Metalloporphyrin mixed-valence π-cation radicals: Solution stability and properties

Article abstract of DOI:10.1021/ja9616950

Solution equilibria and optical spectra of several metallooctaethylporphyrin π-cation radicals have been examined in methylene chloride solution. The several π-cation radical species, [M(OEP^.)]Y (M = Cu, Ni, Zn, Pd, or VO; Y = ClO₄^- or SbCl₆^-), are found to dimerize to form [M(OEP^.)]₂²⁺. These dimeric species are characterized by the appearance of a new, strongly concentration-dependent near-infrared absorption band. This band is found in the region of 900-960 nm for all compounds except for the vanadyl complex which absorbs farther to the red at 1375 nm. The equilibrium constants for the dimerization reaction (2[M(OEP^.)]⁺ (K(D)) [M(OEP^.)]₂²⁺) have been evaluated from multiple-wavelength, concentration-dependent absorption data. Equimolar solutions of these π-cation radicals and their analogous neutral M(OEP) derivatives react to form new binuclear species, [M(OEP(./2)]₂⁺, in which the single radical electron is delocalized over both porphyrin rings. These new species, for which we suggest the term 'mixed-valence' π-cation radical and which bear a formal relationship to the oxidized special pair of photosynthetic reaction centers, also display a near-infrared absorption band. The near-IR band is however distinctly different from that of the π-cation radical dimers. Equilibrium constants for the formation of these new species (M(OEP) + [M(OEP^.)]⁺ (K(MV)) [M(OEP(./2)]₂⁺), have also been determined. The values of K(MV) are 10-100-fold larger than the analogous dimerization constant K(D). Values of ΔH and ΔS have been obtained for both equilibrium processes. The variations in K(MV) and K(D) values are largely the consequence of entropy differences that probably result from a much stronger solvation of the π-cation radical compared to the neutral porphyrin.

Full text of DOI:10.1021/ja9616950

J. Am. Chem. Soc. 1997, 119, 2839-2846
2839
Metalloporphyrin Mixed-Valence π-Cation Radicals: Solution
Stability and Properties
Kristin E. Brancato-Buentello, Seong-Joo Kang, and W. Robert Scheidt*
Contribution from the Department of Chemistry and Biochemistry, UniVersity of Notre Dame,
Notre Dame, Indiana 46556
X
ReceiVed May 20, 1996
Abstract: Solution equilibria and optical spectra of several metallooctaethylporphyrin π-cation radicals have been
•
examined in methylene chloride solution. The several π-cation radical species, [M(OEP )]Y (M ) Cu, Ni, Zn, Pd,
-
-
•
2+
or VO; Y ) ClO4 or SbCl6 ), are found to dimerize to form [M(OEP )]2 . These dimeric species are characterized
by the appearance of a new, strongly concentration-dependent near-infrared absorption band. This band is found in
the region of 900-960 nm for all compounds except for the vanadyl complex which absorbs farther to the red at
•
+
•
2+
1
375 nm. The equilibrium constants for the dimerization reaction (2[M(OEP )] (KD) [M(OEP )]2 ) have been
evaluated from multiple-wavelength, concentration-dependent absorption data. Equimolar solutions of these π-cation
•
/2
+
radicals and their analogous neutral M(OEP) derivatives react to form new binuclear species, [M(OEP )]2 , in
which the single radical electron is delocalized over both porphyrin rings. These new species, for which we suggest
the term “mixed-valence” π-cation radical and which bear a formal relationship to the oxidized special pair of
photosynthetic reaction centers, also display a near-infrared absorption band. The near-IR band is however distinctly
different from that of the π-cation radical dimers. Equilibrium constants for the formation of these new species
•
+
•/2
+
(
M(OEP) + [M(OEP )] (KMV) [M(OEP )]2 ), have also been determined. The values of KMV are 10-100-fold
larger than the analogous dimerization constant KD. Values of ∆H and ∆S have been obtained for both equilibrium
processes. The variations in KMV and KD values are largely the consequence of entropy differences that probably
result from a much stronger solvation of the π-cation radical compared to the neutral porphyrin.
Introduction
is the primary donor for at least one of the photosystem reaction
centers (PS II).
