Electronic Effects in Iridium Pincer Complexes : Structure, speciation and mechanistic pathways in dehydrogenation reactions

Abstract

Iridium pincer complexes are among the most versatile catalysts for bond‑activation chemistry, yet their performance is often governed as much by off-cycle speciation and inhibition as by intrinsic reactivity. This thesis investigates how pincer ligand design influences the structure, dynamics, and catalytic behaviour of iridium complexes and their relevance to acceptorless dehydrogenation.
Initial studies of 1‑phenylethanol dehydrogenation showed that a backbone-modified electron‑poor iridium pincer complex exhibits substantially higher initial turnover frequencies than its parent complex, and a consistently lower experimentally determined barrier of β‑hydride elimination. Complementary density functional theory calculations supported these experimental observations, indicating that CF3 substitution on the aromatic backbone of the pincer-ligand lowers the free‑energy barrier for a rate determining β‑hydride elimination step. Further analysis revealed pronounced inhibition by dissolved hydrogen gas and a transition from chemically to mass‑transfer‑controlled regimes at elevated temperatures.
The reactivity of these catalysts with hydrogen and other inhibiting small molecules was further probed through detailed NMR spectroscopy studies. Spin–lattice relaxation times, H–D coupling constants, and computational modelling showed that both catalysts in their inhibited, tetrahydride complex form exist in an equilibrium of classical and non‑classical hydride structures. The electron-poor metal was also shown to stabilise a dihydrogen‑like character to a greater extent. Exchange spectroscopy experiments quantified hydrogen association/dissociation kinetics, revealed higher rates of H2 dissociation for the electron poor tetrahydride complex, which could further contribute to increased activity.
Finally, the reductive‑elimination/oxidative‑addition processes with arene and alkene ligands were examined to understand the formation and reactivity of transient 14‑electron intermediates. It was found that a slow dissociation, not the formation of the C–H bond, is rate determining. This leads to a slow reductive elimination for phosphinite ligated complexes compared to phosphine ligated analogues, despite faster initial C–H bond formation.
Collectively, these results demonstrate that backbone electronics exert a measurable influence on both on‑ and off‑cycle species in iridium‑mediated dehydrogenation. By complementing kinetic studies with computational analysis, this work provides an initial mechanistic framework for understanding how ligand backbone editing tune hydride speciation, hydrogen inhibition, and ligand exchange in acceptorless dehydrogenation.

Abstract (Swedish)

Iridium pincer complexes are among the most versatile catalysts for bond‑activation chemistry, yet their performance is often governed as much by off-cycle speciation and inhibition as by intrinsic reactivity. This thesis investigates how pincer ligand design influences the structure, dynamics, and catalytic behaviour of iridium complexes and their relevance to acceptorless dehydrogenation.
Initial studies of 1‑phenylethanol dehydrogenation showed that a backbone-modified electron‑poor iridium pincer complex exhibits substantially higher initial turnover frequencies than its parent complex, and a consistently lower experimentally determined barrier of β‑hydride elimination. Complementary density functional theory calculations supported these experimental observations, indicating that CF3 substitution on the aromatic backbone of the pincer-ligand lowers the free‑energy barrier for a rate determining β‑hydride elimination step. Further analysis revealed pronounced inhibition by dissolved hydrogen gas and a transition from chemically to mass‑transfer‑controlled regimes at elevated temperatures.
The reactivity of these catalysts with hydrogen and other inhibiting small molecules was further probed through detailed NMR spectroscopy studies. Spin–lattice relaxation times, H–D coupling constants, and computational modelling showed that both catalysts in their inhibited, tetrahydride complex form exist in an equilibrium of classical and non‑classical hydride structures. The electron-poor metal was also shown to stabilise a dihydrogen‑like character to a greater extent. Exchange spectroscopy experiments quantified hydrogen association/dissociation kinetics, revealed higher rates of H2 dissociation for the electron poor tetrahydride complex, which could further contribute to increased activity.
Finally, the reductive‑elimination/oxidative‑addition processes with arene and alkene ligands were examined to understand the formation and reactivity of transient 14‑electron intermediates. It was found that a slow dissociation, not the formation of the C–H bond, is rate determining. This leads to a slow reductive elimination for phosphinite ligated complexes compared to phosphine ligated analogues, despite faster initial C–H bond formation.
Collectively, these results demonstrate that backbone electronics exert a measurable influence on both on‑ and off‑cycle species in iridium‑mediated dehydrogenation. By complementing kinetic studies with computational analysis, this work provides an initial mechanistic framework for understanding how ligand backbone editing tune hydride speciation, hydrogen inhibition, and ligand exchange in acceptorless dehydrogenation.

• author: Spangenberg, Alice
• supervisor: Ola Wendt
• opponent: Professor Chaplin, Adrian, University of Warwick
• organization: Centre for Analysis and Synthesis
• publishing date: 2026
• type: Thesis
• publication status: published
• subject: Inorganic Chemistry; Organic Chemistry
• keywords: Iridium, pincer complex, acceptorless dehydrogenation, alcohol dehydrogenation, homogenous catalysis, Iridium, pincer complex, acceptorless dehydrogenation, homogenous catalysis, alcohol dehydrogenation
• pages: 82 pages
• defense location: Kemicentrum, Sal B
• defense date: 2026-05-28 09:00:00
• ISBN: 978-91-8104-870-4
• ISBN: 978-91-8104-871-1
• language: English
• LU publication?: yes
• id: 32cf4340-b014-41cd-95cf-13c970e94b55
• date added to LUP: 2026-05-04 12:21:14
• date last changed: 2026-08-11 09:38:12