Probing the Electronic Structure of Spectator Oxo Ligands by 17O NMR Spectroscopy

Abstract

Spectator oxo ligands are ubiquitous in catalysis, in particular in olefin epoxidation and olefin metathesis. Here we use computationally derived 17O NMR parameters to probe the electronic structure of spectator oxo ligands in these two reactions. We show that 17O NMR parameters allow to distinguish between doubly-bonded and triply-bonded oxo ligands, giving detailed insights into the frontier molecular orbitals involved in the metaloxo bonds along the reaction pathway. On the one hand, our study shows that in olefin epoxidation catalysed by methyltrioxorhenium (MTO), the oxo ligand significantly changes its bonding mode upon formation of the oxygen-transferring Re-oxo-bisperoxo-species, changing its nature from a doubly bonded to a triply bonded oxo ligand. On the other hand, only minor changes in the binding mode are found along the olefin metathesis reaction pathway with Mo- and W-based oxo-alkylidene species, in which the oxo ligand behaves as a triply bonded ligand throughout the reaction. This finding contrasts earlier studies that proposed that the change of binding mode of the oxo ligand was key to metallacyclobutane formation.

Origin of Reactivity Trends of an Elusive Metathesis Intermediate from NMR Chemical Shift Analysis of Surrogate Analogs

“…Notably, 1' features a highly shielded 22 and therefore low skew value ( = 0.19) which is unexpected for a group 6 oxo species. [28][29] For instance, 2' has a calculated skew value of 0.79 indicating the axially symmetric, sp-hybridized, oxo ligand in line with previous studies on tetrahedral Schrock-type alkylidenes. 28 The model 3' features an even further lowered skew value of −0.25, pointing to its primarily sp 2 -hybridized oxo ligand.…”

“…Analysis of calculated 17 O NMR parameters: To further substantiate the interplay between basal ligands, we calculated and analyzed 17 O NMR parameters of the oxo ligands in 1' -3' (Figure 7a). [28][29]98 The calculated 17 O NMR parameters and the resulting line shapes vary significantly in the series. Notably, 1' features a highly shielded 22 and therefore low skew value ( = 0.19) which is unexpected for a group 6 oxo species.…”

“…to metal-centered NMR techniques. [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38] Another key step in metathesis, more rarely discussed (and often not written out in reaction mechanisms), is the coordination of the olefin to the metal site. For Schrock-type catalysts [M(E)(=CHR)(X)(Y)], with M = Mo or W, X and Y = monoanionic ligands, and E = imido or oxo (X can be substituted by an N-heterocyclic carbene (NHC) in cationic complexes), olefin coordination requires significant structural reorganization considering that the starting catalysts are tetrahedral d 0 complexes with no empty coordination site.…”

Nuclear Magnetic Resonance: A Spectroscopic Probe to Understand the Electronic Structure and Reactivity of Molecules and Materials

“…This approach has been used to investigate the spectator-oxo effect 64−66 in olefin metathesis and olefin epoxidation. 67 This study suggested that while the formation of peroxo species in olefin epoxidation is assisted by the development of M−O triple-bonding interactions, the metal-oxo bond in olefin metathesis behaves like a triple bond throughout the reaction. Also in that case, the NMR signature of the oxo-ligand is driven by the relative energy of σand πsymmetric orbitals with respect to the M−O bond, thus capturing the essence of the bonding nature of the ligand (Figure 6d).…”

“…An M–O triple bond is characterized by an axially symmetric 17 O chemical shift tensor, whereas an M–O double bond shows a nonsymmetric tensor. This approach has been used to investigate the spectator-oxo effect − in olefin metathesis and olefin epoxidation . This study suggested that while the formation of peroxo species in olefin epoxidation is assisted by the development of M–O triple-bonding interactions, the metal-oxo bond in olefin metathesis behaves like a triple bond throughout the reaction.…”