Porphyrin-diones and porphyrin-tetraones: Reversible redox units being localized within the porphyrin macrocycle and their effect on tautomerism

Article abstract of DOI:10.1021/ja070759b

Porphyrin-2,3-diones and porphyrin-2,3,7,8- and porphyrin-2,3,12,13- tetraones were shown to have a redox-active unit that can function independently of the macrocycle at large. Electroreduction of 5,10,15,20-tetrakis(3,5-di- tert-butylphenyl)porphyrin-2,

Full text of DOI:10.1021/ja070759b

Published on Web 05/01/2007
Porphyrin-Diones and Porphyrin-Tetraones: Reversible
Redox Units Being Localized within the Porphyrin Macrocycle
and Their Effect on Tautomerism
Karl M. Kadish,*,† Wenbo E,† Riqiang Zhan,† Tony Khoury,‡ Linda J. Govenlock,‡
Jognandan K. Prashar,‡ Paul J. Sintic,‡ Kei Ohkubo,§ Shunichi Fukuzumi,*,§ and
Maxwell J. Crossley*,‡
Contribution from the Department of Chemistry, UniVersity of Houston, Houston, Texas
77204-5003, School of Chemistry, The UniVersity of Sydney, NSW 2006, Australia, and
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity,
SORST, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan
Received February 2, 2007; E-mail: kkadish@uh.edu; fukuzumi@chem.eng.osaka-u.ac.jp; m.crossley@chem.usyd.edu.au
Abstract: Porphyrin-2,3-diones and porphyrin-2,3,7,8- and porphyrin-2,3,12,13-tetraones were shown to
have a redox-active unit that can function independently of the macrocycle at large. Electroreduction of
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin-2,3-diones [(P-dione)M] and the corresponding -2,3,-
12,13-tetraones [L-(P-tetraone)M] and -2,3,7,8-tetraones [C-(P-tetraone)M], where M ) 2H, CuII, ZnII, NiII,
and PdII was investigated and the products were characterized by ESR and thin-layer UV-visible
spectroelectrochemistry. Electrochemical and spectroelectrochemical data show that the first two reductions
of the porphyrin-diones and the first three reductions of the porphyrin-tetraones occur at the dione units.
This was confirmed by ESR spectra of first reduction products which show that the electron spin is totally
localized on a semidione unit, independent of the central metal ion and of the number and location of
dione units. ESR spectra of the radical anions derived from free-base porphyrin-2,3-dione [(P-dione)2H]
and porphyrin-2,3,12,13-tetraone [L-(P-tetraone)2H] confirm the trans-arrangement of the two inner protons
and their location on nonsubstituted pyrrolic rings, thereby maintaining an 18-atom 18-π electron
bacteriochlorin-like aromatic delocalization pathway. The redox unit is not similarly isolated in the corner
free-base porphyrin-2,3,7,8-tetraone [C-(P-tetraone)2H]. A one-electron reduction of C-(P-tetraone)2H leads
to the formation of a tautomer with trans inner hydrogens with one residing on the N of the ring with the
reduced unit as the only detectable product. This process is favorable because it creates a more delocalized
18-atom 18-π electron aromatic pathway. This result is consistent with the measured redox potentials which
show the first reduction of C-(P-tetraone)2H to be substantially easier than (P-dione)2H or L-(P-tetraone)2H.
oxidation or reduction.4-7 In all cases, however, the substituents
are simply modulating the redox chemistry of the porphyrin
macrocycle. This study seeks to incorporate electroactive
functionality into the porphyrin structure that can function
independently of the macrocycle. Such porphyrin systems with
independent redox sites might have applications in electron-
transfer reactions and molecular electronics and could make
possible further chemical reactions at the second site that would
modify such properties like metal ion coordination. In this work
we investigate R-dione units for this purpose.
Introduction
Porphyrins have a highly delocalized π system that is suitable
for efficient electron-transfer reactions.1,2 Rich and extensive
absorption features of porphyrins guarantee increased absorption
cross sections and an efficient use of the solar spectrum.1,2 The
excited states of porphyrins can act as both electron donors and
electron acceptors in photoinduced electron-transfer reactions
to produce radical cations and radical anions, respectively.2,3
Extensive efforts have been made to alter the redox potentials
and electron-transfer reactivities by introducing electron-donat-
ing or -withdrawing substituents on the porphyrin ring for easier
(4) Kadish, K. M. Prog. Inorg. Chem. 1986, 34, 435.
(5) Kadish, K. M.; Royal, G.; Caemelbeck, E. V.; Gueletti, L. In The Porphyrin
Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic
Press: New York, 2000; Vol. 9, p 59.
†
University of Houston.
The University of Sydney.
Osaka University.
‡
§
(6) (a) Wijesekera, T.; Matsumoto, A.; Dolphin, D.; Lexa, D. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 1028. (b) Ozette, K.; Leduc, P.; Palacio, M.; Bartoli,
J.-F.; Barkigia, K. M.; Fajer, J.; Battioni, P.; Mansuy, D. J. Am. Chem.
Soc. 1997, 119, 6442. (c) Palacio, M.; Juillard, A.; Leduc, P.; Battioni, P.;
Mansuy, D. J. Organomet. Chem. 2002, 643-644, 522.
(7) (a) Teraxono, Y.; Patrick, B. O.; Dolphin, D. H. Inorg. Chem. 2002, 41,
6703. (b) Bhyrappa, P.; Sankar, M.; Varghese, B. Inorg. Chem. 2006, 45,
4136.
(1) Fukuzumi, S. In The Porphyrin Handbook; Kadish, K. M., Smith, K.,
Guilard, R., Eds.; Academic Press: San Diego, CA; 2000; Vol. 8, p 115.
(2) Gust, D.; Moore, T. A. In The Porphyrin Handbook; Kadish, K. M., Smith,
K. M., Guilard, R., Eds.; Academic Press: New York, 2000; Vol. 8, p
153.
(3) Fukuzumi, S.; Imahori, H. In Electron Transfer in Chemistry; Balzani, V.,
Ed.; Wiley-VCH: Weinheim, 2001; Vol. 2, p 927.
9
6576
J. AM. CHEM. SOC. 2007, 129, 6576-6588
10.1021/ja070759b CCC: $37.00 © 2007 American Chemical Society

Porphyrin-Diones and Porphyrin-Tetraones
A R T I C L E S
Cyclic ketones and quinones have carbonyl groups in which
the carbonyl carbons are both part of the functional group and
part of the ring structure. Electrochemical properties of numer-
ous diones and 1,2-quinones and ESR characterization of the
related semidiones, their one-electron reduction product, have
been extensively studied.8-12 The only macrocycles similar to
a porphyrin to have been investigated were 5,15-dioxopor-
phodimethenes in which the oxo groups are on opposite meso-
positions, akin to an 18- or 22-atom p-quinone, thereby
disrupting the macrocyclic π-electron conjugation. Reduction
of these compounds, however, involves the macrocycle.13
Porphyrin-2,3-diones,14-15 porphyrin-2,3,7,8-tetraones, and
porphyrin-2,3,12,13-tetraones,16-17 which contain R-dione units
across â,â′-pyrrolic positions on the porphyrin periphery, are
versatile building blocks for the synthesis of laterally extended
porphyrin arrays based on linear and square grid architectures
by annulation reactions involving the R-dione units.14-48 Reac-
tion of porphyrin-2,3-diones with 1,2-diamino-arenes gives ring-
fused quinoxalino[2,3-b]porphyrins,14-46 while reaction with
ammonium acetate and aldehydes gives imidazolo[4,5-b]por-
phyrins.47,48 Use of 1,2,4,5-arenetetramines or dialdehydes in
these reactions allows synthesis of corresponding linked bis-
porphyrins.17,18,29,30,32,33,37,39,44 Similar reactions of the porphyrin-
tetraones give linearly annulated diquinoxalinoporphyrins and
diimidazoloporphyrins16,47 and, when 1,2,4,5-arenetetramines are
used, laterally extended porphyrin-based oligomers,16,18,19,31,43,46
an example being a conjugated planar tetrakis-porphyrin that
spans 6.5 nm which provides for the development of new classes
of organic materials for molecular wires.16,19,49-53 The dione
functionality can be transformed into other units to give ring-
expanded and ring-contracted porphyrin systems.14,54
There is no reported data in the literature on the redox
properties of porphyrin-R-diones and porphyrin-tetraones, and
nothing is therefore known about the potentials for their
electroreduction or degree of delocalization of the added
electron. Reduction of the electroactive dione units should lead
first to a semidione radical anion. These semidione radical
anions have an increased double bond character between the
carbonyl carbon atoms relative to diones.11 There are two
possible fates for the newly generated carbon-carbon electron
density; it can become part of an aromaticity of the macrocyclic
ring in a fashion similar to that seen in the one-electron reduction
of ortho-quinones which leads to an aromatic semiquinone
radical anion, or it might function as the isolated semidione
radical anion in which aromaticity of the rest of the macrocycle
is maintained in a chlorin- or bacteriochlorin-like delocalization
pathway.
