Highly Reactive Organolanthanides. A Mechanistic Study of Catalytic Olefin Hydrogenation by Bis(pentamethylcyclopentadienyl) and Related 4f Complexes

Article abstract of DOI:10.1021/ja00312a052

The organolanthanides (Cp'2MH)2, Cp'=η⁵-(CH3)5C5, M=La, Nd, Sm, and Lu, and (Me2SiCp''2MH)2, Cp''=η⁵-(CH3)4C5, M=Nd, Sm, and Lu are highly active catalysts for olefin hydrogenation.In the case of 1-hexene -> n-hexane, activities decrease in the order Cp'2Lu > Cp'2Sm > Cp'2Nd > Me2SiCp''2Lu > Cp'2La > Me2SiCp''2Sm > Me2SiCp''2Nd, with N_t for the most active catalyst exceeding 120 000 h^-1 at 25 deg C and 1 atm of H2.Under the conditions employed, the rate law is ν α ⁰

suggesting rapid, exothermic Lanthanide-H/olefin addition and rate-limiting M-C hydrogenolysis.When D2 uptake is not mass-transport-limited the predominant product is hexane-1,2-d₂.For all catalysts except (Cp'2LuH)2, cyclohexene hydrogenation (to yield predominantly cyclohexane-1,2-d₂ under D2 in a non-mass-transport-limited regime) obeys the rate law ν α ^1/2

⁰, suggesting rate-limiting Lanthanide-H/olefin addition.For Cp'2Lu, the rate law is ν α

⁰, suggesting rate-limiting (Lu-H)2/olefin addition or that the hydride dimer is largely dissociated under these conditions.The relative activities are less than for 1-hexene and follow the order (Me2SiCp''2NdH)2 > Me2SiCp''2SmH)2 > (Cp'2LaH)2 >/= (Cp'2NdH)2 > (Me2SiCp''2LuH)2 >/= (Cp'2SmH)2.Addition of THF significantly decreases the rate both of 1-hexene hydrogenation and (more so) cyclohexene hydrogenation.For all catalysts examined, 3-hexyne hydrogenation follows a two-stage rate law ν α ⁰

(3-hexine -> cis-3-hexene) and ν α ^1/2

⁰ (cis-3-hexene -> n-hexane) except for (Cp'2LuH)2 where ν α

⁰.The rate laws and relative activity trends for trans-2-hexene and trans-3-hexene hydrogenation parallel those for 1-hexene, while cis-2-hexene behaves like cis-3-hexene.While the present olefin and acetylene hydrogenation catalysis can be understood largely on the basis of established or reasonably extrapolated organo-f-element reactivity and bond enthalpy patterns, the steps in the catalytic cycles must proceed with remarkably low activation energies and must be coupled with high efficiency.