Experimental Definition of the S = 1 π vs S = 2 σ Reactivity and S = 2 Character in the Ground State of an S = 1 FeIVO Complex

Augustin Braun; Leland B Gee; Max D J Waters; Michael L Baker; Michael W Mara; Ang Zhou; Thomas Kroll; Dennis Nordlund; Dimosthenis Sokaras; Britt Hedman; Keith O Hodgson; Lawrence Que Jr; Edward I Solomon

doi:10.1021/jacs.4c11034

Experimental Definition of the S = 1 π vs S = 2 σ Reactivity and S = 2 Character in the Ground State of an S = 1 Fe^IVO Complex

J Am Chem Soc. 2024 Dec 25;146(51):35139-35145. doi: 10.1021/jacs.4c11034. Epub 2024 Dec 13.

Authors

Augustin Braun^{1

2}, Leland B Gee^{1

2}, Max D J Waters^{1

2}, Michael L Baker^{1

2}, Michael W Mara^{1

2}, Ang Zhou³, Thomas Kroll², Dennis Nordlund², Dimosthenis Sokaras², Britt Hedman², Keith O Hodgson^{1

2}, Lawrence Que Jr³, Edward I Solomon^{1

2}

Affiliations

¹ Department of Chemistry, Stanford University, Stanford, California 94305, United States.
² Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Stanford University, Menlo Park, California 94025, United States.
³ Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States.

PMID: 39668699
DOI: 10.1021/jacs.4c11034

Abstract

Iron(IV)-oxo intermediates found in iron enzymes and artificial catalysts are competent for H atom abstraction in catalytic cycles. For S = 2 intermediates, both axial and equatorial approaches are well-established. The mechanism for S = 1 sites is not as well understood: an equatorial approach is more energetically favorable, and an axial approach requires crossing from the S = 1 to the S = 2 surface. In this study, we use 1s2p resonant inelastic X-ray scattering (RIXS) and Fe L-edge X-ray absorption spectroscopy on the S = 1 [Fe^IVO(TMC)(CH₃CN)]²⁺ and observe both S = 2 and S = 1 final states, which enables the experimental evaluation of the energetics of the axial and equatorial reactivity of an S = 1 Fe^IVO center on its S = 2 vs S = 1 surface. The observation of S = 2 final states in the RIXS spectrum demonstrates significant S = 2 character spin-orbit mixed into the S = 1 ground state.