Cobalt and Nickel Complexes in a Macrocyclic Ligand Framework with Varying Axial Ligands
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Transition metal complexes are increasingly seen in metalloprotein modeling, energy generation, and materials applications due to their metal–metal interactions. In ongoing research di- and tetranuclear cobalt and nickel complexes were synthesized and characterized using different spectroscopic methods. The binuclear complexes were prepared from the precursor complexes, [M2L(py)2](BF4)2 (M = Co(II), Ni(II); L = imine/phenolate Robson macrocyclic ligand), and two syn pyridine ligands were substituted with bridging organic ligands 7–azaindolate (NN donor atoms), 2– hydroxypyridinate (NO), diphenylacetate (OO). Most of the two-metal-atom structures displayed octahedral coordination geometry. The [M2L(μ–O2CCHPh2)(py)2]BF4 compounds, underwent dimerization when recrystallized and formed tetranuclear complexes [M4L2(μ–O2CCHPh2)2](BX4)2 (X = F or Ph) by substituting the remaining monodentate pyridine ligands while retaining the bridging diphenylacetate donors bound. The coordination geometry is a cubane M4O4 structure. The structural analysis was conducted on the x-ray crystallography data using mercury software. The structural analysis leads us to believe that the syn-pyridine ligands assume spatial orientation away from each other not only because of steric repulsion but also favorable orbital alignment. The spatial alignment of the orbitals is dependent on the bite angle of the bridging ligands as the bridging ligands molecular orbitals interact with the metal. Additionally, infrared spectroscopy was performed to probe the functional groups within the different complexes with varying bridging ligands. Electronic spectroscopy was run to observe the metal–ligand environment of the compounds. The study puts forth questions about synthetic reproducibility and why the tetranuclear species forms for the nickel complex, but not the cobalt counterpart. Moreover, the study furthers our structural understanding of the complexes and delves into the how molecular orbitals and ligand environment affect it.
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