 Hello, welcome. Today, we will discuss Dioxygen reactivity in copper. In other words, if you have a ligand copper complex, how it is going to react with oxygen? We have seen many copper enzymes are present in nature. These metalloenzymes containing copper can effectively promote different oxidation, oxygenation chemistry. Today, we will see the basis of this chemistry. Essentially, we will try to learn what happens in laboratory if we are trying to synthesize these copper complex. Is it going to be reactive as what is seen in the enzyme or is it going to react differently? There are many challenges that is associated in synthetic laboratory reactions compared to the enzyme. In enzyme, we have a completely defined atmosphere where let us say one copper will react with one oxygen or two coppers will react with two oxygen. But such a well defined system cannot be predicted that very easily for a synthetic chemist. Synthetic chemist whenever they are trying to understand the copper oxygen chemistry, iron oxygen chemistry, manganese oxygen chemistry, nickel whatever chemistry we are trying to understand, we usually run into the problems of over reactivity. Sometimes they are not reactive, but often they are uncontrollably reactive. So, one need to really understand how to team the reactivity and get a desired product so that a desired reaction can be performed. Of course, all the copper oxygen chemistry products are not going to be equally reactive. Their reactivity pattern is expected to be different. Those are the patterns we will try to learn today. This is a beautiful enzyme right, hemocyanin. You have two copper centers here, three histidine on each of the copper center. It is found in spiny lobsters, crevice, arachnids. This is the oxygen carrier protein just like we what have in our blood hemoglobin for oxygen carrying purpose. In these lobsters, crevice and arachnids, these are the centers where oxygen binding occurs and oxygen can be carried out reversibly. Oxygen can be delivered reversibly at different part of these species. Each of them are tridentate copper center as you can see and this is the form is called deoxyhemocyanin. This is a colorless compound. These are all copper 1 complexes. The distance between the two copper centers are 4.6 angstrom. Each of them are trigonal planar geometry and copper nitrogen distance is overall nearly 2 angstrom. Once it reacts with oxygen, the species that is formed in the process is called oxyhemocyanin which is blue in color. This compound is blue in color. It has a very characteristic UV visible band at 340 nanometer which is a very strong band and there is another characteristic peak at 570 nanometer which is also quite strong, but nothing like 345. If you look at the resonance Raman spectra, the oxygen-oxygen stretch is arising at 750 wave number which is quite exciting for other purpose I will come back to that in a moment. Now, this complex is going to be diamagnetic meaning that these both of these coppers are now oxidized to copper 2. Each of the copper has given one electron to the oxygen center to make it copper 2, copper 2 paraxomoethic. This dicopper peroxo species is the one which is going to be going to be having a copper-copper distance of 3.6 angstrom. For 4.6 angstrom, the distance shortened to 3.6 angstrom almost it is breathing right. It was far apart oxygen brings them closer, relatively closer to each other right. This oxyhemocyanin is giving blue color and this is very characteristic of this sort of this sort of species both the UV visible spectra, resonance Raman spectra, EPR spectra which shows the diamagnetic structure and also the crystal structure clearly showing that it is a side-on bound peroxo species ok. We will come to that in a moment, but this two copper is binding with both the oxygen equivalently. So, these are very distinct copper oxygen species formation. Now, if you look at another enzyme tyrosinase, it has exactly similar structure. While in this case, the previous case of hemocyanin there is no organic substrate involved in the whole process, only job it does is it is binds with oxygen, carries it and release it wherever it is required. So, it is a completely reversible process, oxygen is getting reduced, but it gets back to its original form when it is need to be delivered right. So, this is not involved in too much of a chemistry it is almost oxygen binding chemistry, of course, electron transfer is occurring, but no organic substrate is getting reacted with this. Now, with tyrosinase we will see what happens with such a molecule or for such a molecule, if an organic substrate is hanging close to this active species. Is this active species going to react with the organic substrate? Answer is yes. And that is what we find in tyrosinase right, this is another enzyme a very very interesting compound. We see our active site metalloenzyme where once again just like in hemocyanin, we have two copper one center which are in the oxy form, it reacts with oxygen to give di-copper peroxo intermediate in an side on bound geometry. So, both the oxygen atoms