Quinone recognition by amide hydrogen bonding in porphyrin systems

Article abstract of DOI:10.1039/a806621c

5.15-cis-Bis(2-trifluoroacetamidophenyl)porphyrin 2c and its pentafluorobenzamidophenyl analogue 2e have significant recognising ability for p-benzoquinone 3a, which is explained by the large negative enthalpy change of the 2c-3a complex and by the small

Full text of DOI:10.1039/a806621c

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Quinone recognition by amide hydrogen bonding in porphyrin
systems
Kiyoshi Tanaka,* Yasuhiro Yamamoto, Isao Machida and Satoru Iwata
Faculty of Engineering, Seikei University, Musashino-shi, Tokyo 180-8633, Japan
Received (in Cambridge) 24th August 1998, Accepted 15th December 1998
5,15-cis-Bis(2-trifluoroacetamidophenyl)porphyrin 2c and its pentafluorobenzamidophenyl analogue 2e have
significant recognising ability for p-benzoquinone 3a, which is explained by the large negative enthalpy change of
the 2c–3a complex and by the small entropy change of the 2e–3a complex, respectively. Porphyrin 2e recognises
electron-rich p-benzoquinone more effectively than electron-deficient p-benzoquinone, which is ascribed to the
enthalpy change of the complexation induced by the hydrogen bonding of amide protons of 2e with oxygen atoms
of the substituted p-benzoquinone.
with respect to perfluoroheptanecarboxamido-, heptanecarbox-
Introduction
amido-, trifluoroacetamido-, acetamido-, pentafluorobenz-
amido-, and benzamidoporphyrins (2a, 2b, 2c, 2d, 2e, and 2f,
respectively), together with the trans-isomer of 2a (2a-trans)
and 5,15-cis-bis(2-aminophenyl)-2,8,12,18-tetraethyl-3,7,13,17-
tetramethylporphyrin (1). Porphyrins 2c and 2d were prepared
by the acylation of 1 with the corresponding anhydride in 78
and 71% yield, respectively.13 Porphyrins 2a, 2b, 2e, and 2f were
obtained by the acylation of 1 with the corresponding acyl
chloride in 73, 80, 63, and 98% yield, respectively (Scheme 1).
The trans-isomer (2a-trans) was prepared from the trans-isomer
of 1 in 80% yield.
In the photoinduced electron transfer in bacterial photo-
synthetic reaction centres, quinone derivatives, which are
bound to the protein by hydrogen bonding to the carbonyl oxy-
gens, play an important role as the electron acceptor.1–4 Con-
structions of the artificial porphyrin–quinone assembly have
been attained by considering noncovalent interactions based on
the two- and four-point hydrogen bonding of meso substituted
naphthols on the porphyrin ring,5–8 the redox coupling of
hydroquinone attached to the porphyrin ring,9 and the amide
hydrogen bonding of the macrocyclic tetraamide which fulfils
the role of a scaffold by the incorporation of a metallo-
porphyrin coordination.10,11 More direct hydrogen bonding
of the amide substituents attached to the porphyrin ring
affords another promising strategy for the porphyrin–quinone
assembly,12 since the acidity of the amide hydrogen can be con-
trolled by varying the attached acyl group and, moreover, the
multipoint hydrogen bonding can be constructed by the intro-
duction of a functionalized acyl group. In this paper, we would
like to demonstrate the p-benzoquinone recognition ability
of 5,15-cis-bis(2-amidophenyl)-2,8,12,18-tetraethyl-3,7,13,17-
tetramethylporphyrins 2 and, in particular, to focus upon the
effect of the introduced acyl group on the association constant
for the porphyrin–p-benzoquinone complexation.
