ENDOR characterization of an iron–alkene complex provides insight into a corresponding organometallic intermediate of nitrogenase. Issue 9 (30th June 2017)

• Record Type: Journal Article
• Authors: Horitani, Masaki; Grubel, Katarzyna; McWilliams, Sean F.; Stubbert, Bryan D.; Mercado, Brandon Q.; Yu, Ying; Gurubasavaraj, Prabhuodeyara M.; Lees, Nicholas S.; Holland, Patrick L.; Hoffman, Brian M.
• Is Part Of: Chemical science. Volume 8:Issue 9(2017)
• Page Start: 5941
• Page End: 5948
• Publication Date: 2017-06-30
• Subjects: Chemistry -- Periodicals; 540.5
• DOI: 10.1039/c7sc01602f
• Languages: English
• ISSNs: 2041-6520

Abstract

Abstract : Comparison of an iron(I)–alkene complex to a nitrogenase intermediate using ENDOR reveals details of the binding geometry. Abstract : A bio-organometallic intermediate, denotedPA, was previously trapped during the reduction of propargyl alcohol to allyl alcohol (AA) by nitrogenase, and a similar one was trapped during acetylene reduction, representing foundational examples of alkene binding to a metal center in biology. ENDOR spectroscopy led to the conclusion that these intermediates have η 2 binding of the alkene, with the hydrogens on the terminal carbon structurally/magnetically equivalent and related by local mirror symmetry. However, our understanding of both thePA intermediate, and of the dependability of the ENDOR analysis on which this understanding was based, was constrained by the absence of reference iron–alkene complexes for EPR/ENDOR comparison. Here, we report an ENDOR study of the crystallographically characterized biomimetic iron(i ) complex1, which exhibits η 2 coordination of styrene, thus connecting hyperfine and structural parameters of an Fe-bound alkene fragment for the first time. A tilt of the alkene plane of1 from normal to the crystallographic Fe–C2–C1 plane causes substantial differences in the dipolar couplings of the two terminal vinylic protons. Comparison of the hyperfine couplings of1 andPA confirms the proposed symmetry ofPA, and that the η 2 interaction forms a scalene Fe–C–C triangle, rather than an isosceles triangle. This spectroscopic study of a structurally characterized complex thus shows the exceptional sensitivity of ENDOR spectroscopy to structural details, while enhancing our understanding of the geometry of a key nitrogenase adduct.