Download THE ELECTROCHEMISTRY OF STRAPPED AND CAPPED PORPHYRIN

J Electroanal Chem. 164 (1984) 335-346
Elsevier Sequoia S.A .• Lausanne - Prmted m The Netherlands
335
THE ELECTROCHEMISTRY OF STRAPPED AND CAPPED PORPHYRIN
MONOMERS, MONO- AND DOUBLY-UNKED DIMERS, AND THEIR Zn
AND Mg COMPLEXES
J.Y. BECKER *, D. DOLPHIN, J B. PAINE and T. WIJESEKERA
Department of ChemIstry. Umverslty of BritIsh ColumbIa. Vancouver. B C. V6T 1 Y6 (Canada)
(Received 29th March 1983, m fmal fonn 30th September 1983)
ABSTRACT
The mteractlon between two "spatially close" porphynns plays Important roles m blOcheffiIcal
functions The present study focuses on the effects of distortion, stenc hmdrance, distance and onentatlon
exerted by a vanety of porphynn monomers (strapped and capped) and dlmers (mono- and doubly-lmked) on their redox properties as well as the stability of posItively charged mtermedlates denved from
them Cychc voltammograms have been conducted m aprolic media on both free bases and their metal
complexes (Zn and Mg). The effect of the nature of supportmg electrolyte on the anodiC peak potentials
and wave separatIOn has been studied m the case of conjugated dlrectly-hnked bls-zmc and bls-magneslUm porphynns
INTRODUCTION
The interactions between two porphyrins, or chlorins, play many and varied roles
'in nature. Thus the transmission of photonic energy within the antennae of both
green plant and bacterial photosynthetic systems is but the first step in conversion of
solar into chemical energy. Having passed along the antennae pigments, a photon
causes an electron to be ejected from a special pair of bacteriochlorophylls and the
manner in which the two tetrapyrrolic macrocycles in this special pair interact both
before, during, and after electron transfer is still under vigorous investigation [1].
The electron which is photochemically lost from the specIal pair passes along a chain
of electron acceptors containing, intera alia, various cytochromes which are hemoproteins [2J. While the cytochromes in these and the related proteins in the
respiratory chains of prokaryotes function via an Fe(II) +:t Fe(III) couple the proximity and orientation of two hemes during electron transfer is still a matter of much
speculation [3,4J.
The above examples describe only a few of the occasions when the interaction of
two "spatially close" tetrapyrrolic macrocycles play important roles in biochemical
*
On leave from Ben-Gunon Uruverslty, Beer Sheva, Israel.
0022-0728/84/$03.00
© 1984 ElseVier Sequoia SA..
336
n M
9
9
Zn
10 H
10 Zn
H
5
4
3
2
I
7
I
Zn
4a
4b
3a
3b
"" Zn
11
8
7
6
M
H
1a
1b
2a
2b
H
R
Et
Et
Et
Et
n-octyl
Et
n-octyl
Et
Et
Et
M
Zn
.!l
5
Zn
Zn
Zn
Zn
Zn
Zn
10
11
Zn
12
Mg
Mg
14
M:: Zn
M:: Mg
4
5
8
6
7
8
M
Zn
Zn
Zn
9
13
M:: Zn
18
19
FIg 1 Structures for the complexes used
20
In
tlus study
15
16
17
337
functions. In order to study the effects of porphynn dlstortIon and of distance and
orientation between two porphyrins we have prepared a variety of unusual porphyrin
monomers and dimers.
The preparation of 'IT-cation radicals of porphyrinic derivatIves by photochemical
[5], chemical [6] and electrochemical methods [7] is well known. The latter two
methods also permit the oxidation of the porphyrin ring (P) to generate dicationic
species from both free bases and divalent metal ion complexes in which the metal is
not readily oxidized. In general, oXldation proceeds by two reversible one-electron
steps:
[M2+P] -.:;,- [M2+p+r -.:;, [M2+P 2 +]2+
+e-
+e-
The present work extends the electrochemical knowledge of structurally and
synthetically more complicated porphyrinic systems which have been prepared in
our laboratory. These compounds include strapped and capped porphynns [8] as
well as mono- and doubly-linked porphyrin dimers [9] and their zinc (and occasionally magnesium) complexes (Fig. 1). Since all the studied compounds, 1-20, are
derived from the skeletal structure of 2,7,12,17-tetraethyl-3,8,13,18-tetramethylporphyrin (etioporphyrin I), theu electrochemical properties are compared with
H 2etio and its ZlflC complex, Zn etio. The goal of this work is to study the effect of
the length of a bndging strap in porphyrin monomers and of chain length in
porphyrin dimers on the anodic peak potentials as well as the stabdity of positively
charged intermediates derived from metal complexes.
