Dianion and Monoanion Ligation of 1,4-Diaza-1,3-butadiene to Barium, Strontium, and Calcium

Two synthetic protocols, a salt metathesis reaction and
a direct
metalation, were developed for preparing 1,4-diaza-1,3-butadiene complexes
of barium, strontium, and calcium, in which 1,4-diaza-1,3-butadiene
serves as a dianionic or monoanionic ligand. A salt metathesis reaction
of BaI2 with the dipotassium salt of N,N′-bis­(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene
(1; abbreviated (Dip)2DAD) afforded the iodide-bridged
dinuclear complex [[K­((Dip)2DAD)­(THF)2]­[Ba­(μ-I)­(THF)2]]2 (2) bearing a dianionic ene-diamide
ligand, while the reaction of MI2 (M = Sr, Ca) with the
dipotassium salt of 1 gave the mononuclear complexes
[M­((Dip)2DAD)­[THF]4] (4, M = Sr; 5, M = Ca). A direct metalation reaction of barium powder
with (Dip)2DAD in the presence of iodine (10 mol%) afforded
an iodide-bridged dinuclear complex, [Ba­((Dip)2DAD)­(μ-I)­(THF)2]2 (3), in which (Dip)2DAD coordinates as a monoanionic ligand to the barium center, as
was evident from the X-ray analysis and the EPR spectral data. The
products from the direct metalation reaction of Sr and Ca powders
with 1 in the presence of a catalytic amount of iodine
(1 mol%) resulted in the formation of mononuclear complexes 4 and 5 bearing the dianionic ene-diamide DAD
ligand.