Push-pull quinoidal porphyrins

Article abstract of DOI:10.1039/c8ob00491a

A family of push-pull quinoidal porphyrin monomers has been prepared from a meso-formyl porphyrin by bromination, thioacetal formation, palladium-catalyzed coupling with malononitrile and oxidation with DDQ. Attempts at extending this synthesis to a push-

Full text of DOI:10.1039/c8ob00491a

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Takeharu Haino et al.
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DOI: 10.1039/C8OB00491A
Organic & Biomolecular Chemistry
PAPER
Push-Pull Quinoidal Porphyrins
Martin J. Smith,a Iain M. Blake,a William Cleggb and Harry L. Anderson*a
A family of push-pull quinoidal porphyrin monomers has been prepared from a meso-formyl porphyrin by bromination,
thioacetal formation, palladium-catalyzed coupling with malononitrile and oxidation with DDQ. Attempts at extending this
synthesis to a push-pull quinoidal/cumulenic porphyrin dimer were not successful. The crystal structures of the quinoidal
porphyrins indicate that there is no significant contribution from singlet biradical or zwitterionic resonance forms. The
crystal structure of an ethyne-linked porphyrin dimer shows that the torsion angle between the porphyrin units is only
about 3°, in keeping with crystallographic results on related compounds, but contrasting with the torsion angle of about
35° predicted by computational studies. The free-base quinoidal porphyrin monomers form tightly π-stacked layer
structures, despite their curved geometries and bulky aryl substituents.
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
D
D
A
A
D
•
D
A
•
A
Introduction
D
+
D
A
–
A
Organic π-systems with small HOMO-LUMO energy gaps have
many potential applications as organic semiconductors and
nonlinear-optical dyes because they are easily oxidized and/or
reduced, and easily polarized by electric fields.1 Quinoidal
molecules have particularly small π-π* gaps due to resonance
with the aromatic singlet biradical form.2,3 If the structure has
a ‘push-pull’ pattern of electron donor and acceptor groups
then a zwitterionic resonance form can also contribute (Fig. 1),
which can further reduce the energy gap between the frontier
orbitals, shifting the absorption to longer wavelengths.4-9 The
energy cost of breaking a double bond to form the biradical,
and separating charge to form the zwitterion, are
compensated by the gain in aromaticity in the core. The effects
of quinoidalization are well illustrated by the UV-vis-NIR
absorption spectrum of the cumulene-linked quinoidal
porphyrin dimer Zn21 (Fig. 2), which exhibits a strong
absorption band at 1080 nm (ε = 1.1 × 105 M–1 cm–1),
compared to a Q band at 724 nm (ε = 5.4 × 104 M–1 cm–1) for
the corresponding alkyne-linked aromatic porphyrin dimer.10
The original aim of the work presented here was to synthesize
a push-pull quinoidal porphyrin dimer M22. Unfortunately, we
were unable to isolate this target compound; however the
project provided some fascinating insights into porphyrin
chemistry. We synthesized a family of push-pull quinoidal
porphyrin monomers, as models for the elusive push-pull
dimers. The crystal structures of these quinoidal porphyrins
N
N
N
quinoidal
non-aromatic
singlet biradical
aromatic
zwitterionic
aromatic
Fig. 1. Possible resonance structures of a quinoidal push-pull system (D and A are
electron donor and acceptor, respectively).
Ar
Zn
Ar
Ar
Zn
Ar
N
N
N
N
N
N
N
N
N
N
N
N
•
•
Zn21
Ar
Ar
N
N
N
N
N
S
S
•
•
M
M
N
N
N
N
N
Ar
M22
Ar
Fig. 2. Structures of the quinoidal dimer Zn21 (reported in ref 10) and the elusive target
of this study, M22. (Ar = 3,5 di-tert-butyl phenyl.)
show that they are extremely curved π-systems, yet,
surprisingly, this curvature does not prevent them from
packing into π-stacked assemblies. The crystal structure of an
ethyne-linked porphyrin dimer shows that it forms
a
bimolecular aggregate in the solid state in which there is
essentially no torsional twist between the two covalently
linked porphyrin units, in contrast to the geometries of ethyne-
linked porphyrin dimers predicted by DFT calculations, which
have a torsion angle of about 35°.
