Helical bis(N-confused porphyrins) with subunits fused by double orthometalation with platinum: Adaptability of an apparently rigid system

Article abstract of DOI:10.1002/anie.200904289

Flexible yet rigid: The flexibility of the porphyrin rings makes racemization or chiral induction still possible with helical metalated bis(N-confused porphyrin) ystems, even though the bipyrrole fragmen itself is rigid. The porphyrins are peripherlly dou

Full text of DOI:10.1002/anie.200904289

Communications
DOI: 10.1002/anie.200904289
Porphyrinoids
Helical Bis(N-Confused Porphyrins) with Subunits Fused by Double
Orthometalation with Platinum: Adaptability of an Apparently Rigid
System**
Piotr J. Chmielewski,* Bartosz Durlej, Marta Siczek, and Ludmiła Szterenberg
Modifications of the interior or periphery of porphyrins allow
major alteration or fine tuning of the properties of the system,
such as cation- or anion-binding ability, redox potential,
absorption, emission, as well as many other properties.
Porphyrin subunits can be also relatively easily linked into
larger two- and three-dimensional arrays, thus extending the
possibility of their application as biomimetic models, cata-
lysts, and materials for the transport of charge, molecules, and
ions. Among the covalently[1] and coordinately[2] linked
oligomers, the most attractive class is constituted by the
systems in which the subunits are directly linked.[3] The
interaction between delocalized p-bond systems appears to be
strongest when the b-pyrrole carbon atoms of the porphyrins
are linked. An important feature of b–b-linked bis(porphyr-
ins) is their intrinsic axial chirality. However, the stability of
the configuration requires restriction of the rotation around
the b–b-bond. This restriction can be provided by the
appropriate choice of metal ion that coordinates within both
macrocyclic cores.[4] The external nitrogen atom of the
confused pyrrole in N-confused porphyrin (NCP) 1[5] and
some of its derivatives[6] and complexes[7] can act as a donor
site.[8] In the readily obtainable bis(N-confused porphyrin)
2,[9] a cis-oriented system of two external nitrogen atoms can
Reaction of bis(NCP) 2 with [Pt(PhCN)Cl2] yields two
isomeric platinum-containing complexes with different coor-
dination modes (Scheme 1). Heating 2 at reflux with an
equimolar amount of [Pt(PhCN)2Cl2] in the presence of
potassium carbonate in toluene for two days resulted in
Scheme 1. External and internal metalation of bis(NCP) 2 with platinum(II),
insertion of silver(III) into the externally platinated bis(NCP) 3, and oxidative
addition of iodine or iodoalkyls to 3.
formation of 3 as the main product (about 50%), as well as
the monoplatinated insertion product 4 (10%). In both cases,
the composition of the complex was established by ESI MS
(m/z 1532), which indicates the presence of one platinum ion
and one bis(NCP) unit. Compound 3 constitutes the first
example of a bis(porphyrinoid) complex that consists of a
metal ion coordinated by double orthometalation.[10] The
twofold symmetry of the 1H NMR spectrum of 3 (CDCl3,
213 K) reveals the same coordination mode for both subunits.
The higher-field signals of protons bound to the internal
carbon atom C21 at d = À3.41 ppm and those attached to the
internal nitrogen atoms N22 and N24 (d = 1.19 and d =
À0.05 ppm, respectively) indicate that the porphyrin cores
are not occupied by metal ions. A doublet signal at d =
potentially serve as a chelating ligand, and coordination of an
inert metal ion in this site may prevent rotation and preserve
configuration of the molecule. Platinum(II) is thus a metal ion
of choice because of its tendency to form inert cis complexes
that consist of a five-membered chelate ring.
[*] Prof. P. J. Chmielewski, B. Durlej, M. Siczek, Dr. L. Szterenberg
Department of Chemistry,University of Wrocław
F. Joliot-Curie Street 14,50 383 Wrocław (Poland)
Fax: (+48)71-32-82348
E-mail: pjc@wchuwr.pl
[**] This work was supported by the Ministry of Science and Higher
Education under Grants PBZ-KBN-118/T09/2004 and N N204
029035.
