Quantifying the Electron Donor and Acceptor Abilities of the Ketimide Ligands in M(NCtBu2)4 (M = V, Nb, Ta)

Addition of 4 equiv of Li­(NCtBu2) to VCl3 in THF, followed by addition of 0.5 equiv of I2, generates the homoleptic V­(IV) ketimide complex, V­(NCtBu2)4 (1), in 42% yield. Similarly, reaction of 4 equiv of Li­(NCtBu2) with NbCl4(THF)2 in THF affords the homoleptic Nb­(IV) ketimide complex, Nb­(NCtBu2)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CtBu2) in THF results in the isolation of a Ta­(V) ketimide complex, Ta­(Cl)­(NCtBu2)4 (5), which can be isolated in 32% yield. Reaction of 5 with Tl­(OTf) yields Ta­(OTf)­(NCtBu2)4 (6) in 44% yield. Subsequent reduction of 6 with Cp*2Co in toluene generates the homoleptic Ta­(IV) congener Ta­(NCtBu2)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 D2d) 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.