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PAPER
HS
(a)
SH
SH
Ar
Ar
Ni
Ar
Ar
Ni
Ar
Ar
or (b)
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
O
O
SH
S
S
POCl3, DMF
82%
NBS, pyridine
88%
Ni
Ar
Br
Ni
Ar
Br
TiCl4, Na2CO3
a: 81%; b: 92%
Ni6a,b
Ni3
Ni4
Ni5
Ar
Ar
Ni
Ar
CH2(CN)2, NaH
Pd2(dba)3, PPh3, CuI
N
N
N
N
N
N
N
N
N
N
S
S
S
S
DDQ
Ni
Ar
a: 67%; b: 94%
a: 79%; b: 42%
N
N
Ni8a,b
Ni7a,b
Ar
Ar
N
N
N
N
HN
N
N
S
N
N
H2SO4, TFA
a: 81%; b: 85%
Zn(OAc)2
S
S
Zn
S
N
a: 93%; b: 82%
NH
N
Ar
Ar
H28a,b
Scheme 1. Synthesis of the quinoidal porphyrin monomers Ni8a, Ni8b, H28a, H28b, Zn8a and Zn8b. (Ar = 3,5 di-tert-butyl phenyl.)
Zn8a,b
Several studies of quinoidal porphyrins have been with benzene-1,2-dithiol in the presence of TsOH under
published recently, motivated by the unusual redox activity Otsubo’s conditions (refluxing toluene)5 led to clean
and near-IR absorption spectra of these compounds, and by debromination and deformylation, regenerating Ni3 in 90%
the possibility of forming stable singlet biradical ground yield. Formation of the thioacetals proceeded smoothly when
states.11 However, to the best of our knowledge, the work using TiCl4 as a Lewis acid in combination with solid Na2CO3 as
presented here is the first investigation of push-pull quinoidal a Brønsted base. Palladium-catalyzed Takahashi coupling14-16
porphyrins.‡,12
converted Ni6a,b to Ni7a,b, then oxidation with DDQ gave the
quinoidal porphyrins Ni8a,b. Nickel(II) was removed using
sulfuric acid to give the free-bases H28a,b, which were treated
with zinc(II) acetate to prepare Zn8a,b. The ‘pull-pull’ quinoidal
monomers Zn9, H29 and Ni9 were also prepared for
comparison (Scheme 2).15
Results and Discussion
Synthesis and Characterization of Push-Pull Quinoidal Monomers
The chemistry of push-pull quinoidal porphyrin monomers was
explored by synthesizing compounds with two types of
electron donor: a) 1,3-dithiolane M8a and b) 1,3-
benzodithiolane M8b, where M = Ni, Zn or H2 (Scheme 1).‡(12)
Vilsmeier formylation of nickel(II) porphyrin Ni3 gave Ni4 in
excellent yield, as reported previously.13 Nickel porphyrins
were used here to avoid demetalation. Bromination with NBS
yielded Ni5. The plan was to condense this aldehyde with 1,2-
ethanedithiol or benzene-1,2-dithiol to generate Ni6a or Ni6b,
respectively. Initially, we attempted this reaction using
Brønsted acid catalysis (TsOH) but these conditions failed to
produce the desired products. Surprisingly, treatment of Ni5
Ar
N
N
N
N
N
N
N
N
Zn9 (M = Zn)
H29 (M = 2H)
Ni9 (M = Ni)
CF3CO2H, 91%
Ni(OAc)2, 99%
M
Ar
Scheme 2. Synthesis of free-base and nickel(II) pull-pull porphyrins from the zinc
complex Zn9 (which was reported in ref 15).
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Attempted Synthesis of a Push-Pull Quinoidal Dimer View Article Online
The non-aromatic quinoidal nature of compounds Ni8a,
Ni8b, H28a, H28b, Zn8a and Zn8b is obvious from their 1H NMR
spectra. For example, the NH protons of H28a, H28b and H29
resonate at 12.23, 11.94 and 13.86 ppm, respectively (in CDCl3,
298 K), whereas a typical free-base porphyrin exhibits an NH
resonance at around –3 ppm. Similarly, the β-pyrrole protons
of these quinoidal porphyrins appear at 6.5–7.5 ppm, whereas
an aromatic porphyrin gives β-signals at 9–10 ppm. The UV-vis-
NIR absorption spectra of Zn8a and Zn8b are slightly more red-
shifted than that of the pull-pull analogue Zn9 (Fig. 3). The
slightly smaller HOMO-LUMO gaps of Zn8b, Ni8b and H28b are
confirmed by the redox potentials (Table 1).
