Ruthenium–Triazene Complexes as Latent Catalysts for UV‐Induced ROMP

Three novel ruthenium(II) complexes, RuX 2 (IMesH 2 )[ N , N ‐diethyl‐ N′ ‐(2‐benzylidene)triazene] [IMesH 2 = 1,3‐dimesitylimidazolin‐2‐ylidene; X = Cl – ( 2 ), CF 3 COO – ( 3 ), C 6 F 5 COO – ( 4 )] are described. Single‐crystal X‐ray structure analysis of compounds 2 – 4 revealed that the triazene ligand exists in the trans form and that the aryl‐connected nitrogen atom of the triazene binds to the ruthenium atom to form a five‐membered chelate ring. Compounds 2 – 4 were applied to the UV‐induced ring‐opening metathesis polymerization (ROMP) of cis ‐cyclooctene ( 5 ) and dicyclopentadiene ( 6 ). Whereas all three precatalysts show pronounced thermal latency in the presence of 5 or 6 , only compound 2 displays significant photo‐ROMP activity. The photoreactivity of precatalysts 2 – 4 , which decreases with increasing electron‐withdrawing character and with decreasing p K a of the X groups, is discussed and correlated with the X‐ray data of 2 – 4 , with low‐temperature 1 H NMR measurements as well as with cyclovoltammetric measurements.