Carbon-13 NMR Chemical Shift: A Descriptor for Electronic Structure and Reactivity of Organometallic Compounds

CONSPECTUS: 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 (delta(11) > delta(22) > delta(33)) 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 (B-0). 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 sigma(M-C) orbital and low-lying vacant orbitals of character. The deshielding at the alpha-carbon hence probes the extent of sigma(M-C) and pi(M=C)* interactions. This molecular orbital view readily explains the strong deshielding and large anisotropy (evidenced by the span Omega = delta(11) - delta(33)) observed in metal alkylidenes and alkylidynes (200 < delta(iso) < 400 ppm). Fischer carbenes are generally more deshielded than Schrock or Grubbs alkylidenes due to their low-lying pi(M=C)* orbital. Chemical shift hence shows their higher electrophilic character, connecting NMR spectroscopy to reactivity patterns. Similarly, the alpha-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 pi-bond character in the metal-carbon bond, making these bonds topologically equivalent to alkylidene pi-bonds. The pi-character in metal-alkyl bonds favors (i) alpha-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 alpha-H agostic interactions, and (iv) C-H bond activation on metal-alkyls via sigma-bond metathesis. The presence of pi-character in the metal-carbon bonds involved in these processes rationalizes the parallel reactivity patterns of metal-alkyls toward olefin insertion and sigma-bond metathesis and the fact that sigma-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 (d(0) Cp2M) or T-shaped L3M, are ubiquitous in these reactions.