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

The ligand precursors (MeOCH(2)CH(2)ImCH(2)CH(2)ImMe]X-2 (X = Cl (2a), Br (2b)) were prepared from reaction of methoxyethylimidazole with a haloethylmethylimidazolium halide. Subsequent conversion to Ag2X2(MeOCH2-CH(2)ImCH(2)CH(2)ImMe) (X = Cl (3a), Br (3b)) was achieved by treatment with Ag2O. Reaction of 3a or 3b with RuCl2(PPh3)(3) (4) afforded RuCl2(PPh3)(MeOCH(2)CH(2)ImCH(2)CH(2)ImMe) (5). In a similar fashion treatment of 3a or 3h with RuCl2(=CHPh)(PCy3)(2) gave the alkylidene complex RuCl2(=CHPh)(MeOCH(2)CH(2)ImCH(2)CH(2)ImMe) (6). In several standard metathesis tests 6 was shown to be a poor catalyst. The related ligand precursor [O(CH(2)CH(2)ImMe)(2)][Br](2) (7) was prepared, transformed to (AgBr)(2)[O(CH2-CH(2)ImMe)(2)] (8) and reacted with RuCl2(=CHPh)(PCy3)(2) to give RuCl2(=CHPh)[O(CH(2)CH(2)ImMe)(2)] (9). Reaction of 9 with B(C6F5)(3) generated the cationic species 10, which was shown to be the dimer [RuCl(=CHPh)(O(CH(2)CH(2)ImMe)(2))](2)[CIB(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 degrees 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.