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

Article abstract of DOI:10.1039/c9sc02526j

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 [Fe^IV(NTs)(MePy₂tacn)](OTf)₂ (1^(IV)

Full text of DOI:10.1039/c9sc02526j

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Octahedral Iron (IV)-Tosylimido Complexes ExhibitingDSOIi: n10.g10l3e9/C9SC02526J
Electron-Oxidation Reactivity
Gerard Sabenya,a Ilaria Gamba,a Laura Gómez,a Martin Clémancey,b Jonathan R.
Frisch,c Eric J. Klinker,c Geneviève Blondin,b Stéphane Torelli,b Lawrence Que, Jr,c,*
Vlad Martin-Diaconescu, a,d* Jean-Marc Latour,b,* Julio Lloret-Fillol,a,d,e* Miquel
Costas.a,*
a) Institut de Química Computacional i Catàlisi (IQCC) and Departament de
Química, Universitat de Girona. Campus Montilivi, E17071 Girona, Spain
miquel.costas@udg.edu
b) Univ. Grenoble-Alpes, CNRS, CEA, IRIG, DIESE, CBM, Grenoble 38000,
(France)
c) Department of Chemistry, University of Minnesota, Pleasant Str 207,
Minneapolis, Minnesota, USA
d) Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of
Science and Technology, Avinguda Països Catalans 16, 43007 Tarragona, Spain.
jlloret@iciq.es.
e) Catalan Institution for Research and Advanced Studies (ICREA), Passeig Lluïs
Companys, 23, 08010, Barcelona, Spain.
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Abstract. 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)=NTs) and [FeIV(NTs)(Me2(CHPy2)tacn)](OTf)2
(2(IV)=NTs), (MePy2tacn = N – methyl – N, N – bis(2-picolyl) –1,4,7 – triazacyclononane, and
2(II)-OTf, L = Me2(CHPy2)tacn = 1 – (di(2 – pyridyl)methyl) – 4,7-dimethyl –1,4,7 –
triazacyclononane, Ts = Tosyl). 1(IV)=NTs and 2(IV)=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)=NTs and
2(IV)=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)=NTs and 2(IV)=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)=NTs and 2(IV)=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)=NTs and 2(IV)=NTs behave as single electron oxidants and Hammet analyses of reaction
rates evidence that tosylimido transfer is more sentitive than oxo transfer to charge effects. In
addition, reaction of 1(IV)=NTs and 2(IV)=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.
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Introduction
High valent iron compounds are of interest in biological and chemical sciences because their high
reactivity makes them competent at performing challenging oxidative transformations in living
organisms and in chemical synthesis.1 Iron – oxo species receive major interest in reactions
entailing oxygenation of organic molecules.2-4 Comparable in synthetic relevance but somewhat
less explored are high valent iron-imidos and iron mediated N – transfer reactions.5-37
Interestingly, the electronic structure of iron – imido complexes appears to be subtler than that of
iron - oxos, and some formal iron (IV) – imidos are best described as ferric imido radical
complexes.38-40 Iron – imido complexes described to date generally contain metal centres with
low coordination numbers, with trigonal and tetrahedral geometries as the most common.32, 41-45
On the other hand, octahedral complexes are very rare. The most thoroughly studied compound
[FeIV(NTs)(N4Py)]2+, 3(IV)=NTs, (N4Py, N, N – bis(2 – pyridylmethyl) – N – bis(2 –
pyridyl)methylamine)),30, 46 exhibits two-electron N – transfer and HAT reactivity. The later
reaction is initiated by an electron transfer (ET) from the substrate forming a ferric – imido
intermediate, rapidly followed by hydrogen atom transfer from the alkyl cation radical.46, 47 More
recent work by Fukuzumi and Nam comparing the reactivity of 3(IV)=NTs and its oxo analogue
[FeIV(O)(N4Py)]2+, (3(IV)=O), concluded that 3(IV)=NTs exhibits a greatly enhanced electron
transfer reactivity arising from a much more positive one electron reduction potential and a
smaller reorganization energy during electron transfer.46 These studies suggest that iron (IV) –
imidos may exhibit a yet unexplored rich and distinctive chemistry from their better known iron
(IV) – oxo counterparts.
Herein we describe two novel octahedral iron (IV) – tosylimido complexes using
pentadentate ligands based on the 1,4,7 – triazacyclononane macrocycle. The reactivity of these
complexes in model N – transfer and HAT reactions has been explored, putting forward novel
mechanistic scenarios, distinct from those previously established for analogous iron (IV) – oxo
complexes and 3(IV)=NTs complexes. Overall, the current study discloses unprecedented single
electron mechanistic scenarios for high-valent iron compounds.
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Results and Discussion
Synthesis and characterization of [FeIV(NTs)(MePy2tacn)](OTf)2 (1(IV)=NTs) and
[FeIV(NTs)(Me2(CHPy2)tacn)](OTf)2 (2(IV)=NTs). Ferrous complexes [FeII(CH3CN)(L)](OTf)2
(1(II)-OTf, L = MePy2tacn = N – methyl – N, N – bis(2 – picolyl) – 1,4,7 – triazacyclononane, and
2(II)-OTf, L = Me2(CHPy2)tacn = 1 – (di(2 – pyridyl)methyl) – 4,7 – dimethyl – 1,4,7 –
triazacyclononane; scheme 1) have previously been described.48, 49 Both complexes contain
aminopyridine ligands based on a triazacyclononane moiety, providing a pentadentate donor set,
leaving a sixth site available for an external labile ligand S (acetonitrile for 1(II) and 2(II)). In 2(II),
the two pyridine rings are in the same plane as the Fe – S axis, while in 1(II), one of the pyridines
is parallel and the second perpendicular to this axis. Reaction of 1(II) and 2(II) with 1.2 equivalents
of N – tosyliminophenyliodinane (PhINTs) in anhydrous CH3CN at 25 ºC, produce
[FeIV(NTs)(MePy2tacn)]2+ (1(IV)=NTs) and [FeIV(NTs)(Me2(CHPy2)tacn)]2+ (2(IV)=NTs), (scheme
1). The reactions can be conveniently monitored by UV – vis spectroscopy; reaction of 1(II) with
PhINTs produces within minutes 1(IV)=NTs, characterized by a low energy feature at λmax = 750
nm ( = 200 M-1 cm-1) and a prominent absorption at λmax = 455 nm ( = 4500 M-1·cm-1). On the
other hand, reaction of 2(II) with PhINTs is slower, and formation of 2(IV)=NTs (λmax = 456 nm; 
= 3600 M-1·cm-1, and 740 nm;  = 170 M-1 cm-1) is mixed with its decay. However, use of acetone
instead of acetonitrile leads to fast and quantitative formation of 2(IV)=NTs (λmax = 455 nm;  =
4000 M-1·cm-1, and 730 nm;  = 230 M-1·cm-1), with good levels of purity (vide infra). Of notice,
the near – IR low absorption bands observed are characteristic of d – d transitions of high – valent
iron (IV) species with a S = 1 state.2 Complexes 1(IV)=NTs and 2(IV)=NTs are metastable and
decompose over time in acetonitrile solution (at 20ºC complex 1(IV)=NTs has a t1/2 of about 40
min while for complex 2(IV)=NTs the t1/2 is on the order of a few hours) to form iron (III) species
(vide infra).
