Two isostructural and isoelectronic group VI azide complexes of the general formula [M(η-allyl)(N)(bpy)(CO)] 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 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 M s, 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 s, which increased in the order of Mo > W and FC-C[triple bond, length as m-dash]C-COOEt > DMAD.
The colourless solid NO(HSO4), known as “lead‐chamber crystals”, was investigated ever since its first preparation more than two centuries ago. Its overall ionic nature now is confirmed by X‐ray crystallography [Pna21, a = 7.3558(4), b = 6.8924(3), c = 7.7017(3) Å, Z = 4]. The next neighbours of the NO+ cations are four hydrogensulfate oxygen atoms, forming a distorted square at a distance of about 2.5 Å from the nitrogen atom. The square pattern next to the nitrogen atom is the most widespread coordination figure about an NO+ ion in a nitrosyl salt. Depending on the anion, the interaction goes along with a decrease of the N–O stretch's excitation energy.
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