Ferrocene functionalized enantiomerically pure Schiff bases and their Zn(ii) and Pd(ii) complexes: a spectroscopic, crystallographic, electrochemical and computational investigation

In this contribution, we describe the synthesis, spectroscopic and structural characterization, as well as the electrochemical behavior of a series of four chiral ferrocene-functionalized Schiff base compounds derived from enantiomerically pure (1R,2R)-(-)-1,2-diaminocyclohexane. The ferrocene enaminones 3 and 4 were obtained upon monocondensation of ferrocenoylacetone Fc-C(O)CH2C(O)R (R = CH3, 1; 4-C6H4OH, 2; Fc = (η5-C5H5)Fe(η5-C5H4)) with one amino group of the primary diamine. The two bimetallic complexes 5 and 6 were prepared via a one-pot three-components template procedure involving half-unit 3, 4-methoxysalicylaldehyde and tetrahydrated zinc(ii) nitrate for 5, and half-unit 4, 4-formyl-3-hydroxybenzoic acid and palladium(ii) acetate for 6. All the syntheses were carried out in refluxing ethanol and compounds 3-6 were isolated as thermally stable, air and moisture compatible colored solids in 71-83% yields. They were characterized by several analytical (EA, ESI-MS) and spectroscopic methods (1H/13C NMR, FT-IR, UV-vis), including the molecular structures of 3 and 4 that were unequivocally established by single crystal X-ray diffraction analyses. Both crystallize in the orthorhombic non-centrosymmetric space group P212121 with two (R,R)-(-)-chiral carbon atoms in the structures. Cyclic voltammetry showed that the greatest anodic shifts (270 mV) of the ferrocenyl Fe(ii)/Fe(iii) redox potentials are observed for the D-π-A push-pull derivative 6. Beyond experimental characterization, DFT- and TDDFT-based theoretical analyses of the new chiral ferrocene-containing Schiff base complexes allow interpretation of their observed spectroscopic, structural and electronic features.