CCC pincer iron complexes: synthesis, reactivity and parahydrogen induced polarization

Najera, Daniel Celis

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https://hdl.handle.net/2142/121304 Copy

Description

Title

CCC pincer iron complexes: synthesis, reactivity and parahydrogen induced polarization

Author(s)

Najera, Daniel Celis

Issue Date

2023-06-14

Director of Research (if dissertation) or Advisor (if thesis)

Fout, Alison R

Doctoral Committee Chair(s)

Fout, Alison R

Committee Member(s)

• Girolami, Gregory
• Mirica, Liviu
• Guironnet, Damien

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Organometallic, Iron, Chemistry, Catalysis, Pincer, CCC, Hydrogenation, Agostic, Silyl, Hydride, Olefin

Abstract

Elements of the platinum group metals like rhodium, palladium, iridium, and platinum have historically held a privileged position in transition metal catalysis due to their predictable and well-defined two-electron reactivity. However, the long-term sustainability of the reliance on precious metals as catalysts of choice has come into question due to their low natural abundance. A shift towards the use of earth-abundant first-row transition metals offers an enticing opportunity, but their propensity to favor one-electron reactivity remains a challenge in this endeavor. Maximizing the strength of the ligand field in first-row transition metal complexes has been adopted as a strategy to promote low-spin configurations and impede one-electron reactivity. In our group, the strongly-donating phenylene-bridged, monoanionic bis(NHC) pincer ligands MesCCC and DIPPCCC (MesCCC = bis(mesityl-benzimidazol-2-ylidene)phenyl and DippCCC = bis(2,6-diisopropylphenyl-benzimidazol-2-ylidene)phenyl) have allowed the investigation of two-electron reactivity with cobalt and nickel. Despite great advances in the hydrofunctionalization of unsaturated molecules with cobalt, and the isolation of nickel complexes in high oxidation states, the chemistry of the related iron system remains underexplored. The present investigation focuses on the application of the electron-rich CCC ligand framework to iron. Chapter 2 describes the synthesis of iron MesCCC complexes via a one-pot metalation with Fe2Mes4. The generated family of diamagnetic complexes, (MesCCC)FeMes(L) (L = Pyridine (Py), 3,5-lutidine, PPh3, PMe3, MeCN, N2, CO) were isolated in excellent to moderate yield. Spectroscopic and structural characterization revealed an agostic interaction between the mesityl methyl hydrogens and the iron center. The strength of this interaction was correlated to the identity of the pendant L-type ligand and reflected as a change in the chemical shift of the corresponding methyl resonance in the 1H NMR spectrum. Density functional theory (DFT) calculations were conducted to rationalize the observed trends. Motivated by our group’s interest in the catalytic hydrogenation of unsaturated substrates, Chapter 3 details the investigation of the activation of H2 by (MesCCC)FeMes(L) leading to the catalytic hydrogenation of olefins. A series of iron(II) hydride complexes of the form (MesCCC)Fe(H)(L)(N2) (L = Py, PMe3 and PPh3), were successfully isolated and characterized. The pyridine variant was found to be a competent catalyst in the hydrogenation of terminal olefins, in some cases rivaling the activity observed for the well-defined cobalt catalyst (MesCCC)Co(N2)(PPh3). The ability of the CCC ligand to impart low-spin configurations with first-row transition metals led to the observation, for the first time, of Parahydrogen Induced Polarization (PHIP) mediated by an iron catalyst. In the most illustrative case, the parahydrogenation of vinyltrimethylsilane led to a 200-fold signal enhancement of the resonances of ethyltrimethylsilane in the 1H NMR spectrum. The specific conditions necessary to observe the PHIP effect informed key aspects of the hydrogenation mechanism. Chapter 4 covers an extension of the reactivity of (MesCCC)FeMes(L) towards the activation of Si–H bonds. A family of iron(II) silyl complexes was synthesized and characterized. Further reactivity revealed these complexes served as precatalysts to dispense a catalytically-active hydride in the presence of H2. This property provides access to efficient hydrogenation catalysis with complexes that are easier to synthesize. Lastly, Chapter 5 revisits the MesCCC cobalt chemistry with the goal of developing a mild reduction protocol to readily access the catalytically-active species from air-stable cobalt precursors. Installation of carboxylate ligands on a cobalt(III) system allowed for facile reduction to cobalt(I) in the presence of silane reagents. This activation proceeds at different temperatures depending on the identity of the silane, allowing for controlled activation of these precursors. The in situ generated catalysts exhibits only marginally reduced efficiency in the hydrogenation of styrene.

Graduation Semester

2023-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Daniel Najera

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