Dipolar organoiron pyranylideneacetaldehyde hydrazone complexes: Synthesis, characterization, crystal structure, linear and nonlinear optical properties

New cationic organoiron(II) pyranylideneacetaldehyde hydrazone complexes were prepared in high yields through condensation reactions of the organometallic hydrazine precursor [CpFe(η⁶-p-MeC ₆H₄NHNH₂)]⁺PF₆ ^- with differently substituted α- and γ- pyranylideneacetaldehydes. The mononuclear hydrazones were stereoselectively obtained as their (E) isomers about the N=C double bond. All the new compounds were thoroughly characterized by a combination of elemental analysis and spectroscopic techniques (¹H and ¹³C NMR, IR, and UV/Vis), and by a single-crystal X-ray diffraction analysis of the 2-tBu-chromene derivative 5. The spectroscopic data suggest that these complexes have a partial pyrylium character due to the electron-accepting ability of the cationic organometallic fragment and to the electron-releasing properties of the methylenepyran-based donor. The crystal structure of 5 shows that the η⁶-coordinated tolyl ring and the pyran ring are coplanar (dihedral angle 3.2°), a favorable situation that allows conjugation between the intracyclic oxygen atom and the cationic iron center through the entire hydrazone backbone. Compounds 4-6 are strongly polarized D-π-A systems that exhibit solvatochromic properties, low-lying intramolecular charge-transfer bands in their electronic absorption spectra, and enhanced second-order NLO properties (μβ), as measured by the EFISH technique at 1.907 μm.