The synthesis of new catalysts and investigation of their activity in polymerization reactions is a field of special importance and ongoing interest. Cationic and in particular dicationic Ru-alkylidene complexes are comparably rare. While this class of compounds has been reported to possess no or low metathetical activity, the highly electrophilic dicationic Ru-alkylidene complex [Ru(DMF)3(1,3-dimesitylimidazolin-2-ylidene)(=CH-2-(2-PrO)-C6H4)]2+(BF4–)2] (1, DMF=dimethylformamide) exhibits high activity in both the ring-opening metathesis polymerization (ROMP) of various functional norborn-2-enes and the cyclopolymerization of various 1,6-heptadiynes, even at low temperature.
It is worth underlining that this propensity to cyclopolymerize 1,6-heptadiynes is unprecedented for dicationic Ru-alkylidenes. Generally, a pronounced difference in reactivity between monomers bearing coordinating groups and those that lack such moieties is observed where high metathetical activity of 1 is observed for monomers containing electron-rich carbon-carbon-multiple bonds. In the presence of coordinating anions such as Br– or I– present in ionic monomers, the parent dicationic complex [Ru(DMF)3(IMesH2)(=CH-2-(2-PrO)-C6H4)]2+(BF4–)2] is converted into the corresponding neutral complexes [Ru(X)2(IMesH2)(=CH-2-(2-PrO)-C6H4)] (X=Br, I).
As shown by Michael R. Buchmeiser and co-workers at the University of Stuttgart, this change in active species leads to changes in the polymerization kinetics. MALDI-TOF measurements demonstrate the presence of both the initiator- and capping agent-derived end groups in the final polymer and thus suggest the absence of chain-transfer or back-biting reactions, at least at low conversion. Most important, [Ru(DMF)3(IMesH2)(=CH-2-(2-PrO)-C6H4)]2+(BF4–)2] essentially displays the same regioselectivity in insertion (i.e. the high α-insertion propensity) compared to results from the previously reported “pseudo-halide”-containing initiators, enriching the existing portfolio of cyclopolymerization-active Ru-alkylidenes.