Upside Down! Crystallographic and Spectroscopic Characterization of an [FeIV(Osyn)(TMC)]2+ Complex

Prakash, Jai; Rohde, Gregory T.; Meier, Katlyn K.; Münck, Eckard; Que, Lawrence

doi:10.1021/acs.inorgchem.5b02011

Cited by 33 publications

(67 citation statements)

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“…Furthermore, the ethylene group orientations in Fe II (TMC)(X) complexes, which are all five-coordinate, are similar to those of 3 and 5 , 55–57 while the ethylene group orientations for both syn and anti isomers of [Fe IV (O)(TMC)(L)] 2+ , which are both six-coordinate, are similar to those found in 4 Cl . 11, 12 So this structural feature likely reflects the Fe−N4 plane distance and the TMC macrocycle conformation required to support this distance.…”

Section: Resultsmentioning

confidence: 99%

“…Following the reported procedure, 11 a 0.35 mM solution of [Fe IV (O syn )(TMC)(NCCH 3 )](OTf) 2 was first generated by adding 1 equiv of (2- t BuSO 2 −C 6 H 4 IO) (dissolved in trifluoroethanol) to the acetonitrile solution of the iron(II)-precursor complex Fe II (TMC)(OTf) 2 at −40 °C. The addition of 1 equiv of Cr(OTf) 2 into the oxoiron(IV) solution resulted in the immediate disappearance of its characteristic 815 nm near-IR band, concomitant with the growth of bands at 395, 446, 515, and 560 nm, indicating the formation of a new species ( 2 , Figure 2).…”

Section: Resultsmentioning

confidence: 99%

“…Recently, our group reported [Fe IV (O syn )-(TMC)(OTf)](OTf), in which the methyl groups on the ligand are oriented syn to the oxo moiety. 11 Inspired by the successful generation of 1 from [Fe IV (O anti )(TMC)-(NCCH 3 )] 2+ and Cr(OTf) 2 , we report herein the reaction between [Fe IV (O syn )(TMC)(NCCH 3 )](OTf) 2 and Cr(OTf) 2 to form a new species, 2 . UV−vis, resonance Raman, Mössbauer, and X-ray absorption spectroscopic methods, as well as electrospray mass spectrometry, were used to confirm 2 as [(TMC)Fe III −O syn −Cr III (OTf) 4 (NCCH 3 )], in which the TMC methyl groups are syn to the oxo bridge and the Fe center is five-coordinate, while in 1 the TMC methyl groups are anti to the oxo bridge and the Fe center is six-coordinate.…”

Section: Introductionmentioning

confidence: 99%

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The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe–O–M Complexes Derived from [Fe^IV(O_anti/syn)(TMC)]²⁺ Isomers

et al. 2016

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Tetramethylcyclam (TMC, 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) exhibits two faces in supporting an oxoiron(IV) moiety, as exemplified by the prototypical [(TMC)-FeIV(Oanti)(NCCH3)](OTf)2, where anti indicates that the O atom is located on the face opposite all four methyl groups, and the recently reported syn isomer [(TMC)FeIV(Osyn)(OTf)](OTf). The ability to access two isomers of [(TMC)FeIV(Oanti/syn)] raises the fundamental question of how ligand topology can affect the properties of the metal center. Previously, we have reported the formation of [(CH3CN)-(TMC)FeIII−Oanti−CrIII(OTf)4(NCCH3)] (1) by inner-sphere electron transfer between Cr(OTf)2 and [(TMC)FeIV(Oanti)(NCCH3)]-(OTf)2. Herein we demonstrate that a new species 2 is generated from the reaction between Cr(OTf)2 and [(TMC)FeIV(Osyn)(NCCH3)]-(OTf)2, which is formulated as [(TMC)FeIII−Osyn−CrIII(OTf)4(NCCH3)] based on its characterization by UV−vis, resonance Raman, Mössbauer, and X-ray absorption spectroscopic methods, as well as electrospray mass spectrometry. Its pre-edge area (30 units) and Fe−O distance (1.77 Å) determined by X-ray absorption spectroscopy are distinctly different from those of 1 (11-unit pre-edge area and 1.81 Å Fe−O distance) but more closely resemble the values reported for [(TMC)FeIII−Osyn−ScIII(OTf)4(NCCH3)] (3, 32-unit pre-edge area and 1.75 Å Fe−O distance). This comparison suggests that 2 has a square pyramidal iron center like 3, rather than the six-coordinate center deduced for 1. Density functional theory calculations further validate the structures for 1 and 2. The influence of the distinct TMC topologies on the coordination geometries is further confirmed by the crystal structures of [(Cl)(TMC)FeIII−Oanti−FeIIICl3] (4Cl) and [(TMC)FeIII−Osyn−FeIIICl3](OTf) (5). Complexes 1–5 thus constitute a set of complexes that shed light on ligand topology effects on the coordination chemistry of the oxoiron moiety.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe–O–M Complexes Derived from [Fe^IV(O_anti/syn)(TMC)]²⁺ Isomers

et al. 2016

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“…[11] The spectral features can be classified into three types:t he 4equivalent Me groups corresponding to the most intense peak, the 16 diastereotopic a-CH 2 protons that by symmetry are subdivided into 4s ubgroups, andt he 4d ia- Chem.E ur.J. [11] The spectral features can be classified into three types:t he 4equivalent Me groups corresponding to the most intense peak, the 16 diastereotopic a-CH 2 protons that by symmetry are subdivided into 4s ubgroups, andt he 4d ia- Chem.E ur.J.…”