7
,8
Electronic interactions between the π systems of two or more
porphyrin molecules in close proximity are believed to be
important in defining the properties of the supramolecular
ensemble. Notable examples of such interacting porphyrin π
systems are those found in photosynthetic proteins and in
linearly stacked arrays that are organic conductors. The
X-ray crystal structures of bacterial photosynthetic reaction
9-12
centers from several organisms
have now provided detailed
information about the ground state geometry of the special pair,
P. The special pair has a cofacial arrangement of the two
bacteriochlorophyll rings with effective overlap of one pyrrole
ring from each macrocycle. The overlap of porphyrin rings has
been frequently observed in the solid-state structures of por-
1
2
photosynthetic proteins include the light-harvesting chlorophyll
3
4
arrays and the photosynthetic reaction center (RC) special
1
3
5
phyrin compounds. However, most porphyrin compounds
exhibit stronger intermolecular interactions than that displayed
by the photosynthetic special pairs as judged by the geometric
criterion of the degree of inter-ring overlap. The apparent
importance of the cofacial bacteriochlorophyll dimer in defining
the photophysics and photoinduced electron transfer of the entire
pairs. The photoinduced formation of the one-electron oxidized
+
reaction center special pair (the primary donor, often called P )
is the first step in the conversion of light energy to chemical
energy. The presence of a strongly interacting pair of bacte-
riochlorophylls in photosynthetic bacterial reaction centers was
originally inferred from spectroscopic measurements. In the
green plant photosynthetic apparatus, a similar, although ap-
6
+
RC system, e.g., the excited state P* to oxidized P transition,
has led to the synthesis and characterization of a number of
bisporphyrin derivatives. Two different, broad classes of
parently less strongly interacting, pair of chlorophyll a molecules
X
Abstract published in AdVance ACS Abstracts, March 1, 1997.
(1) (a) Gregory, R. P. F. Photosynthesis; Chapman and Hall: New York,
(7) (a) Nanba, O.; Satoh, K. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 109.
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Rutherford, A. W. Biochim. Biophys. Acta 1991, 1058, 379.
(8) An alternate view concerning dimers in PS II has been recently
forwarded: Durrant, J. R.; Klug, D. R.; Kwa, S. L. S.; van Grondelle, R.;
Porter, G.; Dekker, J. P. Proc. Natl. Acad. Sci. U.S.A. 1995, 92, 4798.
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Michel, H. Nature 1985, 318, 618.
(10) Deisenhofer, J.; Epp, O.; Sinning, I.; Michel, H. J. Mol. Biol. 1995,
246, 429.
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Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 5730. (b) Chirona, A. J.; Lous, E.
J.; Huber, M.; Allen, J. P.; Schenck, C.; Paddock, M. L.; Feher, G.; Rees,
D. C. Biochemistry 1994, 33, 4584.
1
989. (b) Photosynthesis; Amesz, J., Ed.; Elsevier: New York, 1987.
(2) Hoffman, B. M.; Ibers, J. A. Acc. Chem. Res. 1983, 16, 15.
(
3) (a) K u¨ hlbrandt, W.; Wang, D. N.; Fujiyoshi, Y. Nature (London)
1
994, 367, 614. (b) McDermott, G.; Prince, S. M.; Freer, A. A.;
Hawthornthwaite-Lawless, A. M.; Papiz, M. Z.; Cogdell, R. J.; Isaacs, N.
W. Nature (London) 1995, 374, 517.
(4) Abbreviations used in this paper include the following: RC,
photosynthetic reaction center; PS, photosystem; P, the bacteriochlorophyll
•
special pair; OEP, octaethylporphyrin; OEP , octaethylporphyrin π-cation
•
/2
radical; OEP , octaethylporphyrin mixed-valence π-cation radical.
(
5) The Photosynthetic Reaction Center; Deisenhofer, J., Norris, J. R.,
Eds.; Academic Press: New York, 1993; Vol. 2.
6) (a) McElroy, J. D.; Feher, G.; Mauzerall, D. C. Biochim. Biophys.
(
Acta 1969, 172, 180. (b) Norris, J. R.; Uphaus, R. A.; Crespi, H. L.; Katz,
J. J. Proc. Natl. Acad. Sci. U.S.A. 1971, 68, 625. (c) Norris, J. R.; Scheer,
H.; Katz, J. J. Ann. N. Y. Acad. Sci. 1975, 244, 260. (d) Feher, G.; Hoff, A.
J.; Isaacson, R. A.; Ackerson, L. C. Ann. N. Y. Acad. Sci. 1975, 244, 239.
(12) El-Kabbani, O.; Chang, C.-H.; Tiede, D.; Norris, J.; Schiffer, M.