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Scheme 1. Chemical strategy for the generation of 1(IV)=NTs and 2(IV)=NTs from iron (II)
precursors. 1(II) and 2(II) (S = MeCN, CF3SO3).
High resolution mass spectra (HR-MS) evidence the proposed formulation of 1(IV)=NTs and
2(IV)=NTs, and both show a major peak at m/z = 275.09, with an isotopic pattern that corresponds
to [FeIV(NTs)(L)]2+, (L = MePy2tacn and Me2(CHPy2)tacn) (figures 1 and S1). The spectra also
show a lower intensity peak at m/z = 699.13, which corresponds to monocationic
[[FeIV(NTs)(L)](OTf)]+. The later peaks shift by one mass unit upon formation of 1(IV)=NTs and
2(IV)=NTs with PhI(15N)Ts. Close inspection of these peaks reveals small contributions of the
respective [FeIII(HNTs)(L)]2+ species (~ 10%). Finally, a minor peak at m/z = 550.18 and its
isotopic pattern are consistent with a [FeIII(NTs)(L)]+ formulation.
Spectroscopic characterization of 1(IV)=NTs and 2(IV)=NTs.
Complexes 1(IV)=NTs and 2(IV)=NTs were characterized by Mössbauer spectroscopy using
57Fe enriched samples. The spectrum of a frozen sample prepared after maximum accumulation
of 1(IV)=NTs (followed by UV – vis spectroscopy) is shown in figure 1. The spectrum recorded
at 80 K under zero applied magnetic field, is dominated by a quadrupole doublet which accounts
for 87% of the total iron content. Its simulation yields spin hamiltonian parameters (isomer shift
 = 0.05 mm·s-1, quadrupole splitting EQ = 1.09 mm·s-1), consistent with an iron (IV) centre in
a low – spin (S = 1) configuration.1, 50 A frozen acetonitrile sample prepared after maximum
accumulation of 2(IV)=NTs according to the UV – vis provides a more complex spectrum, showing
only 48% of high – valent iron (IV) species, with  = 0.04 mm·s-1 and EQ = 0.74 mm·s-1 (figure
S2). However, when acetone was used as a solvent, 2(IV)=NTs was obtained in higher purity (81%,
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 = 0.06 mm·s-1, EQ = 0.73 mm·s-1, figure 1). Obtained Mössbauer values are reminiscent of the
previously reported [FeIV(NTs)(N4Py)]2+, 3(IV)=NTs ( = 0.02 mm·s-1 and EQ = 0.98 mm·s-1).30
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Figure 1. Top, Mössbauer spectrum recorded at 80 K in zero-field of 1(IV)=NTs (left) in
acetonitrile and 2(IV)=NTs (right) prepared in acetone. Experimental data are hatched bars and
solid lines represent the contributions of 1(IV)=NTs and 2(IV)=NTs. Centre, UV-vis spectrum of
oxidation of 1(II) to 1(IV)=NTs in CH3CN; kinetic trace shows the evolution of the λmax = 750 nm
feature. Below, HR-MS spectrum of 1(IV)=NTs prepared in CH3CN exhibiting a base peak at m/z
275.09; inset show amplification of major peak for [FeIV(NTs)(MePy2tacn)]2+ species. Proper
simulation of the peak requires inclusion of ≈10 % of [FeIII(NHTs)(MePy2tacn)]2+ in the isotopic
pattern corresponding to [FeIV(NTs)(MePy2tacn)]2+.
1
The H – NMR spectra of 1(IV)=NTs and 2(IV)=NTs in CD3CN or D6 – acetone show
paramagnetically shifted signals between -75 and 50 ppm, with the signals of the pyridine moiety
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as the most distinctive features due to their relative sharpness (figure 2). These could be identified
by comparison with the spectra of the parent iron (IV) – oxo complexes, and also with that of
[FeIV(X)(N4Py)]2+ 3(IV)=X (X = O, NTs) and [FeIV(O)(Bn – TPEN)]2+, 4(IV)=O (Bn – TPEN, N –
benzyl – N,N’,N’ – tris(2 – pyridylmethyl) – 1,2 – diaminoethane).30, 51 Moreover, the presence of
broad signals for both 1(IV)=NTs and 2(IV)=NTs in the high-field region of the spectra, which are
not present in the spectrum of 3(IV)=NTs, suggests that they belong to the N–methyl and N–
methylene protons on the triazacyclononane macrocycle. A bidimensional COSY spectrum of
2(IV)=NTs (inset of figure 2), allowed the assignment of pyridine hydrogens to the signals at 20.87
(), 9.70 () and -6.56 (’) ppm. Of note, the signal pattern closely resembles that of 3(IV)=X(X =
O, NTs), where all the pyridine rings are also parallel to the Fe=O/N axis. On the other hand,
1(IV)=NTs presents a more complex spectrum, in accordance with its reduced symmetry, where
one pyridine is parallel to the Fe=N axis and the other pyridine ring perpendicular to the Fe=N
axis of this complex. The 1H – NMR spectrum and corresponding COSY data identify two distinct
sets of pyridine protons. Subset “a” is assigned to the pyridine parallel to the Fe=N axis, showing
the same pattern as observed for 2(IV)=NTs and 3(IV)=NTs, with the  proton signal downfield
shifted to 41.0 ppm, the ’ proton signal upfield shifted to -13.3 ppm, and the a signal at 12.7
ppm. However, subset “b” belongs to the pyridine perpendicular to the Fe=N axis and shows a
distinct pattern, with both b and ’b proton signals upshifted to -5.6 and -6.9 ppm) and the b
proton at 6.0 ppm, consistent with the pattern observed for the perpendicular pyridine previously
reported for 4(IV)=O.51
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Figure 2. 1H – NMR and COSY spectra of 1(IV)=NTs (top) in acetone-d6 at 0 ºC and 2(IV)=NTs
(bottom) in acetone-d6 at 25 ºC; small peaks at 32 and -9 ppm belong to <5% of 2(IV)=O.
