Comparative Study of the Electronic Structures of μ-Oxo, μ-Nitrido, and μ-Carbido Diiron Octapropylporphyrazine Complexes and Their Catalytic Activity in Cyclopropanation of Olefins

Article abstract of DOI:10.1021/acs.inorgchem.9b02718

The electronic structure of three single-Atom bridged diiron octapropylporphyrazine complexes (FePzPr₈)₂X having Fe(III)-O-Fe(III), Fe(III)-N-Fe(IV) and Fe(IV)-C-Fe(IV) structural units was investigated by M?ssbauer spectroscopy and density functional theory (DFT) calculations. In this series, the isomer shift values decrease, whereas the values of quadrupole splitting become progressively greater indicating the increase of covalency of Fe-X bond in the μ-oxo, μ-nitrido, μ-carbido row. The M?ssbauer data point to low-spin systems for the three complexes, and calculated data with B3LYP-D3 show a singlet state for μ-oxo and μ-carbido and a doublet state for μ-nitrido complexes. An excellent agreement was obtained between B3LYP-D3 optimized geometries and X-ray structural data. Among (FePzPr₈)₂X complexes, μ-oxo diiron species showed a higher reactivity in the cyclopropanation of styrene by ethyl diazoacetate to afford a 95% product yield with 0.1 mol % catalyst loading. A detailed DFT study allowed to get insight into electronic structure of binuclear carbene species and to confirm their involvement into carbene transfer reactions.

Full text of DOI:10.1021/acs.inorgchem.9b02718

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Comparative Study of the Electronic Structures of μ‑Oxo, μ‑Nitrido,
and μ‑Carbido Diiron Octapropylporphyrazine Complexes and Their
Catalytic Activity in Cyclopropanation of Olefins
†
§
§,‡
Lucie P. Cailler, Martin Clemancey, Jessica Barilone, Pascale Maldivi,*,‡ Jean-Marc Latour,*,§
́
and Alexander B. Sorokin*,†
†
́
Institut de Recherches sur la Catalyse et l’Environnement de Lyon IRCELYON, UMR 5256, CNRS - Universite Lyon 1, 2 avenue
A. Einstein, 69626 Villeurbanne cedex, France
‡Univ. Grenoble-Alpes, CEA, CNRS, IRIG, CBM, Grenoble 38000, France
§Univ. Grenoble-Alpes, CEA, CNRS, IRIG-SyMMES, Grenoble 38000, France
S
* Supporting Information
ABSTRACT: The electronic structure of three single-atom
bridged diiron octapropylporphyrazine complexes
(FePzPr8)2X having Fe(III)−O−Fe(III), Fe(III)−N−Fe(IV)
and Fe(IV)−C−Fe(IV) structural units was investigated by
̈
Mossbauer spectroscopy and density functional theory (DFT)
calculations. In this series, the isomer shift values decrease,
whereas the values of quadrupole splitting become pro-
gressively greater indicating the increase of covalency of Fe-X
̈
bond in the μ-oxo, μ-nitrido, μ-carbido row. The Mossbauer
data point to low-spin systems for the three complexes, and
calculated data with B3LYP-D3 show a singlet state for μ-oxo
and μ-carbido and a doublet state for μ-nitrido complexes. An
excellent agreement was obtained between B3LYP-D3 optimized geometries and X-ray structural data. Among (FePzPr8)2X
complexes, μ-oxo diiron species showed a higher reactivity in the cyclopropanation of styrene by ethyl diazoacetate to afford a
95% product yield with 0.1 mol % catalyst loading. A detailed DFT study allowed to get insight into electronic structure of
binuclear carbene species and to confirm their involvement into carbene transfer reactions.
compounds,20 hydroacylation of olefins21 as well as for
INTRODUCTION
■
oxidative defluorination22 and dechlorination23 of halogenated
compounds. Although the catalytic activity of μ-carbido diiron
species has not been published yet, related μ-carbido
diruthenium phthalocyanine catalyzed cyclopropanation of
olefins and N−H bonds.24
For a long time, research in biomimetic oxidation and related
reactions has been associated with mononuclear metal
macrocyclic1,2 and nonheme3,4 complexes. As for binuclear
catalysts, diiron nonheme complexes have attracted significant
attention as possible biomimetic catalysts for challenging mild
oxidation of methane.5 Diiron species supported by nonheme
scaffolds were also shown to be efficient catalysts for nitrene
transfer reactions.6,7 Since 2000, there is growing evidence that
macrocyclic complexes with two single-atom bridged iron sites
exhibit interesting catalytic properties in many reactions. These
complexes bear two iron sites supported by phthalocyanine,
porphyrin, or porphyrazine ligands which can be connected by
μ-oxo, μ-nitrido, and μ-carbido bridges.8 The nature of the
bridging group determines the iron oxidation state. Thus,
neutral μ-oxo, μ-nitrido, and μ-carbido dinuclear complexes are
Fe(III)(μ-O)Fe(III), Fe(III)(μ-N)Fe(IV), and Fe(IV)(μ-C)-
Fe(IV) systems. These complexes can be oxidized or reduced
to attain other oxidation states.8 μ-Oxo diiron phthalocyanines
were found to catalyze the oxidation of aromatic compounds to
corresponding quinones.9,10 In turn, μ-nitrido diiron macro-
cyclic complexes have emerged as effective catalysts11,12 for the
oxidation of methane,13−15 ethane,16,17 alkanes,18,19 aromatic
In all these reactions, the electroinsertion of carbene to
aminenic structures of the iron complexes and their changes
during catalysis play an essential role. In this context, nature
and properties of the macrocyclic ligand as well as bridging
atom can influence the electronic state and catalytic properties
of the single-atom bridged diiron species. In order to discern
the structural and electronic differences, a series of the diiron
phthalocyanine complexes (FePc)2X (X = C, N, O) has been
investigated by X-ray absorption and emission spectroscopies
combined with density functional theory (DFT) calculations.25
The low-spin μ-nitrido dimer (FePc)2N has the highest Fe−X
bond order, whereas (FePc)2C, a closed-shell structure with
FeC double bonds, exhibits lower covalency of bridging
unit. The Fe−X distances change in the sequence Fe−O > Fe−
Received: September 10, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02718
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Inorganic Chemistry
Article
Figure 1. Structures of μ-oxo, μ-nitrido, and μ-carbido diiron octapropylporphyrazines complexes used in this study. R = −CH2CH2CH3.
