Skeletal modification of a non-planar phthalocyanine analogue under very mild conditions

Article abstract of DOI:10.1039/b512442e

A sterically congested tetraphenyl-substituted dibenzo-tetraaza- isobacteriochlorinato nickel derivative unexpectedly reacts with methanol at room temperature to release internal molecular strains, leading to the formation of a ligand having a skeleton bearing two cis-methoxy substituents. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b512442e

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Skeletal modification of a non-planar phthalocyanine analogue under
very mild conditions{
Takamitsu Fukuda, Yusuke Ogi and Nagao Kobayashi*
Received (in Cambridge, UK) 2nd September 2005, Accepted 20th October 2005
First published as an Advance Article on the web 17th November 2005
DOI: 10.1039/b512442e
to the presence of tetrahedral sp3-hybridized carbons. In addition,
A sterically congested tetraphenyl-substituted dibenzo-tetraaza-
we have clarified that the introduction of phenyl groups in the, so-
called, adjacent a-positions produces highly deformed non-planar
Pcs.9 Both of these two types of Pc analogue have, in common,
structural deformations in the aromatic core. These deformations
increase the intramolecular stresses, and reduce the resonance
energy relative to the corresponding planar analogues. In this
study, we have observed that the combination of dehydrogenation
and phenyl-substitution imparts unusual chemical properties to
Pcs, such that tetraphenyl-substituted di-benzo TAiBC (1) shows
interesting chemical reactivities to methanol under very mild
conditions (Scheme 1).
Mixed condensation of 3,6-diphenylphthalonitrile10 and tetra-
methylsuccinonitrile11 in the presence of NiCl2 yielded three
possible macrocycles, i.e. tetraazachlorin (TAC), tetraazabacterio-
chlorin (TABC), and TAiBC, 1.{12 Nickel was chosen for synthetic
reasons.8 The mixture was first purified by column chromato-
graphy in order to remove unassignable by-products, and TAC
and TABC were then removed by stepwise extraction using hexane
and methanol, respectively. Compound 1 is moderately soluble in
both hexane and methanol, while TAC and TABC are very
sparingly soluble in hexane or methanol, respectively. Fig. 1 (solid
line) shows the absorption spectrum of 1 in toluene. The Q band
appears at 711 nm, red-shifted by ca. 40 nm from the
isobacteriochlorinato nickel derivative unexpectedly reacts with
methanol at room temperature to release internal molecular
strains, leading to the formation of a ligand having a skeleton
bearing two cis-methoxy substituents.
Phthalocyanines (Pcs) have an 18p electron aromatic system with
flat molecular structures. Since their accidental discovery in the
early 20th century,1 Pcs have contributed significantly to the dye
and pigment industries due to their prominent applicability. The
versatile spectroscopic and electrochemical properties of Pcs have
also attracted the interest of researchers in many other areas, such
as optical devices, catalysts, electrical conductors, photodynamic
cancer therapy, and chemical sensors.2 It is well known that the
chemical and physical stabilities of typical Pcs are excellent. For
example, commercially available CuPc is soluble in conc. H2SO4
without decomposition, and sublimes with increasing temperature
under vacuum. This robustness, however, restricts the chemical
modification of Pcs under mild conditions, so that Pc core
modifications have rarely been reported. Recently, McGaff and
coworkers have reported that solventothermal reactions of
Ni(CH3COO)2?4H2O with 1,2-dicyanobenzene in alcohol yielded
14,28-di-alkoxy-substituted Pc derivatives.3,4 They concluded,
however, that the formation of the alkoxy-substituted derivatives
occurs during the Pc synthesis as intermediates. Hence, the
modification of the Pc cores in this case occurs as they form, rather
than following the formation of unsubstituted Pcs. 1,2-dicyano-
benzene reacts with indium metal to give the 14,28-bicycloPc
derivative,5,6 although transformation of Pc into the bicycloPc has
not been achieved. As another example, the reaction of TiPcCl2 (or
NbPcCl2) and Na2Pc at 190 (220) uC in chloronaphthalene gives
‘stapled’ TiPc2 (NbPc2).7 These results imply that the 14 and 28
positions are relatively reactive compared to the other segments of
the Pcs during synthesis, but that once the Pcs are formed, these
positions are no longer active.
