Academic literature on the topic 'Fischer carbenes complexes'

An iridium complex supported by a PC^PyP ligand featuring an internal pyridine tether showed selective mono-C–H activation of ethers, which represent intermediates in the synthesis of the corresponding Fischer carbenes.

Fischer carbene complexes of the Group VI transition metals (Cr, Mo and W) containing at least two or three different transition metal substituents, all in electronic contact with the carbene carbon atom, were synthesized and studied both in solution and in the solid state. For the complexes of the type [M(CO)5{C(OR)R’}], the substituents chosen included (hetero)aromatic (benzene or thiophene) rings π-bonded to a chromium tricarbonyl fragment or ferrocene as the R’-substituent, while the OR-substituent was systematically varied between an ethoxy or a titanoxy group, to yield the complexes 1 (M = Cr, R = Et, R’ = Fc), 2 (M = W, R = Et, R’ = Fc), 5 (M = Cr, R = TiCp2Cl, R’ = Fc), 6 (M = W, R = TiCp2Cl, R’ = Fc), 7 (M = Mo, R = TiCp2Cl, R’ = Fc), 12 (M = Cr, R = TiCp2Cl, R’ = 2-thienyl) and 13 (M = Cr, R = TiCp2Cl, R’ = [Cr(CO) 3 (η 6-phenyl)]). Direct lithiation of the ferrocene with n-BuLi/TMEDA at elevated temperatures, followed by the Fischer method of carbene preparation, also resulted, in most cases, in the formation of the novel biscarbene complexes with bridging ferrocen- 1,1’-diyl carbene ligands [μ-Fe{C5H4C(OEt)M(CO) sub>5}2] (3: M = Cr, 4: M = W) or the unusual bimetallacyclic bridged biscarbene complexes [{μ-TiCp2O2-O,O’}{μ- Fe(C5H4)2-C,C’}{CM(CO) 5}2] (8: M = Cr, 9: M = W, 10: M = Mo). It was attempted to prepare the mixed heteronuclear biscarbene complex 11 [W(CO) 5C{μ-TiCp2O2- O,O’}{μ-Fe(C5H4)2-C,C’}CCr(CO) 5], however the complex could not be fully characterized. The investigation was expanded to include Group VII transition metals Mn and Re, and using the same methodology, the manganese complexes isolated included [MnCp(CO2{C(OR)Fc}] (22: R = Et, 24: R = TiCp2Cl), 23 [μ- Fe{C5H4C(OEt)MnCp(CO) 2}2] and 25 [{μ-TiCp2O2-O,O’}{μ- Fe(C5H4)2- C,C’}[CMnCp(CO) 2}2]. The different reactivity of the binary dirhenium decacarbonyl precursor complex, compared to that of the Group VI complexes, resulted in the formation of a range of complexes. The target compounds [Re2 (CO) 9{C(OR)Fc}] (26: R = Et, 31: R = TiCp2Cl), 27 [μ-Fe{C5H4C(OEt)Re2 (CO) 9}2] and 33 [{μ- TiCp2O2-O,O’}{μ-Fe(C5H4)2-C,C’}[CRe2 (CO) 9}2] were isolated displaying a variety of different geometric isomers. In addition, acyl (30) and aldehyde (32) decomposition products, as well as hydrido (29), and hydrido acyl hydroxycarbene (34) complexes and the unique dichloro-bridged biscarbene complex (28) were also characterized. Most of these complexes displayed Re-Re bond breaking, and two probable mechanisms, either radical or ionic, were proposed involving either hydrogen transfer or protonation followed by hydrolysis. Finally, the structural features and their relevance to bonding in the carbene cluster compounds of the Group VI transition metals were investigated as they represent indicators of possible reactivity sites in multimetal carbene assemblies. The possibility of using DFT calculations to quantify the effect of metal-containing substituents on the carbene ligands was tested and correlated with experimental parameters by employing methods such as vibrational spectroscopy, molecular orbital analysis, and cyclic voltammetry. The best results were obtained from the cyclic voltammetric studies, where the localized metal centre’s oxidation potential correlated to both the calculated HOMO energy, and the effect of both the heteroatom substituent and the (hetero)arene substituent, as well as different combinations of the above.
Thesis (PhD)--University of Pretoria, 2010.