C > Fe−N. The most electron-rich μ-oxo complex forms a
single Fe−O bond, whereas μ-carbido unit has too high energy
of p-electrons and donates them to iron sites. The nature of
diruthenium phthalocyanine in combination with ethyl
diazoacetate (EDA) was shown to catalyze cyclopropanation
of olefins and insertion of carbene to amine N−H bonds.24 To
the best of our knowledge, the catalytic properties of
porphyrazine complexes have never been investigated in
carbene transfer reactions. Herein, we evaluate the catalytic
properties of μ-oxo, μ-nitrido, and μ-carbido diiron porphyr-
azine complexes in the cyclopropanation of olefins with ethyl
̈
tetrapyrrolic macrocycle also influences the Mossbauer
parameters. Thus, the values of isomeric shift δ in [LFe(IV)]2C
series are −0.04 and 0.10 mm s−1 for (FePc)2C and
(FeTPP)2C, respectively.26
In continuation of our research project on the catalytic
activity of single-atom bridged dimeric macrocyclic complexes,
we explore the catalytic properties of μ-oxo, μ-nitrido, and μ-
carbido diiron complexes supported by porphyrazine ligand; X-
ray structures of which were previously determined (Figure
1).27
̈
diazoacetate. Detailed Mossbauer studies and DFT calculations
provide an insight into electronic structures of these
complexes. The electronic structures of the three putative
carbene diiron complexes were theoretically characterized for
the first time, and their influence on the reactivity in the
cyclopropanation of styrenes is discussed.
Noteworthy, the catalytic properties of porphyrazine
complexes including their μ-oxo, μ-nitrido, and μ-carbido
dimers have been rarely investigated28 compared to their
porphyrin1,2 and phthalocyanine counterparts.11,12 One
representative example is the oxidation of hydrocarbons by
PhIO, iodosylbenzene sulfate, and KHSO5 catalyzed by μ-oxo
diiron(III) octakis(perfluorophenyl)tetraazaporphyrin.29
In the present work, μ-oxo, μ-nitrido, and μ-carbido diiron
octapropylporphyrazine complexes, (FePzPr8)2X (X = O, N,
C), were evaluated in the reactions of the carbene insertion to
the double bond of olefins which has emerged as well-
established synthetic approach to cyclopropyl compounds.30,31
Many transition-metal macrocyclic compounds activate
carbene precursors to form putative metal-carbene intermedi-
ates which transfer carbenes to olefinic double bonds,32,33
including preparation of chiral compounds.34−37 Porphyrin38,39
complexes of iron,40−43 cobalt,44,45 ruthenium,46,47 iridium,48
osmium,31 and rhodium49 are by far the most studied catalysts
in these reactions. Iron and rhodium corrole complexes are
also efficient catalysts for the cyclopropanation of olefins.50
In sharp contrast, the studies of other tetrapyrrolic catalysts
for the preparation of cyclopropane derivatives are scarce.
Mononuclear metal phthalocyanine complexes of Cu, Fe, Mn,
Ni, Co, Ag, Zn, and Ru were evaluated in the cyclopropanation
of styrenes.51−53 Binuclear single-atom bridged phthalocyanine
complexes recently introduced as efficient catalysts for the
oxidation and other reactions11,12 have never been used as the
catalysts for cyclopropanation except one case. μ-Carbido
EXPERIMENTAL SECTION
■
Materials. Ethyl diazoacetate containing ∼13 wt % of dichloro-
methane was purchased from Sigma-Aldrich. Iron octapropylporphyr-
azine and its μ-oxo, μ-nitrido, and μ-carbido complexes were
synthesized according to published protocols.27
Methods. The UV−visible (UV−vis) absorption spectra were
1
recorded on an Agilent 8453 diode-array spectrophotometer. H and
13C NMR spectra were acquired on a Bruker Ascend 400
spectrometer. The reaction products were identified by NMR and
GC-MS method (Hewlett-Packard 5973/6890 system; electron
impact at 70 eV, He carrier gas, 30 m x 0.25 mm ROTICAP-5
capillary column, polyethylene glycol, 0.25 μm coating).
̈
̈
Mossbauer Experiments. The Mossbauer spectra were recorded
on unenriched powders contained in Delrin cups at ca. 5 and 80 K on
̈
a strong-field Mossbauer spectrometer equipped with an Oxford
Instruments Spectromag 4000 cryostat containing an 8 T split-pair
superconducting magnet. The spectrometer was operated in a
constant acceleration mode in transmission geometry. The isomer
shifts were referenced against that of a room-temperature metallic iron
foil. Analysis of the data was performed with the software WMOSS
̈
Mossbauer Spectral Analysis Software (http://www.wmoss.org),
2012−2013 (Web research, Edina).
Computational Details. Quantum chemistry calculations on
model complexes without propyl substituents (FePz)2X were
performed using DFT in the Kohn−Sham framework. Calculations
̈
on geometry optimizations and Mossbauer parameters were done
using Amsterdam Density Functional (ADF) 201654−56 and Orca
B
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Article
̈
Figure 2. Mossbauer spectra of (FePzPr8)2O (top), (FePzPr8)2N (middle), and (FePzPr8)2C (bottom) recorded (left panel) at 80 K under no
magnetic field and (right panel) under a field of 7 T applied parallel to the γ rays at 80 K (A−C) and 5.5 K (A′−C′). Experimental spectra: hatched
bars, black line: simulation using the parameters listed in Table 1.
3.0.1.57−59 Some bonding analyses were performed with NBO 6.060,61
interfaced with ADF.
70 °C until EDA was consumed. The reaction products were analyzed
1
by H NMR (CDCl3) and by GC-MS.