Biochemistry 1991, 30, 5361.
(
e) Davis, M. S.; Forman, A.; Hanson, L. K.; Thornber, J. P.; Fajer, J. J.
(13) Scheidt, W. R.; Lee, Y. J. Recent Advances in the Stereochemistry
of Metallotetrapyrroles. Struct. Bonding (Berlin) 1987, 64, 1.
Phys. Chem. 1979, 83, 3325.
S0002-7863(96)01695-2 CCC: $14.00 © 1997 American Chemical Society

2840 J. Am. Chem. Soc., Vol. 119, No. 12, 1997
Brancato-Buentello et al.
•
/2
+
bisporphyrin compounds have been studied as RC models:
region. We also report the formation constants for [M(OEP )]2
14
•
2+
covalently linked porphyrin dimers and larger assemblies and
lanthanide and actinide bisporphyrin or “double-decker” deriva-
tives.15 These two systems can in principle be used to form
oxidized (π-cation radical) species with oxidation states that
and the π-cation radical dimers, [M(OEP )]2 , in methylene
chloride solution. Values of ∆H and ∆S are reported for both
equilibrium processes. We have reported the detailed solid-
•
/2
+
state structures of three crystalline [M(OEP )]2 species
+
17
are formally analogous to P , and oxidized double-deckers, in
elsewhere.
The combination of solid-state structural and
particular, have been studied to develop a further understanding
of the electronic structure of P . The double-decker compounds
solution studies reported herein makes it evident that the mixed-
valence π-cation radicals have many properties that are distinct
from those of either neutral or π-cation radical metalloporphyrin
species. The relative ease in preparing the mixed-valence
π-cation radicals demonstrates the stability resulting from
delocalization of the π-radical electron hole over more than one
porphyrin ring.
+
have been particularly attractive for study as RC models because
of their necessarily cofacial structures and the possibility of
varying the porphyrin interplanar spacing by changing the
central metal ion. However, the completely overlapped, cofacial
geometry of these double-deckers is distinctly different from
the slipped conformation of the special pair observed in the
reaction center structures. The geometric connection constraint-
Experimental Section
(
s) in the covalently linked bisporphyrins also leads to structures
General Information. H
Chemicals, Pd(OEP), thianthrene, perchloric acid, tris(4-bromophenyl)-
aminium hexachloroantimonate, and CCl were purchased from Aldrich,
and all other reagents were obtained from Fisher. Dichloromethane
and hexane were distilled from CaH and sodium/benzophenone,
2
OEP was purchased from Midcentury
different than that displayed by the special pair. Nonetheless,
oxidation of covalently linked bisporphyrins does suggest that
the geometry between the two porphyrin rings is important in
yielding derivatives with physicochemical properties similar to
those of photooxidized RC special pairs.
Recently, we have shown that some metallooctaethylpor-
phyrinate derivatives can be oxidized to form π-cation radical
derivatives in which only one electron is removed per two
porphyrin rings rather than the expected one electron per
porphyrin ring. Thus these new dimeric, supramolecular
assemblies of general formula [M(OEP )]2 have the same
formal oxidation level as the photooxidized special pair of
reaction centers. We suggest “mixed-valence π-cation radical”
as a descriptive name for this new class of synthetic porphyrin
derivatives. The apparent stability of the mixed-valence
π-cation radicals, particularly the absence of significant dis-
proportionation in solution, was surprising. Systems of
4
2
respectively. All reactions were performed under an argon atmosphere
with oven-dried Schlenkware and cannula techniques. Except for Pd-
1
6
(
OEP), metals were inserted into the free-base H
2
OEP by standard
18,19
techniques.
CH Cl solutions for equilibrium measurements were
2 2
prepared using volumetric glassware. Absorption spectra samples were
placed in a Teflon-stoppered quartz mixing cell (total path length 0.87
cm) and recorded on a Perkin-Elmer Lambda 19 UV/vis/near-IR
spectrometer. Thianthrenium perchlorate was prepared by literature
procedures.20 Caution! Perchlorate salts can detonate spontaneously
and should be handled only in milligram quantities; other safety
•
/2
+
2
1
precautions are also warranted.
•
Preparation of [M(OEP )][ClO
4
], M ) Cu, Ni, Zn, or Pd. Cu(OEP)
60 mg, 0.101 mmol) and thianthrenium perchlorate (33 mg, 0.105
(
mmol) were placed in a 100-mL Schlenk flask. Dichloromethane (∼50
mL) was added through a cannula and the solution was stirred for 1 h.
