Phenoxyl radical FeIII complex of cis,cis-1,3,5-tris(3′,5′-di-tert-butylsalicylaldimino)cyclohexane, spectro-electrochemical and structural studies

Nairn, Alison K.; Bhalla, Rajiv; Foxon, Simon P.; Liu, Xiaoming; Yellowlees, Lesley J.; Gilbert, Bruce C.; Walton, Paul H.

doi:10.1039/b200571a

Cited by 22 publications

(11 citation statements)

References 21 publications

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“…Both complexes crystallized with five‐coordinate, high‐spin iron(III) centers that are surrounded by fully deprotonated ligands to yield the neutral species (Table S1 and S2 in the Supporting Information). The bond lengths and angles were consistent with other examples in the literature 3c,g,i. 6b,c The ORTEP diagrams with selected bond lengths and angles for 1 and 2 are shown in Figure 1.…”

Section: Methodssupporting

confidence: 88%

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

et al. 2011

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Considerable effort has been directed towards the integration of biomimetic principles into molecular materials that have customized and controllable properties. [1] The notion of stimulus-triggered molecular switching between two or more ground states of comparable energy [1a, 2] 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. [3] 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 [4] and intradiol dioxygenase, [5] 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 N 2 O 3 environments. [6] In these complexes, the assignment of oxidation states [7] becomes challenging because of the contributions of ligand-and 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 [Fe III L 1 ] (1) and [Fe III L 2 ] (2, Scheme 1), in which a low-symmetry ligand field is purposefully enforced around the 3d 5 metal ion by the N 2 O 3 ligands. Ligands [L 1 ] 3À and [L 2 ] 3À both contain N 2 O 3 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 1 or to a methylamine group in L 2 . Both species have four accessible ground states: [Fe III L] 0 /[Fe II L] À , [Fe III LC] + , [Fe III LCC] 2+ , and [...

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Section: Methodssupporting

confidence: 88%

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

et al. 2011

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“…[58] For instance, it has been shown that tert-butyl substituents at the ortho and para positions of the phenolates facilitate one-electron oxidation to the corresponding phenoxyl radicals, because these substituents decrease the oxidation potential of the phenolates and provide enough steric bulk to suppress bimolecular decay reactions of the generated phenoxyl radicals. [59] Several groups have prepared metal-phenoxyl complexes giving new insights into the chemical factors that govern the generation and stability of this type of radicals, [60] temperature X-ray crystallography. [61] In addition, o-iminobenzosemiquinonate(1 -) anions are paramagnetic (S = 1/2) ligands that couple either ferro-or antiferromagnetically when coordinated to a paramagnetic transition metal ion, depending on the symmetry of the magnetic d orbital of the metal ion.…”

Section: Phenoxyl Ligandsmentioning

confidence: 99%

Valence Tautomerism: New Challenges for Electroactive Ligands

2005

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Valence tautomeric complexes combine redox-active ligands and transition metal ions with two or more accessible oxidation states, exhibiting two nearly degenerated electronic states with localized electronic structures. Charge distribution in such electronic isomers has an appreciable sensitivity to the environment so an external perturbation, like photons, temperature and/or pressure, may lead to an intramolecular electron transfer between both redox active units and therefore to a reversible interconversion between the two degenerated electronic states. Moreover, since each electronic isomer exhibits different optical, electronic and/or magnetic properties, these complexes are being proposed as candi-

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“…In reference to previous phenolate-tach complexes, 8 both the first and second reversible oxidation processes observed in the CV of 1 are assigned to the production of first one and then a second phenoxyl-radical. It appears, therefore, that 1 undergoes two reversible ligand-based, phenolate to phenoxyl-radical redox processes, and no metal-based processes are observed within the redox range of the experiment.…”

Section: 1517mentioning

confidence: 99%

“…8 The Fe III complexes exhibit reversible ligand-based oxidation processes to give bound phenoxyl radical species at remarkably low oxidation potentials. The complexes are, however, coordinatively saturated at the metal centre, limiting their ability to act as oxidation catalysts.…”

Section: Introductionmentioning

confidence: 99%

(N-Benzyl-bis-N′,N″-salicylidene)-cis-1,3,5-triaminocyclohexane copper(ii): a novel catalyst for the aerobic oxidation of benzyl alcohol

et al. 2006

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in the formation of a novel Cu II L complex, 1. X-Ray crystallography of 1 shows the Cu II centre coordinated by two phenolate oxygens and two imine nitrogens in a distorted square plane with an elongated bond to the amine nitrogen (2.512 Å ) in the axial position. EPR spectroscopy of 1 gives g values of g 1 = 2.277, g 2 = 2.100, g 3 = 2.025, and A 1 = 15.6 mT which are consistent with the distorted square pyramidal coordination environment determined from the X-ray structure. UV/visible and electrochemical analysis of 1 shows that it undergoes two reversible processes assigned to the successive oxidation of the phenolate oxygens to phenoxyl radicals, the first at + • which is EPR silent due to antiferromagnetic coupling between the Cu II centre and the bound phenoxyl radical. The oxidised species catalyses the oxidation of benzyl alcohol to benzaldehyde.

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Phenoxyl radical FeIII complex of cis,cis-1,3,5-tris(3′,5′-di-tert-butylsalicylaldimino)cyclohexane, spectro-electrochemical and structural studies

Abstract: Complexationof cis,cis-1,3,5-tris(3Ј,5Ј-di-tert-butylsalicylaldimino)cyclohexane (H 3 tBu 2 saltach) with M III (M ‫؍‬ Fe and Ga) followed by subsequent one electron oxidation results in the formation of stable phenoxylradical complexes.

Cited by 22 publications

References 21 publications

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

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

Valence Tautomerism: New Challenges for Electroactive Ligands

(N-Benzyl-bis-N′,N″-salicylidene)-cis-1,3,5-triaminocyclohexane copper(ii): a novel catalyst for the aerobic oxidation of benzyl alcohol

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