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 fieldaround thereaction center in enzymes and molecular catalysis is an importanttopic of research. Herein, we explored the electrostatic field exertedby the alkaline earth metal ions (M2+ = Mg2+, Ca2+, Sr2+, and Ba2+) around Fein Fe-III(Cl) complexes by experimental and computationalinvestigations. M2+ coordinated dinuclear Fe-III(Cl) complexes (1 M-2) were synthesized andcharacterized by X-ray crystallography and different spectroscopictechniques. EPR and magnetic moment measurements exhibited the presenceof high-spin Fe-III centers in the 1 M-2 complexes. Electrochemical investigations revealed Fe-III/Fe-II reduction potential values shifted anodically in 1 M-2 complexes compared to 1. Likewise,2p(3/2) and 2p(1/2) peaks in the XPS data were foundto shift positively in the 1 M-2 complexes,demonstrating that redox-inactive metal ions make Fe-III more electropositive. However, nearly similar lambda(max) values in the UV-vis spectra were observed in 1 and 1 M-2 complexes. The first-principles-basedcomputational simulations further revealed the impact of M2+ on stabilizing 3d-orbitals of Fe. The distortion in Laplacian distribution( backward difference (2)rho(r)) of electron density around M2+ also indicates the possibility of having Fe-M interactionsin these complexes. The absence of a bond critical point between Fe-III and M2+ ions in the 1 M-2 complexes indicates dominant through-space interaction betweenthese metal centers. Experimental and computational studies collectivelyimply that the installation of internal electrostatic fields exertedby M2+ ions in 1 M-2 complexes altersthe electronic structure of Fe-III. An Fe(III)-Cl complex (1) of a bispyridine-bisalkoxideligand and alkaline earth (AE) metal coordinated dinuclear adductsof 1 (1 M-2) has been synthesizedand thoroughly characterized. Short distances between Fe and AE metalions were observed. Experimental observations and theoretical studiesrevealed through-space interactions between Fe and AE metal ions,and such an interaction causes stabilization of Fe d orbital energies.