Modern Main Group Chemistry
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Jones, Cameron
Jerabek, Paul
Hegemann, Julian
Authmann, Andreas
Prof. Jones (Monash University, Australia) talks about new approaches in the field main group chemistry.
 We've been involved in low oxidation state main group chemistry probably for 15 years or so now and about three years ago we began to wonder if we could extend this low oxidation state chemistry of the P block elements to the S block elements and this had largely not been done before there were no examples of compounds containing two S block element or two S block elements bonded to each other covalently and we wondered if we could perhaps extend the chemistry the well-known chemistry of low oxidation state P block compounds to this area and I suppose again this harks back to my undergraduate days where I was looking at the formation of new Grignard reagents and thinking about the mechanism of formation of Grignard reagents and the fact that magnesium-magnesium bonded compounds had been proposed as intermediates in the formation of Grignard reagents. So we wanted to see if we could actually make such systems and we drew some inspiration I suppose from another landmark study in 2004 by a Spanish chemist Ernesto Carmona who managed to prepare the first examples of zinc-zinc bonded compounds. So formally compounds containing zinc in the plus one oxidation state and surprisingly he made these compounds or this particular compound which has a zinc-zinc bond. The two zinc centers are coordinated each by a pentomethyl cyclopentadienyl ligand, a bulky ligand to stabilize this system towards disproportionation. But he made this compound actually by accident and one of the reagents that he used in its preparation was diethyl zinc, the same compound that Franklin made in 1850. So given the chemical similarities I suppose between zinc, a group 12 metal and magnesium, a group 2 metal and the fact that zinc-zinc bonds could be stabilized we then thought well maybe we could use some of these, some of our bulky ligands that we'd developed to stabilize low oxidation state P-block compounds for example gallium-1 compounds and germanium-1 compounds and maybe these compounds could kinetically stabilize magnesium-magnesium-bonded systems and that's what we set out to do. And the main ligand types that we used were di-, sorry, chelating and enkealating mono-anionic ligand systems such as bulky guanidinate ligands which we developed in our laboratory and also beta-dichetanamate ligands which we found had similar stabilizing properties to our bulky guanidinate. And so we began by preparing magnesium-2 precursors to our target magnesium-1 compounds and these were magnesium iodide systems containing or incorporating the bulky guanidinate or beta-dichetanamate ligands. And we simply started by trying to reduce these systems with potassium metal at room temperature and to our surprise in early experiments we managed to prepare these magnesium-magnesium-bonded systems. I think if we didn't have results early on we might have abandoned this study because we thought intuitively that these systems would be pretty hard to stabilize and pretty hard to eventually access but that proved not to be the case. So we managed to prepare a range of such systems. They are quite reactive as you might expect but they're not terribly air and moisture sensitive so these ligands that we've used to stabilize them really do protect them from oxidation and hydrolysis. They're remarkably thermally stable. Theoretical studies have suggested that the magnesium-magnesium bonds in these systems are quite strong but they're not remarkably so, about 45 kcal per mole. But these compounds in some cases can be stable at 300 degrees Celsius which to us seemed really quite remarkable. And once we had prepared these compounds we thought we must really prove that we have magnesium-1 systems. This is quite a big claim and would not look good if we were wrong. And so we spent a long time trying to prove that these systems had magnesium-magnesium covalent bonds and I'm not going to go into what we did to do that but we managed to do that and we published the work I think in the last week of 2007 in science. And after that time we thought well we now have these systems we really must look at their further chemistry and their properties. And really this work is still in its infancy but we have looked in some detail using experimental and theoretical techniques to try and analyze the metal-metal bonding in these systems. And again this work is only in its infancy but we've used DFT calculations in the theory sense and experimental charge density studies in the experimental sense and this technique allows you effectively to see the electrons between the magnesium centers. And this shows that these systems do indeed contain magnesium-magnesium covalent bonds albeit with rather diffuse electron density between the magnesium centers but we believe certainly a covalent bond. And so we've also found that this although there is electron density between, shared between the two magnesium centers it's really quite diffuse, in fact very diffuse and this has led to some strange properties for these compounds. The magnesium-magnesium bond is what we call deformable. We can stretch it quite significantly for example by coordinating the magnesium centers by other Lewis bases. We can stretch the bonds by up to about 8%. So we can elongate them from about 2.85 angstroms in the uncoordinated dimers to about 3.05 angstroms in the coordinated dimers which is a remarkable elongation. So that's one area that we've been looking at. We've also been looking at the use of magnesium-1 systems as reducing