Unexpected but convenient synthesis of soluble meso-tetrakis(3,4- benzoquinone)-substituted porphyrins

Article abstract of DOI:10.1142/S1088424613501071

A new route to 3,4-benzoquinone-substituted porphyrins is reported. In attempted nitration reactions on the copper(II) or nickel(II) complexes of 5,10,15,20-tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin using lithium nitrate in acetic anhydride-acetic acid/chloroform no nitration products could be detected with the main products being the corresponding complexes of 5,10,15,20-tetrakis(3,4-dioxo-5-t-butylcyclohexa-1,5-dienyl)porphyrin. These o-quinone-substituted porphyrins are available in reasonable yield (> 50%), their synthesis is simple and they are of good solubility. The electrochemical and spectroelectrochemical properties of representative o-quinone-substituted Cu(II) and Ni(II) porphyrin derivatives are also reported.

Full text of DOI:10.1142/S1088424613501071

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2014; 18: 173–181
DOI: 10.1142/S1088424613501071
Published at http://www.worldscinet.com/jpp/
Unexpected but convenient synthesis of soluble
meso-tetrakis(3,4-benzoquinone)-substituted porphyrins
Jonathan P. Hill*a,b◊, Wim Van Rossoma, Shinsuke Ishiharaa,c, Navaneetha Subbaiyand,
Francis D’Souza*d◊,Yongshu Xiee, Noelia M. Sanchez-Ballestera
and Katsuhiko Arigaa,b
a International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS),
Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan
b Japan Science and Technology Agency (JST), Core Research for Evolutional Science and Technology (CREST),
1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
c International Center for Young Scientists, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba,
Ibaraki 305-0044, Japan
d Department of Chemistry, University of North Texas, 1155 Union Circle, Denton, #305070, Texas 76203-5017, USA
e Key Laboratory for Advanced Materials and Institute for Fine Chemicals, East China University of Science and
Technology, Meilong Rd. 130, Shanghai 200237, P. R. China
Received 21 August 2013
Accepted 13 September 2013
ABSTRACT:A new route to 3,4-benzoquinone-substituted porphyrins is reported. In attempted nitration
reactionsonthecopper(II)ornickel(II)complexesof5,10,15,20-tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)
porphyrin using lithium nitrate in acetic anhydride-acetic acid/chloroform no nitration products could
be detected with the main products being the corresponding complexes of 5,10,15,20-tetrakis(3,4-
dioxo-5-t-butylcyclohexa-1,5-dienyl)porphyrin. These o-quinone-substituted porphyrins are available in
reasonable yield (>ꢀ50%), their synthesis is simple and they are of good solubility. The electrochemical
and spectroelectrochemical properties of representative o-quinone-substituted Cu(II) and Ni(II)
porphyrin derivatives are also reported.
KEYWORDS: nitration, benzoquinone, porphyrin, lithium nitrate, oxidation.
symptomaticoftheoccurrenceofenergytransferorelectron
INTRODUCTION
migration [4, 5]. Subsequently, the mechanism involved in
Porphyrins substituted with benzoquinone groups
convertinghydroquinone-orphenol-substitutedporphyrins
were first prepared by Milgrom and coworkers [1–3] who
to their quinone analogs was studied intensively [6–8]
recognized their potential due to the presence of functional
with the aim of understanding the structures of any free
moieties similar to those involved in photosynthetic
radical intermediates existing between the oxidized and
processes, namely porphyrin and quinone. Attachment of
reduced states of these molecules. Additionally, oxidation
a quinone unit to porphyrin led largely to the quenching of
of phenol-substituted compounds to the corresponding
fluorescence emission from the latter due to intramolecular
hemiquinonoid-substituted molecules [9–11] has permitted
processes, and this feature is now generally considered as
development of a family of highly colored N-substituted
tetrapyrroles [12–14] suitable for sensing applications
[15–17] and supramolecular chemistry [18–20].
◊SPP full member in good standing
The introduction of a benzoquinone group substituted
*Correspondence to: Jonathan P. Hill, tel: +81 29-860-4399,
at a porphyrin moiety has usually been accomplished by
preparation of the corresponding o-dimethoxyphenyl-
substituted compound followed by its demethylation
fax: +81 29-860-4832, email: Jonathan.Hill@nims.go.jp;
Francis D’Souza, tel: +1 940-369-8832, fax: +1 940-565-4318,
email: francis.dsouza@unt.edu
Copyright © 2014 World Scientific Publishing Company

174
J.P. HILL ET AL.
