title: Spectroscopic and theoretical investigation of a complex with an [O═Fe(IV)-O-Fe(IV)═O] core related to methane monooxygenase intermediate Q.
abstract: 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 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.
authors.affiliation: Department of Chemistry, Carnegie Mellon University , Pittsburgh, Pennsylvania 15213, United States.
authors.forename: Sebastian A
authors.initials: SA
authors.lastname: Stoian
authors.forename: Genqiang
authors.initials: G
authors.lastname: Xue
authors.forename: Emile L
authors.initials: EL
authors.lastname: Bominaar
authors.forename: Lawrence
authors.initials: L
authors.lastname: Que
authors.forename: Eckard
authors.initials: E
authors.lastname: Münck
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D007501:Iron / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D015394:Molecular Structure; D010100:Oxygen / Q000737:chemistry* / Q000378:metabolism; D010105:Oxygenases / Q000737:chemistry* / Q000378:metabolism; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet
chemical_list: D007501:Iron; D010105:Oxygenases; C021555:methane monooxygenase; D010100:Oxygen