Spectroscopic Characterization and Reactivity of Triplet and Quintet Iron(IV) Oxo Complexes in the Gas Phase

Article abstract of DOI:10.1002/anie.201511374

Closely structurally related triplet and quintet iron(IV) oxo complexes with a tetradentate aminopyridine ligand were generated in the gas phase, spectroscopically characterized, and their reactivities in hydrogen-transfer and oxygen-transfer reactions we

Full text of DOI:10.1002/anie.201511374

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201511374
German Edition: DOI: 10.1002/ange.201511374
Spin States
Spectroscopic Characterization and Reactivity of Triplet and Quintet
Iron(IV) Oxo Complexes in the Gas Phase
Erik Andris, Juraj Ja sˇ ík, Laura Gómez, Miquel Costas,* and Jana Roithovµ*
Abstract: Closely structurally related triplet and quintet iron-
IV) oxo complexes with a tetradentate aminopyridine ligand
complexes has not been reported so far because only one
type of the given complex, the more stable one, is usually
(
[7]
were generated in the gas phase, spectroscopically character-
ized, and their reactivities in hydrogen-transfer and oxygen-
transfer reactions were compared. The spin states were
unambiguously assigned based on helium tagging infrared
photodissociation (IRPD) spectra of the mass-selected iron
complexes. It is shown that the stretching vibrations of the
nitrate counterion can be used as a spectral marker of the
central iron spin state.
prepared.
The genuine reactivity of clearly defined complexes can
[12–16]
be directly compared in the gas phase
and it can enlighten
the role of spin in the reactivity of metal oxo complexes. It is
necessary, however, to develop a method for characterization
of the spin-states of such mass-selected complexes in the gas
phase. Herein, we present a spectroscopic characterization of
iron(IV) oxo complexes with a tetradentate ligand denoted
as PyTACN (PyTACN = 1-[2’-(pyridyl)methyl]-4,7-dimethyl-
1,4,7-triazacyclononane, Scheme 1) in the triplet (S = 1) as
well as the quintet (S = 2) state.
T
he search for reagents capable of selectively oxidizing an
alkane CÀH bond, and characterization of their chemistry are
actively pursued and challenging
goals in modern synthesis. One of
the most promising approaches is
based on hypervalent transition-
[1,2]
metal oxo complexes.
An
important advantage of using tran-
sition metals and their oxo com-
plexes is the potential they offer to
[3–6]
explore multistate reactivity.
Metal oxo complexes with a high-
spin ground state are usually con-
[7–10]
sidered to be more reactive,
but also low-spin highly reactive
+
Scheme 1. Generation of [(PyTACN)Fe(O)(NO )] by oxidation of the iron(II) precursor in solution or
by in-source CID during electrospray ionization.
3
[11]
complexes
were
reported.
Direct comparison of the reactiv-
ities of high-spin and low-spin
The reactive ions were formed in acetonitrile solution
from their iron(II) precursor [(PyTACN)Fe(OTf) ] by oxida-
2
tion with peracetic acid. Mçssbauer spectroscopy has dem-
onstrated that this reaction sequence leads to the generation
of triplet state iron(IV) complexes (S = 1). Using electro-
[
*] E. Andris, J. Ja sˇ ík, Prof. J. Roithovµ
Department of Organic Chemistry
Faculty of Science, Charles University in Prague
Hlavova 2030/8, 12843 Prague 2 (Czech Republic)
E-mail: roithova@natur.cuni.cz
[17]
spray ionization (ESI) we were able to transfer the
+
[
(PyTACN)Fe(O)(OTf)] ions to the gas phase and on
+
addition of HNO , the [(PyTACN)Fe(O)(NO )] ions (m/
3
3
Dr. L. Gómez, Prof. M. Costas
z 382) were also detected (Figure S5 in the Supporting
Information). Alternatively, in the gas phase, hypervalent
metal oxo complexes can be generated by radical cleavage of
Departament de Quimica and Institute of Computational Chemistry
and Catalysis (IQCC), University of Girona
Campus Montilivi, Girona 17071 (Spain)
E-mail: miquel.costas@udg.edu
the nitrate counterions (i.e. M-ONO fragments to M-OC and
2
[18]
Dr. L. Gómez
NO C). To generate complexes analogous to those of the
2
Serveis T›cnics de Recerca (STR), Universitat de Girona
Parc Científic i Tecnològic, 17003 Girona (Spain)
condensed phase, the [(PyTACN)Fe(OTf) ] precursor was
2
oxidized by H O to the corresponding iron(III) complex. In
2
2
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
+
the presence of HNO , the abundant [(PyTACN)Fe(NO ) ]
3
3 2
complexes were detected by ESI-MS. Collisional activation of
1
002/anie.201511374.