Tetraazaisobacteriochlorin (TAiBC) is a Pc analogue, in which
two adjacent pyrrole b-carbons are chemically dehydrogenated.
Recently, dibenzo-fused stable TAiBC has been isolated by
employing tetramethylsuccinonitrile as one of the starting
materials.8 The X-ray analysis revealed that the geometry of the
dehydrogenated pyrrole sites deviates slightly from planarity due
Department of Chemistry, Graduate School of Science, Tohoku
University, Sendai 980-8578, Japan.
E-mail: nagaok@mail.tains.tohoku.ac.jp; Fax: +81-22-795-7719;
Tel: +81-22-795-7719
{ Electronic supplementary information (ESI) available: DFT optimized
structures of 1 and 2. See DOI: 10.1039/b512442e
Scheme 1 Chemical reactivities of 1 and 2.
Chem. Commun., 2006, 159–161 | 159
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Fig. 1 Absorption spectra of 1 (solid line), 2 (dotted line), and recovered
1 by annealing (broken line) in toluene. Note that the ordinate is arbitrary.
Fig. 2 ORTEP representation of 2 (50% ellipsoids). Hydrogen atoms are
omitted for clarity. Selected numbering system used in the analysis is
depicted.
corresponding unsubstituted TAiBC. It is known that skeletal
deviation causes a sizable bathochromic shift of the Q band of
porphyrins and Pcs.9b,13 The DFT (B3LYP/6-31G(d)) optimized
structure also suggested a highly deformed aromatic core of 1 due
to steric congestion of the adjacent phenyl groups (see Supporting
Information{). Scheme 2 (top left) shows the color of a solution of
1 in toluene–methanol (1 : 4 (v/v)). The initial deep blue color,
originating from p–p* transitions, changed to yellow on allowing
the solution to stand for ca. 2 weeks at room temperature
(Scheme 2, top right). In the case of TAC and TABC, colored
solids precipitated after standing for 2 weeks. The presence of a
radical trapping reagent (phenyl-N-tert-butyl nitrone) did not
prevent the reaction. As shown in Fig. 1 (dotted line), the Q band
disappeared completely, which indicates that the entire aromaticity
of the macrocycle has been lost. X-ray quality single crystals of this
unknown product were found on the bottom of the sample vessel.§
Fig. 2 represents the ORTEP drawing of the yellow product. The
structure was confirmed as 2, that is, two methoxy groups are
bound to the diagonal pyrrole a-carbons of 1 (C4 and C12). The
presence of sp3 carbons distorts the molecular skeleton signifi-
cantly, and the conjugation order is halved. The C–O bond
shown in the crystal structure, or that the trans isomer also exists in
the solution. In order to compare the deformation mode with
14,28-(MeO)2NiPc, colormap analysis was performed.14 As shown
in Fig. 3, the four meso-nitrogens lie out of the 4Npyrrole plane,
while the two fused benzene rings deviate either positively or
negatively. This type of serpentine isogram is very similar to that
obtained for 14,28-(MeO)2NiPc,14 indicating that 2 is practically
released from the phenyl–phenyl congestion. Compound
2
maintains the yellow color after evaporation of the solvent
(Scheme 2, bottom right). However the color changed to green
after annealing at 120 uC for 24 h (bottom left), and the absorption
spectrum regained Q-band intensity (Fig. 1, broken line).
