Coordination Polymers DOI: 10.1002/ange.200705822 Kinetic Gate-Opening Process in a Flexible Porous Coordination Polymer** Daisuke Tanaka, Keiji Nakagawa, Masakazu Higuchi, Satoshi Horike, Yoshiki Kubota, TatsuoC. Kobayashi, Masaki Takata, and Susumu Kitagawa* Recently, there has been growing interest in the nature of flexible and dynamic porous coordination polymers (PCPs) that reversibly change their structures and properties in response to guest adsorption. [1] After one of the present authors predicted their importance, [2] many flexible PCPs have been prepared in only a decade. So-called structural dynamism has been identified as a key principle for high selectivity, [3] accommodation, [4] and molecular sensing. [5] One of the most interesting phenomena in flexible PCPs is stepwise adsorption caused by the guest-induced framework transition. [3,4,6] In particular, a gate effect occurs when the framework structure changes during the adsorption process from a closed structure to an open one at a specific pressure, which generates an S-shaped or sigmoidal adsorption profile. The onset pressure at which the gates of the closed-structure grooves become open is referred to as the gate-opening pressure (P go ) and is related to the structural transformation of the host framework. [7] Interestingly, the value of P go shows a guest dependency. [8] This difference between adsorbates suggests that flexible PCPs have potential applications in many fields, including separation, sensors, and switching materials. In particular, small gaseous adsorbates, such as O 2 , Ar, and N 2 , are attractive targets for research, first because the differences in sorption behavior between such similar gas molecules have attracted wide commercial interest for processes such as air separation, and also because of scientific interest as a result of their simple structures and small differences in physical properties. [9] However, several questions remain unanswered. Why does adsorption not occur below P go ? What determines P go ? How can the difference in P go for different guests be enhanced? Generally, a sigmoid isotherm with small hyste- resis has been understood to be the result of cooperative phenomena. [10] Several flexible PCPs actually show isotherm discontinuities at high relative pressure, with large hysteresis, where kinetics would play an essential role. [11, 12] In spite of their importance, few attempts have been made to determine the kinetics of the gate effect. [11, 13] Herein, we present the synthesis, crystal structure, and gas sorption properties of a flexible PCP, {[Cd(bpndc)(bpy)]} n (1; bpndc = benzophenone- 4,4’-dicarboxylate, bpy = 4,4’-bipyridyl), which shows a large difference in P go between O 2 , Ar, and N 2 (Figure 1). To understand the mechanism behind the differences between similar gases, we treat this phenomenon with the aid of a new diffusion model in which adsorption proceeds through the formation of an intermediate. Kinetic analysis revealed that the formation of the intermediate, which can be described as a gate-opening process, governs P go and enhances the differ- ence between gases. The solvothermal reaction of Cd(NO 3 ) 2 ·4H 2 O, H 2 bpndc, and bpy in dimethylformamide (DMF) produced the solvated framework compounds {[Cd(bpndc)(bpy)](dmf)(H 2 O)} n (1Solvents). Single-crystal X-ray analysis demonstrated that cadmium ions are connected by bpndc to produce 1D double-chain structures of {[Cd(bpndc)]} n along the c axis, and are linked by bpy along the b axis to give a 2D sheet motif (Figure 2a and b). The 2D layers are mutually interdigitated to create a 3D assembled framework (Figure 2c). This is similar to previously reported PCPs that show a flexible nature. [14] The 3D structure of 1Solvents consists of a Figure 1. Adsorption at low pressure. [*] D. Tanaka, K. Nakagawa, Dr. M. Higuchi, Dr. S. Horike, Prof. Dr. S. Kitagawa Department of Synthetic Chemistry and Biological Chemistry Graduate School of Engineering, Kyoto University Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan) Fax: (+ 81)75-383-2732 E-mail: kitagawa@sbchem.kyoto-u.ac.jp Dr. Y. Kubota Department of Physical Science, Graduate School of Science Osaka Prefecture University, Sakai, Osaka 599-8531 (Japan) Prof. Dr. T. C. Kobayashi Department of Physics, Okayama University Okayama 700-8530 (Japan) Dr. M. Takata Structural Materials Science Laboratory, Harima Institute RIKEN SPring-8 Center and CREST, JST, Sayo-gun, Hyogo 679-5148 and Department of Advanced Materials Science The University of Tokyo (Japan) [**] We thank Prof.Dr. M. Miyahara, Dr. H. Tanaka, Dr. S. Watanabe, H. Sugiyama, and S. Shimomura of Kyoto University and Dr. R. Matsuda of Kyushu University for encouragement of this research and valuable information. We thank Dr. K. Kato and Dr. K. Osaka in JASRI for their experimental help at the SPring-8. This work was supported by ERATO, JST, and a Grant-in-Aid for Scientific Research in a Priority Area “Chemistry of Coordination Space” (434) from the MEXT (Japan). Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Zuschriften 3978 # 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2008, 120, 3978 –3982