Lead corrole complexes in solution: Powerful multielectron transfer reagents for redox catalysis

Article abstract of DOI:10.1016/j.inoche.2010.06.047

The first observation of Pb(II)-complexes of corroles and their photoinduced oxidation to the corresponding Pb(IV)-derivatives is reported. These compounds display metal-centered redox chemistry in solution. Their potential for catalytic oxygen atom trans

Full text of DOI:10.1016/j.inoche.2010.06.047

Inorganic Chemistry Communications 13 (2010) 1187–1190
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Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Lead corrole complexes in solution: Powerful multielectron transfer reagents for
redox catalysis
Wolfgang Schöfberger a, Florian Lengwin a, Lorenz M. Reith a, Manuela List b, Günther Knör a,
⁎
a
Institut für Anorganische Chemie, Johannes Kepler Universität Linz (JKU), Altenbergerstr. 69, A-4040 Linz, Austria
b
Institut für Chemische Technologie Organischer Stoffe, Johannes Kepler Universität Linz (JKU) Altenbergerstr. 69, A-4040 Linz, Austria
a r t i c l e i n f o
a b s t r a c t
Article history:
The first observation of Pb(II)-complexes of corroles and their photoinduced oxidation to the corresponding
Pb(IV)-derivatives is reported. These compounds display metal-centered redox chemistry in solution. Their
potential for catalytic oxygen atom transfer processes is explored.
Received 13 April 2010
Accepted 30 June 2010
Available online 1 August 2010
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Lead complexes
Metal-oxo species
Corroles
Charge transfer
Electron transfer
Photochemistry
Metallocorroles are attractive candidates for redox catalysis, since
their specific electronic structure is very suitable to stabilize high-
valent oxidation states of the coordinated central metals [1].
Compared to the closely related porphyrin-based systems, where
the macrocyclic ring tends to be involved as a noninnocent ligand
forming π-radical species,[2] this feature determines the extraordi-
nary properties of oxygen atom transfer reagents such as manganese
(V)-oxo corroles for the epoxidation of alkenes[3] or the O–O bond
formation step in artificial photosynthetic water oxidation [4]. Quite
unique reactivity patterns have furthermore been observed, when
corrole complexes incorporate heavy-atom central ions [5].
In this context, the investigation of tetrapyrrole complexes with
high-valent main group metals including Pb(IV), Sb(V) or Bi(V) is of
considerable interest due to their intrinsic preference for multielectron
reactivity and their potential to form catalytically active oxo species [6].
Especially, the reversible photochemical formation of very strong
oxidants such as Pb(IV) compounds carrying hydroxo, oxo or
carboxylato groups would be an extremely desirable reaction, since it
is well established that high-valent lead compounds are useful reagents
for mechanistically demanding substrate conversions such as carbon–
carbon-bond formation [7] or electrocatalytic O2-evolution [8].
Interaction of an excess of the low-valent main group metal ions
with the free base corrole ligand in solution resulted in gradual
absorption spectral changes indicating the formation of a sitting-atop
complex, which was allowed to further react under an inert atmosphere
by stirring the mixture for several hours in the dark [10]. The occurrence
of a diagnostic p-type hyper electronic spectrum [11] with band maxima
at 310, 415, 450 and 680 nm including a “split” Soret band served as an
indicator for the presence of a Pb(II) complex of triphenylcorrole. While
at this stage the reaction product was very sensitive to demetallation
and all attempts to completely remove excess salts initially failed, the
obtained NMR- and high-resolution mass spectral data unambiguously
displayed a set of signals and a characteristic isotope pattern consistent
with a mononuclear TPC-compound (Fig. 1).
Interestingly, the positive ion ESI-signal at m/z=731.01 corre-
sponds to the cationic species (TPC)Pb+, which indicates the possibility
We decided to explore this possibility and studied the metallation of
triphenylcorrole[9] H3(TPC) 1 with various Pb(II) salts (Scheme 1)[10].
⁎
Corresponding author. Tel.: +43 732 2468 8800; fax: +43 732 2468 9681.
E-mail address: guenther.knoer@jku.at (G. Knör).
Scheme 1.
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.06.047

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W. Schöfberger et al. / Inorganic Chemistry Communications 13 (2010) 1187–1190
Fig. 1. (a) 1H-NMR spectrum of (TPC)Pb− in CD3OH at 25°C. (b) Electrospray mass spectrum of lead triphenylcorrole. Insert: calculated istope pattern for C37H23N4Pb(M+):
m/z =731.176).
of a metal-centered redox reaction to reach the highly oxidizing Pb(IV)-
state within the intact macrocycle. Such a formation of cationic species
from anionic complexes of divalent metals is an unusual but well-
known phenomenon in metallocorrole mass spectrometry, which was
recently studied in some detail [12].
In addition to this reaction occuring in the mass spectrometer, the
oxidation of the low-valent TPC-complex could also be induced
photochemically. When solutions of Pb(II)-triphenylcorrole were
excited with monochromatic UV-light in the spectral region of the
metal-centeredsp-electronic transitions of the main-group central atom
[14], the bright green colour of the complex caused by the prominent Q-
band absorption at around 680 nm rapidly changed into a brownish-
green. At the same time, within a few minutes of light-exposure, the
irregular split Soret-band feature and the UV-maxima corresponding to
the presence of divalent lead disappeared (Fig. 2). The photoproduct
spectra in EtOH were characterized by a broadened Soret band around