The range of chemical shifts observed for the pyridine protons in this pair of complexes reflects
differences in the orientation of the pyridine ring relative to the Fe=NTs axis. These shifts can be
understood within the context of trends recently identified by Rasheed et al. in a survey of NMR
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data for a group of nonheme iron (IV) – oxo complexes, in which the paramagnetic shifts observed
are found to be inversely dependent on the size of the torsion angle between the pyridine plane
and the FeIV=O unit (table 1).52
Table 1. NMR shifts observed for pyridine protons on related FeIV=X complexes
Py ring to
Fe=X angle
refs


’
41.0 (34)
-5.6 (-12.6)
20.9 (13.9)
12.7 (5.7)
6.0 (-1.0)
9.7 (2.7)
-13.3 (-20.3)
-6.9 (-13.9)
-6.6 (-13.6)
This
work
1(IV)=NTs
2(IV)=NTs
This
work
51
6°
43 (36)
40 (33)
10.6 (3.6)
10.0 (3.0)
8.2 (1.2)
-15 (-22)
-15.5 (-22.5)
-1.2 (-8.2)
[FeIV(O)-
(BnTPEN)]2+ a
16°
91°
-0.3 (-7.3)
51
52
[FeIV(O)-
13.9° (6.8°) c
21.1° (30.4°) c
44 (37)
30 (23)
9.5 (2.5)
8.3 (1.3)
-17 (-24)
-11 (-18)
(N4Py)]2+ b
[FeIV(O)-
(Py5Me2)]2+ b
ave 36.5°
27 (20)
3.5 (-3.5)
-12 (-19)
a Torsion angles based on the crystal structure of the corresponding iron (II) complex.53
b Torsion angles obtained from the crystal structures of [FeIV(O)(N4Py)]2+ and
[FeIV(O)(Py5Me2)]2+.51, 52
c Torsion angles in parentheses from the crystal structure of [FeIV(O)(5Me2N4Py)]2+.52
The largest shifts are found for the pyridines parallel to the Fe=O unit, while the smallest shifts
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 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. On the other
hand, 2(IV)=NTs only exhibits one set of pyridine-derived signals because of the plane of
symmetry relating the two pyridines and the paramagnetic shifts observed for the pyridine protons
have intermediate values that are between the two extremes found in 1(IV)=NTs because of the
intermediate angle found for 2(IV)=NTs.
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Furthermore, complex 1(IV)=NTs exhibits a broad resonance enhanced Raman feature at ca.
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984 cm-1 (figure 3) obtained with 488.0 nm laser excitation, which can be assigned to a vibration
arising from the Fe=N moiety on the basis of the 24 cm-1 downshift observed upon formation of
1(IV)=NTs with PhI15NTs. Similarly, the Raman spectrum of compound 2(IV)=NTs shows a broad
feature around 1016 cm-1 that shifts ca. 22 cm-1 upon formation of 2(IV)=NTs with PhI15NTs. The
observed downshifts approximate the 27-cm-1 shift calculated for a diatomic Fe=N oscillator, but
the frequencies observed are 200 cm-1 higher than found for corresponding FeIV=O units, which
have shorter Fe=O distances than the Fe=N distances found for Fe=NTs complexes.2
A rationale for the higher frequencies observed for 1(IV)=NTs and 2(IV)=NTs complexes
reported here can be found from a comparsion of the vibrational spectra of other Fe=NR
complexes. For example, analogous six – coordinate FeIV(NTs) complexes supported by other
pentadentate N5 ligands, 3(IV)=NTsand 4(IV)=NTs, where N5 = N4Py and BnTPEN, respectively,
also exhibit vibrations at 998 and 984 cm-1 that respectively downshift to 975 and 957 cm-1 upon
15N labelling.54 Related tetrahedral Fe=NR complexes give rise to 15N – sensitive vibrations with
frequencies in the 1100-cm-1 range.55 The higher frequencies observed for these complexes are
rationalized by the coupling of the Fe=N stretching mode to the higher-frequency N–S or N–C
stretching mode of the NTs or NR moiety to account for the upshift in vibrational frequency. This
coupling interaction increases as the Fe=N and N–(S or C) bonds approach linearity. For example,
DFT calculations on 6-coordinate [FeIV(NTs)(N4Py)]2+ predict an Fe=N–S angle of 161°,54 while
an Fe=N–C angle of 179° is found crystallographically for the tetrahedral [(PhBP3)FeIII(NtBu)]
(PhBP3 = [PhB(CH2PPh2)3]– complex.
One exception to the vibrational trends observed in the Fe=NTs complexes discussed above is
the recently reported [FeV(NTs)(TAML)]– complex (TAML = TetraAmido Macrocyclic Ligand),
which exhibits a (Fe=N) mode at 817 cm-1,31 which falls in the range of values reported for Fe=O
vibrations.2 Indeed it is 45 cm-1 lower in frequency than the (Fe=O) mode reported for the
[FeV(O)(bTAML)]– (bTAML
=
biuret TetraAmido Macrocyclic Ligand) complex,56
commensurate with the 0.07-Å longer Fe=NTs distance found for [FeV(NTs)(TAML)]– by
EXAFS analysis.31 DFT calculations on this complex find a more acute Fe=N–S angle at 125°,
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thereby essentially uncoupling the two vibrations and allowing the actual Fe=N stretching
frequency to be observed.
Figure 3. Resonance enhanced Raman spectra for complex 1(IV)=NTs (top) and 2(IV)=NTs
(bottom) upon excitation at 488 nm.
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X – ray diffraction quality crystals could not be obtained for 1(IV)=NTs and 2(IV)=NTs, but
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insight into their structures could be derived from EXAFS analysis of frozen samples in acetone
(figure 4) and supported by comparison to the corresponding amido derivatives 1(III)–NHTs and
2(III)–NHTs which will be described in the next section.
