The racemic and meso diastereomers of an electron-rich binucleating tetraphosphine ligand have been used to prepare homobimetallic rhodium norbornadiene complexes. The racemic bimetallic Rh complex is an excellent hydroformylation catalyst for 1-alkenes, giving both a high rate of reaction and high regioselectivity for linear aldehydes, whereas the meso complex is considerably slower and less selective. A mechanism involving bimetallic cooperativity between the two rhodium centers in the form of an intramolecular hydride transfer is proposed. Mono- and bimetallic model complexes in which the possibility for bimetallic cooperativity has been reduced or eliminated are very poor catalysts.

The cobalt complexes HCo(CO)4 and HCo(CO)3(PR3) were the original industrial catalysts used for the hydroformylation of alkenes through reaction with hydrogen and carbon monoxide to produce aldehydes. More recent and expensive rhodium-phosphine catalysts are hundreds of times more active and operate under considerably lower pressures. Cationic cobalt(II) bisphosphine hydrido-carbonyl catalysts that are far more active than traditional neutral cobalt(I) catalysts and approach rhodium catalysts in activity are reported here. These catalysts have low linear-to-branched (L:B) regioselectivity for simple linear alkenes. However, owing to their high alkene isomerization activity and increased steric effects due to the bisphosphine ligand, they have high L:B selectivities for internal alkenes with alkyl branches. These catalysts exhibit long lifetimes and substantial resistance to degradation reactions.

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Cobalt-rhenium based catalysts were prepared by coimpregnation from two different cobalt precursors: cobalt nitrate [CoRe(N)] and cobalt acetate [CoRe(A)]. They were characterized by H 2 -TPR, ICP, XRD, DRIFTS, and activity/selectivity in CO hydrogenation. The results showed that precursors have a significant effect on the cluster size, dispersion, and CO adsorption/CO hydrogenation activities. XRD showed no bulk crystallinity for the CoRe(A) catalyst, whereas peaks corresponding to a Co 3 O 4 phase were found for the CoRe(N) catalyst. TPR results suggested greater cobalt-rhenium contact for the CoRe(A) catalyst, with Re facilitating reduction of cobalt oxide by hydrogen spillover. Activity/selectivity studies showed that the CoRe(N) catalyst is more active for CO hydrogenation with high selectivity toward hydrocarbons, while the CoRe(A) catalyst has far higher selectivity to oxygenates (but considerably lower overall activity). DRIFTS studies at 25 °C for CO reacting with CoRe(N) showed lower frequency carbonyl bands (2057 and 1942 cm -1 ), whereas CoRe(A) had CO bands at much higher frequencies (2183-2175, 2125, and 2074 cm -1 ). The carbonyl bands in the 2183-2175 cm -1 region are assigned to Co(II)/Co(III)-CO from the presence of nonreduced Co 3 O 4 on the surface of the CoRe(A) catalyst. DRIFTS studies under CO hydrogenation conditions are also presented. Lower wavenumber IR bands seen between 1990 and 1920 cm -1 for CoRe(N) are tentatively assigned to bridging CO's on the cobalt and terminal carbonyls on Re(0) clusters. Only higher frequency CO's are observed for CoRe(A) corresponding to less electron-rich cobalt centers, linear CO coordination, and oxygenate production. The presence of nanoparticle catalysts and highly dispersed Re on the CoRe(A) catalyst is proposed to be key factors in the high oxygenate selectivity. CO is weakly adsorbed on these sites facilitating the M-CO bond dissociation and increasing the CO insertion probability leading to the oxygenate formation. † Part of the "Alfons Baiker Festschrift".