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5406 Chem. Mater. 2009, 21, 5406–5412

Immobilization of Alkali Metal Ions in a 3D Lanthanide-Organic Framework: Selective Sorption and H2 Storage Characteristics
Sudip Mohapatra,† K. P. S. S. Hembram,‡ Umesh Waghmare,‡ and Tapas Kumar Maji*,†

Molecular Materials Laboratory, Chemistry and Physics of Materials Unit and ‡Theoretical Sciences Unit, Jawaharlal NehruCentre for Advanced Scientific Research, Jakkur, Bangalore 560064, India Received May 29, 2009. Revised Manuscript Received September 10, 2009

A lanthanide-alkali (HoIII-KI) bimetallic R-Po type 3D framework {KHo(C2O4)2(H2O)4}n (1) (C2O42- = oxalate dianion) has been synthesized and structurally characterized. Its dehydrated framework 10 , after removal of the K-bound water molecules, is found toexhibit permanent porosity with a clear size selective vapor sorption properties and H2 storage capability. High heat of H2 sorption (approximately -10 kJ/mol) observed in experiment is shown to arise from the preferential interaction of H2 with unsaturated KI sites decorated on the pore surfaces, using first-principles density functional theory-based calculations of energetics as well as thedetailed structure. Our work shows that a material with better hydrogen storage and release properties can be developed through immobilization of unsaturated reactive alkali metal ions at the pore surfaces in a metal-organic framework. Introduction Recent years, the synthesis and characterization of porous coordination polymers (PCPs) or metal-organic frameworks (MOFs) based on d-block metal ions andversatile organic linkers attracted considerable interest due to its applications in gas storage,1 selective separation,2 ion exchange,3 catalysis,4 and in molecular recognition properties.5 In this context, analogous chemistry of MOFs with lanthanide (Ln) ions are attractive because of their versatile coordination geometry,6 unique luminescent7
*Corresponding author. E-mail: (þ91) 80 2208 2826. Fax: (þ91) 80 2208 2766.

and magnetic properties,8 and possible high framework stability.9 Ln-based MOFs typically contain solvent molecules for the gratification of higher coordination number of Ln and removal of these ancillary solvent molecules from the coordination sphere often results collapse of the framework. Therefore, Ln-frameworks without coordinatedsolvent molecules may exhibit high thermal stability and framework rigidity, which are essential for porous functionality. To date, only a handful of examples of Ln-organic frameworks with porous functionality have been reported.10
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