The Nature of the Heavy Alkaline Earth Metal–Hydrogen Bond: Synthesis, Structure, and Reactivity of a Cationic Strontium Hydride Cluster

The
molecular strontium hydride [(Me3TACD)3Sr3(μ3-H)2]­[SiPh3] (2) was isolated as the dark red benzene solvate 2·C6H6 in
69% yield from the reaction of [Sr­(SiPh3)2(thf)3] (1′) with (Me3TACD)H (1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane).
This
reaction can be considered as a redox process, with the Brønsted
acidic amine proton in (Me3TACD)H transformed into the
hydride by the anion [SiPh3]−. Trace
amounts of water resulted in the formation of [(Me3TACD)3Sr3(μ3-H)­(μ3-OH)]­[SiPh3] (2*), which cocrystallized with 2. Single-crystal X-ray diffraction of 2 revealed a substitutional
disorder of a bridging hydride with a hydroxide ligand. Hydride complex 2 was also obtained by hydrogenolysis of [(Me3TACD)­Sr­(SiPh3)] (3), although pure 3 proved difficult
to isolate. In the presence of a 2-fold excess of (Me3TACD)­H,
the reaction with disilyl 1′ gave [(Me3TACD)­SiPh3] (4). Complex 2 underwent
facile H/D exchange with D2 (1 bar), with the anion [SiPh3]− decomposing concurrently. In the reaction
of 2 with 1,1-diphenylethylene (DPE), the anion [SiPh3]− was added to the CC bond in DPE
to give [(Me3TACD)3Sr3H2]­[Ph2CCH2SiPh3] (5),
whereas the cationic cluster [(Me3TACD)3Sr3H2]+ remained unchanged. 9-Fluorenone
underwent one-electron reduction with 2 to give the paramagnetic
ketyl complex [{(Me3TACD)­H}­Sr­(OC13H8•)2(thf)2] (6). These strontium compounds are structurally similar to the lighter
calcium congeners, but more reactive, in particular with regard to
fast H/D exchange and [SiPh3]− anion
decomposition. DFT studies on the cationic hydride clusters suggest
a more pronounced covalent character for strontium compared to calcium.
Disilyl 1, strontium diketyl 6, and the
calcium congener of 6, [{(Me3TACD)­H}­Ca­(OC13H8·)2] (10), were also characterized by X-ray diffraction.