In ADF, geometry optimizations were carried out using the
B3LYP62 functional with Grimme D363 dispersion correction
(B3LYP-D3 below) and the Slater triple-ζ + 2 polarization functions
RESULTS AND DISCUSSION
■
for all atoms (all-electron TZ2P set).64 Mossbauer parameter
Electronic Structures. μ-Oxo, μ-nitrido, and μ-carbido
diiron complexes adopt Fe(III)(μ-O)Fe(III), Fe(III)(μ-N)Fe-
(IV), and Fe(IV)(μ-C)Fe(IV) oxidation states. Mossbauer
̈
calculations on the optimized geometries were performed according
to published procedure.65,66 Geometry optimizations with Orca were
done with B3LYP and the D3BJ correction67 using the Def2-tzvp
basis sets68,69 and the RIJCOSX approximation.70,71
̈
spectra of these complexes have been recorded at 5.5 and 80 K
in absence of magnetic field and under 2 and 7 T magnetic
fields applied parallel to the γ rays. In absence of magnetic field
(Figure 2, left panel), all spectra appear as a quadrupole
doublet. In series (FePzPr8)2O, (FePzPr8)2N, (FePzPr8)2C,
the center of the doublet (isomer shift) moves toward lower
velocity, while the separation between the two lines (quadru-
pole splitting) increases (Table 1). Application of a strong
magnetic field of 7 T at 80 or 5.5 K causes a splitting of these
doublets into multiplets in the velocity range from −2.5 to 2.5
mm s−1 in all cases (Figure 2, right panel). This limited range is
consistent with a low-spin nature of the compounds, S = 0 for
μ-O and μ-C and S = 1/2 for μ-N complexes. For each
compound, all three spectra could be simulated using this
assumption with the parameters given in Table 1 (Figure 2).
The isomer shift and quadrupole splitting values of μ-O, μ-
N, and μ-C bridged diiron tetraphenylporphyrin, phthalocya-
nine, and porphyrazine complexes are assembled in Table 1. A
general decrease of the isomer shift is observed in the series
[(macrocycle)Fe]2(μ-O), [(macrocycle)Fe]2(μ-N),
[(macrocycle)Fe]2(μ-C) for all macrocycles in line with a
progressive increase in the iron overall oxidation state. In
parallel, a notable increase of the quadrupole splitting values
occurs. A possible origin of this trend may be found in the
increased covalency of the Fe−X bond due to the increase in
its double bond character in the μ-O, μ-N, μ-C series.25,27
Particular caution was taken to control local spin densities (see
Supporting Information for details) at each stage (geometry
̈
optimizations and Mossbauer parameters). In particular, all
calculations on singlet states were done in an unrestricted scheme.
The electronic affinities (EA) of the carbene active species were
calculated as the adiabatic difference of electronic energies between
the complex and its one-electron reduced species using the same
conditions as for above-mentioned geometry optimizations (ADF
2016 in gas phase with B3LYP-D3 and all-electron TZ2P sets)
followed by single-points with COSMO72 model solvent using
acetonitrile (ε = 37).
Typical Procedure for Cyclopropanation of Olefins. All
olefins were filtered through basic alumina and silica before use. A
solution containing olefin (1 M) and [FePzPr8]2X (1 mM) in 0.4 mL
toluene was flushed with argon for 5 min. A 2 M solution of EDA in
toluene (0.23 mL, 0.48 mmol, 1.2 equiv) was added to the reaction
mixture under argon atmosphere via a syringe pump over 2, 4, or 6 h
at 70 °C. The reaction mixture was magnetically stirred for 6 h. The
1
reaction products were analyzed by H NMR (CDCl3) and by GC-
MS. Analytical data for the reaction products were identical to those
published in the literature.
Typical Procedure for Competitive Cyclopropanation of
Olefins (Hammett Correlations). A 2 mM catalyst solution and a
solution containing 1 M of styrene and 1 M of substituted styrene
were prepared in toluene separately and were used for the preparation
of the reaction mixture under argon. A 2 M solution of EDA in
toluene (0.05 mL, 0.1 mmol, 0.25 equiv) was added over 1 h to a 0.4
mL solution containing 0.5 M styrene and 0.5 M para-substituted
styrene, and 1 mM (FePzPr8)2X in toluene was added by syringe
pump under argon. The reaction mixture was magnetically stirred at
̈
Analysis of Mossbauer parameters in Table 1 shows that the
electronic structure of the porphyrazine derivatives is closer to
that of the corresponding phthalocyanine than that of the
C
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Inorganic Chemistry
Article
initiating the electronic convergence and to check the final
local spin densities (see Supporting Information for details).
The various possible BS solutions were thus calculated, and
their energetic ordering is summarized in Table 2. In some
cases, the BS solution could not be obtained. For instance, for
X = C, the final wave function was always converging to local
Sz = 0 whatever the initial guess. Some high-spin solutions are
not shown in Table 2 (S = 5 for X = O, S = 3/2 for X = N, and
S = 1 or 2 for X = C) because they converged to wave
functions with nonequivalent Fe ions and were much higher in
energies.
Our exploration of various spin states for the three
compounds led to the conclusion that the low-spin
configuration was always by far the most stable one, that is,
a singlet state for μ-O and μ-C and a doublet state for μ-N
compounds. A good agreement was obtained between B3LYP
with D3 correction optimized geometries and the exper-
imentally resolved X-ray structures,27 allowing for the
Table 1. Isomer Shifts and Quadrupole Splittings of μ-Oxo,
μ-Nitrido, and μ-Carbido Bridged Iron Macrocyclic
Complexes
T
(K)
δ
ΔEQ
(mm s−1
)
(mm s−1
)
ref
[(TPP)Fe]2(μ-O)
[(Pc)Fe]2(μ-O)
[(Pz)Ph8)Fe]2(μ-O)
[(PzPr8)Fe]2(μ-O)
[(TPP)Fe]2(μ-N)
131
77
297
77
131
77
0.40
0.25
0.40
0.23
0.18
0.11
0.62
1.26
0.89
1.31
1.08
1.47
73
74
75
this work
73
[(TPP)Fe](μ-N)[(Pc)
Fe]
76
[(Pc)Fe]2(μ-N)
[(Pz)Ph8)Fe]2(μ-N)
[(PzPr8)Fe]2(μ-N)
[(TPP)Fe]2(μ-C)
[(Pc)Fe]2(μ-C)
77
298
77
77
77
0.06
−0.04
0.03
0.10
−0.03
−0.06
1.76
1.77
1.91
1.89
2.69
2.78
77
78
this work
79
79
this work
a
[(PzPr8)Fe]2(μ-C)
77
a
̈
calculation of Mossbauer parameters which were in good
Spin Hamiltonian parameters gx, gy = 2.18, gz = 2.0; Ax, Ay = −5.75 T,
agreement with experimental data (Table 3).
