Polynuclear Coordination Assemblies : Synthesis, Crystal Structures And Magnetic Behavior
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Construction of polynuclear metal assemblies from discrete 0D clusters to extend 3D networks, comprised of metal ions and bridging organic/inorganic ligands has attracted immense attention, owing to their intriguing network topologies and interesting properties. Proper ligand design and the appropriate choice of the metal center are of vital importance to the design of such polynuclear assemblies. One of the various attributes of polynuclear metal assemblies is magnetism. Magnetic materials can be constructed by incorporating magnetic moment carriers such as paramagnetic metals(V, Cr, Mn, Fe, Co, Ni, Cu) in presence of bridging ligands. Though, one-atom oxo/hydroxo and two-atom cyanide bridges were of popular choices due to their short distance for transmitting strong magnetic coupling between the paramagnetic metal centers, it has been shown that, three-atom bridging ligands like carboxylate and azide (N3 ) are well-fitted moieties for this purpose since they offer a variety of magnetic interactions depending on their versatile bridging modes. It has been well known that incorporation of anionic bridging ligand in presence of azide anion is a challenging task due to the competition between the 2nd anionic ligand with azide in self-assembly process. Incorporating both azide and carboxylate functionalities, a series of polymeric complexes has been synthesized and conversion of 0D discrete clusters to extended networks with the retention of basic core by fine tuning the ligands has been achieved. Single-crystal to single-crystal transformation has received considerable attention in crystal engineering since it is difficult for crystals to retain single crystallinity after removal of the guest at high temperature. Interestingly single-crystal to single-crystal transformation was observed at high temperature for Co(II) formate-formamide complex and change in dimensionality from 3D to 0D was observed at high temperature for Cr(III) formate-formamide complex. Multiferroic materials are those where both ferroelectricity and ferromagnetism coexist in the same phase. In general the transition metal d-electrons which are essential for magnetism reduce the tendency for off-center ferroelectric distortion. First tetrazole based miltiferroic coordination polymer of Co(II) metal ion in presence of azide has been successfully synthesized whereas its analogous Mn(II) complex showed different structural topology with interesting magnetic behavior. It has been also established in the present study, the important role played by hydrazine ligand to prevent oxidation of paramagnetic Co(II) to diamagnetic Co(III) system with the formation of a metal-inorganic assembly of Co(II) which exhibited spin-canted behavior.
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