Authors: Christian Jandl
Source: mediaTUM – the media and publications repository of the Technical University Munich (Technical University Munich) (article, 2018-01-01)
Field: Chemistry — Organic Chemistry

Carbocyclic Carbenes and Beyond: Functionalised Cycloheptatriene Systems as Versatile Ligands in Palladium Chemistry Carbocyclic carbenes represent a relatively unknown class of carbene ligands compared to N-heterocyclic carbenes (NHCs), which have firmly established themselves as one of the most versatile ligand classes in organometallic chemistry.Nevertheless, the group of Pd-cycloheptatrienylidene(CHT) complexes proved to be a highly interesting motif and in this thesis a diverse structural chemistry could be developed around them.In continuation of former studies, the reactivity of the CHT ligand towards N-nucleophiles and the associated rearrangement of the carbene ligand to an allyl binding mode was utilised as the key step to prepare a broad variety of imidazolium-and pyridinium-substituted 3 -cycloheptatrienide-Pd complexes.Unlike previously proposed, the resulting complexes were shown to have a monomeric structure of the type [PdBr2( 3 -C7H6-Im/pyr)] due to the zwitterionic nature of the organometallic ligand.Computational studies revealed a highly dynamic equilibrium between different regioisomers of the allyl bond in solution and the -isomer (with regard to the distance of the central allyl-C atom from the substituent) was found to be most favourable, in agreement with crystallographic results.In order to overcome the drawback of their poor solubility, partial exchange of bromide ligands for non-coordinating anions was identified as a suitable way to obtain dimeric halide-bridged complexes of type [Pd(-Br)( 3 -C7H6-Im/pyr)]2(BF4)2 with greatly enhanced solubility.In the case of the derivative {[PdBr( 3 -C7H6-Im-DiPP)](BF4)}n it could be demonstrated that the dimer co-exists in an equilibrium with a tetramer featuring a Pd4Br4-macrocycle, which might not only have implications for this class of compounds, but also for dimeric Pd complexes with halide bridges in general.In the course of these structural studies, also the -isomer could be crystallographically observed in a few cases, mainly due to packing effects favouring it over the otherwise more stable -isomer.Further investigation of anion exchange reactions showed that a complete removal of bromide leads back to a monomeric structure with coordinating solvent molecules, [Pd(MeCN)2( 3 -C7H6-Im/pyr)](BF4)2.With respect to further synthetic utilisation, particular attention was paid to dimers with a Pd:Br ratio of 2:3, [{PdBr( 3 -C7H6-Im/pyr)}2(-Br)](BF4), featuring two terminal and one bridging bromide ligand, which causes a close Pd-Pd interaction.Although their isolation and full characterisation was only possible in one case, their existence in an equilibrium with a mixture of monomers and Pd:Br = 2:2 dimers made it possible to use such a mixture of readily available compounds instead.Imidazolium-and pyridinium-substituted 3 -cycloheptatrienide-Pd complexes represent a highly interesting platform for Pd chemistry with functionlised cycloheptatriene-based ligand systems.As such, they were successfully employed in the synthesis of functionalised trinuclear Abstract IV Pd sandwich complexes of type [Pd3Br3(C7H6-Im/pyr)2](Br/BF4), for which several strategies were tested.A generally applicable route was developed based on the dimeric precursors with a Pd:Br ratio of 2:3, the particular aptitude of which can be attributed to the metal-metal interaction, which prearranges the Pd atoms for the cluster formation.For such a precursor or intermediate the term "pre-cluster" was introduced.Electrochemical and computational studies gave insights into the electronic structure of the Pd3 sandwich cluster motif, indicating formally triply anionic seven-membered rings and a non-integer formal oxidation state of +2.666 for the Pd atoms.Significant differences in the coordination environments and electron densities allowed the conclusion that the most accurate description based on integer formal oxidation states is a mixed valence cluster of one Pd(II) and two Pd(III) ions.The intermediate formal oxidation state makes them promising candidates for future applications in catalysis.Additionally, the imidazolium-substituted derivatives offer the chance to convert the imidazolium sidearms to NHCs and thus introduce further metal atoms on the sidearms.As a proof of principle, this was performed with Pd, resulting in a Pd3 sandwich cluster complex extended by a fourth Pd(II) between the two NHC sidearms.Electrochemical characterisation showed two different reduction potentials, thus confirming the interpretation that the formal oxidation state of the Pd3 cluster is more positive than for the Pd(II) ion bound on the sidearms.This structural motif could also allow the combination of Pd3 clusters with other metals, which would create a universal platform for heterobimetallic Pd3M systems.The scope of relevant target structures accessible from imidazolium-substituted 3 -cycloheptatrienide-Pd complexes is, of course, not limited to functionalised sandwich clusters.They could also be utilised as precursors in the synthesis of the first hybrid carbocyclic/N-heterocyclic carbene ligand by a two-step deprotonation/hydride