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

The molecular strontium hydride complex [(Me3TACD)3Sr3(?3-H)2][SiPh3] (2) was isolated in 69% yield as dark red crystals of the benzene solvate 2?C6H6 from the reaction of [Sr(SiPh3)2(thf)3] (1') with (Me3TACD)H (1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane). This transformation can be considered a redox process with the Brønsted acidic amine proton in (Me3TACD)H acting as the hydride source. Trace amounts of water gave 2 co-crystallized with [(Me3TACD)3Sr3(?3-H)(?3-OH)]¬[SiPh3] (2*). Substitutional disorder of a bridging hydride by a hydroxide was detected because the bulky Me3TACD ligands efficiently shield the Sr3H2 fragment in 2. The hydride 2 was also obtained by hydrogenolysis of [(Me3TACD)Sr(SiPh3)] (3), although pure 3 proved difficult to isolate. 1' reacted with a twofold excess of (Me3TACD)H to give [(Me3TACD)SiPh3] (4) via the intermediate [Sr(Me3TACD)2]. 2 promoted facile H/D exchange with D2 (1 bar), in tandem with [SiPh3]- anion decomposition. The anion [SiPh3]- in 2 underwent anti-Markovnikov addition to the C=C bond in 1,1-diphenylethylene (DPE) to give [(Me3TACD)3Sr3H2][Ph2CCH2SiPh3] (5). The cationic molecular cluster [(Me3TACD)3Sr3H2]+ is inert against. 9-Fluorenone underwent one-electron reduction with 2 to give the paramagnetic black ketyl complex [{(Me3TACD)H}Sr(OC13H8?)2(thf)2] (6). The strontium compounds are structurally similar to the lighter calcium congeners, but more reactive, in particular the fast H/D exchange and [SiPh3]- anion decomposition. DFT studies on the cationic hydride clusters suggest a more pronounced covalent character for strontium over calcium.