Figure 4. Iron K-edge XANES region (left) for 1(III)–NHTs, 2(III)–NHTs, 1(IV)=NTs and
2(IV)=NTs together with (right) non – phase shift corrected Fourier transforms of EXAFS data for
1(IV)=NTs and 2(IV)=NTs with insets showing the k – space data and fits. Experimental data are
shown in black and fits in red (see fit tables S4-S7).
Iron K – edge X – ray absorption spectroscopy (XAS) of 1(IV)=NTs and 2(IV)=NTs (figure 4)
show rising edges centred at 7123.4 eV and 7123.3 eV, having pre-edge features corresponding
to 1s3d transitions at 7113.2 eV and 7113.3 eV similar to those previously reported iron (IV)
centered complexes (table 2, SI table S8).30, 49, 57, 58 Inspection of the iron K – edge pre–edge and
rising energies of the 1(III)–NHTs (1s 3d 7112.5 eV, E0’ 7122.5 eV) and 2(III)–NHTs (1s 3d
7112.4 eV, E0’ 7122.3 eV) derivatives as well as the recently reported analogous 1(V)-N complex
([FeV(N)(MePy2tacn)]+2 (1s 3d 7114.2 eV, E0’ 7123.8 eV)59 show a systematic shift of the
XANES profile to higher energy strongly suggesting metal centred oxidation for the series.60-63 In
particular, the pre – edge energy shifts in steps of about 0.9 eV across the series, consistent with
a +1 change in formal oxidation state at the metal centre (table 2, and table S8). Additionally, the
pre – edge intensities increase from about 0.11 normalized units for X(III) – NHTs to 0.18 for X(IV)
– NTs and 0.48 for X(V) – N due to both a change in oxidation as well as enhanced p – d mixing
due to more covalent, shorter Fe – N/(NTs) bonds, causing ever increasing tetragonal distortions
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of the pseudo-octahedral environment. This is supported by EXAFS analysis (tables S4 – S7)
showing six – coordinate environments for both X(III) – NHTs and X(IV) – NTs species consisting
of 5 N/O scattering atoms at ~2.0 Å and a shorter Fe – N bond, presumably from the tosylimido
moiety. For X(IV) – NTs complexes this bond, centred at 1.72 Å, is 0.12-0.17 Å shorter than that
determined for X(III) – NTs species (~1.84 Å -1.89 Å) and similar to that of the previously reported
3(IV)=NTs (1.73Å).30
To further our understanding of X(IV) – NTs complexes theoretical models of both 1(IV)=NTs
and 2(IV)=NTs were explored with density functional theory. Geometry optimized structures at
the B3LYP/6-311+g** level together with electronic energies at the B3LYP/cc-pVTZ level favor
a triplet structure for X(IV) – NTs complexes with predicted Fe – NTs bond distances of 1.73 Å
and 1.72 Å respectively with a second shell of 5 N scattering atoms at ~2.03 Å, comparable to
experimentally determined EXAFS metrics (table 2). In the quintet X(IV) – NTs models on the
other hand, the Fe – NTs distance is still centred around 1.72 Å, but the remaining ligands are
split into two scattering shells of 3 N at 2.10 Å and 2 N at 2.20 Å, well outside the EXAFS
experimental fit parameters. In addition, Mössbauer parameters calculated for the triplet
structures are in good agreement with experimentally derived values (table 2), suggesting that in
addition to geometry the electronic structure is also well described by the theoretical models. It is
important to highlight that similar to the iron (III) analogues (vide infra) the quadrupole splitting
for 1(IV)=NTs is larger than that of 2(IV)=NTs, suggesting a more centrosymmetric environment
for 2(IV)=NTs. Finally, a description of the electronic structure in terms of a molecular orbital
picture was carried out at the B3LYP/ def2-TZVP level to facilitate comparison with the
previously described electronic structure of 3(IV)=NTs data30 (figure 5). Similar to 3(IV)=NTs we
find a highly covalent Fe – NTs interaction in the X(IV) – NTs species with spin density shared by
both the iron center (~1.11) and the N of the tosylimido ligand (~0.7). As such the question of a
iron (IV) – N imido or iron (III) – N.- imido radical pair may arise, the latter situation being
recently described by Neidig et al. for a pyrazolyl derived high valent iron – imido complex.38 It
is important to note however, that the Mossbauer isomer shift for the pyrazolyl complex is 0.20
mm/s with a quadrupole splitting of 1.96 mm/s, parameters that may indeed be attributed to an
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iron (III) center, and which indeed is quite similar to that of the corresponding iron (III) – amido
derivative (0.26 mm/s) and to values for X(III) – NHTs complexes reported herein. Secondly, the
experimentally derived XAS rising edges for our family of complexes consistently show metal
based oxidation which when correlated with their corresponding Mössbauer isomer shifts and
rising edges of similar Fe-imido complexes fall in a range best attributable to formally iron (IV)
centers (figure S15).30, 59, 64 Lastly, it is important to note that spin density alone may not be a
good measure for inferring oxidation state for Fe=E (E= O, N) type centers. Experimental and
theoretical descriptions of Fe=O centers, that are isoelectronic to Fe=N, carried out by Que and
Solomon show that the highly covalent nature of the Fe – O bond increases the unpaired spin –
density on the O – atom.65 Systematic theoretical descriptions by Neese et al. further show that
the unpaired spin-density is expected to increase with metal oxidation state and is further favoured
in the * manifold in the case of N due to the smaller effective nuclear charge of N over O
resulting in a more covalent  – interaction for iron – imido complexes.66 Fe=O bonds on the
other hand are predicted to have a stronger -interaction which helps rationalize the shorter Fe=E
bonds in iron-oxo moieties (1.63 Å).30, 49, 66 In the Fe=NTs case the weaker  – interaction results
2
2
in nearly degenerate Fe dx -y2 and Fe dz2 energies as opposed to Fe=O where the Fe dx -y2 and Fe
dz2 are well separated.65 Therefore the X(IV) – NTs complexes are best described as having a
formally +4 oxidation spin 1 iron center similar to the previously reported 3(IV)=NTs complex.30
Employing both unrestricted corresponding orbitals (figure S16), which highlight doubly, singly
or spin paired molecular orbitals based on the spatial overlap of spin up and down pairs, as well
as quasi-restricted orbitals, which allow for the “familiar” picture of doubly and singly occupied
as well as empty orbitals,67 a highly mixed and covalent Fe d – orbital manifold is evidenced. Yet,
the primary contribution to the SOMO orbitals is the iron metal center yielding a
2
(Op)4(Fedxy)2*(Fedxz,yz)2*(Fedx -y2)0*(Fedz2)0 arrangement giving an effective bond order of
2 for the Fe=N moiety (figure 5).