4
8.10 ppm with J = 1.8 Hz is attributed to the meta proton in
the vicinity of the deprotonated coordinating carbon atom of
the para-tolyl substituents on C20 of both subunits. This
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200904289.
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observation is consistent with orthometalation of the tolyl
rings,[10b] while the upfield shift of the 5-tolyl ortho, meta, and
methyl proton signals (d = 6.59, 5.28, and 0.64 ppm, respec-
tively) indicate that orientation of the subunits is similar to
that observed in the free ligand 2.[4c,9]
meta protons of the orthometalated aryl groups indicate 1H–
195Pt coupling (3JHPt = 31 and 36 Hz). A 13C–195Pt coupling
(1JCPt = 638 Hz) can be observed for the methyl carbon atom
in the 1H–13C HSQC spectrum. Similar features are also
present in the spectra of 6b and 6c (Figure 1). Oxidative
addition of iodine to 3 proceeded immediately after mixing
the starting materials. The high-resolution ESI mass spectrum
of the product 6d (m/z 1786.3645) reveals the presence of two
1
The H NMR spectrum of 4 (CDCl3, 213 K) indicates an
asymmetric structure of the system. The spectrum consists of
features typical of a free-base NCP (internal CH and NH
signals at d = À4.91, À2.47, and À2.07 ppm) and those of its
complexes with divalent Group 10 metals (signal for 2-NH at
d = 10.46 ppm).[5a,8a,10b] The geometry of the molecular skel-
eton of 4 is not drastically altered with respect to that of 2 or 3,
as can be inferred from the upfield shifts of some of the
protons caused by the aromatic ring current of the neighbor-
ing subunits.[9,11]
Reaction of 3 with silver(I) acetate under very mild
conditions yields an insertion product 5, which consists of
macrocyclic subunits that are both metalated with silver(III)
(Scheme 1). A peak at m/z 1743.36 in the MALDI spectrum
of 5 is consistent with its composition, while NMR spectra
reveal persistence of the twofold symmetry. The lack of the
“internal” protons and spin–spin coupling of b-pyrrole
protons with 107,109Ag indicate coordination of diamagnetic
d8 silver(III) ions within the porphyrin cores.[7b,11,12]
1
iodide ligands per molecule. The H NMR spectrum of 6d
resembles the spectra of the iodoalkylated species, except for
the obvious lack of the alkyl signals and appearance of only
one set of the ligand resonances, owing to the twofold
symmetry of the bis(iodide) complex.
The single-crystal X-ray diffraction analysis[14] of 6a
reveals the coordination of both methyl and iodide groups,
as well as two external nitrogen and two ortho carbon atoms,
to the metal center (Figure 2). The equatorial environment of
An oxidative addition[13] of methyl iodide to 3 provided
the iodomethylated complex 6a quantitatively within one
hour at room temperature, although for 2-iodoethanol, two
days were required under the same conditions to obtain 6b
and, for the reaction with 1-iodo-2-(S)-methylbutane, heating
of a toluene solution at 908C for about 20 hours was necessary
to obtain 6c in good yield (Scheme 1). The original twofold
symmetry of 3 is lost in the iodoalkylated complexes because
of the non-equivalency of the faces of the compounds, and is
1
reflected by the H NMR spectra of 6a–c, as each spectrum
consists of two sets of NCP signals (Figure 1). The unique
resonance of an apical methyl group at d = 1.71 ppm in the
spectrum of 6a is accompanied by two broader platinum
2
satellites spaced by JHPt = 70 Hz. Also, signals for the two
Figure 2. Stick representations of the molecular structure of 6a. The
position of the methyl ligand is marked with a sphere. All hydrogen
atoms are omitted. The lower view shows the stretched Z shape of the
molecule; all aryl groups are omitted except for the orthometalated
groups.
the platinum ion is essentially planar, with donor atoms lying
alternately over and under the mean plane defined by the
N2C2Pt atom system, with displacements of less than Æ 0.1 ꢀ.
The whole system is helical and the unit cell contains a
symmetry-dependent pair of enantiomers. Similar structural
features are observed for the bis(iodo) complex 6d (see the
Supporting Information).[14] The moleculeꢁs skeleton adopts a
stretched Z shape, in which the porphyrin subunits are almost
parallel to each other with a dihedral angle between the mean
planes of only 1.91(7)8 in 6a and 5.1(1)8 in 6d, and a dihedral
angle of about 558 between these planes and the N2C2Pt plane
in both complexes.
