Phenyl derivative of iron 5,10,15-tritolylcorrole

Article abstract of DOI:10.1021/ic5003572

The phenyl-iron complex of 5,10,15-tritolylcorrole was prepared by reaction of the starting chloro-iron complex with phenylmagnesium bromide in dichloromethane. The organometallic complex was fully characterized by a combination of spectroscopic methods, X-ray crystallography, and density functional theory (DFT) calculations. All of these techniques support the description of the electronic structure of this phenyl-iron derivative as a low-spin iron(IV) coordinated to a closed-shell corrolate trianion and to a phenyl monoanion. Complete assignments of the ¹H and ¹³C NMR spectra of the phenyl-iron derivative and the starting chloro-iron complex were performed on the basis of the NMR spectra of the regioselectively β-substituted bromo derivatives and the DFT calculations.

Full text of DOI:10.1021/ic5003572

Article
pubs.acs.org/IC
Open Access on 04/03/2015
Phenyl Derivative of Iron 5,10,15-Tritolylcorrole
Sara Nardis,† Daniel O. Cicero,† Silvia Licoccia,† Giuseppe Pomarico,† Beatrice Berionni Berna,†
Marco Sette,† Giampaolo Ricciardi,‡ Angela Rosa,*,‡ Frank R. Fronczek,§ Kevin M. Smith,*,§
and Roberto Paolesse*,†
†Department of Chemical Science and Technologies, Universita
̀
di Roma Tor Vergata, 00133 Roma, Italy
‡Dipartimento di Scienze, Universita
̀
della Basilicata, 85100 Potenza, Italy
§Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, United States
S
* Supporting Information
ABSTRACT: The phenyl−iron complex of 5,10,15-tritolylcorrole was prepared
by reaction of the starting chloro−iron complex with phenylmagnesium bromide in
dichloromethane. The organometallic complex was fully characterized by a
combination of spectroscopic methods, X-ray crystallography, and density
functional theory (DFT) calculations. All of these techniques support the
description of the electronic structure of this phenyl−iron derivative as a low-spin
iron(IV) coordinated to a closed-shell corrolate trianion and to a phenyl
1
monoanion. Complete assignments of the H and 13C NMR spectra of the
phenyl−iron derivative and the starting chloro−iron complex were performed on
the basis of the NMR spectra of the regioselectively β-substituted bromo
derivatives and the DFT calculations.
electronic structures of corrole complexes, thereby making the
characterization of its coordination chemistry both challenging
and intriguing.
INTRODUCTION
■
Corrole was one of the first of the porphyrinoids to be reported
in the literature,1 having been prepared during the research
wave that focused on the development of a synthetic route to
vitamin B12; it shares with corrin the molecular framework and
with porphyrin the 18-electron π-aromatic system (Figure 1).2
One of the most debated and interesting examples of the
noninnocent ligand behavior is represented by iron-corrole
derivatives.5 We reported the first iron−corrole complex,
obtained as an Fe(III) derivative,6 and Vogel and co-workers
later noticed that the air oxidation of such a species afforded the
formally Fe(IV) complex, obtained as the μ-oxo species or as its
monomeric chloro derivative, depending on the reaction
workup.7 Reaction of this latter complex with pyridine afforded
the corresponding Fe(III) bis(pyridino) derivative, where the
axial coordination also induced a formal reduction of the
metallic center.7
More recently, the development of synthetic routes for the
preparation of 5,10,15-triarylcorroles has significantly widened
the horizon of corrole chemistry and also the number of metal
complexes,3c presenting more opportunities to tune the ligand
characteristics by synthetic modifications.3b This beneficial
expansion has also been useful for iron corrolates, and both
spectroscopic and theoretical characterization of such com-
plexes has further confirmed that chloro−iron corrole
complexes are better described as Fe(III) [corrole]2•−, where
corrole is oxidized to a macrocyclic π-cation radical, according
to its noninnocent character.5 We have recently exploited the π-
cation radical nature of the complex for the preparation of β-
Figure 1. Molecular structure of porphyrin (a), corrole (b) and corrin
(c).
The corrole macrocycle has recently received an impressive
boost in published research,3 since it shows unusual properties
that are different from those of the corresponding porphyrins
but at the same time suggests promising practical applications.4
One of the corrole peculiarities is related to its coordinative
behavior; being a trianionic ligand, corrole usually stabilizes
coordinated metals in oxidation states formally higher than
those of the corrresponding porphyrin complexes. However,
since corrole is also characterized by low oxidation potentials, a
facile ligand-to-metal electron transfer can occur, introducing
the so-called noninnocent character of the macrocycle as a
ligand. This feature makes it quite difficult to establish the
Received: February 13, 2014
Published: April 3, 2014
© 2014 American Chemical Society
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the positive-ion mode using m-nitrobenzyl alcohol (Aldrich) as a
matrix.
nitrocorrole iron complexes, obtained upon nucleophilic attack
of the nitrite ion on the chloro−iron corrole complex.8,9
On the other hand the μ-oxo dimer is better described as the
Fe(IV) derivative, while the organometallic phenyl−iron β-
alkylcorrole complexes5c show more complex behavior: the
NMR characterization, together with a density functional
theory (DFT) study, show that the corrole assumes a total
−3 charge, but the ligand noninnocence is still present, since
appreciable radical character of the macrocycle has been
reported.5c The stabilization of the high oxidation state of the
metal in the σ-phenyl iron−corrole complex has been explained
based on the strong σ-donor nature of the phenyl axial ligand,
which impedes ligand-to-metal electron transfer while not
completely negating the radical character of the corrole.
Although the σ-phenyl iron−corrole complex shows this
peculiar behavior, it has been limited to the β-alkylcorrole
derivatives7 and has not yet been reported for the triarylcorrole
series.
