Search

The first homogeneous catalytic system where reactions of a relatively low volatility product can be performed in a continuous flow process is demonstrated. This is achieved by using an ionic catalyst dissolved in an ionic liquid and using scCO2 as the transport vector for the substrates and products.

The reduction of Rh[Cl.sub.3]*3[H.sub.2]O by sodium amalgam in the presence of P-i-[Pr.sub.3] has been reexamined. Reaction of Rh[Cl.sub.3]*3 [H.sub.2]O with P-i-[Pr.sub.3] in THF gives i-[Pr.sub.3]PO and [RhH [Cl.sub.2](P-i-[Pr.sub.3]).sub.2]] or [RhD[Cl.sub.2] (P-i-[Pr.sub.3]).sub.2]] if [D.sub.2]O is added. Further reaction of [[RhH[Cl.sub.2][(P-i-[Pr.sub.3]).sub.2]] with [P-i-[Pr.sub.3] gives [i-[Pr.sub.3]PH][[RhH[Cl.sub.3](P-i-[Pr.sub.3]).sub.2]], which reversibly dissociates to [RhH[Cl.sub.2][(P-i-[Pr.sub.3]).sub.2]] and [i-[Pr.sub.3]PH]Cl at high temperatures in solution. Sodium amalgam reduction of [RhH[Cl.sub.2][(P-i-[Pr.sub.3]).sub.2]] in THF containing P-i-[Pr.sub.3] gives [RhH[(P-i-[Pr.sub.3]).sub.3]] if water is rigorously excluded, but [Rh[H.sub.2]Cl[(P-i-[Pr.sub.3]).sub.2]] if water is present. Key words: rhodium, trisiopropylphosphine, hydride, reduction.

Using density functional theory calculations (at the B97‐D2//BP86 level) and measurements of kinetic isotope effects, we explored the mechanism of [RuH2(PPh3)3(CO)] (22) in catalytic acceptor‐less dehydrogenation of methanol to formaldehyde. 22 is found to exhibit a similar activity as the previously studied [RuH2(H2)(PPh3)3] (1 b) complex. On the computed pathway, η2→η1 slippage of Ru‐bound formaldehyde prior to decoordination is indicated to be rate‐limiting, consistent with the low kH/kD KIE of 1.3 measured for this reaction. We also explored computationally the possibility of achieving complete dehydrogenation of methanol (into CO2 and H2), through subsequent decarbonylation of formaldehyde and water‐gas shift reaction of the resulting carbonyl complex. Complete pathways of this kind are traced for 22 and for [RuH2(PPh3)2(CO)2]. An alternative mechanism, involving a gem‐diol intermediate (obtained upon attack of OH− to coordinated formaldehyde), has also been investigated. All these pathways turned out to be unfavourable kinetically, in keeping with the lack of CO2 evolution experimentally observed in this system. Our calculations show that the reactions are hampered by the low electrophilicities of the CO and HCHO ligands, making OH− uptake unfavourable. Consequently, the subsequent intermediates are too high‐lying on the reaction profiles, thus leading to high kinetic barriers and preventing full dehydrogenation of methanol to occur by this kind of mechanism. [ABSTRACT FROM AUTHOR]