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Figure 5. (top) Computationally derived geometries showing first coordination shell bond
distances. (Bottom) Schematic of Fe=NTs molecular orbitals showing the iron d – manifold using
quasi – restricted orbitals (0.05 isovalue) for 1(IV)=NTs (left) and 2(IV)=NTs (right).
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Table 2. Structural and spectroscopic properties of S = 1 high – valent tosylimido ironDOI: 10.1039/C9SC02526J
compounds.
Mössbauer Spectroscopy
X-ray Absorption
Raman v 14N/15N
′
1s3d area
1s3d
E0
 (mm s-1)
EQ (mm s-1)
(cm-1)
Fe-N (Å)
(x102)
(eV)
(eV)
1(IV)=NTs
2(IV)=NTs
b3(IV)=NTs
0.05 (0.03)
0.06 (0.04)
0.02
1.09 (1.01)
0.73 (0.77)
0.98
984/960 (1029)
1016/994* (1063)
998/975
18.5
18.0
18.0
7113.2 7123.4
7113.3 7123.3
7113.1 7123.0
1.71 (1.73)
1.72 (1.72)
1.73
[a] DFT calculated values in parenthesis. [b] Data from ref. 30, 54* Data from complex prepared in
acetonitrile.
To date only a few iron (IV) – imido species have been prepared and structurally characterised.
X – ray diffraction examples include three41 and four coordinate iron complexes.6, 23, 43, 45, 68, 69
Most of these contain a distorted tetrahedral geometry,6, 23, 45, 68, 69 but a complex with a cis –
divacant octahedron geometry has also been recently described.43 Notably, all of them exhibit
significantly shorter Fe=N bonds of 1.61 – 1.64 Å in comparison to octahedral systems. The
aforementioned 3(IV)=NTs (Fe = N 1.73 Å from EXAFS data) is the only example structurally
and electronically comparable to 1(IV)=NTs and 2(IV)=NTs. On the other hand, examples of ferric
– imido radical complexes have been described,38-40 including an example of an octahedral
complex.38 Fe=N distances determined for 1(IV)=NTs and 2(IV)=NTs fall between those of high –
spin three and four coordinate ferric – imido radical complexes (1.76-1.77 Å)18, 40 and the
computed distance for the single example of the low – spin octahedral complex (1.696 Å) recently
described by Neidig and Maron.38 Therefore, the distinctive Mössbauer parameters but not the Fe
– N distance allow the differentiation between the two electronic structures.
Synthesis and characterization of 1(III)–NHTs and 2(III)–NHTs. Attempts to obtain single
crystals of iron (IV) – tosylimido compounds 1(IV)=NTs and 2(IV)=NTs by slow diethyl ether
diffusion to acetonitrile or acetone solutions of the compounds failed, providing instead the
corresponding ferric complexes [FeIII(NHTs)(MePy2tacn)](OTf)2 [1(III)–NHTs](OTf)2, and
[FeIII(NHTs)(Me2(CHPy2)tacn)](OTf)2, [2(III)–NHTs](OTf)2 as crystalline materials. A collection
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of the spectroscopic data obtained for [1(III)–NHTs](OTf)2, and [2(III)–NHTs](OTf)2 is shown in
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DOI: 10.1039/C9SC02526J
table 3.
Table 3. Spectroscopic data for [1(III)–NHTs](OTf)2 and [2(III)–NHTs](OTf)2
S
gx
gy
gz
δ
ΔEQ
Γ
η
Ax/gnµn
Ay/gnµn
Az/gnµn
(T)
%
Eo
(Epre-edge
(eV)
(mm.s-1)
(mm.s-1)
(mm.s-1)
)
[1(III)–NHTs](OTf)2 1/2 2.23
0.28
0.27
2.40
0.57
0.25
0
-36.2
12.8
9.1
100
100
7122.5
(7112.5)
2.23
1.93
[2(III)–NHTs](OTf)2 1/2 2.24
-2.03
0.84
-23
7122.3
2.24
1.95
4.6
3.8
(7112.4)
Single crystals suitable for X – ray diffraction were obtained for [1(III)–NHTs](SbF6)2.
An ORTEP diagram corresponding to its molecular structure along with selected bond distances
are shown in figure 6. The complex contains an iron centre in a distorted octahedral geometry.
Five coordination sites are occupied by nitrogen atoms from the pentadentate ligand with Fe – N
distances typical for iron (III) in low – spin (1.98 – 2.03 Å).70-72 The sixth site is occupied by the
tosylamido N atom. The two pyridine rings are perpendicular one to each other, maintaining the
structure of 1(II), with one of them parallel to the Fe – N(H) axis. Fe – Py bonds are 1.990 and
1.988 Å and Fe – Nalkyl are 2.024, 2.027 and 1.986 Å. Fe – Py bonds are slightly shorter likely due
to the  – acceptor character of pyridine. The Fe – N(H) distance is relatively short (1.89 Å). For
comparison the Fe – N(H) bond is 0.18 Å longer than Fe=N in 1(IV)=NTs.
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S1
N6
N2
Fe1
N1
N4
N5
N3
Figure 6. ORTEP structure of isolated ferric complex [1(III)–NHTs](SbF6)2 (50% ellipsoid
probability). Counterions omitted for clarity. Selected metrical parameters; Fe1–N1 2.027(4) Å,
Fe1–N2 1.986(4) Å, Fe1–N3 2.024(4) Å, Fe1–N4 1.990(4) Å, Fe1–N5 1.988(4) Å, Fe1–N6
1.894(4) Å, Fe1–N6–S1 133.4(3)º.
