Paramagnetic species in catalysis research: An EPR investigation into the photochemistry and reactivity of Cr(I) complexes

Fioco, David ORCID: https://orcid.org/0000-0003-3714-2742 2023. Paramagnetic species in catalysis research: An EPR investigation into the photochemistry and reactivity of Cr(I) complexes. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

In recent years, chromium complexes bearing a bis(diphenylphosphino) ligand have been widely studied for their potential use as catalysts in ethylene oligomerization. These systems are typically activated using a suitable co-cocatalyst such as triethylaluminium (TEA) or modified methylaluminoxane (MMAO). However, despite their potential use as catalysts and the large amount of research conducted on them to date, there is still no consensus regarding the mechanism of operation, the intermediates and active state of the catalyst and even the actual oxidation state of active chromium species. Whilst Cr(III) centres have dominated the ethylene oligomerization research literature, the Cr(I) analogues have received far less attention (primarily due to their paramagnetism and hence difficulties in characterisation by conventional methods). Considering these Cr(I) display a fascinating photochemical activity (and hence potential for photochemical assisted activation) this sparsity for Cr(I) studies is unusual. As Cr(I) is a paramagnetic metal center, EPR is an ideal choice as the primary investigative technique for these systems. In this Thesis [Cr(CO)4(Ph2P(C3H6)PPh2)]+ (using the 1,3 bis-(diphenylphosphino)- propane ligand Ph2P(C3H6)PPh2 = dppp) was found to be converted into a trans- [Cr(CO)2(dppp)2] + complex following UV irradiation in dichloromethane (at 365 nm). The intermediate species involved in this transformation, mer-[Cr(CO)3(κ 1 -dppp)(κ 2 - dppp)]+ , can be identified by EPR following low temperature (140 K) UV radiation. This transformation was found to be wavelength, temperature and concentration dependent. When irradiated with UV light at 380 nm the starting complex only converts to the trans-species, with no evidence for formation of the mer-complex. When irradiated at low temperature in frozen solution and low concentration, the product from an intramolecular reaction [Cr(CO)2(dppp-η 6 -arene)]+ becomes the main species observed whilst UV irradiation in the presence of 1-hexene was found to produce a stable [Cr(CO)3(dppp)(1-hexene)]+ complex. Whilst the common bis-(diphenylphosphino)-propane family of ligands, such as Ph2P(C3H6)PPh2, dominate the literature and this field of study, a novel complex bearing a bis[2-(diphenylphosphino)ethyl]amine pincer ligand (= PNP), [Cr(CO)3(PNP)]+ was synthesized for the first time and examined by EPR and HYSCORE spectroscopy to determine its electronic structure. Similar to the 3 Ph2P(C3H6)PPh2 ligand and its associated Cr(I) complex, this new pincer type Cr(I) complex can also be activated chemically and photochemically to react with 1-hexene, forming a stable [Cr(CO)3(PNP)(1-hexene)]+ complex. The results presented in this work show for the first time that the key step in the activation of the Cr(I) based complexes, namely the removal of the Cr-CO carbonyl groups by TEA or MAO co-catalysts, can potentially be achieved by selective photochemical steps, opening up the possibility for future research opportunities that incorporate UV radiation to enhance or even supplement in a controlled way, the activation and catalysis of these important Cr(I) catalysts for ethylene oligomerisation.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Chemistry
Funders:	Horizon 2020
Date of First Compliant Deposit:	21 May 2024
Last Modified:	21 May 2024 14:00
URI:	https://orca.cardiff.ac.uk/id/eprint/169061

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