Reactivity of Transition Metal Imido Complexes Encoded in their 15N NMR Signatures

Terminal imido complexes, containing metal-nitrogen multiple bonds, have been widely used in organometallic chemistry and homogeneous catalysis. The role of terminal imido ligands span from reactive site to spectator motif, largely depending on the nature of the metal center and its specific coordination sphere. Aiming at identifying reactivity descriptors for M-N multiple bonds, we herein explore solid-state 15N NMR spectroscopy (ssNMR) on early-transition metal terminal imido complexes augmented by computational studies and show that the asymmetry parameter k, (skew, 1 ≥ k ≥ -1), readily available from experiments or calculations, is diagnostic for the reactivity of M-N multiple bonds in imido complexes. While inert imido ligands exhibit skew values (k) close to 1, highly reactive imido moieties display significantly lower skew values (k << 1) as found in metallocene or bis-imido complexes. Natural Chemical Shielding analysis shows that skew values away from 1 are associated with an asymmetric development of pi orbitals around the M-N multiple bond of the imido moiety, with a larger sp2-hybridized character for reactive nitrogen. Notably, this reactivity descriptor does not directly relate to the M-N-C bond angle, illustrating the shortcoming of evaluating hydridization from geometrical parameters alone. Overall, this descriptor enables to obtain direct experimental evidence for pi-loading effect seen in bis(imido) and related complexes, thus explaining their reactivity.