B43: Spin Equilibrium of Cobalt-Salen Complexes

Spin Crossover (SCO), or spin equilibrium, is a well-studied phenomenon in some metal complexes. This phenomenon occurs when changes such as heat, light, or pressure are applied to a metal complex, which the metal center responds by changing its spin state. The spin state of a metal center is the arrangement of its valence electrons, usually d-electrons, which can have 2 different configurations: high-spin, with the maximum number of unpaired electrons, and low-spin, with the minimum number of unpaired electrons. During spin equilibrium, the changes in the environment around the metal complex cause it to change its spin state, from high-spin to low-spin and vice versa. Since the high-spin state of metal complexes have different magnetic properties compared to the low-spin state, SCO compounds have potential in developing molecular magnets and switches, sensors, memory and display devices.

The goal of this study is to explore the SCO behavior of some Cobalt(II)-Salen complexes (denoted Co(1), Co(2), and Co(S,S)) with the presence of pyridine, a nitrogen-containing ligand, by means of UV-Visible and NMR spectroscopy. We hypothesized that as pyridine is introduced to Cobalt-Salen, the complex’s structure changes from a 4-coordinate complex with {N2O2} coordination sphere to 5-coordinate with {N3O2} coordination sphere, and this structural change is associated with the SCO behavior, which would be reflected in both spectroscopic methods. Cobalt-Salen has been reported to have catalytic properties, and understanding this interaction between the complexes and pyridine can provide insights into these complexes’ catalytic ability, as well as develop new catalysts.

The complexes, namely Co(1) and Co(S,S), are synthesized directly from a Co(II) source and corresponding Salen ligands, and spectroscopic titrations are carried out for all the complexes by adding increasing quantities of pyridine into the complex’s solution. We observe spectroscopic evidence of spin equilibrium phenomena occurring in the all of the Co(II)-Salen, and possible SCO behavior in Co(III)-Salen, which can be prepared at ease from the Co(II)-Salen precursor. These changes align with our hypothesis on the effect of structural changes on the spin states of Co(Salen) complexes. This study provides some plans and data to be collected for future work, including synthesizing Co(2) and obtaining crystallographic and EPR spectroscopy data for all the compounds to further confirm our results.

Author(s): Quang Tran, Chemistry Major

Advisor(s): David TIerney, Department of Chemistry and Biochemistry