Az = 6.47 T; η = 0.02.
Although the isomer shifts for (FePzPr8)2N and (FePz
Pr8)2C were less well reproduced, the general trend was in line
with the experimental data, in particular, with the decrease in
isomer shift from μ-O to μ-N and to μ-C complexes. The three
complexes exhibit very different local spin densities rational-
ized according to their formal oxidation state. The (FePz)2O
complex contains two antiferromagnetically coupled FeIII ions
(J = −976 cm−1 calculated with the Yamaguchi formalism80
using as spin Hamiltonian: H = −2J × SA × SB) with local
quartet Fe spin configurations. For (FePz)2N, our calculation
shows a completely delocalized radical over the two equivalent
tetraphenylporphyrin counterparts. In particular, for all
[(macrocycle)Fe]2(μ-X) compounds, δPzPr8 is slightly inferior
to δPc which is 0.13(2) less than δTPP. Consistently, the
electrochemical study of the related nitrido bridged complexes
of alkyl-substituted macrocycles revealed the same trend for
the oxidation of the [(macrocycle)FeI I I (μ-N)-
FeIV(macrocycle)] core: E1/2
< E1/2 ≪ E1/2
.
PzPr8
Pc
TPP
To get a deeper insight into the electronic structure of these
complexes, we performed DFT calculations on (FePz)2X
model compounds containing no propyl groups. The
̈
Fe(+3.5) atoms consistently with our Mossbauer data and
̈
Mossbauer data clearly pointed to low-spin systems, which
previous studies.13,15,27,81 For (FePz)2C with two FeIV sites,
various broken symmetry configurations obtained by spin
flipping from singlet, triplet, or quintet local Fe states always
converged to local singlet FeIV ions. The Kohn−Sham orbital
(MO) diagrams of these species allow us to give a unified
picture of the three species depicted schematically in Figure 3.
The resulting MOs can be analyzed in terms of symmetric (S)
and antisymmetric (AS) combinations of Fe d AOs vs the
plane perpendicular to the mean Fe−X−Fe axis defined as the
are known to be a difficult case in the DFT framework owing
to the multiconfigurational nature of a low-spin state wave
function. Nevertheless, the broken symmetry (BS) approx-
imation may give information on such electronic configuration.
The monodeterminantal solution able to describe such low-
spin wave function depends on the local Fe spin state and thus
related to its individual Sz component. The possible BS
solutions arising for X = O or C are shown in Table 2, together
with their high-spin counterpart.
2
2
z axis and going through the X position. The dx −y AOs are
involved in σ bonds with N atoms from the Pz macrocycles, so
their antibonding combinations are unoccupied and lie high in
energy. The Fe−X−Fe bonding features are thus based on S
For X = N, the X-ray structure27 and Mossbauer data point
̈
to a doublet spin state and to a strict equivalence of the two Fe
ions. This observation implies that one unpaired electron is
delocalized on both atoms. To explore these various electronic
configurations, it was necessary to monitor strictly the guesses
2
and AS combinations of dz , dxz, and dyz overlapping with
symmetry-adapted 2s, 2px, 2py, and 2pz AOs of X to yield
bonding/antibonding MOs (Figure S1). The AS combination
of (dxz, dyz) orbitals is nonbonding as well as the dxy orbitals
(Figure S1). The resulting MO diagrams are shown in Figure 3
focusing only on the nonbonding or antibonding d centered
orbitals, whereas the bonding contributions lie lower in
energies and are not shown.
Table 2. Spin Configurations (BS for S = 0 and High Spin)
Solutions for (FePz)2X with X = O or C Depending on
Individual Fe Sz, and Their Energetic Ordering (ΔE in kcal/
a
mol)
X
total S
BS solutions
ΔE
In all three complexes, the (dxz, dyz)S set forms two fully
occupied π bonds involving 2px and 2py AOs of the bridging X
atom, while their antibonding counterparts are unoccupied.
These two π-bonds are delocalized on the Fe−X−Fe atoms,
one π bond per Fe−X. A Fe−X−Fe σAS bond is formed from a
O
0
0
0
3
0
0
0
2
+1/2, −1/2
+3/2, −3/2
+5/2, −5/2
noCV
0.0
4.8
25.8
0.0
noCV
noCV
64.8
C
0, 0
+1, −1
+2, −2
2AS
dz and 2pz (X) combination, occupied by two electrons,
whereas its antibonding component is unfilled resulting in 0.5
σ
AS bond per Fe−X. The MO mostly concerned by the change
in number of d electrons when going from μ-O to μ-C is the
a
2S
NoCV means that this BS solution could not be obtained.
σS* orbital from dz combined to the 2s (X) orbital. This
D
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Inorganic Chemistry
Article
a
Table 3. Calculated Data with B3LYP-D3 on (FePz)2X (X = O, N, C)
X = O
X = N
X = C
structure
Fe−N(Pz)
Fe−X
Fe−X−Fe
h(Fe)
ρ
exp.
1.955
1.777
158
0.35
calcd
1.95
1.77
155.9
0.4
exp.
1.93
1.655
168.6
0.28
calcd
1.945
1.64
160
0.28
exp.
