 Hello everyone, once again I welcome you all to MSP lecture series on transformative chemistry. I am sure you are enjoying the classification of ligands by donor atoms and in my previous lecture I was telling about how important it is to make right kind of phosphines so that you can perform catalysis very efficiently under homogeneous catalysis. So let me continue from where I had stopped ok. I was telling about the choice of catalyst and for catalysis why we prefer bidentate ligands even when you have bidentate ligands why we prefer unsymmetrical bidentate ligands or dysfunctional ligands having hemilabial nature or of hybrid in nature. So now let me show you these things I showed you and also I showed you are the advantage of bidentate ligands versus monodentate ligands. Now I shall try to convince you using these following cartoons why bidentate ligands are much more efficient and have longer life and can yield products with high turnover number and turnover frequency. Let us assume this palladium tetra crystalline phosphine 4 ligands are there and metal is here and now we have to activate couple of bonds the moment you put into solution what would happen? Two bonds would be getting activated as a result what happens? The bond distance also increases that means they are slightly ready for cleavage you can see and also it is activated. So in the next step they are completely detached completely detached. To imagine a father goes with four children to some sort of meadow where millions of people are there in case if he loses two children it is highly unlikely that they will come back to father. It is highly unlikely that it will they will come back to father and like that if hundred parents are going where a millions of people are gathered I am sure at least few parents would lose their children they go empty handed or go with few children in case if they are coming with four children with them. This is what happens to tetra crystalline phosphine palladium compound in solution when we are using it as a homogeneous catalyst for a particular organic transformation. So that means basically regeneration efficiency is very low as a result what happens? Number of catalytic cycle one can perform will decrease and in case what happens moisture intervenes of course these compounds are highly susceptible for oxidation phosphines can be readily formed phosphine oxide as a result what happens? If the moisture comes what happens the entire metal complex is decomposed and you can never get back this one from this condition. So this is the fate of metal complex having four monotonic ligands when you want to use them in homogeneous catalysis organic chemists what they do is they use large excess of that one as a result what happens they are not worried about the catalytic efficiency what they want is small quantity of catalytic product they should be any problem but if you as an inorganic chemist if you want to use this one in very small quantity with efficiency like 1 mole percent 0.5 mole percent I do not think it is an ideal case when if you think about monotonic ligands. So let us look into bidentate ligand in the same context now you can see the difference between them is these two children are again tied now father knows that it is likely that you know you cannot control all the four together and even if his attention is diverted little bit these two kids can go away as a result what he does is he ties the hands of children like this in that case what happens in case if he loses the grip on two children they cannot go because their hands are tried with another one which he has a firm grip as a result what happens these two try to go away but they cannot go away they are in the vicinity of father and they will try to make bond or try to establish a bond and once the oxidation is over and reductive elimination is also completed they can establish so that father can go happily with four children no matter how many people are gathered it is highly unlikely that he loses children here this is what exactly one can compare with bidentate ligand in homogeneous catalysis due to some reason what happens if it is getting oxidizer that means you get wounded let us say wounded baby or if the phosphine getting oxidizer to form a p o bond and now p o bond means it is a hard donor atom now p is no longer a donor atom but oxygen lone pair is can be donated to the metal sometime it happens if you take THF diethyl ether they can also bind to metals which are soft so as a result what happens now it is no longer a trivalent phosphorus is a pentavalent tetra coordinated phosphorus having p double bond o and o has lone pair and then from this one once the reductive elimination is over still we can establish a bond so that he will go with two wounded children and then they can be taken care so now the bond is established here so that means again you can generate a more active species compared to the parent catalyst and it is much more easy to dissociate these bonds because this is a soft center and these two are hard centers dissociation would be very there is a competition to dissociate this one and this one you know that these bonds are soft soft stable and that is what we want they want to be answer