EXPERIMENTAL
Cychc voltammograms were carried out with a PAR 173 potentiostat and a
Universal programmer, PAR Model 175. J-E responses were recorded on a Tektronix
storage oscilloscope (564B) coupled with a polaroid camera Model C-12. Experiments were conducted in a "mini-cell" especially designed for the precious porphyrins
studied, with which one can use solutions of volumes less than 1 ml (Fig. 2). In this
cell the auxiliary electrode consists of a platinum ring mounted to the walls and the
working electrode is made of a platinum button (0.5 mm in diameter). As a reference
electrode a silver wire immersed in AgCI solution saturated with aqueous KCI was
employed. This electrode was separated from the solvent-electrolyte-substrate
solution by means of agar. For most cyclic voltammograms, solutions of CH 2Ci 2 -O.2
M tetrabutylammonium perchlorate (TBAP) containing a substrate (1 X 10- 3-5 X
10- 4 M) were used. For the less soluble compounds (e.g., 9 and 11) a mixture of
CH 2 Ci 2 + THF (l: 1, vjv) was used. For studying the effect of supporting
electrolyte on the shape of the voltammograms, other electrolytes (n-Bu 4 N+ PF6- ,
n-Bu 4 N+BF4- and n-Bu 4 N+CF3 S03-) were employed. The procedure of metallating
the various porphyrins by Zn2+ or Mg2+ followed standard hterature procedures
[10,11]. In a typical experiment, 21 mg of the free base of 6 were dlssolved in 10 ml
CH 2 Ci 2 and to which - 5 ml of saturated solution of Zn(OAch in methanol was
338
NI wire
sealed to Pt
Teflon cop------
.~
--------- ---------'"
... ---------t mm
-5
60mm
4----- . ------ -_ . ----
.:
Fig. 2 A unum-cell" used for cychc voltammetry measurements.
added. The metallation process was examined by followmg the changes in the visible
spectrum of the mixture. The mitial four maXIma of the free base (in the regIOn
450-700 nm) disappear, and two new maxima of the metallated compound appear
in this region. Usually, the reaction was terminated within 1-3 h after treatment at
ambient temperature. The red precipitate was collected by vacuum filtratIOn. The
product was washed several times With methanol to remove excess of Zn(OAc)2' and
then with water followed by air drymg. For further purification the red solid was
dissolved m tetrahydrofuran (THF) and chromatographed in silica gel (active I)
using a 1 : 2 CH 2CL 2 + THF (vIv) solution as eluant. Addition of methanol to the
solvent caused the precipitation of the final product as red crystals (- 14 mg).
RESULTS
As indicated by the electrochemical data in Table 1, each of the strapped
porphyrin free bases la-3a exhibits two Irreversible oXidation peaks, unlike the
non-strapped porphyrin, H 2etio, which shows two reversible one-electron couples.
Moreover, the first and second anodic peaks of la and 2a are shifted to higher
positive potentials by 50-110 mV relative to the corresponding anodic peaks of
H 2etlO. The capped porphyrin 4a is oxidized at more positive potentials than all the
former compounds. However, It affords two redox couples, both being quasi-reversible.
Unlike the free bases, the electrochemical behaviour of the zinc complexes 2b-4b
is similar to that of Zn etio and all show two reversible couples (each corresponds to
a one-electron process) at about the same respective oxidation potentials of Zn etio_
However, complex Ib is exceptional, since it gives an irreversible wave (at - 0.64 V)
for the first oxidation process.