The Fe – N(H) distance in 1(III)–NHTs is very similar to the values reported by Spasyuk
et al. for octahedral complexes [FeIII(NHR)(B2Pz4Py)] (R = C6H4C(CH3)3 or adamantane, 1.869
and 1.854 Å, respectively).38 This is in line with the low – spin character of the iron center in all
three complexes. By contrast, longer Fe – N(H) distances in the range 1.95 - 2.00 Å are noted for
the high – spin five coordinate ferric amido complexes [FeIII(NHTol)H22] and [FeIII(NH –
SO2C6H4R)TPA2C(O)NHtBu](OTf)2 reported respectively by Borovik73 and Chang.74
EPR studies of 1(III)–NHTs and 2(III)–NHTs at 2 K (figure 7) show axial spectra (geff,
=
2.23 and geff,// = 1.93) and (geff, = 2.21 and geff,// = 1.95) indicative of S = 1/2 systems, characteristic
of low – spin ferric centres. In both samples a minor high – spin signal is observed. Mossbauer
parameters obtained are congruent with the presence of low – spin ferric centers for both
complexes. At 80K in the absence of a magnetic field, both show asymmetric doublets with  ≈
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0.28 mm s-1, but differ only in EQ = 2.40 and 2.03 mm s-1, respectively (figure 7). When recorded
at 4.2 K under a field of 7 T applied parallel to the  – rays, the spectra of both complexes split
into a multiplet spread over a limited velocity range -3 to +3.5 mm s-1 which is consistent with an
S = ½ center. The data could indeed be fitted under this assumption with the parameters listed in
table 3.
Figure 7. Spectroscopic data of isolated 1(III)–NHTs (left) and 2(III)–NHTs (right). (Top) EPR
spectra of 1(III)–NHTs (left) and 2(III)–NHTs (right). Middle, Mössbauer spectra of 1(III)–NHTs (left)
and 2(III)–NHTs (right) recorded at 80 K in absence of field (middle) and at 4.2 K under a field of
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7 T applied parallel to the  – ray (bottom). Experimental spectrum: hatched bars, tDhOeIo: 1r0e.t1i0c3a9l/C9SC02526J
spectrum: solid line (see Table 3 for parameters).
Reactivity of 1(IV)=NTs and 2(IV)=NTs in formal N – atom transfer reactions. 1(IV)=NTs
and 2(IV)=NTs, were freshly prepared in situ in acetonitrile at 293 K by reaction of the starting
ferrous complexes with 1.2 equiv. of solid PhINTs. Reaction mixtures were filtered to remove the
excess of PhINTs and rapidly reacted with thioanisole substrates. Reaction was ascertained by the
bleaching of the characteristic low – energy bands in the visible spectra of the complexes (vide
infra). 1H – NMR analyses at the end of the reactions show formation of sulfilimine products with
yields of 0.5 ± 0.1 and 0.4 ± 0.1 equiv. (table 4) per mol of 1(IV)=NTs and 2(IV)=NTs,
respectively. Alternatively, when excess of PhINTs (e.g. 5 eq.) are used and reaction mixtures
were not filtered, 1(IV)=NTs and 2(IV)=NTs show catalytic behaviour; 1(IV)=NTs reacts with
MeOPhSMe and HPhSMe to provide 1.35 and 2.1 eq of the corresponding sulfilimine product. In
analogous conditions, 2(IV)=NTs provides 3.5 eq and 2.9 eq of the corresponding sulfilimine
products. Blank experiments under analogous conditions but in the absence of iron complexes
generate ≈ 0.1 eq of product. Thus, we conclude that 1(IV)=NTs and 2(IV)=NTs are competent NTs
transfer agents, and that they show catalytic behaviour, although quite modest.
NTs
[Fe]
PhSMe + PhINTs
PhSMe + PhI
Scheme 2. Iron mediated reaction of thioanisole with PhINTs.
Spectroscopic analysis of the reactions. Analysis of iron species formed after reaction with
sulphides. When different p – X– substituted thioanisole substrates, XPhSMe (X = MeO, Me, H,
Cl), were added to freshly prepared solutions of 1(IV)=NTs and 2(IV)=NTs at 293 K, a reaction
with two phases is apparent from the time dependent changes observed in the UV – vis spectra.
In the first phase, the characteristic low energy features at 750 and 740 nm of the respective visible
spectra rapidly decrease in intensity (e.g. 5 – 15 min for MeOPhSMe). Simultaneously a red shift
was observed of the initial iron (IV) – tosylimido bands at 455 and 456 nm towards 470 and 465
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nm, respectively (figure 8). Reaction rates for this first phase are sensitive to the concentration
and nature of the sulphide, being faster as the sulphide is more electron rich, and slower as electron
– poorer sulphides were employed. As stated above sulfilimine products are formed during this
first phase in c.a. 50 % yield relative to the pre-formed X(IV)=NTs. Remarkably, upon complete
consumption of the initial iron (IV) at the end of this phase, as ascertained by the bleaching of the
band at 700 nm, the spectra that result do not match to these of 1(II) and 2(II), and are the same
irrespective of the nature of the p – X – substituted thioanisole substrate employed. A second and
slower phase (on the order of hours) follows and entails formation of 1(II) and 2(II), which show
characteristic UV – vis spectra. This second process is independent of the nature and
concentration of the substrate.
Table 4. Yield of sulfilimine products (2.5 mM on Fe and 0.25 – 1.2 M on substrate), normalised
according to the iron (IV) purity of sample (determined by the UV – Vis spectra, previously
calibrated with Mössbauer spectroscopy).
XPhS(NTs)Me Yields (%)
Complex
1(IV)=NTs
2(IV)=NTs
Solvent
Acetonitrile
Acetone
Acetonitrile
X
MeO
Me
H
57
46
49
44
42
43
53
43
48
44
33
29
Cl
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Figure 8. UV – vis spectra corresponding to the reaction of 1(IV)=NTs (left) and 2(IV)=NTs (right)
upon addition of an excess of MeOPhSMe (reaction at 20 ºC with 100 and 50 eq of substrate added
respectively, [Fe] = 0.25 mM in MeCN). Spectra correspond to the first rapid phase of the
reaction.
Frozen samples were prepared immediately following the first rapid phase of the reaction of
1(IV)=NTs and 2(IV)=NTs with MeOPhSMe, and analysed by Mössbauer and EPR spectroscopy.
These analyses (see supporting information for details) revealed the presence of monoiron species
with an approximate relative ratio of Fe(II):Fe(III) of 1:6 for the decay of 1(IV)=NTs and 1:3 for
2(IV)=NTs. The major ferric species formed after reaction of 1(IV)=NTs and 2(IV)=NTs with the
sulphide can be identified as 1(III)–NHTs and 2(III)–NHTs/2(III)–OH, respectively on the basis of
the agreement with the EPR and Mössbauer parameters of the independently prepared complexes.