abstraction sequence.In the first step, the imidazolium sidearm of the monomeric complex [PdBr2( 3 -C7H6-Im-DiPP)] was transformed into an NHC which prompted an intramolecular coordination to Pd and a rearrangement of the allyl-bound cycloheptatrienide ligand to an alkyl binding mode.This crucial rearrangement allowed the abstraction of the alkyl-H atom as a hydride in the second step, thus converting the cycloheptatrienyl-into a cycloheptatrienylidene-ligand.Spectroscopic and crystallographic data indicate an electronic interaction between the two carbene moieties and allowed the first direct comparison of their properties in a single complex.Reactivity studies revealed a surprising behaviour of the hybrid CHT/NHC-Pd complex towards iodine, leading to dimerisation upon removal of one bromide ligand, which represents an unprecedented reaction of a coordination compound with iodine.As this thesis is fundamentally based on the area of CHT complexes, it also addressed the open questions concerning the ligand precursors of the formula C7H6X2 (X = Cl, Br), which had Abstract V mostly been regarded as 7,7-dihalocycloheptatrienes so far.In fact, this turned out to be the only unfounded representation, whereas all other possible structures, namely the covalent 1,7-, 2,7-and 3,7-dihalocycloheptatriene isomers and the ionic halotropylium halide form, are relevant.In the solid state, crystallographic analysis found the ionic structure exclusively.In solution, however, NMR spectroscopic and computational studies showed a dynamic equilibrium between the three covalent isomers which interconvert via the halotropylium halide structure.With increasing polarity of the solvent, the ionic structure becomes more and more stabilised, so that it actually turns from a minor species functioning as an intermediate to the main species in very polar solvents.The knowledge of the behaviour of these compounds will now permit a better understanding of their reactivity and applications.In the course of these studies, also the crystal structure of hydroxytropylium chloride was obtained serendipitously as a decomposition product and this represents the first crystallographic characterisation of an unstabilised hydroxytropylium ion.This also confirms that such a hydroxytropylium halide species is the intermediate in the hydrolytic decomposition pathway of halotropylium halides to tropone.In continuation of the studies of organometallic Pd clusters, a side project was dedicated to the re-investigation of a protocol from 1964 on the reaction of a Pd(II) salt with triphenylcyclopropenium chloride and ethylene.The true structure of the product was identified as a Pd6Cl8 cluster capped by 3 - 1 : 1 : 3 -C3Ph3 ligands and with triphenylcyclopropenium counterions: [C3Ph3]2[Pd6Cl8(C3Ph3)2].The formation of the ligands could be explained by a partial reduction of Pd(II) to Pd(0) by ethylene and a subsequent ring-opening oxidative addition of the triphenylcyclopropenium ions on Pd(0) leading to the rare binding mode of the capping ligands, which combines two 1 (alkyl)-coordinations and one 3 (allyl)-coordination.The cluster formation then proceeds via self-assembly with excess Pd(II) salt and triphenylcyclopropenium chloride.This cluster from 1964 in fact represents the first isolated organometallic Pd cluster, even though its structure was not recognised until now.Zusammenfassung VI Zusammenfassung Carbocyclische Carbene stellen eine relativ unbekannte Klasse von Carben-Liganden dar im Vergleich zu den N-heterocyclischen Carbenen (NHCs), welche als eine der vielfltigsten Ligandenklassen der Organometallchemie etabliert sind.Nichtsdestoweniger stellte sich die Gruppe der Pd-Cycloheptatrienyliden(CHT)-Komplexe als ein hochinteressantes Motiv heraus und im Rahmen dieser Arbeit konnte auf ihrer Basis eine vielseitige Strukturchemie entwickelt werden.In Fortsetzung vorangegangener Arbeiten wurden die Reaktivitt des CHT-Liganden gegenber N-Nucleophilen und die damit einhergehende Umlagerung des Carben-Liganden zu einem Allyl-Bindungsmodus als Schlsselschritt genutzt, um eine Vielzahl an Imidazoliumund Pyridinium-substituierten 3 -Cycloheptatrienid-Pd-Komplexen darzustellen.Es konnte gezeigt werden, dass die resultierenden Komplexeanders als bislang vorgeschlageneine monomere Struktur des Typs [PdBr2( 3 -C7H6-Im/pyr)] aufweisen, was auf die zwitterionische Natur der metallorganischen Liganden zurckgefhrt werden konnte.Computerchemische Studien deckten ein hochdynamisches Gleichgewicht zwischen verschiedenen Regioisomeren der Allyl-Bindung in Lsung auf und das -Isomer (bezogen auf den Abstand des zentralen Allyl-C-Atoms vom Substituenten) stellte sich in Einklang mit kristallographischen Ergebnissen als das gnstigste heraus.Um den Nachteil ihrer niedrigen Lslichkeit zu berwinden, erwies sich ein teilweiser Austausch der Bromid-Liganden gegen nichtkoordinierende Anionen als geeignete Methode.Dies fhrt zu dimeren, Halog Carbene ; Chemistry ; Pyridinium ; Nucleophile ; Palladium ; Cycloheptatriene ; Ligand (biochemistry) ; Cluster (spacecraft) ; Organometallic chemistry ; Stereochemistry ; Combinatorial chemistry ; Polymer chemistry ; Catalysis ; Medicinal chemistry ; Organic chemistry Catalytic Cross-Coupling Reactions ; Cyclopropane Reaction Mechanisms ; N-Heterocyclic Carbenes in Organic and Inorganic Chemistry