Initiated by Clusters

This section surveys the use of various di-, tri-, and polynuclear ruthenium complexes as precursors for the homogeneous hydroformylation of alkenes. Several arbitrary assumptions have been made so as to include dinuclear starting complexes which are strictly not cluster compounds. Moreover, no distinction is made between neutral and anionic precursors. Also, in several cases, particularly in the patents, information is lacking concerning the intermediate species involved in the catalytic cycles. Interestingly, half of the described systems come from patents, and there are few fundamental studies which clearly establish the implication of cluster species during the catalysis. 

Another interesting example was reported in a patent by Cooper (18). He prepared ketones using [Ru3(CO),2] as the catalyst precursor from essentially ethylene and propylene, for example, at 100°C and 120 bar (Hj/ CO = 2/1). In addition to the expected butyraldehydes, 23% heptanone was obtained with no production of butanols. With an Hj/CO mixture of 3/1 and in acetic acid as solvent, the selectivity to ketone products can reach 50%. Similarly, at 150°C and 120 bar, Hj/CO of 3/1 in glacial acetic acid, ethylene gave approximately 52% penta-3-one and 48% propanal. 

The carbonyl [Ru3(CO),2] is a good cocatalyst for the low pressure hydroformylation of internal alkenes using the classic rhodium phosphine [HRh(CO)(PPh3),] system in the presence of an excess of triphenylphosphine (P/Rh = 200) (22). Starting from a mixture of hex-2- and hex-3-ene, the addition of [Ru3(CO),2l (Rh/Ru = 1/1) increased both the reaction rate and the n/iso ratio of heptanals. More recently, Poilblanc and coworkers (23) have prepared a mixed ruthenium-rhodium complex formulated as [CIRh(/i-CO)(//-dppm)2Ru(CO)2] (dppm is Ph2PCH2PPh2). This species shows catalytic activity in the hydroformylation of pent-l-ene at 40 bar (H2/C0= 1/1) and 75°C. Conversion to hexanals was 90% in 24 hours and the linearity reached 70%. No further report has appeared to determine the role of the two metals in this catalysis. 

A very interesting synergistic effect has been found by Hidai el al. (24- 26) by using [Co2(CO)8]-[Ru3(CO),2] mixtures. Particularly relevant is the hydroformylation of cyclohexene (110°C, 80 bar, H2/CO = 1/1, 4 hours). Under these conditions, [Co2(CO)g] or [Ru3(CO),2] alone gave very poor yields (14 and 3%, respectively) of cyclohexylmethanal, whereas for an Ru/Co ratio of 9.9 an initial rate 27 times as fast as that with [Co2(CO)g] alone was observed. Even a 1/1 ratio did not result in the 

All the work mentioned in this section is silent on the mechanistic aspects of the reactions, except for two species observed by Schultz and Bellstedt (75) which are hydrido tri- or tetranuclear complexes, namely, [H4Ru4(CO),2] and [H4Ru3(CO),o]. 

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