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=C5Ar5, Ar=3,5‐iPr2‐C6H3; 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 (Me3SiCH=CH2), and unactivated terminal alkene (1‐hexene) were evaluated. A set of binuclear heavy alkaline‐earth metal dihydrido complexes with the super‐bulky penta‐arylcyclopentadienyl ligands [(CpAr)Ae(μ‐H)(S)]2 ((CpAr=C5Ar5, Ar=3,5‐iPr2‐C6H3; S=THF, Ae=Ca; S=DABCO, Ae=Sr, Ba) were obtained. These heavy alkaline‐earth metal hydrides could catalyze the hydrogenation of various alkenes under mild conditions, with the activity increasing with the size of the metal center.