Catalyst-free room-temperature iClick reaction of molybdenum(II) and tungsten(II) azide complexes with electron-poor alkynes: Structural preferences and kinetic studies
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Two isostructural and isoelectronic group VI azide complexes of the general formula [M([small eta]3-allyl)(N3)(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 phenyltrifluoromethylacetylene, 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 19F NMR spectroscopic study of the reaction of the fluorinated alkyne with the tungsten azide complex at 27 [degree]C allowed detection of the N1-coordinated intermediate. With this method, the second-order rate constant was determined as (7.3+/-0.1) [times] 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 turned 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 [times] 10-3 s-1, which increased in the order of Mo > W and F3C-C[identical with]C-COOEt > DMAD.