agents. Obviously with this element in this unusual oxidation state you would expect that these dimeric systems would be able to deliver electrons to substrates and we are indeed looking at that with respect to their use as what we call bespoke reducing agents in both organic and organometallic synthesis. So in organic synthesis we've shown that these dimeric systems can act as very facile to center two electron reducing agents. They can deliver if you like two electrons very easily to unsaturated organic substrates. We've looked at many reactions and we've found that we can induce carbon-carbon bond forming reactions, nitrogen-nitrogen bond forming reactions. We can carry out oxidative insertion reactions. We can carry out reductive cleavage reactions and we've seen a whole range of reaction types within organic synthesis if you like using these magnesium-1 systems. And I think most importantly many of the products of these reductions differ from those that you obtain by reducing the same substrates with more classical reducing agents used in organic synthesis such as Sumerium II reagents and Alkaline Metals. And because of this we think that there is a potential use of these systems as selective reducing agents in organic synthesis. And we are developing that at the moment in collaboration with organic chemists. With respect to inorganic chemistry we're also investigating the use of these systems to as reducing agents to access previously unknown examples of low oxidation state P-block complexes. So we're using these magnesium-1 reagents to try and prepare the systems that got us into this area in the first place. And we've had quite a bit of early success and just one example of a compound we've published. I think late 2009 we took an N-heterocyclic carbene adduct of germanium dichloride, a simple compound, and we reduced it with our magnesium-1 compound. This worked and this generated a magnesium-2 chloride system. But the byproduct in this system was a compound that contained two N-heterocyclic carbines coordinated to a germanium-2 fragment. No other substituents on the germanium. So formally this compound contains germanium in the zero oxidation state. So a very unusual compound. You could think of it as a soluble source of this element. So at the moment we're trying to extend it to other elements in the P-block and indeed in the D-block. And we can see these N-hc adducts of the elements if you like as soluble sources of those elements that can be delivering those elements to other reactants in synthesis. And this is something that we've really only just begun work on. There are a number of other groups working on this area of chemistry around the world. Greg Robinson's group in Georgia for example who really whose excellent work got us into this area. So that's another area of chemistry that we've been looking at with our magnesium-1 compounds. And another main area that we are looking at is using these magnesium-1 compounds as soluble models if you like to examine the mechanisms or potentially examine the mechanisms and the kinetics of the hydrogenation of magnesium metal. So magnesium metal reacts with dihydrogen to give magnesium hydride. This is an important reaction. It's a reversible reaction and it's important because magnesium dihydride contains about 7.7% by weight hydrogen. And so therefore it's finding use as a hydrogen storage system in a number of devices such as fuel cells, etc. So it's a reversible hydrogen storage system and in the rapidly developing hydrogen economy, such systems are quite important. So this hydrogenation of magnesium metal to give magnesium hydride has problems. It has kinetic problems. The kinetics are slow and in fact you need a temperature of about 300 degrees Celsius to hydrogenate magnesium metal. And to dehydrogenate magnesium hydride you need about 300 degrees or greater than 300 degrees Celsius as well. So really it's obvious that you can't use these systems in portable devices. And so a lot of work has been carried out to try and improve the kinetics of the hydrogenation of magnesium metal. For example by doping it with transition metals or alloying it with P-block metals, for example aluminium. And this tends to work in some cases and the hydrogenation temperatures of magnesium alloys have been reduced into the hundreds of degrees, which is a usable range. But it's not really known what the mechanisms or the reasons behind this improvement in the kinetics of the hydrogenation of magnesium occur. And so we're beginning to wonder if we can use these magnesium 1 systems as soluble sources of magnesium if you like to try and look at how the magnesium is hydrogenated in the presence of other metals. And this is an area that we have looked at in the last six months or so. We haven't published anything on it yet, but hopefully we will be as soon doing this. So I'm not going to dwell on that. So really that's all I wanted to talk about today. I wanted to give you just a bit of an overview of this rapid development in main group chemistry over the last 30 years or so. I think it's gone from being a rather staid and perhaps boring area of chemistry to what I think is one of the most exciting areas of and rapidly developing areas of inorganic chemistry that is studied today. And I think this renaissance in main group chemistry can tell us one lesson. And that is as chemists, when we see rules in textbooks, maybe we should question those rules because if we do question those rules and we prove that they shouldn't be rules, then we can access new areas of chemistry, very interesting new compounds which have potential applications. And if they don't, well, they have enough fundamental interest to keep us excited.