The volume was reduced to ∼10 mL by vacuum evaporation, and
hexane was added to induce precipitation. The precipitate was filtered
under argon and allowed to dry under vacuum for several hours (yield
•/2
+
[M(OEP )]2 that have been studied include derivatives where
M ) Cu, Ni, Zn, Pd, or VO. A limited set of heterodimeric
systems have also been investigated. In this paper, we examine
the solution properties of these mixed-valence π-cation radical
species including their UV/vis/near-IR spectral properties. Their
spectral properties are distinctly different from those of the
related neutral or classical π-cation radical metalloporphyrin
species; the differences are most pronounced in the near-IR
7
6%). UV/vis/near-IR (CH
2 2
Cl ): λmax 380 (Soret), 512, 563, 614, 914
•
-1
nm. IR (KBr): ν(OEP ) 1595 cm (strong). Other π-cation radical
derivatives were prepared in the same fashion as the copper complexes.
•
[Ni(OEP )][ClO
4
]: UV/vis/near-IR (CH
2
-1
Cl
2
) λmax 386 (Soret), 571, 637,
•
905 nm; IR (KBr) ν(OEP ) 1568 cm (strong); yield 73%. [Zn-
OEP )][ClO
955 nm; IR (KBr) ν(OEP ) 1530, 1555 cm (weak, doublet); yield
8%. [Pd(OEP )][ClO
•
(
4
]: UV/vis/near-IR (CH
2
Cl
2
) λmax 386 (Soret), 571, 637,
•
-1
(14) (a) Senge, M. O.; Vicente, M. G. H.; Gerzevske, K. R.; Forsyth, T.
P.; Smith, K. M. Inorg. Chem. 1994, 33, 5625. (b) Pascard, C.; Guilhem,
J.; Chardon-Noblat, S.; Sauvage, J.-P. New J. Chem. 1993, 17, 331. (c) Le
Mest, Y.; L’Her, M.; Hendricks, N. H.; Kim, K.; Collman, J. P. Inorg.
Chem. 1992, 31, 835. (d) Osuka, A.; Nagata, T.; Maruyama, K. Chem. Lett.
•
7
4
]: UV/vis/near-IR (CH
2
Cl
2
) λmax 375 (Soret),
5
11, 574, 930 nm; yield 73%.
Preparation of [M(OEP )][SbCl
•
6
], M ) Cu, Ni, Zn, VO, or Pd.
Cl was reacted
with tris(4-bromophenyl)aminium hexachloroantimonate (86 mg, 0.105
mmol) in a 100-mL Schlenk flask. After the mixture was stirred for
1
991, 481. (e) Osuka, A.; Nakajima, S.; Nagata, T.; Maruyama, K.; Toriumi,
Cu(OEP) (60 mg, 0.101 mmol) in ∼ 50 mL of CH
2
2
K. Angew. Chem., Int. Ed. Engl. 1991, 30, 582. (f) Rodriguez, J.; Kirmaier,
C.; Johnson, M. R.; Friesner, R. A.; Holten, D.; Sessler, J. L. J. Am. Chem.
Soc. 1991, 113, 1652. (g) Sessler, J. L.; Johnson, M. R.; Creager, S. E.;
Fettinger, J. C.; Ibers, J. A. J. Am. Chem. Soc. 1990, 112, 9310. (h) Cowan,
J. A.; Sanders, J. K. M.; Beddard, G. S.; Harrison, R. J. J. Chem. Soc.,
Chem. Commun. 1987, 55. (i) Dubowchik, G. M.; Hamilton, A. D. J. Chem.
Soc., Chem. Commun. 1986, 1391.
1
h, the volume was reduced to ∼15 mL. The solution was then
transferred from the reaction vessel Schlenk flask into a 50-mL Schlenk
flask by a cannula and then layered with hexane for crystallization.
