Electrochemically controlled ligand shuttling between zinc porphyrin and meso-phenylenediamine substituent

Article abstract of DOI:10.1039/b903374m

ZnTMP-PD was synthesized and fully characterized; by using electrochemical and spectroelectrochemical methods, ligand shuttling between the zinc porphyrin and the meso-phenylenediamine substituent could be monitored; EPR results indicated that the charge

Full text of DOI:10.1039/b903374m

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Electrochemically controlled ligand shuttling between zinc porphyrin
and meso-phenylenediamine substituentw
Kuo Yuan Chiua and Yuhlong Oliver Su*ab
Received (in Cambridge, UK) 17th February 2009, Accepted 13th March 2009
First published as an Advance Article on the web 6th April 2009
DOI: 10.1039/b903374m
ZnTMP-PD was synthesized and fully characterized; by
using electrochemical and spectroelectrochemical methods,
ligand shuttling between the zinc porphyrin and the meso-
phenylenediamine substituent could be monitored; EPR results
indicated that the charge was delocalized on ZnTMP-PD.
PD has two redox couples at E1/2 = +0.59 and +1.01 V
(Fig. S2A, ESIw). As for oxidation, ZnTMP has two redox
couples at E1/2 = +0.79 and +1.11 V (Fig. S3A, ESIw).8b In
the oxidation of ZnTMP-PD (Fig. 2A), three reversible redox
couples could be observed at E1/2
= +0.58, +0.81
and +1.13 V, respectively (Scheme S2, ESIw). These three redox
couples contained those of the metalloporphyrin ring and
para-phenylenediamine (PD) substituent. Besides, differential
pulse voltammetry (DPV) (Fig. S1B, ESIw) was used to
identify the number of electrons oxidized by calculating each
integration area. Thus, it could be inferred that in ZnTMP-PD
the first redox reaction was at the PD substituent, the
second on the zinc porphyrin, and the third redox couple on
both parts.
Metalloporphyrins have been used to mimic biological
systems.1 The understanding of metalloporphyrin properties
in relation to biological processes is of great importance. A
great number of experimental results have been reported over
the past three decades concerning the axial ligation properties
of metalloporphyrins with S, O, P and N bases.1 Recently,
many efforts have been focussed on the molecular recognition
of bio-interest by zinc porphyrins.2,3 A binding mode of the
pyridine group of the nicotine guest coordinated to the Zn ion
of the host was found.4 Detailed studies on the solvation and
axial ligation properties of ZnTPP have been reported.5–8
Addition of imidazole and 2-methylimidazole to zinc porphyrins
caused a cathodic shift of the oxidation potentials.8b,9
After 0.5 equivalent of MeIm was added to PD solution, the
current of the first redox couple increased while that of the
second decreased to aboutꢀhalf (Fig. S2, ESIw). After PD was
oxidized to PD+ꢀ, HN+ in the PD would quickly form
hydrogen bonding with MeIm. When 1.0 equivalent of MeIm
was added, the second redox couple of PD disappeared,
while the current value of the first redox couple became
twice as high. Scheme S3 is proposed (see ESIw).18 For
ZnTMP in the presence of MeIm, the first oxidation potential
shifted cathodically while the second one shifted anodically
(Scheme S4, ESIw).
Hydrogen bonds are one of the most important types of
inter- and intramolecular interactions in biological systems.10
Recently, chemists have studied hydrogen bonds in similar
systems, such as multimolecular assemblies11 or engineering
structures.12 Hydrogen bonds are electrostatic in nature.13 In
1962, the effect of hydrogen bonds on redox potential had
been investigated by Peover.14 In the presence of hydrogen
bonding to flavin in flavinproteins, one redox state would be
stabilized relative to another based on the potential shift.15
Rotello and co-workers also reported hydrogen bonding for
diimide and triimide.16 Using an electrochemical method,
reduced quinone derivatives or arylimides formed hydrogen
bonds with different ureas and diamines as investigated by
Smith and co-workers.17 They also demonstrated a reversible,
redox dependent interaction between nitrobenzene derivatives
and arylureas.
When MeIm was added to ZnTMP-PD solution, the first
two oxidation waves moved towards more negative potentials,
while the third oxidation wave shifted to a more positive
potential (Fig. 2B–D). When MeIm reached 1.5 equivalent,
the oxidation waves no longer shifted. Three oxidation
waves were at E1/2 = +0.51, +0.70 and +1.17 V (Table 1),
with their shifted potentials of À0.07, À0.11 and +0.04 V,
respectively. Based on the results of the cyclic voltammograms
in Fig. S2 and S3 (ESIw), the first redox couple in Fig. 2
was assigned to PD substituent oxidation in ZnTMP-PD
(PD+ and then PD2+-MeIm formation), and the second
ꢀ
We have synthesized a novel zinc porphyrin substituted with
phenylenediamine, ZnTMP-PD (Fig. 1). By electrochemical
oxidation, the oxidized ZnTMP-PD will form hydrogen and
coordinate bonding in the presence of N-methylimidazole
(MeIm). This study should be helpful in providing insight of
the intermediate of heme-dependent enzymatic reactions.
and third redox couples were on the zinc porphyrin.
The absorption peak of ZnTMP-PD at 308 nm,
a
characteristic absorption of the PD substituent, had a molar
a Department of Applied Chemistry, National Chi Nan University 1,
University Road, Puli, Nantou Hsien, 545, Taiwan.
Fax: 886-49-2917956; Tel: 886-49-2910960 ext.4150
b Department of Chemistry, National Chung Hsing University, 250,
Kuo Kuang Rd, Taichung, 402, Taiwan. E-mail: yosu@ncnu.edu.tw;
Tel: 886-4-22840537
w Electronic supplementary information (ESI) available: Details on
synthesis, electrochemical and spectral studies of PD, ZnTMP and
ZnTMP-PD. See DOI: 10.1039/b903374m
Fig. 1 Structure of zinc porphyrin linked with meso-phenylenediamine.
ꢁc
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2884 | Chem. Commun., 2009, 2884–2886