339
TABLE 1
Electrochemical data a of monomenc porphynn denvatlVes 1-4
H2 ello b
la
2a
3a
4a
Zn eUo
tb
2b
3b
4b
E;~/V
E;t/V
E;UV
E;t/V
0.85
090
092
0.86
102
0.79
1.41
152 <
152
1.36
157
1.35
138 d
1.05
1.14
116
1.00
117
098
093
102
0.93
109
090
061
068
064 e
071
071
0.73
0.60
060
0.64
E;j/V
E;f/V
131
123 1
In CH 2C1 2 - TBAP, at 100 m V/s vs. Ag/ AgCl. Cychc voltammograms were scanned from 0 V to + 1 4
or + 1 6 V and then reversed to the mltlal potential E;U E;~d and E;U E;;d couples relate to
radical-cation and dlCatlOn, respectIvely.
b Taken from ref 12 and corrected for Ag/ AgCI reference electrode.
< An Ill-defmed cathodic wave was observed at -1 14 V.
d Observed at sweep rates greater than 2 V/s
e Rem3lned Irreversible even upon scannmg at 10 V/s when reversmg the potential at 0.8 V.
1 An dl-defined wave.
Q
Table 2 describes a systematic study for covalently mono-linked bis-zinc porphyrin
dlmers (5-12), and all of them, regardless of the length of hydrocarbon chain.
exhIbited two reversible couples, similar to Zn etio.
TABLE 2
E1ectrocherrucal data of covalently mono- and doubly-hnked bls-zinc porphynn dlmers 0
Compound
E;;/V
E;f/V
E;UV
E;f/V
5
6
7
8
9
10
11
0.67
0.70
0.67
069
062
068
070
0.71
052
0.57
072
064
065
060
0.64
0.60
060
055
060
060
064
045
0.50
060
055
058
113
1.16
1.07
112
128 <
136 <
132 <
110
0.88
0.92 <
1.25
110
1.06
110
1.01
1.03
1.12 <
1.26 <
1.20 <
1.03
0.81
084
1.03
0.98
12
13
14
15
16
17
105 <
In CH 2 CI 2 +TBAP unless otherwise noted. Scan rate 100 mV/s vs. Ag/AgCI Potential sweep from 0
V to + 1.4 and back to the mitlal pomt Each redox couple consIsts of two-electron process.
b Soluble In CH 2CI 2 + THF (1 : I, vIv).
< An ill-defined peak (usually broad).
Q
340
The bls-magnesIUm complexes, 13 and 14, agam show two reversible couples, but
each oxidation step takes place at less positive potential (by ca. 150-200 mY) than
the correspondmg bls-zinc complexes. However, Mg etio IS oXidized at a potential
260 mV less positive than Zn etio; so, relative to the parent compounds. the bis-Mg
complexes are actually oxidized at a shghtly more positive potential than the bls-Zn
complexes.
When studying the anodic oXidation of two doubly-linked bls-zinc complexes,
15-17, the electrochemical picture basically remains unchanged with respect to Zn
etio and the bis-zinc complexes 5-12.
Table 3 shows electrochemICal data of conjugated mono-linked metalloporphynns, in which the linkage between the rings is at [3,{3' positions (as in 18 and
19) or at meso, [3' positIOns as in 20. Cyclic voltammograms of 18 and 19 afford
mostly three anodic peaks (sometimes four) on the forward scan, and 3-5 reduction
peaks on the reverse scan. In addition. Table 3 describes the effect of various
TABLE 3
Electrochemical data a of conjugated metalloporphynn dlmers and the effects of electrolytes on anodiC
peak potenhals
Expen·
ment
Complex
Anodic peak
potentlaljV
nh
CathodiC peak potential
denved from reverse scan/V
SupportlOg
electrolyte (0.2 M)
18
0.68
081
1.13
080
0.84
125
072; 083
1.06; 1.14
114
0.86
214
1.14
086
2.14
1.84
174
0.62
075
1 0; 0 87 d, 038
072
0.87
111; 099,0.47
065
098; 085; 0.35
TBAP
057
090
061
096
050
0.90
1.69
1.86
1.86
2.07
1.69
2.00
047
0.80, 0 64. 0.19
052
0.85,0.71; 0.24
040
0.80, 0.64, 009
TBAP
0.78
1.11
0.51 d. 0.85
1.16, 1.36
071
1.09
1.82
2.06
0.61
096
0.66
1.04.1.27
054
0.92
TBAP
2
18
3
18
4
19
5
19
6
19
7
20
8
20
9
20
179
2.14
TBAF
TBA +CF)SO)
TBAF
TBA + CF3 SO)
TBAF
TBA + CF)SO)
In CH 2 Cl 2 +THF (2 1. v/v) All potentials are vs. Ag/AgCI.
