Charge-Transfer Salts via Cocrystallization of the Cationic Ferrocenyl Donor with Polyoxometalate Acceptors

Four new charge-transfer (CT) salts based on the cationic ferrocenylmethylpyridinium (fmp) donor (CpFeCp-CH₂-Py⁺) and polyoxometalate (POM) acceptors, Lindqvist-type [M₆O₁₉]²⁻ (M = Mo or W), the decatungstate isopolyanion [W₁₀O₃₂]⁴⁻ and Keggin-type [PMo₁₂O₄₀]³⁻, namely, [CpFeCp-CH₂-Py]₂[Mo₆O₁₉] (1), [CpFeCp-CH₂-Py]₂[W₆O₁₉] (2), [CpFeCp-CH₂-Py]₄[W₁₀O₃₂] (3), and (NBu₄)[CpFeCp-CH₂-Py]₂[PMo₁₂O₄₀] (4), were synthesized by the traditional solution synthetic method. X-ray crystallographic studies of the dark-colored 1 and brownish red 2 reveal that they are isostructural and crystallize in the monoclinic space group P2₁/c. Salt 3 crystallizes in the orthorhombic space group Pbca, and salt 4 crystallizes in the orthorhombic space group Pccn. In salts 1−4, the CpFeCp-CH₂-Py⁺ and the polyoxoanions are cocrystallized by Coulombic forces, and complex C−H···π and π···π stacking interactions also exist between the adjacent ferrocenylmethylpyridinium (fmp) cations. The orbital overlap between the π-donor plane of the pyridyl ring and one oxygen facet of the acceptor octahedron in the solid state is observed in salts 1 and 2. The UV−vis diffuse reflectance spectra indicate the presence of a broad CT band between 500 and 850 nm for 1−4, and the CT character of 1 and 2 is also confirmed by the linear Mulliken correlation between the CT transition energies and the reduction potentials of the POM acceptors. The luminescent spectroscopy of both salts 1 and 2 shows an intense emission at about 394 nm, and they may be excellent candidates for potential solid-state photofunctional materials. The emission band of 3 exhibits weakened fluorescence signals compared to that of the corresponding POM and the cationic donor, while the fluorescence of 4 observed at 400 nm for (NBu₄)₃PMo₁₂O₄₀ was almost totally quenched owing to the occurrence of an efficient CT process from the ferrocenyl donor to POM acceptors.