We attempted to prepare a push-puDllOqI:u1i0n.1o0id39a/lCc8uOmB0u0l4e9n1iAc
porphyrin dimer from Ni6a as shown in Scheme 3.17,18
Sonogashira
coupling
with
trimethylsilylacetylene,
deprotection of the alkyne and Sonogashira coupling with
dibromoporphyrin Zn12 proceeded smoothly to give the
alkyne-linked dimer NiZn13. However, Takahashi coupling of
this bromoporphyrin dimer gave an unexpected product: the
meso-cyano porphyrin NiZn14 instead of the desired
intermediate NiZn17. Nitrile formation has previously been
reported as a side reaction during Takahashi coupling,19 but it
is not clear why it predominates here. Fortunately, changing
the halogen from bromine to iodine solved the problem and
NiZn16 was converted to NiZn17 in good yield. Oxidation of
NiZn17 with DDQ in chloroform at room temperature resulted
in a rapid color change and the appearance of an intense
absorption band in the near-IR (λmax 1034 nm, Fig. 4),
suggesting formation of NiZn2, but all attempts at isolating this
compound gave only complex mixtures of products. Other
oxidizing agents were also tested for conversion of NiZn17 to
NiZn2 (such as chloranil, tris(4-bromophenyl)aminium
hexachloroantimonate and trityl tetrafluoroborate) but none
of them gave more promising results than DDQ.
1.00
0.80
0.60
0.40
0.20
0.00
Fig. 3. The UV-visible absorption spectra of the ‘push-pull’ compounds Zn8a (blue) and
Zn8b (red) and the ‘pull-pull’ compound Zn9 (black), recorded in dichloromethane.
Table 1. Redox potentials.a
Compound
E1ox (V)
E1red (V)
ΔE (V)
Zn8a
Ni8a
H28a
Zn8b
Ni8b
H28b
Zn9
0.36
0.56
0.55
0.28
0.46
0.45
0.87
1.12
[1.15]b
–1.19
–1.04
–1.02
–1.15
–0.99
–1.02
–0.71
–0.45
–0.55
1.55
1.60
1.57
1.43
1.45
1.47
1.58
1.57
[1.70]b
300
400
500
600
700
800
900 1000 1100
wavelength / nm
Ni9
H29
Fig. 4. UV-vis-NIR absorption spectrum of NiZn17 before (black line) and after
(grey dashed line) treatment with DDQ in chloroform (concentration ca. 5 µM).
a
Potentials were measured by square wave voltammetry relative to internal
ferrocene (Fc/Fc+ 0.00 V) in CH2Cl2 containing 0.1 M Bu4NBF4. The oxidation
potential for H29 is approximate because the process is poorly reversible and
occurs close to the limit of the potential window.
b
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DOI: 10.1039/C8OB00491A
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PAPER
Ar
Ar
Ni
Ar
Me3SiC2H, Pd2(dba)3
PPh3, CuI, Et3N
N
N
N
N
N
N
N
N
S
S
S
S
Ni
Ar
Br
SiMe3
80%
TBAF
91%
Ar
Ar
Ni6a
Ni10
Ar
Ni
Ar
N
N
N
N
N
N
N
N
Br
Zn
Br
I
Zn
I
N
N
N
N
S
S
Pd2(dba)3
Pd2(dba)3
PPh3, CuI, Et3N
33%
PPh3, CuI, Et3N
30%
Ar
Ar
Ar
Zn12
Zn15
Ni11
Ar
Ar
Ar
N
N
N
N
N
N
N
N
N
N
N
N
S
S
S
Ni
Ar
Zn
Br
Ni
Ar
Zn
Ar
I
S
N
N
N
N
Ar
NiZn13
NiZn16
CH2(CN)2, NaH,
Pd2(dba)3, PPh3, CuI
CH2(CN)2, NaH,
Pd2(dba)3, PPh3, CuI
Ar
Ni
Ar
Ar
Zn
Ar
77%
55%
Ar
Ni
Ar
Ar
N
N
N
N
N
N
N
N
S
S
N
N
N
N
N
N
N
N
N
N
N
S
S
Zn
Ar
NiZn14
Ar
Ni
Ar
Ar
NiZn17
N
N
N
N
N
N
N
N
N
N
DDQ
S
S
•
•
Zn
Ar
NiZn2
not isolated
Scheme 3. Attempted synthesis of the push-pull quinoidal cumulenic dimer NiZn2. (Ar = 3,5 di-tert-butyl phenyl. For the synthesis of Zn12 and Zn15, see references
17 and 18, respectively.)
