Preparation, electrochemical and spectral properties of N-methyl-pyridylethynyl nickel porphyrins

Article abstract of DOI:10.1039/b414536d

A series of nickel N-methyl-pyridylethynylporphines were synthesized and their electrochemical and absorption properties were studied. UV-visible spectra of these complexes show that the absorption red-shifts of the nickel porphyrins are as significant as the zinc analogues. Although the reduction potential shifts caused by the electron-withdrawing substituents are not as large as the zinc complexes, the first reduction potentials of the nickel porphyrins are more positive than those of the zinc counterparts. In addition, the redox behaviors of these nickel porphyrins are similar to those of the zinc analogues.

Full text of DOI:10.1039/b414536d

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F U L L P A P E R
Preparation, electrochemical and spectral properties of
N-methyl-pyridylethynyl nickel porphyrins
Ching-Yao Lin,* Li-Chieh Chuang, Yu-Chin Lin and Chia-Liang Lin
Department of Applied Chemistry, National Chi Nan University, 302 University Road,
Puli, Nantou Hsien, 545, Taiwan. E-mail: cyl@ncnu.edu.tw; Fax: +886 49 2917956;
Tel: +886 49 2910960 ext. 4152
Received 20th September 2004, Accepted 11th October 2004
First published as an Advance Article on the web 1st November 2004
A series of nickel N-methyl-pyridylethynylporphines were synthesized and their electrochemical and absorption
properties were studied. UV-visible spectra of these complexes show that the absorption red-shifts of the nickel
porphyrins are as significant as the zinc analogues. Although the reduction potential shifts caused by the
electron-withdrawing substituents are not as large as the zinc complexes, the first reduction potentials of the nickel
porphyrins are more positive than those of the zinc counterparts. In addition, the redox behaviors of these nickel
porphyrins are similar to those of the zinc analogues.
Chemical Inc. (MA, USA). Chromatographic purification was
performed with silica gel 60 (230–400 mesh, Merck).
Introduction
Porphyrins exhibit very rich electron-transfer chemistry, and are
potentially good candidates for catalysis, material, biomedicine,
sensor and photosensitizer applications.1 In addition, por-
phyrins with the ethynyl motif are increasing employed for pho-
tonic and electrochemical studies of multichromophore arrays.2
We have recently reported the preparation, electrochemical
and spectroscopic properties of zinc(II) N-methyl-pyridylethynyl
porphyrins.3 In this work, we extend our effort to the synthesis,
electrochemical and absorption spectral properties of nickel(II)
N-methyl-pyridylethynyl porphyrins. Fig. 1 depicts the structure
of these electron-deficient nickel porphyrins.
Instrumentation
Absorption spectra were recorded on an Agilent 8453 UV-
visible spectrophotometric system. NMR data were obtained on
a Varian Unity Inova 300WB NMR Spectrometer. Elemental
analyses were measured at the NSC Instrumentation Center
at National Cheng Kung University. Cyclic voltammetry was
carried out with a standard three-electrode cell. A CH Instru-
ments Electrochemical Workstation 611A was used to record
cyclic voltammograms. For thin-layer and bulk electrolysis, the
potentials were controlled by the same potentiostat. Spectro-
electrochemical measurements were performed with an air tight,
optically transparent thin-layer electrochemical (OTTLE) cell.
The electrochemical cells were all assembled in a glovebox
equipped with a drytrain to exclude moisture and oxygen.
Synthesis procedure
For the unmethylated Ni 3, 4, 5 and 6 precursors, 100 mg of
brominated Ni 0 (NiBPPBr or NiBPPBr2) under nitrogen was
usually mixed with 1–4 equivalents of 3- or 4-pyridylethynye,
20–30 mol% of Pd(PPh3)4, 10 mol% of CuI, 5 mL of Et2NH and
80 mL of freshly distilled THF. The de-gassed reaction mixtures
were stirred under nitrogen at 40 ◦C for 24–72 h (monitored by
TLC and UV-vis spectroscopy), then quenched with NH4Cl(aq)
washes. After separation by chromatography on silica gel, 85–
91% of Ni 3 and 5 precursors (Ni 0 with a 3- or 4-pyridylethynyl
substituent, respectively) were generally obtained. Methylation
of the pyridyl group was carried out with excess of CH3I
Fig. 1 Structural diagram of nickel(II) biphenylporphine (Ni 0 or
NiBPP) and its N-methyl-pyridylethynyl analogues (as Ni 1–6).
