Organometallic-inorganic charge transfer salts containing a cationic iron mixed sandwich and polyoxomolybdate anions: Syntheses, X-ray molecular structures and spectroscopic properties

Two new organometallic-inorganic charge transfer salts formulated as [(η⁵-Cp)Fe(η⁶-MeO-p-C₆H ₄-NHNH₂)]₂[Mo₆O₁₉], 1, and [(η⁵-Cp)Fe(η⁶-MeO-p-C₆H ₄-NHNH₂)]₄[β-Mo₈O ₂₆], 2, were prepared through a metathesis reaction between the organometallic hydrazine precursor [(η⁵-Cp)Fe(η⁶- MeO-p-C₆H₄-NHNH₂)]⁺PF ₆^- and either [n-Bu₄N]₂[Mo ₆O₁₉] or [n-Bu₄N]₄[α-Mo ₈O₂₆] in acetonitrile. In the second case, the [α-Mo₈O₂₆]^4- anion transforms into the [β-Mo₈O₂₆]^4- isomer. These organometallic-inorganic hybrids were characterized by spectroscopic techniques (IR, ¹H NMR and UV-vis). In addition, the UV-vis diffuse reflectance spectra of 1 and 2 in solid state exhibit a band at λ_max = 475 and 470 nm, respectively, not observed in DMSO solution, which have been attributed to a charge transfer transition. On the other hand, the solid state structure of 2, solved by X-ray diffraction analysis shows the formation of hydrogen bonds between the protons of the -NHNH₂ and C-H groups with the terminal oxo groups of the β-octamolybdate anions [β-Mo ₈O₂₆]^4-. Finally, hybrid 3, formulated as [(η⁵-Cp)Fe(η⁶-C₆H₅OMe)] ₄[β-Mo₈O₂₆] was prepared in EtOH under solvothermal conditions. The single crystal X-ray structure shows the elimination of the -NHNH₂ group from the organometallic mixed sandwich reducing its associative ability toward the oxo groups of the counterion only to the electrostatic interactions and to the very weak CH⋯O hydrogen bonds.