Solvent-Dependent Hemilability of (2-Diphenylphosphino)Phenol in a Ru(II) para-Cymene System

The addition of Ph2P­(2-C6H4OH) to [(η6-p-cymene)­RuCl­(μ-Cl)]2 led to the formation of three distinct products, depending on the experimental conditions. The previously reported [(η6-p-cymene)­RuCl­{κ2P,O-PPh2(2-C6H4O)}], 1, was obtained in 90% yield using MeOH as the solvent, in the presence of a base (Et3N). Conversely, [(η6-p-cymene)­RuCl­{κ2P,O-PPh2(2-C6H4OH)}]­Cl, [2]­Cl, was afforded (88% yield) in MeOH without using the base. However, [(η6-p-cymene)­RuCl2{κP-PPh2(2-C6H4OH)}], 3, was the major species in chlorinated solvents. Compounds 1, [2]­Cl, and 3 were characterized by analytical and spectroscopic (IR, NMR) methods. The X-ray structures of [2]+ and 3 were determined, with the latter forming hydrogen bond dimeric units in the solid state. The dissolution of [2]Cl into a series of deuterated solvents, except methanol, led to rapid equilibration between [2]Cl and 3, revealing the hemilabile behavior of the phosphino-phenol ligand. According to Pulsed-Field Gradient Spin Echo (PGSE)-NMR and DFT studies, the dimeric nature of 3 persists in solution; thus, the hydrogen-bonding properties of MeOH are crucial to stabilize [2]Cl with respect to 3, with the latter being 3.4 kcal mol–1 more stable than the former in the gas phase. Compound 1 underwent electrophilic additions by HCl, HBF4, and MeCOCl to give, respectively, [2]Cl (the reverse process being viable with Et3N), [2]­BF4, and the ester [(η6-p-cymene)­RuCl2{κP-PPh2(2-C6H4OCOMe)}], 5. The reaction of 1 with AgNO3 afforded the nitrate complex [(η6-p-cymene)­Ru­(NO3)­{κ2P,O-PPh2(2-C6H4O)}], 4. Compounds [2]­BF4, 4, and 5 were isolated in the solid state in good to high yields and characterized by elemental analysis and IR and NMR spectroscopy.