Non-heme high valent iron(IV) tosylimido complexes displaying oxo-like reactivity.

Abstract

High valent FeIV species serve as important intermediates in many metalloenzymes involved in biological oxidation reactions. For example, in reactions with iron oxo-bleomycin or cytochromes P450, FeIV intermediates are generated as key factors. In addition, they have been implicated as important intermediates during nitrogen fixation and the Haber Bosch process. While FeIV-oxo intermediates are common in biology, the isoelectronic FeIV-imido counterparts are less common and their oxidative and spectroscopic properties are less studied. Therefore, continuous efforts are made to synthesize biomimetic FeII complexes that can function as precursors to FeIV-imido complexes. Here we present three new high-valent complexes with ligands based on the N4Py framework - [(N2Py2IQn)FeIV(NTs)](OTf)2 (1IV=NTs), [(N3PyIQn)FeIV(NTs)](ClO4)2 (2IV=NTs) and [(N2Py2Im)FeIV(NTs)](ClO4)2 (3IV=NTs), (N2Py2IQn = N,N-bis(Isoquinolin-3-ylmethyl)-1,1-di(pyridin-2-yl)methanamine, N3PyIQn = N-(isoquinolin-3-ylmethyl)-1,1-di(pyridin-2-yl)-N-(pyridin-2ylmethyl)methanamine and N2Py2Im = N-bis(1-Methyl-2-imidazolyl)methyl-N-(bis-2-pyridylmethyl)amine. Complexes 1IV=NTs and 2IV=NTs have been characterized through UV-Vis and 1H NMR spectroscopy. The UV-Vis spectrum for both complexes displayed a very similar spectra to the analogous [(N4Py)FeIV(NTs)]2+ imido complex. Product analyses using thioanisole and xanthene as substrates have shown that 1IV=NTs, 2IV=NTs and 3IV=NTs are able to perform both heteroatom transfer and hydrogen atom transfer. 1IV=NTs has also been shown to effect aziridination with low yields. The reactivity kinetics of 1IV=NTs and 2IV=NTs using different aliphatic compounds and para-substituted thioanisoles as substrates has been studied and compared to the analogous oxido complexes 1IV=O and [(N4Py)FeIV(O)]2+ . In both sulfimidation/sulfoxidation and HAT reactions, 1IV=NTs, 1IV=O and [(N4Py)FeIV(O)]2+ displayed similar reactivity. Complex 2IV=NTs, in which the pentadentate ligand contains one isoquinoline substituent as opposed to two such substituents in 1IV=NTs, displayed reactivity rates in-between 1IV=NTs and [(N4Py)FeIV(NTs)]2+. These patterns suggest that introduction of isoquinoline moieties in the ligand framework reduces the electron affinity of the imido complex compared to the N4Py analogue, with two isoquinoline moieties causing the FeIV=NTs complex to have an electron affinity closer to its oxo analogue than what is the case for the analogous FeIV imido and oxido complexes of N4Py. This hypothesis is supported through Hammett analysis of the sulfimidation/sulfoxidation reactivity exhibited by 1IV=NTs and 2IV=NTs. However, further investigation of the electronic influence of the ligands on the reactivities of the FeIV complexes via computational (DFT) modelling, and mechanistic studies into the HAT reactivities of the FeIV complexes are required.

Popular Abstract

Chemistry have always been an important part of human life; the process of oxidizing cellulose for cooking and warmth (i.e. burning wood to create a fire) is arguably the start of our use and reliance on chemistry for survival and everyday life. As human society evolved, increasingly chemical processes became cornerstones for our lives to function, from using and extracting medicines from herbs to combat diseases in the Paleolithic age to the birth of organic chemistry in the late regency era, to the industrial revolution and production of synthetic rubber and plastics. Currently, most important chemical products, such as plastics, pharmaceuticals, and fertilizers are produced from petrol or natural gas. Most petrol and natural gas come in the form of hydrocarbons, which are unreactive (this is why petrol does not expire, for example). However, to quote Walter White: “Chemistry is the study of change”, and these unreactive hydrocarbons are not very interesting, nor very useful as anything but fuel or solvent. To turn these hydrocarbons into more valuable products, the C-H bonds must be functionalized. Currently, this is mainly done industrially, and requires both high temperatures and pressures, such as in the production of methylamine from methane, which first requires the conversion of methane to methanol using an energy-intensive processes and then reacting methanol with ammonia, which in turn is produced through the very energy-intensive Haber-Bosch process. As we face the climate crisis, we must reduce the amount of chemical waste and greenhouse gases produced in these processes.
One way to approach this problem is using catalysis. Catalysts based on metal(s) is common in chemistry and iron-based catalysts in particular is of great interest due to the abundance of this metal in nature, its relatively easy extraction process, and low cost. Designing good catalysts is a tricky process, however, and researchers have looked towards nature for design inspiration for efficient iron catalysts. The abundance of iron also makes it a very common cofactor in enzymes. Enzymes catalyse countless chemical reactions in the body, including C-H and C=C bond functionalization. One of the most well-studied enzymes is cytochrome P-450 (CYP) as it is one of the most versatile enzymes. It contains a single Fe(III) ion with a heme ligand at its active center and is responsible for most drug metabolism, which breaks down drugs and toxins through oxidation, making them more water soluble so that we can excrete them through urination. CYP enzymes are also able to synthesise various useful compounds such as estrogen or ergine (also known as LSA, a psychedelic substance). This versatility and ability to activate C-H bonds at mild, biological conditions is in sharp contrast to the high pressure and temperature environments of industrial chemical processes.

In this work I present three iron-based catalysts using a nitrogen based five-coordinate ligand (where there are 5 nitrogens in the framework available to bond to the central iron) rather than a heme ring. While most of the CYP family of enzymes in nature uses dioxygen to functionalize various compounds with oxygen-containing functional groups, I have used the iso-electronic nitrogen-based compound p-tosylsulfonamine to functionalize various hydrocarbons. The reactivity of the catalysts have been compared to other similar biomimetic pentadentate iron complexes and has shown that changes in the ligand structure and electronic properties can have a drastic effect on reactivity.

• author: Nilsson, Justin
• supervisor: Yong Li
• organization: Department of Chemistry
• course: KEMR30 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: Iron, Catalysis, Aziridination, oxidation, HAT, Atom Transfer, Nitrogen, inorganic chemistry
• language: English
• id: 9183018
• date added to LUP: 2025-01-29 08:46:22
• date last changed: 2025-01-29 08:46:22