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

Abstract

Ferrocenyl Schiff-base derivatives of the form [Fe(η-C₅H₅)(η-C₅H₄CHNR)][R = NCH(C₆H₄NO₂-p)1, C₆H₄CN-p2, C₆H₄NO₂-p3, C₆H₄F-p4, C₆H₄Cl-p5, C₆H₄Br-p6, C₆H₄NO₂-m7, 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.