Exploring the potential of [Fe(phtmeimb)2]+ in photoredox catalysis

Abstract

The aim of this study was the investigation of the photocatalytic properties of Iron{phenyl[tris(3-methylimidazole-2-ylidine)]borate}2 ([Fe(phtmeimb)2]+) by optimising reactions previously established with ruthenium-based catalysts in good yields. For the first time in several decades of research on photochemistry of iron complexes a Fe complex has been made ([Fe(phtmeimb)2]+) that has photophysical properties that are at least in some regards comparable to [Ru(bpy)3]2+, such as charge transfer excited states of high energy and nanosecond lifetime Therefore, we investigated photoredox catalysis with this complex. We investigated the photocatalytic performance of [Fe(phtmeimb)2]+ in aza-Henry and [2+2] cycloaddition reactions. Then optimisation was conducted to make [Fe(phtmeimb)2]+ viable in reactions where currently [Ru(bpy)3]2+ is used as a photocatalyst. For this the commercial photoreactor Photoredox Box from HepatoChem with 450 nm and 525 nm lamps was used and attempts were made to repeat the conditions of the literature reactions and secondly alter the reaction conditions so that [Fe(phtmeimb)2]+ becomes viable. The experiments were predominantly followed by NMR analysis with internal standard.
Initial results from the [2+2] cycloaddition reactions suggest that the reductive pathway is not accessible by [Fe(phtmeimb)2]+ and further investigation should be made on the oxidative pathway. The aza-Henry reactions initially showed some promise. Although, after further control, it could not come close to the performance of [Ru(bpy)3]2+. However, it was found that the addition of a counterion (NH4PF6) showed increased yields in the reactions where [Fe(phtmeimb)2]+ was used. [Fe(phtmeimb)2]+ cannot replace [Ru(bpy)3]2+ as a photocatalyst (PC) so far and more research should be done to further enhance the photophysical properties of iron complexes.

Popular Abstract

Photocatalysts – what are they?

Most of us have heard of catalysts in one way or the other. One of the most prevalent catalysts that come to most people’s mind is the one in cars. Here, harmful gases formed in the combustion engine get transformed into less harmful substances before they are released to the environment. So how does this work? To keep it simple one might envision a trail over a mountain during a trek in the wilds. We start at our dangerous compound (A) and move over the mountain, requiring energy, and then arrive at our destination (B) which is the less harmful substances. A catalyst, in this instance, would lower the amount of energy it takes for us to cross from A to B. How this energy is reduced varies from catalytic system to system. Using the analogy of the mountain again: reducing the energy required to move from A to B could be done by reducing the height of the mountain (reducing the energy of activation between A and B). This is just one example in how a catalyst might work. Photochemistry might not be as prevalent in the minds of the masses and a short introduction is in order. Photochemistry deals with the chemistry involved with light, photons. It is a broad area that encompasses, among other thing, solar cells and chemical reactions driven by the energy from photons. One similar reaction that might come to mind is natural photosynthesis. Here a chromophore (in this case chlorophyll) initiates a charge transfer that drives chemical reactions at specialised catalytic units. A chromophore is a molecule or part of a molecule that can absorb photons. In some cases, a chromophore can function as a photocatalyst directly. So, what then, is a photocatalyst? Simply put, it is a molecule that catalyses a reaction (A to B) by the help of the energy of photons. Photons hit the photocatalyst which thereby gets excited. Simplified, this means that the energy of the molecule is increased until it decays back to the ground state. One decay path is the transfer of this energy to another system that could not have received this energy directly from the photons themselves. So how is a photocatalyst created?

When creating a photocatalyst in a lab we focus on the chromophore and try to synthesise one that absorbs light energy, preferably in the visible spectrum, and can transmit this energy. A lot of research has already been conducted with ruthenium based photocatalysts as these have beneficial properties. One of the drawbacks of ruthenium is that it is a rare transition metal and hence not cheap or available for large scale projects. How can the cost be decreased while the scalability is improved? During the last couple of years researchers have investigated the properties of other, more abundant metal complexes. Our group has focused on iron chemistry and has reached milestones in the development of molecules that might be used to produce dye-sensitised solar cells, among other things. We are currently investigating said molecules ability to act as photocatalysts in a range of chemical reactions where ruthenium complexes have been used previously. The aim of this study is to map out the photocatalytic properties of one of the molecules synthesised in our lab that has recently been published. This will be done by synthesising the photocatalyst and substrates used during the reactions and optimising the reaction conditions.

Preliminary results show that photocatalysis with iron complexes is not as straightforward as with standard photocatalysts. But the study shows that there might be a way to mediate these shortcomings.

• author: Braune, Alexander
• supervisor: Simon Kaufhold
• organization: Department of Chemistry
• course: KEMP30 20192
• year: 2020
• type: L3 - Miscellaneous, Projetcs etc.
• subject: Chemistry
• keywords: Aza-Henry, [2+2] Cycloadditions, Iron based photocatalyst, Photochemistry, Organic chemistry, Organisk kemi
• language: English
• id: 9005358
• date added to LUP: 2020-03-02 10:04:42
• date last changed: 2020-03-02 10:04:42