¹⁸³W NMR Spectroscopy Guides the Search for Tungsten Alkylidyne Catalysts for Alkyne Metathesis

Abstract

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.

Canopy Catalysts for Alkyne Metathesis: Investigations into a Bimolecular Decomposition Pathway and the Stability of the Podand Cap

“…DOSY NMR data showed that the two entities are of considerably different size/molecular weight (D exp = 6.62 × 10 À 10 vs. 9.02 × 10 À 10 m 2 • s À 1 ); both of them are metal alkylidynes as deduced from the characteristic 13 C and 183 W NMR shifts in [D 8 ] toluene (δ C = 264 and 260 ppm; δ W = 114 and 27 ppm, respectively). [26] Equilibration of 6 with a dimeric or (cyclo)oligomeric species was the most plausible explanation for this observation put forward in the original publication, [26] but an unambiguous conformation was difficult to attain because the concentrationdependent composition of the mixture rendered all attempts at crystallizing the individual constituents challenging. After many failed attempts, tiny single crystals could be grown at À 20°C from concentrated solutions in fluorobenzene; for their very small size, however, the intensity of a synchrotron beam was necessary to record suitable diffraction data.…”

Productive Alkyne Metathesis with “Canopy Catalysts” Mandates Pseudorotation

“…It took 1 week for the latter to transform into 6 by what represents a single “turnover”; complex 3 is hence catalytically incompetent. 24 The question arises whether these perplexing observations challenge the consensus mechanism of alkyne metathesis or whether they can be consolidated with it. The answer is deemed critically important for further catalyst development.…”

“… 29 The metallacyclic core is much more distorted in all bond distances; 24 , 34 this distortion persists in solution in that the C α /C α′ -atoms of 5 are inequivalent as manifested in discrete shifts and notably different 1 J C,W coupling constants indicative of substantial “double” bond character for the short W–C α bond but “single” bond character for the longer W–C α′ . 24 The fact that a single tautomer of 5 is detected in solution explains why EXSY-NMR experiments show only the dynamic exchange between the ethyl substituents at C α′ and C β with free 3-hexyne by “unproductive” [2 + 2] cycloreversion that regenerates the starting materials. The obviously much higher barrier of the “productive” cycloreversion is in line with the overly long reaction time of 1 week for 5 to transform into the all-ethyl-substituted tungstenacyclobutadiene 6 .…”

“…The obviously much higher barrier of the “productive” cycloreversion is in line with the overly long reaction time of 1 week for 5 to transform into the all-ethyl-substituted tungstenacyclobutadiene 6 . 24 The core of 6 must be similarly distorted since the C α /C α′ -atoms and their ethyl substituents are inequivalent. However, mutual interconversion of these positions is observed on the NMR time scale: for favorable circumstances, the activation parameters could be deduced.…”

Triple Resonance Experiments for the Rapid Detection of 103Rh NMR Shifts: A Combined Experimental and Theoretical Study into Dirhodium and Bismuth–Rhodium Paddlewheel Complexes

“…A combined experimental and computational approach helps to interpret the now available shift data and in so doing provides insights into the electronic nature of these valuable catalysts. 28 − 30 …”