Magnetic Susceptibility Measurements. The magnetic suscept-
ibility of compounds 1 and 2 was measured in CHCl3 solution using
NMR spectroscopy, following the method of Evans,10 using the
equation of Sur for the geometry of a superconducting magnet,11 and
correcting for the diamagnetic contribution of the corrole ligand.12
The results yielded a triplet ground state, consistent with magnetic
data previously reported for several chloro− and phenyl−iron
corrolates.5a,c,e,7,13
Quantum Chemical Calculations. DFT geometry optimizations
and ground-state electronic structure analyses were performed with
the ADF (Amsterdam Density Functional) program system, release
2013.14 Two exchange-correlation functionals were employed: the
recently implemented generalized gradient approximation (GGA)
functional S12g,15 which includes Grimme’s D3 dispersion energy16,17
and has been shown to yield correct spin-state relative stabilities for
iron(II) complexes,15 and the hybrid B3LYP-D3 functional.16,18,19 The
calculations were performed using the spin-unrestricted approach and
included relativistic effects through the scalar zero-order regular
approximation (ZORA) formalism.20−22 The all-electron ZORA/
TZ2P basis set,23 which is an uncontracted triple-ζ STO basis with two
sets of polarization functions, was used for all atoms.
For these reasons we have been interested in studying such a
triarylcorrole species, and we now report the preparation and
characterization of the phenyl−iron complex 1 of 5,10,15-
tritolylcorrole. Specifically, the X-ray structure, solution
The ground-state geometries of compound 1 and 2 were optimized
in vacuo without any symmetry constraint, assuming an S = 1 ground
state, as indicated by magnetic susceptibility measurements. The
optimized structures were verified to represent local minima by
calculation of harmonic frequencies. Solvent effects on the ground-
state properties were modeled through the conductor-like continuum
solvent model (COSMO)24−26 using the same solvent employed in
the NMR and magnetic susceptibility measurements. For the sake of
comparison with previous results,5d,e a natural orbital (NO) analysis
was also performed for compound 2. The NOs were computed with
TURBOMOLE27 through single-point calculations at the S12g and
B3LYP ADF-optimized geometries. As in TURBOMOLE the S12g
functional is not available, the pure GGA BP86 functional28 was used
instead. In the BP86 and B3LYP single-point calculations the def2-
TZVP basis set,29 which is an extensively polarized basis set of triple-ζ
quality including high angular momentum polarization functions, was
used. Solvent (CHCl3) effects were modeled through the COSMO
model.24,25 The NOs were visualized with gOpenMol.30
1
magnetic measurements, and H and 13C NMR spectra were
acquired, and DFT calculations were performed. To obtain the
complete assignment of the NMR spectra of 1, the 1H and 13
C
NMR spectra of the ad hoc prepared β-substituted chloro−iron
tritolylcorrole complexes 3 and 4 and of the phenyl−iron
derivative 5 were studied. For the sake of completeness the 1H
and 13C NMR spectra of the chloro−iron derivative 2, which is
the precursor of 1, were also investigated and assigned with the
help of the spin-density pattern obtained by DFT calculations
and the NMR spectra of the β-substituted derivatives 3 and 4.
To have a diamagnetic counterpart of the chloro− and phenyl−
iron complexes, the germanium derivatives 6 and 7 were
prepared and investigated by NMR spectroscopy.
Syntheses. 5,10,15-Tritolylcorrole,31 2-bromo-5,10,15-tritolylcor-
role, and 3-bromo-5,10,15-tritolylcorrole,32 as well as the chloro−iron
233 and chlorogermanium 734 derivatives were prepared following
literature methods.
Chloro-iron 2-bromo-5,10,15-tritolylcorrolate 3. The title complex
was prepared as reported for compound 2, starting with 2-bromo-
5,10,15-tritolylcorrole. Yield: 75%. mp > 300 °C. UV−vis (dichloro-
methane): λmax, nm (log ε, M−1 cm−1): 360 (4.66), 420 (4.62), 507
(4.15), 648 (3.05). Anal. Calcd for C40H28BrClFe N4: C, 65.29; H
3.84; N, 7.61. Found: C, 65.31; H, 3.79; N 7.59%.
Chloro-iron 3-bromo-5,10,15-tritolylcorrolate 4. The title complex
was prepared as reported for compound 2, starting with 3-bromo-
5,10,15-tritolylcorrole. Yield: 72%. mp > 300 °C. UV−vis (dichloro-
methane): λmax, nm (log ε, M−1 cm−1): 363 (4.68), 418 (4.60), 508
(4.04), 650 (3.02). Anal. Calcd for C40H28BrClFe N4: C, 65.29; H
3.84; N, 7.61. Found: C, 65.28; H 3.81; N, 7.63%.
Phenyl-Iron 5,10,15-Tritolylcorrolate 1. The complex 2 (65 mg;
0.1 mmol) was dissolved in anhydrous dichloromethane (10 mL) and
treated with 79 μL of a 3 M solution of phenylmagnesium bromide in
diethyl ether. After 10 min water was added, the organic phase was
separated and dried over anhydrous sodium sulfate, and the solvent
was removed under vacuum. The reaction mixture was purified using a
chromatographic column on silica gel, using dichloromethane/hexane
(3:1) as eluent. The first red band was recrystallized from
dichloromethane/methanol (2:1) to give red crystals of the title
product. Yield: 70% (49 mg). mp > 300 °C. UV−vis (dichloro-
methane): λmax, nm (log ε, M−1 cm−1): 382 (4.68), 504 (4.03), 554
(3.93), 651 (3.50). Anal. Calcd for C46H34FeN4: C, 79.08; H, 4.91; N
8.02. Found: C, 79.05; H 4.94; N, 7.98%.
EXPERIMENTAL SECTION
■
General. Reagents and solvents (Sigma-Aldrich, Fluka, and Carlo
Erba Reagenti) for synthesis and purification were of synthetic grade
and were used as received. NMR experiments were performed at T =
300 K on a Bruker Avance 600 MHz with a 5 mm inverse broadband
probe equipped with z-axis gradients. All data were processed with
TopSpin. Two-dimensional (2D) total correlation spectroscopy
(TOCSY), nuclear Overhauser enhancement spectroscopy
1
(NOESY), and H−13C heteronuclear multiple-quantum correlation
(HMQC) were recorded using standard pulse sequences. The Curie
plot was obtained by acquiring the spectra over a temperature range
from −53 to +50 °C. Ultraviolet-visible (UV−vis) spectra were
measured in CH2Cl2 with a Varian Cary 50 spectrophotometer. Mass
spectra (FAB mode) were recorded on a VGQuattro spectrometer in
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Scheme 1
Figure 2. Molecular structure of 1.
