Synthesis and Characterization of Novel Ferrocene Containing aza-BODIPY and Porphyrin Compounds

Abstract

Metal-free (1) and zinc (2) 5,10,15,20-tetra(1′-hexanoylferrocenyl)porphyrins have been prepared using an acid-catalyzed tetramerization reaction between pyrrole and 1’-(1-hexanoyl)ferrocencarboxaldehyde. New orgnometallic compounds were characterized by a combination of 1H, 13C, and variable-temperature NMR, UV-vis, MCD, and high-resolution ESI MS methods. The redox properties of 1 and 2 were probed by electrochemical (cyclic voltammetry and differential pulse voltammetry), spectroelectrochemical, and chemical oxidation approaches. Electrochemical data recorded in the DCM/TBAB(C6F5)4 system (TBAB(C6F5)4 is a weakly coordinating tetrabutylammonium tetrakis(pentafluorophenyl)borate electrolyte) is suggestive of a "1e- + 1e- + 2e-" oxidation sequence for four ferrocene groups in 1 and 2, which is then followed by an oxidation process centered at the porphyrin core. The separation between all oxidation electrochemical waves is very large (150 - 350 mV). The nature of mixed-valence [1]n+ and [2]n+ complexes (n = 1 or 2) was probed by spectroelectrochemical and chemical oxidation methods. Analysis of the inter-valence charge-transfer (IVCT) band in [1]n+ and [2]n+ (n = 1 or 2) is suggestive of the Class II (in Robin-Day classification) behavior of all mixed-valence species. Density Functional Theory – Polarized Continuum Model (DFT-PCM) and Time-Dependent (TD) DFT-PCM methods were applied to correlate redox and optical properties of organometallic complexes 1 and 2 with their electronic structure. 1,1',3,3'-tetraferrocenylazadipyrromethene (3) and corresponding difluoroboryl azaBODIPY (4) were prepared in three and four synthetic steps, respectively, starting from ferrocenecarbaldehyde using chalcone-type synthetic methodology. The novel tetra-iron compounds have ferrocene groups directly attached to both α- and β-pyrrolic positions and shortest Fe-Fe distance determined by the X-ray crystallography was found to be 6.976 Å. New compounds were characterized by the UV-vis, NMR, and HR-ESI MS methods, while metal-metal coupling in these systems was probed by electrochemical, spectroelectrochemical, and chemical oxidation approaches. Electrochemical data suggestive of the well-separated stepwise oxidation of four-ferrocene groups in 3 and 4, while spectroelectrochemical and chemical oxidation results allowed characterization of the mixed-valence forms in the target compounds. IVCT band analysis is indicate that the mixed-valence [3]+ and [4]+ complexes belong to the weakly coupled class II compounds in Robin-Day classification. The electronic structure, redox properties, and UV-vis spectra of new systems were correlated with the Density Functional Theory (DFT) and time-dependent DFT calculations.