Lanthanide (La III /Ho III )-oxalate open framework materials formed by in situ ligand synthesis Sudip Mohapatra a , Sameer Vayasmudri a , Golam Mostafa b , Tapas Kumar Maji a, * a Molecular Materials Laboratory, Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India b Department of Physics, Jadavpur University, Jadavpur, Kolkata 700032, India article info Article history: Received 28 February 2009 Received in revised form 22 May 2009 Accepted 28 May 2009 Available online 6 June 2009 Keywords: La III -complex Ho III -complex Oxalate bridge In situ ligand synthesis Open frameworks abstract Two new lanthanide-organic open framework materials, {La(ox) 0.5 (l-OH) 2 (l-H 2 O)(H 2 O) 2 } n (1) and {[Ho(ox) 1.5 (H 2 O) 3 ](en)} n (2) (ox = oxalate dianion and en = ethylenediamine) have been synthesized and structurally characterized. Both the frameworks obtained from in situ synthesis of oxalate ligand from dicarboxylic acid (2,5-dihydroxyterephthalic acid for 1 and tetrabromoterephthalic acid for 2) and lanthanide ions (Ln III ) at high temperature. The probable route to the formation of oxalate ligand is the decarboxylation from the corresponding dicarboxylic acid and consecutive reductive coupling catalyze by the corresponding Ln III ions. Compound 1 is a 3D framework formed by pillaring of the oxalate ligand to two dimensional La–O–La inorganic layers, resulting 1D channels along the b-axis. Compound 2 is a 2D honeycomb like oxalate bridged framework of Ho III templated by the en molecules. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Coordination polymers of lanthanide ions (Ln III ) have received significant attention in the current research due to their wide range of application such as magnetic materials [1–7], luminescent sensors and light converters [8–10], catalysis [11–12] and in molecular recognition [13]. This makes Ln III ions potential candi- dates for the functional molecular materials. Literature survey shows that lanthanide-organic open frameworks are scarce in comparison to the transition-metal open framework porous mate- rials [14–19]. This is attributed to the variable coordination numbers (6 6 CN 6 12) and geometries of Ln III ions, unlike transi- tion-metal ions, which is controlled by a subtle interplay between electrostatic interactions and inter-ligand steric constraints. How- ever, there is an upsurge for the preparation of stable lanthanide based coordination framework with stimulation of lanthanide coor- dination chemistry in view of novel functional aspects [20–24]. Hydro(solvo)thermal methods are typically employed for the prep- aration of these stable lanthanide-organic frameworks [14–24]. In situ ligand synthesis has become an effective approach and a popular route to new inorganic/organic hybrid materials [25–28]. Under hydrothermal conditions, in situ formation of ligands often relies on the rearrangement or cleavage of organic species [29,30]. The rearranged organic species is then observed in the crystalline reaction products. Very recently, Zhang reported a dif- ferent aspects of in situ ligand synthesis under hydro(solvo)ther- mal conditions, which includes carbon–carbon bond formation, hydroxylation, tetrazole formation, decarboxylation, substitution, hydrolysis etc [31]. The mechanism of the in situ ligand formations remain poorly understood and occurrences are largely serendipi- tous. The in situ formation of oxalate ligands, observed in solid state reaction products, has previously been reported yet the mechanism of formation remains ambiguous [24,32–35]. On the other hand template based synthesis of metal-organic frameworks is in infancy stage. The deliberate use of the large template mole- cule may results versatile framework topology with different pore sizes depending upon the corresponding organic template mole- cules. Such attempts were made in transition-metal based metal- organic framework materials [25]. However, template based synthetic approach is rare in case of lanthanide-organic open framework materials. Here we report two new coordination framework of Ln III , {La(ox) 0.5 (l-OH) 2 (l-H 2 O)(H 2 O) 2 } n (1) and {[Ho(ox) 1.5 (H 2 O) 3 ](en)} n (2) formed by in situ formation of the oxalate ligand at higher tem- perature by the reaction of respective lanthanide salts and dicar- boxylic acids. 2. Experimental 2.1. Materials La(NO 3 ) 3 6H 2 O, Ho(NO 3 ) 3 5H 2 O, 2.5-dihydroxyterephthalic acid and tetrabromoterephthalic acid were purchased from Aldrich Chemical company Inc and used as received. 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.05.055 * Corresponding author. Tel.: +91 80 2208 2826; fax: +91 80 2208 2766. E-mail address: tmaji@jncasr.ac.in (T.K. Maji). Journal of Molecular Structure 932 (2009) 123–128 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc