Synthesis, Structure, and Reactivity of a Stabilized Calcium Carbene: R2CCa.
Calcium chelate dimers with mono- and bis-deprotonated methylenebis(phosphine imine) were prepd. by reaction of the ligands with calcium hexamethyldisilazide and dibenzylcalcium THF complex, resp.; the crystal structures for the carbanionic and carbene species were detd. Reaction of bis(iminophosphorano)methane ligand, H2C(Ph2P:NSiMe3)2 (4-H2), with a calcium hexamethyldisilazide led only to mono-deprotonation, affording mononuclear complex [[HC(Ph2P:NSiMe3)2]Ca], (4-H)2Ca. 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 tetrakis(tetrahydrofuran) complex gave the desired 2-fold deprotonation and formation of [[C(Ph2P:NSiMe3)2]Ca]2 (4-Ca)2, which crystd. as a dimeric complex. Anal. of the calcd. at. and group charges in 4-H2, (4-H)2Ca, and [4-Ca]2 showed that the neg. charge at the imine nitrogens only slightly increases upon successive deprotonation of 4-H2. The electron d. at the central carbon, however, increases considerably: the charge on the carbene carbon in [4-Ca]2 is ca. -1.8. The neg. charge in 42- 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 1-adamantanecarbonitrile gave exclusive formation of the adduct [4-Ca]2.(N.tplbond.CR)2 (R = 1-adamantyl), which did not react further, even at higher temps. Addn. of cyclohexyl isocyanate to a benzene soln. of [4-Ca]2 gave immediate [2 + 2]-cycloaddn. and formation of a dianionic tetradentate ligand that binds to Ca2+ through two nitrogens, the central carbon, and an oxygen. This product crystd. as a dimer with bridging oxygen atoms.
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