Diazadiene Complexes of the Heavy Alkaline-Earth Metals Strontium and Barium: Structures and Reactivity

1,4-Diaza-1,3-diene (=DAD) complexes of the heavy alkaline-earth metals strontium and barium have been synthesized by direct metalation of N,N′-bis­(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (1, =DADDipp). The reaction with Sr metal afforded a mixture of the red enediamide-type derivative (DADDipp)­Sr­(DME)2 (2, DME = 1,2-dimethoxyethane) and black (DADDipp)2Sr­(DME) (3), which contains two coordinated DAD radical anions. With barium, only the radical anion derivative (DADDipp)2Ba­(DME) (4) was formed in 82% yield. For the first time, transfer of a DAD radical anion ligand from an alkaline-earth metal to a rare-earth metal has been achieved. Reaction of 4 with [{(Ph2SiO)2O}2{Li­(THF)2}2]­HoCl (5) afforded the novel (DAD)holmium bis­(disiloxanediolate) complex [{(Ph2SiO)2O}2{Li­(THF)2}2]­Ho­(DADDipp) (6). All new complexes (2–4 and 6) have been structurally characterized by X-ray diffraction. In addition, the radical anion complexes 3, 4, and 6 were characterized by their EPR spectra.