Influence of Copper Substitution on the Interaction of Ethylene over Iron Clusters: A Theoretical Study

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We report the results of density functional theory (DFT) calculations of ethylene adsorption over the most stable pure and bimetallic clusters of Fe(n)Cu(m) (2 ≤ m+n ≤ 4), in two adsorption modes of π and di-σ. Our results show that the quality of interaction of ethylene with iron center in bimetallic clusters of iron-copper is characteristically different from what is found over pure iron. Over the range of our studies for dimers, trimers, and tetramers, whether for π or di-σ mode, alloying iron clusters results in a substantial improvement in adsorption of ethylene over cluster and exhibits a notable increase in binding and interaction energies compared with pure iron clusters. One of the interesting features of this adsorption is that ethylene never orients toward di-σ mode for Cu-Cu or Fe-Cu bonds, and π orientation is strongly preferred. Ethylene adsorption in di-σ coordination is accompanied by the sever restructuring, larger deformation energy, and the larger interaction energy. In the next part, we answer this question of how electronic perturbations induced by copper atoms can enhance the activity of iron toward ethylene. This interpretation is done within the framework of natural bond orbital (NBO) and natural resonance theory (NRT) analyses. Different reaction pathways detected by NRT analysis (donor-acceptor, metallacyclic, and carbanion) reveal interesting aspects of differences between the nature of metal-alkene coordination in bimetallic and purely metallic clusters.