title: Analysis of a pentacoordinate iron dicarbonyl as synthetic analogue of the Hmd or mono-iron hydrogenase active site.
abstract: Pentacoordinate iron dicarbonyls, (NS)Fe(CO)(2)P (NS=2-amidothiophenylate, P=PCy(3) (4), PPh(3), (5), and P(OEt)(3) (6)) were prepared as potential biomimetics of the active site of the mono-iron hydrogenase, [Fe]-H(2)ase. Full characterization including X-ray diffraction, density functional theory (DFT) computations, and Mössbauer studies for complexes 5 and 6 find that, despite similar infrared v(CO) pattern and absorption frequencies as the active site of the [Fe]-H(2)ase, the geometrical distortions towards trigonal bipyramidal, the negative isomer shift parameters, and the differences in CO-uptake reactivity are due to the "non-innocence" of the NS ligand. Ligand-based protonation with a strong acid, HBF(4).Et(2)O, interrupted the extensive pi-delocalization over Fe and NS ligand of complex 4 and switched on CO uptake (1 bar) to form a CO adduct, mer-[(H-NS)Fe(CO)(3)(PCy(3))](+) or 4(CO)-H(+). The extrinsic CO is reversibly removed on deprotonation with Et(3)N to regenerate complex 4. In a (13)CO atmosphere, concomitant CO uptake by 4-H(+) and exchange with intrinsic CO groups provide a facile route to (13)C-labeled 4(CO)-H(+) and, upon deprotonation, (13)C-labeled complex 4. DFT calculations show substantial Fe character in the LUMO of 4-H(+) typical of the d(6) Fe(II) in a regular square-pyramidal geometry. Thus, the Lewis acidity of 4-H(+) makes it amenable for CO binding, whereas the dianionic NS ligand renders the iron center of 4 insufficiently electrophilic and largely of Fe(I) character.
authors.affiliation: Department of Chemistry, Texas A&M University, College Station, TX 77843, USA.
authors.forename: Tianbiao
authors.initials: T
authors.lastname: Liu
authors.forename: Bin
authors.initials: B
authors.lastname: Li
authors.forename: Codrina V
authors.initials: CV
authors.lastname: Popescu
authors.forename: Andrey
authors.initials: A
authors.lastname: Bilko
authors.forename: Lisa M
authors.initials: LM
authors.lastname: Pérez
authors.forename: Michael B
authors.initials: MB
authors.lastname: Hall
authors.forename: Marcetta Y
authors.initials: MY
authors.lastname: Darensbourg
mesh_terms: D001665:Binding Sites; D002384:Catalysis; D018360:Crystallography, X-Ray; D004563:Electrochemistry; D006864:Hydrogenase / Q000737:chemistry* / Q000378:metabolism; D058085:Iron Compounds / Q000737:chemistry*; D007506:Iron-Sulfur Proteins / Q000737:chemistry* / Q000378:metabolism; D008024:Ligands; D015394:Molecular Structure
chemical_list: D058085:Iron Compounds; D007506:Iron-Sulfur Proteins; D008024:Ligands; C118830:iron hydrogenase; D006864:Hydrogenase