A new family of structurally well-defined molybdenum alkylidyne catalysts for alkyne metathesis, which is distinguished by a tripodal trisilanolate ligand architecture, is presented. Complexes of type 1 combine the virtues of previous generations of silanolate-based catalysts with a significantly improved functional group tolerance. They are easy to prepare on scale; the modularity of the ligand synthesis allows the steric and electronic properties to be fine-tuned and hence the application profile of the catalysts to be optimized. This opportunity is manifested in the development of catalyst 1f, which is as reactive as the best ancestors but exhibits an unrivaled scope. The new catalysts work well in the presence of unprotected alcohols and various other protic groups. The chelate effect entails even a certain stability toward water, which marks a big leap forward in metal alkylidyne chemistry in general. At the same time, they tolerate many donor sites, including basic nitrogen and numerous heterocycles. This aspect is substantiated by applications to polyfunctional (natural) products. A combined spectroscopic, crystallographic, and computational study provides insights into structure and electronic character of complexes of type 1. Particularly informative are a density functional theory (DFT)-based chemical shift tensor analysis of the alkylidyne carbon atom and 95 Mo NMR spectroscopy; this analytical tool had been rarely used in organometallic chemistry before but turns out to be a sensitive probe that deserves more attention. The data show that the podand ligands render a Mo-alkylidyne a priori more electrophilic than analogous monodentate triarylsilanols; proper ligand tuning, however, allows the Lewis acidity as well as the steric demand about the central atom to be adjusted to the point that excellent performance of the catalyst is ensured.

Anew type of molybdenum alkylidyne catalysts for alkyne metathesis is described, whichi sd istinguished by an unconventional podand topology.T hese structurally welldefined complexes are easy to make on scale and proved to be tolerant toward numerous functional groups;e ven certain protic substituents were found to be compatible.T he new catalysts were characterized by X-ray crystallography and by spectroscopic means,including 95 Mo NMR.

(Hetero)arylamines constitute some of the most prevalent functional molecules, especially as pharmaceuticals. However, structurally complex aromatics currently cannot be converted into arylamines, so instead, each product isomer must be assembled through a multistep synthesis from simpler building blocks. Herein, we describe a late‐stage aryl C−H amination reaction for the synthesis of complex primary arylamines that other reactions cannot access directly. We show and rationalize through a mechanistic analysis the reasons for the wide substrate scope and the constitutional diversity of the reaction, which gives access to molecules that would not have been readily available otherwise.

Molybdenum alkylidyne complexes of the “canopy catalyst” series define new standards in the field of alkyne metathesis. The tripodal ligand framework lowers the symmetry of the metallacyclobutadiene complex formed by [2 + 2] cycloaddition with the substrate and imposes constraints onto the productive [2 + 2] cycloreversion; pseudorotation corrects this handicap and makes catalytic turnover possible. A combined spectroscopic, crystallographic, and computational study provides insights into this unorthodox mechanism and uncovers the role that metallatetrahedrane complexes play in certain cases.

Triarylsilanolates are privileged ancillary ligands for molybdenum alkylidyne catalysts for alkyne metathesis but lead to disappointing results and poor stability in the tungsten series. 1 H, 183 W heteronuclear multiple bond correlation spectroscopy, exploiting a favorable 5 J-coupling between the 183 W center and the peripheral protons on the alkylidyne cap, revealed that these ligands upregulate the Lewis acidity to an extent that the tungstenacyclobutadiene formed in the initial [2+2] cycloaddition step is over-stabilized and the catalytic turnover brought to a halt. Guided by the 183 W NMR shifts as a proxy for the Lewis acidity of the central atom and by an accompanying chemical shift tensor analysis of the alkylidyne unit, the ligand design was revisited and a more strongly pdonating all-alkoxide ligand prepared. The new expanded chelate complex has a tempered Lewis acidity and outperforms the classical Schrock catalyst, carrying monodentate tertbutoxy ligands, in terms of rate and functional-group compatibility.

Molybdenum alkylidyne complexes with a trisilanolate podand ligand framework ("canopy catalysts") are the arguably most selective catalysts for alkyne metathesis known to date. Among them, complex 1 a endowed with a fence of lateral methyl substituents on the silicon linkers is the most reactive, although fairly high loadings are required in certain applications. It is now shown that this catalyst decomposes readily via a bimolecular pathway that engages the Mo�CR entities in a stoichiometric triple-bond metathesis event to furnish RC�CR and the corresponding dinuclear complex, 8, with a Mo�Mo core. In addition to the regular analytical techniques, 95 Mo NMR was used to confirm this unusual outcome. This rapid degradation mechanism is largely avoided by increasing the size of the peripheral substituents on silicon, without unduly compromising the activity of the resulting complexes. When chemically challenged, however, canopy catalysts can open the apparently somewhat strained tripodal ligand cages; this reorganization leads to the formation of cyclo-tetrameric arrays composed of four metal alkylidyne units linked together via one silanol arm of the ligand backbone. The analogous tungsten alkylidyne complex 6, endowed with a tripodal tris-alkoxide (rather than siloxide) ligand framework, is even more susceptible to such a controlled and reversible cyclo-oligomerization. The structures of the resulting giant macrocyclic ensembles were established by single-crystal X-ray diffraction.

The most active alkyne metathesis catalysts rely on well-defined Mo alkylidynes, X3MoCR (X = OR), in particular the recently developed canopy catalyst family bearing silanolate ligand sets. Recent efforts to understand catalyst reactivity patterns have shown that NMR chemical shifts are powerful descriptors, though previous studies have mostly focused on ligand-based NMR descriptors. Here, we show in the context of alkyne metathesis that 95Mo chemical shift tensors encode detailed information on the electronic structure of these catalysts. Analysis by first-principles calculations of 95Mo chemical shift tensors extracted from solid-state 95Mo NMR spectra shows a direct link of chemical shift values with the energies of the HOMO and LUMO, two molecular orbitals involved in the key [2 + 2]-cycloaddition step, thus linking 95Mo chemical shifts to reactivity. In particular, the 95Mo chemical shifts are driven by ligand electronegativity (σ-donation) and electron delocalization through Mo–O π interactions, thus explaining the reactivity patterns of the silanolate canopy catalysts. These results further motivate exploration of transition metal NMR signatures and their relationships to electronic structure and reactivity.

A combined spectroscopic and theoretical study on triphenyl-and dimethyl-phenyl siloxy molybdenum and tungsten alkylidyne catalysts for alkyne metathesis is reported. Using NMR, X-ray, UV−vis, and resonance Raman spectroscopy and density functional theory calculations, the influence of different ligand systems and metal centers on the geometric and electronic structure and thermochemistry of different intermediates is investigated, that is, the starting alkylidyne and the derived metallacyclobutadiene (MCBD) and metallatetrahedrane (MTd). This includes a mechanistic and kinetic study on the formation and isomerization of MCBDs and MTds. Upon changing from monodentate to tripodal siloxy ligands, higher steric strain is imposed, which modulates the relative energies of the different intermediates. Additionally, intramolecular dispersion interactions between the bound substrate and the ligand can be operative. Tungsten as the central metal leads to stronger M−C σ-bonds, which overstabilize the reaction intermediates and preclude effective turnover. Furthermore, kinetic modeling strongly suggests that MTds are off-cycle intermediates based on the high barriers for direct formation but low barriers for isomerization from MCBDs. We infer from our findings that effective catalysis can only be achieved when factors that (over)stabilize intermediates, such as strong M−C bonds or large dispersion interactions, are prevented by appropriate catalyst design.