ligands they have to be held on to the metal whereas these two bonds should be cleaved so that is what exactly happens now in the second cycle if this happens second cycle even much faster that means some damage that happens to uncoordinated or dangling phosphorus moieties can enhance the catalytic efficiency something unusual unheard of but yes true that happens in case of phosphines and this is the advantage of having unsymmetrical phosphines I am sure now we are convinced why we are very particular about designing appropriate phosphines with appropriate characteristic so that we can use them efficiently and also they can be more durable when we look into the phosphorus bound metal complexes their application are plenty and of course when we talk about phosphines we come across variety of you know groups and phosphorus they can be phosphines are also called phosphines phosphonides phosphonides and phosphorus nitrogen compounds we have and of course pincer chemistry comes into picture and then in coordination argumentally chemistry they are second to none and coordination polymers metal clusters and metal cages phosphines play a major role and of course homogenous catalysis they are the leading ligands and of course with less toxic phosphorus one can think of them in anti-cancer studies and they can also show interesting photophysical studies and also they can be very handy in material applications. Now we should look into this bond enthalpies and if you see here phosphorus to fluorine bonds are quite strong whereas phosphorus to iodine bonds are very weak and of course these compounds are quite expensive and not very easy to handle but on the other hand PCL and PBR bonds can be handled nicely in fact PBR3 is also used as a brominating agent in organic chemistry and the most appropriate and economical compounds are phosphorus chlorine compound that means having three chlorine atoms on phosphorus are phosphorus trichloride. Phosphorus trichloride has optimum properties one can perform a series of nucleophilic substitution reaction to generate desired phosphorus compound either by using lithium reagents or grignard reagents or even Fidelkruff reaction for that matter or several other reactions where one can replace chlorine with right kind of substituents to have moderate or desired steric and electronic attributes on it. Now how to replace in my previous lectures when I was talking about several metal complexes I showed you how you can generate labile complexes and eventually replace those with phosphines or the other ligands. For example, if you have astronitrile, benzonitrile, cycloctadiene or narbornadiene are there they are very easily replaceable ligands one can make those compounds as intermediates and then under mild conditions one can generate a phosphine complex whether monophosphine, bisphosphine or even a phosphorus compound having tridentate properties. So now I will show you how one can replace carbon monoxide using simple condition under mild condition for example one can also perform photochemical reaction you can shine UV light and you can detach or dissociate CO and in its place another phosphorus ligand can come or one can also do thermal reaction by reflecting in appropriate metal carbonyl with ligand in a right kind of solvent. But how to do substitution of carbon monoxide with phosphine under mild condition say room temperature where there is an option one can use trimethylamine N oxide let me show you this trick. So let us consider a hexacarbonyl so it can be molybdenum or tungsten then treat this one with one equivalent of trimethylamine N oxide this is also called as TMNO if somewhere if you read simply TMNO you should understand that it is trimethylamine N oxide then it forms an intermediate of course here you should remember it has something like this bond is polarized here forms a compound of this type here. So now we have best charge and we have negative charge here now what happens it readily loses a molecule of carbon dioxide and a molecule of trimethylamine to generate a vacant site on metal this is a vacant site now. So now if we add at this stage a tripe a tertiary phosphine we can make very conveniently a phosphine complex something like this of course one can do stepwise now we can add one more equivalent of trimethylamine N oxide and we can substitute for one more carbon monoxide we can keep on doing and if the phosphine taken is not very bulky and the phosphor substance are very strong electron withdrawing groups then probably we can replace all carbon monoxide from a hexacarbonyl example if we take trifluorophosphine this is one such ligand whose sigma donor and pi acceptor abilities can be compared to carbon monoxide it can knock off all carbon monoxide and one can form a homolyptic complex like this. So there are not many ligands we have which can replace all CO of course one can also take a nitrogen donor ligand if you ask a question take metal hexacarbonyl at trimethylamine N oxide and it generates a vacant site how about adding ammonia yes you can add ammonia triethylamine you can add you can add astronutrile still you can form but only thing one should remember is if you want to replace carbon monoxide with nitrogen donor