Predicting Catalytic Activity from ¹³C_CH Alkylidene Chemical Shift in Cationic Tungsten Oxo Alkylidene N‐Heterocyclic Carbene Complexes

Abstract

A series of cationic tungsten oxo alkylidene N-heterocyclic carbene (NHC) complexes was synthesized and structurally characterized by single crystal X-ray diffraction. The 13 C NMR chemical shifts of the alkylidene C atoms of these complexes were correlated with the diamagnetic, paramagnetic and spinorbit chemical shifts calculated by DFT. A good correlation (R 2 = 0.90) between the DFT isotropic chemical shifts and the experimental chemical shift as well as a strong correlation between the DFT isotropic chemical shifts and the TOF 1min for the RCM of 1,7-octadiene was found. Further, a comparison of the catalyst geometries allowed for assigning tetracoordinate pseudotetrahedral catalysts to the most deshielded alkylidenes and to the highest TOF 1min , pentacoordinate square-planar catalysts to the intermediate deshielded alkylidenes and intermediate TOF 1min , and hexacoordinate and octahedral catalyst to the most shielded alkylidene and lowest TOF 1min . Analysis of the magnetic shielding tensors allowed for ascribing variations in the chemical shifts to electronic transitions between occupied molecular orbitals corresponding to the alkylidene-C and alkylidene-H σ-bonds and the empty molecular orbital corresponding to the W-alkylidene σ*-bond.

Tungsten Oxo and Tungsten Imido Alkylidene N-Heterocyclic Carbene Complexes for the Visible-Light-Induced Ring-Opening Metathesis Polymerization of Dicyclopentadiene

“…The same trend for the low dose required to gel holds for complexes bearing an oxo ligand ( W-2 , W-5 ). In fact, previous studies showed that tungsten oxo alkylidene NHC complexes possess a remarkably high activity in olefin metathesis. , On the other hand, high doses were required for complexes without electron-withdrawing fluorinated moieties or oxo ligands ( W-6 – W-10 ). Compared to most aryl imido complexes, a tungsten t Bu-imido precatalyst ( W-10 ) required long irradiation until solidification of the mixture commenced, which is in line with previous observations on the poor productivity of tungsten t Bu-imido alkylidene NHC complexes .…”

Cationic molybdenum oxo alkylidenes stabilized by N-heterocyclic carbenes: from molecular systems to efficient supported metathesis catalysts

“…All bond lengths and angles are similar to those observed for five-coordinate cationic W oxo alkylidene NHC complexes. 17 …”

“…All bond lengths and angles are similar to those observed for ve-coordinate cationic W oxo alkylidene NHC complexes. 17 On a nal note regarding the synthesis of cationic Mo oxo alkylidene complexes, the synthesis of further species bearing uorinated alkoxides starting from aryloxide complexes Mo-6 and Mo-7 have been attempted using HOC(CF 3 ) 3 . Unfortunately, the reactions proved unsuccessful, and no conversion was observed.…”

Synthetic and Structural Peculiarities of Neutral and Cationic Molybdenum Imido and Tungsten Oxo Alkylidene Complexes Bearing Weakly Coordinating N‐Heterocyclic Carbenes

“…We already reported on the synthesis of [W-(O)Cl 2 (CHCMe 2 Ph)(PPhMe 2 )(IMes)] [12] ( (B(Ar F' ) 4 À = tetrakis(pentafluorophenyl)borate, Scheme 3), accessible via crystallization from CH 2 Cl 2 /diethyl ether. [15] Diethyl ether-coordinated 17 . Et 2 O crystallizes in the triclinic space group P � 1with a = 1111.85 pm, b = 1553.17 pm, c = 1893.34 pm, α = 92.94, β = 99.14 °, γ = 109.88 °, Z = 2 (Figure 7).…”

“…[12] Immobilization of cationic tungsten oxo alkylidene NHC complexes [12] on silica thermally treated at 700 °C (silica 700 ) offered access to supported cationic tungsten oxo/imido alkylidene NHC catalysts [13] with which turnover numbers (TONs) of > 1,200,000 were achieved in the homometathesis of propene. [14] Based on these findings, we developed new synthetic strategies for tungsten oxo alkylidene NHC complexes, [15] including the addition of water to tungsten alkylidyne NHC progenitors, [16] a route that has successfully been extended to the synthesis of tungsten sulfido alkylidene NHC complexes. [16] In the transition state, cationic molybdenum imido complexes adopt a trigonal bipyramidal (TBP) transition state with the molybdacyclobutane trans to the NHC, [17] while cationic tungsten oxo catalysts can adopt both, a TBP or a square pyramidal (SP) transition state, again with the tungstacyclobutane trans to the NHC.…”