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Water-soluble rhodium-phosphane complexes are very efficient catalysts for the hydrogenation of carbon dioxide to formic acid in water-amine mixtures similar to those used for separation of C02 from flue gases.

The synthesis of colloidal Pt particles obtained from the reaction between platinum acetylacetonate (Pt(acac) 2 ) and Al(CH 3 ) 3 was monitored, and the formation of the Pt colloid via "reductive stabilization" was established by a combination of techniques including NMR, anomalous small-angle X-ray scattering (ASAXS), X-ray absorption spectroscopy (XAS), density functional theory (DFT) computations, and transmission electron microscopy (TEM). For the first time, a defined organometallic compound was identified as the common intermediate in the formation of an organometallic Pt complex or a Pt colloid, depending on the type of ligands or stabilizers present. During the decomposition of this intermediate state, stable colloidal particles of just one size are observed the number of which increases with time.

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The nature of the catalytically active intermediate formed in situ from the tetrameric cluster [{(COD)Rh(μ-H)}4] (COD = 1,5-cyclooctadiene; 1) and the bidentate phosphane Ph2P(CH2)4PPh2 (dppb) during hydrogenation of CO2 to formic acid was investigated. Kinetic measurements suggest the initial formation of a catalyst precursor that reacts with dihydrogen to give the actual active species. NMR spectroscopic investigations of the reaction of 1 with dppb in THF-d 8 reveal three phosphorus-containing products that were fully characterized by one- and two-dimensional techniques, including 2D-(31P,1H)-COLOC spectra. The tetrameric hydride cluster [{(dppb)Rh(μ-H)}4] (2) and the double-phosphane-substituted monomeric rhodium hydride [(dppb)2RhH] (3) are formed as byproducts in low yield. The (phosphane)rhodium η3-cyclooctenyl complex [(dppb)Rh(η3-C8H13)] (4), arising via hydride transfer from rhodium to coordinated COD, is the major product, containing about 80% of the dppb. Complex 4 was isolated from the mixture of products, and its molecular structure was determined by X-ray crystal diffraction. Hydrogenolysis of the allyl moiety in the presence of excess dppb was shown to yield 3 presumably via the 14e species [(dppb)RhH]. The results are most consistent with the formation of 4 as the actual precursor for the active species [(dppb)RhH] in the rhodium-catalyzed hydrogenation of CO2 to formic acid using in situ catalysts consisting of 1 and dppb.

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