The number of electrons per molecule was obtamed by companson of cyclic voltammetric peak heights
per urut of concentral1on With Zn etio.
e Difficult to IOterpret
d An Ill-defmed broad wave.
a
b
341
TABLE 4
VIsible a and mass spectral data of porphynmc denvattves 1-20
Compound
Ama,/nm
--------
ta
2·
3a
4a
tb
2b
3b
5
6
7
8
9
10
tt
12
13
15 c
16 d
17
18'
19 c
20 c
-
M+ (mje)h
-- ----------------------
633.579.552.513.405
626.573.544.507.402
626.572.540,503.401
624,570,537,501,401
586,545,410
579.538,406
572,535,403
573,536,398
568,532,399
567,531,399
568,531,399
572.535.406
572,535,396
572,536,403
569,532,400
580.543.408
572,534,400,387
569,533,385
570,533, 398. 387(sh)
578, 570(sh). 534, 412. 401
590.547.420.408
570,536,418,403
546.3737 (C 37 H 46 N 4 = 546.3724)
5603873 (C3sH4BN4 = 560 3881)
5744040 (C39HsoN4 = 5744038)
692.4866 (C4BH60N4 692.4821)
608.2869 (C37H44N4Zn2 = 608.2859)
622.3034 (C 3B H 46 N 4Zn = 622 3012)
7543943 (C4BHSsN4Zn 754.3955)
11384925 (C6s H7SNsZn2 = 1138.4912)
1124.4733 (~7H76NsZn 2 = 1124.4755)
11l04605(C66H74NsZn2=11104598)
1096.4460 (C6sHnNsZn2 =10964441)
1250.6147 (C76H94NsZn2 = 1250.6169)
10685135 (C63H6SNgZn2 = 1068.4127)
12225861 (C74H9QNSZn2 = 1222 5855)
1040.3810 (C6IH64NsZn2 =10403812)
956.4962 (C6IH64NgMS2 = 956.4957)
1080.4111 (C64H6BNBZn2 = 1080.4127)
1108.4424 (C66 HnNBZn2 =11084441)
1192.5362 (C n HS4NgZn2 = 11925384)
1026.3722 (C60H62NgZn2 = 10263729)
943.9177 (C6oH62NgMg2 = 943.9161)
1026.3739 (C60H62NgZn2 = 1026.3729)
VIsible spectra were measured m CH 2C1 2 except for complexes 9, to and 11 where a 1.1 CH 2C\2 - THF
mixture was used.
b Data are given for the most intense peak in the molecular Ion The spectra were measured on a Kratos
MS-50 usmg an acceleratmg voltage of 70 eV. The source temperature was 220-270°C and the mternal
standard was tns(perfluorononyl)-s-tnazme.
C These compounds show two maXima in the "Soret region"
d Thts compound shows a relatIVely broad Sore! band.
a
electrolytes (n-tetrabutylammonium perchlorate (TBAP), -fluoborate (TBAF) and
-trifluoromethylsulfate (TBA +CF3S0j» on the shape of the voltammograms. In
most cases, the first wave consisting of a two-electron process, formerly mentioned
for the compounds in Table 2, is now separated into two one-electron steps.
Spectral data of all the porphyrin derivatives and their metal complexes are
summarized in Table 4.
DISCUSSION
As the hydrocarbon strap in porphyrins 1-3 is shortened the macrocycle IS
deformed from its normal planar configuration to be severely distorted and nonplanar. Figure 3 shows the deformation observed in la where the. angle between the
mean planes of the strapped pyrrolic rings is 111.5 0 [8]. This loss of coplanarity
342
Fig. 3. ORTEP drawmgs of la [8].
