Phosphine-Induced Ancillary Ligand Orthometalation at a Tantalum−Tantalum Double Bond

The dinuclear tantalum tetrahydride complexes (RPh[NPN]Ta)2(μ-H)4 (RPh[NPN] = (PhNSiMe2CH2)2PR; R = Ph, Cy) promote the asymmetric activation of molecular nitrogen to form
(RPh[NPN]Ta)2(μ-η1:η2-N2)(μ-H)2. The coordinatively unsaturated dinuclear dihydride,
(RPh[NPN]Ta)2(μ-H)2, has been proposed as the reactive intermediate, with a tantalum−tantalum double bond storing four electrons with which to reduce N2. Efforts to trap
(CyPh[NPN]Ta)2(μ-H)2 with excess PMe3 at low temperatures promoted the formation of a
green product, proposed to be (CyPh[NPN]Ta(PMe3))2(μ-H)2. Attempts to isolate this
product promoted the orthometalation of an ancillary ligand N−Ph ring, forming
Addition of 1 equiv of PMe3 to (PhPh[NPN]Ta)2(μ-H)4 immediately produced the analogous orthometalation derivative,