A series of unsymmetrical xylenyl-backboned diphosphine ligands are reported: o-C6H4(CH2ptSU2)(CH2PRR'), R = Ph, R' = 2-Py (L1) and O-C6H4(CH2ptSU2)(CH2PR2), R = 2-Py (L2), R = 2-(3-Me-Py) (L3), R = 2-(4- Me-Py) (L4), R = 2-(6-Me-Py) (Ls) and R = 2-Pyrim (Ls). The ligands have all been characterised by 31p, 1H and 13C NMR spectroscopy and high resolution mass spectrometry. Studies of [PtCI(CH3)(L)] and [PdCI(CH3)(L)] (L = L1-LS) have shown that the favoured isomers have the more strongly a-donating CH3 ligand trans to the bulkier ptSU2 donor. Studies with [PdCI2(L)], [PtCI2(L)] and [Pt(CH3h(L)] (L = L1-LS) show that the donor strength of the two phosphorus centres is not averaged in these complexes, in contrast to the related complexes of o-C6H4(CH2ptSu2)(CH2PPh2) (TP). These complexes have been characterised by a combination of NMR spectroscopy, high resolution mass spectrometry, elemental analysis and X-ray crystallography. The coordination chemistry of PR3, R = 2-(3-Me-Py), R = 2-(4-Me-Py), R = 2-(6-Me-Py), with LiCI and LiSr is also reported. Tripodal coordination to lithium by the three pyridyl nitrogen donors is detected for each ligand. These [LiX(PR3)] complexes have been characterised by 31p, 1H and 13C NMR spectroscopy and X-ray crystallography. Protonation studies with [PdCI2(L)] and [PtCh(L)] (L = L1-LS) suggest that the pendant pyridyl and pyrimidyl groups of L1-LS are protonated in methanolic solutions when treated with acids such as HSF 4, CH3S03H (MSA) and CF3C02H (TFA). The evidence for protonation came from 31p NMR spectroscopy and X-ray crystallography. Protonation studies with [Pt(CH3h(L2)] have shown that tridentate coordination of platinum by L2 (through both P donors and one pyridyl N donor) occurs. Catalysts derived from the new ligands were tested in the palladiumcatalysed methoxycarbonylation of ethene. The systems based upon L1, L2, L4 and Ls are shown to be active and selective for the production of methyl propionate (MeP) over polyketone (PK). Those derived from L2 and Ls are comparable in activity to the best reported system (which is based upon o-C6H4(CH2ptSU2h, dtbpx) when promoted by the strong acid MSA. The L2 and Ls systems are more active than dtbpx with the weaker acids TFA and PA. Complexes derived from L3 and Ls are inactive in methoxycarbonylation catalysis. Catalysts derived from L1-LS are also shown to be active in palladium-catalysed styrene methoxycarbonylation. Mechanistic studies suggest that catalysts derived from L2 produce MeP following the hydride mechanism. Competition between the coordination of palladium by the nitrogen donor of a pendant pyridyl group and acid anions was observed under catalytically relevant conditions. Complexes derived from L3 appear to be inactive in MeP catalysis due to persistent tridentate coordination of palladium (via the two P donors and one pyridyl N donor) blocking substrate coordination. Complexes derived from Ls may be inactive in MeP catalysis due to the electron deficient nature of the PPyrim2 donor. Carbonylation studies with isotopically labelled 13CO support the conclusion that MeP preference is a balance between kinetic and thermodynamic influences - the minor palladium-ethyl complex isomer may react faster to generate the more stable palladium-acyl isomer.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:658207 |
| Date | January 2012 |
| Creators | Turner, Thomas P. W. |
| Publisher | University of Bristol |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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