Syntheses, structures, and reactivity of barium carbene complexes with Chelating bis-iminophosphorano arms

Reaction of (Me3SiN=PPh2)(2)CH2 with Ba[N(SiMe3)(2))(2)](2) in 1:1 ratio gave only monodeprotonation and the homoleptic product [(Me3SiN=PPh2)(2)CH](2)Ba (3-Ba) was formed. Reaction with the strong base dibenzylbarium gave fast and clean 2-fold deprotonation and the dimeric complex [(Me3SiN=PPh2)(2)CBa-THF](2) (1-Ba) was isolated. Also the bulkier ligand (2,6-iPr(2)C(6)H(3)-N=PPh2)(2)CH2 was doubly deprotonated by this reagent, which gave clean formation of the monomeric complex (2,6-iPr(2)C(6)H(3)- N=PPh2)(2)CBa-(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]- ycloaddition 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 pi-Ph center dot center dot center dot Ba contacts.