are bound with both the copper and they are equidistance from each other ok. So, this distance and all these distance are equal to each other ok. Now, if we are placing the same species as you have seen in the hemocyanin, this is the species we have seen in the hemocyanin. Now, if we are having a phenol, then this reactive species can be active with respect to this one and can oxygenate phenol to form the catechol. So, if you are seeing this is the copper center, that is the copper center, these are the three histidine, these are the three histidine and the phenol is sitting over here, it is very close to this copper peroxo intermediate over here and then it is getting overall oxygenated at the orthoposition of the phenol to give the catechol. If one is using the catechol as a substrate, this catechol can react with this further with this peroxo intermediate to give the quinone product. So, this activity is called phenol to catechol formation is called cresolase activity and this catechol to quinone formation is called the catecholase activity. Overall tyrosinase is a fascinating enzyme which can do really beautiful and simple chemistry by utilizing the copper oxygen species, reactive copper oxygen species in the enzyme. Now, how or what has been really known in terms of the copper oxygen chemistry from the synthetic laboratory? These are the studies or the results from the enzyme, but as we understand that the chemistry for synthetic compound as well as the enzymatic compound are not going to differ. Since these are the compounds forming, can we prepare such compound in our laboratory and see that if similar reactivity is found? Actually it is other way around. First the synthetic studies has been done, reactivity pattern and all other studies have been done and subsequently enzyme studies suggested and definitely later on proved that similar chemistry is happening what is known in the synthetic chemistry. Well that is I think is another exciting fact to know that the enzyme studies understanding has directly been influenced by the understanding of the synthetic chemistry and almost always the understanding of the synthetic chemistry setup is going to be much more in detail compared to the enzyme and that is precisely because we can control on the synthetic chemistry compound rather easily controlling the reactivity, understanding the reactivity in enzyme setup in greater detail is always going to be challenging compared to the synthetic ones. Let us look at the overall copper oxygen species that one can think of this is by no means complete list just to give an overview. We have just seen this copper oxygen species giving tyrosinase activity, catecholase and the chrysolase activity. This is the one we were seeing right now, this activity we have seen in the last slide. Of course, there is the oxygen activation or oxygen carrying, oxygen transport is happening at the hemocyanin these two we have seen right now. We will be seeing quite a lot of these two PHM and DBM in maybe 4, 5 class. So, we will discuss about these two enzymes separately. They are reaction mechanism these are very important enzyme these are having although a dinuclear copper side, but a mononuclear copper oxygen species are responsible for these hydroxylation chemistry as you see here alpha position getting hydroxylated and this is going to be quite exciting to learn these mechanism of action for this mononuclear copper oxygen chemistry. Presumably the mononuclear copper superoxo chemistry for these two we will discuss that later. Now, one of the things which we have also learned or you know will be quite interesting to know is this cytochrome C oxidase chemistry which is a beautiful chemistry will not be discussing too much of the particulate methane monoxygenase which convert methane to methanol we will learn this chemistry by utilizing iron right diiron species. Amine oxidase and galactose oxidase are quite interesting on their own rights. Galactose oxidase mechanism we will see briefly and we might will not be discussing too much of this one there is another enzyme which is fascinating to which converts superoxide into oxygen and peroxide. So, this mutation type of reaction happens. So, this enzyme is called copper zinc SOD superoxide dismutase right and then there is ascorbate oxidase which will again briefly mention who can convert oxygen to water where three copper center or multiple copper centers are involved in the process. Overall either a single copper site as you can see single copper site or a di copper site like these are responsible for carrying out many reactions as you can see some of these over here. We will discuss some of them in future for this course, but we will focus today on how the copper is reacting with oxygen in synthetic setup ok. So, all these are some way or the other connected, but we need to understand them separately. Let us briefly look at the galactose oxidase. It converts essentially alcohol to aldehyde ok. This alcohol is converted to aldehyde by galactode oxidase. It has two histidine and these cross linked tyrosine moiety