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

A set of binuclear heavy alkaline‐earth metal dihydrido complexes with the super‐bulky penta‐arylcyclopentadienyl ligands [(Cp^Ar)Ae(μ‐H)(S)]₂ ((Cp^Ar=C₅Ar₅, Ar=3,5‐ⁱ Pr₂‐C₆H_3; 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.

Abstract

Hydrogenolysis of the half‐sandwich penta‐arylcyclopentadienyl‐supported heavy alkaline‐earth‐metal alkyl complexes (Cp^Ar)Ae[CH(SiMe₃)₂](S) (Cp^Ar=C₅Ar₅, Ar=3,5‐ⁱ Pr₂‐C₆H_3; S=THF or DABCO) in hexane afforded the calcium, strontium, and barium metal–hydride complexes as the same dimers [(Cp^Ar)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.

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