OH
OH
HO
HO
N
M
N
N
M
N
O2N
N
N
N
N
OH
OH
HO
HO
1a: M = Cu
1b: M = Ni
(i)
O
O
O
O
HO
O
O
N
N
M
N
others
M
N
N
N
N
N
O
O
O
O
O
O
O
O
2a: M = Cu
2b: M = Ni
3a: M = Cu (not isolated)
3b: M = Ni
OH
O
O
Ph
Ph
N
N
(ii)
N
N
M
N
Ni
N
N
N
Ph
Ph
Ph
Ph
5
4a: M = 2H
4b: M = Ni
Scheme 1. Synthesis of 3,4-dioxo-5-t-butylcyclohexa-1,5-dienyl-substituted porphyrins from metal complexes of 1 and 4. (i) LiNO3
(~9 equiv.)/CHCl3/Ac2O/AcOH/60 °C/3 h.Yield (2): ~50%. (ii) LiNO3 (~2.5 equiv.)/CHCl3/Ac2O/AcOH/60 °C/1.5 h/yield (5): 89%
(using boron tribromide) then oxidation [21] and the utility
of this method has also been demonstrated by Senge and
coworkers [22, 23]. Oxidation of hydroquinone groups has
been achieved in a variety of ways but often simply relies
on interaction of the deprotonated species with ambient
oxygen. This method can be rather inconvenient since it
involves first multiple demethylations in a single step then
potentially low yielding oxidation reactions to the quinone-
substituted porphyrins. Additionally, in our experience,
handling and purification of, for instance, tetrakis(3,4-
dihydroxyphenyl)porphyrin [24] is not so convenient.
In this work, we report a direct access to soluble
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UNEXPECTED BUT CONVENIENT SYNTHESIS OF SOLUBLE MESO-TETRAKIS(3,4-BENZOQUINONE)-SUBSTITUTED
175
benzoquinone-substituted porphyrins by an unexpected
oxidative dealkylation of 5,10,15,20-tetrakis(3,5-di-t-
butyl-4-hydroxyphenyl)porphyrin (1) that yields 5,10,-
15,20-tetrakis(3,4-dioxo-5-t-butylcyclohexa-1,5-dienyl)
porphyrin (2) (see Scheme 1). This transformation was
first observed during attempts to b-nitrate the copper(II)
or nickel(II) complexes of 1 using the method for
nitration described by Richeter et al. [25]. Products due
to stepwise oxidative dealkylation could be detected by
thin layer chromatography and mass spectrometry, and
the final major products of reactions were the respective
Cu(II) and Ni(II) complexes of 2 although products
with lower order conversions could also be obtained
(e.g. 3b which contains a single unreacted phenol
group). Notable features of these compounds are their
brown/black colors in the solid and solution states and
their excellent solubilities in non-polar solvents due
to the remaining t-butyl substituents. Additionally, a
mono-(o-quinone)-substituted compound (5) could also
be prepared by treating the singly (3,5-di-t-butyl-4-
hydroxyphenyl)-substituted porphyrin 4 in the same
manner. This method constitutes a useful method for
introducing a metal chelating group at the periphery of
porphyrins while maintaining good solubilities of the
resulting compounds.
Fig. 1. (a) X-ray crystal structure of precursor 4b and
(b) calculated structure (ChemOffice, Chem3D, MM2) of the
demetalated form of 5
RESULTS AND DISCUSSION
meso-substituents becomes two peaks due to protons
Chemical structures of the compounds prepared
in this work are shown in Scheme 1. Notably, they
are o-quinone-substituted porphyrins analogous with
those prepared by other workers [21, 22, 23, 29].
Quinone-substituted porphyrins have usually been
prepared by synthesis of the porphyrin containing the
appropriate oligo-methoxylated compound followed by
its demethylation usually employing boron tribromide
reagent. The method we describe represents an addition
to the known procedures to the quinone-substituted
porphyrins and is advantageous from the point-of-view
that it uses mild reagents so that a metal complex (albeit
currently only Cu(II) or Ni(II)) can remain intact during
the procedure. During this work it did not prove possible
to prepare suitable crystals of any of the o-quinone
compounds although we could obtain an X-ray crystal
structure of the precursor 4b. This is shown together with
a calculated (ChemOffice, Chem3D, MM2) structure
of the quinone-substituted porphyrin resulting from its
oxidation in Fig. 1.
adjacent to quinone and tBu groups, respectively, with
appropriate integration values and meta-coupling
constants. In 13C NMR spectra the most obvious change
is the appearance of two resonances close to 180 ppm
due to the two carbonyl carbon atoms of the o-quinone
groups. Figure 2 shows spectroscopic data for 5.
The most interesting characteristic of the new
compounds is their intensely brown-black colors in the
solid state (black) and solution (brown-black). In fact,
even the singly-substituted compound 5 exhibits the same
dark hue attesting to the strong effect of the o-quinone
substituent on the porphyrin chromophore relative
to simple phenyl substituents. Electronic absorption
spectra of the compounds are shown in Fig. 3. For
compounds 2a and 2b, the extinction coefficients at
the B-bands are attenuated to about 50% the value
of the parent compounds. Apart from this, the most
important feature is the increased absorbance across the
entire visible range from 350 to nearly 700 nm, which
accounts for their dark colors. The Ni(II) complex 2b
has an additional intense band at 290 nm whose origin is
not currently understood. For 4a, 4b and 5, there are the
expected changes in Q-band structure upon metalation of
the porphyrin and, upon oxidation to the benzoquinone, a
similar increase in extinction across the visible spectrum
from 370–700 nm. This latter feature again confers
a dark brown color on 5 despite its only bearing one
o-quinone functionality.
