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Updated: Sep 13, 2025
Line Shape Analysis of Dynamic NMR Spectra for Characterizing Coordination Sphere Rearrangements at a Chiral Rhenium Polyhydride Complex
Published on: July 27, 2022
Distal Scaffold Flexibility Modulates Eyring Activation Parameters in Re(I) Substitution Reactions: A Case for
Alec T Larson1, Serhii Vasylevskyi1, Michael J Rose1
1Department of Chemistry, University of Texas at Austin, Austin, 105 E 24th St, Austin, Texas 78712, United States.
Abstract:
The use of dynamic ligand scaffolds to construct metal complexes with enhanced reactivity has recently emerged as an important research direction. Previous studies on ligand dynamics lay within the context of catalysis; thus, interpretation of the role dynamics in reactivity remained ambiguous among sequential chemical processes. Herein, we circumvent the complexity of catalytic systems by studying "simple" ligand substitution reactions. We previously reported the rigid Anth(py)2 and dynamic Thianth(py)2 ligands; in this work, we report two new dynamic ligands derived from the phenoxathiin scaffold, OPhxt(py)2 and SPhxt(py)2. Rhenium tricarbonyl complexes of type [(L(py2)Re(CO)3(X))] (where L(py2) = Anth(py)2, Thianth(py)2, OPhxt(py)2, SPhxt(py)2; X = Br, OTf) were synthesized and structurally investigated using IR spectroscopy and X-ray crystallography. Solution dynamics were investigated using variable temperature NMR. The impact of dynamics on reactivity was determined experimentally using Eyring analysis for bromide substitution onto the triflate complexes (OTf-→Br-): a ∼ 10× enhanced rate constant was observed for the most dynamic complex compared with the least dynamic. Eyring activation parameters for ligand substitution displayed an enthalpy-entropy compensation effect, which we posit can be understood in terms of the conformational flexibility (or rigidity) of the complexes. The energetic landscape of the complexes were investigated computationally with potential energy surface scans and conformational searches (using the Conformer-Rotamer Ensemble Sampling Tool, CREST). Our findings suggest that ligand dynamics in transition metal systems enhance their reactivity, and ─ more generally ─ tuning ligand flexibility can be leveraged as a new design principle for the preparation of metal complexes with enhanced reactivity.
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