Syntheses, Structures, and Reactivity of Barium Carbene Complexes with Chelating Bis-iminophosphorano Arms

Reaction of (Me3SiNPPh2)2CH2 with Ba[N(SiMe3)2]2 in 1:1 ratio gave only monodeprotonation and the homoleptic product [(Me3SiNPPh2)2CH]2Ba (3-Ba) was formed. Reaction with the strong base dibenzylbarium gave fast and clean 2-fold deprotonation and the dimeric complex [(Me3SiNPPh2)2CBa·THF]2 (1-Ba) was isolated. Also the bulkier ligand (2,6-iPr2C6H3−NPPh2)2CH2 was doubly deprotonated by this reagent, which gave clean formation of the monomeric complex (2,6-iPr2C6H3−NPPh2)2CBa·(THF)3 (2-Ba). Although organobarium compounds generally show high reactivity, the monomeric barium carbene complex 2-Ba reacted sluggishly with benzophenone or adamantyl cyanide and reaction products could not be identified. Reaction of 2-Ba with cyclohexyl isocyanate, however, cleanly gave the [2+2]-cycloaddition product, which crystallized as a dimer (4). Crystal structures of monomers 2-Ba and 3-Ba and dimers 1-Ba and 4 show in all cases tridentate (C, N, N‘)-coordination of the bis-iminophosphorano ligand. In some cases the large coordination sphere of Ba2+ was saturated by agostic iPr···Ba2+ and π-Ph···Ba contacts.