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

The compounds Cp-2'Ta(CH2B(C6F5)(2))(mu-H)(CH3), where Cp' = C5H5, a series, and C5H4Me, b series, are generated via treatment of Cp-2'Ta(=CH2)CH3 with HB(C6F5)(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 Cp-2'Ta[eta(2)-CH2B(C6F5)(2)] 1 a and 1 b 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 d2 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 a 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 ill-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.