Doctoral Degrees (Chemistry and Polymer Science)

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Browsing Doctoral Degrees (Chemistry and Polymer Science) by browse.metadata.advisor "Cronje, S."

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    Carbene ligand and complex design directed towards application in synthesis and homogeneous catalysis
    (Stellenbosch : Stellenbosch University, 2008-12) Stander-Grobler, Elzet; Raubenheimer, H. G.; Cronje, S.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    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.
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    New transition metal carbene complexes for application in homogeneous catalysis
    (Stellenbosch : University of Stellenbosch, 2005-12) Julius, Gerrit R.; Raubenheimer, H. G.; Cronje, S.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
    This study comprises the preparation and full characterisation of new carbene complexes of group 10 metals (Ni, Pd or Pt), the group 9 metal, rhodium, as well as group 6 metals (Cr and/or W). N-heterocyclic carbene (NHC) complexes of nickel and palladium were prepared via oxidative addition of the corresponding carbene precursors imidazolium-, imidazolinium-, pyridinium- and quinolinium chloride salts, to M(PPh3)4 (M = Ni or Pd). Three types of carbene complexes, namely the standard five-membered two-N carbene complexes, new six-membered NHC complexes and novel six-membered rNHC complexes received attention. In the rNHC complexes the heteroatom (N) is removed from the carbene carbon. These new square planar carbene complexes of the general formula trans-[(PPh3)2MCl(L)]X (M = Ni or Pd; X = BF4 or PF6) L = 1,3-dimethyl-2,3-dihydro-1Himidazol- 2-ylidene, 1,3-dimethyl-2,3,4,5-tetrahydro-1H-imidazol-2-ylidene, 1-methyl-1,2-dihydropyridin- 2-ylidene, 1-methyl-1,2-dihydro-quinolin-2-ylidene, 1,4-dimethyl-1,2-dihydro-quinolin-2- ylidene, 2-methoxy-1-methyl-1,4-dihydro-quinolin-4-ylidene, 1-methyl-1,4-dihydro-pyridin-4- ylidene) have been isolated and characterised. The preparation of the corresponding carbene complexes of platinum was complicated by the formation of [PtCl(PPh3)3]BF4 and the desired carbene complexes could not be isolated in pure form. The investigation of rNHC complexes was extended to include the synthesis of (CO)5M{CSC(CNCMe2CH2O)CHCH} (M = Cr and W). The molecular and crystal structures of thirteen of the new carbene complexes including the structures of both cis- (only formed below –20°C) and trans-chloro(1-methyl-1,2-dihydro-quinolin- 2-ylidene)bis(triphenylphosphine)palladium(II) tetrafluoroborate were determined. The metalcarbene bond distances in both the palladium and nickel carbene families do not differ significantly. The carbene ligands can be arranged in a series of increasing trans-influence, using the metalchloride bond distance as a guideline, as follows: 1,3-dimethyl-2,3-dihydro-1H-imidazol-2-ylidene and 1,3-dimethyl-2,3,4,5-tetrahydro-1H-imidazol-2-ylidene < 1-methyl-1,2-dihydro-pyridin-2- ylidene < 2-methoxy-1-methyl-1,4-dihydro-quinolin-4-ylidene, 1-methyl-1,4-dihydro-pyridin-4- ylidene. The crystal and molecular structures of two platinum compounds, cis-chloro(2-methoxy-1- methyl-1,4-dihydro-quinolin-4-ylidene)bis(triphenylphosphine)platinum(II) tetrafluoroborate and the byproduct [PtCl(PPh3)3]BF4 were also determined. Trans-chloro(2-methoxy-1-methyl-1,4-dihydro-quinolin-4-ylidene)bis(triphenylphosphine) palladium(II) tetrafluoroborate was found to be a very active catalyst, compared to simpler palladium carbene and phosphine complexes, in the Mizoroki-Heck and Suzuki-Miyaura coupling reactions. Quantum mechanical calculations indicated that the rNHC ligand in this complex is bound stronger to the palladium than a standard imidazole-derived NHC ligand. Further calculations suggested that the remote heteroatom carbene (rNHC) complexes of nickel(II) are significantly more stable when compared to the normal carbene (NHC) complexes. Energy decomposition analysis suggested that the rNHC ligands are strong s-donors and weak -acceptors. Unsymmetrical imidazolium-derived bis(carbene) complexes, [Rh(NHC)2COD]Br, bromomono( carbene) complexes, Rh(Br)COD(NHC), and chloro-mono(carbene) complexes, Rh(Cl)COD(NHC) where NHC = 1-R-3-methyl-2,3-dihydro-1H-imidazol-2-ylidene (R = ethyl, propyl or butyl), were formed in each reaction of the corresponding free carbene ligand with [Rh(Cl)COD]2. [Rh(Br)COD(NHC)] formed as a result of substitution of a chloride ligand by a Br-- anion. The carbonyl complexes, cis-[Rh(CO)2X(NHC)] (X = Br or Cl; NHC = 1-ethyl-3-methyl- 2,3-dihydro-1H-imidazol-2-ylidene) were isolated after the substitution of the COD ligand in Rh(X)COD(NHC) (X = Br or Cl) with CO. Isomerisation of these cis-carbonyl complexes to the trans isomers was observed. Cis-[(h4-1,5-cyclooctadiene)bis(1-butyl-3-methyl-2,3-dihydro-1H-imidazol-2-ylidene)rhodium(I)] bromide, bromo(h4-1,5-cyclooctadiene)(1-methyl-3-propyl-2,3-dihydro-1H-imidazol-2-ylidene) rhodium(I) and cis-[(h4-1,5-cyclooctadiene)bis(1-butyl-3-methyl-2,3-dihydro-1H-imidazol-2- ylidene)rhodium(I)]bromide were also characterised by single crystal X-ray diffraction. The synthesis and structural characterisation of a series of acyclic and heterometallacyclic Fischertype carbene complexes in which a soft donor atom (P) attached to the carbene side chain is either uncoordinated, (CO)5M=C(NMe2)CH2PPh2 (M = Cr or W), bonded to the original central metals (Cr or W) in four-membered chelates, (CO)4M=C(NMe2)CH2PPh2, or coordinated to a second M(CO)5 unit (only isolated for chromium) (CO)5Cr=C(NMe2)CH2P(Ph2)Cr(CO)5, were carried out. These compounds were produced by the reaction of the anionic Fischer-type aminocarbene complexes, [(CO)5Cr=C(NMe2)CH2]Li (M = Cr or W), with ClPPh2. The formation of the four-membered chelates, via cyclisation, occurs much faster for Cr than for W. The aminocarbene-phosphine chelates represent the first examples of structurally characterised, four-membered C,P-chelate carbene complexes.
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    Utilisation of various bonding modes of nitrogen-rich heterocycles in gold(I) chemistry
    (Stellenbosch : University of Stellenbosch, 2006-03) Gabrielli, William Fullard; Raubenheimer, H. G.; Cronje, S.; University of Stellenbosch. Faculty of Science. Dept. of Chemistry and Polymer Science.
    This study describes the exploitation of numerous donor ligand possibilities presented by selected nitrogen-rich heterocyclic ligands towards gold(I). The preparation and structural characterisation of novel gold(I) complexes, apart from conventional gold(I) bonding possibilities, also encompassed a study of bi- and polytopic bonding modes within a range of multifunctional ligands.