Chemistry
unrestricted

This organometallic study involves the use organostannanes and organolithiums as precursors to chromium Fischer carbene complexes. Fischer carbenes are typically electrophilic and are stabilized by a single π-donor substituent, and contain low oxidation state metals (often but not always from Group 6). They are highly reactive and can give access to a range of biologically active compounds through cyclopropanations, insertions, coupling and photochemical reactions. Synthesis and characterization of three MOM-protected α-alkoxy organostannanes was successfully carried out via a nucleophilic addition of tributylstannyllithium to suitable aldehydes, and immediate protection of the alcohol with MOM. Two N-BOC protected α-amino organostannanes were successfully synthesized and characterized via α-lithiation and tin-lithium exchange in the presence of TMEDA. Tin-lithium transmetallation of the organostannanes allowed access to the organolithiums required for the synthesis of novel Fischer carbenes. Addition of chromium hexacarbonyl to the organolithiums formed the acylpentacarbonyl chromate salt which was alkylated with Meerwein salt, resulting in the Fischer carbene and a by-product, tetrabutyltin, which proved difficult to remove. Several Fischer carbenes were synthesized and characterized, some simple and known and some novel. In silico work explored the reaction coordinate of the [2+2] cycloaddition towards the formation of β-lactams, and the photoactivation cycle that precedes this process. Computational work also showed the effect of the ligand on the stability and reactivity of the carbene. It was found that in some cases the oxygen on the ligand could negatively influence the stability of the carbene (when compared to a simple methyl carbene). A link between bond orders and back donation in Fischer carbenes was explored in an attempt to theoretically predict the stability of a range of carbenes.
Microsoft� Office Word 2007

Thesis (PhD (Chemistry and Polymer Science))--Stellenbosch University, 2008.
Alkylated acetonitrile that forms during the synthesis of the sulfonium salt, [(Me3)2(MeS)S][BF4], is involved in the formation of new , -unsaturated Fischer-type carbene complexes from (CO)5M=C(OMe)CH2Li (M = Cr, W). Metal migration observed when the substitution product obtained from the reaction of the anionic carbene complexes (CO)5M=C(NMe2)CºC¯ (M = Cr, W) with Ph3PAu+ was left in solution, was also kinetically and theoretically investigated. 1H NMR and quantum mechanical (at the B3LYP level of theory) data indicated a complicated mechanism. The a,b-unsaturated Fischer-type carbene complex, (CO)5Cr=C(OMe)CH=C(Me)NH(Me), obtained from the reaction of (CO)5M=C(OMe)CH2¯ with alkylated acetonitrile, was transformed into the new remote one-N, six-membered, carbene ligand (rN1HC6) complex, (CO)5Cr=C(CH=C(Me)N(Me)CH=C(nBu). The carbene ligand unprecedentedly preferred the softer Rh(CO)2Cl moiety to the Cr(CO)5 metal fragment and transferred readily. A new series of remote and abnormal square planar compounds [r/a(NHC)(PPh3)2MCl]CF3SO3 (M = Pd or Ni) was prepared by oxidative substitution. The various positions for metal-carbon bond formation on a pyridine ring to furnish various ligand types i.e. C2 for nN1HC6, C3 for aN1HC6 or C4 for rN1HC6 received attention. The ligands were arranged in increasing order of carbene character, aNHC < nNHC < rNHC and trans influence, nN2HC5 ~ aN1HC6 ~ nN1HC6 < rN1HC6. In competitive situations, oxidative substitution occurred selectively at C4 of the pyridine ring rather than at C2 and on the aromatic ring containing the heteroatom (C4), rather than on an annealed aromatic ring (C7). Crystal and molecular structure determinations confirmed the preferred coordination sites. Quantum mechanical calculations (at the RI-BP86/SV level of theory) indicated that the chosen carbene ligand has a much larger influence than the metal on the BDE of the M-Ccarbene bond; the farther away the N-atom is from the carbene carbon, the stronger the bond. In complexes that also contain additional external nitrogen atoms, e.g. trans-chloro(N-methyl-1,2,4- trihydro-2-dimethylaminepyrid-4-ylidene)bis(triphenylphosphine)palladium(II) triflate and transchloro( N-methyl-1,2,4-trihydro-2-dimethylaminepyrid-4-ylidene)bis(triphenylphosphine)nickel(II) triflate, stabilisation originates from both the nitrogens. 