Questions about Low-Valence Chemistry
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| Anzahl der Teile | 99 | |
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| Identifikatoren | 10.5446/18755 (DOI) | |
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Chymiatrie56 / 99
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00:00
MagnesiumHeterodimereComputeranimation
00:21
MähdrescherMetallOxidschichtWasserstoffHydroxideHauptgruppenelementMagnesiumChemische VerbindungenAktionspotenzialAtombindungHydrideReaktionsmechanismusReduktionsmittelAlkalimetallNebenproduktMagnesiumhydridKonkrement <Innere Medizin>Systemische Therapie <Pharmakologie>Chemische EigenschaftChemische ReaktionLigandHalogenideIonenbindungLigand <Biochemie>Vorlesung/Konferenz
02:44
BiosyntheseMetallNaturstoffOrganische ChemieTrennverfahrenFlüssigkeitsfilmOberflächenchemieOxideMagnesiumQuerprofilChemische VerbindungenQuarz <alpha->ReduktionsmittelSamariumSterische HinderungSubstrat <Chemie>ReaktivitätSetzen <Verfahrenstechnik>WursthülleSpezies <Chemie>Systemische Therapie <Pharmakologie>VerrottungMetallorganische ChemieLigand <Biochemie>Alkoholische LösungKörpertemperaturGesundheitsstörungIsoliergasStereoselektivitätSekundärstrukturKompressionsmodulEnergiearmes LebensmittelAktivität <Konzentration>Elektronische ZigaretteComputeranimationVorlesung/KonferenzBesprechung/Interview
06:06
Chemische ReaktionLigandMetallOxidschichtPharmazieVitaminCalciumZusatzstoffMagnesiumQuerprofilChemische VerbindungenAtombindungBerylliumCobaltoxideMetallbindungGap junctionReaktivitätSetzen <Verfahrenstechnik>GermaneSpezies <Chemie>Systemische Therapie <Pharmakologie>f-ElementStromschnelleChemische ForschungToxizitätFunktionelle GruppeTerphenyleBenzolringIonenbindungLigand <Biochemie>MultiproteinkomplexComputeranimationVorlesung/KonferenzBesprechung/Interview
08:29
MetallAlkalitätLigand <Biochemie>AdenosylmethioninWursthülleChemische ReaktionDiamantCalciumMagnesiumChemische VerbindungenAzokupplungBerylliumRadikalfängerReaktionsmechanismusReduktionsmittelSetzen <Verfahrenstechnik>IonenbindungPräkursorSystemische Therapie <Pharmakologie>Ligand <Biochemie>Aktivität <Konzentration>MultiproteinkomplexLigandMetallCalciumhydroxidChemischer ProzessFarbenindustrieHalogenideMetallbindungSchmierfettDisproportionierungTubeHeterodimereComputeranimationVorlesung/Konferenz
10:47
RedoxpotentialChemische ReaktionMetallMolekülOxidschichtTieftemperaturtechnikOrganisches LösungsmittelOberflächenchemieMagnesiumPhasengleichgewichtChemische VerbindungenKaliumQuarz <alpha->GraphitOrganischer HalbleiterPeriodateReduktionsmittelSubstrat <Chemie>AlkalimetallNebenproduktPräkursorSystemische Therapie <Pharmakologie>Bukett <Wein>Elektron <Legierung>FilterLigandStöchiometrieLithiumsalzeHalogenideNatriumToluolStereoselektivitätBeta-FaltblattMetallorganische ChemieComputeranimationVorlesung/Konferenz
13:06
AluminiumAmmoniakCVD-VerfahrenChemisches ElementMetallOrganische ChemieOxidschichtWasserstoffWerkstoffkundeHauptgruppenelementInselQuerprofilGermaniumChemische VerbindungenChemischer ProzessFunktionelle GruppeSubstrat <Chemie>ÜbergangszustandWursthülleOrdnungszahlSystemische Therapie <Pharmakologie>OxygenierungAtomclusterÜbergangsmetallEthylenEisbergThermoformenIonenbindungComputeranimationVorlesung/KonferenzBesprechung/Interview
16:05
Quellgebiet
Transkript: Englisch(automatisch erzeugt)
00:00
So when we had prepared the magnesium 1 dimers obviously we needed to prove the
00:29
presence of a metal-metal bond there and more specifically the absence of bridging hydride ligands okay so you in the reduction of main group metal
00:41
halides with alkali metals which is what we were doing to access these systems it's often common that you get hydrogen abstraction from the solvent or the ligand and you end up with hydride ligands attached to the metal you know to try and keep their oxidation state at what they like to be at and so the potential was there that we had ligand magnesium magnesium ligand
01:03
with two bridging hydrides and we really needed to prove that we didn't have any hydride ligands there and they're notoriously difficult to detect crystallographically and in some times spectroscopically and so we spent probably a year and a half trying to make the magnesium hydride compounds
01:22
because we thought that these would have different properties to the magnesium 1 compounds and if we could do that then that was proof and it is remarkable to me that making the magnesium 1 systems was very relatively easy making the magnesium hydride compounds which you would expect
01:40
probably as being the byproducts in these reactions took us a long time to do it but we didn't we managed to do it and we've showed that they are completely different properties and then we also obtained neutron diffraction studies on the magnesium 1 systems which showed the absence of hydride ligands so that only proves the absence of hydride ligands it doesn't
02:03
necessarily the prove the absence or the oxidation state of the magnesium compounds because you could because you could have reduction of the ligand for example and so we then then used DFT calculations with collaborators in combination with these experimental charge density studies to try and
02:22
locate and effectively see the electron density between the magnesium centers and that's that's how we I mean you can never totally prove these things but that's that's what we believe to be the situation now so we believe we have a covalent bond albeit a very diffuse covalent bond between the two
02:40
magnesium centers there's certainly no hydride ligands present well they're more reactive than grignard reagents but as I say they're really remarkably or their reactivity is much less than we expected I mean when we set out to
03:01