The dark purple crystals that formed after ∼4 days were washed with
(15) (a) Buchler, J. W.; Knoff, M. In Optical Properties and Structure
hexane and allowed to dry (yield 77%). UV/vis/near-IR (CH
2
Cl
): λmax
2
of Tetrapyrroles; Blauer, G., Sund, H., Eds.; de Gruyter: West Berlin, 1985;
pp 91-105. (b) Buchler, J. W.; Els a¨ sser, K.; Kihn-Botulinski, M.; Scharbert,
B. Angew. Chem. 1986, 98, 257. (c) Girolami, G. S.; Milam, S. N.; Suslick,
K. S. J. Am. Chem. Soc. 1988, 110, 2011. (d) Buchler, J. W.; Scharbert, B.
J. Am. Chem. Soc. 1988, 110, 4272. (e) Donohoe, R. J.; Duchowski, J. K.;
Bocian, D. F. J. Am. Chem. Soc. 1988, 110, 6119. (f) Buchler, J. W.; De
Cian, A.; Fischer, J.; Hammerschmitt, P.; L o¨ ffler, J.; Scharbert, B.; Weiss,
R. Chem. Ber. 1989, 122, 2219. (g) Duchowski, J. K.; Bocian, D. F. J. Am.
Chem. Soc. 1990, 112, 3312. (h) Bilsel, O.; Buchler, J. W.; Hammerschmitt,
P.; Rodriguez, J.; Holten, D. Chem. Phys. Lett. 1991, 182, 415. (i) Kim,
H.-J.; Whang, D.; Kim, J.; Kim, K. Inorg. Chem. 1992, 31, 3882. (j)
Girolami, G. S.; Gorlin, P. A.; Suslick, K. S. Inorg. Chem. 1994, 33, 626.
•
-1
3
81 (Soret), 512, 562, 614, 914 nm. IR (KBr): ν(OEP ) 1580 cm
(
strong). The other hexachloroantimonate π-cation radical salts were
•
similarly prepared. [Ni(OEP )][SbCl
375 (Soret), 501, 575, 911 nm; IR (KBr) ν(OEP ) 1571 cm (strong);
yield 75%. [Zn(OEP )][SbCl
6
]: UV/vis/near-IR (CH
2
-1
2
Cl ) λmax
•
•
6
]: UV/vis/near-IR (CH
2
Cl
2
) λmax 391
(17) Scheidt, W. R.; Brancato-Buentello, K. E.; Song, H.; Reddy, K.
V.; Cheng, B. Inorg. Chem. 1996, 35, 7500.
(18) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl.
Chem. 1970, 32, 2443.
(19) Schulz, C. E.; Song, H.; Lee, Y. J.; Mondal, J. U.; Mohanrao, K.;
Reed, C. A.; Walker, F. A.; Scheidt, W. R. J. Am. Chem. Soc. 1994, 116,
7196.
(20) Murata, Y.; Shine, H. J. Org. Chem. 1969, 34, 3368.
(21) Wolsey, W. C. J. Chem. Educ. 1973, 50, A335. Chem. Eng. News
1983, 61 (Dec. 5), 4; 1963, 41 (July 8), 47.
(
k) Girolami, G. S.; Gorlin, P. A.; Milam, S. N.; Suslick, K. S.; Wilson, S.
R. J. Coord. Chem. 1994, 32, 173.
16) Scheidt, W. R.; Cheng, B.; Haller, K. J.; Mislanker, A.; Rae, A.
(
D.; Reddy, K. V.; Song, H.; Orosz, R. D.; Reed, C. A.; Cubiernik, F.;
Marchon, J. C. J. Am. Chem. Soc. 1993, 115, 1181.

Metalloporphyrin Mixed-Valence π-Cation Radicals
J. Am. Chem. Soc., Vol. 119, No. 12, 1997 2841
•
Figure 1. Spectral sum (700-3000 nm) of equimolar solutions of M(OEP) and [M(OEP )]Y held in the two compartments of a mixing cuvette
(
“before reaction” spectra (- - -)). “After reaction” spectra (s) are obtained after the two solutions are mixed; these spectra are virtually identical
•
/2
with that of pure [M(OEP )]
2
Y. The oscillations at wavelengths greater than 1600 nm result from interference of CH
2
Cl
2
6
. (a) M ) Cu, Y ) SbCl ,
-
3
-3
-4
concentration ) 2.0 × 10 M; (b) M ) VO, Y ) SbCl
6
, Mc o.ncentration ) 2.0 × 10 M; (c) M ) Zn, Y ) SbCl
6
, concentration ) 8.0 × 10
-
4
M; (d) M ) Zn, Y ) ClO
4
, concentration ) 6.67 × 10
•
Table 1. Summary of Spectral Maxima for the M(OEP),
(
2
Soret), 534, 559, 596, 651, 960 nm; yield 72%. [VO(OH )(OEP )]-
•
•/2
[M(OEP )]Y, and [M(OEP )]
2
Y Systems Including Concentration
[SbCl
6
]: UV/vis/near-IR (CH
Cl
2 2
) λmax 391 (Soret), 474 (sh), 515, 622,
375 nm; IR (KBr) ν(OEP ) 1533 cm (strong); yield 74%. [Pd-
OEP )][SbCl ]: UV/vis/near-IR (CH Cl ) λmax 375 (Soret), 509, 575,
6 2 2
•
-1
Ranges Studied
1
(
9
•
λmax, nma
•
-1
28 nm; IR (KBr) ν(OEP ) 1582 cm (strong); yield 70%.