X-Ray Crystallography
and displacement parameters. Despite the relative high R
factors, the principal features of the molecular structures are
clear, and there are no unaccountable validation alerts.
Crystallographic details are given in the Electronic
Supplementary Information.
The aromatic nickel(II) porphyrin Ni10 crystallizes with four
molecules in the asymmetric unit; the four molecules have
essentially the same geometry (Fig. 5). The angles between the
mean planes of the 1,3-dithiolane ring and the C20N4 porphyrin
cores are in the range 64–67°. Apart from the 1,3-dithiolane
substituent, this structure is very similar to that of a meso-
ethynyl nickel porphyrin reported by Arnold and coworkers.20
Here we compare the crystal structures of compounds Ni8a,
Zn8b, H28b, Zn9, H29, Ni10 and NiZn14. The structures of Zn8b
and Zn9 have been published previously.12,15 Crystals were
grown by vapor diffusion of n-hexane into 1,2-dichloroethane
(H28b), methanol into 1,2-dichloroethane (Ni8a, Ni10) and
methanol into chloroform (Zn8b and NiZn14). All these
compounds gave only very small crystals with weak diffraction,
requiring use of synchrotron radiation. The structure
determinations were hampered by twinning and disorder,
including extensive disorder of solvent molecules, and
required the application of similarity restraints on geometry
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onto an electron deficient carbon (Fig. 7a), although there is
View Article Online
no significant pyramidalization at theDcOaI:rb10o.1n03c9e/Cn8tOerBs0.04T9h1Ae
structures of H28b and H29 both feature columnar stacks which
interlock to generate dense ‘brick walls’ of closely stacked
chromophores. The closest π-stacking distances between rings
are 3.13 Å in H28b (nitrogen to carbon), and 3.38 Å in H29
(carbon to carbon), pyrrole rings being the partners in both
cases. Both structures exhibit many short π-π contacts
suggesting that they might have potential as organic
semiconductors.1
Fig. 5. The conformation of one molecule of Ni10 (50% probability ellipsoids, no H
atoms). Note the twist between the thioacetal group and the tetrapyrrolic core.
The push-pull quinoidal porphyrins Ni8a, Zn8b and H28b
are all extremely curved π-systems, as seen from the
comparison of Ni8a and Ni10 (Fig. 6). Adoption of a saddle-
shaped conformation is attributed to intramolecular steric
repulsion between the β-pyrrole H atoms and the sulfur atoms
or nitrile groups. In Ni8a, the angle between the mean planes
of the thioacetal C2S2C ring and the C(CN)2 unit is 72.5° (where
180° would correspond to a planar geometry). Similar
conformations have been reported for other quinoidal
porphyrins.11a,f
Fig. 6. Comparison of (top) Ni8a (20% probability ellipsoids) and (bottom) Ni10 (50%
probability ellipsoids), both viewed without the meso-aryl substituents and H atoms.
Fig. 7. Crystal packing in free-base quinoidal porphyrins H28b and H29 (a) N···C
interactions. Packing diagrams for (b) H28b and (c) H29. In both these crystals, the
macrocyclic cores form tightly packed layers and the aryl substituents (not shown) form
closely packed layers above and below these layers of cores. Red dashed bonds
indicate C≡N···C(C≡N)2 interactions, with shortest lengths of (b) 3.22 and 3.24 Å; (c)
3.12, 3.19, 3.30 and 3.40 Å. Green dashed bonds indicate C≡N···C=C(C≡N)2 interactions,
with shortest lengths of (b) 3.19 and 3.14; (c) 3.06, 3.14 and 3.13 Å. Thin grey dashed
lines show the unit cells.
The free-base quinoidal porphyrins, H28b and H29, stack
into unusual extended assemblies (Fig. 7). Many C≡N···C(C≡N)2
and C≡N···C=C(C≡N)2 contacts are observed with N···C
distances of 3.0–3.4 Å. Most of these distances are shorter
than the sum of the van der Waals radii (3.41 Å). These short
contacts (marked as red and green dashed lines in Fig. 7)
indicate incipient nucleophilic attack of the nitrile lone pair
The bond lengths in the quinoidal porphyrins are
consistent with the expected pattern of bond orders. The
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differences in bond lengths between the push-pull and pull-
pull systems provide no evidence for a greater aromatic
contribution in the push-pull compounds (Fig. 8).
