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

Abstract

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.