The work presented herein covers the synthesis and coordination chemistry of multidentate
phosphine ligands and their application towards the homogeneous catalytic hydrogenation of
biomass-derived levulinic acid (LA) to gamma-Valerolactone (GVL). Bi- and triphosphine nitrogen-centred ligands of the form R’-N(CH2PR2)2 and N(CH2PR2)3 (N-TriphosR
) were synthesised through
phosphorus-based Mannich reactions. This modular ligand system enabled structural variations (R =
Ph, Cy, tBu; R’ = Me, Bn, CH2CH(OEt)2) to be prepared.
The bisphosphine ligands generated square-planar palladium complexes from [PdCl2(COD)];
analogous complexes were made from N-Triphos ligands followed by oxidation of the pendant
phosphine. Whilst Me-N(CH2PCy2)2 formed a piano-stool complex with [RuCl2(p-cymene)]2, N-TriphosCy
generated poorly defined products. Bidentate coordination of Me-N(CH2PCy2)2 to [Ir(COD)Cl]2 was also
achieved, whilst N-Triphos ligands adopted different coordination modes dependent on phosphine
substituent and solvent. Reactions of Me-N(CH2PCy2)2 and N-TriphosCy with [Rh(COD)(MeCN)2]BF4
generated highly air-sensitive complexes; further reactivity with chloroform generated a tri-chloro
bridged rhodium dimer.
The complexes were tested as homogeneous catalysts for the transfer hydrogenation of LA to GVL,
using formic acid (FA) as the hydrogen source. The PCy2-palladium complexes were the most effective.
Moderate GVL yields were readily achieved in solventless conditions, however, further optimisation
proved challenging. This was largely attributed to the catalysts’ water insolubility promoting FA
dehydration to H2O and CO, reducing the hydrogen supply available and facilitating the formation of
CO-bridged palladium dimers. Attempts to develop water-soluble catalysts gave small improvements.
Using a high-pressure regime accessed quantitative LA hydrogenation to GVL with the palladium
catalysts. Additionally, a Co(BF4)2.6H2O/N-TriphosPh system was effective under hydrogen pressures.
A series of octahedral N(CH2P(O)Ph2)3 complexes were formed with first-row transition metals (M =
Mn, Fe, Co Ni, Cu). Collectively, this work represents an advancement in the coordination chemistry
and applications of nitrogen-centered phosphine and phosphine oxide ligands, demonstrating that
this understudied field has high potential for future study.