Synthesis, Structure, and Reactivity of a Stabilized Calcium Carbene:  R2CCa

Attempted 2-fold deprotonation of the bis(iminophosphorano)methane ligand, H2C(Ph2PNSiMe3)2 (4 −H2), with a calcium amide led only to mono-deprotonation. The crystal structure of (4 −H)2Ca shows two tridentate ligands with short Ca−N and a rather long Ca−C bond. Reaction of 4 −H2 with a dibenzylcalcium complex gave the desired 2-fold deprotonation and formation of 4 −Ca, which crystallized as a dimeric complex. Analysis of the calculated atomic and group charges in 4 −H2, (4 −H)2Ca, and [4 −Ca]2 showed that the negative charge at the imine nitrogens only slightly increases upon successive deprotonation of 4 −H2. The electron density at the central carbon, however, increases considerably: the charge on the carbene carbon in [4 −Ca]2 is ca. −1.8. The negative charge in 4 2- is therefore mainly located on the carbon. Reaction of [4 −Ca]2 with benzophenone in benzene gave the remarkably stable adduct [4 −Ca]2·OCPh2, which was characterized by X-ray diffraction. Reaction of [4 −Ca]2 with adamantylcyanide gave exclusive formation of the adduct [4 −Ca]2·(N≡CR)2, which did not react further, even at higher temperatures. Addition of cyclohexyl isocyanate to a benzene solution of [4 −Ca]2 gave immediate [2 + 2]-cycloaddition and formation of a dianionic tetradentate ligand that binds to Ca2+ through two nitrogens, the central carbon, and an oxygen. This product crystallized as a dimer with bridging oxygen atoms.