prepare these compounds we thought well we weren't terribly optimistic to tell you the truth we thought well we're going to give it a go and we'll spend some time on this but if it doesn't work then we wouldn't surprise us and it turned out that they really are very stable species and of course you can change the reactivity of the compounds by changing the steric
03:22
profile of the ligands that are coordinated to the di magnesium fragment and in fact if we use very bulky ligands and are coordinated to the di magnesium fragment they are almost air stable you can take the compounds and leave them in the air as a crystalline solid not in solution not as
03:42
not as solutions obviously but as a crystalline solid you can take crystals of the compounds leave them out in the air for an hour or so come back and there's just a very thin layer of oxide material on the surface so they're really not terribly air sensitive which is remarkable thermally very stable as I say some of these compounds that they don't
04:04
decompose until 300 degrees Celsius when they do they disproportionate as you might expect so they generate magnesium metal and magnesium two compounds and magnesium two compounds containing two of the ligands coordinated to the magnesium centers that the ligands that we're using and
04:22
in fact in one case you can even sublime the compound okay so you can sublime it at a temperature of about 320 degrees Celsius so it's going from the solid phase into the gas phase at this very high temperature and then re-condensing so very stable compounds for what they are we because
04:42
of that we are looking at their reactivity we're doing the reverse and we're trying to reduce the steric bulk of the compounds of the ligands coordinated to the di magnesium fragments and that that makes them more reactive species and so we're trying to find the limit of balancing the reduction in the steric bulk of the ligands to prepare stable magnesium one
05:06
systems as opposed to compounds that will disproportionate and we think we've done that now and we have several compound types that are more reactive than the first systems that we prepared and we're using those as the reducing agents in the organic synthesis and the organometallic
05:24
synthesis side of things and we we're also looking at trying to develop chiral ligands to coordinate to the magnesium centers because we believe that this might be able to give rise to an antioselectivity in the reductions that we're carrying out of for example organic substrates and
05:41
remember I said that a number of the products that we get from these reductions differ significantly from the the products that you get when you reduce the same substrates with with for example samarium 2 reagents and so we think there's potential there for using these systems for example in
06:00
natural products synthesis and that's that's another area that we are looking at at the moment. No I think it will it will develop quite rapidly and I'm sure these systems will be in the next generation of inorganic texts
06:20
as as you know first examples of stable species containing low oxidation states from the S block and but I think more than that I think they their chemistry will develop if we look at if we compare it to the rapid development of low oxidation state P block chemistry where that went and the applications those systems are finding I think we'll see a similar development
06:42
in S block chemistry again it's the once the first realization that such species can be accessed and have remarkable thermal stability and oxygen and moisture stability then I think people will start to look at that at this area and indeed that's happening already other examples of magnesium 1
07:04
compounds have been reported by a Taiwanese group although they haven't looked at the further chemistry of those systems yet and indeed the first calcium 1 compound was reported last year by a German chemist Westerhausen who is not a it doesn't contain a calcium-calcium bond but it's
07:22
formally a calcium 1 compound formed from the reaction of highly activated calcium metal with triphenyl benzene so this gives rise to a sandwich complex where we have the triphenyl benzene calcium on the top calcium on the bottom and some THF ligands on those calcium centers and it and the the
07:43
the bridging ligand is doubly reduced so it's a paramagnetic system obviously we'll have some interesting reactivity doesn't really contain a true calcium calcium covalent bond obviously we've tried to make such systems extend the chemistry of magnesium to calcium and indeed to
08:01
beryllium forming beryllium beryllium bonded compounds and that chemistry obviously has its own problems with regard to the toxicity of beryllium but we've spent quite a bit of effort to look at these compounds we haven't achieved it yet and we we think we know why that is and so at the moment we're developing a I suppose a second generation of ligand types to
08:23
try and protect the eventual metal metal bonds in these systems from attack definitely the case and that's that's the reason why we haven't been able to access beryllium or calcium 1 compounds we believe at the moment so
08:43
the the magnesium magnesium bonds are about 2.85 angstroms it's been predicted that calcium calcium bonds are going to be about 3.9 angstroms so that's very long an angstrom longer than in the magnesium magnesium systems and in the beryllium case they're much shorter about just a little over two
09:00
angstroms okay so I believe the problem with beryllium is so far that the bond is too short okay so we when we have a bulky ligand attached to the beryllium fragment obviously when we reduce the the ligand beryllium halide precursor that's going to generate a ligand beryllium radical
09:20
