Comparison of the FeO2+ and FeS2+ complexes in the cyanide and isocyanide ligand environment for methane hydroxylation

A general comparison of fundamental distinctions between the FeO²⁺ and FeS²⁺ complexes in an identical cyanide or isocyanide ligand environment for methane hydroxylation has been probed computationally in this work in a series of hypothetical Fe^IV(X)(CN)₅]^3?, Fe^IV(X)(NC)₅]^3?, (X = O, S) complexes. We have detailed an analysis of the geometric and electronic structures using density functional theory calculations. In addition, their σ‐ and π‐mechanisms in C? H bond activation process have been described with the aid of the schematic molecular orbital diagram. From our theoretical results, it is shown that (a) the iron(IV)‐sulfido complex apparently is able to hydroxylate C? H bond of methane as good as the iron(IV)‐oxo species, (b) the O? CN, S? CN complexes have an inherent preference for the low‐spin state, while for the case of O? NC and S? NC in which S = 1 and S = 2 states are relatively close in energy, (c) each of the d block electron orbital plays an important role, which is not just spectator electron, and (d) in comparison to the cyanide and isocyanide ligand environment, we can see that the FeS²⁺ species prefer the cyanide ligand environment, while the FeO²⁺ species favor the isocyanide ligand environment. In addition, a remarkably good correlation of the σ‐/π‐mechanism for hydrogen abstraction from methane with the gap between the Fe‐d_z2 (α) and C? H (α) pair as well as the Fe‐d_xz/yz (β) and C? H (β) pair has been found. © 2012 Wiley Periodicals, Inc.