On the other hand, the parameters of the minor ferrous component observed in the Mössbauer
spectra (figure S7 and table S1) are characteristic of a low – spin ferrous centre and are indicative
of the formation of 1(II) and 2(II), respectively.49 Of interest, no reaction is observed when
1(IV)=NTs is mixed with 1(II) or when 2(IV)=NTs is mixed with 2(II). Therefore, it can be concluded
that 1(III)–NHTs and 2(III)–NHTs do not originate from subsequent comproportionation reactions.
The sum of the spectroscopic data and the stoichiometry of the reactions of 1(IV)=NTs and
2(IV)=NTs with sulphides indicates that transfer of the tosylimido moiety to the sulphide does not
correspond to a 2e- N – transfer where the iron (IV) centre becomes reduced to iron (II), as it has
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75
been previously documented for 3(IV)=NTs, but instead, a competitive and most dominant path
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involves formation of iron (III) species.
Kinetic analysis of the N – transfer reactions. The kinetics of the reactions of 1(IV)=NTs and
2(IV)=NTs (0.25 – 4 mM) with sulphides were explored in acetonitrile. Reactions were monitored
by following the evolution over time of the absorbance of the low – energy bands corresponding
to the iron (IV) species (figure 8). Data was satisfactorily adjusted to first order decay processes
according to equation (1)
d[FeIV]/dt = - kobs[FeIV] (1)
where [FeIV] = [1(IV)=NTs] or [2(IV)=NTs]
The respective pseudo – first order rate constants (kobs) were then calculated from this fitting.
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Chemical Science
Figure 9. Top: Rate dependence on substrate concentration of reactions of (A) 1(IV)=NTs (left)
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DOI: 10.1039/C9SC02526J
with MeOPhSMe substrate (denoted as [S]) and (B) 2(IV)=NTs with MeOPhSMe substrate (right) at
293 K. Bottom: Hammett plot representing log(kX/kH) against Hammett parameter (p) for the
reactions of (C) 1(IV)=NTs (left) and (D) 2(IV)=NTs (right) with p – X – thioanisoles in acetonitrile
at 293 K. kx values correspond to the second order rate constants obtained by measuring kobs
values at different p – X – thioanisole concentrations.
In the first place, reactions of 1(IV)=NTs and 2(IV)=NTs at a fixed concentration (0.25 mM)
against different concentrations of thioanisole substrate, used in large excess (30 – 220 mM) were
analysed (figure 9, panels A and B). Observed reaction rates (kobs) were found to be linearly
dependent on sulphide concentration, indicating that they are pseudo – first order rate constants
with the expression kobs = k’[sulphide].
The k’ values for 1(IV)=NTs and 2(IV)=NTs were determined for para-substituted thioanisole
substrates (XPhSMe, X = MeO, Me, H, Cl) and are shown in table 5. A Hammett plot was
represented by plotting log(k'X/k'H) against Hammett parameters (p). The obtained plots (figure
9, panels C and D) show linearity with a slope of  = -3.12 and  = -3.89 for 1(IV)=NTs (kobs
values were determined in reactions with a common concentration of 1(IV)=NTs and 2(IV)=NTs
respectively. These values are relatively large in magnitude compared to those described for OAT
reactivity examples with iron (IV) – oxo compounds, which range between  = -1 – -2,76-79 but
are in good agreement with that associated with N atom transfer with 3(IV)=NTs (-3.35).47
Moreover, when k’ values were plotted against the oxidation potentials of corresponding
thioanisole substrates, a linear plot was obtained with slopes of -5.9 and -7.8 V-1 for 1(IV)=NTs
and 2(IV)=NTs respectively (figure S8). For comparison, for OAT with iron (IV) – oxo processes
those values range between -2 and -3 V-1. The relatively large slopes exhibited by 1(IV)=NTs and
2(IV)=NTs in comparison with 1(IV)=O and 2(IV)=O indicate that the former reactions proceed with
larger charge transfer in the transition states. In fact, the herein determined values are reminiscent
of reactions where a single electron transfer process is the rate determining step.47, 79 However, as
will be discussed in the following lines, thermodynamic considerations discard this scenario.
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Comparison between reaction rates determined for 1(IV)=NTs and 2(IV)=NTs with respect to
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those previously described for 3(IV)=NTs deserve some comment (table 5). Of note, even when
performed at a higher temperature (293 K vs 273 K), reactions of 1(IV)=NTs and 2(IV)=NTs are
nearly two orders of magnitude slower than those of 3(IV)=NTs. These observations suggest that
1(IV)=NTs and 2(IV)=NTs are milder oxidants. Comparison with reaction rates for oxo – transfer
reactions to sulphides by the corresponding oxo complexes 1(IV)=O and 2(IV)=O is also pertinent.
Comparison between 3(IV)=NTs and 3(IV)=O revealed the former to react with sulphide five times
faster than the latter.75 In contrast, oxo transfer reactions by 1(IV)=O and 2(IV)=O are systematically
faster than tosylimido transfer by 1(IV)=NTs and 2(IV)=NTs, with reaction rates between two to
twenty times larger when comparing complexes with the same pentadentate ligand.
Table 5. Measured second order rate constants for the reaction of 0.25 mM solutions of 1(IV)=NTs
and 2(IV)=NTs with selected XPhSMe at 293 K.
k’ ( x 10-3 M-1·s-1)
a[3(IV)=NTs]
XPhSMe [1(IV)=NTs] [1(IV)=O] [2(IV)=NTs] [2(IV)=O]
CH3O
CH3
H
77 ± 10
20 ± 3
12 ± 2
260 ± 9
126 ± 3
50 ± 2
29 ± 3
207 ± 14
49 ± 1.0
7.9 ± 0.8
2.2 ± 0.5
400 ± 20
201 ± 4
91 ± 4
6794
1560
260
Cl
1.6 ± 0.3
42 ± 4
139
aData from reference 47, 1mM solutions at 273 K.
Reaction mechanism. The proposed mechanism for the reaction of 1(IV)=NTs and 2(IV)=NTs with
p – X – ArSMe substrates is shown in scheme 3. The large slopes in the Hammett plots led us to
initially consider the possibility that the reaction was initiated by an electron transfer (ET).