The black quinone-substituted products have
appropriate changes of infrared spectrum in particular
the appearance of a strong band around 1660 cm-1
due to the new quinone C=O stretching mode and the
disappearance of the hydroxyl stretching mode at 3630
cm-1. Their 1H NMR spectra have resonances due to tBu
groups with the required lower integration values and
the former singlet due to the aromatic protons at the
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176
J.P. HILL ET AL.
Fig. 3. Electronic absorption spectra of compounds 2–5. Top:
2a (solid line) and 2b (dashed line). Bottom: 4a (dotted line),
4b (dashed line) and 5 (solid line)
are reversible as confirmed by cyclic voltammetry. This
suggests occurrence of step-wise electron transfer in
a narrow potential window rather than one-step four-
electron transfer. Scanning the potential more in the
cathodic direction revealed the second quinone based
reductions centred at -1.01V and -0.89V vs.Ag/AgCl, res-
pectively, for 2a and 2b (Figs 4a(ii) and 4b(ii)). Scanning
the potential in the positive direction revealed the first
porphyrin based oxidation at 1.44 V for 2a and 1.30 V
for 2b vs. Ag/AgCl respectively. These oxidations were
more difficult compared to simple Cu and Ni porphyrins
[31] due to the presence of the four electron deficient
o-quinone entities directly attached to the porphyrin
macrocycle. This point (together with the relative ease
of reduction of 2a and 2b) also suggests a reason why
nitration of the metalloporphyrin core does not occur in
this case, i.e. the rapid oxidation during reaction of the
phenol substituents to electron withdrawing o-quinone
groups suppresses macrocyclic nitration since it is an
electrophilic process. We also considered the band
gap and estimated highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital
(LUMO) levels. For 2a and 2b, the estimated values
are similar with an optical band gap around 1.8 eV. The
electrochemical band gap is probably less reliable since it
depends on somewhat broad multiple electron processes
and is narrower at ~1.6 eV. LUMO level estimated from
onset of the first reduction is at -4.19 eV leading to a
Fig. 2. Spectroscopic data for oxidized products revealing the
characteristics before and after transformation of the 3,5-di-t-
butyl-4-hydroxyphenyl substituents to o-quinone groups.
(a) FT-infrared spectra of (i) 4a (note OH (str.) 3632 cm-1,
NH (str.) 3316 cm-1). (ii) 4b (note OH (str.) 3632 cm-1). (iii) 5
(note absent OH (str.) and C=O (str.) 1660 cm-1). (b) 13C NMR
spectra of 4b (lacking C=O) and 5 with two unique carbonyl
1
carbon atoms as required. (c) H NMR spectra of 1b and 2b
revealing the symmetry breaking of the meso-substituent
Electrochemical and spectroelectrochemical data for
2a and 2b are shown in Fig. 4. In agreement with our
earlier report [30], the potential corresponding to the
reductions of o-quinone entities appears at -0.30 V vs.
Ag/AgCl. In this case, the peak is broadened due to the
presence of four quinone entities although reductions
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UNEXPECTED BUT CONVENIENT SYNTHESIS OF SOLUBLE MESO-TETRAKIS(3,4-BENZOQUINONE)-SUBSTITUTED
177
Fig. 4. Electrochemical and spectroelectrochemical data. (a) 2a (i) differential pulsed voltammogram (ii) electronic absorption
spectra under reduction at -0.8 V and (iii) oxidation (+1.7 V). 2b (i) differential pulsed voltammogram (ii) electronic absorption
spectra under reduction at -0.8 V and (iii) oxidation (+1.7 V)
HOMO level (obtained by subtracting the value of the
optically estimated band gap) of -5.99 eV. These values
suggest that these compounds may be of use either as
electron acceptor components of dye-sensitized solar
cells (DSSCs) or bulk heterojunction (BHJ) solar cells.
For comparison, the C70 fullerene derivative PC70BM has
LUMO and HOMO levels respectively of -4.3 eV and
-6.1 eV [32].
both compounds and appearance of new bands at 448 nm
(2a) and 500 nm (2b); to a large extent these spectral
changes were irreversible. The origins of the new spectra
under oxidation are not fully understood but could be
due to oxidative ring cleavage reactions of the oxidized
porphyrin macrocycle, or further oxidative dealkylation
involving the remaining t-butyl groups or reactions with
o-quinone substituents.
Additional spectroelectrochemical studies confirmed
the assigned site of electron transfer. During controlled
potential spectroelectrochemistry at -0.5 V (Figs 4a(ii)
and 4b(ii)), both compounds appear to obtain electronic
absorption spectra similar to their respective metallotetra-
phenylporphyrins with the expected Q-band structure
around 530 nm and a well-defined shift in B-band
wavelength from 405 nm to that usual for the respective
metal complexes around 420 nm. This is perhaps
predictable since the meso-cyclohexadienyl substituents
regain their status as phenyl groups. It is likely that
the cyclohexadienyl groups are relatively strongly
electron withdrawing leading to the hypsochromic shift
in the B-band for the quinone-substituted porphyrins.
The increased absorbance across the visible spectrum
may be due to increased electronic delocalisation (or
increased quinonoid character) of these compounds.
Under oxidation (+1.6 V, Figs 4a(iii) and 4b(iii)), there
are substantial reductions in the B-band intensities for
There are two unexpected aspects of the reaction
and products described here. First, oxidative dealky-
lation occurs at the phenolic meso-substituents. Second,
nitration was not observed to occur to any observable
extent either at the porphyrin macrocycle or at meso-
substituents. These features suggest that conversion of
3,5-di-t-butyl-4-hydroxyphenyl groups to 3,4-dioxo-5-
cyclohexa-1,5-dienyl groups deactivates the porphyrin
against this mild nitration process. In fact, even in the case
of the mono-3,5-di-t-butyl-4-hydroxyphenyl-substituted
compound 4b nitration of the porphyrin macrocycle
was also not observed to occur so that even the presence
of a single meso-3,4-benzoquinone appears sufficient
to preclude nitration at the porphyrin b-positions.
Presumably, the electron-withdrawing effect of even a
single benzoquinone group reduces the propensity of the
macrocycle to undergo electrophilic substitution. The
exclusion of lithium nitrate from the reaction mixture
(i.e. using acetic acid-acetic anhydride or either of these
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178
J.P. HILL ET AL.
reagents alone) leads to no reaction and the starting
metalloporphyrin only was isolated. Thus, the presence
of the nitrating reagent is required for the reaction to the
o-quinone to occur. In fact, formation of o-quinones by
treatment of sterically hindered phenols with nitrating
reagents has been reported [33, 34]. In those cases,
initial reaction involving an oxygen atom of a reactive
NO2 species at the t-butyl group adjacent to the phenol
is followed by elimination of NO and the t-butyl group
(as isobutene) and leads to formation of o-quinone. This
reaction proceeds very rapidly and, if as we suppose a
single benzoquinone substituent suppresses nitration
at the porphyrin macrocycle, this explains the lack of
porphyrin nitration products. Although the lack of any
nitrated products suggests that the presence of a single
o-quinone substituent is sufficient to suppress nitration
at the metalloporphyrin b-positions, we investigated this
point by performing the reaction on a nickel(II) porphyrin
substituted with a single 3,5-di-t-butyl-4-hydroxyphenyl
group. Reaction to the o-quinone-substituted porphyrin
occurred rapidly and cleanly with virtually no other
products including b-nitro compounds. Even addition
of an excess of nitrating agent did not yield the
expected b-nitro derivatives. Since we have a general
interest in the phenol antioxidant substituted porphyrins
it is likely that this method for conversion from
3,5-di-t-butyl-4-hydroxyphenyl to the corresponding
o-quinone will provide a useful and selective synthetic
route for preparation of unusual peripherally substituted
porphyrins.
EXPERIMENTAL
General
Solvents and reagents were obtained from Aldrich
Chemical Co., Fischer Chemical Co., Wako Chemical
Co., Tokyo Kasei Chemical Co. or Kanto Chemical Co.
1H (300.01 MHz) and 13C (75 MHz) NMR spectra were
measuredat298KfromCDCl3 orCDCl3/d6-DMSOsolutions
of the samples using a JEOL model AL300BX spectrometer
with tetramethylsilane as reference. Electronic absorption
spectra were measured from dichloromethane solutions
of the samples using a Shimadzu model UV-3600 UV/
vis/NIR spectrophotometer. FTIR spectra were measured
from samples cast on a barium fluoride disc using a Nicolet
model 760X FTIR spectrometer. Matrix-assisted laser
desorption/ionization time-of-flight mass spectra (MALDI-
TOF-MS) were measured using a Shimadzu-Kratos model
Axima CFR+ mass spectrometer with dithranol as matrix.
Cyclic voltammograms were recorded using a three
electrode system. A platinum button electrode was used
as the working electrode. A platinum wire served as the
counter electrode and an Ag/AgCl electrode was used as the
reference. Ferrocene/ferrocenium redox couple was used
as an internal standard. All the solutions were purged prior
to electrochemical and spectral measurements using argon
gas. Spectroelectrochemical measurements were performed
either on a Princeton Applied Research (PAR) diode array
rapid scanning spectrometer or a Shimadzu UV-visible
spectrophotometer using a homemade cell with optically
transparent (platinum mesh) electrodes. All the experiments
were carried out at 23 1 °C.
Introduction of the benzoquinone group as
a
substituent of porphyrins in turn introduces the possibility
of preparing heterometallic complexes [21, 29] where
the macrocycle binds, for instance, Ni(II) or Cu(II)
leaving the quinone moiety available to bind a different
metal cation. In some cases, complexation at quinone
can lead to formation of semiquinone type coordination
compounds perhaps permitting electronic and magnetic
coupling between the nuclei. Formation of semiquinone
radical substituents has already been demonstrated for
benzoquinone-substituted porphyrins although in that
case diamagnetic Zn(II) cations were present at both
coordination sites. In the course of this work we found that
the Cu(II) and Ni(II) complexes gave by far the best yields
of compounds described here. Other first row transition
metal complexes, although obviously behaving similarly
to the Cu(II) and Ni(II) analogs (judging by the brown/
black colour of the products), so far gave only complex
mixtures with no major product. It is currently unclear
why only Cu(II) or Ni(II) complexes lead to reasonable
yields of the o-quinone-substituted products. Finally, we
note that transition metal complexes of o-quinone ligands
make a major class of transition metal complexes and we
are looking forward to using the present system to bridge
between the photophysical features of the porphyrins
and the exceptional properties of the o-quinone (esp.
semiquinone) metal complexes.
