Water square (uudd) in novel Cu II framework structures built from isomeric (aminomethyl)pyridines and oxalate: Synthesis, structure, spectral and DFT studies Madhulata Shukla a , Bhagwan Kharediya a,b , Nitin Srivastava a , Satyen Saha a , Sailaja Sunkari a,b,⇑ a Department of Chemistry, Faculty of Science, Banaras Hindu University, Varanasi 221005, India b Department of Chemistry, Mahila Maha Vidyalaya, Banaras Hindu University, Varanasi 221005, India article info Article history: Received 14 December 2012 Accepted 9 February 2013 Available online 26 February 2013 Keywords: Water square Cu(II)-amp-oxalate Supramolecular chemistry Density functional theory TD-DFT abstract A perfect water square in uudd configuration, built from solvent waters has been isolated from a hybrid metal–organic host [Cu(3-aminomethyl)pyridine oxalateOH 2 Á2H 2 O] n under self assembly conditions, stabi- lizing the supramolecular framework through cooperative hydrogen bonding interactions, a rare occur- rence among water clusters. The present findings with detailed crystallographic and theoretical studies add to our knowledge regarding the understanding of the contribution of water clusters to the stability and function of biological assemblies as well as the anomalous properties of water. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Water, the major component of planet earth, has stimulated vast interest in the scientific community of various disciplines alike. The curiosity in understanding the basics of water may be largely attributed to its essence for the genesis of life. To under- stand this fact and to mimic nature’s ability, intense research has been undertaken in all disciplines of science. There is immense lit- erature available in diverse forms discussing the theoretical and experimental aspects of water, both in physical and biological sci- ences, as to understand the factors responsible for its nature, behavior under varied environments, hydrogen bonding interac- tions as well as to probe its possible role in the stabilization and functionalization of biomolecules [1–9]. Understanding the intermolecular interactions among water molecules in different clusters can shed light on solid–liquid bulk properties and solvent influenced biological processes. Computa- tional methodologies [8,9] prove to be a powerful tool and have gi- ven insights into such phenomena, but despite its advances the correct description of bulk water has not been achieved yet. Pati and co-workers [5] have made description on cyclic water clusters trapped inside organic or inorganic molecules and attributed the existence of different shapes of water clusters in confined geome- tries to weak supramolecular aromaticity. At this juncture, the only way that remains to understand the behavior of water is to obtain precise structural data of various hydrogen bonded networks in diverse environments. It is this realization that has prompted extensive investigations of water structures in recent years and several water clusters have been realized experimentally under different host environments [10–26]. Hybrid metal–organic supra- molecular systems synthesized under self-assembly conditions of- ten provide good host environment for the isolation of several water clusters either as discrete clusters or extended chains/net- works, often crystallizing as the hydrates of the supramolecular host in the solid state [17–27]. Transition metal supramolecular systems, with their promising material applications [28–31], have geared up new research inter- ests in scientific community for past several years. The scope for generating target specific metal–organic framework/supramolecu- lar structures lie in the judicious selection of metal ions, flexible li- gands with multiple coordination sites and multidenticity and anions. Combination of flexible ligands with multiple bonding sites and anions with varying coordination capacities may lead to 1, 2 or 3D assemblies. (Aminomethyl)pyridines (amp) belong to the class of flexible ligands which may act as chelating or bridging depending upon the relative position of amino group to the pyridine N atom. While bridging as well as chelating modes of coordination have been observed for 2-amp in several Cu(II) and Ag(I) complexes [32–35], only bridging mode of coordination is possible for 3-amp, due to its geometrical constraints [32,34]. Oxalate group with its multi dentate coordination ability towards metal ions and different ligand combinations, has been well explored both experimentally and theoretically for past several years, for its potentiality in carrying over magnetic exchange interactions between metal cen- ters besides generating novel supramolecular structures [36–38]. 0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2013.02.041 ⇑ Corresponding author at: Department of Chemistry, Mahila Maha Vidyalaya, Banaras Hindu University, Varanasi 221005, India. Tel.: +91 9935963366; fax: +91 542 2368127. E-mail addresses: sunkari.s7@gmail.com, ssahabhu@yahoo.com (S. Sunkari). Polyhedron 54 (2013) 164–172 Contents lists available at SciVerse ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly