Structural memory effect in solid-state transformation of kinetic coordination networks Chemical Science 86 The chemistry of porous coordination networks has shown a rapid advance in the last 15 years owing to the hybrid nature of their building blocks (i.e., metal ions and organic ligands) [1]. The selective synthesis of kinetic over thermodynamic products is an important aspect in porous coordination networks. In general, kinetic or thermodynamic structures can be prepared by fast or slow crystallization methods, respectively. However, synthetic routes enabling such selectivity as homogeneous products are not trivial. Fast precipitation can generate metastable products but as microcrystals that are not suitable for single-crystal X-ray analysis. We have recently demonstrated that “instant synthesis” can form kinetic microcrystalline porous coordination networks suitable for ab initio XRPD structure determination [2] and how new porous structures could be obtained by the annealing of kinetic products [3]. Herein, we studied a kinetically controlled family of isostructural porous coordination networks [(ZnX 2 ) 3 (TPT) 2 ] n •5.5(solvent) (where X = I, Br, and Cl), where TPT is tris(4-pyridyl)triazine ( 1-3 ) (Fig. 1) obtained by “instant synthesis” and studied its solid- state reactivity at high temperatures (above 573 K). Insights on the solid-state reactions were obtained using state-of-the-art methods for ab initio XRPD structure determination by synchrotron X-ray diffraction at BL19B2 and BL02B2 beamlines [4]. Upon heating from 300 to 673 K, network 1 undergoes a crystalline-to-amorphous-to-crystalline (CAC) phase transition. In situ XRPD shows that 1 changes to an amorphous phase at 473 K, and upon further heating, a new crystalline porous phase ( 1a ) appears uniformly at 573 K and is stable up to 673 K (Fig. 1). We demonstrated the guest inclusion ability of 1a by immersing it in nitrobenzene and exposing 1a to I 2 vapor. Ab initio XRPD structure determination (BL19B2 beamline) indicated that 1a can reversibly encapsulate nitrobenzene [3] and I 2 [4]. The CAC reaction in 1 involves the guest removal and concurrent shrinking of the networks, followed by the unlocking of interpenetrated networks (cleavage of coordination bonds), and crystallization by bond formation to yield 1a (Fig. 2). To test if the molecular prearrangement in 1 is necessary for the formation of 1a , we performed control experiments by mixing, grinding, and heating using the same stoichiometric amounts of starting materials of 1 as in instant synthesis. X-ray analysis proved that 1a was not formed (Fig. 2). Intrigued by the results, we investigated the isostructural networks 2 and 3 . Microcrystalline 2 was heated to 573 K and monitored by in situ XRPD analysis. The diffraction data shows also two phase transitions through a CAC process. Elemental analysis results suggest the formation of the compound [(ZnBr 2 ) 3 (TPT) 2 ] n •(H 2 O) ( 2a ). The high quality synchrotron XRPD data of 2a was recorded at BL19B2 and structure determination was carried out by ab initio XRPD followed by Rietveld refinement (Fig. 3) [4]. Compound 2a is an interpenetrated network obtained after guest removal and molecular N N N N N N Isostructural 3D Networks [(ZnX 2 ) 3 (TPT) 2 ] n • (solvent) ZnX 2 (1a) t = 30 sec Nitrobenzene/ Methanol TPT 673K 573K 473K 423K 300K 5 10 15 20 25 30 2 θ θ (degree) ( 1 ) X = I ( 2 ) Br ( 3 ) Cl (1) Amorphous Phase 473K E Crystalline Phase 300K Polycrystalline Phase 673K Intensity (arb. units) Fig. 1. Top: Fast crystallization of 1-3 by the “instant synthesis” method. Bottom: XRPD of 1 measured at different temperatures (left). Crystal structures of 1 and 1a and energy plot depicting the transformation from kinetic to thermodynamic products upon heating via an amorphous phase. Interpenetrating networks are shown in blue and orange (right). 87 rearrangement (Fig. 3). To determine whether the unlocking of networks occurs, we investigated a non-interpenetrated ( 2b ) network (not shown here, see Ref. [4]) with the same chemical composition of 2 . Heating 2b to 553 K yields the same XRPD pattern and therefore the crystal structure of 2a [4]. Clearly, the cleavage and formation of coordination bonds occur during the CAC transformation from non- interpenetrated ( 2b ) to interpenetrated ( 2a ). Control experiments performed by mixing the starting materials using the same stoichiometry and grinding followed by annealing did not form 2a , indicating that the atomic-level preorganization in the starting solids is crucial for the formation of the structures obtained after annealing. To our understanding, in the amorphous phase, structural information is retained and is passed via a type of memory effect from metastable ( 2 ) to a more stable ( 2a ) (Fig. 3, bottom). Further heating to 723 K and slow cooling of 2 produced single crystals with a new bromide-bridged network. In this case, the high temperature induced a certain level of calcination and, most likely, partial melt of 2 allowed crystallization upon slow cooling. A similar behavior was observed for the chloride version 3 (see Ref. [4]), which also through a CAC process formed a new phase (nonporous desolvated structure) that was unstable; thus, it transformed to a new chloride-bridged network at 723 K. The different solid-state reactivity can be attributed to the different halides in 1-3 . In summary, we demonstrated that the frequently observed limitation in the single-crystal X-ray analysis of crystalline solids with good crystallinity and size has been tackled using a state-of-the-art ab initio synchrotron XRPD structure solution. In a type of memory effect, the amorphous phase in the CAC process is an intermediate state retaining structural information from the kinetic structure that is passed to a more stable structure upon heating. Clearly, such a detailed structural analysis of organic-inorganic materials is trivial in single-crystal X-ray analysis but not in ab initio XRPD analysis. Intensity (counts) 573 K 473 K 5 10 15 20 25 30 35 45 40 2 θ θ (degree) 2 θ θ (degree) 25000 20000 15000 10000 5000 0 2 2a (2a) 3 8 13 18 23 28 33 38 Retention of Structural Information from 2 via Amorphous Transition: Memory Effect Orthorhombic a =23.089(2) Å c =15.646(1) Å λ =1.3 Å Pbca b =22.702(2) Å V =8201(1) Å 3 T =100 K R WP = 5.39% R F = 2.79% Interpenetrated Not Interpenetrated 1 1a 473 K 573 K Crystalline-to- Amorphous Amorphous-to- Crystalline 3 Znl 2 + 2 TPT Grind; Δ Δ [(Znl 2 ) 3 (TPT) 2 ] n • (solvent) [(Znl 2 ) 3 (TPT) 2 ] n [(Znl 2 ) 3 (TPT) 2 ] n (1) (1a) Fig. 2. Top: Cartoon showing the structural transformation from 1 to 1a . Middle: control experiments showed that it is not possible to obtain the porous network 1a if the building blocks (ZnI 2 and TPT) are mixed, grinded, and heated. Bottom: the molecular prearrangement in 1 is necessary to form 1a , which is only formed upon heating. Fig. 3. Top: Experimental (red), calculated (pale- blue), and difference (dark-blue) XRPD profiles from the final Rietveld refinement of 2a . Middle: crystal structure of 2a with the two interpenetrated circuits in green and red (side view) and viewed along the c -axis (right). Bottom: XRPD showing the amorphous phase upon heating 2 and formation of 2a . The very broad “peaks” indicate that the long-range order allows the passing of structural information from 2 to 2a . References [1] S. Kitagawa et al .: Angew. Chem. Int. Ed. 43 (2004) 2334. [2] M. Kawano et al .: Angew. Chem. Int. Ed. 47 (2008) 1269. [3] K. Ohara et al .: J. Am. Chem. Soc. 131 (2009) 3860. [4] J. Martí-Rujas, N. Islam, D. Hashizume, F. Izumi, M. Fujita and M. Kawano: J. Am. Chem. Soc. 133 (2011) 5853 . Javier Martí-Rujas a and Masaki Kawano b, * a Italian Institute of Technology, Centre for Nano Science and Technology, Italy b The Division of Advanced Materials Science, Pohang University of Science and Technology, Korea *E-mail: mkawano@postech.ac.kr
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