Synthesis and characterization of a stable bismuth(III) A 3-corrole

Article abstract of DOI:10.1021/ic200840m

An efficient metalation procedure for bismuth complexes with meso-substituted corrole ligands is presented. Reaction of 5,10,15-tris- pentafluorophenylcorrole H₃(TpFPC) with Bi{N(SiMe₃) ₃} converts the free ligand H_3<

Full text of DOI:10.1021/ic200840m

ARTICLE
pubs.acs.org/IC
Synthesis and Characterization of a Stable Bismuth(III) A3ꢀCorrole
Lorenz Michael Reith,† Martin Stiftinger,‡ Uwe Monkowius,† G€unther Kn€or,*,† and Wolfgang Schoefberger*,†
†Institute of Inorganic Chemistry and ‡ Institute of Analytical Chemistry, Johannes Kepler University Linz (JKU), Altenberger Strasse 69,
A-4040 Linz, Austria
S Supporting Information
b
ABSTRACT: An efficient metalation procedure for bismuth
complexes with meso-substituted corrole ligands is presented. Reac-
tion of 5,10,15-tris-pentafluorophenylcorrole H3(TpFPC) with
Bi{N(SiMe3)3} converts the free ligand H3(TpFPC) to a neutral
low-valent species Bi(TpFPC), which has been characterized by
different spectroscopic techniques. (Spectro)electrochemical stud-
ies were performed in order to describe the redox potentials of the
Bi(TpFPC) complex and to ascribe the sites of electron transfer.
The first crystal structure of a bismuth corrole is presented and
compared to the geometry-optimized molecular structure obtained
with density functional theory (DFT) calculations. We show an
example of a 4-coordinate metallocorrole with a very large out-of-
plane displacement and significant doming. The electronic structure
of the novel bismuth corrole system is discussed in detail. Time-dependent DFT results support the proposed assignment of electronic
transitions observed for the Bi(TpFPC) derivative. To account for the reactivity we investigated the photocatalytic properties of the
Bi(TpFPC) complex.
’ INTRODUCTION
In comparison to the well-established synthetic routes for
bismuth(III) porphyrins9ꢀ13 and impressive derivatization pro-
cedures performed by Boitrel and co-workers, which were
recently summarized in a review article,14 only two examples of
bismuth corrole complexes have been reported in the literature to
date: bismuth octaethylcorrole Bi(OEC), which was prepared
from H3(OEC) with BiCl3 and potassium acetate in boiling DMF
for 20 min under inert atmosphere,15 and recently, we reported a
metalation reaction of H3(TPC) withdifferent bismuth salts in DMF
and pyridine at elevated temperature.16 Complexation reactions were
indeed successful, but the obtained Bi(III) complexes lacked stability
and demetalation occurred rapidly during purification. Similar results
were obtained during the metalation of H3(TPC) with Pb.16,17 In
order to overcome these obstacles and to prevent undesired degrada-
tion and side reactions by the use of gentle reaction conditions, we
herein wish to report an efficient protocol for the synthesis of meso-
substituted Bi(III)ꢀcorroles at room temperature.
Since the first published one-pot synthesis of corroles reported
in 1999 by Gross and Paolesse,1ꢀ3 the scope of corrole applica-
tion grew tremendously including numerous examples of high-
valent metallocorroles in the fields of catalysis, photochemical
sensors, molecular electronics, and biomedicinal applications.4
In contrast to the corresponding porphyrin systems, free-base
corroles show a considerably lower stability against light and air.
This instability stems from the reduced aromaticity of corroles
and the deformation of the macrocycle from planarity due to the
steric bulk of the three interior hydrogens. Thus, corroles tend
to break down to open chain structures in aerobic solution under
ambient light.5ꢀ7 Depending on the substitution pattern of
the free-base corrole macrocycle the stability varies significantly,
whereby those corroles bearing electronegative substituents dis-
play the highest stability compared to their counterparts bearing
electropositive side groups. Nevertheless, corroles with electro-
negative substituents degrade at room temperature, under air,
and ambient light. Barata et al. recently studied this process on
the free-base 5,10,15-tris(pentafluorophenyl)corrole and ob-
served slow oligomerization throughout the reaction processes.8
Despite this limited stability, many metalation reactions have
to be performed under harsh conditions (e.g., boiling coordinat-
ing and noncoordinating solvents), and partial degradation
reactions during the metalation process on the labile corrole
macrocycle cannot be fully avoided. Moreover, for a few transi-
tion-metal and main-group metallocorroles no proper complexa-
tion procedure exists at present.