2-Chloro-1-methyl-1H-pyrid-4-ylidenephenylammonium triflate afforded complexes with both remote as well as normal nitrogen atoms. New azole complexes of palladium and nickel with remote heteroatoms were also prepared from N-methyl-4',4'-dimethyl-2'-thiophen-3-chloro-2-yl-4,5-dihydro-oxazole. Employing the compound 1,5-dichloroanthraquinone, the product of a double oxidative substitution on two Pd centra could be isolated but not alkylated. The fact that the chemical shift of the metal bonded carbon in the 13C NMR spectrum can not be used as absolute measure of carbene character, was emphasised in a compound where the heteroatom was situated seven bonds away from the carbon donor. In efforts to synthesise a sulphur-bridged complex that contains carbene ligands, crystals of transdi- iodobis(1,3-dimethyl-imidazoline-2-ylidene)palladium were obtained. Bridged thiolato complexes with N1HC6 ligands were unexpectedly found in the attempt to substitute the halogen on chosen square planar carbene complexes of palladium, widening the application possibilities of N1HC6 ligands in organometallic chemistry beyond that of catalysis. A trinuclear cluster, [(PdPPh3)3(μ-SMe)3]BF4 was isolated as a by-product of these reactions. A series normal and abnormal thiazolylidene complexes of nickel and palladium were prepared by oxidative substitution of the respective 2-, 4- and 5-bromothiazolium salts with M(PPh3)4 (M = Pd or Ni), and unequivocally characterised. In a preliminary catalytic investigation, all the thiazolinium and simple pyridinium derived palladium complexes showed activity in the Suzuki-Miyaura coupling reaction. Little variation in activity in the order a (N next to carbon donor) > n > a (S next to carbon donor) was found for the former series, whereas decreased activity was exhibited in the sequence r > a > n of the latter group. The pyridinium derived complexes showed superior activity to the thiazolinium ones. The rNHC complex, trans-chloro(N-methyl-1,2,4-trihydro-2- dimethylaminepyrid-4-ylidene)bis(triphenylphosphine)palladium(II) triflate, showed similar Suzuki-Miyaura activity to the standard N2HC5 carbene complex precatalyst, trans-chloro[(1,3- dimethyl-imidazol-2-ylidene)triphenylphosphine]palladium(II) triflate.

Dissertation (PhD)--University of Stellenbosch, 2003.
ENGLISH ABSTRACT: This study comprises the preparation and characterisation of various novel organometallic species of gold(I) by employing a range of anionic group 6 metal Fischer-type carbene complexes and group 6 metal-acyl complexes as synthons of the organic moieties introduced to the gold(I) electrophiles. The main objectives of this work are to develop the use of Fischer-type carbene complexes as synthons in the preparation of novel organometallic species along unusual reaction pathways and, in doing so, to expand the organometallic chemistry of gold(I), especially Au-C bond formation reactions. By reacting various β-CH acidic Fischer-type alkoxy/dialkylamino/ alkthio(methyl)carbene complexes, first with a base, and then with a gold(I) electrophile (Ph3PAu+), easy access to vinyl ether/dialkylamine/thioether complexes of gold(I) coordinated to M(CO)5 (M = Cr, Mo, W) fragments, is obtained. When methyl alkoxy- or dialkylaminocarbene complexes are employed, coordination of the novel alkoxyvinyl-gold(I)PPh3 and dialkylaminovinyl-gold(I)PPh3 species to the M(CO)5 fragments occurs in an asymmetrical fashion through the vinyl functionalities of the novel gold(I) species. This usually unstable coordination