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

High valent iron species are very reactive molecules involved in oxidation reactions of relevance to biology and chemical synthesis. Herein we describe iron(iv)-tosylimido complexes [FeIV(NTs)(MePy2tacn)](OTf)2 (1(IV)[double bond, length as m-dash]NTs) and [FeIV(NTs)(Me2(CHPy2)tacn)](OTf)2 (2(IV)[double bond, length as m-dash]NTs), (MePy2tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane, and Me2(CHPy2)tacn = 1-(di(2-pyridyl)methyl)-4,7-dimethyl-1,4,7-triazacyclononane, Ts = Tosyl). 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs are rare examples of octahedral iron(iv)-imido complexes and are isoelectronic analogues of the recently described iron(iv)-oxo complexes [FeIV(O)(L)]2+ (L = MePy2tacn and Me2(CHPy2)tacn, respectively). 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs are metastable and have been spectroscopically characterized by HR-MS, UV-vis, 1H-NMR, resonance Raman, Mössbauer, and X-ray absorption (XAS) spectroscopy as well as by DFT computational methods. Ferric complexes [FeIII(HNTs)(L)]2+, 1(III)-NHTs (L = MePy2tacn) and 2(III)-NHTs (L = Me2(CHPy2)tacn) have been isolated after the decay of 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs in solution, spectroscopically characterized, and the molecular structure of [FeIII(HNTs)(MePy2tacn)](SbF6)2 determined by single crystal X-ray diffraction. Reaction of 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs with different p-substituted thioanisoles results in the transfer of the tosylimido moiety to the sulphur atom producing sulfilimine products. In these reactions, 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs behave as single electron oxidants and Hammett analyses of reaction rates evidence that tosylimido transfer is more sensitive than oxo transfer to charge effects. In addition, reaction of 1(IV)[double bond, length as m-dash]NTs and 2(IV)[double bond, length as m-dash]NTs with hydrocarbons containing weak C-H bonds results in the formation of 1(III)-NHTs and 2(III)-NHTs respectively, along with the oxidized substrate. Kinetic analyses indicate that reactions proceed via a mechanistically unusual HAT reaction, where an association complex precedes hydrogen abstraction.

Detection of Indistinct Fe-N Stretching Bands in Iron(V) Nitrides by Photodissociation Spectroscopy.

We report for the first time infrared spectra of three non-heme pseudo-octahedral iron(V) nitride complexes with assigned Fe-N stretching vibrations. The intensities of the Fe-N bands in two of the complexes are extremely weak. Their detection was enabled by the high resolution and sensitivity of the experiments performed at 3 K for isolated complexes in the gas phase. Multireference CASPT2 calculations revealed that the Fe-N bond in the ground doublet state is influenced by two low-lying excited doublet states. In particular, configuration interaction between the ground and the second excited state leads to avoided crossing of their potential energy surfaces along the Fe-N coordinate, which thus affects the ground-state Fe-N stretching frequency and intensity. Therefore, DFT calculated Fe-N stretching frequency strongly depends on the amount of Hartree-Fock exchange potential. As a result, by tuning the amount of Hartree-Fock exchange potential in the B3LYP functional, it was possible to obtain theoretical spectra perfectly consistent with the experimental data. The theory shows that the intensity of the Fe-N stretching vibration can almost vanish due to strong coupling with other stretching modes of the ligands.

Spin-State-Controlled Photodissociation of Iron(III) Azide to an Iron(V) Nitride Complex.

The generation of iron(V) nitride complexes, which are targets of biomimetic chemistry, is reported. Temperature-dependent ion spectroscopy shows that this reaction is governed by the spin-state population of their iron(III) azide precursors and can be tuned by temperature. The complex [(MePy2 TACN)Fe(N3 )]2+ (MePy2 TACN=N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane) exists as a mixture of sextet and doublet spin states at 300 K, whereas only the doublet state is populated at 3 K. Photofragmentation of the sextet state complex leads to the reduction of the iron center. The doublet state complex photodissociates to the desired iron(V) nitride complex. To generalize these findings, we show results for complexes with cyclam-based ligands.

Generation, Spectroscopic, and Chemical Characterization of an Octahedral Iron(V)-Nitrido Species with a Neutral Ligand Platform.

Iron complex [FeIII(N3)(MePy2tacn)](PF6)2 (1), containing a neutral triazacyclononane-based pentadentate ligand, and a terminally bound azide ligand has been prepared and spectroscopically and structurally characterized. Structural details, magnetic susceptibility data, and Mössbauer spectra demonstrate that 1 has a low-spin (S = 1/2) ferric center. X-ray diffraction analysis of 1 reveals remarkably short Fe-N (1.859 Å) and long FeN-N2 (1.246 Å) distances, while the FT-IR spectra show an unusually low N-N stretching frequency (2019 cm-1), suggesting that the FeN-N2 bond is particularly weak. Photolysis of 1 at 470 or 530 nm caused N2 elimination and generation of a nitrido species that on the basis of Mössbauer, magnetic susceptibility, EPR, and X-ray absorption in conjunction with density functional theory computational analyses is formulated as [FeV(N)(MePy2tacn)]2+ (2). Results indicate that 2 is a low-spin (S = 1/2) iron(V) species, which exhibits a short Fe-N distance (1.64 Å), as deduced from extended X-ray absorption fine structure analysis. Compound 2 is only stable at cryogenic (liquid N2) temperatures, and frozen solutions as well as solid samples decompose rapidly upon warming, producing N2. However, the high-valent compound could be generated in the gas phase, and its reactivity against olefins, sulfides, and substrates with weak C-H bonds studied. Compound 2 proved to be a powerful two-electron oxidant that can add the nitrido ligand to olefin and sulfide sites as well as oxidize cyclohexadiene substrates to benzene in a formal H2-transfer process. In summary, compound 2 constitutes the first case of an octahedral FeV(N) species prepared within a neutral ligand framework and adds to the few examples of FeV species that could be spectroscopically and chemically characterized.

