Carbon-13 NMR Chemical Shift: A Descriptor for Electronic Structure and Reactivity of Organometallic Compounds
Résumé
Metal-bonded carbon atoms in metal–alkyl, metal–carbene/alkylidene, and metal–carbyne/alkylidyne species often show significantly more deshielded isotropic chemical shifts than their organic counterparts (alkanes, alkenes, and alkynes). While isotropic chemical shift is universally used to characterize a chemical compound in solution, it is an average value of the three principal components of the chemical shift tensor (δ11 > δ22 > δ33). The tensor components, which are accessible by solid-state NMR spectroscopy, can provide detailed information about the electronic structure (frontier molecular orbitals) at the observed nuclei. This information can be accessed in detail by quantum chemical calculations, most notably by an analysis of the paramagnetic contribution to the NMR shielding tensor. The paramagnetic term mainly results from the coupling of occupied and empty molecular orbitals close in energy—the frontier molecular orbitals—under the effect of the external magnetic field (B0). In organometallic compounds, a large deshielding of the isotropic carbon-13 chemical shift of the metal-bonded carbon atom is commonly related to the coupling between the occupied σM–C orbital and low-lying vacant orbitals of πM═C* character. The deshielding at the α-carbon hence probes the extent of σM–C and πM═C* interactions. This molecular orbital view readily explains the strong deshielding and large anisotropy (evidenced by the span Ω = δ11 – δ33) observed in metal alkylidenes and alkylidynes (200 < δiso < 400 ppm). Fischer carbenes are generally more deshielded than Schrock or Grubbs alkylidenes due to their low-lying πM═C* orbital. Chemical shift hence shows their higher electrophilic character, connecting NMR spectroscopy to reactivity patterns. Similarly, the α-carbon of metal–alkyls display deshielded chemical shifts in specific coordination environments. This deshielding, which is often prominently pronounced for cationic species, indicates the presence of partial π-bond character in the metal–carbon bond, making these bonds topologically equivalent to alkylidene π-bonds. The π-character in metal–alkyl bonds favors (i) α-H abstraction processes in metal bis-alkyl compounds yielding metal alkylidenes, (ii) [2 + 2]-retrocyclization of metallacyclobutanes that participate in olefin metathesis, (iii) olefin insertion in cationic metal alkyls thus explaining polymerization activity trends and the importance of α-H agostic interactions, and (iv) C–H bond activation on metal–alkyls via σ-bond metathesis. The presence of π-character in the metal–carbon bonds involved in these processes rationalizes the parallel reactivity patterns of metal–alkyls toward olefin insertion and σ-bond metathesis and the fact that σ-bond metathesis, olefin insertion, and olefin metathesis are commonly observed with metal atoms in the same ligand field. Because of the similarities in the frontier molecular orbitals involved in these processes, these reactions can be viewed as isolobal. This explains why certain fragments, such as bent metallocenes (d0 Cp2M) or T-shaped L3M, are ubiquitous in these reactions.