Super‐Bulky Penta‐arylcyclopentadienyl Ligands: Isolation of the Full Range of Half‐Sandwich Heavy Alkaline‐Earth Metal Hydrides

: Hydrogenolysis of the half-sandwich penta-arylcyclopentadienyl-supported heavy alkaline-earth-metal alkyl complexes (CpAr )Ae[CH(SiMe3 )2 ](S) (CpAr =C5 Ar5 , Ar=3,5-i Pr2 -C6 H3; S=THF or DABCO) in hexane afforded the calcium, strontium, and barium metal-hydride complexes as the same dimers [(CpAr )Ae(μ-H)(S)]2 (Ae=Ca, S=THF, 2-Ca; Ae=Sr, Ba, S=DABCO, 4-Ae), which were characterized by NMR spectroscopy and single-crystal X-ray analysis. 2-Ca, 4-Sr, and 4-Ba catalyzed alkene hydrogenation under mild conditions (30 °C, 6 atm, 5 mol % cat.), with the activity increasing with the metal size. A variety of activated alkenes including tri- and tetra-substituted olefins, semi-activated alkene (Me3 SiCH=CH2 ), and unactivated terminal alkene (1-hexene) were evaluated.