Browsing by Author "Kruger, Anneke"
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- ItemN-heterosikliese karbeenkomplekse van groep 10 metale : nuwe moontlikhede(Stellenbosch : Stellenbosch University, 2007-03) Kruger, Anneke; Raubenheimer, H. G.; Cronje, S.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.ENGLISH ABSTRACT: This study comprises the synthesis and characterisation of N-heterocyclic carbene complexes of group 10 metals with the main focus on such complexes with the nucleophilic N-atom in a position further than the α-position from the carbene carbon atom. These compounds were synthesised by the initial alkylation of quinoline- and acridine derivatives followed by the oxidative addition of the resulting salts to the complexes M(PPh3)4 (M = Ni, Pd, Pt). To complete the study an investigation of the catalytic activity and stability of such compounds in C,C-coupling catalysis was conducted. The carbene ligands used differed with respect to the position of the N-atom relative to the carbene carbon atom as well as with respect to substituents. The oxidative addition of the quinolinium- and acridiniumchloride salts to Ni(PPh3)4 was initially problematic. The desired product formed only in small amounts and could not be isolated easily. However, by replacing toluene with more polar THF as solvent, the nickel complexes could be synthesised in high yield without any indication of decomposition. Oxidative addition to Pt(PPh3)4 also yielded the desired product, although a number of by-products were formed as well. The molecular structure of trans-chloro(1,3-dimethyl-1,2-dihydroquinoline-2-ylidene)-bis(triphenylphosphine) platinum(II) trifluoromethanesulfonate could nevertheless be determined by X-ray crystallography. The new one-N, six membered, heterocyclic carbene complexes were all unambiguously characterised by NMR spectroscopy, mass spectrometry and X-ray crystallography. The carbene character of the complexes was corroborated by a large downfield chemical shift (δ 203 – 230) of the carbene carbon atoms in their 13C NMR spectra. The analyses furthermore indicated that with the exception of cis-chloro(1-methyl-3-phenyl-1,4-dihydroquinoline-4- ylidene)bis(triphenylphosphine)palladium(II) hexafluorophosphate, all the palladium, nickel and platinum complexes exhibit a trans arrangement of the PPh3 ligands. Both the cis- and the trans isomers of the complexes, chloro(1,3-dimethyl-1,2-dihydroquinoline-2-ylidene)- bis(triphenylphosphine)palladium(II) tetrafluoroborate and chloro-(1,3-dimethyl-1,2-dihydroquinoline- 2-ylidene)bis(triphenylphosphine)palladium(II) trifluoromethanesulphonate are, however, present in solution. The molecular structure of both isomers were determined by Xray crystallography. Crystallographic studies revealed that the new family of cationic complexes exist in a slightly distorted square planar environment with the carbene ligand orientated almost perpendicular to this plane. The metal-carbene bond lengths are insensitive to variations in the substituents on the carbene ligands and are therefore not a suitable parameter to distinguish between the strengths of these bonds. The N-heterocyclic carbene ligands with the N-atom removed from the carbene carbon atom by three bonds, exhibit a greater trans influence in the palladium complexes than those with the N-atom in the α-position. This is an indication of the better σ-donor ability of the former. A significant difference in the Ni-Cl bond lengths could, however, not be detected. The catalytic activity of the new synthesised palladium and nickel complexes in the Mizoroki- Heck and Suzuki-Miyaura coupling reactions, were investigated. The palladium complexes tested show potential as precatalysts for the Mizoroki-Heck reaction, although relatively high temperatures were needed to effect coupling, while coupling of aryl chlorides did not occur at all. The trans-chloro(1,2-dimethyl-1,4-dihydroquinoline-4-ylidene)bis(triphenylphosphine)- palladium(II) trifluoromethanesulphonate complex also displayed high activity in the C,Ccoupling of bromofluorene with arylboronic acids in the Suzuki-Miyaura coupling reaction. It even yielded high conversions in instances where sterically hindered aryl halides were used. The trans-chloro(1,2-dimethyl-1,4-dihydroquinoline-4-ylidene)bis(triphenylphosphine)- nickel(II) trifluoromethanesulphonate complex catalyses the coupling of sterically hindered arylbromides as well as arylchlorides to arylboronic acids. By further optimising the reaction conditions and increasing the streric bulk of the carbene ligands, even better results in both the Mizoroki-Heck and the Suzuki-Miyaura reactions are expected.