Reactions of the cationic diruthenium carbonyl complex [Ru2(μ-dppm)2(CO)4(μ,η2-O2CMe)]+ with bidentate ligands; intramolecularly assisted stereospecific synthesis via the second-sphere face-to-face π–π stacking interactions

The reactions of the diruthenium carbonyl complexes [Ru2(μ-dppm)2(CO)4(μ,η2-O2CMe)]X (X=BF4− (1a) or PF6− (1b)) with neutral or anionic bidentate ligands (L,L) afford a series of the diruthenium bridging carbonyl complexes [Ru2(μ-dppm)2(μ-CO)2(η2-(L,L))2]Xn ((L,L)=acetate (O2CMe), 2,2′-bipyridine (bpy), acetylacetonate (acac), 8-quinolinolate (quin); n=0, 1, 2). Apparently with coordination of the bidentate ligands, the bound acetate ligand of [Ru2(μ-dppm)2(CO)4(μ,η2-O2CMe)]+ either migrates within the same complex or into a different one, or is simply replaced. The reaction of [Ru2(μ-dppm)2(CO)4(μ,η2-O2CMe)]+ (1) with 2,2′-bipyridine produces [Ru2(μ-dppm)2(μ-CO)2(η2-O2CMe)2] (2), [Ru2(μ-dppm)2(μ-CO)2(η2-O2CMe)(η2-bpy)]+ (3), and [Ru2(μ-dppm)2(μ-CO)2(η2-bpy)2]2+ (4). Alternatively compound 2 can be prepared from the reaction of 1a with MeCO2H–Et3N, while compound 4 can be obtained from the reaction of 3 with bpy. The reaction of 1b with acetylacetone–Et3N produces [Ru2(μ-dppm)2(μ-CO)2(η2-O2CMe)(η2-acac)] (5) and [Ru2(μ-dppm)2(μ-CO)2(η2-acac)2] (6). Compound 2 can also react with acetylacetone–Et3N to produce 6. Surprisingly [Ru2(μ-dppm)2(μ-CO)2(η2-quin)2] (7) was obtained stereospecifically as the only one product from the reaction of 1b with 8-quinolinol–Et3N. The structure of 7 has been established by X-ray crystallography and found to adopt a cis geometry. Further, the stereospecific reaction is probably caused by the second-sphere π–π face-to-face stacking interactions between the phenyl rings of dppm and the electron-deficient six-membered ring moiety of the bound quinolinate (i.e. the N-included six-membered ring) in 7. The presence of such interactions is indeed supported by an observed charge-transfer band in a UV–vis spectrum.