This paper reports the electrochemistry and spectroelectro-
chemistry of porphyrin-diones and porphyrin-tetraones that
exhibit unique redox properties in that they have a redox-active
dione unit that can function independently of the macrocycle.
The investigated complexes are shown in Chart 1. The 15
compounds can be divided into three distinct structural types:
porphyrin-2,3-diones [(P-dione)M 2a-e], linear porphyrin-2,3,-
12,13-tetraones [L-(P-tetraone)M 3a-e], and corner porphyrin-
2,3,7,8-tetraones [C-(P-tetraone)M 4a-e], where P is 5,10,15,-
20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and M ) 2H, CuII,
ZnII, NiII, and PdII, a-e, respectively.
A comparison of the properties of porphyrin-diones 2a-e
and porphyrin-tetraones 3a-e and 4a-e with those of the parent
porphyrin and its metalated derivatives [(P)M 1a-e] is made
and shows that initial redox reactions occur at the dione units
and are largely independent of the rest of the macrocycle,
including the central metal ion and number and location of the
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J. AM. CHEM. SOC. VOL. 129, NO. 20, 2007 6577

A R T I C L E S
Chart 1
Kadish et al.
dione units, the only exception being in the case of the free-
base corner porphyrin-2,3,7,8-tetraone 4a. In the other two free-
base cases, 2a and 3a, the semidione unit serves to lock
tautomerism to the tautomer that allows the reduced unit to stay
separate from the rest of the macrocycle.
The electrochemistry of free-base and metallo-5,10,15,20-
tetraarylporphyrins in nonaqueous media has been well char-
acterized in the literature.58 The free-base and metalated
porphyrins 1a-e were expected to undergo well-separated one-
electron oxidations and one-electron reductions at the porphyrin
π-ring system, and this is indeed what was observed. For
example, (P)Cu 1b in CH2Cl2 containing 0.10 M TBAP is
reduced at E1/2 ) -1.36 V and Ep ) -1.88 V and oxidized at
E1/2 ) 0.94 and 1.23 V as shown in Figure 1a. The potential
separation between the reversible first oxidation and reversible
first reduction (the electrochemical HOMO-LUMO gap)
amounts to 2.30 V, and the absolute potential differences
between the two reductions and two oxidations of (P)Cu 1b in
PhCN55 amount to 0.52 V for reduction and 0.29 V for
oxidation. The same type of redox behavior is also seen for the
free-base porphyrin (P)2H 1a in CH2Cl2 (Figure 1c).
Results and Discussion
Synthesis of Metalloporphyrins. The metalloporphyrins 2b-
e, 3b-e, and 4b-e were prepared from the free-base porphyrin-
dione18 2a, free-base linear porphyrin-tetraone16 3a, and free-
base corner porphyrin-tetraone16 4a, respectively. These reaction
conditions are optimized and afford the desired products in high
yield and fast reaction times. This has been achieved by using
high boiling point solvents which help solubilize both the
porphyrins and the metal(II) salt.
Electrochemistry of (P-dione)M 2b-e and (P-dione)2H 2a.
The electrochemistry of each porphyrin-dione 2a-e was
examined in both CH2Cl2 and PhCN containing 0.1 M TBAP.
The initial reductions of all compounds are reversible in both
solvents, and the site of electron transfer could be determined
by UV-visible and/or ESR characterization of the singly and
doubly reduced products (Vide infra). Table 1 contains the E1/2
values of the porphyrin-diones 2a-e in CH2Cl2 and summarizes
also the number of electrons transferred in each oxidation, which
was either one or two, and the potential separations between
the first three reductions of compounds labeled as ∆2-1 and
Quite different redox behavior from that of (P)Cu 1b and
(P)2H 1a, reported above, is seen for (P-dione)Cu 2b and (P-
dione)2H 2a. The difference between the electrochemistry of
the two types of macrocycles is striking. The two one-electron
oxidations of the porphyrin-diones 2a and 2b to generate
π-cation radicals and π-dications in CH2Cl2 are not separated
(55) Ou, Z. E. W.; Shao, J.; Burn, P. L.; Sheehan, C. S.; Walton, R.; Kadish,
K. M.; Crossley, M. J. J. Porphyrins Phthalocyanines 2005, 9, 142.
(56) For the values obtained using PhCN as solvent, see Supporting Information
Table S1.
(57) For cyclic voltammograms of (P-dione)Zn 2c, (P-dione)Ni 2d, and (P-
dione)Pd 2e, see Supporting Information S3.
(58) Kadish, K. M.; Caemelbecke, V.; Royal, G. The Porphyrin Handbook. In
The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;
Academic Press: San Diego, 2000; Vol. 8, pp 1-114.
∆
3-2
.
56 Typical cyclic voltammograms for the (P-dione)Cu 2b
and free base (P-dione)2H 2a in CH2Cl2 are shown in
Figure 1.57
9
6578 J. AM. CHEM. SOC. VOL. 129, NO. 20, 2007

Porphyrin-Diones and Porphyrin-Tetraones
A R T I C L E S
Table 1. Half-Wave Potentials (V vs SCE) of Porphyrin-Diones and -Tetraones in CH2Cl2, 0.1 M TBAPa
oxidation
reduction
b
c
compound
second
first
first
second
third
fourth
∆
2
∆
3-2
-1
(P)2Hd 1a
0.95 (1e)
0.94 (1e)
0.72 (1e)
0.93 (2e)
1.05 (1e)
-1.22
-1.36
-1.42
-1.31
-1.32
-1.55
(P)Cu 1b
(P)Znd 1c
(P)Nid 1d
(P)Pdd 1e
1.23 (1e)
1.06 (1e)
-1.88e
1.52e (1e)
-1.95e
(P-dione)2H 2a
(P-dione)Cu 2b
(P-dione)Zn 2c
(P-dione)Ni 2d
(P-dione)Pd 2e
1.06 (2e)
1.10 (2e)
0.85 (1e)
1.17 (2e)
1.19 (1e)
-0.59
-0.49
-0.57
-0.51
-0.50
-0.95
-1.21
-1.25e
-1.24
-1.26
-1.86e
-1.62
-1.68
-1.61
-1.61
0.36
0.72
0.68
0.73
0.76
0.91
0.41
0.43
0.37
0.35
1.00 (1e)
1.44 (1e)
L-(P-tetraone)2H 3a
L-(P-tetraone)Cu 3b
L-(P-tetraone)Zn 3c
L-(P-tetraone)Ni 3d
L-(P-tetraone)Pd 3e
1.09 (2e)
1.11 (2e)
0.95 (2e)
1.20 (2e)
1.29 (1e)
-0.47
-0.26
-0.34
-0.25
-0.29
-0.72e
-0.70
-0.73
-0.68
-0.64
-1.01
-1.34e
-1.36e
-1.38
-1.34e
-1.80
-1.85
-1.84
-1.84
-1.81
0.25
0.44
0.39
0.43
0.35
0.29
0.64
0.63
0.70
0.70
1.49 (1e)
C-(P-tetraone)2H 4a
C-(P-tetraone)Cu 4b
C-(P-tetraone)Zn 4c
C-(P-tetraone)Ni 4d
C-(P-tetraone)Pd 4e
1.06 (2e)
1.16e (2e)
0.96 (2e)
1.22 (2e)
1.29 (1e)
-0.28
-0.33g
-0.46
-0.32
-0.33
-0.79
-0.67
-0.76
-0.69
-0.69
-1.09f
-1.37e
-1.47e
-1.62e
-1.46e
-1.90e
0.51
0.34
0.30
0.37
0.36
0.30
0.70
0.71
0.93
0.77
a All potentials are good to (10 mV. b Difference between the first and second reduction potentials. c Difference between the second and third reduction
potentials. d Data taken from ref 55. e Peak potential at a scan rate of 0.1 V/s for irreversible reaction. f Unknown irreversible reduction is also observed at
Ep ) -1.60 V at a scan rate of 0.1 V/s. g Unknown irreversible reduction is also observed at Ep ) -0.46 V at a scan rate of 0.1 V/s.
consistent with the electron-withdrawing nature of the oxo
groups which make the porphyrin ring more electropositive,
hence leading to a more difficult oxidation.
