Modeling the Geometric, Electronic, and Redox Properties of Iron(III)-Containing Amphiphiles with Asymmetric [NN′O] Headgroups

Abstract

Two iron(III)-containing amphiphiles 1 and 2 have been synthesized with the [NN'O] ligands HL(tBu-ODA) (2-((octadecyl(pyridin-2-ylmethyl)amino)methyl)-4,6-di-tert-butylphenol) and HL(I-ODA) (2-((octadecyl(pyridin-2-ylmethyl)amino)methyl)-4,6-diiodophenol), respectively. Compound 1 is monometallic, whereas EXAFS data suggest that 2 is a mixture of mono- and bimetallic species. The archetypical [Fe(III)(L(NN'O))(2)](+) complexes 3-9 have been isolated and characterized in order to understand the geometric, electronic, and redox properties of the amphiphiles. Preference for a monometallic or bimetallic nuclearity is dependent on (i) the nature of the solvent used for synthesis and (ii) the type of the substituent in the phenol moiety. In methanol, the tert-butyl-, methoxy-, and chloro-substituted 3, 4, and 5 are monometallic species, whereas the bromo- and iodo-substituted 6 and 7 form bimetallic complexes taking advantage of stabilizing methoxo bridges generated by solvent deprotonation. In dichloromethane, the bromo- and iodo-substituted 8 and 9 are monometallic species; however, these species favor meridional coordination in opposition to the facial coordination observed for the tert-butyl- and methoxy-substituted compounds. Molecular structures for species 5, 7, 8, and 9 have been solved by X-ray diffraction. Furthermore, the electronic spectrum of the amphiphile 1 was expected to be similar to those of facial/cis archetypes with similar substituents, but close resemblance was observed with the profile for those meridional/cis species, suggesting a similar coordination mode. This trend is discussed based on DFT calculations, where preference for the meridional/cis coordination mode appears related to the presence of tertiary amine nitrogen on the ligand, as when a long alkyl chain is attached to the [NN'O] headgroup.