Pierre Kennepohl


Relevant Degree Programs


Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - Nov 2019)
Spectroscopic studies of halogen bonding in model systems : from one end of the electromagnetic spectrum to the other (2019)

At its simplest, chemical bonding involves a combination of two dominant contributions: direct electrostatics (ionic) and electron sharing (covalent). The relative importance of these contributors has been the subject of signif- icant study in primary (intramolecular) chemical interactions. For example, the relevance and importance of covalent contributions has been a primary focus of transition metal chemistry for decades. For weaker secondary chem- ical interactions such as hydrogen bonding (HB) and halogen bonding (XB), the prevailing view in the literature is that electrostatic interactions are so dominant that covalent contributions are negligible. A notable exception is that of so-called symmetric hydrogen bonds, which exhibit large covalent contributions. With X-ray Absorption Spectroscopy (XAS), we have provided the first direct experimental evidence of covalency in XB. From such studies, we ob- serve that XB exhibit a significantly higher degree of covalency compared with HB counterparts of similar bond strength. Notably, the degree of co- valency in certain XBs is equivalent to that observed in transition metal halides. Our studies provide information of the electronic changes that oc- cur in both the charge donor and charge acceptor in model systems, affording us a unique experimental view of these weak interactions. We also demon- strate the importance of covalent contributions in XBs by showing the effect of covalency in the electron transfer properties in XB-modified dye sensi- tised solar cells. These results lead us to conclude that XBs should more generally be classified as coordinate bonds (and thus identified using an ar- row) to distinguish them from significantly less covalent HBs and other weak interactions.

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Investigation of reacted copper(II) species in micronized copper treated wood (2015)

Wood preservatives using micronized particulate copper as the active ingredient recently introduced in the USA has generated controversies due to their limited intrinsic solubility compared to the conventional soluble copper treatments. Because the availability of soluble copper ions is essential for these preservatives to provide an effective treatment, concerns are centered on whether they are able to produce soluble copper, and the copper fixation mechanism of the treatment is little understood. In this thesis, micronized copper treated wood were studied using a combination of Electron Paramagnetic Resonance (EPR) spectroscopy and X-ray Fluorescence (XRF) spectroscopy. The identification and characterization of soluble and chemically fixed copper species were discussed. A calibration standard was developed to quantify the solubilized and fixed copper species in the micronized copper treated wood, which also contains unreacted particulate copper. On the basis of the experimental results, the fixation mechanism is thought to be triggered by the reaction between the carboxylic acid protons in hemicellulose and pectin of wood with the particulate copper, and the quantities of the solubilized and fixed copper species are determined by the availability of the acidic protons. Results from the studies on micronized copper treated earlywood and latewood, as well as the effect of monoethanolamine additive provided further support on the theory of the fixation mechanism. Soil exposure experiment suggested that the Cu fixation may be affected by the moisture level, organic content and metal content in the soil. Study on micronized copper treated heartwood showed that the particulate Cu may react with the resin acids in addition to the major wood components. The effects of fungal colonization and bio-incision on the pre-treatment material were also briefly discussed.

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Electronic structure of sulfur-nitrogen containing compounds : correlations with theory and chemical reactivity (2014)

Molecules containing sulfur-nitrogen bonds, such as sulfonamides, have long been of interest due to their many uses and chemical properties, including the potential release of nitric oxide and nitroxyl. Understanding the factors that cause sulfonamide reactivity is crucial, yet their inherent electronic complex- ity have made them difficult to examine. In this thesis, sulfur K-edge x-ray absorption spectroscopy (XAS) is used in conjunction with density functional theory (DFT) to determine the role of electronic transmission effects through the sulfur-nitrogen bond. A systematic deconstruction of the elements within the sulfonamide moiety is used as an approach to understand critical factors that dictate electronic structure.First, the effect of oxidation state changes and variations in R-group in sulfenamides, sulfinamides and sulfonamides on intramolecular bonding are explored. Next, N-hydroxylation of the sulfonamide amide, in both alkyl sulfonamides and a series of para-substituted aryl sulfonamides with varying Hammett para-sigma constants are studied using structure-function relationships, in conjunction with DFT, to understand the role of electron donation and withdrawal to the sulfonamide moiety. The outcome of these modifications on the sulfonamide framework lead to better insight towards directed drug design and its influence on nitroxyl and nitric oxide release.