’ SYNTHESIS
The free-base H3(TpFPC), 2, was synthesized by employing
the condensation method for reactive aldehydes optimized by
Gryko et al.,18 where the solvent-free condensation of pyrrole
and pentafluorobenzaldehyde is performed at room temperature
under acidic catalysis followed by oxidation with DDQ. First
attempts for the metalation of the free-base H3(TpFPC), 2,
Received: April 22, 2011
Published: June 13, 2011
r
2011 American Chemical Society
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Scheme 1. (a) Preparation of the Bi{N(SiMe3)2}3, 1, Precursor, and (b) Synthesis of Bi(TpFPC), 3
the UVꢀvis absorption spectrum in THF of Bi(TpFPC) com-
pared to the free-base corrole H3(TpFPC). Bi(TpFPC), 3,
exhibits no emission in ethanol at room temperature and at 77 K
presumably due to radiationless intersystem crossing to a non-
luminescent excited triplet state. As a consequence, in such a
heavy atom macrocyclic system an increased triplet to singlet
oxygen conversion is expected.
’ NMR SPECTROSCOPY
1H NMR spectroscopy of Bi(TpFPC), 3, in deuterated THF
(Figure S3, Supporting Information) displayed the characteristic
features for tris-pentafluorophenylꢀcorroles:24 four doublets in
the range between 8.5 and 9.1 ppm are observed with an integral
of two protons each and coupling constants in the range of 4 Hz,
typical for the 8 β-pyrrole protons.
Due to the symmetry of the Bi(TpFPC), 3, molecule two sets
of resonances can be observed with 19F NMR spectroscopy
(Figure 2): those for the fluorine atoms on the substituent on
meso-positions 5 and 15 are chemically and magnetically equiva-
lent and as such give rise to one set, and the fluorine atoms on the
third meso-substituent (10) that give rise to the second set of
resonances. Hence, two triplets are observed for the para-F atoms
(around 157.5 ppm) with an integral ratio of 2:1. For each
resonance, scalar couplings are only resolved to the two directly
Figure 1. UVꢀvis absorption spectra of H3(TpFPC), 2, and Bi-
(TpFPC), 3, in THF.
involved “standard” metalation procedures, which have been
widely used for a variety porphyrins and corroles. Unfortunately,
reactions of 2 with BiCl3 in pyridine and DMF at elevated tem-
perature were not successful, and the question of an alternative
metalation method arose. Our interest soon focused on silylꢀ
amide ligands which are versatile precursors for metalation
agents19 and easily accessible on a multigram scale starting from
the commercially available starting materials Li{N(SiMe3)2} or
HMDS. Nevertheless, examples for application as a metalation
agent for porphyrins and related macrocycles have mainly been
limited to insertion of alkali metals,20,21 and this strategy is unpre-
cedented in the case of a corrole macrocycle.
Synthesis of the metalation agent Bi{N(SiMe3)2}3, 1, was
achieved by reaction of BiCl3 with Li(N(SiMe3)2 at ꢀ10 °C in
abs. THF (illustrated in Scheme 1a).22,23
The preparationof Bi(TpFPC), 3, from Bi{N(SiMe3)2}3, 1, and
H3(TpFPC), 2, proved successful under mild conditions. In the
following, the silyl amide 1 was dissolved in THF at room temper-
ature and added dropwise to a solution of 2 in THF (Scheme 1b)
under inert atmosphere. After workup the desired Bi(TpFPC)
complex 3couldbeisolatedin75%yieldafter column chromatography.
Figure 1 indicates the bathochromic shift of the Soret (B) band in
4
neighboring meta-F atoms (3JFꢀF g 22 Hz), while no JFꢀF
couplings to the corresponding ortho-F atom are observable. Due
to the nonplanarity of the bismuth corrole, an ABCDE spin
system results. Therefore, four signals are observed for the ortho-F
atoms with a 1:2:2:1 integral ratio, which means that within each
of the pentafluorophenyl substituents the fluorine atoms above
and those below the corrole plane are not chemically equivalent.
This fact is also corroborated by analysis of the coupling pattern:
all ortho-F resonances are observed as double doublets, with one
3JFꢀF coupling to the neighboring meta-F atom and an additional
4
4JFꢀF coupling of about 8 Hz. Having already excluded JFꢀF
4
coupling between the ortho- and para-fluorine atoms, JFꢀF
coupling between two ortho-F resonances seems to be the logical
explanation for the observed coupling pattern.
Consequently, a 1:2:2:1 integral ratio is also observed for the
meta-fluorine atoms, whereby two of the resonances are overlapping.
Each meta-fluorine resonance is observed as an octet (ddd),
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Figure 2. 19F NMR spectrum of Bi(TpFPC) with schematic illustration of the observed couplings: (a) ortho-F region, (b) meta-F region, and (c) para-F
region.
3
which is consistent with JFꢀF coupling to the corresponding
neighbors in the ortho and para positions and 4JFꢀF to the second
meta-fluorine atom on the same pentafluorophenyl ring.
In conclusion, Bi(TpFPC), 3, does not only display a dome-
featured structure (Figure 5b) in the solid state but also in solu-
tion, as clear differentiation between fluorine atoms above and
below the corrole plane is possible.
E1/2 = 0.66 and 1.07 V that display reversibility (Figure 3a). The
first reduction of 3 at E1/2 = ꢀ1.28 V is also reversible, followed
by a second irreversible reduction at E1/2= ꢀ1.88 V (Figure 3b).
