10-Heterocorroles: Ring-contracted porphyrinoids with fine-tuned aromatic and metal-binding properties

Article abstract of DOI:10.1002/anie.201300757

A homologous series of ring-contracted corrinoids, the 10-heterocorroles H₂(XCor), is now available as the free-base ligand set. While the higher homologues, 10-thia- and 10-selenacorrole, show a porphyrin-like spectroscopic behavior, 10-oxacorrole displays lower aromatic character and has the smallest N₄ cavity of all free-base porphyrinoids synthesized so far, smaller even than those of corrole and porphycene ligands (see picture). Copyright

Full text of DOI:10.1002/anie.201300757

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DOI: 10.1002/anie.201300757
Contracted Porphyrinoids
10-Heterocorroles: Ring-Contracted Porphyrinoids with Fine-Tuned
Aromatic and Metal-Binding Properties**
Dimitri Sakow, Birte Bçker, Kai Brandhorst, Olaf Burghaus, and Martin Brçring*
A current trend in porphyrinoid chemistry is the development
of ring-contracted macrocycles and the study of their metal
complexes.[1] Corroles clearly play a major role in this field,
particularly with respect to species containing a porphyrin-
like N4 metal binding site.[2] However, many other non-
contracted porphyrinoids, such as the porphyrin isomers and
the azaporphyrins, are in fact also characterized by a smaller
N4 cavity.[3] Reports from coordination compounds of such
macrocycles clearly prove the importance of the size of the
metal binding site for spectacular results in terms of electronic
structure,[4] small-molecule binding,[5] and catalysis.[6]
We are interested in free-base 10-heterocorroles with
a Group 16 element (O, S, Se) in the backbone, as such ligands
may allow a fine-tuning of the electronic properties of the
transition metal through the size of the N4 cavity. For this
purpose, a general and efficient entry into this class of
porphyrinoids (or better: corrinoids) was needed. We report
here the successful syntheses and ligand properties of free-
base 10-oxa-, 10-thia-, and 10-selenacorroles.
The known synthetic protocols leading to 10-heterocor-
roles are only of limited use for a general preparative entry
into this class of porphyrinoids, as they either require a more
than stoichiometric amount of supported palladium as
a reagent, or a thioether precursor, the oxygen and selenium
homologues of which are unavailable. To overcome these
drawbacks we attempted to develop a metal-promoted two-
step macrocyclization approach from a,w-dibrominated
dipyrrins such as 1 (Scheme 2). These precursors were first
coupled to bromine-terminated linear tetrapyrrole copper(II)
chelates by using a nBuLi/CuII protocol, and then cyclized in
a second step to the desired copper(II) complexes such as 2–4,
by using different nucleophilic O, S, or Se transfer reagents,
respectively. During the course of our investigations it
Conceptually, the class of 10-heterocorroles can be
regarded as intermediate between porphyrins (dianionic
ligand) and corroles (ring-contracted ligand; Scheme 1). 10-
Scheme 1. 10-Heterocorroles H2(XCor) as hybrid structures from por-
phyrin and corrole frameworks (with the 18p main conjugation path-
ways shown in bold).
Heterocorroles with an oxygen, sulfur, or nitrogen atom in the
backbone have been known for a long time,[7] and have
recently gained renewed interest.[8] However, the literature
procedures to synthesize these macrocycles are either long
and cumbersome, or yield chelate complexes of Group 10
elements which cannot be demetalated without disintegration
of the porphyrinoid ligand.
[*] D. Sakow, B. Bçker, Dr. K. Brandhorst, Prof. Dr. M. Brçring
Institut fꢀr Anorganische und Analytische Chemie
TU Braunschweig
Hagenring 30, 38106 Braunschweig (Germany)
E-mail: m.broering@tu-bs.de
Homepage:
http://www.tu-braunschweig.de/iaac/personal/broering
Dr. O. Burghaus
Fachbereich Chemie, Philipps-Universitꢁt Marburg
Hans-Meerwein-Strasse, 35032 Marburg (Germany)
Scheme 2. Syntheses of 10-heterocorroles H2L 5–7 via the copper(II)
complexes 2–4; a) Cu(OAc)2·H2O, wet DMF, 1108C, 15 min;
b) Cu(OAc)2·H2O, DMF, 5 min, then Na2S·9H2O, 1108C, 15 min, then
TFA, toluene, 1108C, 3 h; c) Cu(OAc)2·H2O, THF, 15 min, then KSeCN,
658C, 12 h; d) SnCl2·2H2O, HCl(aq), acetonitrile/dichloromethane,
15 min. TFA=trifluoroacetic acid.