On the other hand, the overlapped second oxidation of (P-
dione)Cu 2b is shifted negatively by 130 mV as compared to
(P)Cu 1b in CH2Cl2 indicating a strong ion pairing of the doubly
oxidized Cu(II) porphyrin-dione. This is rarely observed in the
case of simple metalloporphyins having tetraphenylporphyrin
(TPP) or octaethylporphyrin (OEP)-like structures,58 and, with
the single exception of Ni(II) derivatives,59-61 is almost never
observed for any other porphyrin in a solvent such as CH2Cl2.
A negative shift in E1/2 is also observed for the second oxidation
of (P-dione)2H 2a (leading to an overlapping two-electron
oxidation), although the magnitude of the potential shift cannot
be ascertained in CH2Cl2 due to the coupled chemical reaction
following formation of the highly reactive π-cation radical of
(P)2H 1a (see irreversible redox couple in Figure 1c).
Porphyrin-diones 2a-e differ significantly from the unsub-
stituted porphyrins (P)M 1a-e in their reductions where three
one-electron additions are observed for each of the porphyrin-
diones. The first reduction of the five investigated porphyrin-
diones, (P-dione)M 2a-e, is located in a range of E1/2 values
between -0.49 and -0.59 V vs SCE in CH2Cl2 (Table 1), and
between -0.44 and -0.57 V vs SCE in PhCN,56 and is thus
only slightly dependent upon solvent. The second one-electron
reduction of the porphyrin-diones ranges between E1/2 values
of -1.21 and -1.26 V for the metalated complexes 2b-e but
is significantly easier for the free-base porphyrin-dione 2a, E1/2
being -0.95 V in CH2Cl2. Finally the third reduction of the
metalated porphyrin-diones 2b-e spans a narrow E1/2 range of
Figure 1. Cyclic voltammograms of (a) (P)Cu 1b, (b) (P-dione)Cu 2b, (c)
(P)2H 1a, and (d) (P-dione)2H 2a in CH2Cl2, 0.1 M TBAP. The asterisk *
indicates the irreversible cathodic peak due to reduction of an isoporphyrin
formed from the dication in solution.
in potential as in the case of (P)Cu 1b (E1/2 ) 0.94 and 1.23 V)
but are instead overlapped in potential, thus giving an apparent
one-step conversion of the neutral compound to its doubly
oxidized dicationic form (Figure 1b). These processes occur at
E1/2 ) 1.10 V for (P-dione)Cu 2b (Figure 1b) and 1.06 V for
(P-dione)2H 2a (Figure 1d), both values being shifted in a
positive direction from the first one-electron oxidation of (P)-
Cu 1b and (P)2H 1a by 160 and 110 mV, respectively. The
positive shift in E1/2 for oxidation of the porphyrin-diones is
(59) Chang, D.; Malinski, T.; Ulman, A.; Kadish, K. M. Inorg. Chem. 1984,
23, 817-824.
(60) Kadish, K. M.; Lin, M.; Van Caemelbecke, E.; De Stefano, G.; Medforth,
C. J.; Nurco, D. J.; Nelson, N. Y.; Krattinger, B.; Muzzi, C. M.; Jaquinod,
L.; Xu, Y.; Shyr, D. C.; Smith, K. M.; Shelnutt, J. A. Inorg. Chem. 2002,
41, 6673.
(61) Ghosh, A.; Halvorsen, I.; Nilsen, H. J.; Steene, E.; Wondigagegn, T.; Lie,
R.; van Caemelbecke, E.; Guo, N.; Qu, Z.; Kadish, K. M. J. Phys. Chem.
B 2001, 105, 8120.
9
J. AM. CHEM. SOC. VOL. 129, NO. 20, 2007 6579

A R T I C L E S
Scheme 1
Kadish et al.
-1.61 to -1.68 V, while the porphyrin-dione 2a is irreversibly
reduced at more negative potentials; the measured Ep ) -1.86
V in CH2Cl2 for a scan rate of 0.1 V/s. Almost identical values
are obtained for the same processes using PhCN as solvent.56
The electrochemistry of the other three (P-dione)M deriva-
tives, where M ) Zn, Ni, Pd, 2c-e is similar to that of (P-
dione)Cu 2b in the two solvents (see Tables 1 and Supporting
Information S2). Addition of the first electron to the (P-dione)M
2 is significantly easier in each case than reduction of the
analogous (P)M 1 compound with the same metal ion. The
absolute difference in half-wave potentials between the first
reductions in the two series of compounds ranges from 800 to
850 mV in CH2Cl2. The second reduction of (P-dione)M 2 to
give [(P-dione)M]2- is also easier than the second reduction of
the analogous (P)M complex 1, and both of these results are
consistent with the addition of two electrons to the oxo groups
of the dione prior to reduction of the conjugated macrocycle at
more negative potentials.
The relevant electrode reactions of (P-dione)M are given in
Scheme 1 and are confirmed by thin-layer spectroelectrochemi-
cal and ESR data (Vide infra). Prior to discussion of this data,
however, it is important to point out key differences in E1/2
values between the second and third reductions of the four
metalloporphyrin-diones (P-dione)M 2b-e as compared to the
same reactions of the free-base derivative, (P-dione)2H 2a. The
formation of doubly reduced [(P-dione)2H]2- at E1/2 ) -0.95
V in CH2Cl2 and at -0.98 V in PhCN is easier than formation
of doubly reduced [(P-dione)M]2- by more than 250 mV in the
same solvents, but conversely, the formation of triply reduced
[(P-dione)2H]3- at Ep ) -1.86 to -1.92 V is harder by a similar
amount than formation of triply reduced [(P-dione)M]3- under
the same solution conditions. This is illustrated graphically in
Figure 1 and Supporting Information Figure S3 where the
difference between ∆E1/2 values for the stepwise reductions in
CH2Cl2 are indicated on the voltammograms. The differences
range from 680 to 760 and 370-410 mV in the case of (P-
dione)M 2b-e as compared to 360 and 910 mV in the case of
(P-dione)2H 2a.
and ESR spectra of the species generated electrochemically or
chemically, and this data is now presented.
Site of Electron Addition to (P-dione)M 2 and (P-dione)-
2H 2a. The site of electron addition to the (P-dione)M 2b-e
and (P-dione)2H 2a was first examined by thin-layer spectro-
electrochemistry. UV-visible spectra were obtained during the
first two reductions of each compound or during all three
reductions in some cases, an example of which is given in Figure
2 for the stepwise one-electron additions to (P-dione)Cu 2b in
CH2Cl2.
(P-dione)Cu 2b is characterized by a B band at 408 nm and
a weaker intensity band at 486 nm in CH2Cl2 solution containing
0.1 M TBAP. When a controlled potential of -0.60 V is applied
to the (P-dione)Cu 2b solution in the thin-layer cell, the B band
at 408 nm increases in intensity and shifts to 412 nm as the
486 nm band disappears and is replaced by two visible bands
at 536 and 570 nm. These spectral changes are shown in Figure
2a and contrast with the first reduction of (P)Cu 1b (Figure 2d)
where a porphyrin π-radical anion is produced and the B band
absorption significantly decreases in intensity and the Q bands
disappear, being replaced in most cases by a broad band between
600 and 900 nm. The fact that this does not occur in the case
of (P-dione)Cu 2b clearly indicates that the porphyrin macro-
cycle is not reduced during the first one-electron addition. The
significantly small HOMO-LUMO gap of (P-dione)Cu 2b (1.59
eV) as compared with that of (P)Cu 1b (2.40 eV) in Table 1
also indicates that the site of the first reduction is not the
porphyrin macrocycle.
A porphyrin π-radical anion or dianion UV-visible spectrum
is also not obtained during the second one-electron reduction
of (P-dione)Cu 2b at an applied potential of -1.40 V. Here the
412 nm band again increases rather than decreases in intensity
and shifts to 415 nm as new Q bands grow in at 505 and 541
nm. These spectral changes are illustrated in Figure 2b where
the final UV-visible spectrum of the doubly reduced copper(II)
porphyrin-dione resembles a porphyrin spectrum without
the oxo groups in that it has a narrower and more intense B
band.
