Catalyst-free room-temperature iClick reaction of molybdenum(ii) and tungsten(ii) azide complexes with electron-poor alkynes: structural preferences and kinetic studiesElectronic supplementary information (ESI) available: Synthesis of ligand and metal complex precursors; molecular structures of 2, 5, 6, and 11; crystallographic parameters for all X-ray structures; IR spectra of precursors and kinetic traces; DFT-calculated conformational energy diagramm of 6. CCDC 1562700-1562705. For ESI and cr

• Main Authors: Schmid, Paul, Maier, Matthias, Pfeiffer, Hendrik, Belz, Anja, Henry, Lucas, Friedrich, Alexandra, Schönfeld, Fabian, Edkins, Katharina, Schatzschneider, Ulrich
• Format: Journal Article
• Language: English
• Published: 10.10.2017
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/c7dt03096g

Two isostructural and isoelectronic group VI azide complexes of the general formula [M(η 3 -allyl)(N 3 )(bpy)(CO) 2 ] with M = Mo, W and bpy = 2,2′-bipyridine were prepared and fully characterized, including X-ray structure analysis. Both reacted smoothly with electron-poor alkynes such as dimethyl acetylenedicarboxylate (DMAD) and 4,4,4-trifluoro-2-butynoic acid ethyl ester in a catalyst-free room-temperature iClick [3 + 2] cycloaddition reaction. Reaction with phenyl(trifluoromethyl)acetylene, on the other hand, did not lead to any product formation. X-ray structures of the four triazolate complexes isolated showed the monodentate ligand to be N2-coordinated in all cases, which requires a 1,2-shift of the nitrogen from the terminal azide to the triazolate cycloaddition product. On the other hand, a 19 F NMR spectroscopic study of the reaction of the fluorinated alkyne with the tungsten azide complex at 27 °C allowed detection of the N1-coordinated intermediate. With this method, the second-order rate constant was determined as (7.3 ± 0.1) × 10 −2 M −1 s −1 , which compares favorably with that of first-generation compounds such as difluorocyclooctyne (DIFO) used in the strain-promoted azide-alkyne cycloaddition (SPAAC). In contrast, the reaction of the molybdenum analogue was too fast to be studied with NMR methods. Alternatively, solution IR studies revealed pseudo-first order rate constants of 0.4 to 6.5 × 10 −3 s −1 , which increased in the order of Mo > W and F 3 C-C&z.tbd;C-COOEt > DMAD. The azide complexes [M(η 3 -allyl)(N 3 )(bpy)(CO) 2 ] with M = Mo, W smoothly react in a [3 + 2] cycloaddition reaction with electron-poor alkynes.