Diamagnetic Group 6 Tetrakis(di-tert-butylketimido)metal(IV) Complexes

Abstract

The addition of 4 equiv of LiN=C-t-Bu(2) to CrCl(3), MoCl(5), and WCl(6) in diethyl ether produced the complexes M(N=C-t-Bu(2))(4) (M = Cr, Mo, W). Single-crystal X-ray diffraction studies revealed that the molecules have flattened tetrahedral geometries with virtual D(2d) symmetry in the solid state. (1)H and (13)C NMR spectra indicated that the complexes are diamagnetic, and a qualitative MO analysis showed that the orthogonal π-donor and -acceptor orbitals of the ketimide ligand cooperatively split the d(xy) and d(z2) orbitals sufficiently to allow spin pairing in the d(xy) orbital. A more sophisticated quantum-mechanical analysis of Cr(N=C-t-Bu(2))(4) using density functional/molecular mechanics methods confirmed the qualitative analysis by showing that the singlet state is 27 kcal/mol more stable than the triplet state.

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

“…Soriaga et al . did not mention σ-bonding in this class of complexes, 9 but we note that the ketimide ligand is likely a strong σ-donor and this interaction alone could potentially lead to a large (for T d ML 4 ) splitting, ${\Delta }_{\text{tet}}=\frac{4}{3}{\epsilon }_{\sigma }$, between the e ( d z 2 , d x 2 − y 2 ) and t 2 ( d xz , d yz , d xy ) metal ion orbitals. For illustrative purposes, we begin with only σ-bonding and the structure of the Ti(IV) congener, which has θ only ~2° away from ideal tetrahedral (τ = 0.96).…”

“…6 To achieve this description, we define the x and y axes as lying between the M-L bonds (bisecting the L-M-L angles) (Figure 11), which is in any case more appropriate for an axial ( x = y ) system. If the x and y axes were defined along the M-L bonds, as done by Soriaga et al, 9 then the representation of d x 2 − y 2 and d xy would be reversed. Such a definition would be appropriate for C 2 v symmetry, where x and y are symmetry inequivalent.…”

“…Although we have not specifically performed calculations for the Cr(IV) congener, it is clear from our MO diagram that this complex would have a filled b 1 HOMO with the triplet excited state(s) significantly higher in energy, in agreement with earlier work. 9 …”

“…With their syntheses, a complete set of first row M((N=C t Bu 2 ) 4 complexes (M = Ti–Co), as well as two triads, the Group 6 triad (which reported by Hoffman) 9 and now the Group 5 triad, have been isolated and fully characterized. In the solid state, the Group 5 ketimide complexes feature squashed tetrahedral ( D 2d ) geometries.…”

“…8 Our group also synthesized the homoleptic Mn(IV) and Co(IV) ketimides, M(N=C t Bu 2 ) 4 (M = Mn, Co), 4,6 while Hoffman and co-workers reported the isolation of Cr(N=C t Bu 2 ) 4 . 9 In addition, Kiplinger and co-workers reported the formation of a rare U(V) organometallic, Cp* 2 U(NDipp)(N=CPh 2 ) (Dipp = 2,6- i Pr 2 C 6 H 3 ), stabilized by inclusion of the ketimide ligand. 2 The ability of the ketimide ligand to stabilize high oxidation states likely stems from the orbitals available for the metal-ketimide interaction.…”