Clearly differences exist between the metalated porphyrin-
diones and nonmetalated (P-dione)2H 2a in relative stabilities
of the reduced species and the driving force for reduction. The
much easier second reduction of (P-dione)2H 2a as compared
to (P-dione)M 2b-e can be accounted for by a transmission of
some electron density from the oxygens to the porphyrin
macrocycle of the doubly reduced species and, if this occurred,
should also lead to a harder third reduction of (P-dione)2H 2a
as compared to the metalated porphyrins, which is what is
experimentally observed. The source of these differences is
described below. Differences in the site of electron transfer or
electron distribution between the reduced forms of (P-dione)-
Cu 2b and (P-dione)2H 2a should be evident in UV-visible
Finally, during the third reduction of (P-dione)Cu 2b at -1.70
V, the Soret band at 415 nm collapses as is expected for addition
of an electron to the porphyrin π-ring system. These spectral
changes are shown in Figure 2c and are similar to what is seen
during the unambiguous macrocycle-centered reduction of
(P)Cu 1b (Figure 2d). The above-described spectral changes
clearly indicate that the first two one-electron additions to
(P-dione)Cu 2b occur at the dione unit, while the third one-
electron addition occurs exclusively at the porphyrin π-ring
system. The other (P-dione)M complexes, where M ) 2H, Zn,
Ni, or Pd, show similar spectral changes during the three
reductions, thus suggesting the same mechanism of electron
transfer in each case.62
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Porphyrin-Diones and Porphyrin-Tetraones
A R T I C L E S
Figure 2. UV-visible spectral changes of (P-dione)Cu 2b during (a) the first, (b) second, and (c) third reduction and (d) (P)Cu 1b during the first reduction
in CH2Cl2, 0.2 M TBAP.
Scheme 2
For ESR measurements, radical anions of the porphyrin-
diones were produced by photoinduced electron transfer from
dimeric 1-benzyl-1,4-dihydronicotinamide [(BNA)2]63 to the (P-
dione)M derivatives 2a-e in MeCN as shown in Scheme 2.
The (BNA)2 is known to act as a unique electron donor to
produce radical anions of electron acceptors.64
The first one-electron addition to the oxo group of porphyrin-
2,3-diones was confirmed by ESR measurements (Figure 3).65
ESR spectra for the radical anions of (P-dione)Zn 2c and (P-
dione)2H 2a obtained under photoirradiation are shown in
Figure 3. ESR spectra with hyperfine splitting due to one
nitrogen are observed for both compounds, indicating that the
unpaired electron is not delocalized on the porphyrin ring but
rather is localized on the dione unit. The fact that the radical
anion of (P-dione)2H 2a shows no hyperfine splitting due to a
proton (Figure 3b) indicates that hydrogen is not attached to
the nitrogen of the pyrrolic ring which contains the dione radical
anion.
The above data show that, for both free-base and metalated
(P-dione)M 2a-e, the dione unit is reduced and that this leads
to a specific arrangement of the inner hydrogens in (P-dione)-
2H 2a. An explanation of the differences in the redox properties
between the free-base and metalated porphyrins is now ad-
vanced. All free-base porphyrins are known to exist as an
equilibrium mixture of “trans” tautomers in which the hydrogens
of the inner nitrogens of the macrocycle lie on opposite pyrrolic
rings and the aromatic delocalization follows an 18-atom 18
π-electron bacteriochlorin-like “inner-outer-inner-outer”
pathway;66-70 tautomers in which the hydrogens reside on
adjacent “cis” inner nitrogens have been calculated to be of
(62) For UV-visible spectral data of the neutral and reduced porphyrin-diones
in CH2Cl2, 0.1 M TBAP, see Supporting Information S4.
(63) Patz, M.; Kuwahara, Y.; Suenobu, T.; Fukuzumi, S. Chem. Lett. 1997, 567.
(64) Fukuzumi, S.; Suenobu, T.; Patz, M.; Hirasaka, T.; Itoh, S.; Fujitsuka, M.;
Ito, O. J. Am. Chem. Soc. 1998, 120, 8060.
(65) The first one-electron addition to the dione group of porphyrin-2,3-dione
2a is confirmed by thin layer FTIR spectroelectrochemical measurements
showing the disappearance of the carbonyl absorption (see Supporting
Information S5).
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A R T I C L E S
Kadish et al.
Figure 3. ESR spectra for radical anions of (a) (P-dione)Zn 2c and (b)
(P-dione)2H 2a, produced by photoinduced electron transfer from (BNA)2
(2.5 × 10-4 M) to the porphyrins (5.0 × 10-4 M) in CH2Cl2 at 298 K.
1
much higher energy70 and are not detected by H NMR.66-69
Tautomers with a 17-atom 18 π-electron homologous “pyrrole-
like” “inner-inner-inner-outer” aromatic pathway are also of
higher energy than an 18-atom pathway.67 In each tautomer,
rings in which the inner nitrogens are not protonated have an
isolated double bond between the â,â′-positions. In asymmetri-
cally substituted porphyrins, the position of the tautomeric
equilibrium is dependent on the subsituents.66-69
Figure 4. Tautomerism and aromatic delocalization pathways in (P-dione)-
2H 2a and its one- and two-electron reduced forms (aromatic delocalization
pathways are shown as dotted lines).
be as a semiquinone radical anion and is essentially isolated,
while the rest of the macrocycle is essentially neutral and retains
a chlorin-like structure with an 18-atom 18 π-electron bacte-
riochlorin-like “inner-outer-inner-outer” pathway.
(P-dione)2H should exist as an equilibrium mixture of the
forms 5a and 5b (Figure 4), but there is bond localization in
the substituted ring caused by the dione unit which “locks”
single bond character across the â,â′-positions of that ring so
only one “trans” tautomer can show an 18-atom 18 π-electron
“inner-outer-inner-outer” pathway, tautomer 5a. In fact
One-electron reduction of the radical anion 6 is favored
because it liberates a second tautomer in the dianion product,
and in this tautomer 7b the two negatively charged oxygens sit
on the aromatic pathway allowing the charge to be partially
delocalized in the porphyrin structure. Although we have not
determined the position of the tautomeric equilibrium in the
free-base dianions, this delocalization of charge should lead to
tautomer 7b being strongly favored over tautomer 7a. Conse-
quently this delocalization of charge into the porphyrin ring
makes the third reduction, porphyrin ring reduction, more
difficult. In metalated derivatives of a symmetrical porphyrin
all the bond orders across all â,â′-positions are the same.71 There
is thus less extra stabilization to be gained by disrupting the
semidione radical anion, and so it remains effectively isolated
in the first reduction product of (P-dione)M 2b-e. An example
of such isolation is seen in the one-electron reduction of
cyclobutene-diones that has been reported to form an isolated
semidione radical anion, because an antiaromatic compound
would result if the bond was delocalized.72 Similarly the second
reduction is not promoted as much as in the free-base analogue,
and the third reduction is not as destabilized.
1
tautomer 5a is the sole species detected by H NMR of (P-
dione)2H 2a at room temperature.16 This is also supported by
the DFT calculations (UB3LYP/6-31G(d)//UBLLYP/3-21G),
which indicate that 5a is by 6.9 kcal mol-1 more stable than
5b. Furthermore in the one-electron reduced species, there is
what essentially is a localized semidione radical anion that again
“locks” the â,â’-position bonding, this time as a double bond
leading to 6, again by destabilizing the tautomer with an inner
hydrogen on the substituted ring nitrogen. For this reason no
hyperfine splitting due to hydrogen is observed in the ESR
spectrum of the radical anion of (P-dione)2H 2a (Figure 3b),
the species formed as 6 (Figure 4). The semidione radical anion
unit is, of course, resonance stabilized itself. In this tautomer,
the semidione radical anion unit is more stable than it would
(66) Crossley, M. J.; Harding, M. M.; Sternhell, S. J. Am. Chem. Soc. 1986,
108, 3608.
Electrochemistry of Linear and Corner Porphyrin-Tet-
raones, 3 and 4. The electrochemistry of the porphyrin-tetraones
(67) Crossley, M. J.; Harding, M. M.; Sternhell, S. J. Org. Chem. 1988, 53,
1132.
(68) Crossley, M. J.; Field, L. D.; Harding, M. M.; Sternhell, S. J. Am. Chem.
Soc. 1987, 109, 2335.
(69) Crossley, M. J.; Harding, M. M.; Sternhell, S. J. Org. Chem. 1992, 57,
(71) Crossley, M. J.; Harding, M. M.; Sternhell, S. J. Am. Chem. Soc. 1992,
114, 3266.