The product of thissecondreduction shows reoxidation atꢀ0.4 V.
This pattern is similar to the electrochemical behavior of Bi-
(OEC), which shows oxidations at E1/2 = 0.24 and 0.78 V and
one single reduction at E1/2= ꢀ1.74 V vs SCE.15 While a higher
potential for the oxidation of 3 is necessary, reduction is easier
compared to Bi(OEC). This behavior can be attributed to the
three pentafluorophenyl groups which have a strongly electron-
withdrawing effect on the macrocycle, thus facilitating reduction
’ ELECTROCHEMISTRY
Cyclic voltammetry (vs SCE) of 3 in acetonitrile with 0.1 mol
of TBAPF6 (Figure 3) reveals two consecutive oxidations at
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Figure 3. Cyclic voltammograms of Bi(TpFPC), 3, in ACN with 0.1 M TBAPF6 measured with a scanning rate of 50 mV/s: (a) electrochemical
oxidation and reduction processes (overview) and (b) electrochemical reduction process.
three Q bands decrease and a blue shift of the B band from 447 to
443 nm is observed (Figure 4a). During the second oxidation, the
intensity of the B band is decreasing strongly while the intensity
of the Q bands increases, and the most intense Q band shows a
red shift from 635 to 643 nm (Figure 4b).
While metal-centered reduction of Bi(III) to Bi(II) species
in porphyrinoid macrocycles has not yet been observed, it seems
rather unlikely to be reversible due to the tendency of Bi(II)
intermediates to undergo rapid disproportionation processes. As
such, the first reduction can be assigned as a ring-centered pro-
cess. Classification of the two reversible oxidations is not that
straightforward. In our view, arguments in favor of two conse-
cutive ring-centered reactions prevail: For the first oxidation, the
situation seems to be very similar to Bi(OEC),15 as for both deri-
vatives a decrease of B and Q bands is observed and the spectrum
of the oxidized form displays characteristics of a π-radical cation.
In addition, abstraction of one of the Bi 6s electrons at a potential
lower than for the TpFPC-macrocycle would be very atypical,
especially regarding the strong inert pair effect of bismuth and the
accompanying stabilization of the Bi(III) oxidation state.
For the second oxidation, spectroelectrochemistry also does not
reveal any evidence for a metal-centered process. Consequently,
formation of a corrolato dication species seems to occur, where a
second electron is removed from the conjugated π-electron sys-
tem. This means that, at least under the experimental conditions
employed here, the two consecutive oxidations in 3 are ring-centered
processes, which are comparable to the usual electrochemical
oxidation behavior of different bismuth porphyrins reported earlier.11
’ X-RAY CRYSTALLOGRAPHY
Figure 5 illustrates ORTEP plots, representing X-ray crystal
structures of the free-base corrole H3(TpFC), 2, and the meta-
Figure 4. UVꢀvis absorption spectral changes during the electroche-
lated counterpart Bi(TpFPC), 3. To our knowledge, complex 3 is
the first bismuth corrole characterized by single-crystal X-ray
diffraction. The crystallographic data are summarized in Table 1.
The substances were found to crystallize in the monoclinic space
groups P21 (2, Z = 2) and P21/n (3, Z = 4) as chloroform
solvates. Despite many decades of corrole research, only nine
crystal structures of free-base triaryl corroles have been published
according to a CSD query.27ꢀ32 Among them, two crystal struc-
tures of the corrole 2 were reported, one containing m-xylene3
and the other ethyl acetate.6 Analogous to these two structures,
mical oxidation of Bi(TpFPC), 3, in ACN with 0.1 M TBAPF6 at
(a) 0.77 and (b) 1.2 V.
and impeding oxidation, a trend which has already been observed
for other pentafluorophenyl-substituted metallocorrole systems.25
Spectroelectrochemistry was performed in an optically trans-
parent thin layer electrode OTTLE cell26 in order to facilitate the
assignment of the sites of electrochemical oxidations. Along with
the first oxidation, the intensity of the Soret (B) band and the
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Figure 5. Molecular structures of (a) H3(TpFPC), 2, and (b) Bi(TpFPC), 3. (Top views) ORTEP representations with atom labeling, thermal
ellipsoids at 50% probability (H atoms omitted for clarity). (Inserts) Side views along the C1ꢀC19 axis (meso substituents omitted for clarity).
the three NH protons can unambiguously be located on rings A,
B, and D and the macrocylce is considerably distorted from pla-
narity due to the steric repulsion of the inner protons.
is tilted more in the macrocyclic plane compared to the residual
pyrrole moieties, which means that the mean N4 plane is some-
what ill defined compared to the geometry-optimized structure.
As a consequence of these two effects, the calculated structure
leads to partially larger distances between the inner nitrogen
atoms and a larger central cavity, which is, in the absence of axial
ligands, the key factor for influencing the out-of plane displace-
ment of the central Bi atom, explaining the discrepancy between
calculated and measured out-of-plane displacement. To summar-
ize, the optimized structure agrees well with the crystal structure,
whereby the deviations outlined above might result from packing
effects in the solid state.
Electronic Structure and Calculated Properties. Recently,
we discussed the electronic spectra of Bi(TPC), which also can
be adopted in a slightly modified form for Bi(TpFPC), 3.16 The
UVꢀvis spectrum for Bi(TpFPC), 3 (Figure 1), is qualitatively
reproduced by time-dependent density functional theory (TD-
DFT) calculations, and the corresponding transitions are de-
picted in Figure S11, Supporting Information. A simplified mole-
cular orbital diagram is shown in Figure 6 describing the possible
intraligand (πꢀπ*), MLCT, and metal-centered (MC) transi-
tions. For the latter, time-dependent DFT calculations confirm
the presence of a significantly Bi(6s)-localized HOMO in the
πꢀπ* frontier orbital region (Figure 7cꢀf) of the corrole ligand
and also correctly describe the occurrence of a metal-centered
Comparable to the published Bi(III)porphyrin structures,14
the corrole macrocycle exhibits a dome-shaped structure and the
bismuth atom lies ∼1.15 Å above the plane defined by the four
nitrogen atoms of the corrole ring. The Bi atom is coordinated
exclusively by the four nitrogen atoms and no evidence for axial
coordination is found. By comparing the structure of the Bi-
(TpFPC), 3, to other examples of main-group-metal corroles,33
we show the first example of a 4-coordinate metallocorrole with
the largest out-of-plane displacement and most significant dom-
ing of the macrocycle. The molecular geometry of Bi(TpFPC), 3,
was optimized by DFT calculation and indeed also resulted in a
domed structure. An overlay of both structures is found in Figure
S10, Supporting Information (detailed distances and dihedral
angles are listed in Table 2). The out-of-plane displacement
of the central bismuth ion, which differs by about 0.1 Å, and
the calculated BiꢀN distances as well as the dihedral angles χ1 ꢀ
χ4, which are a measure of the doming of the macrocycle, are
comparable for both structures. The geometry optimized struc-
ture slightly overestimates the doming effect, and the pyrrole
moieties are slightly more bended, especially in the region of the
direct pyrroleꢀpyrrole linkage. Additionally, in the crystal struc-
ture one of the pyrrole groups (N3ꢀC11ꢀC12ꢀC13ꢀC14ꢀN3)
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Table 1. X-ray Crystal Data of Compounds 2 and 3
Table 2. Comparison of Selected Distance and Dihedral
Angles of the X-ray Crystal Structure and the Geometry-
Optimized Structure of Bi(TpFPC), 3
H3(TpFPC)
Bi(TpFPC)
formula
C
37H11N4F15, 2 CHCl3 C37H8N4F15, CHCl3
Bi(TpFPC) crystal
structure
Bi(TpFPC) geometry
optimized
MW [g/mol]
cryst size [mm]
cryst syst
space group
a [Å]
1035.23
1121.82
0.55 ꢁ 0.35 ꢁ 0.25
monoclinc
P21/n
0.55 ꢁ 0.32 ꢁ 0.30
monoclinic
P21
out-of-plane distance
BiꢀN(1)
1.15
2.23(1)
2.28(1)
2.24(1)
2.23(1)
2.77
1.05
2.21
2.21
2.21
2.21
2.77
2.89
2.77
2.55
77.7
122.3
70.5
81.6
123.2
77.7
19.4
3.9
BiꢀN(2)
14.816(2)
10.282(1)
14.887(2)
90
15.402(2)
10.862(2)
24.331(4)
90
BiꢀN(3)
b [Å]
BiꢀN(4)
c [Å]
N(1)ꢀN(2)
R [deg]
β [deg]
N(2)ꢀN(3)
2.85
118.435(4)
90
90.261(6)
90
N(3)ꢀN(4)
2.74
γ [deg]
V [Å3]
N(4)ꢀN(1)
2.55
1994.3(4)
1.724
4070.5(11)
1.831
ber [mg cmꢀ3
]
N(1)ꢀBiꢀN(2)
N(1)ꢀBiꢀN(3)
N(1)ꢀBiꢀN(4)
N(2)ꢀBiꢀN(3)
N(2)ꢀBiꢀN(4)
N(3)ꢀBiꢀN(4)
χ1 (C3ꢀC4ꢀC6ꢀC7)
χ2(C8ꢀC9ꢀC11ꢀC12
75.9(3)
118.1(3)
69.8(3)
78.2(3)
118.8(3)
75.6(3)
15.3
F
Z
2
4
μ [mmꢀ1
]
0.54
4.63
T [K]
200
200
Θ range [deg]
λ [Å]
2.5ꢀ25.0
0.71073
19 439
2.3ꢀ23.7
0.71073
6159
reflns collected
)
3.6
unique reflns
3726
6159
χ3 (C13ꢀC14ꢀC16ꢀC17
χ4 (C17ꢀC18ꢀC2ꢀC3)
)
26.8
25.2
1.3
obsd reflns [I > 2σ(I)]
3277
4469
7.4
parameter refined/restraint 577/0
551/0
abs corr
multiscan
multiscan
0.19/0.39
2.41/ꢀ2.03
0.062
Tmin, Tmax
0.76/0.85
0.29/ꢀ0.40
0.038
σ
fin (max/min) [e Åꢀ3
]
R1 [I g 2σ(I)]
wR2
0.092
0.157
CCDC
821890
821891
(MC) sp-transition in the UV region (Figure S18, Supporting
Information). In ethanol solution additional electronic transitions
of Bi(TpFPC), 3, occur in the 305ꢀ340 nm region (32 787ꢀ
29 412 cmꢀ1). The calculated metal-centered (MC) sp and
MLCT transitions at 315ꢀ338 nm occur from the HOMO
and HOMOꢀ1, HOMOꢀ3, HOMOꢀ4, HOMOꢀ6 to the
LUMO, LUMOþ2, LUMOþ3, LUMOþ5, which exhibit high
px,y-orbital character (Figure S18, Supporting Information).
