With functions ranging from simple electron transfer to the biosynthesis of complex natural products, the iron-sulfur (FeS) clusters form an essential and ubiquitous class of metallocofactors. Inasmuch as the physiochemical properties of FeS clusters arise from the behavior of the Fe-based 3d electrons, much effort has been directed at the characterization of their ground-state electronic structures at cryogenic temperatures. At higher temperatures (e.g., physiological conditions), a dense manifold of excited electronic states becomes thermally accessible, but less is known about the factors that influence the valence electronic structure of FeS clusters under these conditions. For example, what factors govern the localization of electrons/holes to particular Fe sites of the cluster, and on what timescales/in what temperature ranges is localization observed? In this talk, we present a combined spectroscopic approach operating in the range from 80 K to 300 K, assisted by broken-symmetry density functional theory calculations, to address how the primary coordination sphere dictates the pattern of localized valences in site-differentiated [Fe₄S₄] clusters. A particular emphasis will be on recently-synthesized alkylated [Fe₄S₄] clusters, which serve as models of an emergent class of organometallic intermediates implicated in a variety of enzymatic reactions. We demonstrate how the presence of a highly covalent, strongly σ-donating alkyl ligand results in partial or complete valence localization at the alkylated subsite as a function of core cluster oxidation state. This effect can persist up to room temperature, as evidenced via variable-temperature paramagnetic ¹H NMR studies. As an extension of these studies, we analyze how tuning the covalency of a terminal Fe–P interaction in an [Fe₄S₄]¹⁺ cluster affects the distribution cluster excited states, using the cluster-bound ³¹P nucleus as a reporter of the local valence state. Significantly, our results underscore the utility of paramagnetic NMR spectroscopy as a probe of electronic structure in antiferromagnetic clusters.

Dr. Niklas B. Thompson

Postdoctoral Fellow, Massachusetts Institute of Technology

Appearances