Chapter 2. Aims of the work

The present study was part of a project targeted at developing new rhodium catalysts for the propene hydroformylation reaction that would favor the formation of the branched aldehyde, isobutanal. The primary focus here was the preparation and characterization of new tertiary phosphane ligands and, while a second objective was to find trends in their behavior as catalytic ligands.

Steric and electronic properties of the phosphane ligand were modified with the aim of producing branched aldehydes in propene and 1-hexene hydroformylation. The first modifications were minor relative to the reference ligand triphenyl­phosphane, but as the work progressed the catalytically most promising features were combined with new modifying pieces. Some of the ligands have been reported earlier, but new preparation was essential to allow investigation of systematically modified groups of ligands. Additionally, most of the spectroscopic data for the ligands is new.

Publication I reports on trifluoromethyl-substituted phenylphosphanes. The main idea in introducing an electron-withdrawing substituent into the para or ortho position(s) was to increase the π -back-bonding from the rhodium atom to phosphorus atom and simultaneous to reduce the rhodium-to-carbonyl π -back-bonding of the trans-CO ligand. This would cause the CO molecule to be less strongly bonded to the rhodium center and facilitate alkene coordination. In the ortho position(s) the trifluoromethyl substituent also caused steric crowding.

Publications II–IV describe the steric modification of arylphosphane ligands through introduction of ortho-alkyl substituents to the aromatic group. The use of ortho-alkyl-substituted ligands was suggested by earlier finding in which (o-methylphenyl)­diphenyl­phosphane ligand, as part of a platinum catalyst in the hydroformylation of 2-butene, exhibited enhanced selectivity to the branched aldehyde [54]. In this work the bulk of the alkyl group was increased step by step, from methyl to ethyl, isopropyl, and cyclohexyl.

Publication V was focused on ortho-alkyl-substituted arylalkylphosphanes. By combining the ortho-alkyl-substituted aryl group through the phosphorus atom with better σ-donor groups, the alkyls, we concomitantly modified the steric and electronic properties of phosphanes.

The third and fifth papers reported on investigations of hydroformylation reactions with ortho-alkyl-substituted pyridylphosphanes. The pyridyl ring(s) was chosen to be a second modifying group since, in earlier investigations, the reaction rates obtained with pyridyl-modified compounds had been higher than the rates of their phenyl analogues. This has been attributed to the electron-withdrawing capacity of the pyridyl ring, which causes the CO molecule to be less strongly bonded to the rhodium center, thus facilitating alkene coordination [48], [55].

Additionally, studies on sterically crowded polyaromatic monodentate phosphanes, anthryl- and naphthylphosphanes, are reported in the first, fourth, and fifth papers, since such ligands have rarely been studied in organometallic chemistry [56], [57], [58], let alone in hydroformylation reactions [59].

During the work it was noted that the catalytic properties of meta-alkyl-substituted triphenylphosphanes had not been extensively studied [60], [61], [62], [63]. π -Electron density of the benzene ring is lower when this is substituted with electron-releasing alkyl group in meta position rather than in ortho or para position [64], and as a consequence the meta-alkyl-substituted phosphanes are poorer electron donor ligands than their ortho and para analogues. Use of m-alkyl-substituted phenylphosphanes as ligands also creates more space in the coordination sphere of a metal, though at the same time the steering effect is less than in the ortho-alkyl-substituted arylphosphanes. This work contains preliminary results of propene hydroformylation tests with three new meta-isopropyl-substituted phenyl­phosphanes.

Further, the synthesis and characterization of three unpublished chromium carbonyl complexes with arylphosphanes containing nitrogen heteroatom are reported for the first time in this publication. The compounds were synthesized to study the coordination chemistry of the prepared tertiary phosphane ligands.

Both the alkyl-substituted arylphosphane and alkyl-substituted mixed ­arylalkyl­phosphane ligands allowed comparative study of their properties and catalytic behavior, and correlations could be sought between spectroscopic and steric properties of the ligands and the hydroformylation results.

The catalytic properties of the ligands in hydroformylation were tested at the Helsinki University of Technology and the University of Joensuu. The modeling work and X-ray crystallographic work were carried out at the University of Joensuu. Some of the hydroformylation results are discussed here since properties of the modified phosphanes can be clarified through reference to their performance in catalysis.