Oxygen Atom Exchange between H2O and Non-Heme Oxoiron(IV) Complexes: Ligand Dependence and Mechanism.

Detailed studies of oxygen atom exchange (OAE) between H2(18)O and synthetic non-heme oxoiron(IV) complexes supported by tetradentate and pentadentate ligands provide evidence that they proceed by a common mechanism but within two different kinetic regimes, with OAE rates that span 2 orders of magnitude. The first kinetic regime involves initial reversible water association to the Fe(IV) complex, which is evidenced by OAE rates that are linearly dependent on [H2(18)O] and H2O/D2O KIEs of 1.6, while the second kinetic regime involves a subsequent rate determining proton-transfer step between the bound aqua and oxo ligands that is associated with saturation behavior with [H2(18)O] and much larger H2O/D2O KIEs of 5-6. [Fe(IV)(O)(TMC)(MeCN)](2+) (1) and [Fe(IV)(O)(MePy2TACN)](2+) (9) are examples of complexes that exhibit kinetic behavior in the first regime, while [Fe(IV)(O)(N4Py)](2+) (3), [Fe(IV)(O)(BnTPEN)](2+) (4), [Fe(IV)(O)(1Py-BnTPEN)](2+) (5), [Fe(IV)(O)(3Py-BnTPEN)](2+) (6), and [Fe(IV)(O)(Me2Py2TACN)](2+) (8) represent complexes that fall in the second kinetic regime. Interestingly, [Fe(IV)(O)(PyTACN)(MeCN)](2+) (7) exhibits a linear [H2(18)O] dependence below 0.6 M and saturation above 0.6 M. Analysis of the temperature dependence of the OAE rates shows that most of these complexes exhibit large and negative activation entropies, consistent with the proposed mechanism. One exception is complex 9, which has a near-zero activation entropy and is proposed to undergo ligand-arm dissociation during the RDS to accommodate H2(18)O binding. These results show that the observed OAE kinetic behavior is highly dependent on the nature of the supporting ligand and are of relevance to studies of non-heme oxoiron(IV) complexes in water or acetonitrile/water mixtures for applications in photocatalysis and water oxidation chemistry.

Iron and Manganese Catalysts for the Selective Functionalization of Arene C(sp(2) )-H Bonds by Carbene Insertion.

The first examples of the direct functionalization of non-activated aryl sp(2) C-H bonds with ethyl diazoacetate (N2 CHCO2 Et) catalyzed by Mn- or Fe-based complexes in a completely selective manner are reported, with no formation of the frequently observed cycloheptatriene derivatives through competing Buchner reaction. The best catalysts are Fe(II) or Mn(II) complexes bearing the tetradentate pytacn ligand (pytacn= 1-(2-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane). When using alkylbenzenes, the alkylic C(sp(3) )-H bonds of the substituents remained unmodified, thus the reaction being also selective toward functionalization of sp(2) C-H bonds.

Triggering the generation of an iron(IV)-oxo compound and its reactivity toward sulfides by Ru(II) photocatalysis.

The preparation of [Fe(IV)(O)(MePy2tacn)](2+) (2, MePy2tacn = N-methyl-N,N-bis(2-picolyl)-1,4,7-triazacyclononane) by reaction of [Fe(II)(MePy2tacn)(solvent)](2+) (1) and PhIO in CH3CN and its full characterization are described. This compound can also be prepared photochemically from its iron(II) precursor by irradiation at 447 nm in the presence of catalytic amounts of [Ru(II)(bpy)3](2+) as photosensitizer and a sacrificial electron acceptor (Na2S2O8). Remarkably, the rate of the reaction of the photochemically prepared compound 2 toward sulfides increases 150-fold under irradiation, and 2 is partially regenerated after the sulfide has been consumed; hence, the process can be repeated several times. The origin of this rate enhancement has been established by studying the reaction of chemically generated compound 2 with sulfides under different conditions, which demonstrated that both light and [Ru(II)(bpy)3](2+) are necessary for the observed increase in the reaction rate. A combination of nanosecond time-resolved absorption spectroscopy with laser pulse excitation and other mechanistic studies has led to the conclusion that an electron transfer mechanism is the most plausible explanation for the observed rate enhancement. According to this mechanism, the in-situ-generated [Ru(III)(bpy)3](3+) oxidizes the sulfide to form the corresponding radical cation, which is eventually oxidized by 2 to the corresponding sulfoxide.