View Article Online
DOI: 10.1039/C8OB00491A
Ar
N
N
N
N
N
S
S
M
N
quinoidal
form
C=C length (Å) C–C length (Å)
Ar
1.388
1.357
1.377
1.451
1.451
1.457
Ni8a
H28b
Zn8b
push-pull
Ar
M
1.376
1.387
1.452
1.472
H29
Zn9
pull-pull
N
N
+
S
N
N
N
N
•
S
–
aromatic
zwitterion
Fig. 9. Two orthogonal views of the NiZn14 aggregate. (a) Side view, omitting meso-aryl
substituents. (b) Plan view omitting coordinated methanol and tert-butyl substituents
(40% probability ellipsoids).
Ar
Fig. 8. Charge-transfer resonance in the push-pull quinoidal porphyrins. Comparison of
the average carbon-carbon bond lengths shown here indicates that there is no
significant increased zwitterionic contribution in the push-pull systems.
The angle between the mean planes of the two C20N4
porphyrin cores of NiZn14 is 10.2° but this should not be
interpreted purely as a torsion angle; it is really a folding
dihedral angle because the axis of rotation between these two
planes is almost perpendicular to the line connecting the
centers of the two porphyrins. A better measure of the
torsional twist in NiZn14 is obtained by connecting the two
aryl-substituted meso-carbon atoms within each porphyrin
core and measuring the angle between these two vectors
when viewed along a line joining their midpoints; this
approach gives a torsion angle of only 3.3°. Thus the barrier to
planarization appears to be small enough to be overcome by
crystal packing effects, as in ortho-unsubstituted biphenyls.28 A
study of conformational bias in crystal structures29 concluded
that higher-energy planar conformers can be favored by
packing effects when the energy barrier is less than about 8–
10 kJ/mol, which is consistent with the calculated barrier
height in these porphyrin dimers.25-26
The aromatic porphyrin dimer NiZn14 crystallizes as an off-
set centrosymmetric anti-parallel bimolecular aggregate (Fig.
9), with one molecule of methanol coordinated to each zinc
center. Disordered solvent also separates these bimolecular
units. The distance between the mean planes of the two C20N4
zinc porphyrin cores is 3.29 Å, and the closest intermolecular
contact (3.34 Å) occurs between the central β-pyrrole carbons
of the zinc porphyrins (c.f. 3.54 Å for twice the van der Waals
radius of carbon). The meso-aryl groups are interdigitated and
they do not hinder this close π-π stacking of the porphyrin
cores. The structure of this aggregate resembles that of a
butadiyne-linked porphyrin tetramer.21 The zinc porphyrin unit
in NiZn14 is essentially planar whereas the nickel porphyrin
unit is noticeably ruffled; the root-mean-square deviations of
the C20N4 cores are 0.12 Å and 0.19 Å, respectively (major
disorder component only). This type of distortion is common in
nickel porphyrin complexes due to the short Ni-N bond
length.22
There has been some debate concerning the torsion angles
in alkyne-linked porphyrin dimers because the twist between
the two porphyrin units limits the extent of electronic coupling
through the alkyne bridge.23-25 Many computational studies
have predicted that steric repulsions between the β-pyrrole
protons in ethyne-linked dimers result in a ground-state
geometry with a torsion angle of θ ≈ 35° in the gas phase, with
a barrier to planarity of about 6 kJ/mol.25-26 In contrast to
these computational results, Therien and coworkers reported
the crystal structures of two ethyne-linked porphyrin dimers
(with palladium and platinum metal centers) in which the
mean planes of the porphyrin units are exactly parallel
(inversion center, torsion angle θ = 0°).27
Conclusions
This work shows that push-pull quinoidal porphyrins can be
synthesized in high yield, and that they are stable under
normal laboratory conditions. They are even stable enough for
nickel(II) to be removed with sulfuric acid, to prepare the free-
base macrocycles. The absorption spectra and redox potentials
of these push-pull systems indicate that their HOMO-LUMO
gaps are not significantly smaller than those of the
corresponding pull-pull or push-push analogues.11,15 It is
surprising that the quinoidal free-base porphyrins H28b and
H29 form infinite closely π-stacked layer structures (Fig. 7),
despite the molecular curvature and bulky meso-aryl
substituents, both of which would have been expected to
prevent π-stacking. This observation suggests that these
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank the Engineering and Physical Sciences Research
Council (EPSRC) and the former Council for the Central
Laboratories of the Research Councils (CCLRC, now STFC) for
support including access to synchrotron facilities, Oxford
University for a studentship for MJS, and the EPSRC UK
National Mass Spectrometry Facility at Swansea University for
mass spectra.
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