For entropic reasons, the synthesis of macrocycles via olefin ring-closing metathesis (RCM) is impeded by competing acyclic diene metathesis (ADMET) oligomerization. With cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes confined in tailored ordered mesoporous silica, RCM can be run with macrocyclization selectivities up to 98% and high substrate concentrations up to 0.1 M. Molecular dynamics simulations show that the high conversions are a direct result of the proximity between the surface-bound catalyst, proven by extended X-ray absorption spectroscopy, and the surface-located substrates. Back-diffusion of the macrocycles decreases with decreasing pore diameter of the silica and is responsible for the high macrocyclization efficiency. Also, Z-selectivity increases with decreasing pore diameter and increasing Tolman electronic parameter of the NHC. Running reactions at different concentrations allows for identifying the optimum substrate concentration for each material and substrate combination.

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...

The influence of nuclearity and charge of chiral Rh diene complexes on the activity and enantioselectivity in catalytic asymmetric 1,2-additions of organoboron reagents to Ntosylimines and 1,4-additions to enones was investigated. For this purpose, cationic dimeric Rh(I) complex [(Rh(1)) 2 Cl]SbF 6 and cationic monomeric Rh(I) complex [RhOH 2 (2)]SbF 6 were synthesized from oxazolidinone-substituted 3-phenylnorbornadiene ligands 1 and 2, which differ in the substitution pattern at oxazolidinone C-5′ (CMe 2 vs CH 2 ) and compared with the corresponding neutral dimeric and monomeric Rh(I) complexes [RhCl(1)] 2 and [RhCl(2)]. Structural, electronic, and mechanistic insights were gained by X-ray crystallography, cyclic voltammetry (CV), X-ray absorption spectroscopy (XAS), and DFT calculations. CV revealed an increased stability of cationic vs neutral Rh complexes toward oxidation. Comparison of solid-state and solution XAS (extended X-ray absorption fine structure (EXAFS), X-ray absorption near edge structure (XANES)) data showed that the monomeric Rh complex [RhCl(2)] maintained its electronic state and coordination sphere in solution, whereas the dimeric Rh complex [RhCl(1)] 2 exchanges bridging chloro ligands by dioxane molecules in solution. In both 1,2-and 1,4-addition reactions, monomeric Rh complexes [RhCl(2)] and [RhOH 2 (2)]SbF 6 gave better yields as compared to dimeric complexes [RhCl(1)] 2 and [(Rh(1)) 2 Cl]SbF 6 . Regarding enantioselectivities, dimeric Rh species [RhCl(1)] 2 and [(Rh(1)) 2 Cl]SbF 6 performed better than monomeric Rh species in the 1,2-addition, while the opposite was true for the 1,4-addition. Neutral Rh complexes performed better than cationic complexes. Microemulsions improved the yields of 1,2-additions due to a most probable enrichment of Rh complexes in the amphiphilic film and provided a strong influence of the complex nuclearity and charge on the stereocontrol. A strong nonlinear-like effect (NLLE) was observed in 1,2-additions, when diastereomeric mixtures of ligands 1 and epi-1 were employed. The pronounced substrate dependency of the 1,4-addition could be rationalized by DFT calculations.

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