W. Schöfberger et al. / Inorganic Chemistry Communications 13 (2010) 1187–1190
1189
Fig. 3. Qualitative MO-diagram for (a) low-valent and (b) high-valent lead corrole
complexes. IL: Intraligand(ππ*) transitionsinvolving electronslocalized at the macrocyclic
ring; MC: Metal centered sp-excitation; MLCT: Metal-to-ligand charge transfer-, and
LMCT: ligand-to-metal charge transfer transitions.
Fig. 2. Spectral changes during the 313nm photolysis of (TPC)Pb−(2) in ethanol at (a) 0, 30,
90, 150, 180, 210, 240, 270, and (i) 300 s reaction times (1 cm cell, 298K, aerobic conditions).
Upon complexation with divalent lead salts, an additional dipole-
allowed metal-to-ligand charge transfer (MLCT) transition occurs
(Fig. 3a), which shows substantial mixing with the intraligand (IL)
bands of the corrole chromophore resulting in an irregular hyper-type
spectrum [11,13]. The presence of a significantly Pb(6 s)-localized
HOMO in the frontier orbital region of the corrole ligand is also
consistent with the results of supporting DFT-calculations. Due to the
large ionic radius of Pb2+ (112 pm)[15] and the contracted tetrapyr-
role core of the triphenylcorrole ligand, a considerable out-of-plane
displacement of the main group metal is expected in the initial sitting-
atop complex, which may explain the tendency for demetallation at
this stage of the reaction. Irradiation in the region of the metal-
centered (MC) bands of the compound removes electron density from
the stereochemically active 6 s lone-pair and leads to photo-metalla-
tion and oxidation. In the photoproduct, which is ascribed to a Pb(IV)-
species, the MLCT bands are no longer present, but a novel ligand-to-
metal charge transfer (LMCT) band is possible due to the availability
of an empty Pb-centered orbital in the tetrapyrrole frontier orbital
region (Fig. 3b). In agreement with the relative intensity and
energetic position, the 505 nm band of the photoproduct (Fig. 2) is
tentatively ascribed to such an allowed LMCT-transition in Pb(IV)
triphenylcorrole, which shows some mixing with the four-orbital
ππ*-transitions of the tetrapyrrole macrocycle.
410 nm, a flat Q-band pattern in the 550–750 nm region, and a
conspicuous new absorption band with a maximum at 505 nm.
Progress of the light-driven reaction was simultaneously followed by
means of mass spectroscopy, which allowed a tentative assignment of the
photogenerated compound as a solvated (TCP)Pb( O) complex due to the
occurrence of diagnostic Pb-signals and fragmentation peaks. Besides the
fragment (TPC)Pb+ with m/z=731.01, the species identified according to
their characteristic isotope pattern include M+·EtOH, M+·H2O and M+,
with M=(TPC)Pb( O) and m/z=793.21, 765.17 and 747.16, respectively.
When the synthesis and the photo-oxidation were alternatively carried
out in MeOH solution, the corresponding (TPC)Pb( O) methanol adducts
such as M+·MeOH wit m/z=779.12 were obtained instead. In all cases,
the presence of sharp tetrapyrrole signals in the aromatic region of the 1H-
NMR spectra and the absence of any free base NH proton signals was
consistent with the formation of diamagnetic metallocorrole species
showing a singlet ground state. The occurrence of several isosbestic points
in the initial phase of the photolysis together with the mass- and NMR-
spectroscopic data of the irradiated samples indicated the clean formation
of the high-valent Pb(IV)-oxo complex of triphenylcorrole with obviously
substitution-labile O-atom donor ligands such as additional solvent
molecules in the axial coordination sphere. Interestingly, aqueous workup
of the irradiated samples and extraction with diethyl ether always
resulted in the occurrence of a novel main peak with m/z=800.43 in the
high resolution mass spectra, indicating the formation of a modified
product with an additional OH-group attached to the molecule, which in
analogy to other high-valent metallocorrole oxo complexes4 is tentatively
assigned to a nucleophilic adduct such as (TPC)PbOOH·2H2O carrying an
axial hydroperoxo ligand [14].
Interestingly, in the course of the Pb(II) to Pb(IV)-conversion, also
a broad metal-to-metal charge transfer (MMCT) band occurs because
of the intermediate presence of both reducing (TPC)Pb− and oxidizing
(TPC)Pb+ metal complexes in solution. Due to the stabilization of the
Pb(IV) state within the corrole ligand, the maximum of this MMCT-
band at around 600 nm (Fig. 2) is red-shifted compared to the
corresponding band in simple mixed-valent chloro complexes of Pb
with a reported MMCT-maximum at 2.51 eV (494 nm) [16].
Although the spectrum of the oxidized compounds gradually
changes within several days in methanol solution, thus indicating a
currently unknown degradation process, the formation of the Pb(IV)
triphenylcorrole species may also be driven as a reversible reaction.
When the high-valent complex is allowed to react with typical oxygen-
atom transfer acceptors in solution, a gradual regeneration of the
characteristic hyper-type electronic spectrum of the low-valent Pb(II)
complex is observed in the course of a slow dark reaction. In the case of
triphenylphosphine as a substrate, this process was followed by 31P-
NMR spectroscopy, which clearly demonstrated the gradual formation
of Ph3P O as the corresponding reaction product. Within the scope of our
present study, no attempts were made yet to accelerate this type of
back-reactions by light, closing a photocatalytic redox cycle.
Oxidation of the central metal to the high-valent state Pb4+ should
be accompanied by a significant contraction to 79 pm [15], thus
reaching a similar size as high-spin Fe2+, which still requires an out-of
plane structure in tetrapyrroles, but readily accepts additional axial
ligands. This is in good agreement with the observation of a quite stable
(TPC)Pb+ core and the occurrence of several different lead(IV)-oxo
species with attached solvent molecules in the coordination sphere. The
large structural changes between low- and high-valent complexes may
also serve to explain their rather slow thermal interconversion rate even
in the presence of potential substrate molecules, since substantial
reorganizational energy contributions are probably influencing the net
two-electron transfer sequence.
In summary, the first examples of lead corroles are presented.
Unlike all the closely related tetrapyrrole derivatives known so far,
these compounds display metal-centered redox chemistry in solution.
The spectroscopic features and reactivity patterns of the lead TPC
complexes can be best rationalized by the following considerations.

