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Pincer-ligated catalysts that can undergo metal–ligand cooperativity (MLC), whereby H 2 is heterolytically cleaved (with proton transfer to the ligand and hydride transfer to the metal), have emerged as potent catalysts for the hydrogenation of CO 2 and organic carbonyls. Despite the plethora of systems developed that differ in metal/ligand identity, no studies establish how variation of the metal impacts the pertinent thermochemical properties of the catalyst, namely the equilibrium with H 2 , the hydricity of the resulting hydride, and the acidity of the ligand. These parameters can impact the kinetics, scope, and mechanism of catalysis and hence should be established. Herein, we describe how changing the metal (Co, Fe, Mn, Ru) and charge (neutral vs. anionic) impacts these parameters in a series of PNP-ligated catalysts (PNP = 2,6-bis[(di- tert -butylphosphino)methyl]pyridine). A linear correlation between hydricity and ligand p K a (when bound to the metal) is found, indicating that the two parameters are not independent of one another. This trend holds across four metals, two charges, and two different types of ligand (amine/amide and aromatization/de-aromatization). Moreover, the effect of ligand deprotonation on the hydricity of (PNP)(CO)(H)Fe–H and (PNP)(CO)(H)Ru–H is assessed. It is determined that deprotonation to give anionic hydride species enhances the hydricity by ∼16.5 kcal mol −1 across three metals. Taken together, this work suggests that the metal identity has little effect on the thermodynamic parameters for PNP-ligated systems that undergo MLC via (de)aromatization, whilst the effect of charge is significant; moreover, ion-pairing allows for further tuning of the hydricity values. The ramifications of these findings for catalysis are discussed.
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Thermodynamic and kinetic hydricity of transition metal hydrides
The prevalence of transition metal-mediated hydride transfer reactions in chemical synthesis, catalysis, and biology has inspired the development of methods for characterizing the reactivity of transition metal hydride complexes. Thermodynamic hydricity represents the free energy required for heterolytic cleavage of the metal–hydride bond to release a free hydride ion, H − , as determined through equilibrium measurements and thermochemical cycles. Kinetic hydricity represents the rate of hydride transfer from one species to another, as measured through kinetic analysis. This tutorial review describes the common methods for experimental and computational determination of thermodynamic and kinetic hydricity, including advice on best practices and precautions to help avoid pitfalls. The influence of solvation on hydricity is emphasized, including opportunities and challenges arising from comparisons across several different solvents. Connections between thermodynamic and kinetic hydricity are discussed, and opportunities for utilizing these connections to rationally improve catalytic processes involving hydride transfer are highlighted.
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- Award ID(s):
- 1953708
- PAR ID:
- 10253898
- Date Published:
- Journal Name:
- Chemical Society Reviews
- Volume:
- 49
- Issue:
- 22
- ISSN:
- 0306-0012
- Page Range / eLocation ID:
- 7929 to 7948
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D0CS00405G