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Fig. 2 Cyclic voltammetry of 1.0 Â 10À3 M ZnTMP-PD in the
presence of N-methylimidazole in CH2Cl2 containing 0.1 M TBAP:
[MeIm] = (A) 0.00, (B) 0.50, (C) 1.00, (D) 1.50 equiv. of ZnTMP-PD.
Working electrode: glassy carbon. Scan rate: 0.1 V sÀ1
.
Table 1 Oxidation potential of PD, ZnTMP and ZnTMP-PD in the
absence and presence of N-methylimidazole
Fig. 3 Spectral changes of 2.0 Â 10À5 M ZnTMP-PD in the presence
of 0.75 equiv. N-methylimidazole in CH2Cl2 containing 0.1 M TBAP
at various applied potentials.
E1/2/V
the first oxidation occurred at the PD substituent with the
second at the zinc porphyrin.
1st
2nd
3rd
PD
PDa
ZnTMP
ZnTMPa
ZnTMP-PD
ZnTMP-PDa
+0.59
+0.62b
+0.79
+0.67
+0.58
+0.51b
+1.01
—
+1.11
+1.32
+0.81
+0.70
—
—
—
—
In the presence of MeIm (Fig. S8, ESIw), MeIm coordinated
with a portion of ZnTMP-PD (lmax = 421 nm) and formed
(MeIm)ZnTMP-PD (lmax = 430 nm). Fig. 3 shows the
spectroelectrochemical results for ZnTMP-PD oxidation in
the presence of 0.75 equivalent MeIm. In the potential range
Eappl. = +0.00–+0.59 V, corresponding to PD substituent
oxidation, the absorbance at 308 nm gradually decreased, and
a new peak at 288 nm was formed. The absorbance at 430 nm
decreased and that at 421 nm increased significantly. The base
lines of the spectra increased in the same way as for without
imidazole. It is noteworthy that the absorption at 430 nm of
(MeIm)ZnTMP-PD disappeared upon PD oxidation and the
421 nm absorbance increased, indicating the removal of
+1.13b
+1.17
a
b
In the presence of 1.0 equiv. N-methylimidazole. Two-electron
transfer.
extinction coefficient of 2.21 Â 104 MÀ1 cmÀ1, and the Soret
band at 421 nm had a value of 2.52 Â 105 MÀ1 cmÀ1. In the
potential range Eappl. = +0.00–+0.59 V, the spectra at
potential increments were obtained by using an optically
transparent thin-layer electrode (OTTLE). (Fig. S7A, ESIw).
As the potential shifted positively, it was observed that the
308 nm peak gradually decreased with an isosbestic point at
340 nm. The Soret band (421 nm), on the other hand, grew
slightly. In the wavelength range 600–1000 nm, there
MeIm from ZnTMP. In the potential range of Eappl.
=
+0.67–+0.93 V, the absorption peaks of the Soret band
(421 nm) and Q band (550 nm) gradually decreased. The
absorbance at 288 nm also decreased, while a broad band at
570–1000 nm grew.
was a small rising in background, which is ascribed to PD+
ꢀ
absorption (Fig. S5A, ESIw). In another potential range at
Eappl. = +0.67–+0.93 V (Fig. S7B, ESIw), the Soret band
(421 nm) and the Q band (550 nm) absorbances decreased
dramatically. A new peak at 415 nm with much smaller
absorbance and broad band at 550–800 nm, conceivably due
to porphyrin ring radical cation, were observed. The spectra
change is consistent with the previous inference from CV that
The change of spectra and absorption peaks in the absence
(Fig. S7A, ESIw) and presence (Fig. 3A) of MeIm for
ZnTMP-PD is discussed as follows (Scheme 1). The coordination
constant of MeIm and ZnTMP-PD is about 104 MÀ1 8b,9 When
.
(MeIm)ZnTMP-PD was oxidized starting at Eappl. = +0.38 V,
the absorbance at 430 nm gradually decreased while that at
421 nm rose. For (MeIm)ZnTMP-PD, MeIm moved to form
ꢁc
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In summary, ZnTMP-PD in the neutral state was ligated by
MeIm to the zinc ion center to form a five-coordinate
species. MeIm then moved to form hydrogen bonding with
oxidized PD and the characteristic absorption spectrum of
four-coordinate ZnTMP was restored. When the electrode
potential was shifted anodically and ZnTMP moiety was also
oxidized, MeIm further moved back to ligate onto the
ZnTMP. The shuttling of MeIm has been determined by the
electrode potential and the formation constant for MeIm with
