Catalytic Activity of Bis-phosphine Ruthenium(II)−Arene Compounds: Structure−Activity Correlations

The phosphine dissociation characteristics of a range of bis-phosphine ruthenium(II)−arene complexes, [Ru(PPh3)(PR3)(η6-arene)]PF6 (arene = p-cymene: PR3 = PPhMe2, PPh3, P(p-tol)3, PPh2iPr; arene = PhMe: PR3 = PPhMe2, PPh3), [Ru(PPh3)(η2-PPh2(C6H4O))(η6-p-cymene)]PF6, and [RuCl(PPh3)(η7-PPh2(CH2)3Ph)]PF6, have been investigated by a combination of ligand exchange kinetics (with P(p-tol)3 in THF) and tandem electrospray ionization mass spectrometry (ESI-MS/MS). Trends in reactivity established from these studies were rationalized in terms of steric bulk, on the arene or phosphine, and conformational freedom of the phosphine ligands. A good correlation is found between these trends, especially from the ESI-MS/MS data, and activity of the complexes as catalyst precursors for the hydrogenation of styrene to ethyl benzene (in THF). The most active catalyst precursors show good activity under comparatively mild conditions (e.g., TOF ≥ 2000 h-1 for styrene hydrogenation in THF at 50 °C under 50 bar of H2). The X-ray structures of [RuCl(PPh3)(PPhMe2)(η6-p-cymene)]PF6, [RuCl(PPh3)(PPh2iPr)(η6-p-cymene)]PF6, [RuCl(PPh3)(P(p-tol)3)(η6-p-cymene)]PF6, and [RuCl(η2-PPh2(C6H4O))(η6-p-cymene)] are also reported.