Fluorophosphites and Related Ligands for Homogeneous Catalysis

Abstract

Naphthalene-derived cyclic fluorophosphites were previously prepared for use as ligands in Rh-catalysed hydroformylation. In this work, a range of novel 2-alkyl and 2,7-dialkyl-derivatised naphthalene-based cyclic fluorophosphites were successfully prepared and characterised. The incorporation of electron-withdrawing CF3 groups in place of alkyl-groups at the 2- and 7-positions on the naphthalene ring proved unsuccessful. The Rh(I) and Mo(0) coordination chemistry of these ligands, in addition to that of related compounds containing phenyl-substituents, generally followed trends previously observed with known cyclic fluorophosphites.
Rh-complexes of the aforementioned ligands, in addition to industrially significant ligands PPh3 and cyclic fluorophosphite Ethanox 398, were applied to the hydroformylation of 1-hexene at various ligand concentrations. In general, an increase in ligand concentration resulted in an increase in n:iso values and a decrease in TOF; these effects were more pronounced with the naphthalene-derived fluorophosphites. The n:iso values tended to increase with increasing ligand bulk up to a point; the results suggest that the 2-ethyl-7-methyl species possesses the optimal steric bulk as ligands with slightly more or slightly less bulk yielded lower n:iso values. Mono-alkylated species delivered surprisingly low values of n:iso and TOF, seemingly not following the expected trends set by ligands possessing similar steric bulk.
DFT calculations of Rh-complexes of PPh3 and selected fluorophosphite ligands were carried out to compare the energetic favourability of the tris- and bis-ligated hydroformylation cycles. Only a weak correlation could be drawn between the n:iso values obtained from Rh-catalysed hydroformylation experiments and DFT results; the fluorophosphites favoured the L3 cycle significantly more than PPh3 but only minor differences were observed between the fluorophosphites themselves. The unusually low n:iso values obtained with the mono-alkylated ligands during hydroformylation experiments were not reflected in the DFT studies, displaying similar energetic preferences to their di-alkylated analogues which delivered much greater n:iso values.
A novel m-carborane-derived phosphinite pincer ligand intended for dehydrogenation catalysis was successfully synthesised but was unable to undergo complexation to Ni. Dehydrogenation of methanol and formic acid using previously reported m-carborane-derived phosphinite Ni pincer complex was unsuccessful.

Date of Award	1 Oct 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Charl F J Faul (Supervisor) & Paul Pringle (Supervisor)