 We've been involved in low oxidation state main group chemistry probably for 15 years  or so now and about three years ago we began to wonder if we could extend this low oxidation  state chemistry of the P block elements to the S block elements and this had largely not  been done before there were no examples of compounds containing two S block element or  two S block elements bonded to each other covalently and we wondered if we could perhaps  extend the chemistry the well-known chemistry of low oxidation state P block compounds to  this area and I suppose again this harks back to my undergraduate days where I was looking  at the formation of new Grignard reagents and thinking about the mechanism of formation  of Grignard reagents and the fact that magnesium-magnesium bonded compounds had been proposed as intermediates  in the formation of Grignard reagents.  So we wanted to see if we could actually make such systems and we drew some inspiration  I suppose from another landmark study in 2004 by a Spanish chemist Ernesto Carmona who managed  to prepare the first examples of zinc-zinc bonded compounds.  So formally compounds containing zinc in the plus one oxidation state and surprisingly  he made these compounds or this particular compound which has a zinc-zinc bond.  The two zinc centers are coordinated each by a pentomethyl cyclopentadienyl ligand, a  bulky ligand to stabilize this system towards disproportionation.  But he made this compound actually by accident and one of the reagents that he used in its  preparation was diethyl zinc, the same compound that Franklin made in 1850.  So given the chemical similarities I suppose between zinc, a group 12 metal and magnesium,  a group 2 metal and the fact that zinc-zinc bonds could be stabilized we then thought  well maybe we could use some of these, some of our bulky ligands that we'd developed to  stabilize low oxidation state P-block compounds for example gallium-1 compounds and germanium-1  compounds and maybe these compounds could kinetically stabilize magnesium-magnesium-bonded  systems and that's what we set out to do.  And the main ligand types that we used were di-, sorry, chelating and enkealating mono-anionic  ligand systems such as bulky guanidinate ligands which we developed in our laboratory and also  beta-dichetanamate ligands which we found had similar stabilizing properties to our  bulky guanidinate.  And so we began by preparing magnesium-2 precursors to our target magnesium-1 compounds and these  were magnesium iodide systems containing or incorporating the bulky guanidinate or beta-dichetanamate  ligands.  And we simply started by trying to reduce these systems with potassium metal at room  temperature and to our surprise in early experiments we managed to prepare these magnesium-magnesium-bonded  systems.  I think if we didn't have results early on we might have abandoned this study because  we thought intuitively that these systems would be pretty hard to stabilize and pretty  hard to eventually access but that proved not to be the case.  So we managed to prepare a range of such systems.  They are quite reactive as you might expect but they're not terribly air and moisture  sensitive so these ligands that we've used to stabilize them really do protect them from  oxidation and hydrolysis.  They're remarkably thermally stable.  Theoretical studies have suggested that the magnesium-magnesium bonds in these systems  are quite strong but they're not remarkably so, about 45 kcal per mole.  But these compounds in some cases can be stable at 300 degrees Celsius which to us seemed  really quite remarkable.  And once we had prepared these compounds we thought we must really prove that we have  magnesium-1 systems.  This is quite a big claim and would not look good if we were wrong.  And so we spent a long time trying to prove that these systems had magnesium-magnesium  covalent bonds and I'm not going to go into what we did to do that but we managed to do  that and we published the work I think in the last week of 2007 in science.  And after that time we thought well we now have these systems we really must look at  their further chemistry and their properties.  And really this work is still in its infancy but we have looked in some detail using experimental  and theoretical techniques to try and analyze the metal-metal bonding in these systems.  And again this work is only in its infancy but we've used DFT calculations in the theory  sense and experimental charge density studies in the experimental sense and this technique  allows you effectively to see the electrons between the magnesium centers.  And this shows that these systems do indeed contain magnesium-magnesium covalent bonds  albeit with rather diffuse electron density between the magnesium centers but we believe  certainly a covalent bond.  And so we've also found that this although there is electron density between, shared  between the two magnesium centers it's really quite diffuse, in fact very diffuse and this  has led to some strange properties for these compounds.  The magnesium-magnesium bond is what we call deformable.  We can stretch it quite significantly for example by coordinating the magnesium centers  by other Lewis bases.  We can stretch the bonds by up to about 8%.  So we can elongate them from about 2.85 angstroms in the uncoordinated dimers to about 3.05  angstroms in the coordinated dimers which is a remarkable elongation.  So that's one area that we've been looking at.  We've also been looking at the use of magnesium-1 systems as reducing agents.  