Probing the Secondary Coordination Sphere of Zn(II) and Fe(II) Pyridinediimine (PDI) Complexes

The transformation of many small, abundant molecules is necessary both in a biological setting and in the chemical industry. In Nature, the activation of small molecules is promoted by metalloenzymes. However, many of these chemical transformations are thermodynamically demanding and consist of multi-electron redox processes. Understanding the secondary coordination sphere has played an integral role in determining the catalytic activity and selectivity of such transformations and has led to the development of bioinspired catalysts in order to mimic the native active site of the metalloenzyme. Due to its extensive modularity, the utilization of the pyridinediimine (PDI) metal complexes was targeted in this work to study the secondary coordination sphere and its relationship to the reactivity at the metal active site. The redox-active PDI ligand scaffold containing a pendant base was used to synthesize a series of Fe(II) and Zn(II) complexes consisting of H-bond acceptors/donors in the secondary coordination sphere. The Zn(II) complexes are able to be protonated in the secondary coordination sphere, forming metal halogen hydrogen bonds (MHHBs). The use of these intramolecular H-bonds in the Zn complexes also serve to provide stabilization of the hydrosulfide (HS-) ligand, forming a six-coordinate Zn complex. The Fe(II) complexes were reduced under CO atmosphere, followed by protonation in the secondary coordination sphere. This resulted in stable, doubly reduced protonated species, capable of moving protons and electrons in and out of the system. The protonated Fe(II) complex was poised to deliver protons and electrons necessary to investigate nitrite (NO2-) reduction for the formation a dinitrosyl iron complex (DNIC). The reduction of nitrate (NO3-) for the synthesis of the DNIC was also explored and used to further investigate products of the reaction. Fe(II) PDI complexes with an incorporation of Lewis acids in the secondary coordination sphere were also synthesized and characterized in order to provide a better understanding of how redox inactive metals in the secondary coordination sphere of the PDI scaffold of alters the redox activity of the complex