different moieties of ZnTMP-PD at various oxidation states.
The authors thank the National Science Council of the
Republic of China for support of this work.
Scheme 1
hydrogen bonding with HN+ after PD oxidation to PD+
The formation constant (B105 MÀ1 17,19
of the hydrogen
bonding was larger than the coordination constant
(B104 MÀ1 8b,9
of MeIm with zinc porphyrin. After MeIm
.
ꢀ
ꢀ
Notes and references
)
1 K. M. Kadish, K. M. Smith and R. Guilard, The
Porphyrin Handbook, Academic Press, San Diego, 2000.
2 A. Robertson and S. Shinkai, Coord. Chem. Rev., 2000, 205, 157.
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2000, 305, 111.
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5 M. Nappa and J. S. Valentine, J. Am. Chem. Soc., 1978, 100, 5075.
6 O. W. Kooling, Anal. Chem., 1982, 54, 260.
)
moved over, the center of zinc porphyrin was restored to a
four-coordinated state, causing the original zinc porphyrin
absorption peak (421 nm) to increase upon PD moiety
oxidation.
7 R. M. Wang and B. M. Hoffmann, J. Am. Chem. Soc., 1984, 106,
4235.
8 (a) G. S. Marbury, C. Brewer and G. Brewer, J. Chem. Soc., Dalton
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J. Electroanal. Chem., 2002, 531, 155.
Fig. S7B (ESIw) shows a change in the spectrum when
ZnTMP-PD was oxidized, and the spectral change was similar
to ZnTMP cation radical formation. In the absence of MeIm,
those of the Soret band and the Q band decreased. A broad
band at 500–800 nm characteristic for the porphyrin cation
radical grew. However, in the presence of MeIm (Fig. 3B), the
absorption peak at 288 nm decreased. In the applied potential
9 K. M. Kadish, L. R. Shiue, R. K. Rhodes and L. A. Bottomley,
Inorg. Chem., 1981, 20, 1274.
10 (a) J. Rebek, Angew. Chem., Int. Ed. Engl., 1990, 29, 245;
(b) F. P. Schmidtchen and M. Berger, Chem. Rev., 1997, 97, 1609.
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13 K. Morokuma, Acc. Chem. Res., 1977, 10, 294.
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Chem., 1995, 34, 1090.
15 E. Breinlinger, A. Niemz and V. M. Rotello, J. Am. Chem. Soc.,
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range, [ZnTMP+ꢀ-PD2+ 3+
]
was formed; ZnTMP+ and
ꢀ
ꢀ
MeIm have a very large coordination constant of about
106–107 MÀ1 8b,9
larger than the constant for MeIm and
,
PD+ꢀ. At this point, MeIm would move back to the zinc
ion center of the metalloporphyrin.
Cation radicals PD+ꢀ, ZnTMP and ZnTMP-PD+ were
obtained by the electrochemical oxidation at 298 K in CH2Cl2
and were stable at room temperature. The EPR spectrum of
+
ꢀ
ꢀ
PD+ at room temperature exhibited a broad six-line pattern
ꢀ
(Fig. S9A, ESIw) with g = 2.0030. A number of hydrogen nuclei
in PD+ had various hyperfine coupling (hfc) constants
ꢀ
resulting in broad line width.18 Fig. S9B (ESIw) shows the
16 J. B. Carroll, M. Gray, K. A. McMenimen, D. G. Hamilton and
V. M. Rotello, Org. Lett., 2003, 5, 3177.
17 (a) Y. Ge, R. R. Lilienthal and D. K. Smith, J. Am. Chem. Soc.,
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C. E. Nohrden, K. T. Hoang, D. Truong and D. K. Smith,
J. Am. Chem. Soc., 2005, 127, 6423.
ZnTMP+ EPR spectrum, which could be distinguished as a
ꢀ
nine-line pattern. ZnTMP was oꢀxidized at the a2u orbital with
g = ꢀ2.0030. The ZnTMP-PD+ spectrum included those of
PD+ and ZnTMP+
, and gave a twelve-line pattern
ꢀ
18 S. R. Miller, D. A. Gustowski, Z.-H. Chen, G. W. Gokel,
L. Echegoyen and A. E. Kaifer, Anal. Chem., 1988, 60, 2021.
19 Y. Hirao, A. Ito and K. Tanaka, J. Phys. Chem. A, 2007, 111,
2951.
(Fig. S9C, ESIw) with g = 2.0033. The results indicated that in
ZnTMP-PD+ꢀ, the radical did not rest on the PD substituent,
but delocalized across the meso-position to the zinc porphyrin.
ꢁc
This journal is The Royal Society of Chemistry 2009
2886 | Chem. Commun., 2009, 2884–2886