Probably, the annealing induced a partial decomposition of 2,
and the formed by-products are responsible for the relatively large
absorbance in the 300–500 nm region. This is why the recovered 1
appears green rather than the original blue. Taken together, the
above data suggest that 1 and 2 reversibly transform between each
other by choosing suitable reaction conditions (Scheme 1, top).
Compound 2 was also characterized by ESI-TOF-Mass
spectrometry. The samples for the measurements were prepared
by dissolving 2 in CHCl3–ethanol. Measurements were performed
both immediately after and 24 h after sample preparation. The first
˚
distances are 1.414(4) and 1.413(3) A for O1–C4 and O2–C12,
respectively. It is conceivable that one of the methoxy groups binds
at the fused-benzene site (O2) so that the intramolecular stresses
could be reduced. We are unsure whether the other methoxy group
(O1) binds selectively on the same side of the molecular plane as
Fig. 3 Colormap analysis of the crystal structures of 2. Hydrogen atoms,
and phenyl and methoxy groups are omitted for clarity. Hollow marks
indicate sp3 carbons.
Scheme 2 Color appearances in toluene–MeOH (1 : 4 (v/v)) (top) and in
the solid state (bottom) for 1 (left) and 2 (right).
160 | Chem. Commun., 2006, 159–161
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a catalytic amount of ammonium molybdate was refluxed for 2 h in
quinoline under nitrogen. After cooling to room temperature, the reaction
mixture was diluted with EtOH–H2O (1 : 1 v/v, 50 mL). The resulting
precipitate was filtered, and washed with hot EtOH–H2O until the
washings were colorless. The crude residue was passed through a short
silica column (CHCl3) in order to remove polymeric by-products, and the
obtained eluate was evaporated. A succession of extractions of the mixture
with hexane and methanol followed by chromatographic separation (silica,
toluene–cyclohexane (4 : 1 v/v)) gave the desired 1. Spectral data: Mass
(MALDI-TOF): m/z 890 (M+); elemental analysis calcd for C56H48N8Ni:
C, 75.43; H, 5.43; N, 12.57; found: C, 73.33; H, 5.58; N; 11.60%.
§ Crystal data for 2: C58H54N8NiO2, M = 953.80, a = 11.817(5), b =
˚
15.359(6), c = 15.504(6) A, a = 73.82(2), b = 75.54(2), c = 64.453(18)u,
3
˚
¯
U = 2410.9(17) A , T = 223(2) K, space group P1(no. 2), Z = 2, m(Mo-
Ka) = 0.456 mm21, 18990 reflections collected, 8331 unique (Rint = 0.0257),
R1 = 0.0534 [I . 2s(I)], wR2 = 0.1215 (all data). CCDC 283235. For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
b512442e.
1 A. Braun and J. Tcherniac, Chem. Ber., 1907, 40, 2709.
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Smith, and R. Guilard, Academic Press, San Diego, 2003; (b)
Phthalocyanine Materials-Synthesis, Structure and Function, ed. N. B.
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Phthalocyanines-Properties and Applications, Vol. 1–4, ed. C. C.
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Thomas, CRC Press, Boca Raton, FL, 1983.
Fig. 4 Calculated isotopic patterns for 2–4 (top). ESI-TOF mass spectra
of 2 in CHCl3–EtOH measured immediately after (middle) and 24 h after
(bottom) sample preparation.
3 C. D. Molek, J. A. Halfen, J. C. Loe and R. W. McGaff, Chem.
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4 The 14 and 28 positions represent diagonal pyrrole a-carbons of Pcs.
5 J. Janczak and R. Kubiak, J. Chem. Soc., Dalton Trans., 1994, 2539.
6 14,28-bicycloPc consists of one indium ion and six isoindole units. That
is, an isoindole dimer unit binds to the Pc skeleton at the 14 and 28
positions.