1.915
1.67
175.1
0.22
calcd
1.94
1.64
179.8
0.21
Fe1
Fe2
X
2.48
−2.48
0.0
0.43
0.44
0.14
0
0
0
̈
Mossbauer
ΔEQ
δ
1.32
0.23
1.02
0.29
1.93
0.03
1.90
0.22
2.82
−0.07
2.31
0.13
main structural parameters (distances and height from the mean Pz plane h(Fe) in Å, angle Fe−X−Fe in deg) compared to X-ray data,27 Mulliken
spin densities (ρ), and Mossbauer parameters (mm s ). X = O and C: singlet state; X = N: doublet state.
a
−1
̈
Figure 3. Schematic Kohn−Sham orbital diagrams of (FePz)2X species for (a) X = O (d10); (b) X = N (d9); (c) X = C (d8). The valence
configuration for μ-O, μ-N or μ-C is s2p6 in all cases (corresponding to formally “O2−”, “N3−” or “C4−” valencies). For the sake of simplicity, Fe−X
bonding orbitals and orbitals localized on the macrocycles are not shown except for HOMO and LUMO (shown in red), which are made up of π
and π* orbitals from the Pz rings. Bottom: σ and π Fe−X bond composition as well as Mayer bond orders (b. o.).
Scheme 1. Cyclopropanation of Styrene by EDA Catalyzed by (FePzPr8)2X (X = O, N, C)
antibonding orbital contains two electrons for X = O (Figure
3a) leading to a net bond order of 0 for the Fe−O−Fe σS
bond. In turn, for X = C the σS* orbital is unoccupied yielding
a Fe−C−Fe σS bond order of 1. In case of X = N, the σS*
orbital is the SOMO containing the unpaired electron equally
one electron fills the antibonding σS* and is absent for μ-C
species which is almost linear. The Mayer bond orders (Figure
3) illustrate the increase in Fe−X bond orders from (FePz)2O
to (FePz)2C. These electronic configurations are also in
agreement with the evolution of quadrupole splitting values
from μ-O to μ-C complex, pointing to an increase of the
covalency and with the electronic equivalence of both Fe sites
2
delocalized over the two Fe dz orbitals and the observed spin
densities of ca. 0.5 on each Fe. Careful examination of this σS*
orbital rationalizes the decrease in Fe−X−Fe bending angle
from X = O to C (see Table 2). It is doubly occupied in
(FePz)2O, and the bending tends to release somewhat the
energetic destabilization of this orbital, by favoring weak
bonding overlaps between an O hybrid orbital (2s + 2p) and
̈
revealed in the Mossbauer study. This in-depth study confirms
and completes the previous calculations that were published on
single-atom bridged species.25,27
Evaluation of Catalytic Properties in Cyclopropana-
tion of Styrene. The aforementioned difference in the
electronic properties of the three dimeric complexes might
result in the different catalytic behavior. Therefore the catalytic
2
the in-plane ring of both dz orbitals. This behavior is still
present with a somewhat less extent for μ-N complex, as only
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activity of the three dimeric complexes was investigated in the
cyclopropanation of styrene by ethyl diazoacetate (EDA) using
0.1 mol % catalyst loading (Scheme 1).
with a 97% yield after 1 h reaction. The (FePzPr8)2N complex
showed a lower catalytic activity. All three dimeric complexes
exhibited similar moderate trans/cis ratios of cyclopropanation
products. In the case of sterically unhindered porphyrin
complexes, the high trans/cis ratios (>8) were associated with
the involvement of the less-electrophilic Fe(II) carbene
intermediates, while low trans/cis selectivity could be
considered as an indication for more electrophilic Fe(III)
species.82 Thus, relatively low trans/cis selectivity of the
cyclopropanation mediated by (FePzPr8)2X complexes is
compatible with their high oxidation states.
Several diazo carbene precursors were evaluated in
combination with (FePzPr8)2O. Benzyl diazoacetate provided
a 95% yield of cyclopropanation products with increased
selectivity to trans-cyclopropyl derivative with trans/cis ratio of
84:16 (Table 4, entry 10). However, t-butyl diazoacetate was
less suitable for cyclopropanation of styrene owing to the
preferential formation maleate and fumarate derivatives (Table
4, entry 11). Trimethylsilyldiazomethane furnished 1-phenyl-2-
trimethylsilylcyclopropanes with a 47% yield in 83:17 trans/cis
ratio (Table 4, entry 12).
A 2 M EDA solution was added slowly to the solution of
styrene and catalyst in toluene at 70 °C to favor cyclo-
propanation of styrene derivatives over EDA dimerization.
Importantly, the high yield of cyclopropanation products can
be achieved when styrene was the limiting reagent and 1.2
equiv EDA was used. It should be noted that in many studies, a
large excess of olefin substrate has been used to limit the
formation of undesirable EDA dimerization products which
resulted in low cyclopropanation yields based on the initial
substrate amounts.30,33,35,37,48,50 Importantly, the comparison
of the UV−vis spectra of (FePzPr8)2O, (FePzPr8)2C, and
(FePzPr8)2N recorded before and after catalytic reactions
confirmed that all complexes retained their dimeric structures
under catalysis conditions (Figures S2−S4). Thus, (FePz-
Pr8)2O, (FePzPr8)2C, and (FePzPr8)2N rather than their
mononuclear FePzPr8 counterpart are involved in the cyclo-
propanation reaction. When the reactions were carried out
without styrene substrate, diethyl maleate and fumarate were
formed, and the intensities of UV−vis spectra of all dimeric
complexes were decreased indicating their partial degradation
in the absence of substrate (Figures S5−S7). The time courses
of the cyclopropanation of styrene in the presence of the three
dimeric complexes clearly show superior activity of (FePz-
Pr8)2O (Figure 4).
The scope of the reaction was extended using (FePzPr8)2O
catalyst under optimal conditions: 0.1 mol % catalyst loading,
1.2 M EDA, and 1 M amine in toluene at 70 °C for 2 h
reaction time (Table 5).