is there and this essentially it is a phenol 2 histidine and phenol moiety is there and we have oxygen reaction with this copper center to give this copper 2 superoxo intermediate one electron transfer to the oxygen to give the copper 2 superoxo intermediate. This copper 2 superoxo intermediate then abstract hydrogen atom from this phenol to give this phenoxy radical and copper 2 hydro peroxo intermediate. Subsequently oxygen-oxygen bond cleavage and formation of water takes place or in this case particularly hydrogen peroxide goes out upon protonation of this hydro peroxo species to give rise to a phenoxy radical intermediate. Oxygen-oxygen bond cleavage is a possibility, but did not happen in this case and it then underwent this phenoxy radical formation that has happened over here. This phenoxy radical and copper 2 now bound with phenolate can modulate the reactivity of the alcohol. For example, by utilizing this phenoxy radical one can think of or it can be shown in the enzyme setup that we can do these chemistry quite efficiently. This is the PHM chemistry that we will discuss in next after next 4, 5 class I guess yeah approximately 4 classes. We will discuss that after 4 more classes we will discuss that multi copper oxidize has at least 3 copper presence in there the oxygen binding is not 100 percent understood, but this is going to be predict this is has been predicted to be one of these 2 and among many other possibilities that is out there. Now, this di copper iron site in tyrosinase is the one which we were discussing earlier. So, we will not be focusing too much on this right now. In the last slide we have seen we will see this one again we have seen a little bit of this one before, but well maybe we can discuss briefly there is this 2 copper center bound by 2 histidine or you know it could be 2 histidine, it could be even tetradent tridentate ligand system, but overall we can see that oxygen can react to give you the copper peroxo species. If you want to go back for example, tyrosinase you see the 3 high histidine is there 3 histidine is there. So, more correct you know the correct drawing would be with the 3 histidine. Anyway this is to show the plane of the molecule and we find that a peroxo species is formed during this reactivity of 2 copper 1 and 1 oxygen. Now phenol is ligated with the copper center and position in such a way so that an electrophilic aromatic substitution reaction can go on and that is what is happening over here. A electrophilic aromatic substitution reaction giving rise to the oxygenation of the orthophogestion of the phenol that is in turn is very very exciting that means, that we are going to form the benzoquinone directly from this intermediate. That is fascinating we might we will discuss some of these or similar reaction mechanism little later. So, 2 copper centers they form the peroxo, peroxo participate in oxygenating the phenol substrate. There are quite a few names quite a few big groups that have been that have been involved all over the world for solving and understanding these copper oxygen chemistry. What became increasingly clear by these studies is this is far more than simple. This is indeed very very complicated to understand what goes on. Upon decades of studies shows that the ligand is controlling the geometry around the copper center. If a tetradentate ligand is present it is likely that if this end on peroxo species is going to be the reactive intermediate. If a bidentate or tridentate ligands are present as you have seen earlier a Psydon peroxo species is formed. So, both of these are the peroxo species and both of these copper centers and these copper centers are going to be copper 2 plus, but the way they are bound with the metal center is differing. So, in this case they are bound in an Psydon geometry in this case they are bound in an end on geometry. This is going to be nucleophilic in nature these are going to be electrophilic in nature or these centers are quite interesting and aromatic hydroxylation chemistry as I was mentioning in the last slide can also be feasible. So, these are going to give you nucleophilic reactivity ok. So, these are delta minus over here and these are delta plus over here. So, a protonation perhaps would be possible over here and a protonation will not be possible over here. These are nucleophilic as you can see these are electrophilic both of them are electrophilic in nature. Now, these two copper centers are in plus 2 oxidation state. If a suitable ligand exists usually with the bidentate ligand what we see that even this oxygen-oxygen bond of the peroxo can be cleaved to give you copper 3 copper 3 bismeoxo intermediate which can obstruct again the hydrogen atom. So, all this is telling that when we are taking a suitable ligand copper complex and reacting it with oxygen there is possibility of formation of many different types of copper oxygen chemistry. The species that is going to be formed is going to be dictated by the ligand for the metal center. If it is a tetradentate ligand present over there we can expect an end-on geometry as