7 (a) C. Ercolani, A. M. Paoletti, G. Pennesi, G. Rossi, A. Chiesi-Villa
and C. Rizzoli, J. Chem. Soc., Dalton Trans., 1990, 1971; (b)
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Soc., 2001, 123, 10740; (b) T. Fukuda, S. Homma and N. Kobayashi,
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11 W. Barbe, H.-D. Beckhaus, H.-J. Lindner and C. Ru¨chardt, Chem. Ber.,
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measurement detected molecular ion peaks at 952 amu, corre-
sponding to 2.15 However, the second run clearly indicates the
existence of methoxy-ethoxy (3) and bis-ethoxy (4) derivatives
(966, 980 amu, respectively). Neither 3 nor 4 were detected in the
first run (Fig. 4). Therefore, these are clearly formed not in the
mass spectrometer, but in the sample solution during standing
(Scheme 1, bottom).
In summary, we have found that the structurally stressed
dibenzo-fused TAiBC derivative, 1, reacts with methanol to give a
bis-methoxy adduct, 2, under very mild conditions. The color of 1
changes from deep blue to yellow as a result of the reaction.
Furuta treated N-fused porphyrins with NaOMe–MeOH to give
methoxy-substituted N-confused porphyrins.16 Senge reported that
treatment of metal-free tetra-meso-tert-butyl porphyrin with
HClO4 and methanol yielded the meso-methoxy-substituted
porphyrin (porphodimethene).17 To our knowledge, however, this
is the first example in which an aromatic Pc core reacts directly
with an alcohol. Although further studies are necessary in order to
determine the structural requirements of Pcs to induce this type of
reaction, we suggest that the relatively high solubility of 1 in
methanol is a likely reason.
12 Detailed characterization of the TAC and TABC derivatives synthesized
in this study will be reported elsewhere.
13 (a) A. K. Wertsching, A. S. Koch and S. G. DiMagno, J. Am. Chem.
Soc., 2001, 123, 3932; (b) S. G. DiMagno, A. K. Wertsching and
C. R. Ross, J. Am. Chem. Soc., 1995, 117, 8279; (c) H. Ryeng and
A. Ghosh, J. Am. Chem. Soc., 2002, 124, 8099; (d) R. E. Haddad,
S. Gazeau, J. Pe´caut, J. Marchon, C. J. Medforth and J. A. Shelnutt,
J. Am. Chem. Soc., 2003, 125, 1253; (e) T. Fukuda, T. Ishiguro and
N. Kobayashi, Tetrahedron Lett., 2005, 46, 2907.
This research was supported by a Grant-in-Aid for the COE
project, Giant Molecules and Complex Systems, 2005, and Grant
for Encouragement of Young Scientists (B) No. 17750029 and
Grant-in-Aid for Scientific Research (B) No. 17350063 from the
Ministry of Education, Culture, Sports, Science, and Technology,
Japan.
14 T. Fukuda and N. Kobayashi, J. Porphyrins Phthalocyanines, 2004, 8,
1251.
15 Ion peaks corresponding to water adducts were also detected. Water
and/or proton adducts are often observed for ESI-Mass measurements.
16 (a) H. Furuta, T. Ishizuka, A. Osuka and T. Ogawa, J. Am. Chem. Soc.,
2000, 122, 5748; (b) H. Furuta, T. Ishizuka, A. Osuka and T. Ogawa,
J. Am. Chem. Soc., 1999, 121, 2945.
17 (a) T. Ema, M. O. Senge, N. Y. Nelson, H. Ogoshi and K. M. Smith,
Angew. Chem., Int. Ed. Engl., 1994, 33, 1879; (b) M. O. Senge,
I. Bischoff, N. T. Nelson and K. M. Smith, J. Porphyrins
Phthalocyanines, 1999, 3, 99.
Notes and references
{ 1: A mixture of 3,6-diphenylphthalonitrile (1.0 g, 3.6 mmol), tetra-
methylsuccinonitrile (780 mg, 5.73 mmol), NiCl2 (500 mg, 3.86 mmol) and
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Chem. Commun., 2006, 159–161 | 161