Anilines bearing electron-donating or electron-withdrawing
groups afforded cyclopropanation products with excellent to
quantitative yields with similar trans/cis ratio accompanied by
minor amounts of EDA dimerization products. Importantly,
the presence of electron-withdrawing substituents was well
tolerated. Even pentafluorostyrene was cyclopropanated with a
89% yield, and the dimerization of EDA was limited to 11%
(Table 5, entry 6). Bulky 1,1-diphenylethylene was converted
to corresponding cyclopropyl derivative with a quantitative
yield (Table 5, entry 8). However, cyclohexene was unreactive,
and a significant amount of diethyl maleate was formed (Table
5, entry 9). In contrast, n-butylvinylether furnished a good
cyclopropanation yield. The presence of methyl substituent at
double bond in α-methylstyrene did not prevent the reaction,
and corresponding cyclopropane was obtained with quantita-
tive yield (Table 5, entry 7). In turn, 2,3-dimethyl-1,3-
butadiene afforded mono- and double cyclopropanation
products with 66 and 33% yields, respectively (Table 5,
entry 11).
Hammett Correlations. The influence of the electronic
structure of styrene derivatives on the reactivity was
investigated for the three (FePzPr8)2X complexes. The relative
cyclopropanation rates kX/kH were determined in competitive
reactions using 0.5 M concentrations of the para-substituted
styrene and styrene and 0.25 M EDA. The relative rates were
calculated as the molar ratio of cyclopropanation products
obtained from the para-substituted styrene to that derived
from styrene (Figure 5).
Figure 4. Time courses of the styrene cyclopropanation with EDA in
the presence of (FePzPr8)2O, (FePzPr8)2N and (FePzPr8)2C at 70
°C.
The reaction between styrene and EDA at 70 °C without
catalyst produced ∼1% and 8% yields of cyclopropanation
products in a 64:36 trans/cis ratio after 1 and 6 h, respectively
(Table 4).
83,84
+
Different σ polar effect parameters (σp , σp, σmb)
were
In the presence of 0.1 mol % (FePzPr8)2O, a 95% yield of
the cyclopropanation products was achieved at 70 °C after 1 h.
The reaction was also efficient at 60 °C, although the product
yield decreased to 73% accompanied by the increase of the
amount of EDA dimerization products. The (FePzPr8)2O
complex did not react with EDA at 25 °C. The corresponding
μ-carbido and μ-nitrido diiron complexes furnished lower
yields of cyclopropanation products at 70 °C after 8 h: 46 and
33%, respectively. Interestingly, (FePzPr8)2C was as efficient as
(FePzPr8)2O at 100 °C affording cyclopropanation products
correlated with log(kX/kH). The better correlations for all the
+
complexes were obtained with σp parameter which is
consistent with the development of the partial positive charge
in the transition state stabilized by resonance effects. However,
(FePzPr8)2O and (FePzPr8)2C showed a moderate linearity
(R2 = 0.86), and no clear trend in the cyclopropanation of
electron-rich and electron-deficient olefins was observed for
(FePzPr8)2N (Figure S8). Next, we performed a double
parameter correlation using an approach introduced by Jiang
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Table 4. Catalytic Cyclopropanation of Styrene by EDA Catalyzed by (FePzPr8)2O, (FePzPr8)2C and (FePzPr8)2N Complexes
cyclopropanation
a
b
entry
catalyst
temperature (°C)
reaction time
yield (%)
trans/cis
EDA dimerization yield (%)
1
2
3
4
5
6
7
8
9
no catalyst
no catalyst
70
100
60
70
100
70
100
70
100
1 h (8 h)
1 h (6 h)
75 min
1 h
1 h
8 h
1 h
8 h
<1 (8)
3 (24)
73
64:36
64:36
77:23
77:23
70:30
78:22
70:30
75:25
64:36
0
1 (2)
37
8
18
36
21
38
1
(FePzPr8)2O
(FePzPr8)2O
(FePzPr8)2O
(FePzPr8)2C
(FePzPr8)2C
(FePzPr8)2N
(FePzPr8)2N
95
95
46
97
33
1 h (6 h)
30 (46)
Benzyl Diazoacetate
10
11
12
(FePzPr8)2O
(FePzPr8)2O
(FePzPr8)2O
70
70
70
2 h
95
17
84:16
88:12
83:17
8
40
23
t-Butyl Diazoacetate
2 h
Trimethylsilyldiazomethane
2 h
47
a
b
Yields of cyclopropanation products are based on the initial amount of styrene. Yields of EDA dimerization products are based on the initial
amount of EDA.
•
σJJ spin delocalization parameter. Although a better
Table 5. Catalytic Cyclopropanation of Aromatic and
Aliphatic Olefins by EDA Catalyzed by (FePzPr8)2O
•
+
correlation was obtained with σJJ /σp pair, no notable
improvement in linearity was obtained compared with classical
Hammett correlations (R2 = 0.86, 0.57, and 0.87 for μ-O, μ-N,
and μ-C complexes, respectively) (Figure S9).
cyclopropanation
a
yield
(%)
trans/cis EDA dimerization, yield
b
entry
substrate
ratio
of cis/trans (%)
The observed nonlinearity might be indicative of inter-
mediates that have significant radical character.85 Notable
improvement of correlation was obtained when Creary’s
radical σc• parameters were used. These parameters were
determined in the thermal rearrangement of 1,1-dimethyl-2-
methylenecyclopropanes in C6D6 at 80 °C (set A)86 and in iso-
octane at 100 °C (Set B).87 Using set B parameters,
significantly better linearity was achieved for (FePzPr8)2C
(R2 = 0.99) and (FePzPr8)2O (R2 = 0.97) whereas the
correlation obtained for (FePz)2N (R2 = 0.86) was still
moderate (Figure 6).
