Heavy Alkaline-Earth Metal Formazanate Complexes and Their Catalytic Applications

The synthesis and characterization of three alkaline-earth metal complexes comprising formazanate ligands are described. Transamination between the neutral formazan [PhNNC(4-tBuPh)NNHPh] (LH) and the bis(amide) precursors [M{N(SiMe3)(2)}(2)(thf)n] (M = Ca, Sr, Ba; n = 0 or 2) in 2:1 or 1:1 stoichiometric ratio gave either the THFcoordinated bis(formazanate) compounds [L2Ca(thf)(2)] and [L2Sr(thf)(2)], or the dimeric heteroleptic mono(formazanate) compound [{LBaN(SiMe3)(2)}(2)]. Depending on the size of the central metal ions, the coordination mode of the formazanate ligand is different. For the smaller Ca ion, each formazanate is symmetrically coordinated to the central metal ion, forming six-membered CaN4C rings. In the case of the larger Sr and Ba ions, each ligand coordinates unsymmetrically in an "open" mode to the metal atoms. The catalytic activities of the three compounds were studied in the hydroboration of carbonyl compounds and in the hydrophosphination of styrenes. Among all three complexes, the Ba amide [{LBaN(SiMe3)(2)}(2)] exhibited the highest catalytic efficiency.