concn range
(×10 M)
“before
reaction”
“after
•
/2
Preparation of [Pd(OEP )]
2
[ClO
4
]. Pd(OEP) (70 mg, 0.101
-3
M
Y
reaction”
mmol) and thianthrenium perchlorate (17 mg, 0.055 mmol) were placed
in a 100-mL Schlenk flask. Dichloromethane (∼75 mL) was added
through a cannula and the solution was stirred for 1 h. The volume
was reduced to ∼10 mL by vacuum evaporation, and hexane was added
to induce precipitation. The precipitate was filtered under argon and
Ni
Ni
SbCl
ClO
SbCl
ClO
SbCl
ClO
SbCl
ClO
SbCl6
SbCl6
SbCl
SbCl
6
6
6
6
2.00-0.25
2.00-0.25
2.00-0.25
2.00-0.25
1.20-0.15
1.20-0.15
0.80-0.16
0.80-0.16
1.30-0.16
0.60-0.15
2.00-0.25
2.00-0.25
911
905
914
914
928
930
960
955
1375
960
914
909
1540
1537
1541
1529
1472
1471
974
925
1446
930
4
Cu
Cu
Pd
Pd
Zn
Zn
VO
4
allowed to dry under vacuum for several hours (yield 80%). UV/vis/
4
•
near-IR (CH
1
2
Cl
674 cm (weak).
Preparation of [Pd(OEP )]
mmol) in ∼75 mL of CH Cl was reacted with tris(4-bromophenyl)-
2
) 392 (Soret), 512, 545, 1471 nm; IR (KBr) ν(OEP )
-
1
4
•
/2
2
[SbCl
6
]. Pd(OEP) (70 mg, 0.101
2
2
H ,Zn
2
aminium hexachloroantimonate (45 mg, 0.055 mmol) in a 100-mL
Schlenk flask. After the mixture was stirred for 1 h, the volume was
reduced to ∼15 mL. The solution was then transferred from the
reaction vessel Schlenk flask into a 50-mL Schlenk flask by a cannula
and then layered with hexane for crystallization. The dark purple
crystals that formed after ∼4 days were washed with hexane and
Ni,Cu
Cu,Ni
6
1527
1525
6
a
Relative uncertainties in absorption maxima are estimated to be
5 nm. All spectra were measured in methylene chloride solution.
∼
•
π-cation radical, [M(OEP )]Y, were each placed in one of the
allowed to dry (yield 76%). UV/vis/near-IR (CH
2 2
Cl ) 392 (Soret), 512,
compartments of a Teflon-stoppered quartz mixing cell (total path length
5
46, 1472 nm.
)
0.87 cm). Eight different concentrations of the neutral complex and
Equilibrium Constant Determinations. All equilibrium constant
measurements, except those involving the palladium complexes,22 were
the π-cation radical were used in a typical experiment. Most experi-
ments used the same concentrations for the two species, but some
experiments were also performed in which the concentration of the
two were different by factors of 1.5 or 2. The “before reaction” spectra
were recorded from 700 to 3000 nm. The cell was then inverted, the
two solutions were mixed thoroughly, and an “after reaction” spectrum
was taken over the same spectral range. A new near-IR band is
observed in all after reaction spectra. Table 1 presents the range of
before reaction concentrations used along with wavelength maxima for
conducted by titration of the neutral complex and the π-cation radical
in a quartz mixing cell. CH
2 2
Cl solutions of neutral, M(OEP), and the
(
22) Neutral Pd(OEP) is minimally soluble in CH2Cl2, and therefore,
mixing experiment titrations could not be carried out. Rather, various
concentrations of π-cation radical solutions were prepared directly, equi-
librium measurements were carried out as they were for the other metallo
systems, and the procedure was repeated for the mixed valence π-cation
radical complexes.

