 Hello everyone, I once again welcome you all to MSB lecture series on Transfer Metallic Chemistry. I am sure you are having good time reading and understanding chemistry. This is 37th lecture in the series and in my previous lecture, I started discussion on phosphorus ligands and also I did mention about the importance of phosphines in both coordination chemistry and ergonomically chemistry and also their utility in homogeneous catalysis. How important phosphines when compared to other similar sigma donor and pi acceptor ligands. So let me continue from where I had stopped two important aspects revolve around phosphorus chemistry when we want to use them as ligands. One is electronic properties and other one is steric properties. When we talk about electronic properties, we can readily alter the sigma donor ability and pi acceptor capability simply by changing the phosphorus substituents. And if we put more electron donating groups on phosphorus, it becomes a good sigma donor and poor pi acceptor. On the other hand, if you put more electron withdrawing groups on phosphorus, it makes relatively less or poor sigma donor but very strong pi acceptor. So this kind of facility, you do not come across among carbon monoxide, that is the reason tertiary phosphines, whether it is monophosphines, bisphosphines or polyphosphines play an important role in stabilizing coordination compounds and also ergonomically compounds in different oxygen states and also coordination number and hence their utility in homogeneous catalysis. So I mentioned about electronic properties and also steric attributes can be measured in terms of cone angle and I explained what is cone angle. So for example, let us consider a phosphine bound to metal and the average metal to phosphorus distance is about 228 p-commeter or 2.28 angstrom units. So let us imagine a conical surface at metal that encloses the van der Waals surfaces of all ligand substituents or all rotational orientations. That means if the phosphine is less bulky or phosphorus have small groups on it, your cone angle will be very small. On the other hand, if you put more bulky groups, cone angle increases. Once again I repeat, it is a conical surface defined or imagined at metal with a metal to phosphorus distance of 228 p-commeter that encloses van der Waals surfaces of all ligand substituents or all rotational orientations. So how we can use this one in utilizing phosphines in catalysis? Let us look into it. Now I have shown here and also I have listed corresponding cone angle for various phosphines as the bulkiness is increasing, cone angle is also increasing. Now you can see for example, if you take a metal complex having carbon monoxide as well as phosphines and how the stretching frequency of carbon monoxide varies that determines to what extent a phosphine is acting as a sigma donor and pi acceptor. For example, if you take here, I have put all alkyl phosphines here. If you see here, when we have alkyl phosphines in a metal complex having carbon monoxide, stretching frequencies are very low. On the other hand, if you can see here trifluorophosphine, very strong electron withdrawing group here we have as a result what happens CO stretching frequency is very high. That means in this type of ligands, there is a computation between both carbon monoxide and tertiary phosphine to take electrons through back bonding and if they are equally computed then what happens less electron density goes to the pi star of carbon monoxide as a result there is not much change in the stretching frequency. So that means this can give a measure of the sigma donor and pi acceptor capability of phosphines with respect to carbon monoxide. Now I have listed 3 ligands and the corresponding tetrachys metal complexes. So one such complex I have taken is Ni L4 where L is a neutral ligand is a phosphine and nickel is in zero valence state that means it is an 18 electron complex. Let us say if you want to use this one in catalysis since it is 18 electron species we cannot perform any oxidative addition as a result what happens one has to take out or dissociate one or more ligands before we use it as a catalyst. There is a typical example I have shown sterically demanding phosphine ligands can be used to create empty coordination sites or 16 valence electron complexes which is an important trick to fine tune the catalytic activity of phosphine complexes. That means you see here if you just look into the dissociation constant here for various ligands that can be compared with tolman cone angle for example if you take trimethoxy phosphine or trimethylphosphite it is very slow tolman angle is 107. So that means dissociation is little endothermic in nature. On the other hand if you go for slightly bulkier ligands such as dimethylphenylphosphine and here rate is increasing you can see here. It is 5 into 10 to the power of minus 2 and tolman angle is increasing that means you can see how tolman cone angle acids in understanding how quickly how easily a ligand can be dissociated from metal. When you go to trimethylphosphine its cone angle is 145 it completed dissociation happens within no time the moment you put into the solution. So that means in case if you take bulky ligands and even if you make it is coordinatively saturated with 8 electron species the moment we put into solution dissociation of 1 or 2 ligands would be very easy. In that case what happens we can generate 14 electron species or 16 electron species readily that is essentially a active catalyst and then oxidization can be