Element–organic frameworks with high permanent porosityw Marcus Rose, a Winfried Bo¨hlmann, b Michal Sabo a and Stefan Kaskel* a Received (in Cambridge, UK) 10th December 2007, Accepted 25th February 2008 First published as an Advance Article on the web 26th March 2008 DOI: 10.1039/b718925g Microporous hydrophobic polysilanes with high specific surface areas (700–1100 m 2 g 1 ) for applications in gas adsorption are obtained using an organolithiation route. The search for novel porous materials with narrow pore size distribution, high accessible surface area and well defined functional groups on the inner surface is crucial for the development of applications in adsorption, separation, gas storage, and heterogeneous catalysis. A rational design was achieved in metal–organic frameworks (MOFs), coordination polymers consisting of connectors (metal ions or clusters) and linkers (organic molecules with functional groups) defining the network topology and pore diameter 1–4 and in covalent organic frameworks (COFs) as an extension of the modular concept using boronic acids as building blocks. 5,6 They sur- pass traditional molecular sieves such as zeolites and activated carbons in terms of surface area and specific pore volume. A disadvantage of these materials is the low hydrothermal stability in some cases. 7 Metals such as chromium in MIL-101 have a high toxicity. 8 Other porous organic–inor- ganic hybrid materials of great interest obtained in the last years are the periodic mesoporous organosilicas (PMOs) 9 and porous polymers. 10 Hypercrosslinked polymers (HCPs) were obtained by hypercrosslinking of polymer chains in a swollen polymer to generate a rigid, porous polymer network. 11–13 Another approach is the polymerization of large rigid mole- cules to form chains and networks with inefficient space packing and voids in the range of micropores, realized in the polymers of intrinsic microporosity (PIMs). 14,15 Recently Cooper et al. published the synthesis of conjugated micropor- ous polymers (CMPs) using Sonogashira–Hagihara coupling of alkynes with aryl halogens. 16 They showed that the pore size of the obtained amorphous microporous polymers depends on the size of the organic linker. Our interest was to develop a new class of microporous materials (d o 2 nm) with high hydrophobicity, high stability against water and good thermal stability using the modular concept of connectors and linkers but an organome- tallic polymer synthesis route. In the following, we report the integration of elements such as silicon and their use as connectors. At the same time, organic linkers are used to tailor the pore size, resulting in a porous, highly hydro- phobic and thermally stable element organic framework (EOF). Tetrakis(4-bromophenyl)silane (TBPS), was used as the primary building block. The synthesis of TBPS was reported earlier by Fournier et al. 17 TBPS was lithiated fourfold by reaction with n-butyllithium under inert conditions. Subse- quent reaction with tetraethylorthosilicate (TEOS) at 263 K resulted in the formation of the porous network poly(1,4- phenylene)silane (EOF-1, Scheme 1). The white product is separated from the solution by centrifugation. EOF-1 forms as an X-ray amorphous precipitate composed of very small particles. Instead of TEOS, SiCl 4 can be used for the frame- work formation but the specific surface area is slightly re- duced. A biphenylene linker was used to obtain larger pores in poly(4,4 0 -biphenylene)silane (EOF-2). Since 4,4 0 -dibromobi- phenylene can be lithiated twofold, EOF-2 can be synthesized in a one-step reaction (Scheme 1), thus the synthesis of the tetrahedral precursor tetrakis(4-bromobiphenylene)silane is unnecessary. The particle size and the morphology of the resulting particles was characterized by SEM analysis (Fig. 1). EOF-1 consists of spherical particles with a diameter of 0.1–0.5 mm while EOF-2 forms a dendritic network of flat particles. Both polymers are X-ray amorphous. The para-substituted phenylene group (EOF-1) is detected in the 13 C CP MAS NMR spectrum (d = 131.7, 124.1 ppm). A shoulder at 139.5 ppm indicates the presence of non-symmetric substituted linkers due to incomplete conversion of Ph–Br groups. In the 29 Si MAS NMR spectrum one sharp peak at 17.9 ppm reflects the majority of SiPh 4 -groups but a broader region at 33 to 43 ppm indicates the presence of some aliphatic substitution of Si atoms (see supporting information). EOF-1 and EOF-2 have a good thermal stability in air up to 673 K. A complete degradation is only observed above 873 K. They show no decomposition by air, moisture, or aqueous solutions. Both compounds are highly porous. From the nitrogen physisorption isotherms measured at 77 K (Fig. 2), the specific surface areas determined using the BET equation are 780 m 2 g 1 (EOF-1) and 1046 m 2 g 1 (EOF-2). Using the t-plot method, a specific micropore volume is determined for EOF-1 with 0.32 cm 3 g 1 and for EOF-2 with 0.45 cm 3 g 1 . The external specific surface area is high for both compounds (167 m 2 g 1 (EOF-1) and 201 m 2 g 1 (EOF-2)). a Department of Inorganic Chemistry, Dresden University of Technology, Mommsenstr. 6, D-01069 Dresden, Germany. E-mail: stefan.kaskel@chemie.tu-dresden.de; Fax: +49-351- 46337287; Tel: +49-351-46334885 b Faculty of Physics and Earth Science, University of Leipzig, Linne ´str. 5, D-04103 Leipzig, Germany. E-mail: bohlmann@physik.uni-leipzig.de; Fax: +43-341-9732769; Tel: +49-341-9732613 w Electronic supplementary information (ESI) available: Experimental procedures, NMR, IR and Raman spectra, DTA/TG diagrams and H 2 and CH 4 physisorption isotherms for EOF-1 and EOF-2. See DOI: 10.1039/b718925g 2462 | Chem. Commun., 2008, 2462–2464 This journal is  c The Royal Society of Chemistry 2008 COMMUNICATION www.rsc.org/chemcomm | ChemComm