Section: Resultsmentioning

confidence: 99%

On the Lewis Acidity of the Oxoiron(IV) Unit in a Tetramethylcyclam Complex

Klein

Draksharapu

Shokri

et al. 2017

Chemistry A European J

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The correlation between oxidation state and Lewis acidity is well established for hexaquairon complexes in the +II and +III oxidation state, in which the higher oxidation state leads to a lower pK for the bound H O ligand. This article addresses the Lewis acidity of the oxoiron(IV) complex [Fe (O)(TMC)(OH )] (1-OH ; TMC=1,4,8,11-tetramethylcyclam) by determining the pK of the H O ligand. We establish that 1-OH has a pK of 6.9±0.5, a value that falls in between those found for [Fe (OH ) ] and [Fe (OH ) ] . This intermediate value can be readily rationalized by the presence of the highly basic oxide ligand that mitigates the Lewis acidity of the iron(IV) center. Although the oxo ligand occupies only one position in 1-OH , anti to all four methyl groups that protrude from the same face of the nonplanar TMC ligand, its conjugate base 1-OH exists as a mixture of syn and anti tautomers, which are related by proton transfer between the oxo and the hydroxo ligands.

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“…Although a number of synthetic nonheme Fe IV (O) complexes have been reported, only a few have been crystallographically characterized to date. 39–45 …”

Section: Resultsmentioning

confidence: 99%

Aromatic C–F Hydroxylation by Nonheme Iron(IV)–Oxo Complexes: Structural, Spectroscopic, and Mechanistic Investigations

Sahu

Zhang²,

Pollock³

et al. 2016

J. Am. Chem. Soc.

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The synthesis and reactivity of a series of mononuclear nonheme iron complexes that carry out intramolecular aromatic C–F hydroxylation reactions is reported. The key intermediate prior to C–F hydroxylation, [FeIV(O)-(N4Py2Ar1)](BF4)2 (1-O, Ar1 = −2,6-difluorophenyl), was characterized by single-crystal X-ray diffraction. The crystal structure revealed a nonbonding C–H···O=Fe interaction with a CH3CN molecule. Variable-field Mössbauer spectroscopy of 1-O indicates an intermediate-spin (S = 1) ground state. The Mössbauer parameters for 1-O include an unusually small quadrupole splitting for a triplet FeIV(O) and are reproduced well by density functional theory calculations. With the aim of investigating the initial step for C–F hydroxylation, two new ligands were synthesized, N4Py2Ar2 (L2, Ar2 = −2,6-difluoro-4-methoxyphenyl) and N4Py2Ar3 (L3, Ar3 = −2,6-difluoro-3-methoxyphenyl), with –OMe substituents in the meta or ortho/para positions with respect to the C–F bonds. FeII complexes [Fe(N4Py2Ar2)(CH3CN)]-(ClO4)2 (2) and [Fe(N4Py2Ar3)(CH3CN)](ClO4)2 (3) reacted with isopropyl 2-iodoxybenzoate to give the C–F hydroxylated FeIII–OAr products. The FeIV(O) intermediates 2-O and 3-O were trapped at low temperature and characterized. Complex 2-O displayed a C–F hydroxylation rate similar to that of 1-O. In contrast, the kinetics (via stopped-flow UV–vis) for complex 3-O displayed a significant rate enhancement for C–F hydroxylation. Eyring analysis revealed the activation barriers for the C–F hydroxylation reaction for the three complexes, consistent with the observed difference in reactivity. A terminal FeII(OH) complex (4) was prepared independently to investigate the possibility of a nucleophilic aromatic substitution pathway, but the stability of 4 rules out this mechanism. Taken together the data fully support an electrophilic C–F hydroxylation mechanism.

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Upside Down! Crystallographic and Spectroscopic Characterization of an [Fe^IV(O_syn)(TMC)]²⁺ Complex

Cited by 33 publications

References 20 publications

The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe–O–M Complexes Derived from [Fe^IV(O_anti/syn)(TMC)]²⁺ Isomers

The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe–O–M Complexes Derived from [Fe^IV(O_anti/syn)(TMC)]²⁺ Isomers

On the Lewis Acidity of the Oxoiron(IV) Unit in a Tetramethylcyclam Complex

Aromatic C–F Hydroxylation by Nonheme Iron(IV)–Oxo Complexes: Structural, Spectroscopic, and Mechanistic Investigations

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Upside Down! Crystallographic and Spectroscopic Characterization of an [FeIV(Osyn)(TMC)]2+ Complex

Cited by 33 publications

References 20 publications

The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe–O–M Complexes Derived from [FeIV(Oanti/syn)(TMC)]2+ Isomers

The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe–O–M Complexes Derived from [FeIV(Oanti/syn)(TMC)]2+ Isomers

On the Lewis Acidity of the Oxoiron(IV) Unit in a Tetramethylcyclam Complex

Aromatic C–F Hydroxylation by Nonheme Iron(IV)–Oxo Complexes: Structural, Spectroscopic, and Mechanistic Investigations

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Product

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Upside Down! Crystallographic and Spectroscopic Characterization of an [Fe^IV(O_syn)(TMC)]²⁺ Complex

The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe–O–M Complexes Derived from [Fe^IV(O_anti/syn)(TMC)]²⁺ Isomers

The Two Faces of Tetramethylcyclam in Iron Chemistry: Distinct Fe–O–M Complexes Derived from [Fe^IV(O_anti/syn)(TMC)]²⁺ Isomers