07/15/2010

Elusive high-spin Fe=O complex crystallized

Recent research from the research group of Professor Lawrence Que.

Understanding how iron enzymes activate O₂ to break strong C-H bonds on the substrate.

High-valent oxoiron(IV) species are considered to be the active oxidants in the catalytic cycles of a wide range of mononuclear non-heme oxygen activating enzymes. Several of these high-valent intermediates have now been spectroscopically characterized and found to have high-spin (S = 2) iron(IV) centers. In contrast, the overwhelming majority of synthetic oxoiron(IV) complexes exhibit an intermediate spin state (S = 1), with the only exceptions being the highly unstable [Fe^IV(O)(H₂O)₅]²⁺ and the trigonal bipyramidal complex [Fe^IV(O)(TMG₃tren)]²⁺ (1). As a follow-up to their paper detailing the synthesis and spectroscopic characterization of the latter, Que group post-doc Dr Jason England together with collaborators at Carnegie Mellon University recently published an article in the Journal of the American Chemical Society (JACS, 2010, 132, 8635-8644, DOI: 10.1021/ja100366c) detailing the crystal structure of this complex, the first such example for a high-spin oxoiron(IV) complex. This was achieved as a consequence of a detailed analysis of the self-decay pathway of this complex, which indicated that the highly reactive oxoiron(IV) center reacted intramolecularly with the methyl substituents of the TMG₃tren ligand. Retardation of this process by exploitation of the large kinetic isotope effect (24 at 25 °C) accompanying perdeuteration of the aforementioned methyl groups allowed crystals to be grown that were suitable for X-ray analysis. The solution of this first crystal structure of a high-spin (S = 2) oxoiron(IV) center represents a fundamental step on the path towards a full understanding of these pivotal biological intermediates.