Strontium Hydride Cations Supported by a Large NNNNN Type Macrocycle: Synthesis, Structure, and Hydrofunctionalization Catalysis.

The use of the 15-membered NNNNN macrocyclic ligand Me 5 PACP (Me 5 PACP = 1,4,7,10,13-pentamethyl-1,4,7,10,13-pentaazacyclopentadecane) allowed the isolation of two cationic strontium hydride complexes by hydrogenolysis of benzyl precursors. Treatment of sparingly soluble [(Me 5 PACP)Sr(CH 2 Ph) 2 ] with dihydrogen gave free Me 5 PACP, toluene, and oligomeric strontium hydride [SrH 2 ] n , while hydrogenolysis in the presence of 1 equiv of the benzyl cation [(Me 5 PACP)Sr(CH 2 Ph)][B(C 6 H 3 -3,5-Me 2 ) 4 ] enabled isolation of the thermally unstable trihydride cation [(Me 5 PACP) 2 Sr 2 (μ-H) 3 ][B(C 6 H 3 -3,5-Me 2 ) 4 ]. When the benzyl cation [(Me 5 PACP)Sr(CH 2 Ph)][BAr 4 ] 2 (Ar = C 6 H 3 -3,5-Me 2 or C 6 H 4 -4- n Bu) was reacted with dihydrogen or n -octylsilane, dihydride complexes [(Me 5 PACP) 2 Sr 2 (μ-H) 2 ][BAr 4 ] 2 containing a dinuclear core bridged by two hydride ligands were obtained. The soluble dihydride complex [(Me 5 PACP) 2 Sr 2 (μ-H) 2 ][B(C 6 H 4 -4- n Bu) 4 ] 2 was tested in olefin hydrogenation and hydrosilylation catalysis. Kinetic analyses for [(Me 5 PACP) 2 Sr 2 (μ-H) 2 ] 2+ showed lower catalytic activity as compared to that of the isostructural calcium homologue [(Me 5 PACP) 2 Ca 2 (μ-H) 2 ] 2+ . This is explained by a shift in the monomer-dimer equilibrium which precedes the catalytic cycle.