 rings, in which M2L2 rings are more thermodynamically favored in solution than

Zhang et al.

M2L3 cages. Addition of one ligand L to an M2L2 ring also leads to structural diversification, i.e. formation of either an M2L3 0-D cage or [M2L3]n 1-D/2-D polymers. Such structural diversification is due to ring-opening isomerization during crystallization, i.e. structural change of the ligand from the cis- to trans-conformation. As exemplified in Scheme 1, single ring-opening of 0-D cage 1-C gives rise to polymerization of 1-D polymer 1-P, while double ring-opening of 0-D cage 2-C (intermediate) gives rise to polymerization of 2-D polymer 2-P. Control of such ring-opening isomerization and polymerization in crystal growth has been investigated on the basis of their differentially kinetic and thermodynamic preference as well as the Ksp of the product, which suggests that better understanding of the ROI structural relationship and the ROP crystallization process is of help to predictable and controllable syntheses of 1-D to 3-D MOFs or CPs from solution 0-D precursors, such as M2L2, M2L3, and even M2L4 cyclic species. Experimental Section General Procedures. All starting materials and solvents were obtained from commercial sources and used without further purification. Ligands L1 and L2 were synthesized as previously described.11 IR spectra were measured on a Bruker Tensor 27 FT-IR spectrometer with KBr pellets. 1H NMR spectra were recorded on a VARIAN Mercury-Plus 300 instrument. The X-ray powder diffraction was recorded on a D/Max-IIIA diffractometer with a Cu-target tube (λ = 1.5418 A˚) and a graphite monochromator. Thermogravimetric analysis (TG) was performed in N2 at a heating rate of 10 C/ min on a NETZSCH TG209F3. [Ag2(L1)2](CF3SO3)2 (1-R). See ref 11a. [Ag2(L1)3](CF3SO3)2 (1-C). This complex was prepared in an alternative way different from previous methods with higher purity.11a L1 (0.117 g, 0.3 mmol) in EtOH (5 mL) and AgCF3SO3 (0.052 g, 0.20 mmol) in MeCN (5 mL) were mixed and filtered rapidly to obtain a white precipitate. Yield: 65%. IR (KBr, cm-1): 3055w, 1612w, 1495 m, 1455 m, 1384w, 1324w, 1279 m, 1236 m, 1202w, 1171w, 1031w, 885w, 825w, 740s, 634w, 430w. {[Ag2(L1)3](CF3SO3)2 3 CH3CN}¥ (1-P). A solution of AgCF3SO3 (0.052 g, 0.20 mmol) in MeCN (5 mL) was layered carefully onto a solution of L1 (0.117 g, 0.3 mmol) in EtOH (5 mL). Slow diffusion between two solutions at ambient temperature gave colorless block crystals. Yield: 42%. IR (KBr, cm-1): 3054w, 1504w, 1482w, 1459w, 1391w, 1269s, 1221 m, 1149 m, 1031 m, 893w, 763 m, 637 m, 573w, 515w, 429w. [Ag2(L2)2](ClO4)2 3 1.5CH3CN (2-R). A solution of AgClO4 (0.052 g, 0.20 mmol) in MeCN (5 mL) was layered with a solution of L2 (0.038 g, 0.10 mmol) in EtOH (5 mL). Slow diffusion between two solutions at ambient temperature gave colorless block crystals. Yield: 66%. IR (KBr, cm-1): 3115m, 2971m, 1756w, 1613m, 1512m, 1460m, 1395m, 1327w, 1294m, 1237m, 1202m, 1087s, 923w, 876w, 745s, 622m, 575w, 525w, 473w, 427w. [Ag2(L2)3](ClO4)2 (2-P). Method I: L2 (0.114 g, 0.3 mmol) in EtOH (5 mL) and AgClO4 (0.040 g, 0.20 mmol) in MeCN (5 mL) were mixed and filtered rapidly to obtain a white precipitate. Recrystallization of the rude product from EtOH-DMSO (v:v = 1:1) yielded colorless block crystals. Yield: 70%. Method II: A solution of AgClO4 (0.052 g, 0.20 mmol) in MeCN (5 mL) was layered carefully onto a solution of L2 (0.117 g, 0.30 mmol) in EtOH (5 mL). Slow diffusion between two solutions at ambient temperature gave colorless block crystals. Yield: 56%. IR (KBr, cm-1): 3129w, 1609w, 1502m, 1459m, 1393m, 1326w, 1291w, 1244m, 1202w, 1204m, 1095s, 903w, 856w, 749m, 623m, 430w. Caution! Perchlorate metal salt and its complex in the presence of organic ligands are potentially explosive. Only a small amount of material should be used and handled with care. X-ray Crystallography. Experimental details of the X-ray structural analyses as well as the crystallographic data are provided in Table 1. Selected bond distances and angles are listed in Table 2. The diffraction data were collected on a Bruker SMART APEX