results in a lowering of the aromaticity. This can be observed m the diminished ring
current as exemplified by nmr spectroscopy [8] and from the present work. As the
planarity decreases so will the electron delocalization. After a one-electron oxidation
the remaining electron of the 'IT-cation radical will be less delocalized. as the
porphyrin ring is bent. 3a is less strained than la and 2a and therefore oxidIZed at
lower potentials (this obtained for both oxidation waves (Table 1». However, none
of the three compounds will permit good delocalization, and thus each gives two
irreversible waves for the first and second oxidation steps, unlike the coplanar
H 2etio. The peak potentials of the totally irreversible waves exhibited by each of the
above compounds are shifted anodically with increasing sweep rate. This observation
343
may be explained by two plausible mechanisms: an EE reaction to form radical-cation and dlcationic species or an ECE mechanism in which the chemical process (e.g.
a cleavage of the strap and/or loss of the conjugation in the porphyrin ring) is very
fast relative to the scan rate [13]. In this case the second electrochemical step
involves the oxidation of a chemical product (it would be necessary to carry out a
product identification in order to distinguish between the two mechanisms) *.
In porphyrin 4a the strap containing the tetramethylbenzene (durene) moiety is
longer than in 3 hence the porphyrin is less stramed and it affords two quasi-reversible couples. Nevertheless, the two anodic potentials of 4a are considerably hIgher
than those of H 2etIO. We attribute this behaviour to the shielding effect of the
durene which will prevent the close approach, at least on one SIde of the porphyrin,
of the amon of the supporting electrolyte which would otherwIse more readily
stabIlize the cationic species.
Introducing a metal into a strapped porphyrin forces the ring to be more planar
than without it. Consequently, more stable oxidized intermediates are to be expected. Indeed, the cyclic voltammograms of the least strained zinc complexes, 3b
and 4b, show stable radical-cation and dicatIOn species for both compounds,
respectively, similar to that observed for Zn etio. The more stramed complex. 2b.
affords a quasi-reversible couple for the radical-cation and an irreversible couple for
the dication, whereas the most strained complex, Ib, exhibits a totally irreversible
wave for Its radical-cation. The latter shifts positively with increasing scan rate and
may either undergo a fast follow-up chemical reaction to form electroactive species
which show two oxidation waves (at 1.14 and 1.31 V) and reduction waves on the
reverse scan, or undergo further anodic oxidation.
By linking two porphyrin rings together, via a saturated hydrocarbon chain of
various lengths, we hoped to learn whether a mutual interaction" through space"
exists between the rings at a given chain length. As indicated in Table 2, each of the
covalently mono-linked bis-zinc porphyrin dimers (5-12) behaved as the monomeric
denvative, Zn etio. This means that each of the metalloporphyrin rings in the dimer
behaves independently with no interaction between them, regardless of the length of
the hydrocarbon chain Joining them together. It seems that the two rings in the
dimer "prefer" conformatIOns which place them away from each other. However,
the fact that both rings undergo electrochemIcal oxidation concomitantly may
suggest that they approach the electrode surface simultaneously on the time scale of
the CV measurements.
The magnesium complexes 13 and 14 behave sImilarly to the zinc complexes and
each also gives two reversible couples. However, both 13 and 14 are oxidized at less
positive potentials than the respective bis-zinc complexes, 6 and 12. This is to be
expected since the divalent ion, Mg2+, is larger than Zn2'. Consequently the latter
.. Smce the pOS1lI0ns of the Ep values of compounds la-3a are not very extreme relatIVe to that of the
standard (Table 1). It cannot be excluded that the lfreversIble peak pOSItIOns may be deternuned by
homogeneous chemIcal (rather than heterogeneous change transfer) kmelIcs. so that structural dIfferences
nught be masked.
344
Ion withdraws more negative charge from the rmg towards Its nucleus. resultmg m a
relatively more electron-deficient porphynn ring in the zinc complexes than in the
magnesium ones.
We have also examined the effects of constraining two porphyrins to be cofacial
by doubly hnkmg them with two methylene chains. In our hands the bls-zinc
complexes (15-17) behave sirrularly to Zn etio II. It appears that the two rings are
sufficiently mdependent that no mutual interaction IS observed between them.
However, the broadened peak scan for the second oxidabon wave may result from
two overlapping one-electron processes. Others [14] however. have observed that the
bis-cobalt complexes of cofacial dimers exhibit redox chemistry at the metal which
clearly demonstrates a mutual interaction between the two redox centres. Similar
results have been found for bimetallic Schiff-base type macrocycles [15].
