Phosphine-induced ancillary ligand orthometalation at a tantalum-tantalum double bond

The dinuclear tantalum tetrahydride complexes ((RPh)[NPN]Ta)(2)(mu-H)(4) ([NPN] = (PhNSiMe2CH2)(2)PR; R = Ph, Cy) promote the asymmetric activation of molecular nitrogen to form ((RPh)[NPN]Ta)(2)(mu-eta(1):eta(2)-N-2)(mu-H)(2). The coordinatively unsaturated dinuclear dihydride, ((RPh)[NPN]Ta)(2)(mu-H)(2), has been proposed as the reactive intermediate, with a tantalum-tantalum double bond storing four electrons with which to reduce N-2. Efforts to trap ((CyPh)[NPN]Ta)(2)(mu-H)(2) with excess PMe3 at low temperatures promoted the formation of a green product, proposed to be (CyPh [NPN]Ta(PMe3))(2)(mu-H)(2). Attempts to isolate this product promoted the orthometalation of an ancillary ligand N-Ph ring, forming (CyPh)[NPN]Ta(mu-H)(2)[mu-N(C6H4)]Ta[PN](H)(PMe3). Addition of 1 equiv of PMe3 to ((PhPh)[NPN] Ta)(2)(mu-H)(4) immediately produced the analogous orthometalation derivative, [NPN]Ta(mu-H)(2)[mu-N(C6H4)]Ta[PN](H)(PMe3).