−
Experimental
in DMF. After replacing the counter ion with ClO4 and
crystallization from DMF/CH2Cl2, 86–88% of Ni 3 and 5 were
usually obtained. For Ni 4 and 6 precursors, severe precipitation
Materials
◦
Air-sensitive solids were handled in an MBraun Uni-lab glove
box. A vacuum line and standard Schlenk techniques were
employed to process air-sensitive solutions. Solvents used in
the synthesis (ACS Grade) were obtained from Mallinckrodt
Baker, Inc. (CH2Cl2, CHCl3 and ethyl acetate; Kentucky, USA),
Haltermann (hexanes; Hamburg, Germany), and Merck (THF;
Darmstadt, Germany). These solvents were used as received
unless otherwise stated. All other chemicals were ordered from
Acros Organics (NJ, USA). Tetrahydrofuran (THF), DMF
and tetrabutylammonium perchlorate (TBAP) were purified
according to literature methods.4 All NMR solvents were used
as received. Pd(PPh3)4 catalyst was purchased from Strem
was observed after stirring at 40 C for 48 to 60 h. The green
precipitate from the Ni 4 precursor reaction seemed to have
no solubility in organic solvents, resulting in the infeasibility of
chromatographic purification or any further reactions. On the
other hand, the blue precipitate of Ni 6 precursor was found
pure enough and was directly methylated in excess CH3I/DMF
without prior purification. After replacing the counter ion with
ClO4− and crystallization from DMF/CH2Cl2 (3 times, until the
color of the powders turned from green to blue–purple), 35% of
Ni 6 was usually obtained (based on the amount of NiBPPBr2
used). Ni 1 precursor was prepared from H2 1 precursor by reflux
in DMF with excess NiCl2·6H2O. 45% of Ni 1 was collected
4 0 0 6
D a l t o n T r a n s . , 2 0 0 4 , 4 0 0 6 – 4 0 0 9
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
©

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after metallation, methylation, exchanging the counter ion and
crystallization.
of this interesting phenomenon. As shown in the scheme,
nickel biphenylporphine with 2-pyridylacetyl substituent was
the major product and the desired compound, Ni 1 precursor,
was the minor product. Because (1) a similar reaction was
not observed for Zn 1 precursor and (2) de-metallating Zn 1
precursor is easy, we prepared Ni 1 via H2 1 from Zn 1 precursor.
For the synthesis of Ni 2 precursor, nickel porphyrin with one
2-pyridylethynyl and one 2-pyridylacetyl substituents was the
only major product from the cross-coupling reaction. Moreover,
de-metallating Zn 2 precursor in CHCl3/HCl produced free-
base biphenylporphine with one 2-pyridylethynyl and one 2-
pyridylacetyl substituents.7 Consequently, complex Ni 2 was not
prepared for study in this report.
Cross-coupling NiBPPBr2 with 3-pyridylethyne generated
severe precipitation that showed no solubility in most of the
organic solvents. Moreover, synthesizing Ni 4 via H2 4 from Zn
4 precursor yielded impurities that could not be removed nor
separated. Hence, Ni 4 was not included in this study.
The synthesis of Ni 6 was straightforward with a few inter-
esting twists: Cross-coupling NiBPPBr2 with 4-pyridylethyne at
40 ◦C for 48 h afforded many blue precipitates. The reaction
solution was found to contain very little of Ni 6 precursor at that
point. Therefore, only the blue powders were collected and the
reaction solution was discarded. These blue powders were found
to be pure enough for methylation without prior purification.8
Finally and puzzlingly, we found that Ni 6 seemed to decompose
very quickly in DMSO.9 Fortunately, Ni 6 seem to be quite stable
in DMF.
Characterization data
Ni 1: 1H-NMR (d6-DMSO at 2.50 ppm), 10.04 (d, J 5 Hz, 2H),
9.98 (s, 1H), 9.88 (d, J 5 Hz, 2H), 9.34 (d, J 5 Hz, 4H), 9.21 (d, J
6 Hz, 1H), 8.73 (m, 2H), 8.17 (t, J 6 Hz, 1H), 8.04 (m, 4H), 7.83
(m, 6H), 4.65 (s, 3H). Elemental analysis, C40H26ClN5O4Ni·H2O,
calculated C 63.82%, H 3.75%, N 9.30%; found C 63.86%, H
3.69%, N 9.34%.