initiated. So this is the advantage and this is how tolman cone angle can be correlated with catalytic efficiency of tertiary phosphines. Let us look into the influence of bite angle on catalytic efficiency in a catalytic reaction. So what is bite angle when you use a bidentate ligand when it collates to the same metal the angle generated by these two ligands is called bite angle and this separation between the two donor atoms is called bite separation. If the bite separation is larger the angle will also be larger that means if we have a very bulky linker between two phosphorus atoms then obviously what happens bite angle also increases as a result of increase in bite separation. So now if you take a typical square planar complex the angle should be 90 degrees and if the ligand when it forms a square planar complex so if you just see with bulky ligands what happens if the angle is increased here then what happens the distance between these two will shrink to compensate this one. So that means basically very bulky ligands would what happens decrease the distance between two groups if they are leaving groups then they come very closer and then in the reductive elimination process three bond concerted elimination would be very facile. So that means the ideal bite angle should be around 102 to 121 will make a bisphosphine ideal catalyst for both oxidative addition and also speed up the reductive elimination that is the important step in the coupling reactions. So larger bite angle facilitates the reductive elimination and hence the turnover numbers and also another advantage with these things are in case if we have some alkyl groups with beta hydrogen atoms it can also minimize beta hydrogen elimination because in the beta hydrogen elimination it is not giving any scope for the expansion of the coordination number of metal it can also minimize and also increase the stability of a metal complex especially if it is an organometallic compound. So now I have shown you with these graphics the advantage with large bite angles or large bite ligands for example you can see here this is of course this say 16 electron species with you know metal in a plus 2 state it is a D8 system and now you can see just focus your attention to this separation between these two ligands. Let us assume these two are ready for reductive elimination through the coupling between these two and in this case when you increase this bite angle so they come very closer and hence facilitate the concerted elimination so that reductive elimination will be much more facile it can come out. Now it is a photoelectron species and it is ready for second cycle of catalysis. Advantage with short bite ligands so now let us say we have a short bite this is then advantage with large bite ligand depending upon how we use it whether we have a large bite ligand we have a short bite ligand we can use according to our convenience if you know how we can use them in particular reaction. For example let us say we have short bite ligands when you short bite ligands with two phosphines separated by a small linker like DPPM say bisadiphenyl phosphino-methane where it forms a very strain 4 membered ring these chelate rings are unstable because of strain as a result if you generate a tetradentate tetra coordinated metal complex like this this should be tetrahedral in nature if it is nickel palladium or platinum the 10 system with 18 electrons. Now what happens the moment you put into the solution and try to add some reagents for oxidative addition because of ring strain it is likely that one of the phosphine to metal bond from each ligand would be cleaved to release the strain so that now it forms a photon electron species and still these phosphors are in close vicinity and you can perform oxidative addition and once reductive elimination happens you can get back this one. So that means whether we have a large bite ligand whether we have a short bite ligand if you know how to use them certainly we can use them in metal complexes and hence their utility in homogeneous catalysis. So here eta 2 ligand bidentate ligand becomes temporally modernated ligand and coordination sites are generated here and it is a photoelectron species and now you can see once reductive elimination is over this dangling phosphorous dora or atoms will come back and this can be regenerated. Now let us look into the specialty of phosphines I will give you a very interesting example here just look into this titanium octahedral complex two methyl groups are in axial position and four phosphorous atoms from two ligands they are this dimethyl phosphinoethane they are in the plane. So now these are all anionic ligands so this is a D2 system you know that 3D2 4S2 two electrons are gone it is a D2 system and D2 system means we have two electrons in the valence shell and we know from simple counting electrons that up to three electrons if we have they can be always whether we use strong field ligand or weak field ligand they always tend to be paramagnetic D1 D2 D3 system irrespective of what type of ligands we are using whether we are using strong field ligands or weak field ligands they remain paramagnetic. Now the question is this is an octahedral D2 complex is it possible to think it as a diamagnetic species very strange right how it happens we have two electrons are there under the influence of the sigma donor pi acceptor ligands what happens that degeneracy of T2G is destroyed in T2G we have DXY DXZ and DXY let us say two electrons would occupy DXY and as a result the energy of this one