these complexes led to elimination of NO C radicals and the
2
ꢀ
2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
+
generation of the desired [(PyTACN)Fe(O)(NO )] com-
3
KGaA. This is an open access article under the terms of the Creative
Commons Attribution-NonCommercial License, which permits use,
distribution and reproduction in any medium, provided the original
work is properly cited and is not used for commercial purposes.
plexes (Figures S4 and S5).
We have analyzed the structures and spin states of the
+
gaseous [(PyTACN)Fe(O)(NO )] complexes by helium tag-
3
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[
19]
[23]
ging infrared photodissociation (IRPD) spectroscopy. The
complexes were generated either by the oxidation in solution
and transferred to the gas phase by ESI, or by in-source
fragmentation during ESI of the iron(III) nitrate precursor
complexes. The mass-selected complexes are trapped in
a cryo-cooled ion trap (2.5 K) with helium buffer gas and
ated in the gas phase. The spectra clearly show that we are
working with two distinct groups of ions. The major group is
À1
À1
characterized by the peaks at 1582 cm and 1603 cm ,
whereas the band at 1570 cm belongs to the other group.
The helium complexes from the first group are depleted by
irradiation of the ion cloud at 1603 cm . The spectrum
À1
À1
+
[20]
form [(PyTACN)Fe(O)(NO )(He)] complexes.
Helium
generated by subsequent probing with a second photon beam
(black line in Figure 1b) resembles that of the ions generated
in solution (blue line in Figure 1b). Under the assumption
that the experimental IRPD spectra result from a linear
combination of the spectra of two species, we have extracted
the corresponding base spectra (Figure 1c). They can be
linearly combined in a 0.75:0.25 ratio to provide the IRPD
spectrum of the ions transferred from the oxidized solution,
and in a 0.35:0.65 ratio to provide the IRPD spectrum of ions
obtained by in-source fragmentation (Figure S11). Note that
the ratios are reversed, so the ions characterized by the blue
spectrum in Figure 1c are mostly formed by the oxidation in
solution, whereas the ions characterized by the pink spectrum
in Figure 1c are mostly formed by in-source fragmentation of
the iron(III) precursors. We were not able to completely
suppress the in-source frag-
3
atoms serve as swift leaving tags where an increase of the
complexes internal energy results in their dissociation.
Monitoring helium elimination from the tagged complexes
can be thus used to sensitively detect the absorption of IR
photons. Dependence of the depletion of the helium complex
on the wavenumber of incident IR photons provides IRPD
[
21]
[22]
spectra of the complexes.
Comparison of the IRPD spectra of the complexes
generated in solution and in the gas phase reveals distinct
differences in the range of the nitrate antisymmetric stretch-
ing mode (bands around 1590 cm ) and around 900 cm
Figure 1a). To assign the individual peaks, we have studied
À1
À1
(
À1
À1
the range from 1540 cm to 1640 cm by two-color IRPD
spectroscopy (Figure 1b and Figure S10) of the ions gener-
mentation of the iron(III)
precursors, therefore it also
partly contributed to the
ions formed by in-solution
oxidation.
Interpretation of the
IRPD spectra is based on
comparison with theoretical
IR spectra of possible iso-
mers for the [(PyTACN)Fe-
+
(
O)(NO )] complex (Fig-
3
ure 1d). We have assumed
that the oxygen atom can
either be in plane with the
pyridine (equatorial posi-
tion) or perpendicular to
the plane of the pyridine
(axial position; isomers
1
and 2 in Scheme 1, green
panel). Both isomers can be
present either in the triplet
state or the quintet state.
We have performed a series
of DFT calculations. It can
be stated that the correct
ordering of the spin states
cannot be reliably deter-
mined by DFT methods
(
Figure 2).