beryllium one radical and two of those radicals need to couple to form the beryllium beryllium bonded system and and we've tried some some reductions and we see in the early stages of these reductions very bright deep blue colors to those reductions and we're almost certain that this is due to beryllium one radicals however the ligands we're using don't don't allow
09:44
the coupling of these two radicals to form beryllium one dimers and instead intramolecular CH activation reactions are going on effectively the beryllium radical is eating the ligand and that's what gives rise to very complex reaction mixtures in the calcium case we believe the ligands aren't big
10:03
enough so we're reducing the ligand calcium halide precursors and potentially the dimers are forming but we have this very long bond calcium calcium bond about 3.9 angstroms it's also theory suggests it's going to be weaker than the magnesium magnesium bonds the magnesium magnesium bonds are
10:20
about 45 kilo cals per mole I think the calcium calcium bonds should be about 30 kilo cals per mole so they're open for attack they're also weaker so we need to protect those metal metal bonds upon formation and so we're looking at more three-dimensional ligands if you like to to form a tube of grease if you like around the magnesium around the calcium calcium bond to
10:43
protect it from attack and from disproportionation processes I think with respect to the reduction of organometallic compounds you know to generate low oxidation state p-block systems these generally involve the
11:04
reduction of ligand metal halide systems and the classical reducing agents that have been used for this purpose are the alkali metal so potassium metal sodium metal or lithium metal or other systems like potassium graphite okay so these these reducing agents are firstly quite harsh
11:22
they they have quite they're quite strong reducing agents they're also solids okay so you're getting reactions at the surface of the metal or the potassium graphite so it's not terribly controlled it's also sometimes difficult to deliver them stoichiometrically so you need to weigh out very small amounts sometimes of of these alkali metals or potassium
11:43
graphite to stoichiometrically reduce compounds because if you have an excess of these reagents you can perhaps generate the compound you want but that goes on to react further with the excess alkali metal that is present so with the magnesium one systems firstly the I think they're milder reducing
12:04
agents than the alkali metals they're also soluble okay so you can do them in coordinating or largely in non-coordinating solvents so they're soluble in in toluene where a lot of the precursor molecules are so you can carry out reductions at low temperature in a single phase with a
12:22
milder reducing agent and you can stoichiometrically deliver electrons if you like to those substrates and I think that's the big advantage in these systems and also the byproducts so the beta diketanate magnesium halide byproducts are dimeric systems they encapsulate the the magnesium
12:42
halide if you like that's being generated and these these byproducts are often not very soluble so they precipitate out from the reaction mixture you can filter off the product and simply crystallize so I think they're they're more selective they're stoichiometric they're milder
13:01
reducing agents and all of these things together we like to call them bespoke reducing agents. Well that of course remains to be seen but I think we're just starting to get a hint of their potential uses at the moment I
13:20
mean they as I say they have we've now shown that we can prepare main group compounds with main group elements in a range of oxidation states not just one or two oxidation states and we can see that we can shuttle between those oxidation states for example in the reversible addition of ethylene to low oxidation state germanium systems as has been previously or
13:46
recently shown if we can do the same with hydrogen and ammonia and other substrates we can use these compounds for catalysis and indeed this is starting to be shown already main group compounds these so-called frustrated
14:01
Lewis pairs that have been developed by Doug Stefan's group in in Canada and now being used as catalysts for example for the hydrogenation of unsaturated organic substrates this is just this is just the tip of the iceberg there are many many possibilities to come I think and I think if we're going to be
14:21
very optimistic we could say that p-block systems could be used to replace many transition metal catalysts in the future and if this is the case then it will affect all of us because as we know everything around this is is has some involvement with transition metal catalysts in their in their
14:40
production but of course it really does remain to be seen I think there's also a good future for low oxidation state systems in in the preparation of materials and the understanding of materials and just one example is the work of Schnurkel in Karlsruhe who's developed these huge group 13 clusters
15:04
so he can form clusters of for example aluminium atoms which contains 70 in one case in it you know in his famous example 77 aluminium atoms I think 57 of those only have aluminium aluminium bonds so it's effectively a blob of
15:24
aluminium metal with an organic coat so what he's doing is is looking at how metal is building up upon its upon its deposition and using these systems to model for example how metal is deposited in chemical vapor deposition processes which are important in micro electronics and and other industries so I
15:46
think it it remains open to be seen what the future of this area is but I think it's I think it's quite exciting and I think even if even if the applications are not realized the fundamental interest in the compounds necessitate the future development of the field
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