Naphthalene-fused metallo-porphyrins-synthesis and spectroscopy

Article abstract of DOI:10.1039/c1ob06281f

Oxidative aromatic coupling of meso-substituted porphyrins bearing one electron-rich naphthalene unit has been studied in detail. After thorough optimization of oxidant, naphthalene-fused porphyrins were prepared in high yield without contamination from c

Full text of DOI:10.1039/c1ob06281f

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Naphthalene-fused metallo-porphyrins–synthesis and spectroscopy†
a
a
a
b
b
a
´
Jan P. Lewtak, Dorota Gryko, Duoduo Bao,‡ Ernest Sebai, Olena Vakuliuk, Mateusz Scigaj and
Daniel T. Gryko*a,b
Received 28th July 2011, Accepted 13th September 2011
DOI: 10.1039/c1ob06281f
Oxidative aromatic coupling of meso-substituted porphyrins bearing one electron-rich naphthalene unit
has been studied in detail. After thorough optimization of oxidant, naphthalene-fused porphyrins were
prepared in high yield without contamination from chlorinated side-products using Fe(ClO4)3·2H2O.
Copper and nickel complexes were successfully transformed into p-expanded porphyrins in 40–83%
yield.
Porphyrins and other porphyrinoids, by nature of their electronic
and redox properties, are extremely versatile architectures that
figure prominently in applications ranging from materials1 to
biomedicine.2 The relationship between the structure of porphyri-
noids and their spectroscopic and photophysical properties is
a complex and deeply intriguing issue.3 Among other changes,
p-expansion of a porphyrin chromophore usually leads to a
certain bathochromic shift in its absorption (and hence emission)
spectrum. The magnitude of this alteration depends more on the
type of conjugation than on the actual number of double or
triple bonds added. The subtle nature of this relationship can
be exemplified by Osuka’s expanded porphyrin which, in spite of
possessing an extra double bond, displays hipsochromic shift.4 The
vast diversity of p-expanded porphyrinoids5 attracts attention not
only because of the theoretical issues involved, but also because of
the opportunities to explore their linear and non-linear properties
in technology and medicine.6 Among various synthetic strategies,
a breakthrough approach was demonstrated in 1997 by Osuka
et al. Porphyrins that are unsubstituted at the meso positions
were firstly oxidized to form a meso-meso link7 and then treated
with DDQ/Sc(OTf)3 to form two additional bridges between
the b positions. This structural change caused an extension of
the conjugated p-system and a concomitant strong bathochromic
absorption shift.8,9
porphyrin and possessing high electron density should be able to
form analogous products via oxidative aromatic coupling which
was proved by seminal contribution by Anderson et al.11 This
concept was also successfully realized for such units as azulene,12a
BODIPY,12b and pyrene.13 Very recently, elaborated examples of
oxidative coupling were presented by Wu, Wang and Thompson.14
The attachment of simpler units using oxidative aromatic coupling
is a less explored subject. An alternative approaches based on
acylation14f,g and direct arylation14h were also presented.
We were intrigued by the question of what kind of changes
can be introduced by fusion of the porphyrin core with the most
fundamental two-ring aromatic unit–naphthalene. Naphthalene-
fused porphyrins have been prepared sporadically in the past.
In 2005 Cammidge and co-workers unexpectedly found that
direct aromatic coupling occurs when a porphyrin bearing one
naphthalene unit at the meso-position and a triflate group at
position 8 is exposed to typical Suzuki reaction conditions. A fused
porphyrin lacking any substituents on the naphthalene moiety
was isolated in 14% yield.15 Shortly afterwards, Imahori and co-
workers revealed that Ni-porphyrin bearing a naphthalene unit
at the meso-position (and possessing an additional ester group)
undergoes oxidative aromatic coupling in the presence of FeCl3
to give fused-porphyrin in 16% yield.16a Scott and co-workers
proposed an alternative route based on rearrangement of cyclic
bis-dipyrrines.16b
Naphthalene-fused porphyrinoids are simple systems that allow
us to study the chemical, spectroscopic, and photophysical effects
of this type of p-expansion. We were also intrigued by the relation
between the output of oxidative aromatic coupling reactions and
the nature of the oxidizing agent and the central metal cation.