(72) Rieke, R. D.; White, C. K.; Rhyne, L. D.; Gordon, M. S.; McOmie, J. F.
W.; Hacker, N. P. J. Am. Chem. Soc. 1977, 99, 5387.
1833.
(70) Reimers, J. R.; Lu¨, T. X.; Crossley, M. J.; Hush, N. S. J. Am. Chem. Soc.
1995, 117, 2855.
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Porphyrin-Diones and Porphyrin-Tetraones
A R T I C L E S
Figure 5. Cyclic voltammograms of (a) (P-dione)Cu 2b, (b) L-(P-tetraone)Cu 3b, and (c) C-(P-tetraone)Cu 4b in CH2Cl2, 0.1 M TBAP (the peak indicated
by a star is due to a side product of the first reduction).
is similar to that of the porphyrin-diones in that the first two
one-electron additions occur at the easily reducible oxygen units,
forming semidiones. Six electrons might be added to each
porphyrin-tetraone (two at the conjugated macrocycle and
one at each of the four dioxo groups), but only three or four
electron additions could be observed within the negative
potential limit of the utilized solvent, the remainder occurring
at potentials too negative to be monitored under our experimental
conditions. A summary of half-wave potentials for oxidation
and reduction of the 10 investigated porphyrin-tetraones 3a-e
and 4a-e is given in Table 1,56 and examples of cyclic
voltammograms for the Cu(II) derivatives in the two series of
porphyrin-tetraones are shown in Figure 5 which also includes
a cyclic voltammogram of (P-dione)Cu 2b under the same
experimental conditions.
The electrochemistry of the porphyrin-tetraones can be
interpreted in terms of a system containing two equivalent redox
centers (the dione units) that interact with each other across a
bridge, the bridge in this case being the conjugated porphyrin
macrocycle. Under these conditions the initial formation of the
semidione radical anion in the porphyrin-tetraones 3 and 4 is
split into separated one electron-transfer processes; one is easier
for reduction of (P-dione)M 2 with the same metal ion, and the
other is harder, the absolute potential difference between the
two E1/2 values of L-(P-tetraone)M 3 and C-(P-tetraone)M 4
being a measure of the degree of interaction between the two
redox centers.
The splitting of the two dione-centered redox reactions in
the Cu(II) porphyrin-tetraones 3b and 4b is shown graphically
in Figure 5 where the midpoint potential between the first and
second reductions of the linear tetraone Cu 3b (E1/2 ) -0.26
and -0.70 V) and the corner tetraone Cu 4b (E1/2 ) -0.33 and
-0.67 V) are -0.48 and -0.50 V, respectively in CH2Cl2. The
average of the first two reduction potentials is -0.49 V, exactly
the same E1/2 value as that for reduction of Cu 2b, the compound
having a single dione unit on the porphyrin.
On the basis of the combined electrochemical and spectro-
scopic data, the first three one-electron reductions of L-(P-
tetraone)Cu 3b and C-(P-tetraone)Cu 4b can be unambiguously
assigned as occurring at the dione unit rather than at the
macrocycle. The product of the first two reductions yields a
di[semidione radical anion] where a single negative charge
resides on each of the two separated oxygen atoms of the
compound, while the third one-electron addition then gives a
trianionic species where at least three out of the four oxygen
atoms on the porphyrin have been reduced prior to accessing
the conjugated π-ring system of the porphyrin. These reactions
are shown in Schemes 3 and 4. The site of reduction in the
fourth one-electron addition could not be determined due to the
proximity of this reaction to the negative potential limit of the
solvent.
The splitting of the two dione-centered redox processes is
larger for L-(P-tetraone)Cu 3b (440 mV) than for C-(P-tetraone)-
Cu 4b (340 mV), consistent with a greater interaction between
the two equivalent redox centers in the linear Cu(II) tetraone
3b than in the corner Cu(II) tetraone 4b and the same can be
said for all metalated porphyrin-tetraones in the two series where
the separation in potentials averages 403 mV between the first
two reductions of L-(P-tetraone)M 3b-e and 343 mV between
the first two reductions of C-(P-tetraone)M 4b-e.
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A R T I C L E S
Kadish et al.
Figure 6. Cyclic voltammograms of (a) (P-dione)2H 2a, (b) C-(P-tetraone)2H 4a, and (c) L-(P-tetraone)2H 3a in CH2Cl2, 0.1 M TBAP.
Scheme 3
Scheme 4
Interestingly, the splitting in potentials between the first two
dione-centered redox processes of the free-base C-(P-tetraone)-
of corner porphyrin-tetraones, while the opposite is true in the
case of the linear free-base L-(P-tetraone)2H 3a where the first
two reductions occur at E1/2 ) -0.47 and -0.72 V giving a
∆E1/2 ) 250 mV as compared to an average 342 mV for the
2H 4a (E1/2 ) -0.28 and -0.79V) is much larger (∆E1/2
)
510 mV) than that for the same reductions in the metalated series
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A R T I C L E S
Figure 8. ESR spectra for the radical anions of (a) L-(P-tetraone)H2 and
(b) C-(P-tetraone)H2, produced by photoinduced electron transfer from
(BNA)2 (2.5 × 10-4 M) to the porphyrins (5.0 × 10-4 M) in CH2Cl2 at
298 K.
Figure 7. ESR spectra for the radical anions of (a) L-(P-tetraone)Zn 3c
and (b) C-(P-tetraone)Zn 4c produced by photoinduced electron transfer
from (BNA)2 (2.5 × 10-4 M) to the porphyrins (5.0 × 10-4 M) in CH2Cl2
at 298 K.
other metalated compounds in the series (Figure 6). It is due to
the first reduction of C-(P-tetraone)2H 4a being easier as it leads
to greater aromaticity (vide infra).
The spectroscopic data indicate that the third reduction of
the metalated porphyrin-tetraones involves an electron addition
to one of the already reduced diones, thus generating the
trianionic species with an unreduced porphyrin macrocycle.73
The third electrode reaction is easier for the L-(P-tetraones)M
3b-e than for the C(P-tetraones)M 4b-e derivatives by an
average of 120 mV in CH2Cl2 (see Table 1). In addition, the
separation between the second and third reductions is close to
700 mV, about the same potential separation observed between
the first and second reductions of the metalated porphyrin-diones
where the second electron goes onto an adjacent already-reduced
oxygen atom.
The site of the first one-electron addition to a dione group of
porphyrin-tetraones was also confirmed by ESR measurements.
In the case of the radical anions of L-(P-tetraone)Zn 3c and
C-(P-tetraone)Zn 4c (Figure 7), the ESR spectra exhibit hyper-
fine splitting due to only one nitrogen, indicating that the
unpaired electron is localized on only a single dione unit of the
porphyrin-tetraone.
In the case of the radical anion of C-(P-tetraone)2H 4a, there
is additional hyperfine splitting due to one proton, whereas no
hyperfine splitting due to the proton is observed for the radical
anion of L-(P-tetraone)2H 3a (Figure 8). There must be in the
case of C-(P-tetraone)2H a hydrogen attached to one of the
pyrrolic ring nitrogens containing the semidione radical anion
unit. A substantially easier first reduction of C-(P-tetraone)2H
4a is seen in both solvents (E1/2 ) -0.28 V in CH2Cl2 and
-0.26 V in PhCN).
Figure 9. Tautomerism and aromatic delocalization pathways in L-(P-
tetraone)2H 3a and its one- and two-electron reduced forms (aromatic
delocalization pathways are shown as dotted lines).
The relative ease in the first reduction of C-(P-tetraone) 4a
can be understood in terms of the effect of reduction on the
tautomeric processes that operate in the free-base porphyrin-
tetraones 3a (Figure 9) and 4a (Figure 10). Linear porphyrin-
tetraone 3a exists as the tautomer 8a as is readily discerned
(73) For UV-visible spectral data of the neutral and one-, two-, three-, and
four-electron reduced porphyrin-tetraones in CH2Cl2, 0.1 M TBAP, see
Supporting Information S6 (L-(P-tetraone)Ni 3d) and S7 (C-(P-tetraone)-
Ni 4d).