Synthesis
1 and its metal complexes were prepared according to
previously reported methods [26]. Nitration was attempted
bythemethodofRicheteretal.[25]byadditionof9–12fold
excessesoflithiumnitratedissolvedina~50:50v/vmixture
of acetic acid and acetic anhydride to ~2 mM solutions
of the appropriate porphyrin in chloroform. Reaction
progress was monitored using thin layer chromatography
(silica plates; CH2Cl2/1–2% CH3OH). Complexes of
2 and 3 were isolated by column chromatography on
silica gel eluting with dichloromethane/1–2% methanol
or using preparative thin layer chromatography with the
same solvent system. 4 was prepared as a by-product
from another project [27] and it was prepared as its Ni(II)
complex by using the acetate method with monitoring by
t.l.c. (silica; CH2Cl2). Final purification was achieved by
using column chromatography (silica; CH2Cl2) yielding
4b which was subjected to nitrating conditions with
monitoring using t.l.c. (silica; CH2Cl2).
Preparation of 5,10,15,20-tetrakis(3,4-dioxo-5-t-
butylcyclohexa-1,5-dienyl)porphyrinatocopper(II)
(2a). 5,10,15,20-tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)-
porphyrininatocopper(II) (0.5 g, 0.42 mmol) was dissolved
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UNEXPECTED BUT CONVENIENT SYNTHESIS OF SOLUBLE MESO-TETRAKIS(3,4-BENZOQUINONE)-SUBSTITUTED
179
in chloroform (200 mL) and was warmed to 60 °C with
stirring. A solution of lithium nitrate (0.26 g, 3.77 mmol,
~9 equiv.) in a mixture of acetic acid (50 mL, 0.87 mol) and
acetic anhydride (50 mL, 0.525 mol) was added dropwise
over a period of 3 h with monitoring of the reaction mixture
using thin layer chromatography (silica plates; CH2Cl2/1–
2% CH3OH). The reaction mixture was then allowed to
cool to ambient temperature then neutralized by careful
addition of aqueous saturated NaHCO3. The mixture was
partitioned and the chloroform layer collected then dried
(anhydrous Na2SO4) and the solvents removed under
reduced pressure. The resulting black residue was applied
to a silica gel column and chromatographed eluting with
CH2Cl2/1–2% CH3OH. The highest yield brown/black
band (judging from t.l.c.) was collected (following the
elution of products still containing phenol groups). Solvent
was removed by evaporation under reduced pressure. A
more pure sample for analyses was obtained by preparative
thin layer chromatography on silica. 2a: black amorphous
powder. Yield 56% (240 mg). UV-vis (CHCl3): l, nm
(e, M-1.cm-1) 401 (182,400), 501 (35,200), 621 (18,900).
FT-IR (BaF2): n, cm-1 2959.8, 2920.3, 2869.9 (m, all C–H
str.), 1660.2 (s, C=O str), 1562.9 (w), 1536.4 (w), 1482.6
(w), 1466.8 (w), 1367.6 (m, C–H def.), 1342.5 (m, C–H
def.), 1297.8 (m), 1267.3 (m), 1238.2 (m), 1207.3 (w),
1144.7 (w), 1081.7 (w), 998.3 (m), 955.3 (m), 909.6 (m),
1562.0 (w), 1541.8 (w), 1481.8 (w), 1466.9 (w), 1433.4
(w), 1367.6 (m, C–H def.), 1346.4 (m, C–H def.), 1299.9
(m), 1267.7 (m), 1237.7 (m), 1147.4 (w), 1121.8 (w),
1080.2 (w), 1003.9 (m), 956.0 (w), 910.3 (w), 883.6
1
(w), 826.5 (w), 797.2 (m). H NMR (CDCl3, 300 MHz,
298 K): d, ppm 1.36 (s, 18H, tBu-H, quinone adj. to
phenol), 1.37 (s, 9H, tBu-H, quinone opp. phenol), 1.56
(s, 18H, tBu-H, phenol), 5.59 (s, 1H, phenol O–H), 6.94
4
4
(d, J = 2.10 Hz, 3H, quinone Ar-H), 7.56 (d, J = 2.10
Hz, 3H), 7.78 (s, 2H, phenol Ar-H, quinone Ar-H), 8.96
3
3
(d, J = 5.10 Hz, 2H, b-H), 9.11 (d, J = 4.80 Hz, 2H,
b-H), 9.23 (s, 4H, b-H). 13C NMR (CDCl3, 300 MHz,
298 K): d, ppm 29.39 (tBu methyl, quinone), 30.55
(tBu methyl, phenol), 34.58 (tBu-C, phenol), 35.76
(tBu-C, quinone), 115.17, 115. 70, 119.82, 123.56,
130.54, 130.78, 131.01, 131.44, 131.63, 131.69, 134.84,
134.88, 135.04, 138.68, 140.21, 140.55, 144.73, 148.52,
152.63, 179.53 (C=O), 180.41 (C=O). MALDI-TOF-MS
(dithranol): m/z 1058.38 (calcd. for C64H64N4O7Ni [M +
2H]+ m/z 1058.41).
