The organolanthanides (Cp'2MH)2Cp’ = η5-(CH35C5, M = La, Nd, Sm, and Lu, and (Me2SiCp“2MH)2Cp” η5-(CH34C5, 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 > Me2SiCp2Lu > Cp'2La > Me2SiCp2Sm > Me2SiCp”2Nd, with N1for the most active catalyst exceeding 120 000 h-1at 25 °C and 1 atm of H2. Under the conditions employed, the rate law is v ∝ [olefin]°[Lanthanide] [H2] suggesting rapid, exothermic Lanthanide-H/olefin addition and rate-limiting M-C hydrogenolysis. When D2uptake 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-d2under D2in a non-mass-transport-limited regime) obeys the rate law v [olefin] [Lanthanide]1/2[H2]°, suggesting rate-limiting Lanthanide-H/olefin addition. For Cp’2Lu, the rate law is v ∝ [olefin] [lanthanide] [H2]°, 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 v ∝ [alkyne]°[Lanthanide][H2] (3-hexyne → cis-3-hexene) and v ∝ [olefin] [Lanthanide] 1/2[H2]° (m-3-hexane ∝ n-hexane) except for (Cp'2LuH)2where v ∝ [olefin] [Lanthanide] [H2]°. 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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry