Issue 9, 2017

ENDOR characterization of an iron–alkene complex provides insight into a corresponding organometallic intermediate of nitrogenase

Abstract

A bio-organometallic intermediate, denoted PA, 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 η2 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 the PA 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) complex 1, which exhibits η2 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 of 1 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 of 1 and PA confirms the proposed symmetry of PA, and that the η2 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.

Graphical abstract: ENDOR characterization of an iron–alkene complex provides insight into a corresponding organometallic intermediate of nitrogenase

Supplementary files

Article information

Article type
Edge Article
Submitted
10 4 2017
Accepted
17 6 2017
First published
30 6 2017
This article is Open Access

All publication charges for this article have been paid for by the Royal Society of Chemistry
Creative Commons BY license

Chem. Sci., 2017,8, 5941-5948

ENDOR characterization of an iron–alkene complex provides insight into a corresponding organometallic intermediate of nitrogenase

M. Horitani, K. Grubel, S. F. McWilliams, B. D. Stubbert, B. Q. Mercado, Y. Yu, P. M. Gurubasavaraj, N. S. Lees, P. L. Holland and B. M. Hoffman, Chem. Sci., 2017, 8, 5941 DOI: 10.1039/C7SC01602F

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