Preparation of 5-(3,5-di-t-butyl-4-hydroxyphenyl)-
10,15,20-triphenylporphyrin (4a). This compound
remained from another project [26]. UV-vis (CHCl3):
l, nm (e, M-1.cm-1) 420 (400,500), 517 (20,100), 554
(13,100), 590 (9,300), 648 (9,600). FT-IR (BaF2): n,
cm-1 3631.8 (m, O–H str.), 3316.4 (m, N–H str.), 3054.7,
3024.6 (w, C–H str.), 2957.1, 2908.3 (m, C–H str.), 1596.7
(w, C=C str), 1556.0 (w, C=C str.), 1470.3 (m, C–H def.),
1440.4 (m), 1430.9 (m), 1399.9 (w), 1350.2 (m), 1311.1
(w), 1226.9 (m), 1186.4 (w), 1176.9 (w), 1154.2 (m),
1121.1 (w), 1071.4 (w), 1031.4 (w), 1001.7 (m), 971.9
(m), 963.4 (m), 926.9 (w), 889.5 (w), 874.6 (w), 847.2
1
880.9 (w), 826.4 (w), 798.3 (m). H NMR (CDCl3, 300
MHz, 298 K): d, ppm 1.34 (s, br, tBu-H), 6–8 (br., Ar-H).
HRMS (ESI): m/z 1020.2994 (calcd. for C60H51N4O8Cu
[M – H]- m/z 1020.2985).
Preparation of 5,10,15,20-tetrakis(3,4-dioxo-5-t-
butylcyclohexa-1,5-dienyl)porphyrinatonickel(II)
(2b). This was obtained in a similar manner to 2a as a
byproduct. 2b: black amorphous powder.Yield 44% (188
mg). UV-vis (CHCl3): l, nm (e, M-1.cm-1) 275 (107,000),
397 (145,000), 511 (36,000), 635 (17,000). FT-IR (BaF2):
n, cm-1 2959.1, 2919.9, 2869.2 (m, all C–H str), 1660.0
(s, C=O str), 1625.0 (m), 1561.9 (w), 1541.8 (w), 1481.8
(w), 1466.8 (w), 1367.7 (m, C–H def.), 1345.6 (m, C–H
def.), 1297.6 (m), 1267.7 (m), 1238.2 (m), 1207.5 (w),
1145.6 (w), 1082.3 (w), 1002.8 (m), 956.2 (m), 909.5
(m), 882.9 (w), 826.7 (m), 795.9 (m). 1H NMR (CDCl3,
300 MHz, 298 K): d, ppm 1.38 (s, 36H, tBu-H), 6.94 (d,
1
(w), 800.4 (s). H NMR (CDCl3, 300 MHz, 298 K): d,
ppm -2.77 (br. s, 2H, NH), 1.63 (s, 18H, tBu-H), 5.55
(s, 1H, phenol O–H), 7.75 (m, 9H, phenyl-H), 8.04 (s,
2H, phenol Ar-H), 8.23 (m, 6H, phenyl-H), 8.40 (m, 6H,
b-H), 8.93 (d, 3J = 4.50 Hz, 2H, b-H). MALDI-TOF-MS
(dithranol): m/z 743.35 (calcd. for C52H47N4O [M + H]+
m/z 743.37).
Preparation of 5-(3,5-di-t-butyl-4-hydroxyphenyl)-
10,15,20-triphenylporphyrinatonickel(II) (4b). 4a
(0.25 g, 0.34 mmol) was dissolved in chloroform (70 mL)
and warmed to 50 °C. A solution of nickel(II) acetate
tetrahydrate (0.4 g, 1.61 mmol) in methanol (10 mL) was
then added and the mixture was brought to reflux. Heating
was continued for 8 h then the solvents were removed
under reduced pressure. The resulting red-orange
residue was subjected to column chromatography (silica;
CH2Cl2). Product containing fractions were combined and
solvents were removed under reduced pressure and the
product recrystallized by dissolving in dichloromethane
(20 mL) and layering the resulting solution with the same
volume of methanol. Crystals suitable for single crystal
X-ray analysis were obtained by a further crystallization
on a smaller scale using the same method. 4b: red-purple
crystalline plates. Yield 85% (228 mg). UV-vis (CHCl3):
l, nm (e, M-1.cm-1) 418 (273,600), 530 (25,000). FT-IR
4
4J = 2.10 Hz, 4H, quinone Ar-H), 7.54 (d, J = 2.10 Hz,
4H, quinone Ar-H), 9.28 (s, 8H, b-H). 13C NMR (CDCl3,
300 MHz, 298 K): d, ppm 29.42 (tBu-methyl), 35.87
(tBu-C), 116.50, 131.95, 132.32, 138.38, 141.05, 149.02,
152.06, 179.31 (C=O), 180.27 (C=O). HRMS (ESI):
m/z 1015.3005 (calcd. for C60H51N4O8Ni [M – H]- m/z
1015.3015).