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A R T I C L E S
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energy “inner-inner-inner-outer” 17-atom 18 π-electron
pathway. One-electron reduction leads to the tautomer 11a being
formed as the overwhelmingly dominant product, thereby
“turning off” the tautomeric reaction and creating a new, more
stable “outer-inner-outer-inner” 18-atom 18 π-electron aro-
matic pathway (Figure 10). This tautomer, with an inner
hydrogen on the nitrogen of the ring at which electron-transfer
occurred, is detected by ESR (Figure 8b), and the energy gained
is reflected in it being the easiest of the compounds in this study
to be reduced by one electron. The alternate tautomer 11b still
has a higher energy “inner-inner-inner-outer” 17-atom 18
π-electron pathway. The second reduction gives the degenerate
tautomers 12a and 12b and turns the tautomeric process “on”
again. The only previous example of such a switching off of
tautomerism was observed in a complex system involving the
compound 2-hydroxy-5,10,15,20-tetraphenylporphyrin where a
secondary enol-keto tautomerism leads to a 5:3:2 equilibrium
mixture of essentially a single tautomer of the keto form and
two tautomers of the enolic hydroxy form.67 The present case
of C-(P-tetraone)2H 4a is the only case of the process leading
to essentially a single entity.
Conclusions
In contrast to porphyrins that exhibit two one-electron
reductions, three one-electron reductions are observed for
reduction of the porphyrin-2,3-diones. The electrochemical,
UV-visible spectroelectrochemical, and ESR data indicate that
the first two one-electron reductions involve the oxo groups of
the dione unit, followed by the third reduction on the porphyrin
ring at more negative potentials. In the case of porphyrin-2,3,7,8-
tetraones and porphyrin-2,3,12,13-tetraones, the first three one-
electron reductions occur at dione units prior to accessing the
conjugated π-ring system of the porphyrin. Thus, the dione unit,
while involving carbons that are part of the porphyrin macro-
cyclic framework, can show independent redox chemistry from
that of the porphyrin ring. Such unique redox properties of
porphyrin-diones and -tetraones will make it possible to vary
the reduction potentials more significantly by binding of metal
ions to the dione units without changing the oxidation potentials.
This will certainly expand the scope of electron-transfer
chemistry of porphyrins. One-electron reduction of the corner
tetraone C-(P-tetraone)2H 4a causes an unprecedented “locking”
of the tautomeric inner hydrogen exchange process.
Figure 10. Tautomerism and aromatic delocalization pathways in C-(P-
tetraone)2H 4a and its one- and two-electron reduced forms (aromatic
delocalization pathways are shown as dotted lines).
from the 1H NMR spectrum in CDCl3 where the four equivalent
â-pyrrolic protons resonate as a doublet (4J ) 1.8 Hz) at 8.50
ppm that collapses to a singlet upon irradiation of the two-proton
broad singlet (N-H) at -1.80 ppm. This compound has a
striking visible spectrum with a blue-shifted B band at 386 nm
and a red-shifted Qy band at 772 nm and resembles that of a
bacteriochlorin which has a locked 18-atom 18 π-electron
aromatic delocalization pathway.16 The alternate tautomer 8b
has a very unfavorable 16-atom 18 π-electron aromatic delo-
calization pathway (Figure 9). The DFT calculations (UB3LYP/
6-31G(d)//UB3LYP/3-21G) indicate that 8a is by 18.1 kcal
mol-1 more stable than 8b.
Experimental Section
The first reduction gives 9a in which the electron has reduced
a dione unit which remains isolated from the 18-atom 18
π-electron aromatic delocalization pathway; tautomer 9b is very
unfavorable as before. The DFT calculations indicate that 9a is
by 5.2 kcal mol-1 more stable than 9b. The second reduction
gives 10 on electrostatic grounds as there is no advantage in
reducing the first semidione radical anion unit further, as at best
it could create an alternate 17-atom 18 π-electron aromatic
delocalization pathway having much higher energy than 10. The
third reduction also takes place on the peripheral unit for the
same reason.
In the case of the corner tetraone, C-(P-tetraone) 4a, the first
example of tautomerism being “switched off”, here by a
reversible one-electron process, is observed (Figure 10).
Corner tetraone C-(P-tetraone) 4a exists as a 1:1 mixture of
the degenerate tautomers 10a and 10b. The aromatic delocal-
ization of these tautomers necessarily involves a relatively higher
General. Microanalyses were performed by the Campbell Microana-
lytical Laboratory, University of Otago, New Zealand. Infrared spectra
were recorded on a Perkin-Elmer model 1600 FT-IR spectrophotometer
and on Fourier transform infrared spectrophotometer FTIR-8400S
SHIMADZU of solutions in the stated solvents. Ultraviolet-visible
spectra were routinely recorded on a Cary 5E UV-vis-NIR spectro-
photometer in chloroform that was deacidified by filtration through an
1
alumina column. H NMR spectra were recorded on either a Bruker
AC-200 (200 MHz) or a Bruker DPX-400 (400 MHz) spectrometer as
stated. Samples were dissolved in deuteriochloroform (CDCl3), and the
chloroform peak at 7.26 ppm was used as an internal reference unless
otherwise stated. Deuterated solvents were used as received except for
deuterated chloroform, which was deacidified by filtration through a
plug of alumina.
Matrix assisted laser desorption ionization time-of-flight (MALDI-
TOF) mass spectra were recorded on a VG TofSpec spectrometer. For
all the porphyrin derivatives described in this paper no matrix was
9
6586 J. AM. CHEM. SOC. VOL. 129, NO. 20, 2007

Porphyrin-Diones and Porphyrin-Tetraones
A R T I C L E S
required. Mass spectra were obtained as an envelope of the isotope
peaks of the molecular ion. The mass corresponding to the maxima of
the envelope is reported and was compared with the maxima of a
simulated spectrum. Electrospray ionization (ESI) and atmospheric
pressure chemical ionization (APCI) mass spectra were recorded on a
ThermoQuest Finnigan LCQ DECA instrument. High-resolution elec-
trospray ionization Fourier transform ion cyclotron resonance (HR-
ESI-FT/ICR) spectra were either acquired at the School of Chemistry,
The University of New South Wales on a Bruker Daltonics BioAPEX
II FT/ICR mass spectrometer equipped with a 7 T MAGNEX
superconducting magnet and an Analytica external ESI source or
acquired at the Research School of Chemistry, Australian National
University on a Bruker Daltonics BioAPEX II FT/ICR mass spectrom-
eter equipped with a 4.7 T MAGNEX superconducting magnet and an
Analytica external ESI source. Electrospray ionization high-resolution
mass spectrometry (ESI-HRMS) data were either recorded on a VG
Quattro II triple quadrupole mass spectrometer at the Research School
of Chemistry, Australian National University or recorded on a Finnigan
MAT 900 × L mass spectrometer at the School of Molecular and
Microbial Sciences, The University of Queensland.
All commercial solvents were routinely distilled prior to use. Solvent
mixture proportions are given by volume ratios. Light petroleum refers
to the fraction with bp 60-80 °C.
Electrochemical Measurements. Absolute dichloromethane (CH2Cl2)
and pyridine were received from Aldrich Co. and used as received
without further purification. Tetra-n-butylammonium perchlorate (TBAP)
was purchased from Sigma Chemical or Fluka Chemika Co., recrystal-
lized from ethyl alcohol, and dried under vacuum at 40 °C for at lease
1 week prior to use.
Cyclic voltammetry was carried out by using an EG&G Princeton
Applied Research (PAR) 173 potentiostat/galvanostat. A homemade
three-electrode cell was used for cyclic voltammetric measurement and
consisted of a platinum button or glassy carbon working electrode, a
platinum counter electrode, and a homemade saturated calomel reference
electrode (SCE). The SCE was separated from the bulk of the solution
by a fritted glass bridge of low porosity, which contained the solvent/
Supporting electrolyte mixture. UV-visible spectroelectrochemical
experiments were performed with a home-built thin-layer cell that had
a light transparent platinum net working electrode. Potentials were
applied and monitored with an EG&G PAR model 173 potentiostat.
Time-resolved UV-visible spectra were recorded with a Hewlett-
Packard model 8453 diode array spectrophotometer. Thin-layer FTIR
spectroelectrochemical measurements were carried out with an FTIR
Nicolet 550 Magna-IR spectrometer using a specially constructed light
transparent FTIR spectroelectrochemical cell.