1
Ni 3: H-NMR (d6-DMSO at 2.50 ppm), 10.13 (s, 1H), 9.91
(d, J 5 Hz, 2H), 9.82 (s, 1H), 9.53 (d, J 5 Hz, 2H), 9.18 (d, J
8 Hz, 1H), 9.07 (d, J 6 Hz, 1H), 9.00 (d, J 4 Hz, 2H), 8.93 (d, J
5 Hz, 2H), 8.31 (dd, J1 8 Hz, J2 6 Hz, 1H), 8.05 (m, 4H), 7.81
(m, 6H), 4.51 (s, 3H). Elemental analysis, C40H26ClN5O4Ni·H2O,
calculated C 63.82%, H 3.75%, N 9.30%; found C 64.34%, H
3.75%, N 8.87%.
Ni 5: 1H-NMR (d6-DMSO at 2.50 ppm), 10.10 (a singlet peak
overlapped with a doublet, 3H), 9.75 (d, J 5 Hz, 2H), 9.20 (d, J
5 Hz, 2H), 9.16 (d, J 5 Hz, 2H), 9.12 (d, J 6 Hz, 2H), 8.68 (d, J
7 Hz, 2H), 8.06 (m, 4H), 7.83 (m, 6H), 4.41 (s, 3H). Elemental
analysis, C40H26ClN5O4Ni·H2O, calculated C 63.82%, H 3.75%,
N 9.30%; found C 64.57%, H 3.74%, N 8.90%.
Ni 6: 1H-NMR (d7-DMF; TMS at 0.00 ppm), 10.21 (d, J 5 Hz,
4H), 9.42 (d, J 7 Hz, 4H), 9.21 (d, J 5 Hz, 4H), 8.98 (d, J 7 Hz,
4H), 8.18 (m, 4H), 7.90(m, 6H), 4.69(s, 6H). Elemental analysis,
C48H32Cl2N6O8Ni·2H2O, calculated C 58.44%, H 3.68%, N
8.52%; found C 58.62%, H 3.66%, N 8.52%.
1H-NMR of Ni 6 in d6-DMSO (2.50 ppm) is also presented
for comparison with the spectrum in d7-DMF: 9.23 (d, J 6 Hz,
4H), 8.91 (d, J 6 Hz, 4H), 8.29 (br, 4H), 7.97 (br, 6H), 4.45
(s, 6H).
Absorption spectra
Fig. 2 illustrates the UV-visible spectra of Ni 0, 1, 3, 5 and 6
in DMF. The absorption maxima and extinction coefficients are
compared in Table 1. These porphyrins exhibit characteristic
Results and discussion
Synthesis
The preparation of pyridylethynylporphines involves the reac-
tion of mono- or di-brominated biphenylporphines with 2, 3 or
4-pyridylethyne according to the cross-coupling method.5 This
well developed synthetic strategy works well for most of the zinc
complexes. However, the synthesis of the nickel(II) complexes
was slightly more complicated: Although porphyrin Ni 3 and 5
were prepared according to the literature method at fairly high
yields, we found that it was more effective to modify the synthesis
methods of Ni 1 and 6. In addition, we found that Ni 2 and 4
were unattainable for study at this point (see below).
The synthesis of complexes Ni 1 and 2 by cross-coupling
NiBPPBr or NiBPPBr2 with 2-pyridylethyne afforded unex-
pected products.6 Scheme 1 summarizes our understanding
Fig. 2 Absorption spectra of Ni 0 to 6 in DMF.
Scheme 1 Reactions of brominated NiBPP with 2-pyridylethyne.
D a l t o n T r a n s . , 2 0 0 4 , 4 0 0 6 – 4 0 0 9
4 0 0 7

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Table 1 Absorption maxima (in nm) and the extinction coefficients
(in cm−1M−1) of Ni 0 to 6 in DMF
Table 2 Reduction potential comparison of Ni 0 to 6.
1/2/V
E
Entry
Soret (log e)
Q (log e)
Entrya
Red (1)
Red (2)
Red (3)
Ni 0
Ni 1
Ni 3
Ni 5
Ni 6
400 (5.35)
439 (5.11)
427 (5.26)
442 (5.07)
454 (5.30)
515 (4.20), 546
Ni 0
Ni 1
Ni 3
Ni 5
Ni 4
Ni 6
−1.16 (76)
−0.72 (Epc)
−0.87 (Epc)
−0.68 (80)
−0.82 (Epc)
−0.50 (70)
546 (4.04), 587 (4.31)
539 (4.13), 575 (4.13)
547 (4.04), 587 (4.32)
625 (4.70)
−0.94 (88)
−1.11 (106)
−0.89 (80)
−0.99 (Epc)
−0.63 (70)
−1.24 (120)
−1.11 (170)
−1.14 (110)
−1.23 (70)
porphyrin absorption spectra: strong Soret bands in the higher
energy region and weaker Q bands in the lower energy region.
