Alkene and Alkyne Reactivity Towards a Bisruthenium (II) µ2-Dinitrogen Complex Containing the 'Pincer' ligand 2,6-Bis[(dimethylamino)methyl]pyridine(NN'N). The X-ray Crystal Structures of [Ru(=C=CHPh)Cl2(NN'N)] and [Ru(=C=CHPh)(OTf)(NN'N)(PPh3)][OTf] (OTf =Trifluoromethane Sulfonate)

Abstract

The binuclear Ru(II) 2-dinitrogen complex [{RuCl2(3-NN'N)}2(-N2)] (1, NN'N = 2,6-bis[(dimethylamino)methyl]pyridine) is an excellent starting material for the synthesis of mononuclear 2-alkene complexes: mer,trans-[RuCl2(3-NN'N)(2-H2C=CHR)] (R = H, CH=CH2, Ph, CH2Ph, CH2Br, CH2OH, CHO, CN, CO2Me; yields 73-93%). These new 2-alkene Ru(II) compounds are stable at room temperature even when the alkene carbon atoms are substituted with potentially reactive functional groups. Of particular note is the reaction of 1 with allyl bromide, which leads to a rare example of a stable 2-allyl bromide Ru(II) complex. Disubstituted alkenes are unreactive, but in contrast, maleic anhydride forms a stable 2-alkene complex. Also with a terminal alkyne such as phenylacetylene, the 2-N2 Ru(II) complex 1 reacts cleanly, leading to the formation of the neutral Ru(II) vinylidene complex mer,cis-[Ru(=C=CHPh)Cl2(NN'N)] (12, X-ray structure). The corresponding 2-alkyne complexes could not be detected. Via an alternative pathway, related mono- and dicationic vinylidene derivatives with phosphine ligands were synthesized using the neutral compound mer,trans-[RuCl2(NN'N)(PPh3)] (13) as starting material. Its treatment with silver salts in the presence of phenylacetylene yielded the complex mer-[Ru(=C=CHPh)(OTf)(NN'N)(PPh3)][OTf] (17, OTf = trifluoromethane sulfonate; X-ray). Both Ru(II) starting materials, 1 and 13, used in this study are unreactive toward internal alkynes such as diphenylacetylene, while the use of acetylene gas leads to the formation of complex product mixtures.