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

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 (M 2+ = Mg 2+ , Ca 2+ , Sr 2+ , and Ba 2+ ) around Fe in Fe III (Cl) complexes by experimental and computational investigations. M 2+ coordinated dinuclear Fe III (Cl) complexes ( 1 2 M) were synthesized and characterized by X-ray crystallography and different spectroscopic techniques. EPR and magnetic moment measurements exhibited the presence of high-spin Fe III centers in the 1 2 M complexes. Electrochemical investigations revealed Fe III /Fe II reduction potential values shifted anodically in 1 2 M complexes compared to 1 . Likewise, 2p 3/2 and 2p 1/2 peaks in the XPS data were found to shift positively in the 1 2 M complexes, demonstrating that redox-inactive metal ions make Fe III more electropositive. However, nearly similar λ max values in the UV-vis spectra were observed in 1 and 1 2 M complexes. The first-principles-based computational simulations further revealed the impact of M 2+ on stabilizing 3d-orbitals of Fe. The distortion in Laplacian distribution (∇ 2 ρ(r)) of electron density around M 2+ also indicates the possibility of having Fe-M interactions in these complexes. The absence of a bond critical point between Fe III and M 2+ ions in the 1 2 M 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 M 2+ ions in 1 2 M complexes alters the electronic structure of Fe III .