Preparation of 5-(3,5-di-t-butyl-4-hydroxyphenyl)-
10,15,20-tris(3,4-dioxo-5-t-butylcyclohexa-1,5-dienyl)-
porphyrinatonickel(II) (3b). This compound was
obtained as a minor product from the reaction to obtain
2b. 3b: black amorphous solid.Yield < 5%. FT-IR (BaF2):
n, cm-1 3628.9 (w, O–H str.), 2958.1, 2920.1, 2869.5 (m,
all C–H str.), 1660.6 (s, C=O str.), 1635.7 (m, C=C str.),
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 179–181

180
J.P. HILL ET AL.
(BaF2): n, cm-1 3631.9 (m, O–H str.), 3055.5, 3024.7 (w,
C–H str.), 2957.2, 2910.6, 2871.1 (m, C–H str.), 1599 (m,
C=C str.), 1574.4 (w), 1546.9 (w), 1440.11 (m), 1433.2
(m), 1391.1 (w), 1352.7 (m), 1311.4 (m), 1223.9 (m),
1205.2 (w), 1177.5 (w), 1155.0 (m), 1121.3 (m), 1073.2
(m), 1006.1 (s), 942.9 (w), 889.1 (w), 832.9 (w), 825.8
refinement against F2 (all data) were carried out
using the SHELXTL package [28]. The structure was
refined as a two component inversion twin. All ordered
non-H atoms were refined anisotropically. X-ray
crystallography data for 4b: C52H44N4NiO, M = 799.63
g.mol-1, space group P-1, a = 6.190(5), b = 10.831(5),
c = 32.528(5) Å, a = 80.588(5), b = 85.784(5), g =
77.922(5)°, U = 2102(2) Å3, Z = 2, rcalcd = 1.264 mg.m-3,
m(MoKa) = 0.070 mm–1, 64713 reflections measured,
9816 were unique (Rint. 0.0357); refinement against F2
to wR2: 0.1627, R1 = 0.0650 (8093 reflections with I >
2s(I)), 531 parameters.
1
(w), 797.7 (s), 751.8 (m). H NMR (CDCl3, 300 MHz,
298 K): d, ppm 1.57 (s, 18H, tBu-H), 5.48 (s, 1H, phenol
O–H), 7.66 (m, 9H, phenyl-H), 7.82 (s, 2H, phenol Ar-H),
8.01 (m, 6H, phenyl-H), 8.73 (s, 4H, b-H), 8.74 (d, 3J =
4.80 Hz, 2H, b-H), 8.83 (d, 3J = 5.10 Hz, b-H). 13C NMR
(CDCl3, 300 MHz, 298 K): d, ppm 30.62 (tBu methyl),
34.55 (tBu-C), 118.66, 118.80, 120.55, 126.86, 127.69,
131.09, 131.95, 132.03, 132.08, 132.62, 133.72, 134.22,
134.31, 134.55, 141.06, 142.53, 142.65, 143.26, 153.61.
MALDI-TOF-MS (dithranol): m/z 797.60 (calcd. for
C52H45N4ONi [M – H]+ m/z 797.28).
CONCLUSION
In conclusion, we have observed the unexpected
oxidative dealkylation of some metal complexes of
5,10,15,20-tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)-
porphyrin (1), which results directly in the analogous
complexes of 5,10,15,20-tetrakis(3,4-dioxo-5-t-butylcy-
clohexa-1,5-dienyl)porphyrin (2) in reasonable yields.
These compounds have several significant features
including strong electronic absorption across the visible
range (they appear almost black in solution and the solid
state) and high solubilities leading to easy handling
for film formation or other processing, and reversible
electrochemical reduction processes.
We are also currently investigating the synthesis of
discrete heterometallic complexes of these porphyrins
in order to investigate magnetic interactions between
macrocyclic core metal cation and those complexed at the
rather remote periphery of these molecules. We believe
that the inter-metal-cation coupling interactions will be
mediated by semiquinone radicals where an appropriate
metal cation is complexed at the benzoquinone moieties.
Details of the structures as well as the catalytic and
magnetic properties of these complexes will be reported
later. We note that o-quinones are a versatile starting
point for other functionalities including ring-fused or
ring-opened products [22, 23]. We will also report our
investigations of those possibilities in due course. Finally,
several other 3,5-di-t-butyl-4-hydroxyphenyl-substituted
chromophores are available in our laboratory including
phthalocyanine, corroles and porphyrins with the
antioxidantgroupsnotdirectlyattachedtothemacrocycle.
We will shortly report on the transformations of those
compounds in the context of the reaction reported here.
Preparation of 5-(3,4-dioxo-5-t-butylcyclohexa-1,5-
dienyl)-10,15,20-triphenylporphyrinatonickel(II) (5).
4b (168 mg, 0.21 mmol) was treated with lithium nitrate
(36 mg, 0.52 mmol, ~2.5 equiv.) using the same method
as that leading to 2b and using proportional volumes of
acetic anhydride/acetic acid. The reaction was monitored
using t.l.c. (silica; CH2Cl2). Upon completion the
reaction was neutralized and 5 was isolated by using the
same method as for 2b. 5: dark purple microcrystalline
powder. Yield 144 mg (89%). UV-vis (CHCl3): l, nm
(e, M-1.cm-1) 410 (255,000), 528 (19,300), 630 (10,300).
