Quantifying the Electron Donor and Acceptor Abilities of the Ketimide Ligands in M(N=C^tBu₂)₄ (M = V, Nb, Ta)

Addition of 4 equiv of Li(N=C^tBu₂) to VCl₃ in THF, followed by addition of 0.5 equiv of I₂, generates the homoleptic V(IV) ketimide complex, V(N=C^tBu₂)₄ (1), in 42% yield. Similarly, reaction of 4 equiv of Li(N=C^tBu₂) with NbCl₄(THF)₂ in THF affords the homoleptic Nb(IV) ketimide complex, Nb(N=C^tBu₂)₄ (2), in 55% yield. Seeking to extend the series to the tantalum congener, a new Ta(IV) starting material, TaCl₄(TMEDA) (3), was prepared via reduction of TaCl₅ with Et₃SiH, followed by addition of TMEDA. Reaction of 3 with 4 equiv of Li(N=C^tBu₂) in THF results in the isolation of a Ta(V) ketimide complex, Ta(Cl)(N=C^tBu₂)₄ (5), which can be isolated in 32% yield. Reaction of 5 with Tl(OTf) yields Ta(OTf)(N=C^tBu₂)₄ (6) in 44% yield. Subsequent reduction of 6 with Cp∗₂Co in toluene generates the homoleptic Ta(IV) congener Ta(N=C^tBu₂)₄ (7), although the yields are poor. All three homoleptic group 5 ketimide complexes exhibit squashed tetrahedral geometries in the solid state, as determined by X-ray crystallography. This geometry leads to a dx2-y2¹ (²B₁ in D_2d) ground state, as supported by DFT calculations. EPR spectroscopic analysis of 1 and 2, performed at X- and Q-band frequencies (∼9 and 35 GHz, respectively), further supports the ²B₁ ground-state assignment, whereas comparison of 1, 2, and 7 with related group 5 tetra(aryl), tetra(amido), and tetra(alkoxo) complexes shows a higher M-L covalency in the ketimide-metal interaction. In addition, a ligand field analysis of 1 and 2 demonstrates that the ketimide ligand is both a strong π-donor and strong π-acceptor, an unusual combination found in very few organometallic ligands.