Spectroscopic and Theoretical Investigation of a Complex with an [OFe^IV–O–Fe^IVO] Core Related to Methane Monooxygenase Intermediate Q

Previous efforts to model the diiron­(IV) intermediate Q of soluble methane monooxygenase have led to the synthesis of a diiron­(IV) TPA complex, 2, with an O=Fe^IV–O–Fe^IV–OH core that has two ferromagnetically coupled S_loc = 1 sites. Addition of base to 2 at −85 °C elicits its conjugate base 6 with a novel OFe^IV–O–Fe^IVO core. In frozen solution, 6 exists in two forms, 6a and 6b, that we have characterized extensively using Mössbauer and parallel mode EPR spectroscopy. The conversion between 2 and 6 is quantitative, but the relative proportions of 6a and 6b are solvent dependent. 6a has two equivalent high-spin (S_loc = 2) sites, which are antiferromagnetically coupled; its quadrupole splitting (0.52 mm/s) and isomer shift (0.14 mm/s) match those of intermediate Q. DFT calculations suggest that 6a assumes an anti conformation with a dihedral OFe–FeO angle of 180°. Mössbauer and EPR analyses show that 6b is a diiron­(IV) complex with ferromagnetically coupled S_loc = 1 and S_loc = 2 sites to give total spin S_t = 3. Analysis of the zero-field splittings and magnetic hyperfine tensors suggests that the dihedral OFe–FeO angle of 6b is ∼90°. DFT calculations indicate that this angle is enforced by hydrogen bonding to both terminal oxo groups from a shared water molecule. The water molecule preorganizes 6b, facilitating protonation of one oxo group to regenerate 2, a protonation step difficult to achieve for mononuclear Fe^IVO complexes. Complex 6 represents an intriguing addition to the handful of diiron­(IV) complexes that have been characterized.