mode for vinyl ethers is stabilised by delocalisation of partial positive charges from the α-gold vinyl carbon atoms to either the gold(I)PPh3 fragment [for η2-{alkoxyvinyl-gold(I)PPh3}M(CO)5 complexes] or the nitogen atoms of the vinyl amine group [for η2-{dialkylaminovinylgold( I)PPh3}M(CO)5 complexes]. In the latter complexes this delocalisation occurs to such an extent that these complexes are best described as zwitterions. The corresponding negative charges in the bimetallic complexes reside on the M(CO)5 fragments. As a representative example, uncoordinated ethoxyvinyl-gold(I)PPh3 was isolated in high yield via a ligand replacement reaction with PPh3. When Fischer-type alkthio(methyl)carbene complexes are employed in this conversion, novel sulphur coordinated {alkthiovinyl-gold(I)PPh3}Cr(CO)5 complexes are formed.The reaction mechanism involved in these conversions is believed to be the gold(I) analogue of the hydrolysis of Fischer-type carbene complexes. In this mechanism the bimetallic η2-vinyl ether coordinated {alkoxyvinyl-gold(I)PPh3}M(CO)5 complexes represent stabilised gold(I) analogues of postulated transition states in the hydrolytic decomposition of Fischer-type alkoxycarbene complexes. The term aurolysis is conceived to describe the conversion when Ph3PAu+ is employed as electrophile instead of H+. The formation of the bimetallic η2-vinyl ether coordinated complexes in the current conversion, furthermore, strongly supports the existence of similar transition states in the hydrolytic decomposition of Fischer-type alkoxycarbene complexes. This mechanism is also accepted for the formation of analogous η2-{dialkylaminovinyl-gold(I)PPh3}M(CO)5 and {alkthiovinyl-gold(I)PPh3}-S Cr(CO)5 complexes when β-CH deprotonated Fischer-type dialkylamino- and alkthiocarbene complexes are employed in this reaction. The reaction of anionic group 6 metal-acyl complexes and their nitrogen analogues, N-deprotonated Fischer-type aminocarbene complexes, leads to the formation of acylgold(I) and novel imidoylgold(I) complexes coordinated to M(CO)5 (M = Cr, W) fragments. In the previous complexes poor stabilisation of the M(CO)5 fragments allows halide anions to readily form ionic adducts with these groups. This characteristic of these products provides a useful reaction pathway to the first example of a free acylgold(I) complex, benzoyl-AuPPh3. Coordination of the imine nitrogen atom to the M(CO)5 fragments in the analogous bimetallic imidoylgold(I)-M(CO)5 complexes is much stronger. These complexes are remarkably stable and could even be effectively isolated by means of low temperature silica gel chromatography. As a preliminary reaction mechanism for this conversion we propose a mechanism that is closely related to the aurolysis mechanism described above. The only difference is that, instead of formal reductive elimination of vinyl ethers/amine/thioether complexes of gold(I) from the M(CO)5 fragments, acyl and imidoyl complexes of gold(I) are produced in this step. Furthermore, the (Z)- stereoisomers of the bimetallic imidoylgold(I)-M(CO)5 complexes generated in this conversion are exclusively obtained.A second N-deprotonation-auration reaction sequence performed on suitable examples of the bimetallic imidoylgold(I)-M(CO)5 complexes yields, as the only isolable product, a novel triangular Au2Cr cluster complex, cis-{η2-(Ph3PAu)2} PPh3Cr(CO)4. This complex is the isolobal equivalent for the unstable molecular hydrogen complex, (η2-H2)PPh3Cr(CO)4, and exhibits the shortest known Au-Au separation between two gold atoms in cluster complexes of the type Au2M. Finally, two novel and vastly different molecular structures of closely related anionic benzoylpentacarbonyl tungstates, one with Li+ as counterion and another in which exactly half the Li+-cations have been replaced by protons, highlight the importance of hydrogen bonding and ion-dipole interactions in determining the solid state structure of such complexes.
