CO2 activation .7. Formation of the catalytically active intermediate in the hydrogenation of carbon dioxide to formic acid using the [{(COD)Rh(mu-H)}(4)]/Ph(2)P(CH2)(4)PPh(2) 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(mu-H>}(4)] (COD = 1,5-cyclooctadiene; 1) and the bidentate phosphane Ph(2)P(CH2)(4)PPh(2) (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-(P-31,H-1)-COLOC spectra. The tetrameric hydride cluster [{(dppb)Rh(mu-H)}(4)] (2) and the double-phosphane-substituted monomeric rhodium hydride [(dppb)(2)RhH] (3) are formed as byproducts in low yield. The (phosphane)rhodium eta(3)-cyclooctenyl complex [(dppb)Rh(eta(3)-C8H13)] (4), arising via hydride transfer from rhodium to coordinated GOD, 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.