C^N^N Coordination Accelerates the iClick Reaction of Square-Planar Palladium(II) and Platinum(II) Azido Complexes with Electron-Poor Alkynes and Enables Cycloaddition with Terminal Alkynes

Organometal azido complexes with a cyclometalating C^N^N coligand of the general formula [M­(N3)­(phbpy)] with M = Pd­(II) or Pt­(II) and Hphbpy = 6-phenyl-2,2′-bipyridine were prepared in very good yield and reacted in a catalyst-free iClick reaction with internal alkynes 4,4,4-trifluoro-2-butynoic acid ethyl ester and dimethyl acetylenedicarboxylate and also methyl propiolate as a prototypical terminal alkyne. At room temperature, the two internal alkynes reacted too fast to study the kinetics with NMR spectroscopy, as the first spectrum obtained after mixing of the reactants showed only the product. Only in the case of methyl propiolate, second-order rate constants were obtained as 7.7 × 10–3 and 5.6 × 10–3 M–1 s–1 for the Pd­(II) and Pt­(II) azido complexes, respectively. As an alternative method with a shorter lag time, solution IR spectroscopy monitoring the disappearance of the azido vibration was used to obtain pseudo-first-order rate constants in the range of 3–4 × 10–4 s–1 for the terminal alkyne and 10–2 s–1 for the internal alkynes, with a general trend in the reactivity of Pd > Pt and CF3,COOEt > COOCH3,COOCH3 ≫ H,COOCH3.