2842 J. Am. Chem. Soc., Vol. 119, No. 12, 1997
Brancato-Buentello et al.
the before and after reaction species. Typical before and after reaction
spectra, in the near-IR region for M ) Cu, are shown in Figure 1a.
The after reaction spectra primarily reflect the reaction:
Table 2. Equilibrium Constants (log KMV, 291 K) for the
following Reactiona
KMV
•
•/2
M(OEP) + [M′(OEP )]Y y z [M,M′(OEP ) ]Y
2
KMV
•
+
•/2
+
M(OEP) + [M(OEP )] y z [M(OEP )]2
(1)
M
M′
Y
log KMV
Ni
Ni
Ni
Ni
SbCl
6
6
3.18 ( 0.05
3.19 ( 0.05
3.72 ( 0.03
3.78 ( 0.05
3.58 ( 0.01
3.54 ( 0.04
3.39 ( 0.07
3.42 ( 0.10
4.31 ( 0.09
3.77 ( 0.04
4.00 ( 0.12
3.2 ( 0.3
although due consideration of the additional equilibrium
ClO
SbCl
ClO
4
Cu
Cu
Ni
Cu
Pd
Pd
Zn
Zn
VO
Cu
Cu
Cu
Ni
Pd
Pd
Zn
Zn
VO
Zn
KD
•
+
•
2+
4
2
[M(OEP )] y\ z [M(OEP )]2
(2)
SbCl
SbCl
SbCl
6
6
6
must be made for completeness of the analysis. The value of this
second equilibrium constant can be evaluated independently by an
analysis of the before reaction spectra or direct measurements.
The equilibrium constants for these equilibria were calculated with
the nonlinear least-squares program SQUAD2 on a DEC VAXstation
ClO
4
SbCl
ClO4
SbCl
SbCl
6
3
6
6
4
000-90A. SQUAD calculates best values for the equilibrium
H
2
constants by employing multiple-wavelength absorption data for varying
concentrations of the reactants. A particularly useful feature of SQUAD
is that it can accommodate multiple equilibria. Absorbance data, at
a
2 2
All reactions in CH Cl .
, 291 K) for the Reactiona
D
Table 3. Equilibrium Constants (log K
3
0-50-nm increments spanning the near-IR region, were input into
KD
\
SQUAD. Computation of K
D
, eq 2, used the absorbance data from
•
•
2
[M(OEP )]Y y
z [M(OEP )] Y
2
2
2
4
the before reaction spectra.
monomer and dimer radicals over the entire spectral range (700-3000
nm) were then calculated using this K . The after reaction absorbance
data were then used to calculate KMV, eq 1, while holding K and the
The extinction coefficients for the
M
Y
log K
D
D
Ni
Ni
SbCl
ClO
SbCl
ClO
SbCl
ClO
SbCl
ClO
SbCl6
6
6
6
6
2.00 ( 0.06
1.75 ( 0.09
2.09 ( 0.07
2.02 ( 0.03
2.73 ( 0.06
3.05 ( 0.06
2.68 ( 0.11
2.30 ( 0.18
2.40 ( 0.09
D
4
associated extinction coefficients fixed at the values determined in the
first step.
Cu
Cu
Pd
Pd
Zn
Zn
VO
4
Selected equilibrium constants were determined as a function of
temperature over a 23 or 34 deg temperature range. Temperature
control was provided by a Fisher Scientific Model 900 Isotemp
Refrigerated Circulator bath. The solution temperatures were measured
using an Omega Model 199 series digital thermometer fitted with an
iron-constantan thermocouple immersed in the solutions. Van’t Hoff
plots were then used to estimate the values of the enthalpy and entropy
of formation.
4
4
a
2 2
All reactions in CH Cl .
