| The reactivity of the highly electrophilic borane bis(pentafluorophenyl)borane, 1, with Schrock's methylidene, Cp′2Ta(=CH 2)(CH3) (37) was investigated. Attack at the methylidene moiety with one equivalent leads to a methyl hydride species, 42, that is unstable to methane loss. Solid state structural characterization on carbonyl (46a) and isocyanide (45) adducts of the putative complex that forms after methane loss has revealed η 2 coordination of a borataalkene ligand, [CH2B(C6 F5)2]−. In related compounds, the borataalkene exclusively binds the metal center in an η1 fashion. With the aid of Density Functional Theory calculations, the bonding characteristics of the borataalkene ligand have been investigated and are similar to bonding displayed by isoelectronic olefin complexes. These characteristics have been used for the explanation of isotopic exchange in 42 and a related dihydride 40.; The reactivity of the coordinated borataalkene ligand has also been investigated. Analogous to early metal olefin complexes, [CH2B(C6F 5)]− undergoes coupling reactions with added alkyne reagents to form boron containing, zwitterionic, tantalacyclopentene derivatives. Tantalacycle species formed with sterically bulky alkynes, decompose to give zwitterionic vinylalkylidene complexes (102–104). Two major mechanistic possibilities for the formation of the vinylalkylidene compound were proposed, but could not be distinguished.; Finally, the formation of α-agostic zwitterions was achieved through reaction of Schrock's methylidene, 37a, with tris(pentafluorophenyl)borane, 2a. The fundamental isocyanide insertion and η2-iminoacyl isomerization chemistry of these zwitterions, as well as the dimethyltantalocene cation, 130, was explored and compared with known examples in the current literature. |
