Charge Distribution in Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes: A Combined X-ray, XAS, XES, DFT, Mössbauer, and Catalysis Approach

Abstract

The charge delocalization between the N-heterocyclic carbene (NHC) and the metal in cationic molybdenum imido alkylidene NHC mono(nonafluoro-tert-butoxide) complexes has been studied for different NHCs, i.e., 1,3-dimesitylimidazol-2ylidene (IMes), 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene (IM-esCl 2 ), 1,3-dimesityl-4,5-dimethylimidazol-2-ylidene (IMesMe 2 ), and 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (IMesH 2 ). The binding situation in the corresponding cationic complexes Mo(N-2,6-Me 2 C 6 H 3 )(CHCMe 2 Ph)(NHC)(OC(CF 3 ) 3 ) + B(Ar F ) 4 − (NHC = IMes (1), IMesCl 2 (2), IMesMe 2 (3), and IMesH 2 (4) was compared to that of the analogous neutral Schrock catalyst Mo(N-2,6-Me 2 C 6 H 3 )(CHCMe 2 Ph)((OC(CF 3 ) 3 )) 2 (5). Singlecrystal X-ray data were used as a starting point for the optimization of the geometries of the catalysts at the PBE0-D3BJ/def2-SVP level of theory; the obtained data were compared to those obtained from X-ray absorption (XAS) and emission spectroscopy (XES). The very similar X-ray spectroscopic signatures of the XANES (X-ray absorption near-edge structure) and Kβ-XES of catalysts 1, 2, and 5 suggest that a similar oxidation state and charge are present at the Mo center in all three cases. However, charge delocalization is more pronounced in 1 and 2 compared to 5. This is supported by quantum chemical (QC) calculations, which reveal that all NHCs compensate to a very similar extent for the cationic charge at molybdenum, leading to charge model 5 (CM5) partial charges at Mo between +1.292 and +1.298. Accordingly, the partial charge in the NHCs was in the range of +0.486 to +0.515. This strong delocalization of the positive charge in cationic molybdenum imido alkylidene NHC (nonafluoro-tert-butoxide) complexes is also illustrated by the finding that the analogous neutral Schrock catalyst 5 has a more positive charge at molybdenum (+1.435) despite being a neutral 14-electron complex. Complementarily, charge analysis on complexes 1 and 2 and the acetonitrilecontaining derivatives 1•MeCN and 2•MeCN revealed that a small partial positive charge of about +0.1 was found on acetonitrile, accompanied by an increase in positive charge on Mo. Accordingly, the partial charges at the imido, the alkoxide, and NHC ligands decreased slightly. Finally, the catalytic activity of complexes 1−4 was determined for a number of purely hydrocarbon-based substrates in a set of olefin metathesis reactions. A correlation of the Tolman electronic parameter (TEP) with catalyst activity, expressed as the turnover frequency after 3 min, TOF 3min , was found for complexes 1−3 based on imidazol-2-ylidenes. 57 Fe-Mossbauer measurements on Mo(N-2,6-Me 2 C 6 H 3 )(CH-ferrocenyl)(NHC)(OTf) 2 and Mo(N-2,6-Me 2 C 6 H 3 )(CH-ferrocenyl)-(NHC)(OTf) + B(Ar F ) 4− (NHC = IMes (6, 8) and IMesH 2 (7, 9)) revealed significant changes in the quadrupole splitting of these complexes. These suggest a significantly more efficient charge distribution between the cationic molybdenum center and an imidazol-2-ylidene-based NHC compared t...

Olefin Metathesis in Confinement: Towards Covalent Organic Framework Scaffolds for Increased Macrocyclization Selectivity

“…A change in the electronic structure or oxidation state by immobilization in the COF can thus be excluded Results of extended X-ray absorption fine structure (EXAFS) analysis, probing the local geometric structure around an X-ray absorbing atom of the Ru catalyst under homogeneous and immobilized conditions are shown in Figure 4, b. [20] The corresponding first shell scattering paths combined with coordination numbers, bond distances and Debye-Waller factors, which describe the static and dynamic disorder in the coordination shell, are collected in Figure S16 and Table S7. The results of the structure analysis for all samples are in good…”

On the Origin of E-Selectivity in the Ring-Opening Metathesis Polymerization with Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes

“…Oskam and Schrock also found that the less electron-withdrawing the alkoxide ligand is, the faster the rate of interconversion. As the NHC ligand in 1 is a sigma-donor that effectively stabilizes the positive charge of Mo by donating an electron, 12 it can be assumed that the rate of syn – anti interconversion is of similar rate or faster than metathesis.…”

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

“…In general, all NHCÀ W bond lengths are shorter than in their neural precursors, which can be attributed to a delocalization of the positive charge by the NHC. [7] In the same way, the alkoxide ligands are more tightly bound to the cationic metal centers, which is reflected in bond distances of only between 185.0 and 194.2 Å, whereas in the neutral complexes those are mostly > 200 Å. The alkylidene-W and W-oxo bond distances, on the contrary, do not significantly differ from those observed in neutral W oxo alkylidene NHC complexes.…”

“…In earlier studies, this strategy was successfully utilized for the correlation of charge distribution and reactivity of cationic molybdenum imido alkylidene NHC complexes. [7] The kinetic profiles as well as TONs, E/Z-selectivity and reaction conditions are available from the Supporting Information. In addition to that, the turnover frequencies after 1 and 3 min (TOF 1min and TOF 3min ) were determined as average of three independent measurements.…”

“…In general, the highest TOFs were observed for the RCM of 1,7-octadiene, which is in accordance with previous kinetic studies with cationic molybdenum imido alkylidene complexes. [7] Judging from the kinetic profiles of most catalysts, the reaction with 1,7-octadiene is virtually complete after a few minutes. A linear correlation (R 2 = 0.953) between the 13 C CH alkylidene shift of all tested complexes 5 a-d, 5 f-g, 10 a-b, 12, 5 b-MeCN and 5 c-MeCN and the TOF 1min was found (Figure S60, S.I.…”