drops and these two will remain doubly degenerate at DXZ and DYZ now these electrons are paired unlike a typical D2 case where we have one electron each in two orbitals in that case it remains paramagnetic whereas here these two are paired now this orbital will conveniently overlap with sigma star of phosphine through to back donation so back donation happens that means this ligands these electrons will be taken by phosphorus sigma star orbitals phosphine mighty sigma star orbitals and back bonding happens because of this back bonding what happens despite having two electrons and D2 electronic configuration this titanium 2 plus compound is diamagnetic so now we do not have any unpaired electron these kind of unusual things can only happen with phosphines and of course D2 is a very strong pi donor favours metal to phosphorus pi bonding and MTPR sigma star are more stable and lower in energy they can readily interact with this one and take away their electron density and make it diamagnetic. So now let us compare transfer metal organic compounds or transfer metal or organometallic compounds with main group compounds having carbon. Let us look into the thermodynamic stability versus kinetic liability of organometallic compounds and also main group compounds where we have element to carbon bond so if you consider binary transfer metal complexes such as alkyl or aryl compounds complexes they are highly unstable and could not be made under normal conditions and also could not be stored at room temperature. For example, if you take tetraethyl methiam or tetraethyl titanium they can be stable only up to minus 60 degree centigrade and again among tetromethyl titanium and tetraethyl titanium tetraethyl is much more reactive and less stable compared to tetromethyl titanium. So but if you look into the bond parameters there is no difference between the transfer metal to carbon bond enthalpies and also a main group to carbon bond enthalpies. Transfer metal to carbon sigma bonds can be stabilized by including additional ligands such as cyclopentadienyl group or CO group or a phosphine or even halides that means transfer metal to carbon bond is weaker than transfer metal to halogen bond. So that means if you consider any of these things or even oxygen for that matter more electronegative these bonds are weaker compared to transfer metal to carbon bond. Despite these bonds are stable with respect to this one the reason and if you just see here the force constant measurements for metal to carbon sigma bonds shows that main group element to carbon bond as well as transfer metal to carbon sigma bonds can be comparable in strength having energy anywhere between 120 to 135 kilo joules per mole. That means why these transfer metal to carbon bonds are highly unstable compared to main group elements to carbon bond and if the bond enthalpies are essentially same the difficulties in handling organometallic compounds is not due to low thermodynamic stability but rather due to high kinetic liability. I repeat again the difficulties in handling and storing organometallic compounds is not due to low thermodynamic stability but rather due to their high kinetic liability. So what is the aspect that is responsible for them to make kinetically levied the culprit is beta hydrogen elimination. So they have easy pathways for decomposition and one such mechanism involving beta hydrogen elimination is shown in this figure. So let us say we have a metal bond group like this where we have beta hydrogen and it can establish a 4 membered intermediate in this fashion so that hydrogen starts interacting beta hydrogen starts interacting and in this one what happens this eventually this CH bond is cleaved and metal to hydrogen bond is established and now it has olefin formation takes place olefin binding and further what happens a hydrated compound comes and then your olefin comes out and then eventually this may decompose depending upon metal to hydrogen bond enthalpy. So that means here this is exactly opposite to hydrogenation reaction we come across on a metal center. So this kind of reactions are responsible for destabilizing organometallic compounds how to prevent that one yes you have to have some bulky groups like phosphine cyclopentadienyl or pentamethycyclo pentadienyl groups on metal so that further formation of this kind of 4 membered intermediate can be minimized or one can also look for organic moieties having no beta hydrogen atoms and of course how to know that beta hydrogen elimination is happening in a particular reaction you can see here for example you can take the labeled one this beta hydrogen is labeled with deuterium and if you see that one eventually that beta hydrogen should stay on metal so CUD is there and other one goes with this one. So this is how one can also analyze the beta hydrogen elimination process and is it reversible of course beta hydrogen elimination is reversible you can see here take this for example just look into this example here where we have 2 cyclopentadienyl groups on niobium and one ethylene group is there and one ethyl group is there this one on heating eliminates one of the ethylene group and it forms a then this C2H5 and it was beta hydrogen elimination to form a compound like this on addition of ethylene you can regenerate and of course this beta hydrogen elimination is reversible if not the utility of metal complexes in hydrogenation reaction would not have come very handy for homogeneous catalysts. So now the choice of the catalyst let us look into the choice of the catalyst