Previous studies (most notably by Osuka and Anderson)7–11
and general knowledge of aromatic oxidative coupling17 both
indicate that the planned concept can only be realized if the
naphthalene porphyrin bears an electron-donating substituent at
a suitable position (Scheme 1). Macrocycles of this type could in
principle be synthesized either via Suzuki coupling starting from
meso-bromo porphyrins or from the corresponding aldehydes via
mixed condensation. The second route is more straightforward
An elegant example of such a multiplied effect is shown in
the so-called meso-meso, b-b, b-b triply linked ‘porphyrin tapes’
(10 nm molecular size), which display absorption at 2850 nm.10
In principle, any aromatic unit located at the meso-position of a
aInstitute of Organic Chemistry Polish Academy of Sciences, Kasprzaka
44/52 01-224, Warsaw, Poland. E-mail: dtgryko@icho.edu.pl; Fax: +48 22
632 66 81; Tel: +48 22 343 20 37
bWarsaw University of Technology, Noakowskiego 3, 00-664, Warsaw,
Poland; Fax: +48 22 628 27 41; Tel: +48 22 343 30 63
† Electronic supplementary information (ESI) available: Experimental
details, 1H NMR spectra of compounds 5, 5a, 5c, 6, 6a, 6c, 7c, 8a-H2,
8c and 13C NMR spectra of compounds 5, 5c, 6, 6a, 6c, 8c data in PDF
format. See DOI: 10.1039/c1ob06281f
‡ On the leave from University of California Riverside.
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pyrrole (2) and mesitaldehyde (3) under Lindsey’s18 conditions
for sterically hindered aldehydes gave the desired A3B-porphyrin
5 in 20% yield (Scheme 1). This porphyrin was subsequently
metallated under standard conditions resulting in zinc, copper
and nickel complexes 5a–c in excellent yields. Zinc porphyrin 5a,
when subjected to standard Scholl conditions (FeCl3/CH3NO2,
substrate/CH2Cl2), gave on TLC a distinct green spot representing
probably fused complex 7a.
Chromatography of that porphyrin revealed its instability, and
in spite of many attempts its purification could not be achieved.
In light of this result we replaced the moderately electron-
rich mesityl unit with an electron-neutral 3,5-difluorophenyl.
An analogous mixed condensation performed under Lindsey’s19
conditions gave A3B-porphyrin 6 in 28% yield (Scheme 1). After
transformation into the zinc complex 6a and subsequent treatment
under FeCl3/CH3NO2 conditions free-base porphyrin 8a-H2 was
obtained in 47% yield as green solid, which turned out to be the
product of oxidative aromatic coupling followed by demetalation.
Since zinc complexes proved to be unstable in the presence of
a Lewis acid (FeCl3), subsequent research focused on copper(II)
and nickel(II) complexes, which are significantly more robust.
Complexes 5b, 5c, 6b, and 6c were synthesized using the standard
procedure (Scheme 1). Not only porphyrins with mesityl units
(5, 5a, 5b and 5c) but also these bearing 3,5-difluorophenyl
substituents (6, 6a, 6b and 6c) possessed good solubility. We were
delighted to see that all four compounds underwent intramolecular
oxidative ring closure with FeCl3 to give 7b, 7c,8b, and 8c in
50%, 77%, 86%, 72% yields, respectively, and no demetalation
was observed. Unfortunately, fused products were contaminated
with mono-chlorinated by-products, which were clearly visible
by FD/MS as well as by elemental analysis. The chlorination
of electron-rich aromatic compounds using FeCl3 was reported
for the first time by Niementowski.20 This serious problem was
mentioned before by Osuka21 and Anderson.11 Since separation
of these side-products from the desired fused porphyrins is
basically impossible, efforts were directed toward suppressing
chlorination. We have chosen porphyrin 5c as a model, and
oxidative aromatic coupling was performed on this compound
using various conditions (Table 1).
Replacement of FeCl3 with the standard procedure for prepa-
ration of meso-meso, b-b, b-b triple-fused porphyrins (i.e.
DDQ/Sc(OTf)3) led to drastically lower yields (Table 1, entries
4–6). The use of copper(II)22 or molybdenum(V) salts resulted in
no desired product (Table 1. Entries 7–8).
Osuka et al. reported that addition of silver triflate suppresses
formation of chlorinated fused porphyrins.21 When applied to our
compounds, this procedure still resulted in chlorinated impurities.
(Table 1, entry 2). A survey of other oxidation methods suggested
that iron perchlorate dihydrate may offer a solution to this
problem—indeed, pure 7c was obtained in satisfactory yield. In
spite of running >20 reactions with iron(III) perchlorate hydrate
we never experienced any problems, however this compound may
be explosive in large quantities.
The detailed analysis revealed that this product is free of any
chlorinated side-products. The reaction with Fe(ClO4)3·2H2O gave
compounds 7b, 7c, 8b and 8c in 83%, 60%, 60%, and 40% yields,
respectively (Scheme 1).
