Exploring the Formation of Copper-Ruthenium Bimetallic Complexes in Olefin Metathesis

Copper(I) halides are often added to olefin metathesis reactions to inhibit catalyst degradation, control product isomerization, enhance catalyst activation, or facilitate catalyst dimerization. In each of these examples, the copper salt is presumed to operate as an independent species separate from the ruthenium center. We have discovered, however, that certain copper salts can form complexes with the ruthenium catalyst itself, forming heterobimetallic copper-ruthenium olefin metathesis catalysts. We confirmed the formation of two complexes through single-crystal X-ray crystallography and NMR spectroscopy. The crystal structure revealed the presence of a four-membered ring containing ruthenium, carbon, copper, and chlorine or bromine. The heterobimetallic catalyst displayed higher latency and lower activity in ring-opening metathesis polymerization (ROMP) of norbornene compared to analogous monometallic catalysts. For example, norbornene polymerization catalyzed by the monometallic complex reached 80% conversion after 4 h but only 12% conversion when catalyzed by the heterobimetallic copper-ruthenium complex under the same conditions. Conversion increased to 63% when the temperature increased to 50 degrees C for 1 h, indicating that the bimetallic complex retains activity but requires a higher temperature to activate. The formation of these copper-ruthenium bimetallic complexes suggests the possibility of multimetallic olefin metathesis catalysts, potentially with different activity and properties than their traditional monometallic counterparts.