is in here this is called end-on peroxo, di-copper peroxo or mu 1 to peroxo. If it is a tridentate or bidentate ligand usually for the tridentated one we get the side-on peroxo species. These are similar species, but the reactivity pattern is completely different. If you have usually a bidentate ligand and suitable solvent and depending on the ligand we can also in addition to this peroxo we can also get this bismeoxo intermediate. These two species can be in equilibrium in fact, these and that can also be in equilibrium. Let us not get into too much of the details, but overall what we realize that this bismeoxo species can be formed upon leaving the oxygen-oxygen bond and both the copper centers are now oxidized to copper 3 plus right. Now, this is nucleophilic these are electrophilic in nature these are all dinuclear species right. There is indeed a possibility of forming the mononuclear species. In fact, while forming these dinuclear species first form intermediates are these mononuclear species. Depending on the ligand steric and electronic properties we can expect one of these species is forming as a first step. These reaction these species formations while reacting with ligand copper 1 complex and oxygen these are very very fast. In fact, these are also very very unstable these species need low temperatures such as minus 80 minus 90 minus 120 where it would be possible to perhaps detect it and study to some extent. Compared to that these are I would say these are still thermodynamically preferred product, but kinetic formation or the first formation of this species is definitely there which a lot of studies has shown that in recent years, but overall these are going to be much more reactive intermediate and these are the one which is known as mononuclear copper oxygen species. This is going to be an end on superoxo species, this is going to be a side on peroxo species, this is going to be depending on the charge. So, this is going to be the side on peroxo species this is in going to be side on eta to peroxo with copper 3 and this is going to be obviously the side on superoxo species. So, end on superoxo as you see only one end of the oxygen is bound with copper the other one is not in this case both the oxygens are bound with copper equally. So, this is going to be the side on superoxo species. We will see this slide in next 4-5 class again where we will discuss this mononuclear copper oxygen chemistry, but overall just to tell you that there is possibility of forming many different types of copper oxygen species once we are reacting them under laboratory setup. Perhaps in enzyme we have very important formation of only one species, but in case of synthetic setup we have very limited option of controlling this reactivity pattern. So, all of these species are likely the control is going to be with the temperature solvent, but more importantly ligand ok. So, that we have briefly seen let us look at little bit of their reactivity pattern. So, as I was trying to mention that these end on peroxo this is the end on peroxo these are going to be nucleophilic in nature that means, these are delta minus delta plus on the other hand these are going to be side on peroxo in nature and they are going to be electrophilic in nature it is going to be delta plus delta plus delta minus delta minus. So, as expected if we are bringing a cation such as H plus protonation it would be possible to protonate because this is delta minus delta minus to give the hydrogen peroxo. Overall if you are bringing an acyl moiety acyl moiety can also react with this because this is minus delta minus this is a cation it is going to react with this. If we are reacting with carbon dioxide that is also a feasible reaction can give you this carbonate species finally, and overall this is quite interesting even the phenol can be deprotonated to give the phenolate intermediate. On the other hand this side on bound copper oxygen intermediate which is known as the side on peroxo cannot react with proton because this is delta plus this is delta plus this is a cation ok. This cannot react with proton it cannot react with acyl equivalent it cannot react with CO2 because once again these are delta plus delta plus no reaction. But once we are reacting with phenol we get the phenoxy molecule from these species and from their own once a phenoxy radical is generated radical radical coupling can go on and copper 2 complex can form. So, overall in this class therefore, we have seen that different type of copper oxygen species can form if the ligand copper complex is reacted with oxygen. And their reactivity pattern is also varying as you have seen end on peroxo species are nucleophilic in nature and side on peroxo species is electrophilic in nature. Do keep studying these different complexes and their reactivity pattern we will discuss from here in the next class and we will try to see the reactivity of the copper ligand complexes in the reduced form to give the oxygenated product, oxygen chemistry and their reactivity pattern of these species in the next class as well. See you soon.