The absorptions of Ni 1 to 6 are significantly red-shifted from
those of Ni 0 due to the extended conjugation and the positive
charge on the pyridinium ring. In addition, the red-shifts of these
nickel porphyrins’ Soret bands are as significant as those of the
zinc analogues: 54 nm (or 2973 cm−1) between Ni 0 and Ni 6 vs.
54 nm (or 2787 cm−1) between Zn 0 and Zn 6. On the other hand,
the lowest-energy Q band are not shifted as dramatically for the
nickel porphyrins as for the zinc counterparts. For example, the
lowest-energy Q-band shifts between Ni 0 and Ni 6 were found
to be only 79 nm (or 2315 cm−1) and the shifts between Zn 0 and
Zn 6 were 115 nm (or 2835 cm−1). Nonetheless, significant red-
shifts of the absorptions were indeed observed for these electron-
deficient nickel porphyrins.
a Reaction conditions: 0.5 mM of Ni 0 to 6 in DMF/0.1M TBAP; Pt
working and counter electrodes; SCE reference electrode; scan rate = 100
mV s−1. For Fc+/0 in the same condition, E1/2 = +0.49 V vs. SCE. Peak-
to-peak separations (mV) are put in parentheses for quasi-reversible
reactions. For irreversible reductions, only the cathodic peak potentials
(Epc) are shown.
species. However, the two reduction potentials were found to be
too similar to each other to be distinguished (−1.37 V for Zn 6
and −1.39 V for Zn 0). Notably, two reduction waves with very
small current intensities occur at about −0.30 V and −0.75 V for
Ni 6 (Fig. 3b). These tiny waves are possibly due to a very small
amount of degraded Ni 6 in DMF. Note that Ni 6 decomposes
very quickly in DMSO, and this phenomenon might take place in
DMF at a much slower rate. Finally, no significant dimerization
was observed for these nickel porphyrins under the experimental
conditions.11
Electrochemistry
Fig. 3 shows the cyclic voltammograms (CVs) of Ni 0, 1, 3, 5 and
6 in DMF/0.1M TBAP. The reduction potentials are included
in Table 2 for comparison. The shape and current intensity of
these CVs are nearly identical to those of the corresponding zinc
complexes (only differing in the potentials). Thus, we suggest
that these nickel porphyrins should undergo reduction reactions
similar to those suggested for the zinc complexes:3 (1) Ni 0, 1, 3,
5 and 6 receive one electron for their first reduction reactions and
form the anion radicals as for most of the nickel porphyrins.10
The irreversibility of the Ni 1’s, 3’s and 5’s reductions is likely
to be caused by the chemical reactions following the electron-
transfer steps. (2) Ni 6 undergoes three reversible, one-electron
reduction reactions, forming the anion radical, the di-anion
and probably the tri-anion (see below). Significantly, the first
reduction potential of Ni 6 (−0.50 V) is much more positive than
that of Ni 0 (−1.16 V), demonstrating the electron-withdrawing
effect of the two N-methyl-4-pyridylethyne groups. Remarkably,
the potential of Ni 6’s third reduction reaction (−1.23 V) is
quite different from that of Ni 0’s first reduction (−1.16 V). This
is consistent with our previous suggestion that these reduction
reactions are in fact two different processes. It was suggested
that the first reduction of Zn 0 was to generate its anion radical
while the third reduction of Zn 6 was to form the triply reduced
Spectroelectrochemistry of Ni 6
Fig. 4 demonstrates the spectral changes of Ni 6 in DMF/0.5M
TBAP upon its first and second reductions. Unlike the zinc
counterpart, we found that it was much more difficult to control
the applied potential in order to resolve the absorption spectrum
of Ni 6 anion radical form those of the neutral species and the di-
anion. Upon applying –0.30 V vs. SCE, the absorptions of Ni 6
gradually and isosbestically changed to longer wavelengths: The
intensity of the Soret band decreased, a red shoulder appeared
to the right of the Soret band and the Q band shifted from
626 to 778 nm (Fig. 4, thin solid line). This spectrum is similar
to that of the Zn 6 anion radical and is consistent with the
formation of other porphyrin anion radicals.3,12 Isosbestic points
were observed at 299, 366, 402, 470, 572 and 656 nm, suggesting
no intermediate being observed during the reduction reaction.
