Templating Metal Clusters via Resorcinarene- and Cyclen- Based Ligand Platforms

Polynuclear metal sites, found in both metalloenzymes and heterogeneous catalysts, play a critical role in small molecule activation by enabling multi-electron redox transformations through cooperative reactivity. One of the most compelling examples is biological nitrogen fixation, mediated by the nitrogenase enzyme, which offers a blueprint for sustainable alternatives to the energy-intensive Haber-Bosch process. Additionally, the Nitrous Oxide reductase presents a Cuz/Cuz* site that facilitates the reduction of Nitrous Oxide to environmentally benign forms. At the heart of metalloenzyme function are complex polynuclear clusters capable of storing and delivering multiple electrons while supporting diverse substrate binding modes across its metal centers. To bridge the knowledge gap between structure and reactivity as well as what intermediates ensue after substrate activation, this thesis employs polynucleating ligand scaffolds to template the synthesis of a series of copper and iron-based clusters and investigates their (multi)electron reactivity. In Chapter I introduce the role multimetallic centers in metalloenymes and heterogenous catalysis play in substrate activation and conversion. I also cover the current method of developing templating ligands to drive the synthesis of clusters and their reactivity. In Chapter 2, I discuss the synthesis of a tetraamine scaffold RL(NH2)4, where L is a rigidified resorcin[4]arene ligand and R = Methyl, Phenyl and Isobutyl groups. We found that treatment of this ligand scaffold with a Cu4Mes4(py)2 leads to the formation of C4 ii symmetric RL(NH)4Cu4 clusters with Cu−Cu distances indicative of metal-metal interactions. The one-electron oxidized [Cu4]+ species was probed by variable temperature X-band continuous wave-electron paramagnetic resonance (CW-EPR), which displays a multiline spectrum at room temperature as well as DFT calculations. In Chapter 3, I demonstrate that, by ligand modification, we can tune the cavity size of these clusters. While the local coordination environment and electronic structure at the [Cu4] core remains largely the same, the imparted cavity size engenders reversible host guest interactions with acetonitrile (MeCN) and molecular oxygen (O2), with O2 binding corroborated by variable-temperature NMR, resonance Raman spectroscopy, and DFT calculations. In Chapter 4, I introduce a new 1,4,7,10-tetraazacyclododecane (cyclen)-derived octaamine ligand LH4. To demonstrate the versatility of this ligand platform, we isolate a family of divalent first-row dinuclear transition metal complexes—LCr2, LMn2, LFe2, LCo2, and LZn2. Crystallographic and DFT analysis reveals varying degree of metal-metal interactions from weak to strong quadrupole bonding. Lastly, in Chapter 5, LH4 is used as a starting point in the synthesis of hexanuclear iron cluster, LFe6Mes4, Reaction with organic and inorganic azides yields a terminal μ4-nitride cluster, LFe6Mes4(μ4-N), via a net three-electron oxidation to a mixed-valent [FeII3FeIII3] core. Comparative analysis reveals a significant structural reorganization in the [Fe4] unit upon nitride incorporation. Cyclic voltammetry demonstrates that LFe6Mes4 is capable of traversing at least eight distinct oxidation levels across a 2.5 V window—approaching the electron stoichiometry of enzymatic nitrogen fixation. These findings highlight the potential of this ligand scaffold to stabilize redox-active, high-nuclearity iron clusters relevant to small molecule activation and multi-electron redox transformations.

Name thesis:degree_name

Doctor of Philosophy

Level thesis:degree_level

Doctoral

Discipline thesis:degree_discipline

Natural Sciences

Grantor

Rice University

Year dc:date.issued

2025

Author dc:creator

• Osei, Manasseh Kusi

Advisor dc:contributor.advisor

• Hernandez Sanchez, Raul

• Copper and Iron Clusters

Statement dc:rights

• Copyright is held by the author, unless otherwise indicated. Permission to reuse, publish, or reproduce the work beyond the bounds of fair use or other exemptions to copyright law must be obtained from the copyright holder.

Language dc:language.iso

eng

Handle dc:identifier.uri

https://hdl.handle.net/1911/118628

OAI identifier oai:identifier

oai:repository.rice.edu:1911/118628