Co(II) Coordination Polymers with Co-Ligand Dependent Dinuclear to Tetranuclear Core: Spin-Canting, Weak Ferromagnetic, and Antiferromagnetic Behavior Prem Lama, † Jerzy Mrozinski, ‡ and Parimal K. Bharadwaj* ,† † Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016, India ‡ Faculty of Chemistry, University of Wroclaw, 14 F. Joliot-Curie Street, 50-383 Wroclaw, Poland * S Supporting Information ABSTRACT: Four new Co(II) containing coordination polymers have been synthesized using an ether bridged tricarboxylic acid ligand, o- cpiaH 3 (5-(2-carboxy-phenoxy)-isophthalic acid). This ligand readily reacts with CoCl 2 ·6H 2 O in the presence of different nitrogen donor ligands such as 1,10-phenanthroline (phen), 4,4′-bipyridyl (bpy), 4,4′- azopyridine (apy), and 1,4-bis(4-pyridinylmethyl)piperazine (bpmp) under hydrothermal conditions to afford three 3D and one 2D coordination polymers, {Co 3 (o- cpia) 2 (phen)(H 2 O)· 5H 2 O} n (1), {Co 1.5 (o-cpia)(bpy)} n (2), {Co 2 (o-cpia)(OH)(apy) 0.5 } n (3), and {Co(o-cpiaH)(bpmp) 0.5 } n (4). Single crystal X-ray studies show that these coordination polymers contain homometallic clusters varying from dimeric to tetrameric depending upon the co-ligand used. In complexes 1 and 2, Co(II) ions form an angular and linear trimeric unit that extends along all directions to generate an overall 3D structure. In contrast, 3 forms a 3D coordination polymer containing a tetranuclear Co(II) unit. When the distance between the two donor N atoms of the co-ligand is further increased as in 4, a (4,4) net connected 2D coordination polymer results where the Co(II) ions form a dimeric paddle wheel unit. In addition to single-crystal X-ray crystallography, the complexes are also characterized by IR spectroscopy, thermogravimetry, and elemental analysis. Variable temperature magnetic susceptibility measurements on the complexes were carried out over the temperature range 1.72-300 K. Complex 1 exhibits ferromagnetic interactions due to uncompensated magnetic moments of the system leading to spin-canted antiferromagnetic behavior, while 2 and 3 show predominantly antiferromagnetic interactions. Complex 4 exhibits weak ferromagnetic behavior below 9 K. ■ INTRODUCTION Studies on coordination polymers have witnessed an upsurge in recent years due to their novel architecture as well as potential applications as functional materials. 1 In particular, these materials can be engineered via ligand design to impart useful magnetic properties. 2 It has been our interest to construct coordination polymers with interesting magnetic properties using carboxylate as well as mixed carboxylate and nitrogen donor linkers. Herein, we have used a tricarboxylate ligand with di fferent N-donor co-ligands to construct coordination polymers with Co(II) metal nodes. Organic linkers bearing carboxylate as bridging ligands have been used frequently to synthesize magnetic materials because of their diverse binding capability and efficiency in transmiting magnetic coupling. 3 In 1999, Li and co-workers reported 4 one of the first examples of carboxylate donor ligands with N-donor co-ligands that led to a series of two-dimensional (2D) coordination polymers with the general formula, [M(ox)(bpy)] (where ox = oxalate and bpy = 4,4′-bipyridyl and M = Fe(II), Co(II), and Ni(II)). Coordina- tion polymers of paramagnetic metal ions have led to an interesting range of systems exhibiting magnetic phenomena such as ferromagnetism, 5 antiferromagnetism, 6 spin-canting, 7 metamagnetism, 8 single-chain magnetism, 9 and so on. Con- struction of coordination polymers with metal ions capable of showing magnetic anisotropy is particularly attractive. 10 In the present work, we have chosen Co(II) as the paramagnetic metal ion, that can readily bind to the ligand, o- cpiaH 3 (5-(2-carboxy-phenoxy)-isophthalic acid) with multiple bridging capability (Scheme 1) to form multinuclear clusters of Co(II) capable of magnetic interactions. Several N-donor co- ligands were used to probe how the overall architecture of the complexes are influenced by these co-ligands. Synthesis and characterization of four new coordination polymers are described along with their interesting variable temperature magnetic properties. Received: March 9, 2012 Revised: April 24, 2012 Published: April 26, 2012 Article pubs.acs.org/crystal © 2012 American Chemical Society 3158 dx.doi.org/10.1021/cg300330p | Cryst. Growth Des. 2012, 12, 3158-3168