Rhodium complexes bicarbonates

When investigating the aqueous-phase bicarbonate hydrogenation with ruthenium and rhodium complexes, Benyei and J06 observed certain activity for the reverse reaction, that is, formate decomposition. [RuCl2(mTPPMS)2]2 (mTPPMS = meta-monosulfonated triphenylphosphine) decomposed sodium formate and formic acid (41), while RhCl(mTPPMS)3 slowly decomposed calcium formate and promoted calcium carbonate precipitation (42). 

The use of CO2 as a reagent for synthetic purposes would be highly desirable, due not only to the vast availabiUty of this gas but also its environmental concerns. The stoichiometric activation of CO2 has been achieved with the iridium-PCP complex 29 comprising an alkyl rather than an aryl skeleton (Scheme 12.12) [32]. The addition of CO2 to the dihydride complex results in C=0 insertion into the iridium-hydride bond, and affords the formate complex 30. However, this complex is not stable and disproportionates spontaneously into the virtually insoluble bicarbonate complex 31 and the carbonyl dihydride 32. Such disproportionation is suppressed when the iridium metal center is replaced by rhodium [33], which is generally assumed to have a lower hydride affinity than iridium. 

Reaction 6 has been documented for H2Rh(02C0H)(PCy3)2 (16).) Other IR bands at 1580(s) and 1235(m) cm"1 that result from the solution C02 reactions could arise from a C02 complex (54), a formate complex formed via C02 insertion into Rh-H (16), or even a bicarbonate formed via Reaction 7, which has been documented again for a rhodium/PCy3 system (16). 

Similar to the hydroacylation with aldehydes, the hydroformylation of alkenes with paraformaldehyde is efficiently catalyzed by the rhodium bicarbonate complex RhIl2(OOCOH)[P(r-Pr)3]2 and gives the corresponding aldehyde in neutral solution55. 

Yoshida T, Thom DL, Okano T, Ibers JA, Otsuka S (1979) Hydration and reduction of carbon dioxide by rhodium hydride compounds. Preparation and reactions of rhodium bicarbonate and formate complexes, and the molecular structure of RhH2(02COH)(P(i-Pr)3)2. J Am Chem Soc 101 4212-4221... 

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