for example if we have several very interesting variable key tertiary phosphines with us then why we should go for a bidentate ligand even when you go for bidentate ligand why we should not go for symmetric bidentate ligand instead we should go for unsymmetrical bidentate ligand or even with unsymmetrical bidentate ligand why we should go for heterodonor ligands donors with heterodonor functionalities that means one phosphorus can be there other one can be a nitrogen oxygen or sulfur donor. So let us look into the choice of the catalyst and choice of the ligands and choice of the combination of one or more donor atoms in such a way that that really facilitates homogeneous catalysis through various processes let us look into that one for catalysis why these phosphines are bidentate ligands why not monophosphines obviously this question would come into the mind and next yes if I answer satisfied and convince you the next question is why unsymmetrical are diphanxial ligands dppm is there fine why I should go for something else where two phosphorus varieties are different or one is phosphorus and another one is something else and why not symmetric phosphine I shall convince you about choice of the catalyst advantage of each one through these cartoons I am sure at the end of seeing these couple of slides you would be convinced about what I say about the choice of the catalyst or choice of a particular ligand in a metal complex for its utility in catalytic process. Now advantage of bidentate ligand first let us look into monodentate ligand so it assume this is a typical tetra case triphenyl phosphine palladium complex many organic commits a variety of organic transformation they use it is commercially available it is very unstable and especially it has to be handled under net atmosphere so when you take this one put into solution initially what happens it has to get rid of two ligand to generate an active species something like this and once you generate this one and these two ligands will be in solution whatever the organic medium you are taking and then it is not very easy to regenerate by bringing this back to establish this kind of coordination we are not talking about an isolated molecule here if you take even one more we have 6.523 into 10 to power of 23 molecules of complexes are there and then we have so much of solvent molecules are there in that case what happens we also talk about efficient collision frequency and all those things so considering all those things when these two ligands are dissociated again going back to establish this one is next to impossible or efficiency may drop as a result what happens in the first round let us have 100 such molecules were there in the second stream we may be left with 40 in the third we may be 20 so this catalytic cycle will diminish and within no time it becomes catalyst is no longer available for further catalytic process so that means catalytic turnover number and turnover frequency are going to be very very low and as regeneration is less effective when you use modern dead ligands on the other hand just look into these two bidentate ligands are there very similar to this one except its chelation is there and in this case two ligands are there here and two ligands are there bidentate ligand again in the 18 electron species so before we use it for catalysis we have to dissociate two bonds and when we dissociate two bonds what happens still to donor atoms in close vicinity of the metal center they are like dangling dangling and not going away from the metal center and they are continuously making efforts to come back to the metal to establish this chelation in that process what happens when this catalytic process is over initial oxidative addition and the retroalumination is over what happens they are still in the close vicinity and they are making an attempt they will come back and once they come back this catalyst can be regenerated very easily because it does not require much energy to bring back them to establish the bond this is the advantage of bidentate ligands regeneration is very effective on the other hand due to some reason what happens this dangling phosphorus atoms getting oxidized then it becomes p o when it forms p o what happens now between the soft metal is their heart center is there and of course in the absence of any other better ligand what happens even there can be a soft hard interaction and this chelation is completed a chelate effect will ensures that even a heart center like oxygen that can establish a bond as a result again 18 electron species is generated in fact this 18 electron species is much more reactive is going to be much more reactive compared to the previous compound here because your dissociation will be very facile because we have to cleave a hard and soft combination not soft soft combination so that means due to some reason what happens a freed coordination site getting oxidized to generate a p o and then when the catalyst is regenerated having o bonds to metal then that becomes double effective because dissociation can happen within no time so that is the advantage of unsymmetrical or heterodyne functional ligands or having a combination of soft hard donor ligands in this context what happens all people who are generating or designing ligands give importance to make not symmetrical ligands but unsymmetrical ligands or having labile donor functionalities are also called hemilabile ligands so let us discuss more about these things in my next lecture until then have an excellent time reading chemistry.