Structural implications of the paramagnetically shifted NMR signals from pyridine H atoms on synthetic nonheme FeIV=O complexes

Spectroscopic and Reactivity Comparisons between Nonheme Oxoiron(IV) and Oxoiron(V) Species Bearing the Same Ancillary Ligand

“…The chemical shift pattern observed for the pyridine β and γ protons of 2b can shed light on the orientation of the pyridine ligand relative to the FeO unit, based on comparisons with the patterns associated with structurally well characterized Fe IV O complexes in the literature. 43 Most published examples have pyridines bound cis to the FeO unit and oriented parallel to the FeO axis, namely [Fe IV (O)- Neither of the shift patterns described above matches that observed for 2b, which exhibits a relatively small downfield paramagnetic shift of 8 ppm for the β protons and a larger upfield paramagnetic shift of 32 ppm for the γ proton. Such a pattern is associated with a pyridine bound trans to the oxo atom, as reported for the pendant pyridine ligand trans to the FeO unit in [Fe IV (O)(TMC-py)] 2+ (TMC-py = 1-(pyridyl-2-methyl)-4,8,11-trimethyl-1,4,8,11-tetrazacyclotetradecane).…”

“…Studies have shown that 1 H NMR spectroscopy can be a useful method for determining how the pyridine donors are bound relative to the FeO unit. − They could be coordinated either cis or trans to the oxo unit, and in the case of the cis -bound mode, the ring may be oriented parallel or perpendicular relative to the FeO axis or somewhere in between (Table ). Indeed we show below that this technique can distinguish between the two geometrical isomers of 2 , as the orientation of the pyridine with respect to the FeO unit is different in the two options (Scheme b).…”

Octahedral iron(iv)–tosylimido complexes exhibiting single electron-oxidation reactivity

“…These shis can be understood within the context of trends recently identied by Rasheed et al in a survey of NMR data for a group of nonheme iron(IV)-oxo complexes, in which the paramagnetic shis observed are found to be inversely dependent on the size of the torsion angle between the pyridine plane and the Fe IV ]O unit (Table 1). 52 The largest shis are found for the pyridines parallel to the Fe]O unit, while the smallest shis are found for the pyridines perpendicular to the Fe]O unit (Table 1). A perusal of this table indicates that this pattern can also be used to interpret the NMR 52 data for 1 (IV) ]NTs and 2 (IV) ]NTs.…”

“…52 The largest shis are found for the pyridines parallel to the Fe]O unit, while the smallest shis are found for the pyridines perpendicular to the Fe]O unit (Table 1). A perusal of this table indicates that this pattern can also be used to interpret the NMR 52 data for 1 (IV) ]NTs and 2 (IV) ]NTs. There are two sets of pyridine peaks in the spectrum of 1 (IV) ]NTs, because one pyridine is nearly parallel to the Fe]N bond while the other is almost perpendicular to the Fe]N bond.…”

Hydrogen-atom and oxygen-atom transfer reactivities of iron(iv)-oxo complexes of quinoline-substituted pentadentate ligands

“…The proton resonances of the pyridine and quinoline rings are assigned based on the spectra of previously reported iron(IV)oxo complexes. 12,16,28 The oxidation state of the iron center of 8 was confirmed by zero-field 57 Fe Mössbauer spectroscopy at 293 K. The complex exhibits an isomeric shift (δ) value of −0.05 mm s −1 , and a quadrupole splitting (ΔE Q ) value of 0.66 mm s −1 (Fig. 5).…”