1
Figure 1. Low- and high-field regions of the H NMR spectra
(600 MHz, CDCl3, 300 K) of platinum(IV) complexes 6a (a), 6b (b),
6c, (c), and 6d (d). Positions of the meta-proton resonances of the
orthometalated para-tolyl groups (20m’) are joined by the curved lines.
The insets in spectrum (a) (6a) show expansions of the signals of
20m’ and the apical methyl protons with platinum satellites. The
asterisks above spectrum (c) indicate positions of some of the
resonances of the minor diastereomer of 6c.
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The optical spectra of the macrocycles are strongly
details). Racemization upon crystal growth at room temper-
ature from the solution of enantiopure 6a prevents assign-
ment of the absolute configuration of the complex by X-ray
crystallography, but indicates the flexibility of systems that
contain platinum(IV) complexes.
affected by the external coordination of the platinum ion
(see Figure S51 in the Supporting Information). The broad
band centered at 982 nm observed for 3 is absent in the
spectrum of isomeric 4 or the ligand 2. Also, the spectra of the
platinum(IV) complexes 6a–e exhibit relatively strong bands
between 800 and 1000 nm. These features, together with the
considerable splitting of the Soret-type band in the spectra of
3, 5, and 6 suggest the presence of more effective interactions
between the p-electron systems of aromatic subunits in these
complexes than in the nonfused 2 or 4.
Systems 2–6 are intrinsically chiral because of their helical
structures. The configuration stability is provided by fusion of
the subunits with coordinated platinum ions in 3, 5, and 6. We
succeeded in enantiomer separation for the systems 3 and 5 by
means of enantiomer-resolving HPLC, and obtained about
96% ee for each fraction. The nonracemic 6a or 6d could be
obtained by addition of MeI or iodine, respectively to the
separated enantiomers of 3. Metalation of the enantiomers of
3 with silver(III) leads to the respective stereoisomers of 5.
The sign of the Cotton effect in the CD spectra of each
enantiomer of 3 is preserved in those of derivatives 6a and 6d
(Figure 3), thus indicating helicity retention upon oxidative
The unexpected flexibility of the platinum-fused bis-
(NCP) is reflected by its accommodation of a chiral carboan-
ion to form 6c. The reaction of a racemic solution of 3 with
enantiopure 1-iodo-2-(S)-methyl-butane was monitored by
1H NMR spectroscopy and proceeded slowly at room temper-
ature to give initially an equimolar mixture of diastereomers
of 6c (about 5% of conversion after 5 h) but, during a 10 day
period, the ratio of diastereomers altered from 1:1 to 1:0.6 and
the overall substrate conversion rose to 50%. When the
reaction was carried out in toluene at 908C, a full conversion
of 3 with about a threefold molar excess of one of the
diastereomers was obtained after 18 h of heating.[16] Further
enhancement of the diastereomeric excess (up to 0.8 molar
ratio of the major component) could be achieved by heating a
solution of 6c without the chiral alkyl iodide in chloroform at
408C for two days. The CD spectrum of 6c closely resembles
that of one of the enantiomers of 6a or 6d (Figure 3).
Evidently, the chiral alkyl ligand induces unidirectional
alteration of the helicity of the bis(porphyrin) system after
coordination to the platinum(IV) ion.
In conclusion, we have shown the formation of a helical
bis(NCP) system in which the subunits are fused by peripheral
coordination and double orthometalation of platinum(II),
which stabilizes the configuration of the system and facilitates
separation of the enantiomers. The subunits retain their
ability to coordinate trivalent metal ions within the macro-
cyclic interior. An efficient oxidative addition to form
hexacoordinated platinum(IV) complexes allows further
modification of the system. The system can undergo chiral
induction owing to the adaptive properties of the platinum-
fused bis(porphyrin). The potential of the system in the
synthesis of new chiral catalysts by insertion of redox-active
metal ions into the porphyrin cores is currently under
investigation.