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Investigating the role of metal-ligand bonding on biological activity of metallotherapeutics (2013)

Defining and understanding the structure activity relationship and mechanisms of action of pharmaceutical agents in a biological environment is vital for their faster and successful clinical development. The mode of activation is often complicated for metalloanticancer drugs due to their wide spectrum of activity and interactions with biomolecules, during and after transport to cancer cells. The research herein describes the investigation of thiolate ligand oxygenation and its potential role in the mechanism of action for a family of “half-sandwich RuII arene” anticancer complexes. X-ray absorption spectroscopy (XAS), in concert with density functional theory (DFT) calculations, has been used to describe the influence of thiolato oxygenation on the nature of the Ru-S bond with the effect on ancillary ligands modifications in the parent thiolato (M-SR) and oxidized sulfenato (M-SOR) and sulfinato (M-SO₂R) species respectively. This study suggests that the sulfenato species are most susceptible to ligand exchange, but only via activation by protonation of the terminal oxo group. Perturbations of the sulfenato and sulfinato species can be achieved via either protonation or Lewis acid interaction; however the effect is greater in the sulfenato compared to that of sulfinato. DFT calculations are in agreement with the experimental data. Further studies of the electronic structure of a broader series of OsII and RuII arene complexes have been performed using a similar combined spectroscopic and computational approach. Results from these studies provide important new insights into the chemical and biochemical properties of these complexes. Finally, these studies uncover the correlation between the electronic structure and reactivity that is important and must be considered when investigating the transition metal complexes in medicinal chemistry.

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Probing the electronic structure of dioxygen as a ligand : using x-ray absorption spectroscopy to quantify backbonding (2011)

The search for more economical and environmentally friendly oxidation catalysts for organic functional group transformations is currently one of the most prevalent research areas in chemistry. The coordination and activation of dioxygen by transition metal complexes holds particular promise and has thus been thoroughly investigated. Although work has predominantly focused on synthesis of new transition metal dioxygen complexes, determining how the dioxygen ligand interacts with transition metals is also particularly critical. The nature of the M-O2 bond should have important implications on both the chemical and structural properties of the complexes. This thesis focuses on developing and exploring the spectroscopic characteristics of a series of newly reported M-O2 with highly unusual bonding. Such complexes, best described as singlet dioxygen adducts, represent a new class of metal dioxygen complexes with characteristics that are very different from typical metal-superoxo and peroxo complexes. The electronic properties of a variety of rhodium and ruthenium complexes were explored utilizing a combination of synchrotron-based X-ray Absorption Spectroscopy (XAS) techniques in conjunction with Density Functional Theory (DFT) calculations. To quantitatively investigate the complexes, a new fitting methodology was developed, and is described herein. For several of the rhodium dioxygen complexes, the Rh L2,3-edge data provided evidence that no formal oxidation occurred at the metal center upon dioxygen coordination. The data extracted from the experimental spectra provided the first quantitative π-backbonding information for second row transition metal complexes known thus far. Both Rh K-edge XAS and DFT results corroborate the findings from the L-edge spectra. A set of complementary ruthenium complexes, thought to have similar M-O2 binding characteristics, were studied in an analogous manner. Ultimately, this thesis provides the first example of utilizing second row transition metal L2,3-edge XAS data to experimentally determine π-backbonding. Although the research described herein focuses on π-backbonding between dioxygen and transition metals, it provides a basis for applying similar experimental strategies for investigating π-backbonding interactions in other systems of catalytic interest.

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Electronic structure studies of ruthenium-based catalysts for olefin metathesis : an x-ray absoprtion spectroscopy perspective (2010)

Interest in olefin metathesis has increased over the years with the development of ruthenium-based catalysts. Their unique properties have allowed their use in numerous industrial and laboratory processes in relatively mild conditions and in combination with a wide range of solvents. Several studies have provided insights into how these catalysts work, but very little has been done in order to understand why they work that way; an important aspect that has the potential of benefiting chemists while designing new catalysts. The research introduced here has focused on the fundamental understanding of their reactivity by exploring their electronic structure, using a combination of synchrotron-based X-ray-absorption (XAS) techniques in combination with DFT calculations and multiplet simulations. As part of the experimental work, samples from various ruthenium-based catalysts classified as first-generation (whenever the ancillary ligand is a phosphine) or as second-generation analogues (whenever this ligand is an N-heterocyclic carbene, NHC) were used. The Ru K-edge XAS data have revealed that the ruthenium centre in second-generation analogues is more positively charged than the corresponding first-generation counterparts. This offers a rationale for previously observed kinetic results, which have shown a slower initial step for the second-generation Grubbs catalyst. At the same time, they raise questions in a more fundamental level on whether or not NHCs are truly better charge donors than phosphine ligands. DFT results are consistent and the ongoing analyses of the Cl K- and C K-edge XAS data indicate similar overall bonding structures between first- and second- generation analogues. In addition, from preliminary results on these edges, two possible identities of substantially different nature have emerged for the LUMO orbital. In this regard, the final conclusion should provide important insights on through which orbital the metathesis reaction gets started. As a side product, the analyses of the challenging Cl K-edge XAS data have inspired the development of a new methodology and a Matlab-based computer program for fitting. Ultimately, the methods and techniques detailed here can serve as the foundation for the comprehensive study of other related systems relevant to olefin metathesis, or in general, to the field of homogeneous catalysis.

