Novel Cyclohexyl-Based POCOP-Pincer Ligands for C(sp3)-C(sp3) Bond Activation

Abstract

Alkanes offer great and barely utilised potential for usage as raw materials in large-scale chemical processes. Their lack of reactivity, however, warrants methods of manipulating and transforming them in an efficient and mild way, which can potentially be realised by employing homogeneous catalysis using pincer complexes. A reliable procedure for the synthesis and diastereomer-separation of 2-methyl-1,3-cyclohexanediol was established, paving the way for the preparation of three novel cyclohexyl-based POCOP-pincer ligands, which have been characterised via 1H-, 31P{1H}- and 13C{1H}-NMR spectroscopy. These systems had been proposed as candidates for the ultimate goal of scission of unstrained, unactivated C(sp3)-C(sp3) bonds in an intermolecular reaction. Therefore, their cyclometallation behaviour was thoroughly studied under varying conditions employing i.a. different metallating agents. The results of these investigations revealed a trend of facile metal-induced ligand decomposition occurring with two of the ligands accompanied by a distinct lack of cyclometallation in all tested systems, clearly indicating an underlying issue, wherein bidentate coordination appears to not be feasible. While the desired bond cleavage has not been achieved, these studies provided an increased understanding of the general aspects of cyclometallation and ligand stability/flexibility as well as the influence these factors exert on each other. Further mechanistic investigations were initiated as to elucidate the potential of the proposed concept, affording novel insight into differences in reactivity between these POCOP-systems and their PCP-analogues.

Popular Abstract

Catalysts are substances, which are known to be at the root of many large-scale chemical processes, due to their ability to either facilitate specific steps in the reaction sequence or by altogether rendering certain transformations possible, which would not have occurred otherwise, by lowering the required energy input and increasing the overall-rate of the reaction. Among such chemical processes which employ catalysts are well-known representatives such as the Monsanto process for acetic acid production, the Wacker process for large-scale synthesis of aldehydes, the Müller-Rochow process for the generation of chlorosilanes, which are further used for the production of silicones as well as various catalysts that are being used in polymerisation reactions leading to synthesis of ubiquitous plastic materials.
In recent decades a process known as “C-C bond activation” has been granted increasingly more attention in research. C-C activation is a term that describes the scission of a carbon-carbon bond in a molecule that often involves a catalyst based on a metal compound that induces this breakage. Having cleaved such a bond, a variety of subsequent transformations of the molecule can occur, providing grounds for achieving a great versatility of substances based on one molecule.
One very impactful application of the C-C bond activation could be the manipulation of so-called “alkanes”. Alkanes are a substance class that is mainly afforded from crude oil and are isolated and purified through refining processes.
These products serve mainly as the constituents for different fuel types and lubricants. These applications do not warrant complex transformations of the alkanes, which are generally used as received after isolation, with solely long-chain compounds being transformed into their shorter-chain analogues, due to their limited utility of the former.
Their sheer abundance however would make them convenient candidates for usage as starting materials in large-scale chemical processes, with C-C bond activation providing an efficient way to overcome difficulties in transforming them into other chemical substances. Furthermore, this route could provide less energy-consuming methods for controlled degradation of long-chained products, while simultaneously rendering the targeted and controlled transformations of these compounds possible.
Recent studies in this field have offered promising insight into the reaction behaviour of alkyl-groups in so-called PCP-pincer complexes, which contain an organic molecule harbouring phosphorus-groups bound to a metal centre. These findings have given rise to the herein conducted project work, wherein a system analogous to that PCP-pincer complex, known as a POCOP-pincer complex, was proposed as a possible candidate for investigation of intermolecular C-C bond cleavage in this class of compounds, which would afford the very first example of such a reaction being successfully conducted. Thus, the required molecules used as ligands were synthesised and their reaction behaviour was thoroughly studied in the context of the desired activation, providing insight into whether this particular approach could prove promising in the future.

• author: Ilic, Aleksandra
• supervisor: Ola Wendt
• organization: Department of Chemistry
• course: KEMR10 20192
• year: 2020
• type: H2 - Master's Degree (Two Years)
• subject: Chemistry
• keywords: C-C bond activation, POCOP, pincer, cyclometallation, inorganic chemistry, organometallic chemistry
• language: English
• id: 9003878
• date added to LUP: 2020-02-25 12:50:12
• date last changed: 2020-02-25 12:50:12