Understanding the Effect of Internal Electrostatic Fields Created by Alkaline Earth Metal Ions Poised over Secondary Coordination Sphere of Molecular Iron Complexes.

Abstract

Understanding the effect of the local electrical field around the reaction center in enzymes and molecular catalysis is an important topic of research. Herein, we explored the electrostatic field exerted by the alkaline earth metal ions (M2+ = Mg2+, Ca2+, Sr2+, and Ba2+) around Fe in FeIII(Cl) complexes by experimental and computational investigations. M2+ coordinated dinuclear FeIII(Cl) complexes (12M) were synthesized and characterized by X-ray crystallography and different spectroscopic techniques. EPR and magnetic moment measurements exhibited the presence of high-spin FeIII centers in the 12M complexes. Electrochemical investigations revealed FeIII/FeII reduction potential values shifted anodically in 12M complexes compared to 1. Likewise, 2p3/2 and 2p1/2 peaks in the XPS data were found to shift positively in the 12M complexes, demonstrating that redox-inactive metal ions make FeIII more electropositive. However, nearly similar λmax values in the UV-vis spectra were observed in 1 and 12M complexes. The first-principles-based computational simulations further revealed the impact of M2+ on stabilizing 3d-orbitals of Fe. The distortion in Laplacian distribution (∇2ρ(r)) of electron density around M2+ also indicates the possibility of having Fe-M interactions in these complexes. The absence of a bond critical point between FeIII and M2+ ions in the 12M complexes indicates dominant through-space interaction between these metal centers. Experimental and computational studies collectively imply that the installation of internal electrostatic fields exerted by M2+ ions in 12M complexes alters the electronic structure of FeIII.