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Table 1. Crystallographic Data and Structure Refinement Summary for Complexes complex chemical formula formula weight space group a/A˚ b/A˚ c/A˚ R/deg β/deg γ/deg V/A˚3 Z D/(g cm-3) μ/mm-1 T/K Rint R1 wR2

1-P C82H81Ag2F6N13O6S2 1738.46 P21/c 16.6799(8) 13.4356(6) 17.4060(8) 90 99.9890(10) 90 3841.6(3) 2 1.503 0.642 150.0(2) 0.0786 0.0548 0.1150

2-P C75H72Ag2Cl2N12O8 1556.09 P3c1 14.6142(3) 14.6142(3) 18.4967(9) 90 90 120 3421.17(19) 2 1.511 0.718 293(2) 0.0317 0.0450 0.1130

Table 2. Selected Bond Distances/A˚ and Bond Angles/deg for 1-P, 2-P, and 2-Ra 1-P

Ag-N(3)a

2.156(5)

N(1)-Ag-N(5)

94.3(2)

Ag-N(1) Ag-N(5) N(1)-Ag-N(3)a N(3)a-Ag-N(5)

2.172(6) 2.465(6) 157.2(2) 103.3(2)

Ag-N(1) N(1)-Ag-N(1)b

2-P 2.247(3) Ag-O(1B) 119.971(5) N(1)-Ag-O(1B)

sAg(1)-N(1) Ag(1)-O(40 ) Ag(2)-N(8) N(1)-Ag(1)-N(5) N(5)-Ag(1)-O(40 )

2.076(6) 2.54(2) 2.113(6) 170.8(2) 89.9(5)

2.606(12) 89.01(8)

2-R Ag(1)-N(5) Ag(2)-N(4)

2.088(6) 2.111(6)

N(1)-Ag(1)-O(40 ) N(4)-Ag(2)-N(8)

96.9(5) 172.1(2)

a Symmetry transformations: (a) -x þ 2, -y þ 1, -z. (b) -x þ y, -x þ 1, z.

CCD-based diffractometer (Mo KR radiation, λ = 0.71073 A˚) for 1-P at 150 K and 2-R and 2-P at 293 K. The raw data frames were integrated with SAINTþ14 and applied corrections for Lorentz and polarization effects. An empirical absorption correction based on the multiple measurements of equivalent reflections was applied with the program SADABS.15 The structure was solved by a combination of direct methods and difference Fourier syntheses, and it was refined by full-matrix least-squares against F2 (SHELXTL).16 In 1-P, two independent triflate anions (S1 and S2) were located, both of which are disordered over inversion centers. An acetonitrile molecule in a crystal lattice is disordered in the same region as triflate S1. A total of 41 restraints were used in modeling the triflate disorder. The disorder of triflate S2 was severe, and the reported atomic positions should be regarded only as approximate. The largest residual electron density peaks of ca. 1.2 e/A˚3 are in the vicinity of S2, indicative of further disorder, which could not be modeled successfully. All non-hydrogen atoms were refined with anisotropic displacement parameters except the S2 triflate (sulfur only). Hydrogen atoms were placed in geometrically idealized positions and included as riding atoms. In 2-P, the Ag and Cl and O1 atoms are located on 3-fold axes. The L2 ligand is situated on a 2-fold rotational axis. The ClO4- anion is disordered about its 3-fold axis and was modeled as occupying two distinct orientations. The Cl-O distances for both components were restrained to be similar. Non-hydrogen atoms were refined with anisotropic displacement parameters; hydrogen atoms were idealized and included as riding atoms. In 2-R, one ClO4- anion is disordered about the central Cl atom with O atoms occupying two positions. While the other ClO4- anion is disordered at two separate positions with fractional occupancy. All non-hydrogen atoms were refined with anisotropic displacement parameters, with ClO4- anion and MeCN molecules modeled by 274 restraints. H atoms were included in the calculated positions. The CCDC reference numbers are CCDC 767085-767087.

2-R C106H105Ag4Cl4N19O16 2474.37 C2/c 39.172(3) 9.9604(9) 34.221(3) 90 121.969(2) 90 11327.0(17) 4 1.451 0.845 293(2) 0.0893 0.0886 0.1889

Acknowledgment. This work was supported by the 973 Program of China (2007CB815302) and NSFC Projects 20821001, 20773167, 20731005, and U0934003. Supporting Information Available: TG curves and X-ray crystallographic data (CIF format). This material is available free of charge via the Internet at http://pubs.acs.org.

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Ring-Opening Isomerization Based on the 3-Connecting Node

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