Versatility of Azide in Serendipitous Assembly of Copper(II) Magnetic Polyclusters

Abstract

Engineering at the molecular level is one of the most exciting new developments for the generation of functional materials. However, the concept of designing polynuclear extended structures from bottom up is still not mature. Although progress has been made with secondary building units (SBUs) in metal organic frameworks (MOFs), the control seems to be just an illusion when it comes to bridging ligands such as the azide ion. When we say that the azido ligand is versatile in its bridging capabilities, what we mean is that it would be difficult to predict or control its bridging properties. However, this kind of serendipity is not always bad news. For example, scientists have shown that the azido ligand can mediate magnetic exchanges between paramagnetic metals in a predictable fashion (usually depending upon the bonding geometries). Therefore, it is a well-respected ligand in polynuclear assemblies. Serendipitous assemblies offer new magnetic structures that we may not otherwise even think about synthesizing. The azido ligand forms a variety of complexes with copper(II) using different blocking amines or pyridine based ligands. Its structural nature changes upon changing the substitution on amine, as well as the amount of blocking ligand. In principle, if we take any of these complexes and provide more coordination sites to the bridging azido ligands by removing a fraction of the blocking ligands, we can get new complexes with intricate structural networks and therefore different magnetic properties with the same components as used for the parent complex. In this Account, we mainly discuss the development of a number of new topological and magnetic exchange systems synthesized using this concept. Not all of these new complexes can be grouped according to their basic building structures or even by the ratio of the metal to blocking ligand. Therefore, we divided the discussion by the nuclearity of the basic building structures. Some of the complexes with the same nuclearities have very similar or even almost identical basic structures. However, the way these building units are joined together (by the azido bridges) to form the overall extended structures differ almost in every case. The complexes having the Cu6 core are particularly interesting from a structural point of view. Although they have almost identical basic structures, some of them are extended in three dimensions, but two of them are extended in two dimensions by two different bridging networks. In the complexes having linear Cu4 basic units, we find that using similar ligands does not always give the same bridging networks even within the basic building structures. These complexes have also enriched the field of molecular magnetism. One of the complexes with a Cu3 building unit has provided us with the opportunity to study the competing behavior of two different kinds of magnetic exchange mechanism (ferromagnetic and antiferromagnetic) acting simultaneously between two metal ions. Through density functional theory calculations, we showed how they work i...

Anion directed cation templated synthesis of three ternary copper(II) complexes with a monocondensed N2O donor Schiff base and different pseudohalides

“…Pseudohalides, especially azide, cyanate and thiocyanate, have widely been employed for the syntheses of such systems because of their ability to coordinate metal ions with different modes [21][22][23]. Dicyanamide is sometimes called bent pseudohalide [24][25] and the coordination polymers based on dicyanamide (dca) have also attracted much attention in the past few years for their interesting extended architectures and magnetic properties as well [26-.…”

A polynuclear and two dinuclear copper(II) Schiff base complexes: Synthesis, characterization, self-assembly, magnetic property and DFT study

“…Focusing on copper(II), pseudohalides have widely been employed for the syntheses of such complexes because of their ability to coordinate copper(II) in different modes [18][19][20][21][22]. Both basalbasal and basal-apical modes are observed in the binding of adjacent copper(II) centres by end on pseudohalides [23].…”

Crystal structure, characterization and magnetic properties of a 1D copper(II) polymer incorporating a Schiff base with carboxylate side arm

“…[1][2][3][4][5][6][7][8][9][10][11] Careful selection of ligands having capacities to combine more than one metal centre or by using different spacer ligands for connecting metals in combination with other co-ligands are considered as important strategies for the synthesis of these complexes in addition to the effect of reaction conditions and solvents. [2][3][4][5] Sometimes, suitable secondary building units (SBU) are also used as important tools for the design and synthesis of these coordination polymers.…”

“…Role of various Schiff bases/blockers and azide as bridging ligand in a * For correspondence number of copper(II) poly-clusters/assemblies may be mentioned in this regard not only for the structural diversities of the resulting complexes but also for their potential applications in the field of molecular magnetism. 7,8 The role of the Schiff base ligands with free carboxylate arm also deserved attention in this regard for their abilities to act simultaneously as chelator and bridge to the metals leading to wide varieties of structures. Amino acid-based Schiff bases, a class of the Schiff bases with carboxylate side arm, and their metal complexes were synthesized and studied extensively for their wide applications in biology, pharmacy and industry.…”