Charge-transfer salts based on Lindqvist and Keggin polyoxoanion acceptors and ferrocenyl cationic donors

Abstract

Four new charge-transfer salts based on the ferrocenyl cationic donors ((CpFeCpCH₂N⁺(CH₃)₃, CpFeCpCH₂N⁺(CH₃)₂C₂H₅ and CpFeCpCH₂P⁺(Ph)₃)) and Lindqvist and Keggin polyoxoanion acceptors (([M₆O₁₉]²⁻ (M = Mo, W) and [PMo₁₂O₄₀]³⁻)) with the ratio of ferrocenyl : polyoxoanions of 1 : 1, 2 : 1 or 4 : 1, [NBu₄][CpFeCpCH₂N(CH₃)₃][Mo₆O₁₉] (1), [CpFeCpCH₂P(Ph)₃]₂[Mo₆O₁₉] (2), [CpFeCpCH₂P(Ph)₃]₂[W₆O₁₉] (3) and [CpFeCpCH₂N(CH₃)₂C₂H₅]₄[PMo₁₂O₄₀] (4), were synthesized and characterized by IR, solid diffuse reflectance spectroscopy, fluorescence spectroscopy, electrospray ionization mass spectrometry (ESI-MS) and single crystal X-ray diffraction. X-Ray crystallographic studies reveal that 1 crystallizes in the monoclinic space group Pn, 2 and 3 are isomorphous and crystallize in the triclinic space group P, and 4 crystallizes in the monoclinic space group P2₁/n. In 1–4, the ferrocenyl cations and the polyoxoanions are cocrystallized by Coulombic forces and C–H⋯O hydrogen bonds. Diffuse reflectance spectra indicate the presence of a broad charge-transfer band between 300 and 800 nm for 1–4. Studies of the photoluminescent properties show that formation of the charge-transfer salts 1–4 leads to the fluorescence quenching of the starting materials owing to the occurrence of an efficient electron transfer process from the ferrocenyl donors to POM acceptors. ESI-MS studies of 1–4 dissolved in acetonitrile show the formation of ion pairs of [NBu₄][Mo₆O₁₉]⁻, [CpFeCpCH₂N(CH₃)₃][Mo₆O₁₉]⁻, [CpFeCpCH₂P(Ph)₃][Mo₆O₁₉]⁻, [CpFeCpCH₂P(Ph)₃][W₆O₁₉]⁻, [CpFeCpCH₂N(CH₃)₂C₂H₅]₂[PMo₁₂O₄₀]²⁻ and H[CpFeCpCH₂N(CH₃)₂C₂H₅]₂[PMo₁₂O₄₀]⁻. All the results provide the proof of the close interactions and the charge-transfer between the ferrocenyl and polyoxometalate moieties in 1–4 both in the solid state and in solution.