AFRIKAANSE OPSOMMING: Hierdie studie behels die bereiding en karakterisering van verskeie nuwe organometaalkomplekse van goud(I). Hierdie komplekse is berei deur gebruik te maak van n wye reeks anioniese groep 6 metaal Fischer-tipe karbeenkomplekse asook anioniese groep 6 metaal asielkomplekse as sintetiese ekwivalente vir die organiese fragmente wat gedurende die sintese aan die goud atoom gebind word. Die hoofdoel van hierdie studie is om die gebruik van Fischer-tipe karbeenkomplekse as sintetiese voorgangers in die bereiding van nuwe organometaalverbindings te ontwikkel en om sodoende ook die organometaalchemie van goud verder uit te bou. Veral die ontwikkeling van nuwe sintetiese metodologieë vir die bereiding van Au-C bindings is hier van belang. Verskeie Fischer-tipe alkoksie-/dialkielamino-/alktio-(metiel)karbeenkomplekse met suuragtige waterstofatome geleë op die β-metallo-koolstofatoom is eers onomkeerbaar gedeprotoneer. Byvoeging van die goud(I) elektrofiel, Ph3PAu+, lei - volgens n ongewone reaksiemeganisme - tot die vorming van onderskeie vinieleter-, dialkielvinielamien- en vinieltioeterkomplekse van goud(I). Hierdie komplekse is verder ook op verskillende wyses aan M(CO)5 fragmente (M = Cr, Mo, W) gekoördineer. Die vinieleter- en vinielamienkomplekse van goud(I), wat vorm wanneer alkoksie- en dialkielaminokarbeenkomplekse onderskeidelik in hierdie sintese aangewend word, koördineer onsimmetries deur hulle viniel dubbelbindings aan die vrygestelde M(CO)5-groepe. Hierdie normaalweg onstabiele vorm van vinieleterkoördinasie, word gestabiliseer deur delokalisering van positiewe lading vanaf die α-goud viniel koolstofatoom na die AuPPh3-fragment [vir die η2-{alkoksievinielgoud( I)PPh3}M(CO)5 komplekse] óf na die stikstofatoom van die dialkielvinielamien groep [vir die η2-{dialkielaminoviniel-goud(I)PPh3}M(CO)5 komplekse]. Laasgenoemde komplekse kan as zwitterione beskryf word. Die onderskeie negatiewe ladings in hierdie komplekse bevind hulle hoofsaaklik op die M(CO)5 groepe. Sterk koördinerende ligande (bv. PPh3) verplaas die onsimmetriese viniel eter vanaf die M(CO)5-fragment. Só kon, as n voorbeeld, die vrye etoksievinielgoud( I)PPh3-kompleks met n hoë opbrengs berei word. Wanneer β-gedeprotoneerdeFischer-tipe tiokarbeenkomplekse met Ph3PAu+ reageer, vorm swawel gekoördineerde {tioviniel-goud(I)PPh3}Cr(CO)5 bimetalliese komplekse in stede van die π-komplekse. Dit word voorgestel dat in die bogenoemde reaksies die goud(I)elektrofiel dieselfde rol vervul as die proton gedurende die hidrolise van Fischer-tipe alkoksiekarbeenkomplekse. Die bimetalliese, η2-vinieleter-gekoördineerde {alkoksieviniel-goud(I)PPh3}M(CO)5-komplekse hier berei verteenwoordig dus stabiele goud(I) analoë van voorgestelde tusseprodukte in so ’n meganisme. Die term aurolise word voorgestel om die geval waar Ph3PAu+ in stede van H+ as elektrofiel aangewend word te beskryf. Die vorming van bimetalliese, η2-vinieletergeko ördineerde komplekse in die huidige reaksie ondersteun die moontlike vorming van die voorgestelde tussenprodukte tydens die hidrolise van Fischer-tipe alkoksie(metiel)karbeenkomplekse. ’n Soortgelyke meganisme kan ook gebruik word om die vorming van die η2-{dialkiellamienviniel-goud(I)PPh3}M(CO)5- en {alktioviniel-goud(I)PPh3}-S Cr(CO)5-komplekse vanuit β-CH gedeprotoneerde Fischer-tipe dialkielamino- en tiokarbeenkomplekse en Ph3PAuCl te interpreteer. Die reaksie van anioniese groep 6 oorgangsmetaal metaal-asielkomplekse en hulle stikstofanaloë, N-gedeprotoneerde Fischer-tipe aminokarbeenkomplekse, lewer onderskeidelik asiel- en imidoielkomplekse van goud(I) wat aan M(CO)5 fragmente (M = Cr, W) koördineer. Die goud(I)asiel-M(CO)5 koördinasie deur die asielsuurstofatoom is baie swak en die M(CO)5-eenheid in hierdie komplekse word maklik deur haliedanione en sekere oplosmiddel molekules verplaas. Die haliedanione vorm anioniese addukte met the M(CO)5 fragmente. Hierdie eienskap van die bimetalliese komplekse verskaf sodoende n gerieflike sintetiese roete na die eerste voorbeeld van n vrye asielgoud(I)-kompleks, bensoiel-AuPPh3. Koördinasie van die imienstikstofatoom aan M(CO)5-groepe in die bg. imidoielkomplekse is egter veel sterker. Die bimetalliese {imidoielgoud(I)}M(CO)5-komplekse is verbasend stabiel en kan selfs effektief deur middel van lae temperatuur SiO2-kolomkromatografie geïsoleer word. n Soortgelyke reaksie meganisme as wat voorgestel word vir die aurolise van Fischer-tipe karbeenkomplekse word voorgestel vir hierdie reaksie. Die enigste verskil is dat die formele reduktiewe eliminasie van n viniel-eter, -amien of -tioeter vervang word met die vorming van asiel- of imidoielkomplekse van goud(I). Verder word die (Z)-isomere van die bimetalliese {imidoielgoud(I)}M(CO)5-komplekse uitsluitlik in hierdie