Interpretation of the natural bond orbital (NBO) population
analysis on the bismuth atom and the corresponding pyrrole
nitrogens in Bi(TpFPC), 3, reveals relatively small NBO charges
of 1.914 e and ꢀ0.698 e, respectively. The significantly lower
NBO charge compared to the previously studied Bi(TPC)
(NBO charge of 2.517e) indicates that the BiꢀN bonds have a
larger covalent character.34
Figure 6. Qualitative MO diagram of Bi(TpFPC) 3. IL: Intraligand
(πꢀπ*) transitions involving electrons localized at the macrocyclic ring.
MC: Metal-centered sp transitions. MLCT: Metal-to-ligand charge
transfer.
the significant doming of the macrocycle might be responsible for
the relatively small 1.94 V difference in redox potentials.
’ CHEMICAL OXIDATION AND PHOTOCHEMISTRY
The HOMO and LUMO energy levels were calculated from
the peak potentials of the first oxidation and reduction processes
obtained from cyclovoltammetry (Figure 3), leading to energy
Chemical oxidation of Bi(TpFPC), 3, with an excess of H2O2,
iodosobenzeneꢀdiacetate, or (NH4)2Ce(NO3)6 was hampered by
destruction of the macrocycle, while no evidence for the formation
of higher valent bismuthꢀoxo species could be observed.
values of the two states (EHOMO = ꢀ5.06 eV and ELUMO
=
ꢀ3.12 eV). The resulting HOMO and LUMO energy gap of
Bi(TpFPC), 3, obtained from the cyclovoltammogram matches
reasonably well with those obtained from gas-phase TD-DFT
(ΔHOMOꢀLUMO,CV = 1.94 eV, ΔHOMOꢀLUMO,TD-DFT = 2.12 eV,
ΔHOMOꢀLUMO,UVꢀvis = 1.96 eV).
In order to photochemically oxidize Bi(TpFPC), 3, to the
corresponding Bi(V) species, as already demonstrated in the
case of Bi(TPC),16 Bi(TpFPC), 3, was dissolved in ethanol and
irradiated (λ > 455 nm) under aerobic conditions. Unlike Bi(TPC),
where the high-valent state of bismuth could be reached via
sensitization of the metal-centered sp excited states, mass spec-
trometry of the irradiated solution of 3 suggests formation of a
The unique structural motif of Bi(TpFPC), 3 (Figure 5b),
with the large out-of-plane displacement of the central Bi ion and
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Scheme 2. (a) Oxidation of Thioanisole Sensitized by Bi-
(TpFPC) (3), and (b) Singlet-Oxygen Ene (SOE) Reaction of
Cyclohexene
between dioxygen activation at the metal site and generation of
high-valent oxo derivatives.37 While this latter transformation
was already observed for other bismuth corrole complexes,16 in
the case of 3 the bismuth(V) oxidation state seems to be in-
accessible without decomposition.
Singlet Oxygen Formation. In order to evaluate the role of
singlet oxygen generation sensitized by the low-valent complex
Bi(TpFPC), 3, irradiation experiments with different sensitizer
concentrations in aerated ethanol solutions have been performed
and compared with the reference photosensitizer Rose Bengal
(ϕΔ= 0.6838) whereby 1,3-diphenylisobenzofurane (DPBF) was
used as acceptor. Figure S14, Supporting Information, displays
the concentration-dependent time course of the degradation
compared with the reference sensitizer. To reassure the fact that
singlet oxygen is the reactive species for decomposition of DPBF,
control experiments with addition of NaN3 were performed.
Azide acts as quencher for the formed singlet oxygen, and thus,
decomposition of DPBF should be slowed down with increasing
azide concentration.39 Indeed, as shown in Figure S15, Support-
ing Information, with increasing azide concentration degradation
is slowed down by up to a factor of 7.