X-ray. Crystallographic data were collected using Cu Kα radiation
(λ = 1.541 78 Å) at T = 90 K on a Bruker Kappa APEX-II DUO
diffractometer equipped with a IμS microfocus source. Crystal data:
C46H34FeN4. (0.45CH2Cl2), Mr = 736.84, monoclinic space group
P21/c, a = 13.5056(4), b = 44.6178(14), c = 13.2577(4) Å, β =
116.803(2)°, V = 7130.6(4) Å3, Z = 8, Dx = 1.373 Mg m−3, θmax
=
60.1°, R = 0.057 for 10 535 data (8121 with I > 2σ(I)) and 925 refined
parameters. Disordered solvent contribution was removed from the
data using the SQUEEZE procedure,35 amounting to 0.45 dichloro-
methane molecules per corrole complex. CCDC 975295.
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Phenyl-Iron 2-Bromo-5,10,15-tritolylcorrolate 5. The title com-
plex was prepared as reported for compound 1, starting from the
chloro−iron derivative 3. Yield: 71%. mp > 300 °C. UV−vis
(dichloromethane): λmax, nm (log ε, M−1 cm−1): 384 (4.65), 507
(4.08), 547 (3.87), 648 (3.20). Anal. Calcd for C46H33BrFeN4: C,
71.06; H, 4.28; N 7.21. Found: C, 70.98; H 4.25; N, 7.24%.
Phenyl-Germanium 5,10,15-Tritolylcorrolate 6. Chlorogermanium
corrolate (100 mg; 0.15 mmol) was dissolved in anhydrous
dichloromethane (15 mL) and treated with 119 μL of a 3 M solution
of phenylmagnesium bromide in diethyl ether. After 10 min water was
added, the organic phase was separated, dried over anhydrous sodium
sulfate, and then the solvent removed under vacuum. The crude
mixture was filtered through a silica gel plug, using dichloromethane as
eluent, and the collected band was crystallized from dichloromethane/
methanol (2:1) to give crystals of the title product. Yield: 76% (81.5
mg). mp > 300 °C. UV−vis (CH2Cl2): λmax, nm (log ε, M−1 cm−1):
401 sh (4.53), 419 (5.18), 514 (3.68), 531 (3.77), 570 (3.91), 612
(4.44). Anal. Calcd for C46H34GeN4: C, 77.23; H, 4.79; N 7.83.
Found: C, 77.05; H 4.81; N, 7.80%.
atoms on the side of the molecule opposite the coordinated
phenyl group. In both octaethyl compounds, the Fe
coordination is also square pyramidal, and as compared to 1,
the Fe out-of-plane distance is slightly larger: 0.272(1) Å for
SUMXED and slightly smaller, 0.242(2) Å for ZOKCIL. The
Fe−C(phenyl) distance in 1 agrees well with that in SUMXED,
1.984(7) Å, but it is slightly longer than that in the π cation
radical, 1.965(5) Å. The Fe−N distances in 1 also agree well
with those of SUMXED, but those of ZOKCIL are systemati-
cally smaller by an average value of 0.012 Å.
The same synthetic pathway adopted for the preparation of 1
was used to synthesize the Ge derivative 6. This complex was
prepared to provide a diamagnetic species analogous to the iron
derivative and thereby to provide help for the NMR
characterization of 1.
DFT Calculations. Experimental and theoretical studies on
β-substituted phenyl−iron corrolates have led to a consensus
about the electronic structure of these complexes, namely, a
low-spin iron(IV) that is coordinated to a closed-shell corrolate
trianion and to a phenyl monoanion.5a,7,37−41 Our DFT-ZORA
calculations on complex 1 support this view. A spin-coupled
metal−radical system situation can be excluded on the basis of
the computed ⟨S2⟩ value that was found to be very close to the
value expected for a pure triplet state (2.05 and 2.10 at S12g
and B3LYP-D3 level, respectively). Quite different is the
situation for 2. In agreement with previous experimental and
theoretical studies,5 our DFT-ZORA calculations on complex 2
point to the existence of appreciable radical character of the
corrole macrocycle. In this case the value computed for ⟨S2⟩
deviates significantly from that expected for a pure triplet state,
particularly at B3LYP-D3 level. The calculated ⟨S2⟩ values are
2.37 and 2.76 at S12g and B3LYP-D3 level, respectively.
Molecular Structure. Table 1 lists selected structural
parameters obtained for complex 1 from the S12g and B3LYP-
D3 optimized geometries together with the X-ray structural
data determined for the two independent molecules forming
the crystalline asymmetric unit. The S12g optimized structure
of 1 is shown in Figure 3. The two functionals yield rather
similar geometries, save for the iron−phenyl distance, the
displacement of the iron atom out of the (Np)4 and the 23-
atom corrole planes, and the degree of tilting of the axial phenyl
ligand with respect to the N2−Fe−N4 plane. Compared to
S12g, B3LYP-D3 predicts a larger Fe−Ph distance and a less
pronounced extrusion of the metal from both the (Np)4 and 23-
atom corrole planes, in better agreement with the experiment.
However, B3LYP-D3 largely underestimates the degree of
tilting of the axial phenyl ligand with respect to the N2−Fe−N4
plane. Both, the S12g and B3LYP-D3 functionals reproduce the
experimental Fe−N distances quite satisfactorily and account
well for the occurrence of two short (Fe−N1 and Fe−N4) and
two long (Fe−N2 and Fe−N3) Fe−N bonds in the complex.
The calculations also account for the trend observed for the
Cmeso−CTol distances and the degree of tilting of the tolyl
groups with respect to the 23-atom corrole plane. Indeed, both
functionals predict a lengthening of the C10−CTol bond relative
to the C5−CTol and C15−CTol bonds, and a less pronounced
tilting of the tolyl group bound to C10, both features reflecting
somewhat less effective conjugation between this tolyl group
and the corrole π system.42
RESULTS AND DISCUSSION
■
Synthesis and X-ray Characterization. The preparation
of the corrole phenyl−iron derivative 1, as indicated in Scheme
1, is quite straightforward, and follows the protocol already
reported for the analogous β-octalkylcorrole complexes.7 The
visible absorption spectrum of 1 is quite similar to those
reported in the literature for related β-octalkylcorrole
complexes, showing a Soret band at 382 nm and a broad Q
band at 504 nm, with a shoulder at 554 nm. The FAB mass
spectrum of 1 did not show the molecular ion, with a parent
peak corresponding to the Fe−corrole residue, generated by
the loss of the axial phenyl group under the ionization
conditions. Complex 1 is air stable, and slow crystallization
from methanol diffusion into a CHCl3 solution of the iron
derivative allowed formation of single crystals suitable for X-ray
characterization.