reaksie verkry. Wanneer geskikte voorbeelde van bimetalliese {imidoielgoud(I)}M(CO)5-komplekse n tweede keer gedeprotoneer word en gereageer word met Ph3PAuCl, is die enigste isoleerbare produk van die reaksie n driehoekige Au2Cr troskompleks, nl. cis-{η2- (Ph3PAu)2}PPh3Cr(CO)4. Hierdie verbinding dien as n isolobale model vir die onstabiele molekulêre waterstof kompleks , (η2-H2)PPh3Cr(CO)4, en besit verder die kortste Au-Au afstand tussen twee goud atome in driehoekige troskomplekse wat nog tot dusvêr gerapporteer is. Laastens is die kristalstrukture van twee nou verwante anioniese {bensoiel}W(CO)5- komplekse bepaal. Die enigste verskil tussen die hierdie twee verbindings is dat die een slegs Li+ as teenioon bevat terwyl presies die helfte van die Li+-teenione in die tweede struktuur deur protone verplaas is. Hierdie klein verskil in samestelling veroorsaak egter drastiese verskille in die kristalstrukture van hierdie verbindings. Die belangrikheid van waterstof bindings en ioon-dipool interaksies in die bepaling van die vastetoestandstrukture van sulke verbindings word hierdeur beklemtoon.

En aquesta tesi s'han estudiat mecanismes de reaccions de cicloanulació en carbens de Fischer a través de mètodes teòrics, concretament fent servir el nivell de teoria B3LYP/(Wachters' basis / 6-31G**). Els alcoxi- i amino carbens de pentacarbonil crom, ja siguin vinílics o aromàtics, reaccionen amb acetilè per produir fenols, naftols o derivats ciclopentadiè o indè substituïts amb el Cr(CO)3 coordinat, d'una manera regioselectiva. L'objectiu d'aquest treball és discutir ambudes reaccions competitives particularment a la reacció de Dötz, la qual durant els darrers anys ha estat explorada experimentalment per W.D. Wulff, C.P. Casey, R. Aumann i J. Barluenga entre altres diferents propostes mecanístiques. A més K.H. Dötz va demostrar que la coordinació del Cr(CO)3, un cop l'anell ja està format, pot patir canvis haptotròpics, és a dir, la caminada del complex metàl·lic d'un anell a un altre -generalment almenys substituït- canviant la seva hapticitat (coordinació pi amb els membres de l'anell). Llavors, s'han estudiat les migracions haptotròpiques intramoleculars en petits hidrocarbons aromàtics policíclics amb l'objectiu d'analitzar les rutes de reacció per les quals aquestes reaccions es porten a terme

The synthesis and structure elucidation of fourteen novel Fischer ethoxy- and amino carbene complexes of molybdenum and chromium metals of the type [(CO)3L2M=C(X)R] were performed. Substitution of the parent pentacarbonyl complex [(CO)5M-C(OEt)(R)]; M = Mo, Cr; R = 2-thienyl, 2-furyl; with mono- and bidentate phosphine ligands yielded the corresponding tetracarbonyl complexes [(CO)4(PR’3)M-C(OEt)R] with M = Mo, Cr, R = 2-thienyl, 2-furyl and R’ = Ph, Cy or tricarbonyl complexes [(CO)3(DPPE)Mo-C(OEt)(2-furyl)], respectively. Aminolysis of these novel complexes resulted in substitution of the ethoxy substituent with an amino group. Full characterisation of these novel complexes included infrared spectroscopy, NMR (1H, 13C, 31P) spectroscopy, x-ray crystallography and mass spectral analyses. From the characterisation data, it was found that the cis isomer was dominant for the tetracarbonyl carbene complexes whereas the mer isomer was favoured in most cases for the tricarbonyl carbene complexes. Structural elucidation of nine of the novel complexes was confirmed with x-ray crystallography. The amino carbene complexes showed preference for a syn conformation of the oxygen atom of the furyl moiety and the nitrogen atom of the amino moiety in the crystal form. The ethoxy carbene complexes showed a preference for the anti conformation in the crystal form. The NMR analysis showed that resonance between the metal-carbene carbon-heteroatom stabilisation is more prevalent in the case of the amino carbene complexes. DFT calculations were performed for eleven of these complexes and their isomers. The infrared stretching modes were calculated and compared to experimental data. The HOMO and LUMO position and energy gaps were calculated. It was observed that the HOMO in all calculated complexes lay on the metal centre, while the LUMO was centred on the carbene carbon. It was found that the HOMO-LUMO gap was larger for the amino Fischer carbene complexes than for the ethoxy Fischer carbene complexes. From frontier orbital considerations, it is concluded that ligand substitution of carbonyls by phosphines as well as carbene substituent alterations does not seem to improve the reactivity of the novel complexes of this study particularly for the potential application as metathesis catalysts. The performed DFT calculations, in the future, can help determine which type of catalytic reactions these fourteen complexes would be most effective for.