ESR Measurements. Semidione radical anions were prepared by
the photoreduction of porphyrin-diones and porphyrin-tetraones with
(BNA)2 in deaerated MeCN. Typically, a porphyrin dione was dissolved
in CH2Cl2 (5.0 × 10-4 M in 300 µL) containing (BNA)2 (2.5 × 10-4
M) and purged with argon for 10 min. The solution was bubbled with
Ar gas through a syringe that has a long needle. ESR spectra were
recorded on a JEOL JES-RE1XE spectrometer under irradiation of a
high-pressure mercury lamp (USH-1005D) by focusing at the sample
cell in the ESR cavity at 298 K. Semidione radical anions were also
produced by the electron-transfer reduction of porphyrin-diones and
porphyrin-tetraones by a tetramethyl-p-benzosemiquinone radical anion
(Me4Q•-) in CH2Cl2. The magnitude of modulation was chosen to
optimize the resolution and signal-to-noise (S/N) ratio of the observed
spectra under nonsaturating microwave power conditions. The g values
were calibrated using a Mn2+ marker.
chloride (32.0 mg, 0.180 mmol), and sodium acetate (46.0 mg, 0.561
mmol) were dissolved in toluene (8 mL) and glacial acetic acid (18
M, 8 mL), and the mixture was heated at reflux for 2 min. Dichloro-
methane (30 mL) was added to the mixture, and the organic phase
was separated; washed with water (200 mL), sodium carbonate solution
(10%, 150 mL), and water (200 mL); dried over anhydrous sodium
sulfate; and filtered, and the filtrate was evaporated to dryness. The
product was purified by column chromatography over silica (CH2Cl2-
light petroleum; 1:1). The front running green band afforded 2e (24.5
mg, 99%) as a green microcrystalline solid, mp >300 °C. IR (CHCl3):
2965s, 2869m, 1724s, 1593s, 1477m, 1364s, 1300m, 1108m, 1020w
cm-1. UV-vis (CHCl3): 269 (log ꢀ 4.35), 404 (5.16), 478 (4.34), 611
(3.72), 683 (3.73) nm. 1H NMR (400 MHz, CDCl3): δ 1.44 (36 H, s,
tert-butyl H); 1.47 (36 H, s, tert-butyl H); 7.58 (4 H, d, J 1.8 Hz, Ho);
7.69 (2 H, t, J 1.7 Hz, Hp); 7.73 (2 H, t, J 1.8 Hz, Hp); 7.86 (4 H, d,
J 1.8 Hz, Ho); 8.22 (2 H, d, J 5.1 Hz, â-pyrrolic H); 8.46 (2 H, d, J 5.1
Hz, â-pyrrolic H); 8.50 (2 H, s, â-pyrrolic H). MS (ESI) (m/z): 1197.6
(M+ requires 1198.0). HR-ESI-FT/ICR Found: [M + H]+ 1197.6166.
C76H90N4O2Pd requires 1197.6196.
{2,3,12,13-Tetraoxo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-
bacteriochlorinato}zinc(II) (3c). Tetraone 3a (13.2 mg, 0.0117 mmol)
and zinc(II) acetate dihydrate (100 mg, 0.456 mmol) in chloroform
(12 mL) and methanol (6 mL) was heated at reflux for 52 h. The solvent
was removed under vacuum, and the residue was purified by column
chromatography over silica (CH2Cl2). The major green band was
collected, and the solvent was removed to afford 3c (7.6 mg, 55%) as
a green microcrystalline solid, mp >300 °C. Found: C, 73.01; H, 7.33;
N, 4.56. C76H88N4O4Zn + 1/2CH3CN + 1/2CHCl3 requires C, 73.46; H,
7.16; N, 4.97%. IR (CHCl3): 3061w, 2964s, 2905m, 2868m, 1717s,
1593s, 1477m, 1464w, 1429w, 1394m, 1364m, 1348w, 1298w, 1265w,
1248m, 1204w, 1126w, 1078m, 1034m, 1016w cm-1. UV-vis
(CHCl3): 398 (log ꢀ 5.08), 424sh (4.74), 465sh (4.41), 546sh (3.46),
1
582 (3.52), 674sh (3.74), 750sh (3.90), 829 (3.93) nm. H NMR (400
MHz, CDCl3): δ 1.43 (72 H, s, tert-butyl H); 7.56 (8 H, d, J 1.8 Hz,
Ho); 7.69 (4 H, t, J 1.8 Hz, Hp); 8.24 (4 H, s, â-pyrrolic H). MS (ESI)
(m/z): 1187.5 (M+ requires 1186.9). MS (MALDI-TOF) (m/z): 1186.4
(M+ requires 1186.9).
{2,3,12,13-Tetraoxo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-
bacteriochlorinato}nickel(II) (3d). Tetraone 3a (62.9 mg, 0.0560
mmol), nickel(II) acetate dihydrate (300 mg, 1.410 mmol), and sodium
acetate (200 mg, 2.438 mmol) were dissolved in toluene (13 mL) and
glacial acetic acid (18 M, 13 mL), and the mixture was heated at reflux
for 48 h. Workup was as that above, and column chromatography over
silica (CH2Cl2-light petroleum; 1:1) gave 3d (19.9 mg, 30%) as a green
microcrystalline solid, mp >300 °C. IR (CHCl3): 2964s, 2905m, 2868m
1717s, 1593m, 1541w, 1477w, 1464w, 1431w, 1408w, 1394w, 1364m,
1356m, 1302w, 1271w, 1248w, 1121w, 1084w, 1041m, 1026w, 1018w
cm-1. UV-vis (CHCl3): 327sh (log ꢀ 4.44), 373 (4.76), 397 (4.76),
464 (4.31), 492sh (4.08), 588sh (3.41), 642sh (3.63), 712 (3.87), 874sh
(3.75), 949 (3.91) nm. 1H NMR (400 MHz, CDCl3): δ 1.38 (72 H, s,
tert-butyl H); 7.30 (8 H, d, J 1.7 Hz, Ho); 7.59 (4 H, t, J 1.7 Hz, Hp);
8.14 (4 H, s, â-pyrrolic H). MS (ESI) (m/z): 1180.8 (M+ requires
1180.2). ESI-HRMS Found: [M + Na]+ 1201.607. C76H88N4NiO4 +
Na requires 1201.605. The second brown band was collected, and the
solvent was removed to afford unreacted 3a (22 mg, 35%).
{2,3,12,13-Tetraoxo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-
bacteriochlorinato}palladium(II) (3e). Tetraone 3a (60.0 mg, 0.0530
mmol), palladium(II) chloride (64.0 mg, 0.361 mmol), and sodium
acetate (110 mg, 1.341 mmol) were dissolved in toluene (13 mL) and
glacial acetic acid (18 M, 13 mL), and the mixture was heated at reflux
for 5 min. Workup was as that above, and column chromatography
over silica (CH2Cl2-light petroleum; 1:1) gave 3e (58.4 mg, 90%) as
a green microcrystalline solid, mp >300 °C. IR (CHCl3): 2964s,
2905m, 2868m, 1720s, 1593m, 1477w, 1464w, 1414w, 1394w, 1364m,
Synthesis. Unless stated otherwise, new compounds were crystallized
from a chloroform-acetonitrile solution. Synthesis of porphyrin-diones
2a-d and linear porphyrin-tetraones 3a,b have been described
earlier.16-18,29,30
{2,3-Dioxo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)chlorinato}-
palladium(II) (2e). Dione 2a (22.5 mg, 0.0206 mmol), palladium(II)
1354m, 1302w, 1248w, 1121w, 1097w, 1045s, 1030w, 1020w cm-1
.
UV-vis (CHCl3): 308sh (log ꢀ 4.40), 362sh (4.64), 386 (4.83), 448
9
J. AM. CHEM. SOC. VOL. 129, NO. 20, 2007 6587

A R T I C L E S
Kadish et al.
(4.46), 479sh (4.15), 581sh (3.50), 641 (3.91), 805sh (3.87), 868 (4.02)
nm. 1H NMR (400 MHz, CDCl3): δ 1.40 (72 H, s, tert-butyl H); 7.44
(8 H, d, J 1.8 Hz, Ho); 7.65 (4 H, t, J 1.8 Hz, Hp); 7.99 (4 H, s, â-pyrrolic
H). MS (ESI) (m/z): 1228.5 (M+ requires 1228.0). MS (MALDI-TOF)
(m/z): 1228.3 (M+ requires 1228.0). ESI-HRMS Found: [M + Na]+
1249.575. C76H88N4O2Pd + Na requires 1249.576.
{2,3,7,8-Tetraoxo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-
isobacteriochlorinato}copper(II) (4b). Tetraone 4a (20.0 mg, 0.0178
mmol) and copper(II) acetate monohydrate (40.0 mg, 0.200 mmol) in
chloroform (12 mL) and methanol (6 mL) were heated at reflux for 2
min. The solvent was removed under vacuum, and the product was
purified by column chromatography over silica (CH2Cl2-light petro-
leum; 1:1). The major green band afforded 4b (18.0 mg, 85%) as a
green microcrystalline solid, mp >300 °C. Found: C, 76.8; H, 7.7; N,
4.5. C76H88N4CuO4 requires C, 77.0; H, 7.5; N, 4.7%. IR (CHCl3):
2965s, 2868w, 1724s, 1594m, 1540w, 1476w, 1364m, 1346m, 1296w,
1265w, 1218s, 1214s cm-1. UV-vis (CHCl3): 389 (log ꢀ 4.87), 411sh
(4.69), 447sh (4.37), 473 (4.43), 552 (4.05), 597 (4.05) nm. MS
(MALDI-TOF) (m/z): 1185 (M+ requires 1185). MS (ESI) (m/z):
1185.5 (M+ requires 1185.1).
t, J 1.8 Hz, Hp); 7.58 (1 H, d, J 1.8 Hz, Ho); 7.64 (2 H, t, J 1.8 Hz, Hp);
7.78 and 7.97 (4 H, ABq, J 4.8 Hz, â-pyrrolic H). MS (MALDI-TOF)
(m/z): 1181 (M+ requires 1180). MS (ESI) (m/z): 1180.5 (M+ requires
1180.2).
{2,3,7,8-Tetraoxo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-
isobacteriochlorinato}palladium(II) (4e). Tetraone 4a (55.8 mg,
0.0497 mmol), palladium(II) chloride (43.0 mg, 0.243 mmol), and
sodium acetate (61 mg, 0.744 mmol) were dissolved in toluene (7 mL)
and glacial acetic acid (18 M, 7 mL), and the mixture was heated at
reflux for 3 min. Workup was as that above, and column chromatog-
raphy over silica (CH2Cl2-light petroleum; 1:1) gave 4e (60.5 mg, 99%)
as a green microcrystalline solid, mp >300 °C. Found: C, 74.3; H,
7.3; N, 4.3. C76H88N4O4Pd requires C, 74.3; H, 7.2; N, 4.6%. IR
(CHCl3): 2965s, 2905m, 2868w, 1723s, 1594m, 1546w, 1477w, 1363m,
1351m, 1300w, 1247w, 1215s, 1183w, 1111w, 1080w cm-1. UV-vis
(CHCl3): 385 (log ꢀ 4.99), 444 (4.40), 467 (4.46), 545sh (3.99), 579
(4.07), 718 (4.72) nm. 1H NMR (400 MHz, CDCl3): δ 1.39 (18 H, s,
tert-butyl H); 1.42 (36 H, s, tert-butyl H); 1.46 (18 H, s, tert-butyl H);
7.23 (2 H, d, J 1.8 Hz, Ho); 7.45 (4 H, d, J 1.8 Hz, Ho); 7.64 (1 H, t,
J 1.8 Hz, Hp); 7.67 (2 H, t, J 1.8 Hz, Hp); 7.71-7.74 (3 H, m, Ho, Hp);
7.96 and 8.19 (4 H, ABq, J 4.9 Hz, â-pyrrolic H). MS (MALDI-TOF)
(m/z): 1229 (M+ requires 1228). MS (ESI) (m/z): 1228.7 (M+ requires
1228.0).
{2,3,7,8-Tetraoxo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-
isobacteriochlorinato}zinc(II) (4c). Tetraone 4a (52.0 mg, 0.0463
mmol) and zinc(II) acetate dihydrate (34.0 mg, 0.155 mmol) in
chloroform (20 mL) and methanol (10 mL) were heated at reflux for
3 min. The solvent was removed under vacuum, and the residue was
purified by column chromatography over silica (CH2Cl2). The major
green band afforded 4c (54.5 mg, 99%) as a green microcrystalline
solid, mp >300 °C (from CH2Cl2-light petroleum). Found: C, 74.7;
H, 7.4; N, 4.5. C76H88N4O4Zn + 1/2CH2Cl2 requires C, 74.7; H, 7.3; N,
4.5%. IR (CHCl3): 2964s, 2905m, 2868m, 1722s, 1593s, 1526w,
Acknowledgment. The support of the Robert A. Welch
Foundation (KMK, Grant E-680) and the Texas Advanced
Research program to K.M.K. under Grant No. 003652-0018-
2001 are gratefully acknowledged. This work was also partially
supported by a Discovery Research Grant (DP0208776) to
M.J.C. from the Australian Research Council and a Grant-in-
Aid (No. 17750039) to K.O. from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
1477m, 1464w, 1364m, 1342m, 1294m, 1248w, 1101w, 1072w cm-1
.
UV-vis (CHCl3): 396 (log ꢀ 4.94), 423sh (4.75), 457sh (4.43), 479sh
1
(4.22), 545sh (4.10), 606 (4.04), 644sh (3.95), 724sh (3.71) nm. H
NMR (400 MHz, CDCl3): δ 1.39 (18 H, s, tert-butyl H); 1.42 (36 H,
s, tert-butyl H); 1.45 (18 H, s, tert-butyl H); 7.25 (2 H, d, J 1.2 Hz,
Ho); 7.47 (4 H, d, J 1.2 Hz, Ho); 7.61 (1 H, t, J 1.2 Hz, Hp); 7.65 (2 H,
t, J 1.2 Hz, Hp); 7.70 (1 H, t, J 1.2 Hz, Hp); 7.75 (2 H, d, J 1.2 Hz, Ho);
7.85 and 8.12 (4 H, ABq, J 4.6 Hz, â-pyrrolic H). MS (ESI) (m/z):
1186.6 (M+ requires 1186.9).
Supporting Information Available: Table S1: Half-wave
potentials (V vs SCE) of porphyrin-diones and -tetraones in
PhCN, 0.1 M TBAP. Figure S1: Cyclic voltammograms of (a)
(P-dione)Ni 2d, (b) (P-dione)Zn 2c, and (c) (P-dione)Pd 2e in
CH2Cl2, 0.1 M TBAP. Table S2: UV-visible spectral data of
neutral and reduced porphyrin-diones in CH2Cl2, 0.1 M TBAP.
Figure S2: Thin-layer FTIR (a) spectra of (P-dione)2H 2a before
reduction in a thin-layer FTIR cell and (b) differential spectral
changes during the first reduction at -0.70 V in CH2Cl2, 0.2
M TBAP. Figure S3: UV-visible spectral changes of L-(P-
tetraone)Ni 3d during the (a) first, (b) second, (c) third, and (d)
fourth reduction in CH2Cl2, 0.2 M TBAP. Figure S4: UV-
visible spectral changes of C-(P-tetraone)Ni 4d during the (a)
first, (b) second, (c) third, and (d) fourth reduction in PhCN,
0.2 M TBAP. This material is available free of charge via the
Internet at http://pubs.acs.org.
{2,3,7,8-Tetraoxo-5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)-
isobacteriochlorinato}nickel(II) (4d). Tetraone 4a (18.0 mg, 0.0160
mmol) and nickel(II) acetate dihydrate (40.0 mg, 0.188 mmol) were
dissolved in toluene (8 mL) and glacial acetic acid (18 M, 8 mL), and
the mixture was heated at reflux for 2 min. Workup was as that above,
and column chromatography over silica (CH2Cl2-light petroleum; 1:1)
afforded 4d (18.5 mg, 98%) as a green microcrystalline solid, mp
>300 °C. Found: C, 77.1; H, 7.6; N, 4.5. C76H88N4O4Ni requires C,
77.3; H, 7.5; N, 4.7%. IR (CHCl3): 2950s, 2919s, 2857s, 1721m,
1459m, 1377w, 1351w, 1295w, 1218s cm-1. UV-vis (CHCl3): 333sh
1
(log ꢀ 4.49), 399 (4.66), 476 (4.26), 571 (3.82), 599 (3.83) nm. H
NMR (400 MHz, CDCl3): δ 1.31 (18 H, s, tert-butyl H); 1.36 (36 H,
s, tert-butyl H); 1.41 (18 H, s, tert-butyl H); 6.95 (2 H, d, J 1.8 Hz,
Ho); 7.26 (4 H, d, J 1.8 Hz, Ho); 7.49 (1 H, t, J 1.8 Hz, Hp); 7.56 (2 H,
JA070759B
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6588 J. AM. CHEM. SOC. VOL. 129, NO. 20, 2007