The structure of compound 1 is shown in Figure 2. There are
two independent molecules in the asymmetric unit, and both
have very similar dimensions (Supporting Information, Figure
S1). The Fe has a square pyramidal coordination geometry with
Fe−N distances in the range of 1.858(3)−1.890(3) Å and Fe−
C(phenyl) distances of 1.984(4) and 1.987(4) Å. The Fe atoms
lie out of the corrole N4 planes by 0.2524(6) and 0.2640(5) Å.
The Fe−N bonds to the directly linked pyrroles, 1.858(3)−
1.872(3), mean 1.864 Å, are slightly shorter than they are to the
others, 1.884(3)−1.890(3), mean 1.888 Å. The 23-atom
corrole cores of both independent molecules have nonplanar
conformations, with small saddle distortions. For one molecule,
the mean and maximum deviations from coplanarity are 0.077
and 0.199(3) Å, while for the other, these values are larger,
0.127 and 0.325(3) Å. The Cβ atoms (C2, C3, C7, C8, etc.) are
alternately above and below the best planes in pairs by average
deviations of 0.128 and 0.251 Å, respectively, for the two
molecules. The two molecules form a slipped, stacked dimer
with Fe···Fe distance 5.153(1) Å. However, the two corrole
planes are not parallel, forming a dihedral angle of 7.5(1)°.
Closest intermolecular contacts are C···C 3.293(5) Å and Fe···
C 3.517(4) Å.
It is of interest to compare the structure of 1 with those of
octaethylcorrolato−phenyl−iron,7 (OEC)FePh, refcode
SUMXED and its corresponding oxidized π cation radical
[(OEC)FePh]ClO4,36 refcode ZOKCIL. Unlike the saddle
corrole conformation of 1, both of the octaethyl derivatives
have more planar corroles (mean deviations of 0.037 Å for
SUMXED and 0.022 Å for ZOKCIL), with almost all distal C
As for the chloro−iron complex, the relevant structural
parameters obtained from the S12g and B3LYP-D3 optimized
geometries are gathered in Table 2 and are compared to the X-
ray data available for the chloro−iron corrolate analogue, the
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Table 1. Selected Bond Lengths (Å), Dihedral Angles (deg),
and Metrical Parameters(Å) Calculated for Complex 1
a
DFT-ZORA
exp
S12g
B3LYP-D3
A
B
Fe−N1
Fe−N2
Fe−N3
Fe−N4
1.872
1.899
1.896
1.871
1.954
1.480
1.485
1.482
67.3
1.864
1.899
1.889
1.876
1.973
1.484
1.488
1.482
57.4
1.858(3)
1.883(3)
1.890(3)
1.867(3)
1.987(4)
1.487(5)
1.490(6)
1.486(5)
48.88(7)
51.28(11)
46.40(11)
46.9(3)
1.860(3)
1.884(3)
1.890(3)
1.872(3)
1.984(4)
1.481(5)
1.486(6)
1.479(5)
41.36(7)
46.78(13)
44.65(11)
52.6(4)
Fe−Ph
C5−CTol
C10−CTol
C15−CTol
b
θ1
c
θ2
74.8
63.6
d
θ3
63.7
60.3
e
θ4
45.5
35.0
f
ΔFe
0.328
0.451
0.279
0.198
0.2521(15)
0.2639(16)
0.2485(9)
0.2864(9)
g
ΔFe
a
Experimental values for 1 (this work); A and B refer to the two
b
independent molecules in the crystal. Dihedral angle between the
plane of the tolyl group bound to the meso C5 atom and the 23-atom
corrole plane. Dihedral angle between the plane of the tolyl group
c
d
bound to the meso C10 atom and the 23-atom corrole plane. Dihedral
angle between the plane of the tolyl group bound to the meso C15
e
atom and the 23-atom corrole plane. Dihedral angle between the
f
phenyl plane and the plane passing through N2−Fe−N4. Displace-
g
ment of the Fe atom out of the (Np)4 plane. Displacement of the Fe
atom out of the 23-atom corrole plane.
TPFCorrFeCl complex.43 Just as in the case of the phenyl−iron
complex, S12g and B3LYP-D3 calculations yield rather similar
geometries, except for the Fe−Cl distance that at B3LYP-D3
level is predicted to be ca. 0.04 Å longer than at S12g level.
What we find is in line with previous DFT calculations on
axially ligated iron corroles, which indicate that B3LYP tends to
predict larger iron−ligand distances compared to pure GGA
functionals such as BP86.5f While the Fe−N distances and the
displacement of the iron atom out of the (Np)4 and the 23-
atom corrole planes match well with the experiment, the Fe−Cl
distance is somewhat underestimated, particularly at S12g level.
Electronic Structure and Spin Densities. Figure 4 shows
a molecular orbital (MO) diagram of complex 1 obtained at the
S12g level of theory. A similar MO pattern was obtained using
the B3LYP-D3 functional. In the complex, the phenyl−
iron(IV) (S = 1) moiety is, just as the oxoiron(IV) (S = 1)
moiety is in oxoiron(IV) porphyrin complexes,44 a d4 system
with a (dxy)2(dxz)1(dyz)1 electronic configuration. The Fe−Ph
bond is formed by a σ interaction between the (nominally
Figure 3. Top view (a) and side view (b) of the molecular structure of
1 optimized at DFT-ZORA/S12g/TZ2P level of theory, in vacuo.
Table 2. Selected Bond Lengths (Å), Dihedral Angles (deg),
and Metrical Parameters (Å) Calculated for Complex 2
DFT-ZORA
a
S12g
B3LYP-D3
exp
Fe−N1
Fe−N2
Fe−N3
Fe−N4
1.892
1.927
1.919
1.896
2.177
1.478
1.489
1.479
56.2
1.900
1.938
1.929
1.905
2.219
1.479
1.484
1.480
61.4
1.882(7)
1.919(6)
1.922(7)
1.880(6)
2.238
Fe−Cl
C5−CTol
C10−CTol
C15−CTol
2
unoccupied) Fe 3dz orbital and the highest occupied (HOMO)
b
θ1
of the Ph− fragment having large amplitude on the C1−2pz
orbital (cf. the σ and σ* MOs in Figure 4), and a π interaction
between the (nominally half occupied) Fe 3dxz,yz orbitals and
the occupied π MOs of the Ph− fragment, mainly the HOMO−
1 and HOMO−4. In Figure 4 only the Fe−Ph π-antibonding
MOs (which are nominally the dπ orbitals and are denoted as
πxz and πyz) are reported. Of the Fe dδ orbitals, the fully
occupied dxy is substantially a nonbonding orbital, whereas the
c
θ2
60.7
67.9
d
θ3
57.2
62.7
e
ΔFe
0.423
0.510
0.428
0.512
0.3670(11)
f
ΔFe
0.4034(11)
a
b
Experimental values for TPFCorrFeCl, from reference 43. Dihedral
angle between the plane of the tolyl group bound to the meso C5 atom
c
and the 23-atom corrole plane. Dihedral angle between the plane of
the tolyl group bound to the meso C10 atom and the 23-atom corrole
2
2
d
empty dx −y , with lobes along the axes, is destabilized by
antibonding interaction with the σ lone pairs of the equatorial
nitrogens. This MO, denoted as σ′*, ends up above the corrole
“eg*-like” MOs (the β-spin component of the σ′* is too high in
energy to enter the diagram of Figure 4).
plane. Dihedral angle between the plane of the tolyl group bound to
e
the meso C15 atom and the 23-atom corrole plane Displacement of
f
the Fe atom out of the (Np)4 plane. Displacement of the Fe atom out
of the 23-atom corrole plane.
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Figure 4. Diagram of selected energy levels and relevant molecular orbitals of complex 1 in the S = 1 spin state obtained from DFT-ZORA/S12g/
COSMO/TZ2P calculations, in CHCl3 solution.
Table 3. Mulliken Gross Population of Iron 3d Orbitals Computed for Complexes 1 and 2 in CHCl3 Solution
1
2
S12g
B3LYP-D3
S12g
B3LYP-D3
a
orbital
spin α
spin β
spin α
spin β
spin α
spin β
spin α
spin β
2
3dz
0.68
0.48
0.96
0.98
0.98
0.52
0.40
0.94
0.23
0.24
0.74
0.47
0.98
0.99
0.99
0.51
0.37
0.96
0.15
0.16
0.83
0.49
0.97
0.98
0.98
0.35
0.38
0.94
0.21
0.21
0.95
0.46
0.98
1.00
1.00
0.29
0.34
0.96
0.16
0.16
2
2
3dx −y
3dxy
3dxz
3dyz
a
Summation over all MOs, multiplied by occupations.
The nature of the above-described bonding interactions is
participation of this orbital in the occupied Fe−Ph σ MOs
(see the plots of these orbitals in Figure 5) and, to a lesser
extent, from donation by the “a2u-like” corrole orbital.
Dissimilar from the a1u-like MOs, the two components of the
a2u-like orbitals, especially the α spin component, are in fact not
substantiated by the Mulliken gross population of the Fe 3d
orbitals reported in Table 3. It is clear from the data in the
Table that both spin components of the 3dxy and the α spin
component of the 3dxz and 3dyz are fully occupied, in
agreement with a formal low spin d4 electron configuration of
the iron.
2
pure ligand orbitals. They have some (5−7%) Fe 3dz character,
as is evident from the plot of the α spin a2u-like MO in Figure 5.
This interaction, which is enabled by the displacement of the
iron atom out of the corrole plane, induces a little spin
polarization of the a2u-like orbital. This explains why the spin
density distribution in the tritolylcorrole moiety largely reflects
the atomic orbital composition of the a2u-like MO. As a matter
of fact the nitrogen and meso-carbon atoms carry a (small)
negative spin density, while the Cα atoms of the pyrroles to
which they are attached show smaller positive spin densities,
except for the Cα atoms bound to C10 that, at S12g level, have
negative spin density. All Cβ atoms have positive spin densities,
with values larger at S12g than at B3LYP-D3 level. Small spin
The β spin component of the 3dπ orbitals shows the charge
accumulating as a consequence of the π donation by the axial
phenyl ligand. According to the gross population of the 3d
orbitals in Table 3, ca. 0.9 electrons are distributed over the two
2
2
spin components of the Fe 3dx −y , owing to its participation in
the low-lying Fe−Nlp (lp = lone pair) MOs, although one
classifies these as “ligand” (Nlp) orbitals. The empty α and β
2
2
spin σ′* MOs are nominally the “3dx −y ” but are certainly not
pure metal orbitals, having no less than 40% of ligand (Nlp)
character. The considerable population of the α and β spin
2
components of the 3dz orbital arises mainly from the
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The m-, o-, and p-carbon atoms of the axial phenyl ligand carry
appreciable spin densities with alternating signs, the spin
density on the carbanion C1 amounting to −0.142 and −0.126
at B3LYP-D3 and S12g level, respectively.
As discussed in the NMR section, the comparison between
the measured isotropic shifts and the computed spin densities is
overall quite satisfactory. The atomic spin densities obtained
with the pure S12g functional better account, however, for the
isotropic shifts measured for the Cβ and tolyl protons.
The MO diagram of the chloro−iron derivative obtained at
the S12g level of theory is shown in Figure 6. As in the phenyl−
iron complex, the bond with the axial ligand consists of a σ
2
interaction between the (nominally unoccupied) Fe 3dz orbital
and the Cl 3pz orbital (cf. the σ and σ* MOs in Figure 6) and a
π interaction between the (nominally half occupied) Fe 3dxz,yz
orbitals and the occupied Cl 3px,y. The metal−corrole bond
interaction is significantly different in the two complexes,
however. According to the orbital diagram of Figure 6, in the
spin-up manifold the corrole a2u-like MO mixes to some extent
2
with the Fe 3dz /Cl 3pz σ-antibonding orbital (σ*). Of the
resulting occupied 170a and the empty 172a MOs, the former
has a dominant σ* character, while the latter is largely the
corrole a2u-like orbital (see the plots of these MOs in Figure 6).