Determination of the association constant for the
porphyrin–3a complexation was first attempted by UV titra-
tion; however, it was found that the absorption change in the
presence of varying amounts of 3a was too small for the titra-
tion. The Stern–Volmer plot for the fluorescence quenching of
porphyrin with 3a indicates a linear correlation, which must be
based on intermolecular collision of photoexcited porphyrin
and a quinone molecule. The reliable intracomplex quenching,
which can make possible the Benesi–Hildebrand treatment, is
not realised. The 1H NMR technique is another plausible
1
method for the titration. The H NMR spectrum of a mixture
of porphyrin 2a and 3a showed sharp and single resonances for
downfield-shifted amide protons of 2a and for upfield-shifted
protons of 3a, which are consistent with the face-to-face struc-
ture 4. The association constant for the porphyrin–3a complex-
ation was evaluated from the 1H NMR titration for the change
of the downfield-shifted amide protons of porphyrin, the con-
Results and discussion
The recognising ability for p-benzoquinone (3a) was evaluated
R'
R'
O
O
R'
R
N
R'
R'
R
N
R'
R'
O
O
O
O
K
3a:R' = H, 3b: R' = CH3, 3c: R' = F
H
H
R'
R
N
+
R
N
N
N
N
H
H
O
O
N
H
H
N
N
N
N
H
H
4
2a
2b
2c
2d
2e
2f
R = C7F15 C7H15
CF3 CH3 C6F5 C6H5
Scheme 1
J. Chem. Soc., Perkin Trans. 2, 1999, 285–288
285

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Table 1 Association constants (K/MϪ1) for porphyrin–p-benzoquinone 3a complexation in C6D5CD3–CDCl3 (10:1)a
2a
2b
2c
2d
2e
2f
2a-trans
1
(2.4 0.3) × 10b
(3.5 0.4) × 10b
(1.5 0.1) × 102 c
(9.7 0.4) × 10c
(1.7 0.2) × 102 c
8.3 4.6c
2.7 0.5b
6.2 1.2b
1
a The association constants were determined by H NMR titration, the concentration of porphyrin being 2.5 × 10Ϫ3 M. b Determined at 293 K.
c Determined at 298 K.
Table 2 Association constants (K/MϪ1) and thermodynamic parameters for porphyrin–p-benzoquinone complexation in C6D5CD3–CDCl3 (10:1)a
K
∆GЊ298
/
∆HЊ/
T∆SЊ298/
253 K
263 K
273 K
298 K
313 K
kcal molϪ1
kcal molϪ1
kcal molϪ1
2c–3a (1.3 0.1) × 103 (7.5 0.3) × 102 (4.6 0.1) × 102 (1.5 0.1) × 102 (8.9 0.6) × 10 Ϫ3.0 0.1
2d–3a (7.2 0.7) × 102 (4.2 0.4) × 102 (2.6 0.2) × 102 (9.7 0.4) × 10 (5.8 0.2) × 10 Ϫ2.7 0.1
2e–3a (9.0 0.1) × 102 (5.6 0.7) × 102 (3.8 0.3) × 102 (1.7 0.2) × 102 (1.1 0.1) × 102 Ϫ3.1 0.1
2e–3b (1.7 0.3) × 103 (1.0 0.1) × 103 (5.6 0.3) × 102 (2.2 0.1) × 102 (1.3 0.1) × 102 Ϫ3.2 0.1
2e–3c (1.1 0.1) × 102 (7.6 0.5) × 10 (6.1 0.4) × 10 (3.3 0.4) × 10 (2.9 0.2) × 10 Ϫ2.1 0.1
Ϫ7.1 0.1
Ϫ6.6 0.1
Ϫ5.4 0.1
Ϫ6.7 0.2
Ϫ3.5 0.2
Ϫ2.7 0.2
Ϫ4.1 0.1
Ϫ3.9 0.1
Ϫ2.3 0.2
Ϫ3.5 0.2
Ϫ1.5 0.3
Ϫ1.5 0.5
2f–3a (2.0 0.5) × 10 (1.5 0.5) × 10 (1.2 0.4) × 10
8.3 4.6
6.7 3.7
Ϫ1.3 0.3
a The association constants were determined by 1H NMR titration, the concentration of porphyrin being 2.5 × 10Ϫ3 M.
centration of porphyrin being 2.5 × 10Ϫ3 M and that of 3a
being varied from 1 to 200 equimolar amount in C6D5CD3–
CDCl3 (10:1).14 The results are collected in Table 1.
