Spectroscopic and Theoretical Investigation of a Complex with a O=FeIV–O–FeIV=O Core Related to Methane Monooxygenase Intermediate Q

摘要

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 Sloc = 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 (Sloc = 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 Sloc = 1 and Sloc = 2 sites to give total spin St = 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 pre-organizes >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