Non-carboxylate based metal-organic frameworks (MOFs) and related aspects Srinivasan Natarajan * , Partha Mahata Framework solids Laboratory, Solid State and Structural Chemistry Unit, Indian Institute of Science, Sir C.V. Raman Avenue, Bangalore560012, India article info Article history: Received 21 April 2009 Accepted 20 May 2009 Keywords: Metal-organic frameworks Non-carboxylate frameworks Crystal engineering Host guest compound Phosphonate framework abstract The metal-organic frameworks, in recent years, show a variety of new developments that includes new methods of preparation, post synthesis modifications and novel class of compounds. Though most of the developments happened in the carboxylate based family of compounds, the other related systems are also equally interesting. In this article, we have highlighted some of the developments that have taken place in the family of non-carboxylate metal-organic frameworks. We have also highlighted some of the recent attempts at modifying the surfaces and pores of the MOFs by careful chemical manipulations. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Metal-organic frameworks (MOFs), a subset of two-dimensional or three-dimensional coordination polymers that comprises metal ions or metal ion clusters and bridging organic ligands, have emerged as an important family of porous materials due to their unique structural and functional properties [1]. The design of MOF materials has been greatly facilitated by the node-and-con- nector model where chemical components, i.e., metal ions and organic ligands, are conceptualized as points, lines, polygons or polyhedra [2,3]. This approach greatly helps in our understanding and appreciation of complex MOF structures. MOFs are prepared using a variety of soft chemical approaches such as slow diffusion and direct reaction under solvothermal conditions of the metal ion and the organic ligands. The aforementioned design principles have helped not only in our understanding but also design newer MOFs and provide useful clues towards the synthesis of such framework compounds. Some of the MOFs find uses in the areas such as catalysis, separations and gas storage, which place them among some of the interesting family of solid-state materials. The carboxylate and pyridyl based compounds represent the most studied within the family of MOFs, and have been reviewed in many important articles. In the present paper we will not pro- vide much discussion about the MOF compounds based on carbox- ylate, but concentrate on other ligands such as phosphonate, sulphonate, imidazolate, porphyrin, carborane etc. In addition, some recent developments towards the post-synthetic modifica- tion of the MOFs are also discussed as they appear to be the emerg- ing directions. This review will examine the advances in the area of metal-organic frameworks (non-carboxylate based) reported mostly during 2007–2008. 2. Phosphonate frameworks The use of phosphonate ligands for the formation of framework structures are less common compared to the carboxylate ones. This striking anomaly could have arisen due to many factors; (1) the ease of forming simple phosphonate structures as dense layered materials, (2) the growth of single crystals is difficult as the phos- phonate phases precipitate much more rapidly as less ordered, insoluble phases and (3) the coordination chemistry of the phos- phonate is more complex due to varying stages of deprotonation. In spite of all these difficulties, the phosphonate ligands are attrac- tive to prepare interesting compounds with different structures and properties. The organo phosphonic acids, generally, require mild synthetic conditions to form the metal phosphonates, which encouraged many researchers to explore the formation of phos- phonate frameworks. The structures depend on the many synthesis variables such as the phosphonate source, the metal source, the metal/P ratio, the solvent, the concentration, the pH, and impor- tantly the reaction temperature. The early simple metal phospho- nate structures have been reviewed by Clearfield (Clearfield A. Metal phosphonate chemistry. Prog Inorg Chem 1998;47:371– 510). The first three-dimensional metal phosphonate with one- dimensional channels have been observed in b-Cu(O 3 PCH 3 ) and has been discussed in the review. Phosphonate framework struc- tures are now known with a variety of transition metals, p-block elements as well as lanthanide elements [4–10]. Shimizu and co-workers reported microporous behavior in a phosphonate, [Cu 3 (H 3 L)(OH)(H 2 O) 3 ]H 2 OMeOH (H 8 L = 1,3,5,7-tetrakis(4-phospho- natophenyl)adamantine) [4]. The structure has an interpenetrated 1359-0286/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cossms.2009.05.002 * Corresponding author. Fax: +91 80 2360 1310. E-mail address: snatarajan@sscu.iisc.ernet.in (S. Natarajan). Current Opinion in Solid State and Materials Science 13 (2009) 46–53 Contents lists available at ScienceDirect Current Opinion in Solid State and Materials Science journal homepage: www.elsevier.com/locate/cossms