Bioinspired Five-Coordinate Iron(III) Complexes for Stabilization of Phenoxyl Radicals

Autor: Marco M. Allard, Mary Jane Heeg, H. Bernhard Schlegel, Bruce R. McGarvey, Cláudio N. Verani, Jason A. Sonk
Rok vydání: 2011
Předmět:
Zdroj: Angewandte Chemie. 124:3232-3236
ISSN: 0044-8249
DOI: 10.1002/ange.201103233
Popis: Considerable effort has been directed towards the integration of biomimetic principles into molecular materials that have customized and controllable properties. The notion of stimulus-triggered molecular switching between two or more ground states of comparable energy is particularly relevant because such switching leads to detectable electronic and structural changes. Coordination complexes that merge transition-metal ions with ligands that stabilize organic radicals are among the most promising candidates for redox-responsive switching processes. Among the electroactive ligands that have been well characterized, those that contain phenolate moieties are significant because of their synthetic versatility and redox accessibility. This importance has been highlighted by studies on metal–phenoxyl complexes that have several geometries. Iron(III) complexes that contain phenolates tend to favor an octahedral geometry and are electrochemically reversible, but usually do not withstand multiple redox cycles. Thus, an understanding of the alternative geometries of such complexes becomes a necessary strategy for the future development of redox switches. We are investigating bioinspired designs that incorporate the basic geometries that are present in redox-versatile enzymes, such as tyrosine hydroxylase and intradiol dioxygenase, in which five-coordinate iron(III) centers support radical-based mechanisms for generating l-3,4-dihydroxyphenylalanine (l-DOPA) and cleaving catechol-type rings, respectively. We have reported the behavior of high-spin iron(III) complexes that are confined to low-symmetry, pentadentate N2O3 environments. [6] In these complexes, the assignment of oxidation states becomes challenging because of the contributions of ligandand metal-centered orbitals to the same redox process, and the presence of five unpaired electrons. Nonetheless, we have shown that high oxidation states are unavailable to the metal ion, and that the ligand supports up to three consecutive oxidations, which leads to antiferromagnetic interactions. Relative to octahedral fields, these fivecoordinate environments are expected to yield low-degeneracy molecular orbitals (MOs) that are sensitive to subtle but noticeable structural changes in the ligands. These changes should lead to orbital rearrangements that modify the sequence by which phenolate oxidations occur. Herein, we investigate the behavior of the five-coordinate species [FeL] (1) and [FeL] (2, Scheme 1), in which a low-symmetry ligand field is purposefully enforced around the 3d metal ion by the N2O3 ligands. Ligands [L ] and [L] both contain N2O3 environments with three phenolate moieties, denoted A, A’, and B; phenolates A and A’ share the same amine group and are chemically equivalent, whereas phenolate B is attached to either an azomethine group in L or to a methylamine group in L. Both species have four accessible ground states: [FeL]/[FeL] , [FeLC], [FeLCC], and [FeLCCC]. The aim of this study is to determine the sequence in which each of the phenolate rings is oxidized in the presence of the azomethine and the methylamine groups, and to test the feasibility of consecutive, multielectronic oxidations by ion-pairing effects with the supporting electrolyte. This study is intended to contribute to the fundamental understanding of the redox and electronic behavior of high-spin 3d 5 ions in five-coordinate ligand fields, and provide significant insight into bioinspired redox cycling. Complexes 1 and 2 were synthesized as previously described and crystals that were suitable for analysis by [*] M. M. Allard, J. A. Sonk, Dr. M. J. Heeg, Prof. H. B. Schlegel, Prof. C. N. Verani Department of Chemistry, Wayne State University 5101 Cass Ave. Detroit, MI 48202 (USA) E-mail: cnverani@chem.wayne.edu Homepage: http://chem.wayne.edu/veranigroup/
Databáze: OpenAIRE
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