Figure 7. Comparison of structural parameters of the geometry-opti-
mized structure (a) and the crystal structure (b) of Bi(TpFPC), 3. Bond
lengths and bond angles are indicated in the units of Ångstroms and
degrees, respectively. The highest occupied molecular orbitals (HOMO,
HOMOꢀ1) and lowest unoccupied molecular orbitals (LUMO,
LUMOþ1) are depicted from cꢀf. A C2 point group was assumed for
Bi(TpFPC), 3, and corresponds to a 2-fold rotational symmetry. The
symmetry labels are added accordingly.
mixture of dioxo-open-chain structures (Figure S12b, Supporting
Information), which also explains the UVꢀvis spectral changes
during irradiation: new broad absorption bands between 360 and
380 as well as 520ꢀ670 nm with low intensity and a decrease of
the characteristic B-band are observed (Figure S13, Supporting
Information).
The strong ꢀCHꢀ to ꢀCdO correlations observed in a long-
Application of Bi(TpFPC) as Photocatalyst for Selected
Oxidation Reactions. Although a degradation reaction during
irradiation was observed as outlined above, the potential of
Bi(TpFPC), 3, to act as photocatalyst was evaluated by means of
the oxidation of thioanisole and cyclohexene with singlet oxygen.
Both reactions are well suited for evaluation of photocatalysts and
their mechanisms have been examined extensively.40,41
Oxidation of thioanisole with singlet oxygen (Scheme 2a) to
methyl-phenyl-sulfoxide was evaluated and proved successful in
ethanol under O2 atmosphere, with a 1000-fold excess of the
reactant over the photocatalyst. After 8 h of reaction time, nearly
full conversion of the reactant was observed (Figure S16, Supporting
Information) in a clean reaction, where neither nameable amounts
of side products nor significant degradation of the catalyst was
observed. Further oxidation of the sulfoxide to the sulfone was only
observed in traces. Test reactions without catalyst or irradiation did
not lead to detectable product formation.
13
range correlation 1Hꢀ C HMBC spectrum, illustrated in Figure
S14b, Supporting Information, supports our tentative assignment
of the formation dioxo-open-chain tetrapyrrolic compounds.
Similar degradation products of iron corroles and porphyrins
catalyzed by heme oxygenases have been presented recently.
Nevertheless, additional ring-opening reactions at the meso-posi-
35
ꢀ
tions as reported by Swider et al. cannot be ruled out.
Additionally, a Bi-peroxo species could be assigned with mass
spectrometry as HOOꢀBi(TpFPC) (m/z = 1035.03) (Figure
S11, Supporting Information), which seems to represent an
intermediate in the course of the degradation of 3. Thisissupported
by the fact that no accumulation of the HOOꢀBi(TpFPC) species
is observed upon longer irradiation time, while the amount of the
open-chain product is growing constantly. Though it could not be
proven unambiguously, axial ligation of Bi with the peroxo moiety
would fit most logically into the reaction profile. In this context, it
is interesting to note that similar metalꢀhydroperoxo species are
discussed as critical intermediates formed in the course of
hemeꢀoxygenase-catalyzed tetrapyrrole ring cleavage.36 These
compound 0-type species represent an intermediate stage
Under similar conditions, oxidation of cyclohexene (“ene re-
action”) to the corrersponding allylic hydroperoxide (Scheme 2b)
was examined and monitored via 1H NMR spectroscopy (Figure
S17, Supporting Information). In this case, only partial conversion
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of the substrate to the desired product could be observed,
accompanied by significant degradation of the catalyst within the
8 h of reaction time.
Similar to the photocatalytic reactions observed for antimony
corroles, all evidence points towards singlet-oxygen as the exclu-
sive oxidant in these reactions performed with Bi(TpFPC), 3.
platinum wire as a counter electrode, and a silver/silver-chloride pseudo-
reference electrode. The materials were dissolved in acetonitrile with 0.1 M
TBAPF6 as the supporting electrolyte, and scanning rates of 50ꢀ200 mV/s
were applied. Ferrocene was employed as internal standard for potential
referencing, and the potentials were subsequently referenced vs SCE
accordingly.43
Spectroelectrochemistry was performed in a thin layer OTTLE cell26
with a platinum working electrode, a platinum wire as a counter elec-
trode, and a silver reference electrode. The corresponding UV absorp-
tion spectra were collected with a Jasco V670 Spectrometer.
’ CONCLUSION
In the present work, we have shown the synthesis of Bi(III)-5,
10,15-tris(pentafluorophenyl)corrole, Bi(TpFPC), 3, using a
novel procedure employing Bi{N(SiMe3)2}3 as the metalation
agent, which can be employed under very gentle conditions. The
crystal structure of the complex exhibits considerable out-of-
plane displacement of the central Bi atom and a slight out-of-
plane bending of the corrole macrocycle. Geometry optimiza-
tions employing the density functional theoretical approach gave
an almost identical structural motif and natural bond orbital
(NBOꢀ) analysis gave a central natural charge state of þ1.914e,
which means that BiꢀN(C) bonds in the corrole complex possess
a relatively large component of covalent bonding.