The appreciable association constant of 2a, compared with
that of 2a-trans, supports the two-point hydrogen bonding
described in structure 4. The negligible association constant of
porphyrin 1 indicates not only the weak hydrogen bonding of
amino protons but also the weak π–π stacking of the amino-
phenyl substituted porphyrin ring and of the aminophenyl ring
itself with 3a.15 Porphyrin 2a is expected to show more rigid
complexation than 2b, since the amide protons of 2a are more
acidic than those of 2b because of the strongly withdrawing
fluorine group. However, a slight preference of 2b over 2a
in association constants is in evidence, which indicates that
the complexation is affected by both the acidity of the amide
protons and the steric effect of the substituent, because the
non-flexibility of the perfluoroalkyl group, other than its
electron-withdrawing effect, is well known.16
Trifluoroacetamidoporphyrin 2c, the trifluoromethyl group
of which is expected to be small enough for the complexation,
was next evaluated; the association constant was attained and
was found to be greater than that of acetamidoporphyrin 2d. It
is remarkable that pentafluorobenzamidoporphyrin 2e recog-
nises 3a effectively, its association constant being comparable to
that of trifluoroacetamidoporphyrin 2c, and this is in sharp
contrast to the negligible association constant of benz-
amidoporphyrin 2f.
Fig. 1 van’t Hoff plots for porphyrin–p-benzoquinone complexation
in C6D5CD3–CDCl3 (10:1). ᭺, 2e–3b (r = 0.9985); ᭜, 2c–3a (r =
0.9998); ᭞, 2e–3a (r = 9990); ᭿, 2d–3a (r = 0.9998); ᭝, 2e–3c (r =
0.9947); ×, 2f–3a (r = 0.9936).
of 2f reflects the considerable decrease in the enthalpy change,
rather than the entropy change, and this seems to be caused by
the weakly electron-withdrawing property of the phenyl group
in 2f.
The complexation equilibria of the selected porphyrins 2c,
2d, 2e, and 2f with p-benzoquinone 3a were next followed
by similar 1H NMR titration at various temperatures. The
obtained association constants are summarized in Table 2 and
the satisfactorily linear van’t Hoff plot, giving the relationship
between the association constants and temperature, is described
in Fig. 1. The thermodynamic parameters, which are evaluated
by the van’t Hoff equation, are also collected in Table 2. As was
expected, the sufficiently negative free energy change of the
complex with 2c is mainly affected not by entropy change but
by enthalpy change, which shows that the trifluoromethyl group
is acting as a strongly electron-withdrawing group of compar-
able size to the methyl group in 2d. On the contrary, it is of
interest that the small entropy change of the complex with 2e
overcomes a slight decrease in enthalpy change, compared with
2d, and facilitates the formation of the complex. It is likely that
the small entropy change is due to the mesomeric delocalization
between the pentafluorophenyl group and the carbonyl group
in both 2e and its complex with 3a, resulting in the restriction
of the free rotation of the pentafluorophenyl group. It should
be added that the small negative free energy change in the case
The recognising ability for electron-rich tetramethyl-p-benzo-
quinone (3b) and electron-deficient tetrafluoro-p-benzoquinone
(3c) was next evaluated using pentafluorobenzamidoporphyrin