[**] We gratefully acknowledge support for this work by the Deutsche
Forschungsgemeinschaft (DFG).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300757.
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became apparent that the first step is not always necessary. In
fact, heating the dipyrromethene 1 with copper(II) acetate in
wet DMF for 15 minutes results in the formation of the
copper–10-oxacorrole 2, without the observation of any linear
tetrapyrrole intermediate. After reductive demetalation with
SnCl2·2H2O/HCl in a mixture of acetonitrile and dichloro-
methane,[9] the desired 10-oxacorrole 5 was isolated as a pure
compound in an overall yield of 39% (Scheme 2). Treatment
of 1 with copper acetate in DMF, followed by the addition of
four equivalents of sodium sulfide, leads to the formation of
several sulfur-containing macrocycles, as evident by mass
spectrometry. Treatment of this mixture with additional
copper acetate in a THF/methanol mixture, followed by
trifluoroacetic acid in boiling toluene, and subsequent reduc-
tive demetalation results in the formation of the free-base 10-
thiacorrole 6 as the exclusive product (16% overall yield;
Scheme 2). Potassium selenocyanate in THF was found to
serve as the reagent of choice for the introduction of
a selenium atom into the ligand backbone. The product
mixture from the treatment of 1 with copper acetate and
KSeCN can be purified by flash chromatography, and affords
the pure copper chelate 4. Chelate 4 is finally demetalated as
above under reductive conditions to yield 42% of the desired
10-selenacorrole 7. All free-base compounds are obtained in
analytical pure form after chromatography and recrystalliza-
tion.
À
Mechanistically, the formation of the direct C C bonds
without any apparent reductive coupling appears particularly
interesting. In the case of 10-thiacorrole, a thorough mech-
anistic and theoretic investigation of this process was recently
published.[8d] According to these results, the reaction proceeds
via the initial formation of a 5,15-dithiaporphyrin, from which
the 10-thiacorrole is formed by sulfur extrusion. A similar
reaction pathway also appears plausible for the formation of
the 10-selenacorrole derivative 4.
Crystallographic analyses of the 10-heterocorroles 5–7
have been undertaken, and the resulting molecular structures
are presented in Figure 1. The measurements reveal basically
planar macrocycles with small saddle-shaped distortions. This
nonplanar conformation is accompanied by the binding of
both inner hydrogen atoms at the bipyrrolic N-donor atoms,
and attenuated by the sterically encumbered substitution
À
À
pattern of the macrocycles. The C C and C N bonds within
the five-membered rings clearly distinguish two pyrrolic and
two pyrrolenine-like C4N subunits through alternating bond
lengths (see the Supporting Information). This feature is
typically observed in tetrapyrrolic macrocycles, although,
with the exception of corrphycene, a transoid arrangement of
the NH moieties is predominant. The heteroatom X at the 10-
Figure 1. Molecular structures of the free-base macrocycles 5–7 from
single-crystal X-ray diffraction studies (top views, and front views
without substituents). Thermal ellipsoids are set at 50% probability.
Carbon-bound hydrogen atoms are omitted for clarity.
À
position is bound almost symmetrically with C X bond
lengths of 1.360(2)/1.361(2) ꢀ (5, X = O), 1.730(3)/1.739(3) ꢀ
(6, X = S), and 1.880(3)/1.887(3) ꢀ (7, X = Se) in the ligand
backbone, and the C-X-C angle at this position decreases
from 125.0(1)8 (5, X = O) to 112.8(1)8 (6, X = S) to 110.3(1)8
(7, X = Se), as expected. In combination, these changes result
in a continuous increase in the size of the N4 cavity from 5 to 6
to 7 (see below), and in a progressively trapezoid shape.
Corroles also show a nonplanar macrocyclic structure because
of the overcrowded center carrying three hydrogen atoms.[10]
The optical spectra of the 10-heterocorroles 5–7 show an
unexpected feature. As depicted in Figure 2, the typical
porphyrinoid spectrum, which is also found for corroles[11] and
azacorroles,[7a] with a strong Soret band at about 400 nm and
several weaker Q bands around 580 nm is only observed for
the higher homologues 6 and 7. For the 10-oxacorrole 5,
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The influence of the N4 cavity size of the 10-heterocor-
roles 5–7 on the metal–ligand interaction was investigated for
the copper complexes 2–4. The optical spectra of the copper
chelates suggest the same trend for the electronic properties
of the ligands as for the free-base macrocycles, that is, the
aromatic character increases in the order O ! Se < S (see the
Supporting Information). Q-band EPR spectra of the three
complexes (Figure 3), however, reveal an increasing rhom-
bicity and the development of the ACu hyperfine interaction of
the z component of the g tensor from 242 G (2, X = O) over
228 G (3, X = S) to 221 G (4, X = Se). These data clearly
follow the order of the cavity size. Apparently, the metal
properties are governed largely by the steric properties of the
macrocycle cavity, and not (or to a much lesser extent) by the
electronic properties of the ligand.
Figure 2. UV/Vis spectra (CH2Cl2) of the 10-heterocorroles 5 (X=O;
c), 6 (X=S; b), and 7 (X=Se; g).
The special nature of 10-heterocorroles as ligands for
metal complexes is particularly evident from a comparison of
the N4 cavity sizes[17] [7.05 ꢀ2 (5, X = O), 7.57 ꢀ2 (6, X = S),
and 7.77 ꢀ2 (7, X = Se)] with those of the well-established
corrole and of the porphyrin isomers.[18] For comparison, the
respective values found in a series of octaethyl derivatives of
porphyrin and its isomers are 8.50 ꢀ2 [porphyrin(1.1.1.1)],
8.27 ꢀ2 [corrphycene(2.1.0.1)], 8.23 ꢀ2 [hemiporphycene-
(2.1.1.0)], 7.64 ꢀ2 [porphycene(2.0.2.0)], and 7.41 ꢀ2 for an
octaalkylcorrole.[10,19] Iron and manganese complexes of the
smallest of these ligands, that is, of porphycene and corrole,
have so far provided the most spectacular results, such as non-
innocent behavior,[20] stabilization of uncommon electronic
states,[21] strong O2 binding and CO discrimination,[22] and
efficient O-transfer catalysis.[23] We expect the 10-heterocor-
however, the broadened bands and in particular the blue-
shifted and relatively intense Q bands indicate diminished
aromaticity and significant polyenic character, similar to
linear tetrapyrrole metal chelates.[12] 1H NMR spectroscopy is
only of limited value for the characterization of 5–7 because
of the high degree of substitution. For this reason the
octaethyl-10-heterocorroles 5’–7’ were prepared by analogous
reactions (see the Supporting Information), and the chemical
shifts of the meso-situated as well as the N-bound protons
were determined from these derivatives.
Table 1 presents the results from these measurements on
5’–7’. The chemical shifts of both the NH protons and the
meso protons are in agreement with the 10-thiacorrole 6’
being the compound with the highest degree of aromaticity,
followed by the 10-selena derivative 7’ and the 10-oxacorrole
5’. Interestingly, an analogous trend (S > Se > O) has long
been known for the five-membered ring aromatic compounds
furan, thiophene, and selenophene.[13] As the measured CH
and NH resonances of 5’–7’ are also in good agreement with
those predicted by GIAO NMR calculations[14] on the
unsubstituted heterocorroles (Table 1; for details see the
Supporting Information), we assume that the p conjugation of
the macrocycle and thus the aromatic character increases in
the order O ! Se < S. A similar conclusion has been drawn
recently from NICS calculations on nickel complexes of
meso-phenyl-10-oxacorrole and -thiacorrole, although the
overall aromatic character appears to be more pronounced in
the metal chelates.[1c,8d] As is apparent from recent publica-
tions, aromaticity in porphyrin and related nonbenzoid
macrocycles is still an unsolved problem and a topic of
current interest.[15] The fine-tuned aromatic behavior of the
10-heterocorroles described here may therefore be a welcome
subject for future in-depth theoretical studies.
Table 1: Selected 1H NMR chemical shifts d (ppm) of the octaethyl
derivatives 5’–7’ (CDCl3, 200 MHz).[a]
Compound
H2(OCor) 5’
H2(SCor) 6’
H2(SeCor) 7’
meso-CH
NH
8.28 (9.01)
7.07 (6.80)
8.91 (9.58)
3.60 (4.19)
8.55 (9.33)
5.00 (5.08)
Figure 3. Q-band EPR spectra of copper–10-heterocorroles 2–4 in
frozen toluene at 100 K (34.22 GHz, 80 mW/19 dB, modulation fre-
quency: 100 kHz, modulation amplitude: 0.4 mT; c) and simulated
spectra (program hyperfinespectrum,[16] g).
[a] Calculated values for unsubstitued 10-heterocorroles are shown in
parentheses (GIAO, B3LYP/6-311+G(2d,p), CHCl3).
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Received: January 28, 2013
Revised: March 4, 2013
Published online: April 8, 2013
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Keywords: 10-heterocorroles · aromaticity · copper · N ligands ·
porphyrinoids
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