However, this scenario is discarded on the basis of thermochemical considerations; DFT
computed E1/2(FeIV/FeIII) values for the pairs 1(IV)=NTs/1(III)=NTs and 2(IV)=NTs/2(III)=NTs are
0.87 and 0.97 V vs SHE (see section 5 in the SI for details, attempts to determine the values
experimentally by cyclic voltammetry and by red-ox titrations with ferrocene proved
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unsuccessful), while the computed potential for the single electron oxidation of thioanisole to the
corresponding sulfide cation radical (p – H – ArS·+Me) is 1.32 V vs SHE. These values indicate
that the ET reaction is highly endergonic, discarding its viability. In addition, DFT computed
E1/2(FeIV/FeIII) values for the pairs 1(IV)=O/1(III)=O and 2(IV)=O/2(III)=O are 0.08 and 0.19 V vs
SHE respectively (see supporting information for details). These values indicate that they are
milder single electron oxidants than the corresponding tosylimido analogues and that ET from
thioanisole to these iron (IV) – oxo complexes is also highly endergonic.
Alternatively, reaction of 1(IV)=NTs and 2(IV)=NTs with p – X – ArSMe substrates is proposed to
take place via a two – step process and involves two molecules of the iron (IV) – tosylimido
complex (1(IV)=NTs or 2(IV)=NTs) for each molecule of sulphide. In the first, rate determining
step, the sulphide reacts via a nucleophilic attack on the tosylimido N atom of a first molecule of
iron (IV) – tosylimido complex. This attack results in tosylimido transfer to the substrate, forming
a putative [FeII(p – X – ArS(NTs)Me)(L)], 1-2(II)ArS=NTs intermediate that may undergo
sulfilimine dissociation and acetonitrile binding to yield [FeII(CH3CN)(L)]2+, as earlier
documented for 3(IV)=NTs.47 However, in the case of 1(IV)=NTs and 2(IV)=NTs, the latter appears
to be a minor path (k3 < k2), and instead 1-2(II)ArS=NTs can be oxidized by a second molecule of
1(IV)=NTs or 2(IV)=NTs, to form one equivalent of sulfilimine, and two equivalents of ferric
species. One of the two retains a tosylimido ligand and converts to 1(III)–NHTs or 2(III)–NHTs
presumably by proton transfer from adventitious water. Formation of additional amounts of 1(III)
–
NHTs or 2(III)–NHTs /2(III)–OH from the five coordinate ferric species [FeIII(L)] may originate
from reaction with residual H2NTs and adventitious water.
Therefore, while the three iron (IV) – imido complexes 1-3(IV)=NTs react with sulphides
via an initial two-electron tosylimido transfer step, the reactions that follow this step are
dependent on the nature of L. Structural/steric factors depending on the specific complex may
account for the different reactivity, by limiting bimolecular reactions or favouring dissociation.
Specifically, the N4Py ligand may provide a sterically more demanding scaffold than the tacn
based ligands (MePy2tacn and Me2(CHPy2)tacn). This factor presumably favors rapid sulfilimine
dissociation from 3(II)ArS=NTs and disfavors its reaction with 3(IV)=NTs.
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Ts
N
Ts
S
Ts
NH
N
N
Ar
N
FeIV
X
N
N
N
N
N
N
N
N
HX
-H+
N
N
FeIII
N
FeIII
FeIII
N
+
N
N
N
N
N
Ar
N
k2
S
Ts
H+
Ts
N
FeIV
N
HX = H2NTs or H2O
N
FeII
N
k'
N
N
N
N
S
+
Ar
Solv
k3
Solv
N
N
N
N
N
N
N
Ts
S
N
FeII
N
+
Ar
N
Scheme 3. Mechanism proposal for the first phase of the reactions of iron (IV) – tosylimido
complexes with thioanisole substrates. Solv. stands for solvent molecule.
Reactivity of 1(IV)=NTs and 2(IV)=NTs toward H – atom abstraction reactions. The
potential HAT reactivity of 1(IV)=NTs and 2(IV)=NTs against substrates containing weak C – H
bonds was studied by monitoring the bleaching of the characteristic visible bands (with maxima
at 740 and 730 nm, respectively) upon addition of solutions of selected substrates (xanthene,
dihydroantracene (DHA), cyclohexadiene (CHD) and fluorene). No reaction was observed with
toluene and ethylbenzene. Reactions were performed in acetone and in acetonitrile at 293 K under
a N2 atmosphere. UV – vis monitoring shows that the reactions lead to the formation of 1(III)
–
NHTs or 2(III)–NHTs along with substrate oxidation products resulting from a formal
dehydrogenation process (-H2). Ferrous complexes 1(II) or 2(II), and products resulting from
tosylimido transfer were not detected. Nearly quantitative reaction yields for the formation of
anthracene from DHA for 1(IV)=NTs and 2(IV)=NTs were obtained (0.47 and 0.49 equiv.
anthracene/equiv. of iron), confirming the involvement of two iron (IV) molecules for the
formation of one molecule of product. Therefore, Fe(IV)=NTs complexes behave as 1e- oxidants
in C – H oxidation reactions, as also occurs with their Fe(IV)=O analogs.3, 48, 80, 81
Reactions were studied kinetically. The decay of 1(IV)=NTs and 2(IV)=NTs exhibits an
exponential behavior upon addition of different substrate concentrations. Unexpectedly, the
observed rates (kobs, s-1), extracted from the exponential fits, do not vary linearly with the substrate
concentrations and instead saturation profiles are observed (figure 10).
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Page 29 of 37
Chemical Science
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DOI: 10.1039/C9SC02526J
Figure 10. Observed reaction rates (kobs) derived from the reaction of a) 1(IV)=NTs and b)
2(IV)=NTs against different substrates in acetonitrile solution at 293 K under N2. Substrates: Black
= xanthene; red = DHA; blue = CHD; green = fluorene. c) 1(IV)=NTs and d) 2(IV)=NTs against
DHA (circles) and DHA-d4 (stars).
Conditions that lead to rate saturation profiles involve the specific mechanistic feature of
presenting a fast reversible reaction step that precedes a slower irreversible step. Thus, by fitting
the data with the pre – equilibrium approximation model:
kobs = (Keq*k*[CH])/(1+Keq*[CH])
Equation (1)
The values of k(s-1) and Keq(M-1) reported in table 6 could be derived.
Table 6. k(s-1) and Keq(M-1) values for the reaction of 1(IV)=NTs and 2(IV)=NTs against different
hydrocarbons in acetonitrile at 293 K.
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