CO2 Activation. 7.† Formation of the Catalytically Active Intermediate in the Hydrogenation of Carbon Dioxide to Formic Acid Using the [{(COD)Rh(μ-H)}4]/Ph2P(CH2)4PPh2 Catalyst: First Direct Observation of Hydride Migration from Rhodium to Coordinated 1,5-Cyclooctadiene

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-d8 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...