Noteworthy, in the case of (FePzPr8)2N, the kX/kH values
are not very sensitive to the electronic nature of para-
substituent of styrene excepting p-methoxystyrene in all
attempted correlations (Figures 6, S8, and S9). This finding
suggests that in this case, the rate-limiting step might be the
formation of (FePzPr8)2N-carbene species rather than its
reaction with styrene derivative. μ-Nitrido complex with
Fe(III)Fe(IV) site is probably not sufficiently nucleophilic to
assist N2 release from the complexed EDA to generate carbene
species. The similar reactivity of (FePzPr8)2N at 70 and 100
°C (Table 4, entries 7 and 8) is in accordance with this
suggestion. The increased activity of p-methoxystyrene can be
1
2
3
4
5
6
7
8
4-methoxystyrene
4-methylstyrene
3-methylstyrene
4-fluorostyrene
4-cyanostyrene
pentafluorostyrene
α−methylstyrene
100
100
99
82
100
89
76:24
77:23
76:24
73:27
68:32
80:20
63:37
−
6/0
6/0
8/<1
17/1
9/<1
11/1
7/<1
5/<1
100
100
1,1-
diphenylethylene
9
10
11
cyclohexene
n-butylvinylether
0
58
66
−
55:45
71:29
76/3
19/1
12/<1
2,3-dimethyl-1,3-
c
butadiene
a
Yields of cyclopropanation products are based on the initial amount
of olefin. Yields of EDA dimerization products are based on the
initial amount of EDA. Yield of the products of double cyclo-
propanation was 33%.
b
c
to take into account spin-delocalization effect occurring when
radical species are involved.84 Double parameter linear
regressions were carried out to evaluate the relative impact
of both polar effect and spin delocalization using different σ
+
polar effect parameters (σp , σp, σmb) in combination with the
Figure 5. Determination of the relative rates of the cyclopropanation of para-substituted styrene with respect to styrene.
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+
Figure 6. Correlation plots of log(kX/kH) vs (ρJJ• σc• + ρp+ σp ) for the cyclopropanation of p-Y-substituted styrenes by ethyl diazoacetate catalyzed
by (FePzPr8)2X.
Figure 7. Top panel: Structures of carbene (FePz)2X-based complexes (left: X = O, middle: X = N, right: X = C) optimized with B3LYP-D3 and
NPA spin densities at iron, carbene, and bridging atoms (not shown for closed-shell μ-C complex). Bottom panel: NBO analysis showing only the
proximal iron d-antibonding orbitals and py AO of the carbene involved in its radical character. Key orbitals are displayed for μ-O and μ-C
complexes and not shown for μ-N complex for the sake of simplicity because they are very similar to those of μ-O complex.
explained by its nucleophilic properties strong enough to attack
directly the (FePzPr8)2N-EDA intermediate before N2 release
according to the mechanism proposed by Gross for nitrogen-
and sulfur-containing nucleophilic compounds.88 The negative
values of ρp+ (−0.43, −0.45, and −0.34 for μ-O, μ-C, and μ-N
complexes, respectively) are consistent with a nucleophilic
attack of olefin on the iron carbene intermediate. Excellent
Creary’s correlations obtained with (FePzPr8)2O and
(FePzPr8)2C suggest radical stepwise mechanism of the
Electronic Structure of Putative Carbene Complexes:
Relationship with Reactivity. Several experimental and
theoretical studies published on the olefin cyclopropanation
mediated by EDA and mononuclear iron porphyrin-like
catalysts point to an active intermediate having carbene axially
bonded to the Fe site.33,36,82,89−91 The observed catalytic
activity of the three diiron complexes suggests that related
carbene intermediates can be formed at binuclear platforms.
We performed theoretical modeling on the putative (FePz)2X-
carbene species using the same DFT approach successfully
used for the initial (FePz)2X species. The Fe site bearing the
carbene group is noted Fep (proximal Fe) and another Fe site
is noted Fed (distal Fe). Starting from the low-spin
configuration of the (FePz)2X complex, the carbene brings
two valence electrons which can be involved into bonds with
the proximal Fe site. Thus, the resulting spin states of the
carbenic species are expected to be open-shell singlet or triplet
for (FePz)2O and (FePz)2C and doublet or quartet for
+
cyclopropanation reaction. The negative ρp values for all
three complexes are comparable to those determined for this
reaction mediated by iron porphyrin complexes,40 indicating
their electrophilic nature and the development of the positive
charge at the olefin carbon atoms in the transition state. The
+
ρp /ρC′ ratios of 0.95 and 1.09 obtained for μ-O and μ-C
species, respectively, suggest that both polar and spin effects
are important.85
H
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(FePz)2N. Indeed, thorough exploration of the various spin
states using broken-symmetry approaches showed that the
ground state at the B3LYP level is a singlet for (FePz)2O and
(FePz)2C and doublet for (FePz)2N carbene species. The main
structural parameters and spin densities are shown in Table S4,
and their structure is shown in Figure 7.
in the LUMO eigenvalue from (FePz)2O to (FePz)2N and
(FePz)2C species could be observed (Figure 8).
As expected, both Fe−X bonds of binuclear carbene
complexes are not equivalent due to the axial bonding of the
carbene group to Fep site. Mulliken as well as NPA spin
densities clearly point to a radical character of the carbene in μ-
O and μ-N species (see Table S4).
The bonding features were examined through a NBO
analysis, and the most significant ones are described here,
whereas detailed analyses of the localized orbitals are given in
Supporting Information (Tables S5−S7). In the carbene
(FePz)2O complex, the Fed site is an FeIII ion in the
intermediate spin local configuration. The Fep site in S = 1
FeIV local configuration has one σ bond with the carbene group
and 2 π bonds with the bridging oxygen only for β orbitals.
This σ bond between Fep and carbene is made up of the
Figure 8. LUMO of the three (FePz)2X-carbene species calculated
with B3LYP/TZ2P, eigenvalues in eV.
The LUMOs for the (FePz)2N and (FePz)2C carbene
complexes are localized on π* orbitals of the distal Pz ligand
that can explain their lower reactivity in the reaction with
styrene. In the case of (FePz)2O, LUMO is the unoccupied β
2py of the radical carbene species, which should be more
favorable for the initial radical addition to double bond of
styrene. The electron affinity of these three carbene species
reveals a monotonic decrease: 101.1 kcal/mol for (FePz)2O,
91.1 kcal/mol for (FePz)2N, and 82.9 kcal/mol for (FePz)2C.
This is consistent with the trend in eigenvalues of the LUMO.
