Molecular materials containig donor and acceptor groups. Synthesis, structure and spectroscopic properties of ferrocenyl Schiff bases

Ferrocenyl Schiff-base derivatives of the form [Fe(η-C₅H₅)(η-C₅H₄CH=NR)] [R = NCH(C₆H₄NO₂-p) 1, C₆H₄CN-p 2, C₆H₄NO₂-p 3, C₆H₄F-p 4, C₆H₄Cl-p 5, C₆H₄Br-p 6, C₆H₄NO₂-m 7, NH(C₆H₄NO₂-o) 8, NH(C₆H₄NO₂-p) 9 or NH(C₆F₅) 10], have been prepared from ferrocenecarbaldehyde. Proton, ¹³C NMR, UV/VIS and ⁵⁷Fe Mössbauer spectroscopic data are presented. A number of these derivatives contain the donor-π-acceptor-(D-π-A) structural motif desired for non-linear optical materials. The behaviour of the ferrocenyl moiety as a donor is compared to that of the 4-dimethylaminophenyl group. The UV/VIS spectra of compound 1 showed considerable solvatochromism. As a result of this and its extended donor-π-acceptor nature, 1 was tested for non-linear optical properties, specifically, second harmonic generation. The results, however, were negative. A single-crystal X-ray study revealed 1 to crystallize in a centrosymmetric space group P2₁/n, with a = 5.885(1), b = 30.745(3), c = 8.662(1) Å, β = 96.40(2)° and Z = 4. The most striking feature of the molecular structure is the coplanarity of the substituent group with the η-C₅H₄ ring of the ferrocenyl moiety. The crystal structure reveals stacks of ferrocenyl, phenyl, phenyl, ferrocenyl moieties with inter-ring distances of 3.529 Å between the C₅-C₆ rings and 3.478 Å between the C₆-C₆ ring planes. The observation of a DAAD in contrast to a DADA stack is discussed.