FT-IR (BaF2): n, cm-1 3102.4, 3055.8, 3025.6, 2994.7 (all
weak, C–H str.), 2958.9 (m, C–H str.), 2868.2 (w, C=H
str.), 1659.8 (s, C=O str.), 1621.4, 1599.1 (w, C=C str.),
1562.1 (w), 1550.4 (w), 1481.8 (w), 1459.6 (w), 1440.8
(m), 1367.0 (m), 1347.5 (m), 1299.6 (w), 1266.9 (w),
1238.3 (m), 1206.4 (w), 1177.1 (w), 1074.6 (m), 1006.1
(s), 957.2 (w), 908.7 (m), 833.9 (w), 827.7 (w), 797.7
1
(m), 752.5 (w), 730.8 (w), 698.3 (w). H NMR (CDCl3,
300 MHz, 298 K): d, ppm 1.34 (s, 9H, tBu-H), 6.96 (d,
4J = 1.20 Hz, 1H, quinone H), 7.68 (m, 10H, phenyl-H,
quinone H), 7.97 (m, 6H, phenyl-H), 8.72 (s, 4H, b-H),
3
3
8.86 (d, J = 5.10 Hz, 2H, b-H), 9.11 (d, J = 4.80 Hz,
2H, b-H). 13C NMR (CDCl3, 300 MHz, 298 K): d, ppm
29.42 (tBu methyl), 35.65 (tBu-C), 113.89, 119.99,
120.45, 126.84, 126.94, 127.00, 127.97, 128.02, 129.85,
131.27, 132.65, 132.85, 133.46, 133.63, 133.65, 139.29,
139.74, 140.48, 140.50, 143.08, 143.21, 143.41, 147.75,
153.52, 179.97 (C=O), 180.69 (C=O). MALDI-TOF-MS
(dithranol):m/z757.42(calcd. forC48H34N4O2Ni[M+H]+
m/z 756.20).
Acknowledgements
X-ray crystallography
This research was partly supported by the World
Premier International Research Center Initiative on
Materials Nanoarchitectonics from MEXT, Japan,
and by the Core Research for Evolutional Science and
Technology (CREST) program of JST, Japan. This work
was also supported by the National Science Foundation
(Grant No. 1110942 to FD).
Crystals of 4b suitable for X-ray diffraction were
grown by diffusion of hexane into a solution of 4b in
dichloromethane. X-ray data for 4b were collected at
200 K on a Rigaku XtaLAB P200 diffractometer using
graphite-monochromated MoKa radiation. Structure
solutions by direct methods and full-matrix least-squares
Copyright © 2014 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2014; 18: 180–181

UNEXPECTED BUT CONVENIENT SYNTHESIS OF SOLUBLE MESO-TETRAKIS(3,4-BENZOQUINONE)-SUBSTITUTED
181
Supporting information
16. Shundo A, Labuta J, Hill JP, Ishihara S and Ariga K.
J. Am. Chem. Soc. 2009; 131: 9494–9495.
17. Labuta J, Ishihara S, Šikorský T, Futera Z, Shundo
A, Hanyková L, Burda JV, Ariga K and Hill JP. Nat.
Commun. 2013; DOI: 10.1038/ncomms3188
18. Xie Y, Hill JP, Schumacher AL, Sandanayaka ASD,
ArakiY, Karr PA, Labuta J, DSouza F, Ito O, Anson
CE, Powell AK and Ariga K. J. Phys. Chem. C
2008; 112: 10559–10572.
19. Geng F, Gao H, Meng Q, Dong Z, Wakayama Y,
Akada M, Ariga K and Hill JP. Chem. Commun.
2011; 47: 8533–8535.
20. Subbaiyan NK, Hill JP, Ariga K, Fukuzumi S and
D’Souza F. Chem. Commun. 2011; 47: 6003–6005.
21. Jose DA, Shukla AD, Kumar K, Ganguly B, Das
A, Ramakrishna G, Palit DK and Ghosh HN. Inorg.
Chem. 2005; 44: 2414–2425.
1H and 13C NMR spectra for new compounds; atomic
coordinates for calculated structure of the free base form
of 5; crystallographic information file for 4b (Figs S1–S9)
are given in the supplementary material. This material is
available free of charge via the Internet at http://www.
worldscinet.com/jpp/jpp.shtml.
Crystallographic data (excluding structure factors)
for the structures in this paper have been deposited at
the Cambridge Crystallographic Data Centre (CCDC)
under number CCDC-956468. Copies can be obtained on
request, free of charge, via www.ccdc.cam.ac.uk/data_
request/cif or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax:
+44 1223-336-033 or email: deposit@ccdc.cam.ac.uk).
22. Speck M, Kurreck H and Senge MO. Eur. J. Org.
Chem. 2000; 2303–2314.
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J. Porphyrins Phthalocyanines 2014; 18: 181–181