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W. Schöfberger et al. / Inorganic Chemistry Communications 13 (2010) 1187–1190
[4] Y. Gao, T. Åckermark, J. Liu, L. Sun, B. Åckermark, J. Am. Chem. Soc. 131 (2009)
8726.
[5] Reports on other metallocorroles with “heavy” 5d and 6 s central atoms are still limited
to Re, Ir and Bi:
Their potential for photoredox catalysis and oxygen atom transfer
processes seems very promising. Further investigations are currently
underway which will focus on a more detailed characterization of
the high-valent species and on the photocatalytic reactivity of these
interesting multielectron transfer photosensitizers.
(a) J.H. Palmer, A. Mahammed, K.M. Lancaster, Z. Gross, H.B. Gray, Inorg. Chem. 48
(2009) 9308 and references cited therein;
(b) K.M. Kadish, C. Erben, Z. Ou, V.A. Adaminan, S. Will, E. Vogel, Inorg. Chem. 39
(2000) 3312.
[6] G. Knör, Chem. Eur. J. 15 (2009) 568.
Acknowledgements
[7] M.G. Moloney, Main Group Met. Chem. 24 (2001) 653.
[8] R. Amadelli, L. Armelao, A.B. Velichenko, N.V. Nikolenko, D.V. Girenko, S.V.
Kovalyov, F.I. Danilov, Electrochim. Acta 45 (1999) 713.
[9] B. Koszarna, D.T. Gryko, J. Org. Chem. 71 (2006) 3707.
[10] In a typical procedure, 39 mg triphenylcorrole and 26 mg tetrabutyl ammonium
tetrafluoroborate were dissolved in 50 ml of absolute ethanol. 370 mg lead nitrate
were added, and the resulting suspension was stirred for 15 h at 45 °C under a
nitrogen atmosphere. Photoinduced oxidation was performed with 313 nm light
from a 1 kW Hg/Xe lamp equipped with a Schoeffel high-intensity monochromator.
[11] P. Sayer, M. Gouterman, C.R. Connell, Acc. Chem. Res. 15 (1982) 73 and references
cited therein.
Financial support of this work by the Austrian Science Fund (FWF
project P21045: “Bio-inspired Multielectron Transfer Photosensitizers”
and FWF project P18384: “Solid state and Liquid NMR of biomolecular
Metalcomplexes”) is gratefully acknowledged.
Appendix A. Supplementary material
Electronic Supplementary Information (ESI) available: further exper-
imental details, spectroscopic data and electronic structure calculations at
the DFT-level. Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.inoche.2010.06.047.
[12] J.F.B. Barata, C.M. Barros, M.G.O. Santana-Marques, M.G.P.M.S. Neves, M.A.F.
Faustino, A.C. Tomé, A.J. Ferrer Correia, J.A.S. Cavaleiro, J. Mass Spectr. 42 (2006)
225.
[13] G. Knör, A. Vogler, Inorg. Chem. 33 (1994) 314.
[14] Alternatively,
a six-coordinate oxo-hydroxo Pb(IV)-species (TPC)Pb( O)
(OH)·2H2O or a hydroxyphlorin type product resulting from nucleophilic attack
of hydroxide at the meso-carbon of the dipyrrine subunit of the corrole ligand are
also compatible with a calc. m/z=800.18.
References
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(1976) 751.
[16] R.D. Cannon, Electron Transfer Reactions, Butterworths, Sydney, 1980 p. 309.
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