ligands which are only sigma donor you cannot go beyond 3 you can not go beyond 3 and at most you can get M CO 3 L thrice L 3 where L can be any nitrogen donor ligands whereas in case of phosphines it is possible. So this is one mild reaction method where a carbon monoxide can be replaced at room temperature using a solvent such as astronutrile or even toluene. Now the next question is is there remain always ancillary ligands or silent spectators phosphines or is it possible to perform some reactions on coordinated phosphines yes I showed you while talking about orthometallation triphenylphosphine can undergo orthometallation through CH activation and then it can be added acceleratively and then if the condition is suitable HCl can be eliminated or H if the metal has a halide that can still retain having higher coordination number or higher access state but if we can perform reaction on coordinated phosphorus yes it is possible then again we are thinking of an interesting catalytic system. So let us look into what would happen if we make an attempt to do PC bond cleavage. So let us consider a complex like this we have some other ligands along with a triphenylphosphine and hydride. So if we take this one there can be cleavage of PC bond to generate a rhodium to phenyl bond and now we have a phosphido group is there and this lone pair is intact now and then it can generate a species of this type. So now let us say we are adding an organic compound it can give something like this this one can eventually eliminate triphenylphosphine as a different ligand now which can eliminate this diphenylphosphine moiety through this coupling reaction that means it is possible to activate a PC bond of coordinated tertiary phosphine. Now what happened earlier we had a phenyl group was there here if you see and now phenyl group has come out and then in its place we have a new moiety here. So that means this is a typical a PC bond cleavage and also this is also called as r group reshuffle or r group shuffle. So this also comes very handy in generating a new kind of tertiary phosphines having different substituents on phosphorus. So another advantage I shall tell you more about this kind of reactions at later stage in fact in our own group we have seen PC bond cleavage and also migration of phosphorus bond group on to the metal very interesting chemistry carried out recently I shall tell you sometime before I conclude phosphorus donor ligands. And another advantage with phosphines is 31p NMR. So NMR spectroscopy comes very handy in diagnosing the complexes and nature of the complexes and also reactivity of the complexes because phosphorus depending upon what kind of groups we have and also what is the access state whether trivalent phosphorus or pentavalent phosphorus they have very distinct chemical shifts that vary that range from plus 250 to minus 300 in general or it can go from plus 600 to minus 600 in special cases or in some cases it can also go from plus 800 to minus 800 and here 31p NMR nuclear spin is half and it is 100% abundant. As a result carrying out or performing NMR phosphorus NMR would be very easy and here we are using 85% phosphory acid as a reference and that chemical shift is considered as zero whatever comes on the left is called downfield region whatever comes on right region is called upfield region or this is shielded and this is D shielded and low frequency shift and high frequency shift we also call. I shall if time permits I shall introduce interpreting NMR spectroscopy to make you familiar to interpret spectra while characterizing metal complexes when we have nuclear active species in it. I shall try to do that at the end and now just you can see here when you have different substituents they have very distinct chemical shifts for example trichlorophosphorous or phosphorus trichloride appears 220 ppm whereas when you start replacing chlorides what happens it will be more shielded and when it comes here it is more shielded and it appears at 36 and as I mentioned phosphoric acid has zero reference and then when you have this one minus 2 is there triphenyl phosphine minus 6 like that what happens they have a distinct chemical shifts are there when you react a metal complex with phosphine whether the metal complex has formed or not or if we have any unreacted phosphine or whether it is decomposed where it is oxidized all this information comes simply by looking into 31 PNMR of the species. So this is where it comes very handy in diagnosing the reactions and reaction sequences and also sometimes one can also perform kinetics using 31 PNMR spectroscopy. So now I will show you very interesting feature here for example when we have 4-membered ring okay this is a bisphosphine having one methylene group is there and it said DPPM bis-diphenyl phosphine or methane we have bis-diphenyl phosphine and ethane we have bis-diphenyl phosphine or propane we have. So all of them are forming 4-membered or 5-membered or 6-membered rings and now again chelate ring size also has influence on chemical shift that means electron density and the phosphorus can also vary okay with the ring size okay. So that means that also sometime assist us in understanding the reaction sequence and also how