While
the
+
Figure 1. a) IRPD spectra of [(PyTACN)Fe(O)(NO )] (m/z 382) generated by oxidation in solution (blue trace,
PBEPBE functional ener-
getically largely favors the
triplet states of both iso-
mers, B3LYP predicts 1 as
the most stable isomer fol-
lowed by 2. The OPBE
functional predicts 2 as the
most stable species and
3
shifted on the y-scale by 0.4 for clarity) or by in-source collisional activation during ESI (red trace) G.P.=gas
phase. b) Top panel: Two-color IRPD experiment with the ions generated in the gas phase, one optical
À1
paramagnetic oscillator (OPO) was set to 1603 cm , the second OPO was scanned (black trace). Lower panel
3
shows one-color IRPD spectra from (a) in detail. c) Separated spectra of the two complexes contributing to
the IRPD spectra shown in (a). The spectra are normalized to the original N . d) Theoretical IR spectra of 1,
, 2, and 2 calculated with B3LYP-D3/6-311+ +G**. e) The most stable structures of 1 and 2. The red
arrows denote the NO and FeO stretching vibrations responsible for the bands at around 900 cm and the
3
0
5
5
3
5
3
5
1
5
À1
À1
blue arrows show the antisymmetric stretch of the NO group, located at about 1600 cm
.
2
3
638
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À1 [17]
frozen acetonitrile solution (831 cm ). The experimentally
determined Fe=O stretches of iron(IV) complexes in con-
À1
densed phase expand from 780 to 900 cm and a spin
[24]
correlation with the spin state cannot be established.
The IRPD spectra do not give strong evidence for which
isomers are formed. Nevertheless, DFT methods are much
more reliable in determination of relative stabilities of
[25]
isomers than in the prediction of spin states. We can state
3
that all tested DFT methods predict that the 1 isomer is
3
substantially more stable than the 2 isomer (at least
À1
1
0 kJmol ). Hence, we conclude that by electrospray ioniza-
Figure 2. Results of DFT calculations (the functionals are specified at
the bottom of the data, the basis set was always 6-311+ +G**, for
the OPBE functional also def2-TZVP was tested—denoted by asterisk).
The red and blue lines denote relative energies of the isomers (left
axis); the pink and light blue lines show the unscaled antisymmetric
3
tion of the ions oxidized in solution we generate the
1
isomers. Under the assumption that the spin-isomerization at
[
3,5]
the iron atom is fast,
it can be also concluded that the
triplet state is the ground state of the isomer 1. Significantly,
1 reproduces the spin and isomeric structure previously
determined in acetonitrile solution.
NO stretch for quintet and triplet states, respectively (dashed lines
2
are for the 1 isomers and solid lines are for the 2 isomers).
5
For the quintet states, the 2 isomer is consistently more
5
stable than the equatorial 1 isomer. The energy differences
are smaller than those for the isomers in the triplet state
À1
finally the M06 functional largely stabilizes both quintet
states with respect to the triplet states (results obtained with
other DFT functionals can be found in the Supporting
Information, Figures S14, S15, Table S3). It should be stressed
that not only the ordering of the spin states is changing in
dependence of the used DFT functional, but also optimized
geometries of triplet and quintet states differ and favor the
particular state. The correct theoretical treatment thus not
only requires high-level multi-reference calculations but also
full geometry optimizations at the given level are necessary.
Contrary to the energy assignments, the IR characteristics
of the given spin-isomers are predicted consistently and can
be used for the assignment of the ions studied experimentally.
(4 kJmol on average). Theoretically, we could thus gener-
5
ate, by in-source fragmentation, a mixture of both isomers 1
and 2. Again, under the assumption that spin-isomerization
at the iron atom is fast, the 1 isomer is expected to cross over
to the 1 ground state. We are thus left with the conclusion
that the ions in the quintet state generated by in-source
fragmentation of the iron(III) precursors correspond to the 2
ions. We note in passing that we have excluded interference
from other isomers of the generated complexes such as those
with an oxidized ligand (Figure S16, S17).
Analysis of the IRPD spectra thus suggests that we are
studying a mixture of 1 and 2. The separated IRPD spectra
shown in Figure 1c characterize the triplet isomer 1 (the blue
spectrum) and the quintet isomer 2 (the pink spectrum). The
ratio of 1 and 2 among the ions transferred from the solution
and among those formed by in-source fragmentation is
0.75:0.25 and 0.35:0.65, respectively, as determined from the
linear combinations of the spectra.
