The coordination chemistry and catalytic applications of nitrogen-centred triphosphine ligands
File(s)
Author(s)
Phanopoulos, Andreas
Type
Thesis
Abstract
Tripodal triphosphine ligands featuring a heteroatomic apical moiety of general formula R'E(CH2PR2)3 are under-investigated compared to mono- or bidentate phosphines, and to the carbon-centred analogue CH3C(CH2PPh2)3. The work presented in this thesis is an investigation of the nitrogen-centred triphosphine ligands N(CH2PR2)3 (NP3R), including coordination behaviour and catalytic applications. behaviour and catalytic applications.
The modular synthesis of NP3R ligands allowed the substituents at phosphorus to be replaced, generating novel symmetrical ligands (with three identical phosphine arms), as well as unsymmetrical "mixed-arm" derivatives with PPN coordinating moieties. Coordination of these, and previously reported ligands to tungsten(0) precursors afforded complexes with the NP3R ligands in either a bi- or tridentate coordination mode. Evaluation or the steric and electronic properties of the NP3R ligands suggest that the diff erent coordination modes observed are primarily electronically controlled, with more electron-donating phosphines coordinating in a bidentate fashion.
A comparative study of the phenyl and cyclopentyl ligands NP3Ph and NP3Cyp coordination to ruthenium precursors displayed similar trends to those observed during tungsten coordination. The electronic parameters of the ligands, as well as oxidation state of the metal were crucial in determining the denticity of NP3R coordination. A facile route to synthesise the dihydride complexes, including [RuH2(CO)(k3-NP3Ph)], was established, which proceeded under mild conditions and via the air- and moisture-stable carbonate complex [Ru(CO3)(CO)(k3-NP3Ph)].
Three pre-formed ruthenium complexes, as well as systems generated in situ were evaluated as catalysts for the hydrogenation of the biomass-derived levulinic acid to value-added compounds. In the absence of acidic co-catalyst, quantitative conversion of levulinic acid to 1,4-pentanediol was achieved using the pre-formed ruthenium complex [RuH2(PPh3)(k3-NP3Ph)], which crucially featured a labile PPh3 ligand. The addition of catalytic amounts of a strongly acidic component that also features a highly non-coordinating conjugate base (HNTf2) allowed the conversion of levulinic acid to 2-MTHF in 87% yield, making this one of the most active systems reported for this reaction under relatively mild conditions.
The modular synthesis of NP3R ligands allowed the substituents at phosphorus to be replaced, generating novel symmetrical ligands (with three identical phosphine arms), as well as unsymmetrical "mixed-arm" derivatives with PPN coordinating moieties. Coordination of these, and previously reported ligands to tungsten(0) precursors afforded complexes with the NP3R ligands in either a bi- or tridentate coordination mode. Evaluation or the steric and electronic properties of the NP3R ligands suggest that the diff erent coordination modes observed are primarily electronically controlled, with more electron-donating phosphines coordinating in a bidentate fashion.
A comparative study of the phenyl and cyclopentyl ligands NP3Ph and NP3Cyp coordination to ruthenium precursors displayed similar trends to those observed during tungsten coordination. The electronic parameters of the ligands, as well as oxidation state of the metal were crucial in determining the denticity of NP3R coordination. A facile route to synthesise the dihydride complexes, including [RuH2(CO)(k3-NP3Ph)], was established, which proceeded under mild conditions and via the air- and moisture-stable carbonate complex [Ru(CO3)(CO)(k3-NP3Ph)].
Three pre-formed ruthenium complexes, as well as systems generated in situ were evaluated as catalysts for the hydrogenation of the biomass-derived levulinic acid to value-added compounds. In the absence of acidic co-catalyst, quantitative conversion of levulinic acid to 1,4-pentanediol was achieved using the pre-formed ruthenium complex [RuH2(PPh3)(k3-NP3Ph)], which crucially featured a labile PPh3 ligand. The addition of catalytic amounts of a strongly acidic component that also features a highly non-coordinating conjugate base (HNTf2) allowed the conversion of levulinic acid to 2-MTHF in 87% yield, making this one of the most active systems reported for this reaction under relatively mild conditions.
Version
Open Access
Date Issued
2015-06
Date Awarded
2015-10
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Long, Nicholas
Miller, Philip
Publisher Department
Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)