This appears to be counterintuitive because the formal redox
states of Fe ions in these complexes increase from μ-oxo to μ-
carbido species. However, it should be taken into account that
the whole molecules stay neutral and the bridging ligand
becomes more and more donating from μ-oxo to the μ-nitrido
and to the μ-carbido bridge. Recently, Wang et al. reported
that electron-withdrawing substituents of quinoid carbene
complex of Ru porphyrin lowered its electrophilicity in carbene
transfer reaction.96
2
combination of the dz and a hybridized sp2 orbital of the
carbene group with one electron provided by Fep and one by
the carbene. The second carbenic valence electron remains as
unpaired electron localized in a py AO. The bonding of Fep in
the μ-N complex is very similar to that in μ-O species with a
more covalent Fep-μN bond compared to Fep-μO as attested
by the Wiberg bond orders (Table S8) and relative
contributions of Fep and O/N orbitals (Tables S5 and S6).
Both Fe sites in (FePz)2N are FeIV in local intermediate spin
configurations.
The NPA population in the μ-carbido counterpart shows
low-spin proximal FeII and distal FeIV sites. This Fep
configuration is due to charge transfers from both μ-carbido
and carbene carbon atoms through strong covalent bonds
(Table S7). Indeed, the NPA charges (Table S9) are negative
for μ-X and carbene group in μ-O and μ-N species, whereas
they are both positive in the μ-C analogue, indicating esssential
electron transfer to Fep. The carbene group in (FePz)2C is
bonded to Fep by one π dative bond formed by combination of
Fep dyz and carbene 2py orbitals and occupied by two electrons
provided by carbene group. The Wiberg bond orders (Table
S8) reflect all these bonding features and are consistent with
the trends in Fe−X and Fe−C (carbene) optimized distances.
Previous theoretical studies have been done on carbenic
species issued from mononuclear FeII porphyrins,89,92−94 while
only a few studies on FeIII porphyrin carbene species have been
described.36,95 Interestingly, we observe a similar behavior as
that of FeII porphyrin carbene species described by Sharon et
al.93 In (FePz)2O and (FePz)2N species, the unpaired electron
of carbene is localized on a py orbital which exhibits some
delocalization onto the O atoms of the ester group yielding a
planar structure of both Fe−C(H)COO moieties. For the
closed-shell (FePz)2C species, the two electrons from the
carbene are localized in a π bond formed with dyz, and there is
no more delocalization at the ester group. The resulting
carbene structure is thus bent to the Pz ring, which is quite
similar to what Sharon et al. observed for the mononuclear
closed-shell singlet structure.93
SUMMARY AND CONCLUSIONS
■
In this study, we have demonstrated for the first time that
single-atom bridged diiron tetrapyrrolic complexes are capable
of catalyzing the carbene transfer to styrenes affording
cyclopropane derivatives. The order of the catalytic activity is
FeIII−O−FeIII ≫ FeIVCFeIV > FeIII−NFeIV. The μ-oxo
diiron complex showed excellent catalytic efficiency providing
a 95% yield of cyclopropanation products based on substrate in
77:23 trans/cis ratio with 0.1 mol % catalyst loading (TON =
950). The reaction scope was successfully extended to
aromatic olefins bearing either electron-donating or electron-
withdrawing substituents and to aliphatic olefins. Other
carbene precursors such as benzyl diazoacetate, trimethylsi-
lyldiazomethane, and t-butyl diazoacetate can also be used,
though the latter with less efficiency. The carbene transfer
reactivity is usually associated with the involvement of metal
carbene species.30,40 Several mononuclear iron carbene
complexes have been described though their electronic
structure and is still a matter of debate in the literature.89−94
To get insight into the possible involvement of putative
carbene species on binuclear diiron platform, we have first
performed a detailed DFT study of the three diiron
porpyrazine complexes differing in the nature of bridging
atom (μ-O, μ-N and μ-C) and in the iron oxidation state,
FeIIIμOFeIII, FeIIIμNFeIV, and FeIVμCFeIV, respectively (Figure
1). DFT calculations provided useful information on the
electronic properties of these complexes. The DFT-optimized
structures of (FePzPr8)2X (X = O, N, C) are consistent with
The experimental reactivity data clearly point to a
nucleophilic attack of the olefin double bond to carbene
complex. We examined the frontier orbitals of these three
complexes with a special focus on the LUMO. A slight increase
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their crystal structures with discrepancies in bond distances
A0020807648 and A0040807648). P.M. and J.-M.L. thank
Labex ARCANE and CBH-EUR-GS (ANR-17-EURE-0003)
for partial support of this work.
̈
<0.03 Å (Table 3). The simulated Mossbauer spectra compare
̈
well with the experimental data. Detailed analysis of Mossbauer
data of diiron complexes involving different bridging atom and
macrocyclic ligands clearly indicates that the electronic
structure of the porphyrazine complexes is closer to that of
the phthalocyanine rather than porphyrin analogs.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b02718.
■
S
Experimental and additional computational details,
correlation data on reactivity, Cartesian coordinates of
computed structures (PDF)
(12) Sorokin, A. B. μ-Nitrido diiron phthalocyanine and porphyrin
complexes: unusual structures with interesting catalytic properties.
Adv. Inorg. Chem. 2017, 70, 107−165.
̇ ̈
(13) Isç i, U.; Faponle, A. S.; Afanasiev, P.; Albrieux, F.; Briois, V.;
AUTHOR INFORMATION
Corresponding Authors
*E-mail: pascale.maldivi@cea.fr.
*E-mail: jean-marc.latour@cea.fr.
*E-mail: alexander.sorokin@ircelyon.univ-lyon1.fr.
■
Ahsen, V.; Dumoulin, F.; Sorokin, A. B.; de Visser, S. P. Site-selective
formation of an iron(IV)-oxo species at the more electron-rich iron
atom of heteroleptic μ-nitrido diiron phthalocyanines. Chem. Sci.
2015, 6, 5063−5075.
(14) Kudrik, E. V.; Afanasiev, P.; Alvarez, L. X.; Dubourdeaux, P.;
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ORCID
Alexander B. Sorokin: 0000-0002-2699-6665
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Research support was provided by the Agence Nationale de
Recherche (ANR, France, grant ANR-16-CE29-0018-01).
■
̀
L.P.C. is indepted to Ministere de l’Education Nationale, de
́
l’Enseignement Superieur et de la Recherche, France for his
Ph.D. fellowship. This work was performed using HPC
resources from GENCI-CINES and IDRIS (grants
J
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