In the spin-down manifold the corrole a2u-like MO remains,
instead, an almost pure ligand orbital. The mixing between the
corrole a2u-like MO and the σ* occurring in the spin-up
manifold reflects on the Mulliken gross population of the α spin
2
component of the Fe 3dz orbital that is in the chloro−iron
complex larger than it is in the phenyl−iron analog (Table 3) in
spite of the lower σ donor capability of the axial ligand in 2
(note in Table 3 the diminished population of the β spin Fe
2
3dz when moving from 1 to 2). The population of the α spin
2
component of the Fe 3dz orbital is particularly large at B3LYP-
D3 level as an indication of a more effective charge transfer
2
from the corrole a2u-like MO into the Fe 3dz and hence of a
more pronounced radical character of the corrole macrocycle.
Anyway, both the pure and hybrid functionals agree on that, at
variance with the phenyl−iron analog, the chloro−iron complex
does not feature a closed-shell corrolate trianion. This does not
mean, however, that in complex 2 an entire electron has been
transferred from the corrolate trianion into the formally empty
σ*, turning the [FeIV(corrolate3−)]Cl system into a
[FeIII(corrolate2•−)]Cl diradical and the corrole into a non-
innocent ligand. In fact, the electron donation from the
corrolate trianion into the σ* may be accomplished not only
through antiferromagnetic exchange coupling between the
unpaired spins residing on (weakly) overlapping orbitals but
also through a covalent interaction. To assess the noninnocence
of the corrole macrocycle one should be able to distinguish
between covalent and diradical character contributions to the
ground-state wave function. Pierloot et al.5f have recently
suggested a procedure to evaluate the diradical character of a
bond based on the natural orbital occupation numbers
(NOON) involved in the bond. This procedure has been
applied by these authors to provide a quantitative estimate of
the diradical character of the metal−corrole bond in some
copper and iron corroles. In particular, using the NOON of the
Figure 5. Atomic spin density distribution for complex 1 in the S = 1
spin state obtained from DFT-ZORA/COSMO/TZ2P calculations in
CHCl3 solution using B3LYP-D3 (a) and S12g (b) functionals.
densities with alternating signs on adjacent atoms are also
found on the carbon atoms of the tolyl groups, on account of
the a2u-like MO having some amplitude also on the o- and p-
carbons of the tolyl groups (see the plot of the α spin
component of the a2u-like MO in Figure 5).
2
bonding and antibonding combinations of the Fe 3dz and the
corrole a2u-like MOs previously computed for unsubstituted
chloro−iron corrole by Roos, Ghosh, and co-workers at
CASSCF level,5e Pierloot et al.5f predicted for this complex a
diradical character of only 59%. For the sake of comparison
with these results, we applied the procedure suggested by
The spin density on the metal is typical for Fe(IV) S = 1 and
arises mainly from the 3dxz and 3dyz contributions. Compared
to B3LYP-D3, S12g gives a smaller spin density on the metal
(1.864 vs 2.072). This is in line with the well-known tendency
of GGA functionals to overestimate metal−ligand covalency.5e
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Figure 6. Diagram of selected energy levels and relevant molecular orbitals of complex 2 in the S = 1 spin state obtained from DFT-ZORA/S12g/
COSMO/TZ2P calculations, in CHCl3 solution.
Pierloot et al. to the chloro−iron complex here investigated. To
this end we performed for complex 2 a NO analysis, both at
B3LYP and at BP86 levels. The calculations showed the
occurrence of two NO with occupation numbers that strongly
deviate from either 2 or zero, the 169a and 172a, with
occupations of 1.48 and 0.52 at B3LYP level, 1.84 and 0.16 at
BP86 level. As can be inferred from the plots of the 169a and
172a in Figure 7, these NOs are the bonding and antibonding
combination of the corrole a2u-like MO and the σ*. The
diradical character of 2 obtained from the B3LYP NOON
amounts to 52%, while that obtained from the BP86 NOON is
only 16%, a value that is significantly smaller than that
predicted for the unsubstituted chloro−iron complex using
CASSCF natural orbitals. As it is presumable that the diradical
character obtained from the S12g NOON would not be much
different from that obtained from the BP86 NOON, is not
surprising that the spin density pattern obtained at B3LYP-D3
level is quite different from that obtained at S12g level (Figure
8). Both functionals predict large negative spin densities at the
meso positions, larger than in complex 1. However, the B3LYP-
D3 spin densities are significantly more negative than the S12g
ones, which is consistent with the more pronounced diradical
character predicted for complex 2 by the hybrid functional. On
account of the enhanced corrole-to-metal charge transfer in the
chloro−iron complex the metal carries a larger spin density
than in the phenyl−iron analog, particularly at the B3LYP-D3
level, in which case the spin density approaches that of a ferric
iron of intermediate spin.
1
NMR Characterization. The H NMR characterization of
phenyl−iron derivatives of corrole has been reported for β-
alkylcorroles,37 while only the analogous phenyl−iron(IV)
meso-arylporphyrin complexes can be found in the literature.45
1
Figure 9a shows the H NMR spectrum of complex 1. Two
distinct signals are observed for both the o- and m-protons of
the tolyl substituents of the two compounds. This is due to a
relatively high rotational barrier that causes the appearance of
two separated signals for protons pointing in the two different
directions with respect to the plane of the macrocycle.
Assignment of the tolyl resonances was possible using a
combination of TOCSY and NOESY spectra. Resonances
belonging to the axial Phe ligand were assigned by comparison
with data already available for similar compounds.37 In
particular, the p-proton of the Phe ligand was readily assigned,
being the only proton showing an integral equal to 1. The
spectrum of complex 1 shows large isotropic shifts for the axial
Phe ligand and three β-pyrrolic protons (positions 2/18, 7/13
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Figure 7. Natural orbitals and their occupation numbers (in
parentheses) involved in the interaction between the σ* and the
corrole a2u-like orbitals obtained for complex 2 from unrestricted
B3LYP (a) and BP86 (b) calculations. The contour values are 0.04
e/au3.
and 8/12), which ended in the −40 to −160 ppm spectral
region. Smaller isotropic shifts were observed for the tolyl
protons. No cross peak was detected for the β-pyrrolic protons
in COSY and NOESY spectra, precluding an unambiguous
assignment of these signals. To overcome this problem, it was
decided to prepare the phenyl−iron derivatives of the
monosubstituted complex 5, taking advantage of the
regioselectivity showed by corrole in the substitution reactions.
