Neutral and Cationic Tridentate Bis(N-heterocyclic carbene) Ether Ruthenium Alkylidene Complexes in Metathesis

The ligand precursors (MeOCH2CH2ImCH2CH2ImMe]­X2 (X = Cl (2a), Br (2b)) were prepared from reaction of methoxyethylimidazole with a haloethylmethylimidazolium halide. Subsequent conversion to Ag2X2(MeOCH2CH2ImCH2CH2ImMe) (X = Cl (3a), Br (3b)) was achieved by treatment with Ag2O. Reaction of 3a or 3b with RuCl2(PPh3)3 (4) afforded RuCl2(PPh3)­(MeOCH2CH2ImCH2CH2ImMe) (5). In a similar fashion treatment of 3a or 3b with RuCl2(CHPh)­(PCy3)2 gave the alkylidene complex RuCl2(CHPh)­(MeOCH2CH2ImCH2CH2ImMe) (6). In several standard metathesis tests 6 was shown to be a poor catalyst. The related ligand precursor [O­(CH2CH2ImMe)2]­[Br]2 (7) was prepared, transformed to (AgBr)2[O­(CH2CH2ImMe)2] (8) and reacted with RuCl2(CHPh)­(PCy3)2 to give RuCl2(CHPh)­[O­(CH2CH2ImMe)2] (9). Reaction of 9 with B­(C6F5)3 generated the cationic species 10, which was shown to be the dimer [RuCl­(CHPh)­(O­(CH2CH2ImMe)2)]2[ClB­(C6F5)3]2 in the solid state. Compound 9 was also ineffective as a metathesis catalyst; however, 10 proved to be a moderately active metathesis catalyst at 45 and 80 °C in RCM, ROMP, and CM test reactions. Structural data for 3a, 5, 6, 9, and 10 are reported, and the implications of this work in catalyst development are considered.