M C. White, Chem 153 Hydrogenation -156 Week of october 15. 2002 Mechanism of h, activation Base promoted heterolytic cleavage o-donation>> MH+bh heterolytic cleavage note: there is no generally observed for ele xidation state metals that are in their his change at tI oxidation state within the g-complex heir ligand framework Complexation of dihydrogen to the electrophilic, cation Donation of electron density from the H-H o-bond to an empty Ir orbital leaves the H-H with a partial positive (BF4)internal base effecting heterolytic cleavage of the acidified as an dihydrogen a-complex via deprotonation. When L=PPh3, Hδ+ is observed. When more basic alkyl phosphines are used PBu3)the equilibrium lies to the left with the H2 complex 1 being observed exclusively by NMR. It was hypothesized that moving to a more basic phosphine increases the electron density at the metal center. This makes the metal a less effective a-acceptor and attenuate its ability to effectively acidify the dihydrogen complex Crabtree chem. commn. 1999 297 o-bond metathesis. the base is effectively one of the ligands on the metal base may still drive the rxn forward by forming insoluble amine hydrochloride salts HCI RuCl(PHab 8+H Ruci(PPh3) Uhci(PPh3)3 H bond metathesis Crabtree The Organometallic Chemistry of the Transition Metals: 3rd Edition, Wiley: New York; 2001M.C. White, Chem 153 Hydrogenation -156- Week of October 15, 2002 Mechanism of H2 Activation σ-bond metathesis: the base is effectively one of the ligands on the metal RuCl2(PPh3)3 H2 RuCl(PPh3)3 Cl H H δ− δ+ RuHCl(PPh3)3 HCl σ-bond metathesis external amine base may still drive the rxn forward by forming insoluble amine hydrochloride salts Crabtree The Organometallic Chemistry of the Transition Metals: 3rd Edition; Wiley: New York; 2001. Base promoted heterolytic cleavage: H H Mn σ-donation>> π-backbonding heterolytic cleavage σ-complex Mn H BH+ + note: there is no oxidation state change at the metal generally observed for electrophilic metals that are in their highest stable oxidation state within the context of their ligand framework. Example: N NH2 Ir(III) L H L H H + (BF4)- N NH3+ Ir(III) L H H L + (BF4)- Complexation of dihydrogen to the electrophilic, cationic Ir(III) center is predominantly σ-donating in nature. Donation of electron density from the H-H σ-bond to an empty Ir orbital leaves the H-H with a partial positive charge. The pendent NH2 group is thought to act as an internal base effecting heterolytic cleavage of the acidified dihydrogen σ-complex via deprotonation.When L = PPh3, the equilibrium lies far to the right and only the dihydride 2 is observed. When more basic alkyl phosphines are used (L= PBu3) the equilibrium lies to the left with the H2 complex 1 being observed exclusively by NMR. It was hypothesized that moving to a more basic phosphine increases the electron density at the metal center. This makes the metal a less effective σ-acceptor and attenuates its ability to effectively acidify the dihydrogen complex. 1 2 Crabtree Chem. Commun. 1999 297. δ+