Pentamethylcyclopentadienyl organoiron(II) hydrazone complexes: Synthesis, spectroscopic characterization, and second-order nonlinear optical properties. X-ray crystal structure of [(η⁵ - C₅ Me₅) Fe (η⁶ - C₆ H₅) NHNH₂] ⁺ PF₆^-

A series of novel pentamethylated sandwich complexes based on the [Cp^*Fe(η⁶-C₆H₅)]⁺ core (Cp^* = η⁵-C₅Me₅) has been prepared. The new organometallic π-conjugated push-pull chromophores [Cp^* Fe (η⁶ - C₆ H₅) -NHN {double bond, long} CHR] ⁺ PF₆^- (R = 2,4,6-Me₃C₆H₂, 4; (η⁵-C₅H₅)Fe(η⁵-C₅H₄), 5) were prepared through condensations between the organometallic hydrazine precursor [Cp^* Fe (η⁶ - C₆ H₅ NHNH₂)] ⁺ PF₆^- (3), and either the mesitaldehyde or the ferrocenecarboxaldehyde, respectively. Their original design combines the cationic mixed sandwich acceptor (A) associated with an organic or organometallic donor (D) through the asymmetric hydrazone spacer -NH-N{double bond, long}CH-. The mesityl ring of 4 has been complexed by the arenophile Cp^*Ru⁺, leading to the first η⁶:η⁶-coordinated dinucleating hydrazone complex, {Mathematical expression} (6). Both the mono- and dinuclear hydrazones were stereoselectively obtained as their trans-isomers about the N{double bond, long}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). In addition, the solid-state structure of the organometallic hydrazine precursor 3 has been determined by X-ray diffraction study. Spectroscopic and electrochemical data of the organometallic hydrazones 4 and 5 clearly indicate a mutual donor-acceptor electronic influence resulting from conjugation between the end groups through the entire hydrazone backbone. Compounds 4 and 5 are strongly polarized D-π-A systems exhibiting low-lying intramolecular charge transfer bands in their electronic absorption spectra and enhanced second-order NLO properties (μβ), as measured by EFISH technique at 1.907 μm.