We found that with the selected corrole ligand system the first
two electrochemical oxidation processes occur on the macrocyclic
ring. Attempts for a chemical oxidation of the central bismuth(III)
ion promptly resulted in demetalation and degradation of the
corrole macrocycle.
Singlet-Oxygen Assay. Evaluation of photosensitized singlet
oxygen generation by Bi(TpfPC) was performed in ethanol solution
at room temperature, whereby degradation of the 1O2 scavenger (DPBF,
1,3-diphenylisobenzofurane) was monitored by the decline of the
411 nm absorption band. For all experiments the concentration of
DPBF was adjusted to 150 μmol, and experiments with different
sensitizer concentrations were performed. Rose Bengal was used as
reference photosensitizer (ϕΔ= 0.6838) in a way that the absorbance of
the main Q-band maximum of Bi(TpfPC) and the absorption maximum
of rose Bengal were set to approximately identical values. After aeration
of the samples, irradiation was performed with a 100 W high-pressure
mercury lamp using a Schott GG 495 nm cutoff filter. As a control, no
bleaching of DPBF occurred under anaerobic irradiation conditions.44
For the control experiments, NaN3 was additionally added in order to
inhibit singlet oxygen formation.
Crystal Structure Determination. Crystals of 2 and 3, mounted
on a Mitegen Micromount, were automatically centered on a Bruker
SMART X2S benchtop crystallographic system. Intensity measurements
were performed using monochromated (doubly curved silicon crystal)
Mo KR radiation (0.71073 Å) from a sealed microfocus tube. APEX2
software was used for preliminary determination of the unit cell.45
Determinations of integrated intensities and unit cell refinement were
performed using SAINT.46 Data were corrected for absorption effects
with SADABS using the multiscan technique.47 The structures were
solved by direct methods (SHELXS-97)48 and refined by full-matrix
least-squares on Fo2 (SHELXL-97).48,49 The interior hydrogen atoms of
2 were located using the difference map and refined isotropically without
restraint. All other H atoms were calculated geometrically, and a riding
model was applied during the refinement process. Crystals of the Bi
compound contain two molecules of chloroform, one of which is
disordered. The disorder could not be resolved successfully. Hence,
the program SQUEEZE,50 a part of the PLATON package of crystal-
lographic software, was used to calculate the solvent disorder area and
remove its contribution to the overall intensity data.
DFT Calculations. All calculations were performed using the
Gaussian 03 package version E02.51 Electronic structure calculations
were based on KohnꢀSham density functional theory (KS-DFT) with
Becke’s three-parameter hybrid functional (B3LYP)52,53 and a com-
pound basis set, where the Pople’s 6-311þG(d,p) basis sets were used
for C, H, N, F.54,55
For our system, we first performed a tight structural optimization,
followed by a frequency calculation to confirm that the optimized struc-
ture was indeed a minimum (with no imaginary frequencies). Single-
point frequency calculations and NBO analyses were carried out with
tight SCF convergence and ultrafine grids in the structural optimizations.
Los Alamos National Laboratory 2 (LANL2) relativistic effective core
potentials (RECPs)56 were used to describe the core electrons of Bi
atoms, and split-valence (double-ζ) basis sets were used to describe s-
and p-valence electrons of Bi. The LANL2DZ basis set was augmented
by adding one set of polarization functions on all atoms and one set of
diffuse functions on all non-hydrogen atoms. To gain insight into the
vertical singlet electronic states, time-dependent density functional theory57
(TD-PBE0 method) calculations were performed. Energies reported herein
Bi(TpFPC), 3, displayed photocatalytic activity (λ > 455 nm)
during oxidation of thioanisole and cyclohexene by singlet oxygen.
In order to broaden the scope of the presented metalation
procedure and to study the substituent effect toward catalytic and
photophysical properties, synthetic work on differently substi-
tuted Bi(III) A3-corroles and Bi(III) A2B-corroles is underway,
as well as extension of the metalation procedure on other main-
group metals.
’ MATERIALS AND METHODS
All chemicals were purchased from Sigma Aldrich or Acros Chemicals
and used without further purification. Reagent-grade solvents were
purchased from Fischer Chemicals and distilled prior to use. THF was
distilled over sodium and benzophenone under an argon atmosphere
and stored over molecular sieves (4 Å) upon use.
