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

Abstract

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.