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

The molecular strontium hydride [(MeTACD)Sr(μ-H)][SiPh] (2) was isolated as the dark red benzene solvate 2·CH in 69% yield from the reaction of [Sr(SiPh)(thf)] (1') with (MeTACD)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 (MeTACD)H transformed into the hydride by the anion [SiPh]. Trace amounts of water resulted in the formation of [(MeTACD)Sr(μ-H)(μ-OH)][SiPh] (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 [(MeTACD)Sr(SiPh)] (3), although pure 3 proved difficult to isolate. In the presence of a 2-fold excess of (MeTACD)H, the reaction with disilyl 1' gave [(MeTACD)SiPh] (4). Complex 2 underwent facile H/D exchange with D (1 bar), with the anion [SiPh] decomposing concurrently. In the reaction of 2 with 1,1-diphenylethylene (DPE), the anion [SiPh] was added to the C═C bond in DPE to give [(MeTACD)SrH][PhCCHSiPh] (5), whereas the cationic cluster [(MeTACD)SrH] remained unchanged. 9-Fluorenone underwent one-electron reduction with 2 to give the paramagnetic ketyl complex [{(MeTACD)H}Sr(OCH)(thf)] (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 [SiPh] 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, [{(MeTACD)H}Ca(OCH·)] (10), were also characterized by X-ray diffraction.