Ferrocene functionalized enantiomerically pure Schiff bases and their Zn() and Pd() 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 (1 R ,2 R )-(−)-1,2-diaminocyclohexane. The ferrocene enaminones 3 and 4 were obtained upon monocondensation of ferrocenoylacetone Fc-C(&z.dbd;O)CH 2 C(&z.dbd;O)R (R = CH 3 , 1 ; 4-C 6 H 4 OH, 2 ; Fc = (η 5 -C 5 H 5 )Fe(η 5 -C 5 H 4 )) 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 ( 1 H/ 13 C 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 P 2 1 2 1 2 1 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. A combination of X-ray diffraction, IR, UV-vis and NMR spectroscopy together with computational methods was used to characterize and study the properties of the title compounds.