Based on the intensity of the Soret band, 88% of the Ni 6 was
recovered by re-oxidation at 0.10 V. Upon applying −0.60 V,
these spectrum further shifted to 420, 494, 510 (split Soret)
and greater than 1100 nm (Q, Fig. 4, bold grey line). This
Fig. 3 Cyclic voltammograms of (a) Ni 0, 1, 3 and 5, and (b) Ni 0
and 6 in DMF/0.1M TBAP. Experimental conditions: 0.5 mM of the
porphyrins, Pt working and counter electrodes, SCE reference electrode
Fig. 4 Absorption spectral changes of Ni 6 upon reduction. Dotted
line: the neutral species, thin solid line: mainly the anion radical, bold
grey line: the dianion. Experimental conditions: Pt working and counter
electrode, SCE reference electrode, in DMF/0.5M TBAP/N2.
and the scan rate = 100 mV s−1
.
4 0 0 8
D a l t o n T r a n s . , 2 0 0 4 , 4 0 0 6 – 4 0 0 9

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spectrum is similar to that of the Zn 6 dianion and is consistent
with the formation of a porphyrin di-anion.3,13 Nearly 75%
of Ni 6 was recovered by re-oxidation at +0.10 V. However,
no isosbestic points were observed during this process. This is
probably because (1) the solid line in Fig. 4 represents mainly the
anion radical with minor contribution from the neutral species.
The weak absorption near 618 nm could be attributed to the
residual neutral species. (2) The poorer recovery rate from the
di-anion to the neutral species translates to more Ni 6 being
decomposed during the process, compromising the formation
of the isosbestic points. The electrolysis of the third reduction
showed an even poorer recovery rate, therefore, its spectrum
changes were not addressed.
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Conclusion
In this work, we report the synthesis of a series of nickel(II)
N-methyl-pyridylethynylporphines. The absorption red-shifts
of these porphyrins are as significant as the zinc analogues.
Electrochemical data showed that the reduction potentials of
these nickel porphyrins are more positive than those of the
zinc counterparts and the electrochemical behaviors of these
electron-deficient nickel porphyrins are very similar to those of
the zinc analogues: (1) They all receive one electron for their
reduction reaction. (2) Chemical reactions very likely follow
the electrochemical steps for Ni 1, 3 and 5, resulting in their
irreversible reduction waves. (3) Three reversible, one-electron
reduction reactions were observed for Ni 6, and they were
suggested to be the formation of the anion radical, the di-anion
and possibly the tri-anion. (4) Noteworthy, the third reduction
potential of Ni 6 is very different from the first reduction
potential of Ni 0, supporting our previous suggestion that these
reactions are two different processes.
7 This phenomenon is similar to the reaction reported as the acid-
hydrolysis of 2-pyridylethyne: T. Sakamoto, Y. Kondo, M. Shiraiwa
and H. Yamanaka, Synthesis, 1984, 3, 245–247.
8 Chromatographic purification of these blue precipitates on silica gel
was possible. However, we found that the purification was difficult
(due to the low solubility of the blue precipitates in the eluent, 10%
of MeOH in CH2Cl2) and seemed unnecessary (the yields of Ni 6 did
not seem to be largely affected).
9 The B and Q bands of Ni 6 in DMSO (freshly distilled and degassed)
were found to be at 449 and 608 nm, respectively. These absorptions
are at shorter wavelengths than those of Ni 6 in DMF (454 and
625 nm, Table 1). In addition, these absorptions irreversibly blue-
shifted to 432 (B), 540 and 569 (Q) nm after standing under N2
for 1 h, suggesting the decomposition of the porphyrin.3 On the
other hand, the 1H-NMR spectrum of Ni 6 in d6-DMSO showed
very weak intensities and fewer peaks than it should have (see
the Characterization section for details). Based on the blue-shifted
absorptions and the lack of expected NMR signals, we suggest that
Ni 6 is not stable in DMSO.
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Acknowledgements
This work was supported by the National Science Council (NSC
91-2113-M-260-008).
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D a l t o n T r a n s . , 2 0 0 4 , 4 0 0 6 – 4 0 0 9
4 0 0 9