2e. As shown in Table 2, the electron-releasing methyl group
brings about an outstanding increase in association constants
and conversely the electron-withdrawing fluorine atom leads to
a decrease in the association constant, compared with that of
3a. This order 3b > 3a > 3c in association constants is primarily
caused by enthalpy change. It is realised that the small enthalpy
change in the case of 3c demonstrates the negligible contribu-
tion of the π–π stacking between electron-donating porphyrin
and electron-accepting tetrafluorobenzoquinone 3c. Hydrogen
bonding of the amide protons of 2e with the oxygen atoms of
p-benzoquinones seems to play a main role in the complexation
and it is supported by the calculated net charge of the oxygen
atoms of p-benzoquinones; that is, the order of net charge
Ϫ0.54 for 3b > Ϫ0.52 for 3a > Ϫ0.46 for 3c is parallel to the
decreasing association constants 2e–3b > 2e–3a > 2e–3c.17
286
J. Chem. Soc., Perkin Trans. 2, 1999, 285–288

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In conclusion, (2-trifluoroacetamido- and 2-pentafluoro-
benzamido)porphyrins 2c and 2e have significant recognising
ability for p-benzoquinone 3a and it should be noted that their
recognising ability is as good as or slightly superior to that of
(2-hydroxy-1-naphthyl)porphyrin analogues, association con-
stants of which are reported to be K = 5.5 × 10 MϪ1 in CDCl3
2.65 (s, 12H), 3.90 (q, J = 7.6 Hz, 8H), 6.90 (s, 2H), 7.25 (t,
J = 7.6 Hz, 2H), 7.60 (m, 4H), 9.20 (d, J = 8.8 Hz, 2H), 10.27 (s,
2H); UV λmax nm (log ε/dm3 molϪ1 cmϪ1) 626.5 (3.18), 575
(3.48), 540.5 (3.52), 506.5 (3.86), 406 (4.94). Anal. Calcd. for
C60H76N6O2: C, 78.90; H, 8.39; N, 9.21. Found: C, 78.80; H,
8.11; N, 8.95%.
1
(based on H NMR titration) and 1.4 × 102 MϪ1 in benzene
(based on UV–VIS titration) at 298 K. Moreover, 2e recognises
electron-rich p-benzoquinone more effectively than electron-
deficient p-benzoquinone, which is ascribed to the enthalpy
change of the complex, caused by the hydrogen bonding
of amide protons of 2e with oxygen atoms of the substituted
p-benzoquinone.
5,15-cis-Bis(2-trifluoroacetamidophenyl)-2,8,12,18-tetraethyl-
3,7,13,17-tetramethylporphyrin (2c)
Similar acylation of 1 using trifluoroacetic anhydride afforded
78% yield of 2c: mp 335–336.5 ЊC (dec.) (recrystallized from
hexane–chloroform); IR 3375 (NH), 1735 (C᎐O), 1150 cmϪ1
᎐
1
(CF3); H NMR δ Ϫ1.90 (br s, 2H), 1.67 (t, J = 7.6 Hz, 12H),
2.49 (s, 12H), 3.82 (m, 8H), 7.24 (m, 2H), 7.51 (dd, J = 7.6, 1.5
Hz, 2H), 7.56 (m, 2H), 8.04 (s, 2H), 8.87 (dd, J = 8.5, 1.0 Hz,
2H), 10.28 (s, 2H); UV λmax nm (log ε/dm3 molϪ1 cmϪ1) 626.5
(3.47), 575 (3.79), 540.5 (3.82), 506.5 (4.18), 406 (5.26). Anal.
Calcd. for C48H46N6F6O2: C, 67.58; H, 5.44; N, 9.86. Found:
C, 67.20; H, 5.30; N, 9.48%.
Experimental
The IR spectra were recorded on a JASCO A-100 spectrometer
and samples were run as potassium bromide pellets. UV–Visible
spectra were recorded with a JASCO Ubest-50 spectrometer
and measured in CH2Cl2. The 1H NMR spectra were recorded
with a JEOL JNM-GX270 (270 MHz) or -LA400 (400 MHz)
spectrometer using tetramethylsilane as an internal standard,
the chemical shifts being given in δ/ppm downfield. Samples
were taken in C6D5 CD3–CDCl3 (10:1) unless otherwise noted.