Scheme 1
for the preparation of A3B-porphyrins. We have therefore chosen
the readily available 4,7-dimethoxy-1-naphthaldehyde (1) as a
precursor, which possesses high electron density at the required
position 8 due to the suitably positioned methoxy groups. Mesi-
taldehyde (3) was chosen as the second coupling partner since its
presence in porphyrin molecules ensures their high solubility. This
aspect is of paramount importance as the products are expected
to possess a larger p-electron system, which will substantially
decrease their solubility. A mixed condensation of aldehyde 1,
Purification and characterization of 7b, 7c, 8b and 8c proved
to be challenging due to their low solubility and tendency to
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Table 1 Results of oxidation of porphyrin 5c into 7c
Entry
Oxidizing reagent
Solvent
T/◦C
Time (h)
Yield of 7c (%)
1
2
3
4
5
6
7
8
FeCl3
FeCl3/AgOTf
MeNO2/DCM
MeNO2/DCM
Toluene
Toluene
Xylenes
1,2-Dichlorobenzene
THF
DCM
DCM
THF/18-crown-6
DCM
RT
RT
110
110
140
170
50
RT
RT
RT
RT
RT
RT
RT
0.5
0.5
5
86a
80a
46
12.5
20
dec.
0
0
0
0
0
PIFA/BF3·OEt2
DDQ, Sc(OTf)3
DDQ, Sc(OTf)3
DDQ, Sc(OTf)3
CuCl2,O2/[iPrMgCl·LiCl+TMP]
molybdenum 2-ethylhexanoate
Fe(acac)3
K3[Fe(CN)6]
Cu(ClO4)2
Mn(CH3COO)3
Fe(ClO4)3·2H2O
Fe(OTf)3
72
72
72
48
48
48
48
48
48
0.33
0.5
9
10
11
12
13
14
DCM
MeNO2/DCM
MeNO2/DCM
14
60
42
a Product contaminated with chlorinated porphyrins.
aggregate. The traces of some solvents could not be removed from
these complexes, even after prolonged drying in 80 ◦C.
All fused porphyrins have distinct deep green color. The
bathochromic shift that resulted from attaching the dimethoxy-
naphthalene unit at positions 1 and 3 is ~60–70 nm for the
Soret bands and ~100–130 nm for the Q-bands, depending on
the type of complex (Fig. 1–3). For nickel complexes red-shift
of Q-bands is higher than for corresponding copper complexes.
For both copper and nickel fused-complexes the least energy Q-
band are more significantly intense than in case of their non-fused
counterparts. In accord with previous findings of Imahori16a et al.
we observed that molar absorptivity at the Soret band of fused
porphyrins is lower than that of non-fused porphyrin complexes.
At the same time broadening of both the Soret band and the Q-
bands is observed. The free-base fused-porphyrin 8a-H2 produces
the most complex absorption spectrum (Fig. 3): not only does it
possess five well resolved peaks (with additional shoulders), but the
lowest energy Q-band has intense (e = 27000) absorption at 701 nm
with a shoulder up to 750 nm. 8a-H2 also displays fluorescence in
the near-infrared region (lmax = 710 and 760 nm, Ufl = 10%).
Fig. 2 Absorption of 6b (dotted line) and 8b (solid line).
Fig. 3 Absorption (solid line) and normalized fluorescence (dotted line)
of porphyrin 8a-H2 in CH2Cl2.
Replacing the commonly used FeCl3 with readily available
Fe(ClO4)3 as an oxidizing reagent was an effective strategy for
preventing chlorinated by-products, and thus opens new horizons
in oxidative aromatic coupling of porphyrins and other electron-
rich aromatic compounds. Oxidation of a Zn-coordinated por-
phyrin resulted in demetalation of the complex, while replacement
of zinc by copper and nickel allowed stable fused porphyrin
complexes to be synthesized. The fusion of a small naphthalene
unit with a porphyrin causes a ~100 nm red shift in the absorption
spectrum. Free-base p-expanded porphyrins display relatively
Fig. 1 Absorption of 5c (dotted line) and 7c (solid line).
Conclusions
In conclusion, fused electron-rich naphthalene-porphyrins have
been synthesized via intramolecular oxidative aromatic coupling.
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Acknowledgements
This work was supported by the Ministry of Science and Higher
Education (Contract N204 123837). We thank Eva Nichols
(Caltech) for amending the manuscript and Mitsubishi Japan for
generous gift of mesitaldehyde
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