METAL CLUSTERS .21. SYNTHESIS OF RHODIUM PHOSPHITE CLUSTERS

Reaction of η3-C3H5Rh-1, 5-cyclooctadiene with phosphites and phosphines has through the simple expedient of stoichiometry control yielded a series of quasi-four-coordinate and five-coordinate η3-allylrhodium complexes, η3-C3H5Rh η3-C3H5Rh η3-C3H5Rh η3-C3H5RhL2 and η3-C3H5 RhL3. All members of these two classes exhibited a fast hydrogen cleavage of the allylrhodium bond at 20 °C to form propene (initially) and rhodium hydrides. The cleavage reactions were always preceded or accompanied by ligand exchange so that the reactions typically yielded η3-C3RhL4, H3RhL3, (HRhL3)2, and an unidentified hydride; the yield of the polynuclear η3-C3H5RhL2 species was highest with the η3-C3H5RhL2 complexes. Fully identified were the polynuclear hydrides: |HRh[P(OCH3)3]2)3, |HRh[P(OC2H5)3]2|3, and (HRh[P(0-i-C3H7)3]2)2. Stability of the phosphine analogues was substantially lower and rhodium metal was a pervasive and significant product in the H2 + η3 -C3H5Rh[PR3]„ reactions. The polynuclear hydride complexes were very active catalyst precursors for olefin and acetylene hydrogenation reactions. Turnover rates for olefin hydrogenation were greater than ~two/s at 23 °C at olefin to polynuclear hydride ratios of 10 000: 1. Because acetylenes competed far more effectively than olefins for coordination sites in the polynuclear hydrides, the catalyzed acetylene hydrogenation reactions, although much slower than olefin hydrogenation, yielded largely olefins. The dimer (HRh[P(0-i-C3H7)3]2j2 was a catalyst precursor for arene hydrogenations: the rates were low, the hydrogen addition mode was nonselective (large amounts of trans-dimethylcyclohexanes were produced from xylenes), and in D2-arene reactions there was substantial H-D exchange. In the reactions of jHRh[P(OR)3]2l« with hydrogen, there was no detectable cluster fragmentation. The intermediate in the case of the dimer was identified as H[(/-C3H7O)3P]2RhH3Rh[P(0-;-C3H7)3]2 and as H5Rh3[P(OCH3)3]6 for the trimer. Both these hydrogen adducts were fluxional molecules; hydride ligand migration occurred in each adduct in two mechanistically distinct processes. In the catalytic reaction, the hydrogen addition step and the olefin addition step appear to be largely centered on adjacent rhodium sites. © 1979, American Chemical Society. All rights reserved.