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Investigation of cysteine and methionine oxidation using x-ray absorption spectroscopy (2010)

Cysteine (Cys) and methionine (Met) are sulfur containing amino acids with various oxidation forms. Oxidation of Cys yields cysteinyl radicals that have been postulated as intermediates in several biological contexts including enzymatic catalysis, long-range electron transfer, peptide post-translational modification and cellular redox signaling. The challenges of detecting sulfur-based radicals with electron paramagnetic resonance (EPR) have led to the development of Sulfur K-edge X-ray absorption spectroscopy (S K-edge XAS) as a spectroscopic tool. The reactivity of sulfur-based radicals was studied in a Pseudomonas aeruginosa azurin protein system to probe the electronic structure of isolated cysteinyl radicals, which are characterized by S 3p ← 1s pre-edge transition. S K-edge XAS has shown to be a sensitive method in detecting these cysteinyl radicals in hydrophobic and hydrophilic protein environments. The pre-edge feature of the cysteinyl radicals in hydrophobic environments was lower in energy than their hydrophilic counterparts due to hydrogen bonding interactions. Additionally, S K-edge XAS was employed to study the redox photochemistry of Met and its oxidized forms methionine sulfoxide (MetSO) and methionine sulfone (MetSO₂). Met is easily photooxidized to MetSO and MetSO₂ in the presence of O₂. In the absence of O₂, photoirradiation leads to the one-electron-oxidized Met cation radical (MetS•⁺), suggesting an alternative mechanism for photooxidation of thioethers through direct oxidation. The photoirradiation of MetSO leads back to Met under both aerobic and anaerobic conditions while MetSO₂ is photochemically inert. These findings provide new insights into the formation of age-related cataracts. Finally, the metal-induced Met oxidation in amyloid-β (Aβ) peptide was investigated. Much of the research to date has focused on the redox chemistry of Cu²⁺ in Aβ peptide with inconsistent findings with regards to the role of Met₃₅ and the oxidation state of the Met₃₅. Findings reported here indicate that in the presence of Cu²⁺ alone, Met₃₅ was oxidized to MetSO, but surprisingly Fe³⁺ failed to oxidize the Met. These differences in the oxidation behaviour lead to the investigation of the metal binding site in Aβ. Fe³⁺ found to be in a six-coordinate environment with oxygen-rich ligands while Cu²⁺ is in a five-coordinate environment with histidine-rich ligands.

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X-ray absorption spectroscopy as a tool for characterizing sulfur based reactive intermediates (2009)

Sulfur K-edge X-ray absorption spectroscopy (XAS) has proven to be a great tool for the investigation of sulfur oxidation states and sulfur-metal ligand bonding. In thisthesis, XAS has been applied in the detection and characterization of sulfur-based reactive intermediates and products of photo-reacted sulfur species, with applications in both bioinorganic and inorganic chemistry. Low molecular weight thiols and their derivatives have important proteinmodulation, signal transduction and antioxidant activities. This includes glutathione (GSH), nitrosoglutathione (GSNO), and lipoic acid (LA), which are involved in complex redox pathways resulting in a variety of intermediates and products that can be difficult to characterize. These compounds have been used as models for thiol nitrosation and oxidation reactions, and their reactivity was probed with sulfur K-edge XAS, which has been developed into a valuable tool for the investigation of sulfur-containing radicalspecies and related non-radical intermediates.XAS was also applied to investigate the reactivity of p-toluene sulfonyl chloride, an initiator in metal catalyzed living radical polymerization, to explore the effect of hyperconjugation on the reactivity of the S-Cl bond. A series of model compounds of the form RSO₂G(G = -Cl, -OH, -alkyl) were used to evaluate the effect of aryl versusalkyl R groups on the photo-reactivity and orbital mixing of the S-G bond.

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