Figure 3. CD spectra (CH2Cl2, RT) of the slower migrating enantiomers
of 3 (c), the product of the reaction of 3 with MeI 6a (a) and
with I2 6d (g), and 6c, that is, the product of the reaction of
racemic 3 with 1-iodo-2-(S)-methylbutane (d). The CD spectra for
the pairs of enantiomers of the complexes 3, 5, 6a, and 6d are given
in the Supporting Information (Figure S52–S55).
Received: August 1, 2009
Published online: October 13, 2009
Keywords: helical structures · orthometalation ·
.
oxidative addition · platinum · porphyrinoids
addition.[15] A very slow racemization at room temperature
could be observed for a solution of a pure enantiomer of 3 in
chloroform, and results in the appearance of about 15% of
the other stereoisomer after 50 days, as established on the
basis of the HPLC analysis. The racemization process
accelerates in a benzene solution heated to 808C to yield a
racemic mixture after 24 h. Apparently, the isomerization
takes place even though the rotation of the subunits is
disabled by double chelation of the platinum ion. Molecular
modeling studies and DFT calculations were used to create a
structural model, which has an energy 23 kcalmolÀ1 above the
optimized Z-shaped arrangement of 3 and adopts a U shape
that allows the subunits to exchange their positions, and may
result in racemization (see the Supporting Information for
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2004, 43, 1885. In the case of 2, analysis of the reaction
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two platinum ions, and two chloride ions. When the reaction is
carried out without any proton scavenger, tetramers that are
difficult to separate from each other are formed. However, the
mixture can be easily converted into 3 by heating a solution of
the tetramers in toluene to reflux in the presence of K2CO3
without an additional platinum source.
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Mr = 2132.35, T= 100 K, MoKa radiation, triclinic, space group
ꢀ
P1, a = 15.798(5), b = 17.063(7), c = 19.413(8) ꢀ, a = 70.56(3),
b = 88.54(3), g = 88.51(3), V= 4932(3) ꢀ3, Z = 2, 1calcd
=
1.436 MgmÀ3
, , F(000) = 2146,
l = 0.71073 ꢀ, m = 2.05 mmÀ1
KUMA KM4 CCDK-geometry Diffractometer with Sapphire
CCD detector, 3.0 ꢀ q ꢀ 36.88, 58578 collected reflections, 30072
independent reflections, 1193 parameters, R1(F) = 0.063,
wR2(F2) = 0.164, S = 1.03, largest difference peak and hole 2.08
and À1.98 eꢀÀ3. Crystal data for 6d: C96H72I2N8Pt·1.5(C6H14)·2
(CHCl3) Mr = 2154.50, T= 100 K, MoKa radiation, triclinic, space
´
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ꢀ
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71.72(5), b = 86.01(5), g = 88.19(5), V= 4841(4) ꢀ3, Z = 2,
1calcd = 1.478 MgmÀ3
, , F(000) =
l = 0.71073 ꢀ, m = 2.30 mmÀ1
2158, diffractometer Oxford Diffraction Xcalibur PX with
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wR2(F2) = 0.190, S = 1.01, largest difference peak and hole 3.50
and À1.70 eꢀÀ3. CCDC 742635 (6a) and 742636 (6d) contain
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Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
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[15] This result contrasts with other helical bis(porphyrin)systems
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and coordination number is combined with helicity inversion,
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[16] Formation of
a small fraction (10–15%) of the bis-
[9] P. J. Chmielewski, Angew. Chem. 2004, 116, 5773; Angew. Chem.
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(iodide)complex 6d can be observed upon reaction of 3 with 1-
iodo-2-(S)-methyl-butane carried out at elevated temperatures.
Compound 6d can be easily separated from 6c by column
chromatography. Interestingly, 6d formed under such conditions
displays optical activity expressed as molar CD amplitude, which
is about 20% of that recorded for 6d obtained from nonracemic
3.
[10] In the dimeric platinum(II) complexes reported previously for
NCP or its alkylated nickel(II) complexes, one of the subunits
coordinates through the external nitrogen atom of the confused
pyrrole and through an ortho-carbon of the adjacent aryl ring,
while the other subunit coordinates solely through the external
nitrogen atom. Consequently, the overall symmetry of the
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