TLC was performed using Fluka silica gel (0.2 mm) on aluminum
plates. Silica gel columns for chromatography were prepared with silica gel
60 (0.060ꢀ0.20 mesh ASTM) from Acros. Proton (1H NMR) and
carbon (13C NMR) spectra were recorded on a Bruker DRX 500
spectrometer equipped with a cryogenically cooled probe (TXI) at 500
and 125.8 MHz, respectively, 19F NMR spectra were recorded ona Bruker
Avance 600 MHz or a Varian Inova 300 MHz spectrometer at 564.7 or
282.4 MHz. The chemical shifts are given in parts per million (ppm) on
the delta scale (δ) and are referenced to TMS (δ = 0 ppm) for 1H and
TFA for 19F. Mass spectra were collected on a Finnigan LCQ DecaXPplus
Ion trap Mass spectrometer with ESI ion source, and MALDI-TOF
measurements were collected with a Bruker Autoflex III Smartbeam
spectrometer, whereby a matrix:sample ratio of 10:1 was used. High-
resolution MS measurements were performed with a 6510 quadrupole/
time-of-flight (Q-TOF) instrument (Agilent, Palo Alto, CA) using direct
infusion of the sample solution at a flow rate of 40 μL/min by means of a
model 22syringe pump(Harvard Apparatus, South Natick, MA). UVꢀvis
absorption spectra were measured on a Varian CARY 300 Bio spectro-
photometer. All ε values are given in L molꢀ1cmꢀ1
.
Cyclic voltammetry was carried out under inert conditions in a
glovebox (CV) using an Eco Autolab-type potentiostat with a self-made
three-electrode cell consisting of a platinum working electrode, a
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were evaluated using the fourth-order MøllerꢀPlesset perturbation theory
[MP4(SDQ)] in combination with PBE0 parametrization.
’ ASSOCIATED CONTENT
S
Supporting Information. This material is available free
b
Synthesis of the Compounds. Tris(bis(trimethylsilyl)amido)-
bismuth Bi{N(SiMe3)2}3 (1). Synthesis in accordance to refs 22 and 23:
2.05 g (19.5 mmol) of BiCl3 dissolved in 20 mL of THF were added
dropwise to a solution of 3.27 g (6.5 mmol) of LiN(SiMe3)2 in 10 mL of
THF at 10 °C under inert atmosphere. The solution was stirred for 3 h,
during which the solution turned shimmering orange, followed by
evaporation of the solvent. Subsequently, the residue was dissolved in
pentane, filtered, and evaporated to dryness, yielding the pale yellow
solid product. Analytical data are consistent with literature values22
(2.31 g (3.34 mmol); 51.5% yield).
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Phone:þ43-732-2468-8811 (W.S.); þ43-732-2468-8800 (G.K.).
E-mail: wolfgang.schoefberger@jku.at (W.S.); guenther.knoer@
jku.at (G.K.).
Tris-Pentafluorophenylcorrole H3(TpFPC) (2). Synthesis in accor-
dance to ref 18: 1.52 g (8 mmol) of pentafluorobenzaldehyde was mixed
with 80 μL of a 10% TFA in CH2Cl2 and 840 μL (12 mmol) of pyrrole at
40 °C and vigorously stirred. After 10 min, 80 mL of CH2Cl2 and 2.2 g
(9.6 mmol) of DDQ in 16 mL of THF/toluene 1:1 were added and
stirred for further 5 min. Subsequently, the solution was passed over a
chromatography column (silica; hexane:dichloromethane 2:1) and
further purified by column chromatography using the same solvents
(230 mg (0.288 mmol); 10.8% yield). Crystals suitable for single-crystal
structure determination were obtained by slow evaporation of CHCl3.
Complementary data: 1H NMR (500 MHz, CDCl3, 25 °C): δ = 9.10
(d, J = 4.1 Hz, 2H, H2 þ H18), 8.76 (d, J = 4.5 Hz, 2H, H7 þ H13),
8.55ꢀ8.60 (4H, 2d overlapping, H8 þ H12 and H3 þ H17). 19F NMR
(564.7 MHz, CDCl3, 30 °C): ꢀ137.2 (2F), ꢀ137.7 (4F), ꢀ152.2 (2F),
ꢀ152.7 (1F), ꢀ161.4 (4F), ꢀ161.9 (2F). MS (ESIꢀ): m/z calcd for
C37H10F15N4, 795.07; found, 795.20 [M ꢀ H]ꢀ. UVꢀvis (THF):
λmax = 408, 418(sh), 563, 604 nm.
Bismuth tris-Pentafluorophenylcorrole Bi(TpFPC) (3). Under N2
atmosphere, 50 mg (0.063 mmol) of H3(TpFPC) (2) was dissolved
in 15 mL of abs. THF and 55 mg (0.078 mmol) of Bi{N(SiMe3)2}3 dis-
solved in 2 mL of THF were added dropwise under permanent stirring.
After 12 h at room temperature, during which the color of the reaction
mixture changed from dark violet to dark green and the intense red fluo-
rescence of the free-base corrole completely vanished (the reaction pro-
gress was followed with UVꢀvis spectroscopy), the solvent was
evaporated and the crude product was purified with column chroma-
tography (silica, hexane:dichloromethane 2:1) (45 mg (0.045 mmol);
71.3% yield).
’ ACKNOWLEDGMENT
Financial support of this work by the Austrian Science Fund
(FWF project P18384 “Solid state and Liquid NMR of biomo-
lecular Metalcomplexes” and FWF project P21045 “Bio-inspired
Multielectron Transfer Photosensitizers”) is gratefully acknowledged.
We wish to thank Dr. Roland Fischer for valuable discussion and
measurement of the 19F NMR spectra.
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