The free base 2-bromo-5,10,15-tritolylcorrole, together with the
corresponding 3-Br regioisomer, was prepared in satisfying
yields as previously reported;32 other than having the possibility
of obtaining both regioisomers, the choice of the Br
substituents was designed to minimize the impact of the β
substituents on the electronic characteristics of the correspond-
ing iron complexes. The introduction of the Br atom at the 2
position renders the 3,17 protons nonequivalent, and
consequently the signal was split into two, with the
concomitant reduction of the intensity of the signal related to
the residual 18 proton (Figure 9b). In Supporting Information,
Figure S2 are shown the chemical shifts of the protons of
complex 1 as a function of 1/T. All resonances extrapolate to
the diamagnetic region of the NMR spectrum, suggesting that
the spin state of complex 1 is pure, with no thermally accessible
excited state available. The magnetic moment of 1 was
determined using the Evans method; the value obtained was
3.0 μB, consistent with a triplet ground state.
Figure 8. Atomic spin density distribution for complex 2 in the S = 1
spin state obtained from DFT-ZORA/COSMO/TZ2P calculations in
CHCl3 solution using B3LYP-D3 (a) and S12g (b) functionals.
substituted complexes 3 (with a bromine in position 2) and 4
(bromine in position 3) gave us the opportunity to perform the
1
complete assignment of the H NMR spectrum of complex 2
Figure 10a shows the 1H NMR spectrum of the chloro−iron
complex 2. The unambiguous assignment of the β-pyrrolic
proton resonances is not possible, since also in this case no
cross peak was detected for these signals in COSY and NOESY
spectra, due to their short relaxation times. In the literature this
assignment has been achieved on the basis of DFT
calculations5b,13 or by regioselective partial deuteration.46 The
by selective substitution, as was done in the past for the
complete assignments of the resonances of the analogous β-
alkyl corrole complexes.5a While in the spectrum of complex 3
it is possible to observe the reduction of intensity of the signal
around −40 ppm, in the spectrum of compound 4 in this
region it is possible to observe two resonances due to the
presence of the Br in position 3, which makes the 2,18 protons
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Figure 9. 1D 1H NMR spectrum in CDCl3 at 300 K of complex 1 (a)
and comparison with the spectrum of the 2-Br derivative 5 (b).
Figure 10. 1D 1H NMR spectrum in CDCl3 at 300K of 2 (a) and the
comparison with the corresponding spectra of the substituted
complexes 3 and 4 (b).
no more equivalent (Figure 10b). These results consequently
assign the high-field resonance to the 2,18 protons and not to
the 3,17 analogous, as previously reported.
Table 4 shows the values for the measured 1H isotropic shifts
for all the protons in compounds 1 and 2. Isotropic shifts were
calculated using the corresponding Ge complexes as reference,
and the estimated contact shifts were derived using an
approximate value for the dipolar contribution.23 It is
interesting to note that the spin densities obtained by DFT/
S12g calculations nicely support the assignments of the
chemical shifts for complexes 1 and 2, as reported in Table
4, and they better account for the isotropic shifts measured for
the Cβ and tolyl protons than do the spin densities calculated
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by DFT/B3LYP-D3. In the case of complex 2, for example, the
Cβ protons did not show an alternating sign in the isotropic
shifts, expected from the spin density obtained calculated by
DFT/B3LYP-D3, in accord with the pattern obtained by DFT/
S12g calculations. NMR data show an alternating sign in the
isotropic shift for o- and m-positions of the tolyl substituents,
the shift for the o-protons being larger than that of the m-
protons. This observation is in agreement with DFT/S12g
calculations showing alternating signs for the spin density at
these two positions. This is consistent with the presence of
negative spin density at the meso carbons, although the effect is
much smaller than it is in complex 2, as suggested by the
smaller observed isotropic shifts of meso-phenyl protons of
complex 1 compared to 2, and the relatively low calculated spin
densities (Σ|ρmeso = 0.07 for complex 1). On the other hand,
DFT/S12g calculations predict spin densities at the β carbons
larger for complex 1 (Σ|ρpyrrole| ≈ 0.18) than for 2 (Σ|ρpyrrole| ≈
0.09), which is consistent with the relatively large isotropic
shifts observed for the β-pyrrolic protons of 1.
density and are more readily converted into quantitative
estimation.5b The two sources contributing to the 13C
paramagnetic shifts are those of dipolar nature and the contact
contribution due to spin delocalization. The first, in the case of
13C, is divided into two contributions: the metal- and the
ligand-centered pseudocontact shifts. Compared to the large
isotropic shifts normally observed for 13C, their values are
relatively small,47−49 accounting for about 10% of the total
paramagnetic shift. For this reason, one can draw conclusions
based on the second source, which in turn has two different
contributions: the delocalization of metal electrons to the
macrocycle through Corr → Fe π donation, and the residual
due to the corrole π radical electron. For meso substituents,
however, only the latter is important for determination of the
13C isotropic shifts, because the corrolate orbitals with which
the metal dπ orbitals interact to delocalize the spin to the
macrocyclic ring have nodes at the meso positions.5c
In fact, it was calculated that for one or two delocalized
electrons, the contribution to the o- and m-position of meso-
phenyl substituent carbons accounts only for −3 to −6 ppm,
respectively, of the total anisotropic shift.5c So the large
isotropic shifts observed for the o-carbons (in the range of
−100 to −400 ppm), both for complexes 1 and 2 (see Table 5),
are clear evidence of the presence of a negative spin density at
the meso-carbons. Isotropic shifts of the o-carbons of the tolyl
substituents are about 4 times larger for complex 2 than they
are for 1, reflecting the higher noninnocent character of the
corrole ligand in 2.