Dissertation (MSc)--University of Pretoria, 2014.
tm2015
Chemistry
MSc
Unrestricted

The central focus of this study was the synthesis, structural investigation and characterisation of multiple chromium carbene complexes. Fifteen novel chromium(0) complexes were synthesised. The synthesis of the primary monocarbene starting material [Cr(CO)5{C(OEt)(heteroaryl)}], heteroaryl = thiophene, furan, 2,2’-bithiophene, was carried out utilising typical Fischer methodology. A wide variation of spacer ligands were reacted to obtain different carbene substituents. The ligand substitution reaction between carbonyl ligands and the bidentate ligands followed the techniques proposed in literature and produced distinctive chelated monocarbene complexes with the resulting structure mer- [Cr(CO)3(dppe){carbene)}]. An extensive collection of more sophisticated monocarbene complexes was synthesised via modification pathways (aminolysis). Conversion of original ethoxy-bearing monocarbenes through aminolysis provided the corresponding amine analogues, possessing both novel structure and unique chemical reactivity. The aminolysis reactions involved different sized amino reagents; both ammonia and cyclohexyl amine (bulky, cyclic chair amines) were employed to produce derivatives of the monocarbene starting complexes. Lastly, the synthesis of unique N-heterocyclic carbene (NHC) complexes was envisaged. Synthesis of both the pentacarbonyl-bearing and phosphine-bearing NHC complexes was attempted utilising an adapted version of a methodology proposed in literature. The synthesis of chelate NHC complexes, however, proved difficult and the resulting products were not obtained. All Fischer and N-heterocyclic carbene complexes were characterised using infrared spectroscopy, nuclear magnetic resonance spectroscopy and mass spectrometry. In cases where single crystals were obtained, X-ray crystallography was used to confirm molecular structures. X-ray crystallographic studies indicated that carbonyl substitution reactions performed on monocarbene starting material, will favour the formation of the meridonial isomer in molecules where the carbene substituents are less bulky. Due to steric considerations, the substitution of labile carbonyl ligands in the trans position to the carbene moiety will be favoured. Density functional theory (DFT) calculations were performed on the complexes synthesised in this study. The results obtained indicated the favoured isomeric form to be facial in some cases whereas crystallographic data signified the meridonial isomer as the more stable product, irrespective of the bulkiness of the carbene substituents. Bond lengths, geometry and bond angles were all comparable to those of the single-crystal X-ray data. Single-point energy calculations show clearly that modelling methods provide good estimations of energetically favourable geometries, and accurate DFT calculations also predict the HOMO and LUMO orientations around the metal or ligand spheres. The majority of the structures provided by the computed model, illustrated that the 3d atomic orbitals of the metal contributed significantly to the HOMO, whereas the LUMO was mostly orientated around the carbene carbon atom. Metathesis and polymerisation catalytic reactions were attempted on [Cr(CO)3(dppe){C(OEt)(thiophene)}], 2, whereas only metathesis studies were employed for [Cr(CO)3(dppe){C(NHCy)(thiophene)}], 6. Both complexes presented as inert to either reaction and no catalytic capability was witnessed. Gas chromatography was used to indicate the level of progression of the reaction and the chromatogram verified that neither pre-catalyst found application in metathesis or, in the case of 2, in polymerisation.
Dissertation (MSc)--University of Pretoria, 2012.
Chemistry
MSc
Unrestricted