The elemental analyses were measured with a Perkin-Elmer
2400 II CHN Analyzer.
5,15-cis-Bis(2-acetamidophenyl)-2,8,12,18-tetraethyl-3,7,13,17-
tetramethylporphyrin (2d)
Similar acylation of 1 using acetic anhydride afforded 71% yield
of 2d: mp 313–314 ЊC (dec.) (recrystallized from hexane–ethyl
acetate); IR 3400 (NH), 1695 cmϪ1 (C᎐O); 1H NMR δ Ϫ1.84 (s,
᎐
2H), 0.60 (s, 6H), 1.70 (t, J = 7.6 Hz, 12H), 2.64 (s, 12H), 3.89
(m, 8H), 6.79 (s, 2H), 7.25 (m, 2H), 7.57 (dd, J = 7.6, 1.7 Hz,
2H), 7.64 (m, 2H), 9.14 (d, J = 8.5 Hz, 2H), 10.37 (s, 2H); UV
λmax nm (log ε/dm3 molϪ1 cmϪ1) 625.5 (3.21), 574 (3.77), 540.5
(3.73), 506.5 (4.18), 406.5 (5.29). Anal. Calcd. for C48H52N6O2–
CH3CO2C2H5: C, 74.96; H, 7.26; N, 10.09. Found: C, 75.11; H,
7.17; N, 10.49%.
5,15-cis-Bis(2-perfluoroheptanecarboxamidophenyl)-2,8,12,18-
tetraethyl-3,7,13,17-tetramethylporphyrin (2a)
Triethylamine (200 mg, 1.98 mmol) was added dropwise to a
solution of aminophenylporphyrin 1 (100 mg, 0.15 mmol) and
perfluorooctanoyl chloride (800 mg, 1.85 mmol) in 10 mL of
THF. After being stirred at room temperature for 1 h, the solu-
tion was evaporated to leave a residue which was dissolved in
chloroform. The mixture was washed with water and brine,
dried over magnesium sulfate, and evaporated. The resulting
solid was chromatographed on silica gel (hexane–ethyl acetate,
5:1) to give 160 mg (73% yield) of 2a. Further purification was
carried out by recrystallization from hexane–chloroform: mp
5,15-cis-Bis(2-pentafluorobenzamidophenyl)-2,8,12,18-tetra-
ethyl-3,7,13,17-tetramethylporphyrin (2e)
Similar acylation of 1 using pentafluorobenzoyl chloride
afforded 63% yield of 2e: mp 312–313 ЊC (dec.) (recrystallized
from hexane–chloroform); IR 3390 (NH), 1695 cmϪ1 (C᎐O); 1H
᎐
191–193 ЊC; IR 3380 (NH), 1730 (C᎐O), 1210 and 1240 cmϪ1
NMR δ Ϫ1.96 (br s, 2H), 1.68 (t, J = 7.6 Hz, 12H), 2.62 (s, 12H),
3.87 (m, 8H), 7.24 (m, 2H), 7.46 (dd, J = 7.6, 1.5 Hz, 2H), 7.62
(s, 2H), 7.63 (m, 2H), 9.22 (d, J = 8.0 Hz, 2H), 10.30 (s, 2H); UV
λmax nm (log ε/dm3 molϪ1 cmϪ1) 624.5 (3.44), 573 (3.83), 541
(3.83), 507 (4.20), 406.5 (5.28). Anal. Calcd. for C58H46N6O2F10:
C, 66.39; H, 4.42; N, 8.01. Found: C, 66.15; H, 4.24; N, 8.05%.
᎐
(C–F); 1H NMR δ Ϫ1.98 (s, 2H), 1.70 (t, J = 7.6 Hz, 12H), 2.48
(s, 12H), 3.85 (q, J = 7.6 Hz, 8H), 7.30 (t, J = 7.6 Hz, 2H), 7.60
(m, 4H), 8.0 (s, 2H), 8.85 (d, J = 8.6 Hz, 2H), 10.30 (s, 2H); UV
λmax nm (log ε/dm3 molϪ1 cmϪ1) 626.5 (3.05), 575 (3.34), 540
(3.37), 506.5 (3.72), 405.5 (4.79). Anal. Calcd. for C60H46N6-
F30O2: C, 49.58; H, 3.19; N, 5.79. Found: C, 49.29; H, 3.12;
N, 5.98%.