The chemical shifts for 13C nuclei with favorable relaxation
rates were measured using the 2D HMQC correlation
experiment. Attempts to observe the remaining 13C nuclei
using one-dimensional (1D) experiment failed. Figure 11 shows
Peculiar behavior is observed for the sign of the isotropic
shift of m-carbons in the two compounds. Those in positions
5,15 show positive isotropic shifts, as expected, due to the
alternation in sign between the o- and m-positions. However,
the tolyl substituent in position 10 shows small negative
isotropic shifts. Since the expected contribution to the isotropic
shift arising from the delocalization of metal electrons to the
macrocycle is in the order of −3 ppm (one electron) or −5
ppm (two electrons),5c the observed positive isotropic shifts of
the m-tolyl carbons in positions 5,15 must be the consequence
of a contribution of +26 ppm (2) or +10 ppm (1) from the
corrolate radical. The same contribution to substituents at
position 10 is around 0, leaving isotropic shifts with similar
values to that expected merely for the delocalization of the
metal electrons. Table 5 also shows the ratio between the 13C
1
and H isotropic shifts. These ratios are expected to be always
negative, due to the negative sign of the constant relating the
paramagnetic shifts of the two nuclei. As can be seen, this is the
sign for all the ratios except for that of the 13C and 1H nuclei in
the meta position of the 10-tolyl group. Once again, this is
pointing to a peculiar value of the total spin density at these
positions, different from that observed for the other two tolyl
groups meta nuclei. This result is in agreement with DFT
calculations, which predict a less-pronounced conjugation of
the 10-tolyl substituent with the corrole ring.
This behavior can also give some insights into the different
reactivity observed for the meso positions of the corrole ring;
for example, usually oxidation50 and also the oxidative ring-
opening34 or corrole ring expansion to give hemiporphycene51
or azahemiporphycene52−54 rings have been always observed to
involve the 5-position.
1
Figure 11. Selected regions of the 2D H−13C HMQC correlation
spectra of complexes 1 (a) and 2 (b).
selected regions of these experiments for complexes 1 and 2.
Table 5 shows the observed chemical shifts and the calculated
isotropic shifts using the corresponding Ge complexes as the
source of the diamagnetic shifts. In general, the 13C isotropic
1
shifts show the same trend as those of H, reflecting the
differences already discussed between the two compounds. 13C
shifts carry additional information about the pattern of spin
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Table 5. 13C Chemical Shifts of Tolyl Substituents in Complexes 1 and 2 in CDCl3 at 300 K, and the Corresponding Ratios
1
between 13C and H Isotropic Shifts
1
2
a
chemical shift ppm
isotropic shift ppm
13C/1H
chemical shift ppm
isotropic shift ppm
13C/1H
assignment
41.3, 37.4
14.4, 11.8
133.9, 132.5
124.2, 123.1
26.4
−93.3, −96.6
−120.1, −122.0
5.4, 4.0
−14.1, −16.9
−30.7, −31.3
−3.4, −2.1
+1.8, +2.1
−2.6
−189.9, −177.9
−278.9, −265.6
149.8, 149.4
126.1, 122.6
51.5
−324.2, −312.2
−413.1, −399.4
20.8, 21.2
−5.7, −2.2
30.1
−18.4, −19.0
−26.3, −25.3
−2.0, −2.0
+0.5, +0.2
−2.1
5,15-o-Ph−C
10-o-Ph−C
5,15-m-Ph−C
10-m-Ph−C
5,15-p-Ph−CH3
10-p-Ph−CH3
−4.0, −5.1
5.0
28.2
6.8
−3.2
46.9
25.5
−2.1
a
Isotropic shift = observed chemical shift − diamagnetic shift. Diamagnetic shifts were taken from the corresponding Ge derivatives.
(8) Stefanelli, M.; Nardis, S.; Tortora, L.; Fronczek, F. R.; Smith, K.
ASSOCIATED CONTENT
* Supporting Information
CIF file of 1, structure of the asymmetric unit cell, and H
NMR Curie plot of 1. This material is available free of charge
via the Internet at http://pubs.acs.org
■
M.; Licoccia, S.; Paolesse, R. Chem. Commun. 2011, 47, 4255−4257.
(9) Nardis, S.; Stefanelli, M.; Mohite, P.; Pomarico, G.; Tortora, L.;
Manowong, M.; Chen, P.; Kadish, K. M.; Fronczek, F. R.; McCandless,
G. T.; Smith, K. M.; Licoccia, S.; Paolesse, R. Inorg. Chem. 2012, 51,
3910−3920.
S
1
(10) Evans, D. F. J. Chem. Soc. 1959, 2003.
AUTHOR INFORMATION
Corresponding Authors
*E-mail: roberto.paolesse@uniroma2.it. (R.P.)
*E-mail: angela.rosa@unibas.it. (A.R.)
*E-mail: kmsmith@lsu.edu. (K.M.S.)
(11) Sur, S. K. J. Magn. Reson. 1989, 82, 169.
(12) Kahn, O. Molecular Magnetism; VCH Publishers: Weinheim,
■
Germany, 1993; pp 2−4.
(13) Zakharieva, O.; Schunemann, V.; Gerdan, M.; Licoccia, S.; Cai,
̈
S.; Walker, F. A.; Trautwein, A. X. J. Am. Chem. Soc. 2002, 124, 6636.
(14) (a) ADF2013, SCM, Theoretical Chemistry; Vrije Universiteit:
Amsterdam, The Netherlands, http://www.scm.com; (b) te Velde, G.;
Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Fonseca Guerra, C.;
Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22,
931. (c) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Snijders, J. G.
Theor. Chem. Acc. 1998, 99, 391.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the MIUR (R.P. Grant PRIN
2009 2009Z9ASCA) and U.S. National Institutes of Health
(K.M.S. Grant CA 132861). Upgrade of the diffractometer was
made possible by Grant No. LEQSF(2011−12)-ENH-TR-01,
administered by the Louisiana Board of Regents. A.R. and G.R.
(15) Swart, M. Chem. Phys. Lett. 2013, 580, 166.
(16) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys.
2010, 132, 154104.
(17) Grimme, S. WIREs Comput. Mol. Sci. 2011, 1, 211.
(18) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(19) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785.
(20) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys.
1993, 99, 4597.
thank the Universita
(RIL_2011 funds).
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della Basilicata for financial support
(21) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys.
1994, 101, 9783.
(22) van Lenthe, E.; Ehlers, A. W.; Baerends, E. J. J. Chem. Phys.
1999, 110, 8543.
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