In the next step, we have compared the reactivities of the
ions and extracted reaction rates associated with the triplet-
and quintet-state complexes (Table S1 in the Supporting
Information). The 1 and 2 complexes differ not only in spin,
but also in the orientation of the oxo group with respect to the
plane of the pyridine ring. It was previously shown that the
orientation of the oxo ligand perpendicular to the plane of the
pyridine ligand can contribute to its larger reactivity. As we
have shown experimentally, it also contributes to the stabi-
lization of the high-spin state. It might be possible that these
two effects are interconnected (for example via lowering of
the high-spin energy barrier for a reaction). In both isomers,
1 and 2, an aliphatic amine is trans to the oxo ligand, and
therefore this aspect can be reasonably discarded to differ-
5
5
3
5
3
5
3
5
The antisymmetric stretching of the NO unit (cf. Figure 1e) is
2
3
5
blue-shifted for the quintet states with respect to the triplet
states (pink vs. light blue line in Figure 2). Natural bond
orbital (NBO) analysis shows that there is consistently greater
spin localization at the nitrate counterions for the quintet
states which is reflected in the blue shift of the antisymmetric
NO2 stretch (Table S3). Another effect observed for the
quintet-state complexes is a red shift of the NÀO vibration
À1
3
5
below 900 cm (cf. Figure 1d,e). Analogous band shifts are
also observed experimentally. Hence, the ions generated in
the gas phase by in-source fragmentation reveal a blue-shifted
À1
À1
band at 1603 cm and a red-shifted band at 900 cm with
respect to the bands observed for the ions obtained by
oxidation in solution. Therefore we conclude that the ions
generated by in-source fragmentation of the iron(III) pre-
[26]
5
5
cursors are in the quintet state ( 1 and/or 2). The ions
oxidized by peracetic acid in solution and transferred to the
3
3
gas phase are then triplets ( 1 and/or 2). Note that this is in
agreement with the previous results obtained by Mçssbauer
[17]
[27]
spectroscopy.
The expected Fe=O stretch is predicted in the computed
entiate the reactivity between isomers. It was reported that
a change of the ligand trans to the oxo ligand from an aliphatic
amine to pyridine can lead to differences in reactivities for
isospin iron complexes of an order of magnitude.
The reaction was investigated with 1,4-cyclohexadiene
and with partially deuterated 1,4-cyclohexadiene-[D ] with
both methylene groups bearing one H and one D atom
3
5
3
5
À1
spectra of 1, 1, 2, and 2 at approximately 900 cm . These
are commonly low-intensity vibrational features and cannot
be discerned in the experimental IRPD spectra. They are blue
shifted with respect to the value obtained by resonance
[25]
6
2
+
Raman spectroscopy for [(PyTACN)Fe(O)(CH CN)] in
3
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Acknowledgements
This research was supported by the European Research
Council (StG ISORI and BIDECASEOX), the Grant agency
of the Czech Republic (14-20077S), the Spanish MINECO
(CTQ2012-37420-C02-01/BQU), and an ICREA Academia
Award to M.C. Computational resources were provided by
the MetaCentrum under the program LM2010005 and the
CERIT-SC under the program Centre CERIT Scientific
Cloud, part of the Operational Program Research and
Development for Innovations, Reg. no. CZ.1.05/3.2.00/
0
8.0144. We thank Andrew Gray for his help in preparing
Figure 3. Mass spectra corresponding to the reaction of [(PyTACN)Fe-
the manuscript and Michael J. Bojdys for the graphics.
+
(
O)(NO )] (m/z 382, these ions were generated by in-source fragmen-
3
tation) with a) 1,4-cyclohexadiene and b) 1,4-cyclohexadiene-[D ] (pres-
6
Keywords: CÀH activation · IR spectroscopy · iron complexes ·
sure of the neutral reactant was 0.2 mTorr). c) Collision-energy depend-
ence (center-of-mass) of the cross-sections s, and d) pressure depend-
ence of the relative cross section A of HAT, DAT, and OAT in the
reaction with 1,4-cyclohexadiene-[D ] (A is defined as Àln-
mass spectrometry · spin state
How to cite: Angew. Chem. Int. Ed. 2016, 55, 3637–3641
Angew. Chem. 2016, 128, 3701–3705
6
i
(
1ÀSI /(SI +I ))(I /SI ), where I and I are intensities of the frag-
n
n
P
i
n
n
P
ments and the parent, respectively). DAT=deuterium-atom transfer.
[
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[
À12
3
À1
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À12
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(
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Chemie
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Received: December 8, 2015
Published online: February 15, 2016
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3641