5,15-cis-Bis(2-benzamidophenyl)-2,8,12,18-tetraethyl-3,7,13,17-
tetramethylporphyrin (2f)
5,15-trans-Bis(2-perfluoroheptanecarboxamidophenyl)-2,8,12,
18-tetraethyl-3,7,13,17-tetramethylporphyrin (2a-trans)
Similar acylation of 1 using benzoyl chloride afforded 98%
yield of 2f: mp 294–295 ЊC (recrystallized from hexane–ethyl
1
acetate); IR 3410 (NH), 1680 cmϪ1 (C᎐O); H NMR (CDCl )
In a similar manner to the above, acylation of 5,15-trans-bis(2-
aminophenyl)-2,8,12,18-tetraethyl-3,7,13,17-tetramethylpor-
phyrin with perfluorooctanoyl chloride gave 80% yield of 2a-
trans: mp 239–241 ЊC (recrystallized from hexane–chloroform);
᎐
3
δ Ϫ2.37 (s, 2H), 1.75 (t, J = 7.6 Hz, 12H), 2.60 (s, 12H), 4.02 (m,
8H), 6.49 (m, 4H), 6.65 (m, 4H), 6.80 (tt, J = 7.6, 1.2 Hz, 2H),
7.55 (m, 2H), 7.81 (dd, J = 7.6, 1.4 Hz, 2H), 7.90 (m, 2H), 8.06
(s, 2H), 9.05 (dd, J = 8.3, 1.0 Hz, 2H), 10.28 (s, 2H); UV λmax nm
(log ε/dm3 molϪ1 cmϪ1) 626.5 (3.32), 575.5 (3.76), 541.5 (3.74),
508 (4.14), 409 (5.22). Anal. Calcd. for C58H56N6O2: C, 80.14;
H, 6.50; N, 9.67. Found: C, 80.11; H, 6.46; N, 9.43%.
IR 3375 (NH), 1730 (C᎐O), 1260, 1200, and 1140 cmϪ1 (C–F);
᎐
1H NMR δ Ϫ2.0 (br s, 2H), 1.70 (t, J = 7.6 Hz, 12H), 2.50 (s,
12H), 3.85 (q, J = 7.6 Hz, 8H), 7.30 (t, J = 7.6 Hz, 2H), 7.60 (m,
4H), 8.00 (s, 2H ), 8.90 (d, J = 8.6 Hz, 2H), 10.30 (s, 2H); UV
λmax nm (log ε/dm3 molϪ1 cmϪ1) 626.5 (3.04), 575 (3.34), 539.5
(3.37), 506 (3.73), 405 (4.79). Anal. Calcd. for C60H46N6F30O2:
C, 49.58; H, 3.19; N, 5.79. Found: C, 49.57; H, 2.83; N, 5.56%.
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᎐
δ Ϫ1.90 (br s, 2H), 0.3–0.95 (m, 30H), 1.70 (t, J = 7.6 Hz, 12H),
J. Chem. Soc., Perkin Trans. 2, 1999, 285–288
287

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the plot based on the following equation, 1/∆δobsd(NH) = {1/∆δcomp
-
(NH)} ϩ (1/K) {1/∆δcomp(NH)} {1/[p-benzoquinone]total}, where
∆δobsd(NH) = δobsd(NH) Ϫ δ2(NH),
∆δcomp(NH) = δcomp(NH) Ϫ
δ2(NH), and δ2(NH) is the chemical shift of the amide protons of the
free porphyrin 2.5
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Paper 8/06621C
288
J. Chem. Soc., Perkin Trans. 2, 1999, 285–288