Metal to Ligand Charge Transfer (MLCT) in [PyX][CuI2] systems

A charge transfer (CT) occurs when electron density in the highest occupied molecular orbital (HOMO) of the metal complex is excited to the lowest unoccupied molecular orbital (LUMO) of the complex. As one might expect, the nature of the CT behavior in a metal complex varies significantly with the choice of the metal and the ligands in the complex. Metal to Ligand Charge Transfer (MLCT) is a specific type of charge transfer (CT) process that can occur in metal complexes upon electronic excitation. The “ML” part of MLCT denotes that the HOMO in the CT is metal like in character, and the LUMO has the character of whatever ligands are part of the metal coordination complex. When the electron density relaxes (i.e., falls from the LUMO to HOMO), luminescence in the UV range of the electromagnetic spectrum is often observed. However, it is important to note that the nature of the relationship between the metal complex structure and constituent parts (i.e, what M and L are) is not well understood. This project will attempt to shed light on the relationship between the size of the metal center and the nature of CT (particularly MLCT) that can occur in metal cluster.

 

This relationship will be primarily studied through the copper iodide alkyl pyridinium system (CuI2PyX, where X is an alkyl), which is a system that is extremely conducive to MLCT for two reasons. The first is that the metal – Cu – is very electron rich, and therefore has a higher energy HOMO. Secondly, pyridine (Py)  is a good pi acceptor – in other words, it’s LUMO (a π* orbital) is quite stable and therefore relatively low lying in energy. The combination of these two factors (an electron rich metal and a good pi acceptor ligand) allow for a relatively small energetic gap between the HOMO (which will take on the characteristics of the Cu) and the LUMO (which takes on the characteristics of the pyridine) of the overall complex. The “X” – or alkyl – will allow us to vary the size of the pyridinium cation without changing the electronic behavior of the said cation too much.

 

This background amounts to a project that will have three overall steps. First, I must synthesize many different alkyl pyridiniums of various sizes; the general process for this involves a neat reaction of pyridine with an alky iodide to produce a salt (i.e, XPy+I). Secondly, I will then need to react these alkyl pyridinium salts with CuI to produce the CuI2PyX complex and obtain crystals of the products; this process will vary significantly with alkyl pyridinium salt used. Finally, I will need to determine the structure of the final (hopefully) crystal structure of each complex through single crystal X ray crystallography.

 

I have two main goals for this project. The first one is to collect enough “data” to publish a paper in an appropriate journal. This is an ambitious goal, but not an implausible one – while there is always the potential for setbacks (I guess it wouldn’t be research if there weren’t….), I’ve been working on this project for the past academic year, and (hopefully) have most of the bumps in the project steps ironed out. The second goal is a more personal one – I hope that, through working on this project, I can develop a better sense of “chemical intuition”. I’ve heard “chemical intuition” described in several ways; my loose definition of it is that “chemical intuition” refers to an ability to predict how different compounds/reagents will behave in different situations, based on a deep understanding of what the characteristics of these different compounds are. This is something that is, to be frank, challenging to achieve – I think anyone who’s taken a chemistry test that had a “predict the products” section will agree. However, this is something that that I hope to develop through the experience that I will gain this summer.