What can NMR spectroscopy of selenoureas and phosphinidenes teach us about the π-accepting abilities of N-heterocyclic carbenes?

Abstract

The electronic nature of the interaction of NHCs with metal centres is of interest when exploring their properties, how these properties influence those of metal complexes, and how these properties might depend on ligand structure. Selenourea and phosphinidene complexes have been proposed to allow the measurement of the π-accepting ability of NHCs, independent of their σ-donating ability, via the collection of ⁷⁷Se or ³¹P NMR spectra, respectively. Herein, the synthesis and characterisation of selenoureas derived from a range of imidazol-2-ylidenes, 4,5-dihydroimidazol-2-ylidenes and triazol-2-ylidenes are documented. Computational studies are used to explore the link between the shielding of the selenium centre and the electronic properties of the NHCs. Results show that δ_Se is correlated to the energy gap between a filled lone pair orbital on Se and the empty π* orbital corresponding to the Se-NHC bond. Bond energy decomposition analysis indicated no correlation between the orbital σ-contribution to bonding and the chemical shielding, while a good correlation was found between the π-contribution to bonding and the chemical shielding, confirming that this technique is indeed able to quantify the ability of NHCs to accept π-electron density. Calculations conducted on phosphinidene adducts yielded similar results. With the link between δ_Se and δ_P and π-back bonding ability clearly established, these compounds represent useful ways in which to fully understand and quantify this aspect of the electronic properties of NHCs.

Fig. 1. Methods used to quantify the electronic properties of carbenes.

Fig. 2. Selenourea compounds considered in this study, and their ⁷⁷Se chemical shift values (δ _Se) obtained in acetone-d ₆ (red) and chloroform-d (blue); not all compounds were sufficiently soluble in acetone-d ₆ for ⁷⁷Se{¹H} NMR analysis.

Fig. 3. δ _C (carbene C2) plotted versus δ _Se for the selenourea compounds considered in this study.

Fig. 4. X-ray crystal structures for selenoureas derived from IAd, ICy, IDD, IMe, IMes, IPr^Cl, IPent, SIMes, and Tr2–Tr5. All H atoms except the backbone H atoms have been excluded for clarity; thermal ellipsoids are drawn at 50% probability.

Fig. 5. Calculated chemical shielding versus experimental NMR chemical shift for the selenium centres in the selenoureas in Fig. 2, where the red point corresponds to [Se(Tr5)] and is not included in the trendline.

Fig. 6. (a) Schematic representation of the filled Se(p_y) and empty Se–NHC(π*) orbitals; (b) and (c) isodensity plots for the Se(p_y) and Se–NHC(π*) orbitals of [Se(IⁱPr^Me)].

Fig. 7. Se(p_y) → Se–NHC(π*) energy gap versus calculated Se NMR shielding for 23 selenoureas.

Fig. 8. (a) Hirschfeld charge on Se versus calculated Se NMR shielding (excl. [Se(Tr5)]), with outliers highlighted in red; (b) Hirschfeld charge on Se versus calculated Se NMR shielding (excl. [Se(Tr5)]) with Se–H distances constrained to ≥3.5 Å.

Fig. 9. Optimized geometries for (a) [Se(IPr)], (b) [Se(IPr*)] and (c) [Se(IPent)], showing Se–H distances.

Fig. 10. Schematic representation of the frontier molecular orbitals involving the Se(p) and NHC(π) orbitals. Se to NHC back-bonding occurs through in phase combination between a filled p orbital on Se with an empty π orbital of the NHC (Se–NHC (π)). Out of phase combination of the same orbitals (Se–NHC (π*)) is instead responsible for the paramagnetic shielding (Fig. 6(a)).

Fig. 11. Calculated NMR shielding versus E _π for (4,5-dihydro)imidazol-2-ylidenes (black points), with the exception of [Se(IAd)] and [Se(I^tBu)] which are highlighted in red, and for triazol-2-ylidenes (purple points).

Fig. 12. Phosphinidene adducts considered computationally.

Fig. 13. (a) Calculated P chemical shielding versus the experimental P chemical shift of carbene–phosphinidene adducts; (b) calculated NMR shielding versus the Hirshfeld charge on P; (c) calculated NMR shielding versus E _π.

Fig. 14. (a) Calculated chemical shielding of phosphinidenes versus selenoureas and (b) calculated E _π for 11 phosphinidenes (see Fig. 12) versus calculated E _π in the corresponding 11 selenoureas.