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 3 sigma* 3d(z)(2)-2p(z), antibonding orbital. Both the energetic position of this orbital and the spin state of the system (which in turn also affects the 3 sigma* 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 sigma-donating equatorial ligands, which is consistent with the literature. The high-spin configuration is more reactive because of significant exchange stabilisation of the crucial 3 sigma* up arrow 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 3 sigma* orbital by varying the nature of the axial ligand. A linear relation between the sigma-donor properties of the axial ligand (estimated from the magnitude of the orbital interaction between the sigma lone pair and the 3 sigma* orbital) and the activation barrier for the abstraction reaction is observed, and is related to a "push effect" of the sigma donors that destabilises the 3 sigma* 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 sigma donor weaker than H2O or by preventing ligands from coordinating to iron in an axial position. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007).