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.