The Role of Equatorial and Axial Ligands in Promoting the Activity of Non‐Heme Oxidoiron(IV) Catalysts in Alkane Hydroxylation

The key electronic structural feature of the FeO2+ moiety, which determines its activity as an alkane hydroxylation catalyst, is the presence of low‐lying acceptor orbitals, namely the 3σ* 3d –2pz antibonding orbital. Both the energetic position of this orbital and the spin state of the system (which in turn also affects the 3σ* energy) depend on the surrounding ligands. We present results of density functional theory (DFT) calculations performed on a series of gas‐phase complexes of composition [FeO(H2O)n(L)5–n]2+ (n = 4, 1, 0) derived from the recently characterised aqueous [FeO(H2O)5]2+ by substitution of ligand water molecules with L = NH3, CH3CN, H2S and BF3. The calculations reveal that the high‐spin (quintet) state is favoured by the weaker σ‐donating equatorial ligands, which is consistent with the literature. The high‐spin configuration is more reactive because of significant exchange stabilisation of the crucial 3σ*↑ orbital. Once the quintet state is formed by a judicious choice of equatorial ligands, the reactivity can be fine‐tuned by modulating the energy of the 3σ* orbital by varying the nature of the axial ligand. A linear relation between the σ‐donor properties of the axial ligand (estimated from the magnitude of the orbital interaction between the σ lone pair and the 3σ* orbital) and the activation barrier for the abstraction reaction is observed, and is related to a “push effect” of the σ donors that destabilises the 3σ* orbital. We propose that species with enhanced activation properties for hydrogen abstraction relative to [FeO(H2O)5]2+ might be obtainable by either replacing the axial ligand with a σ donor weaker than H2O or by preventing ligands from coordinating to iron in an axial position. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)