M.C. White, Chem 153 Hydrogen ation -151 Week of october 15. 2002 Mechanism of monodentate cationic complexes pathway for their catalyst. Halpern notes some significant differences in the reactivities towards H2 of the catalysts w bidentate and monodentate phosphine ligands? Ph In the absence of olefin substrate. no further uptake of H2 can be detected. The only species observed k vate bserved by Nmr Treatment of the monodentate PPh3 of the rh(illl-dihydride con PhaF observed by NMR Halpern JACS 1977(99)8055 Schrock& Osborn JACS 1976(98)2135 The Trans Effect ties towards H] of the catalysts, Halpern invokes the trans effect. The trans effect is defined as the labilization of ligands trans to certain other ligands. The trans effect often arises when a ligand shares an orbital with another ligand of strong o-bonding character. Because phosphine forms a strong o bond with Rh, trans Rh-H bonds formed will be weak because the orbital is not as available for bonding to H. In the case of the bidentate complex, cis addition of H2 requires that one hydride share an orbital with a phosphine Since both hydride and phosphine are strong o-bonding ligands, the dihydride adduct, once formed, is highly unstable and thus rapidly reverts back via reductive elimination to the solvated 4-coordinate species. In the case of the monodentate phosphine complex, a H2 adduct can form where neither H ligand is trans to a phosphine Classic example of the trans effect: synthesis of "cis-platinum"a chemotherapeutic agent 2 C NH H H3 N H3N H NH3 Cl has a stronger"trans CI has a stronger"trans only the trans Influence"than nh isomer is formed influence "than NH3 isomer is formM.C. White, Chem 153 Hydrogenation -151- Week of October 15, 2002 Mechanism of monodentate cationic complexes Halpern notes that the hydrogenation mechanism for bidentate ligated cationic complexes where olefin substrate coordination precedes oxidative addition of H2 may not be operating for cationic catalysts with monodentate ligands. Schrock-Osborn invoke involvement of the dihydride complex (below) in the principle hydrogenation pathway for their catalyst. Halpern notes some significant differences in the reactivities towards H2 of the catalysts w/ bidentate and monodentate phosphine ligands. Rh(I) PPh3 PPh3 Ph3P Rh(III) S PPh3 H H S + + Rh(I) Ph2 P P Ph2 H2 S Rh(I) S Ph2 P P Ph2 + + In the absence of olefin substrate, no further uptake of H2 can be detected. The only species observed by NMR is the cationic, 4-coordinate solvated species. Treatment of the monodentate catalyst with H2 resulted in detection of the Rh(III)-dihydride complex. Halpern JACS 1977 (99) 8055. Schrock & Osborn JACS 1976 (98) 2135. Ph3P Rh(I) S PPh3 S + only species observed by NMR k1 k-1 H Rh(I) S Ph2 P P Ph2 + k1 k-1 H2 only species observed by NMR H S To explain the difference in reactivities towards H2 of the catalysts, Halpern invokes the trans effect. The trans effect is defined as the labilization of ligands trans to certain other ligands. The trans effect often arises when a ligand shares an orbital with another ligand of strong σ-bonding character. Because phosphine forms a strong σ bond with Rh, trans Rh-H bonds formed will be weak because the orbital is not as available for bonding to H. In the case of the bidentate complex, cis addition of H2 requires that one hydride share an orbital with a phosphine. Since both hydride and phosphine are strong σ-bonding ligands, the dihydride adduct, once formed, is highly unstable and thus rapidly reverts back via reductive elimination to the solvated 4-coordinate species. In the case of the monodentate phosphine complex, a H2 adduct can form where neither H ligand is trans to a phosphine. Classic example of the trans effect: synthesis of "cis-platinum" a chemotherapeutic agent Cl Pt(II) Cl Cl Cl 2- NH3 Cl Pt(II) Cl Cl NH3 - Cl has a stronger "trans influence" than NH3 NH3 Cl Pt(II) Cl NH3 NH3 only the cis isomer is formed H3N Pt(II) H3N NH3 NH3 2+ Cl H3N Pt(II) H3N Cl NH3 + Cl has a stronger "trans influence" than NH3 Cl Pt(II) H3N Cl NH3 only the trans isomer is formed Cl The Trans Effect: