Hydroformylation of formaldehyde to glycolaldehyde with homo- and hetero-metallic catalytic systems involving metal carbonyl species in different oxidation states

Abstract

Hydroformylation of paraformaldehyde and aqueous (37%) formaldehyde to glycolaldehyde with selectivity up to ca. 80% was carried out in several organic solvents at 90 – 140 °C and 100 – 145 atm of carbon monoxide and hydrogen (CO:H 2=1:1) with halide-promoted Rh 4(CO) 12 as catalyst precursor. The co-catalytic behaviour of the halides follows the trend Cl − > I − > Br − > F −, the highest activities and selectivities being observed with Cl −/Rh molar ratios in the range 0.3 – 0.7. IR monitoring of the catalytic solutions with peak activity and selectivity, once returned to ambient conditions, systematically shows the presence in solution of variable mixtures of the [Rh(CO) 2X 2] −, [Rh 5(CO) 15] − and [Rh 5(CO) 14X] 2− (X = halide) carbonyl derivatives. Corresponding mixtures of the above species, prepared by mixing preformed [Rh(CO) 2X 2] −, [Rh 5(CO) 15] − and [N(PPh 3) 2]X salts, enable the hydroformylation of formaldehyde to be carried out with activities and selectivities comparable to those observed with Rh 4(CO) 12 and halides as precursor. Optimization of the [Rh(CO) 2Cl 2] −/[Rh 5(CO) 15] −/Cl − molar ratios allowed the selectivity to be increased up to 90%. An investigation of the catalytic behaviour of each individual carbonyl species taken alone seems to suggest [Rh(CO) 2Cl 2] − as the more probable species responsible for the activation of CO and the selectivity of the reaction, whereas the [Rh 5(CO) 15] −-Cl − pair is mainly responsible for the activation of hydrogen and the overall activity of the above systems. Mixtures of these two rhodium species in different oxidation states give rise to a limited synergetic effect, which is significantly enhanced by addition of extra halides. CIR-FTIR experiments under pressure suggest that activation of hydrogen results from unexpectedly facile fragmentation of [Rh 5(CO) 15] −, probably promoted by chlorides, to a mixture of [Rh(CO) 2Cl 2] −, [Rh(CO) 4] − and HRh(CO) 4. Attempts to substitute either [Rh(CO) 2Cl 2] − or [Rh 5(CO) 15] −, as well as both the above derivatives, with corresponding species of other metals have also been investigated, either in the search for more active catalytic systems, or in the hope to trap some possible intermediate which could give a better insight in the mechanism of the reaction. However, under strictly comparable experimental conditions, these other homo- or hetero-metallic systems do not catalytically perform as effectively as the homometallic rhodium system, with the notable exception of bimetallic [Rh(CO) 2Cl 2] −-HRu 3(CO) 11] −-X − mixtures. Infrared analysis of catalytic samples after being returned to ambient conditions points out the occurrence in solution of a redox reaction between the two precursors, which gives rise to the additional presence of significant amounts of [Ru(CO) 3X 3] − and [Rh 5(CO) 15] −. This heterometallic system, however, rapidly loses selectivity upon increasing the temperature or recycling, probably as a consequence of formation of the [RuRh 4(CO) 15] 2− bimetallic cluster.