Synthesis and Intramolecular and Interionic Structural Characterization of Half-Sandwich (Arene)Ruthenium(II) Derivatives of Bis(Pyrazolyl)Alkanes

Arene ruthenium(II) complexes containing bis(pyrazolyl)methane ligands have been prepared by reacting the ligands L′ (L′ in general; specifically L1 = H2C(pz)2, L2 = H2C(pzMe2)2, L3 = H2C(pz4Me)2, L4 = Me2C(pz)2 and L5 = Et2C(pz)2 where pz = pyrazole) with [(arene)RuCl(μ-Cl)]2 dimers (arene = p-cymene or benzene). When the reaction was carried out in methanol solution, complexes of the type [(arene)Ru(L′)Cl]Cl were obtained. When L1, L2, L3, and L5 ligands reacted with excess [(arene)RuCl(μ-Cl)]2, [(arene)Ru(L′)Cl][(arene)RuCl3] species have been obtained, whereas by using the L4 ligand under the same reaction conditions the unexpected [(p-cymene)Ru(pzH)2Cl]Cl complex was recovered. The reaction of 1 equiv of [(p-cymene)Ru(L′)Cl]Cl and of [(p-cymene)Ru(pzH)2Cl]Cl with 1 equiv of AgX (X = O3SCF3 or BF4) in methanol afforded the complexes [(p-cymene)Ru(L′)Cl](O3SCF3) (L′ = L1 or L2) and [(p-cymene)Ru(pzH)2Cl]BF4, respectively. [(p-cymene)Ru(L1)(H2O)][PF6]2 formed when [(p-cymene)Ru(L1)Cl]Cl reacts with an excess of AgPF6. The solid-state structures of the three complexes, [(p-cymene)Ru{H2C(pz)2}Cl]Cl, [(p-cymene)Ru{H2Cpz4Me)2}Cl]Cl, and [(p-cymene)Ru{H2C(pz)2}Cl](O3SCF3), were determined by X-ray crystallographic studies. The interionic structure of [(p-cymene)Ru(L1)Cl](O3SCF3) and [(p-cymene)Ru(L′)Cl][(p-cymene)RuCl3] (L′ = L1 or L2) was investigated through an integrated experimental approach based on NOE and pulsed field gradient spin−echo (PGSE) NMR experiments in CD2Cl2 as a function of the concentration. PGSE NMR measurements indicate the predominance of ion pairs in solution. NOE measurements suggest that (O3SCF3)− approaches the cation orienting itself toward the CH2 moiety of the L1 (H2C(pz)2) ligand as found in the solid state. Selected Ru species have been preliminarily investigated as catalysts toward styrene oxidation by dihydrogen peroxide, [(p-cymene)Ru(L1)(H2O)][PF6]2 being the most active species.