Synthesis of Piano Stool Complexes Employing the Pentafluorophenyl-Substituted Diphosphine (C6F5)2PCH2P(C6F5)2 and the Effect of Phosphine Modifiers on Hydrogen Transfer Catalysis

Ruthenium, rhodium, and iridium piano stool complexes of the pentafluorophenyl-substituted diphosphine (C6F5)2PCH2P(C6F5)2 (2) have been prepared and structurally characterized by single-crystal X-ray diffraction. The η5,κP-Cp−P tethered complex [{(η5,κP-C5Me4CH2C6F4-2-P(C6F5)CH2P(C6F5)2}RhCl2] (9), in which only one phosphorus is coordinated to the rhodium, was prepared by thermolysis of a slurry of [Cp*RhCl(μ-Cl)]2 and 2 and was structurally characterized by single-crystal X-ray diffraction. The tethering occurs by intramolecular dehydrofluorinative coupling of the η5-pentamethylcyclopentadienyl ligand and κP,κP-coordinated 2. The geometric changes that occur on tethering force dissociation of one of the phosphorus atoms. The effects of introducing phosphine ligands to the coordination sphere of piano stool hydrogen transfer catalysts have been studied. The complexes of fluorinated phosphine complexes are found to transfer hydrogen at rates that compare favorably with leading catalysts, particularly when the phosphine and cyclopentadienyl functionalities are tethered. The highly chelating η5,κP,κL-Cp−PP complex [(η5,κP,κP-C5Me4CH2-2-C5F3N-4-PPhCH2CH2PPh2)RhCl]BF4 (1) was found to outperform all other complexes tested. The mechanism of hydrogen transfer catalyzed by piano stool phosphine complexes is discussed with reference to the trends in activity observed.