Dipolar Organoiron Pyranylideneacetaldehyde Hydrazone Complexes: Synthesis, Characterization, Crystal Structure, Linear and Nonlinear Optical Properties

Abstract

Abstract New cationic organoiron( II ) pyranylideneacetaldehyde hydrazone complexes were prepared in high yields through condensation reactions of the organometallic hydrazine precursor [CpFe(η 6 ‐ p ‐MeC 6 H 4 NHNH 2 )] + PF 6 – 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 ( 1 H and 13 C NMR, IR, and UV/Vis), and by a single‐crystal X‐ray diffraction analysis of the 2‐ t Bu‐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 η 6 ‐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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)