much electron density resides on phosphorus and all those things all those things. For example now if we look into this chelate complex and let us say in case of DPPM if you want to imagine two simple mononotentate ligands the best would be having so we have methylene group is there so the best mononotent ligands that come very close to the property of bidentate ligand or methyl diphenyl phosphine okay for example it is here okay. This one is more or less comparable to two donor atoms present in these three chelate rings and now what I have done here is to show you chemical shift for this metal complexes here okay and then plus or minus charge indicates the shielded or deshielded. So this is for the complex and this is for the free ligand DPPM and similarly this is for DPP and this is for DPP and this is for the free ligand here and then the difference what is this this is called coordination shift coordination shift is nothing but the difference in the chemical shift between free ligand and complex okay so that one is given here what is this one this is called coordination shift of a chelate complex. So here this value also give you some information this is taken by considering this value from this one okay for example if you take this value here and then the difference would come here in the ligand chemical shift and this one should give you this one okay. Now we can see DPPM is more shielded and DPPAD shielded and again this is shielded and this one is shielded. So that means some information it gives okay so that information we can effectively use in analyzing some of these reactions and also to understand the nature of donor properties and also the stability and all those things. Let us okay try to do these things as and when okay we get such okay opportunities. Now just I will show you the donor and acceptor properties of phosphines how one can analyze or how they can impact the bond parameters okay I have taken two examples here with cobalt having one CP group okay cyclopentadienyl and two triethyl phosphines one is positive okay and one is neutral okay so okay. So that means here we are taken a cobalt one and here we are taken a cobalt two and now similarly in iron case also we have taken one neutral okay and then one had taken so that means one a positive one and a neutral that means in this one electron density is less and here electron density is more okay. Now let us look into the metal to cobalt to phosphorus bond cobalt to phosphorus bond distress is 2.218 amstang units here in this one whereas in this one it is 2.230 okay 2.230 that means okay the bond is little bit elongated why it is elongated there is less back bonding from cobalt to phosphorus because it already electron deficient. So that means this one certainly gives a hint that yes there is back bonding is there and back bonding can strengthen metal to phosphorus bond the way we see in case of metal carbonates where excessive back bonding from metal to carbon monoxide okay increases the strength of metal to carbon bond and decreases its length and also CO bond length is increased or it is elongated okay. So in the same context you can also compare here okay in this one bond is little shorter whereas here little longer because CO is not a very good donor here because of positive charge same thing one can see here okay in iron compound okay 2.146 and 2.261. So that means again here iron because of one electron less it is a weak pi donor and then how that impacts of course here what happens if the back bonding is more metal to phosphorus bond is strengthened on the other hand what would happen to phosphorus to carbon bond okay you can see here in this one it is 1.846 here 1.846. So that means bond is getting elongated okay PC bond is getting elongated of course that should happen I will show you later and then here okay the bond not much back bonding is there as a result what happens the bond okay PC bond is little stronger here okay 8 1.829. So that means the impact has not only alters metal to phosphorus bond but also dramatically it alters metal to carbon bond also that can be seen here okay in extreme cases what happens one can think of leaving PC bond this is what exactly happens I showed you in my previous slide okay so same thing analysis you can make it that means okay if we have careful analytical observation we can see all those things and we can understand the properties in a much better way okay like this okay for example we take when excessive back bonding is there and that means more and more electrons go to the sigma star as a result what happens this R group can come out as a carbon anion and then it will carry a positive charge if that happens whether we can use this moiety okay in a catalytic manner for as an alkalizing agent okay yes we can do it no one has done so far but if you want to do it we have to see how again we can bring back another alkyl group here if we can catalytically using a suitable reagent if we can furnish again formation of PR bond yes we can use this one very interestingly as an alkylating agent okay let me stop at this juncture and continue talking or discussing interesting phosphorus chemistry in my next lecture until then enjoy reading phosphorus chemistry.