Quantifying the electron donor and acceptor ability of the ketimide ligands in M(N=CtBu2)4 (M = V, Nb, Ta)

Addition of 4 equiv of Li(N=C t Bu 2 ) to VCl 3 in THF, followed by addition of 0.5 equiv I 2 , generates the homoleptic V(IV) ketimide complex, V(N=C t Bu 2 ) 4 ( 1 ), in 42% yield. Similarly, reaction of 4 equiv of Li(N=C t Bu 2 ) with NbCl 4 (THF) 2 in THF affords the homoleptic Nb(IV) ketimide complex, Nb(N=C t Bu 2 ) 4 ( 2 ), in 55% yield. Seeking to extend the series to the tantalum congener, a new Ta(IV) starting material, TaCl 4 (TMEDA) ( 3 ), was prepared via reduction of TaCl 5 with Et 3 SiH, followed by addition of TMEDA. Reaction of 3 with 4 equiv of Li(N=C t Bu 2 ) in THF results in a isolation of a Ta(V) ketimide complex, Ta(Cl)(N=C t Bu 2 ) 4 ( 5 ), which can be isolated in 32% yield. Reaction of 5 with Tl(OTf) yields Ta(OTf)(N=C t Bu 2 ) 4 ( 6 ) in 44% yield. Subsequent reduction of 6 with Cp* 2 Co in toluene generates the homoleptic Ta(IV) congener Ta(N=C t Bu 2 ) 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 d x 2 − y 2 1 ( 2 B 1 in D 2 d ) 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 2 B 1 ground state assignment, while 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.