Strontium Hydride Cations Supported by a Large NNNNN Type Macrocycle: Synthesis, Structure, and Hydrofunctionalization Catalysis
Résumé
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(CH2Ph)(2)] with dihydrogen gave free Me(5)PACP, toluene, and oligomeric strontium hydride [SrH2](n), while hydrogenolysis in the presence of 1 equiv of the benzyl cation [(Me(5)PACP)Sr(CH2Ph)]-[B(C6H3-3,5-Me-2)(4)] enabled isolation of the thermally unstable trihydride cation [(Me(5)PACP)(2)Sr-2(mu-H)(3)][B(C6H3-3,5-Me-2)(4)]. When the benzyl cation [(Me(5)PACP)Sr(CH2Ph)][BAr4](2) (Ar = C6H3-3,5-Me-2 or C6H4-4-Bu-n) was reacted with dihydrogen or noctylsilane, dihydride complexes [(Me(5)PACP)(2)Sr-2(mu-H)(2)][BAr4](2) containing a dinuclear core bridged by two hydride ligands were obtained. The soluble dihydride complex [(Me(5)PACP)(2)Sr-2(mu-H)(2)][B(C6H4-4-Bu-n)(4)](2) was tested in olefin hydrogenation and hydrosilylation catalysis. Kinetic analyses for [(Me(5)PACP)(2)Sr-2(mu-H)(2)](2+) showed lower catalytic activity as compared to that of the isostructural calcium homologue [(Me5PACP)(2)Ca-2(mu-H)(2)](2+). This is explained by a shift in the monomer-dimer equilibrium which precedes the catalytic cycle.