Synthesis and chemistry of zwitterionic tantala-3-boratacyclopentenes: olefin-like reactivity of a borataalkene ligand.

The compounds, where Cp' = C(5)H(5), a series, and C(5)H(4)Me, b series, are generated via treatment of with HB(C(6)F(5))(2). When allowed to undergo irreversible methane loss in the presence of an excess of the sterically modest alkynes 2-butyne or phenylacetylene, the putative intermediates 1a and 1b are trapped as the tantala-3-boratacyclopentene compounds 2 and 3, respectively. In these complexes, the alkyne and borataalkene ligands have reductively coupled at the d(2) tantalum center. For the unsymmetrical alkyne, a kinetic product resulting from coupling in the opposite regiochemical sense is observed; the thermodynamic products 3-t incorporate the phenyl group in the alpha position of the tantalaboratacyclic ring. Two of these compounds (2b and 3b-t) were characterized crystallographically. For bulkier alkynes (diphenylacetylene, 1-phenyl-1-propyne, and 3-hexyne), intermediates with similar spectroscopic properties to the tantala-3-boratacyclopentenes were observed, but the ultimate products were the vinylalkylidene compounds 5-(R,R'). Compound 5-(Ph,Me) was characterized crystallographically, and it was found that the vinylalkylidene binds to the metal in an eta(1)-bonding mode, with the tantalum center receiving further ligation through a hydridoborate moiety. Mechanistic studies suggest that these products arise via retrocyclization of the tantala-3-boratacyclopentenes formed kinetically. These studies represent the first studies concerning the reactivity of a borataalkene ligand at a transition metal center and show that it can behave in an "olefin-like" manner, despite having a more flexible array of bonding modes available to it than an olefin.