A Distinctive PdCl 2 ‑Mediated Transformation of Fe-Based Metallogels into Metal−Organic Frameworks Published as part of the Crystal Growth & Design virtual special issue IYCr 2014 - Celebrating the International Year of Crystallography Harshitha Barike Aiyappa, Subhadeep Saha, Bikash Garai, Jayshri Thote, Sreekumar Kurungot,* and Rahul Banerjee* Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India *S Supporting Information ABSTRACT: Simple, efficient conversion of viable Fe 3+ -based metallogels into Fe-metal−organic frameworks (MOFs) has been achieved by PdCl 2 -mediated gel degradation. The metallogels and the resulting MOFs have been characterized, and a probable mechanism for the event has been elucidated. M etal−organic frameworks (MOFs) have emerged as promising materials for a myriad of applications viz., gas adsorption, separation and storage, sensors, catalysis, drug delivery, luminescence, etc., owing to their tunable surface area and pore size. 1 MOF synthesis begins with the meticulous choice of suitable metal ions and ligands which dictate their ultimate properties. Among the most widely studied MOFs, Co-, Ni-, Cu-, and Zn-based MOFs mainly dominate the list followed by Ca-, Mn-, and Mg-based MOFs. 2 In spite of being a cheap and abundantly available ion, limited examples of Fe- based MOFs exist in the literature owing to the easy oxidation of Fe 2+ and ready hydrolysis of Fe 3+ under hydrothermal conditions which poses a grand challenge for their bulk synthesis. 3 Currently, a handful of protocols exist for large-scale synthesis of Fe-MOFs, particularly the MIL-n (Fe 3+ )series. 3c,d However, most of these procedures suffer from synthetic constraints such as the use of toxic HF/HNO 3 , and most of their structures are determined from the powder X-ray diffraction (PXRD) patterns owing to the difficulty in the formation of mountable-sized single crystals. 3c This demands a look out for a facile method to improve the scalability as well as size of such MOF crystals so as to make the crystal structure determination easier, which is essentially needed to understand the MOF structure−property relationship. One of the ways to improve the crystal size and growth is the use of traditional sol−gel technique wherein an inert gel matrix maintains the crystal nuclei in its position of formation and growth. 4 On attainment of a mountable size, the crystals are recovered by dissolution of the gel matrix, induced by the external physical or chemical factors. However, in the case of MOFs, which essentially needs an inorganic metal ion along with an organic strut, screening an inert matrix that could be later dissolved leaving the MOFs unharmed would be a daunting task. One such synthesis was attempted by Yaghi et al., wherein an inert nonaqueous gelling solvent was used for growing a three-dimensional (3D) MOF, Zn (HBTC)- (NC 5 H 5 ) 2 ·C 2 H 5 OH. 4c Recently Lloyd and co-workers reported crystallization of small coordination compounds such as metallocycles from the gelling solvent itself. 4d This phenom- enon could be used in the case of Fe-MOF systems wherein gelation of a Fe-ligand solution is observed in some of the regularly used organic solvents such as DMF, ethanol, methanol, etc. 5 On degradation, such metallogel systems could act as a reservoir for furnishing the needed metal ions as well as organic linkers for in situ MOF formation. Nonetheless, this metallogel to MOF conversion remains largely unexplored for MOF synthesis. Moreover, inducing crystallization in such stable metallogel systems would be challenging as they are well-known for their high thermal and chemical stability. 5c,d Herein, we present a new strategy for the synthesis of Fe-MOFs via PdCl 2 -mediated gel-to-crystal transformation from substituted formamide analogues, namely, Received: March 17, 2014 Revised: May 27, 2014 Published: May 28, 2014 Article pubs.acs.org/crystal © 2014 American Chemical Society 3434 dx.doi.org/10.1021/cg500368q | Cryst. Growth Des. 2014, 14, 3434−3437