The general picture described above changes considerably in the case of monolinked metalloporphynn dimers (17-20), in which the two rings are conjugated and
linked directly. As shown m Table 3, at least the first of the two-electron processes
previously discussed for the other mono-linked dimers (Table 2) is now being split
into two separate waves, each representing a one-electron oxidation. For mstance,
the voltammogram of 18 (Fig. 4) shows, in the presence of TBAP, two stepwise
one-electron reversible couples, which may be attributed to catIOn-radical and
dication-radical species (at formal E lo/ 2 of 0.65 and 0.78 V, respectively), and a third
irreversible two-electron process at - 1.13 V. The latter is coupled with two
electrochemical reduction waves at 1.0 and 0.38 V. Our interpretation of these results
is that once one of the zinc porphyrin rings is oxidized, it behaves as an electronWithdrawing group with respect to the other, causing the latter to be oxidized at a
more positive potential. Probably both inductive and resonance effects are present,
the former bemg predominant, since the methyl groups at the ,a-positions of the
porphyrin rings do not allow the rings to be co-planar. Consequently, one of the
rings lies in a plane essentially perpendicular with respect to the plane of the second
porphyrin ring (Fig. 5). The shape of the cyclic voltammograms remained unchanged
when other electrolytes (TABF or TBA +CF3 S03 ) were bemg used.
Surprisingly, the magnesium analog, 19, does not behave as 18, but more as the
other mono-linked porphyrin dimers (5-14) and gives only two two-electron anodic
peaks, however at less positive potentials, for the reasons discussed earlier. Since
l/)JA
0.2
I
o
1.2
E/v
Fig. 4 CV of compound 18 m CH 2CI 2 + THF (2 1, v /v )-0.2 M TBAP. Scan rate' 100 mVIs.
345
Fig. 5 Spatial view of compound 20
Mg2+ is less electropositive than Zn2+, the difference in peak potentials between the
first two consecutive one-electron steps is probably small and therefore they overlap
each other. However, the second two-electron step is similar to that obtained for 18.
Again, a follow-up chemical reaction takes place and the products exhibit three
reduction peaks on the reverse scan. Principally, the shape of the voltammograms
and number of peak potentials remained unchanged upon replacing CIO; by BF4or CF3 SO)-.
The zinc complex 20, with a linkage between the meso and ,B'-positions, shows
two oxidation waves, each provides one reduction peak with AEp of 170 and 150
mY, respectively, in the presence of TBAP or TBA +CF3 S0 3 . However, in the
presence of TBAF, four anodic peaks are observed. On the reverse scan, three
reduction waves emerged from the four oXldation peaks. Since the ratio between the
anodic peak currents is not equal (Fig. 6), one cannot exclude the possibility that
chemical reactions (due to partial decomposition of electrogenerated intermediates)
may be involved. It is noteworthy that Kadish et al. [16] also found that in weakly
coordinating solvents the E1/2 of an iron porphyrin derivative varies with changing
counter ion.
Finally, a careful examinatIOn of the data in Table 3 reveals that in general, the
oxidation potentials of complexes 18-20 are simllar in the presence of CI04- or
CF3 SO; , but somewhat higher in the presence of BF4-. Perchlorate and trifluoromethylsulfate have comparable ionic radii, which are larger than that of tetrafluorob orate. Smce the oxidized porphyrins represent large delocalized cation their
interaction, and stabilization, by larger anions is expected and may account for the
observed decrease in oxidatIOn potential of the porphyrin ring in the presence of
such counter ions.
I/~A
0.51
01--_ _
0.4
1.2
EV
a
b
0f--_ _
OA
1.2
EV
Fig. 6. CV of compound 20 m CH 2C1 2 , (a) m the presence of TBAP; (b) m the presence of TBAF.
346
ACKNOWLEDGEMENTS
This work was supported by the Canadian Natural Sciences and Engmeenng
Research Councll and the United States National Institutes of Health (AM 17989).
We also wish to thank the Chemistry Department at U.B.C. for the partial support
of J.Y.B. during his sabbatical leave from the Ben-Gurion University of the Negev,
Beer Sheva, Israel.
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