Coordination chemistry of unusual oxidation states of nickel and palladium relevant to homogeneous catalysis

Coordination complexes of Group 10 transition metals – nickel (Ni) and palladium (Pd) hold privileged positions in the realm of metal-mediated catalysis. Transition metal complexes containing these metals are some of the most active catalysts that have been developed for important areas including, but not limited to, cross-coupling catalysis, C-H activation, olefin polymerization, and cross-electrophile coupling. In addition, several metalloenzymes found in Nature that catalyze important small-molecule activation processes feature Ni at their active site. The commonly accepted mechanism of Pd-catalyzed cross-coupling, the most well-developed technology among these processes, involves diamagnetic intermediates. However, over the past two decades, researchers have become increasingly interested in how nickel-catalyzed reactions differ, particularly because Ni undergoes one-electron (radical) processes more readily. Ni and Pd coordination complexes that contain a single unpaired electron (NiI, PdI, NiIII, and PdIII) are thus subjects of tremendous interest due to their fundamental electronic structure as well as their role as intermediates in the various classes of reactions outlined above. Understanding and controlling the reactivity of these ‘unusual’ oxidation states of Ni and Pd promises to expand their roles in various bond-breaking and bond-making processes catalyzed by such complexes. This is the central question that this thesis aims to answer, with the focus being on the chemistry of low-valent Ni and Pd compounds that have one unpaired electron. The first part of the thesis presents investigations into the electrochemistry of Ni and Pd compounds supported by thiapyridinophane ligands that have been developed in the Mirica group. These ligands feature mixed hard-soft donors pairs. This feature was hypothesized to be critical for stabilizing both low and high-valent states that are relevant to reductive small molecule activation reactions – particularly the proton and carbon dioxide reduction reactions. Key contributions include the development of a small-molecule mimic of the redox-active Ni center of [NiFe] hydrogenase. The small-molecule model was shown to switch between NiI and NiIII via a C-H activation process, mimicking the enzyme’s key redox states. It was also an active electrocatalyst for proton reduction. Further modifications of the ligand design led to interesting Ni and Pd coordination chemistry, which helped in the identification a decomposition pathway in a moderately active Pd carbon dioxide reduction catalyst, observation of unusual properties and reactivity in Ni and Pd coordination complexes. The second section of the thesis details the coordination chemistry of Ni-isocyanide complexes. Isocyanides are most commonly form coordinatively saturated, inert transition metal coordination compounds. It was found that simple, commercially available isocyanide ligands were capable of stabilizing dinuclear NiI complexes – a much sought after synthetic target in the catalysis community. Unlike existing NiI compounds, these dinuclear complexes display thermal stability and rapid ligand substitution reactivity. They can also function as catalysts or pre-catalysts for a range of cross-coupling reactions. These results transform the position of isocyanides as ancillary ligands in coordination chemistry to potentially privileged ligands in cross-coupling catalysis. Furthermore, the development of NiII precatalysts bound to pivalonitrile ligands is also disclosed. Overall, this section introduces the first general source of NiI in cross-coupling catalysis and expands the repository of NiII precursors available to the synthetic chemist. Finally, defining the role of NiI in catalysis begets the question – can such radical chemistry be performed by Pd catalysts? One area where PdI intermediates have been routinely proposed is photoexcited Pd catalysis. This chemistry allows the activation of alkyl halides, substrates that are typically inaccessible to ground-state Pd chemistry due to competing β-hydride elimination. Yet, direct structural or spectroscopic proof of such intermediates are absent. The synthesis and characterization of Xantphos-ligated PdI complexes are described, which were shown to be critical intermediates in photodriven elementary steps involving Pd0 and PdII-methyl complexes. This intermediate demonstrates the role of paramagnetic states in Pd catalysis.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemistry

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Chakrabarti, Sagnik

Contributors dc:contributor

• Mirica, Liviu M
• Rauchfuss, Thomas B
• Gewirth, Andrew A
• Snyder, Benjamin E. R.

• Cross-coupling
• Paramagnetic
• Nickel
• Palladium
• EPR

Statement dc:rights

• Copyright 2025 Sagnik Chakrabarti

Language dc:language

en

Handle dc:identifier

https://hdl.handle.net/2142/132616

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/132616