Obviously with this element in this unusual oxidation state you would expect that these  dimeric systems would be able to deliver electrons to substrates and we are indeed looking at  that with respect to their use as what we call bespoke reducing agents in both organic  and organometallic synthesis.  So in organic synthesis we've shown that these dimeric systems can act as very facile  to center two electron reducing agents.  They can deliver if you like two electrons very easily to unsaturated organic substrates.  We've looked at many reactions and we've found that we can induce carbon-carbon bond  forming reactions, nitrogen-nitrogen bond forming reactions.  We can carry out oxidative insertion reactions.  We can carry out reductive cleavage reactions and we've seen a whole range of reaction types  within organic synthesis if you like using these magnesium-1 systems.  And I think most importantly many of the products of these reductions differ from those that  you obtain by reducing the same substrates with more classical reducing agents used in  organic synthesis such as Sumerium II reagents and Alkaline Metals.  And because of this we think that there is a potential use of these systems as selective  reducing agents in organic synthesis.  And we are developing that at the moment in collaboration with organic chemists.  With respect to inorganic chemistry we're also investigating the use of these systems  to as reducing agents to access previously unknown examples of low oxidation state P-block  complexes.  So we're using these magnesium-1 reagents to try and prepare the systems that got us  into this area in the first place.  And we've had quite a bit of early success and just one example of a compound we've published.  I think late 2009 we took an N-heterocyclic carbene adduct of germanium dichloride, a  simple compound, and we reduced it with our magnesium-1 compound.  This worked and this generated a magnesium-2 chloride system.  But the byproduct in this system was a compound that contained two N-heterocyclic carbines  coordinated to a germanium-2 fragment.  No other substituents on the germanium.  So formally this compound contains germanium in the zero oxidation state.  So a very unusual compound.  You could think of it as a soluble source of this element.  So at the moment we're trying to extend it to other elements in the P-block and indeed  in the D-block.  And we can see these N-hc adducts of the elements if you like as soluble sources of  those elements that can be delivering those elements to other reactants in synthesis.  And this is something that we've really only just begun work on.  There are a number of other groups working on this area of chemistry around the world.  Greg Robinson's group in Georgia for example who really whose excellent work got us into  this area.  So that's another area of chemistry that we've been looking at with our magnesium-1  compounds.  And another main area that we are looking at is using these magnesium-1 compounds as  soluble models if you like to examine the mechanisms or potentially examine the mechanisms  and the kinetics of the hydrogenation of magnesium metal.  So magnesium metal reacts with dihydrogen to give magnesium hydride.  This is an important reaction.  It's a reversible reaction and it's important because magnesium dihydride contains about  7.7% by weight hydrogen.  And so therefore it's finding use as a hydrogen storage system in a number of devices such  as fuel cells, etc.  So it's a reversible hydrogen storage system and in the rapidly developing hydrogen economy,  such systems are quite important.  So this hydrogenation of magnesium metal to give magnesium hydride has problems.  It has kinetic problems.  The kinetics are slow and in fact you need a temperature of about 300 degrees Celsius  to hydrogenate magnesium metal.  And to dehydrogenate magnesium hydride you need about 300 degrees or greater than 300  degrees Celsius as well.  So really it's obvious that you can't use these systems in portable devices.  And so a lot of work has been carried out to try and improve the kinetics of the hydrogenation  of magnesium metal.  For example by doping it with transition metals or alloying it with P-block metals, for example  aluminium.  And this tends to work in some cases and the hydrogenation temperatures of magnesium alloys  have been reduced into the hundreds of degrees, which is a usable range.  But it's not really known what the mechanisms or the reasons behind this improvement in  the kinetics of the hydrogenation of magnesium occur.  And so we're beginning to wonder if we can use these magnesium 1 systems as soluble sources  of magnesium if you like to try and look at how the magnesium is hydrogenated in the presence  of other metals.  And this is an area that we have looked at in the last six months or so.  We haven't published anything on it yet, but hopefully we will be as soon doing this.  So I'm not going to dwell on that.  So really that's all I wanted to talk about today.  I wanted to give you just a bit of an overview of this rapid development in main group chemistry  over the last 30 years or so.  I think it's gone from being a rather staid and perhaps boring area of chemistry to what  I think is one of the most exciting areas of and rapidly developing areas of inorganic  chemistry that is studied today.  And I think this renaissance in main group chemistry can tell us one lesson.  And that is as chemists, when we see rules in textbooks, maybe we should question those  rules because if we do question those rules and we prove that they shouldn't be rules,  then we can access new areas of chemistry, very interesting new compounds which have  potential applications.  And if they don't, well, they have enough fundamental interest to keep us excited.
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