Table 4. Experimental Enthalpy and Entropy Values in Methylene
Chloride Solution
Results
chemical
speciesa
∆H
(kcal/mol)
∆S
The reaction of equimolar dichloromethane solutions of the
M
(cal/(mol‚K))
ref
•
+
π-cation radical [M(OEP )] and neutral M(OEP) leads to the
appearance of a new, broad, near-IR band in the electronic
spectrum. The near-IR absorption maxima are listed in Table
Ni
dimer
dimer
dimer
dimer
mixed-valence
mixed-valence
mixed-valence
mixed-valence
-11
-28
-20
-38
-9.5
-4.9
-5.6
-3.1
-4.9
this work
this work
Cu
Zn
VO
Ni
Cu
Zn
VO
-8.4
-18
b
30
-5.6
-5.7
-6.8
-6.8
-6.8
this work
this work
this work
this work
this work
1
(“after reaction”) for a number of different systems with
varying metal and anion. We associate the new near-IR band
with the formation of a novel species, [M(OEP )]2 , which is
in equilibrium with the starting species:
•
/2
+
a
•
2+; mixed-valence is the
KMV
Dimer is the formation of [M(OEP )]
2
•
+
•/2
+
•
/2
+ b
2
M(OEP) + [M(OEP )] y z [M(OEP )]2
(1)
formation of [M(OEP )]
.
3
Measured in MeOH:CHCl (10:1).
The constant for this equilibrium process can be determined
by an analysis of “mixing experiment” spectral titrations; these
calculated values are listed in Table 2. Another near-IR band,
which is associated with the π-cation radical (Table 1, “before
reaction”), diminishes in intensity upon reaction. This near-IR
band is associated with a dimerization of the π-cation radical:
between sets of measurements showed larger spreads. Accord-
ingly, we have calculated weighted means and weighted
variances rather than simple averages and these weighted
25
values are reported in Tables 2 and 3.
We have also determined KMV and KD values over a range
of temperature. The temperature of the experiments was
restricted to the range -13 to 20 °C, because of the need to
use dichloromethane as solvent and the limited solubility at
lower temperatures. Nonetheless, the van’t Hoff plots were
linear (linear correlation coefficients > 0.99) with reasonably
large slopes. Table 4 gives enthalpy and entropy parameters
derived from the van’t Hoff plots for both equilibrium processes.
KD
•
+
•
2+
2
[M(OEP )] y
\
z [M(OEP )]2
(2)
Again, an equilibrium constant can be determined from an
analysis of the spectral data and the constants are listed in Table
3. The quoted equilibrium constants are based on at least
duplicate determinations. Although the agreement in the value
of K’s between runs was quite good, the values of the variances
We have also prepared and isolated a number of the mixed-
valence π-cation radical species as crystalline solids. The
spectra obtained by dissolving these isolated complexes are
identical to those obtained above by mixing experiments. We
(23) Leggett, D. J. In Computational Methods for the Determination of
Formation Constants; Leggett, D. J., Ed.; Plenum: New York, 1985; Chapter
6
.
(
24) The spectral range used initially included the entire main portion
of the peak (a range of ∼450 nm). There is no contribution to the absorbance
(25) Hamilton, W. C. Statistics in Physical Sciences; Ronald: New York,
1964; pp 37-42.
from M(OEP) over the near-IR spectral region.

Metalloporphyrin Mixed-Valence π-Cation Radicals
J. Am. Chem. Soc., Vol. 119, No. 12, 1997 2843
suggest the term “mixed-valence π-cation radical” for these new
compounds.
conclude that a mixed metalloporphyrin system indeed results
and that its preparation is independent of reaction path.28 The
fact that a new product is formed in these Ni/Cu mixed systems
suggests that the unpaired electron must be delocalized over
both metalloporphyrin rings. However, the unpaired electron
may not be delocalized equally over the two rings in mixed
systems; any asymmetry in electron delocalization would be
related to differences in the ring oxidation potentials. It is
interesting to note that the two KMV values for the mixed Ni/
Cu system (Table 2) are quite close, but perhaps not exactly
equal, to the arithmetic mean of the homo nickel and copper
systems; the deviation from the exact mean may indicate
asymmetry in the electron delocalization. This issue is under
active consideration.
Discussion
Solution Equilibria. The initial preparations of species with
•
/2
+
the chemical formula [M(OEP )]2 were accomplished by the
oxidation of metalloporphyrin derivatives with 0.5 equiv of an
appropriate one-electron oxidant and were isolated as crystalline
solids. Structural analyses1 of crystalline species show that
the compounds exist as discrete binuclear units in the solid.
The stability of such species in solution might have been
expected to be rather minimal with decomposition readily
occurring by disproportionation:
6,17
Kdisp
•
/2
+
2
•
+
[
M(OEP )]
y
z M(OEP) + [M(OEP )]
We have also determined the formation constant for the mixed
•
/2
Zn(OEP), free base species which we write as [Zn(OEP )-
•
/2 +
However, the reverse reaction, the comproportionation reaction,
written previously as eq 1, actually dominates the solution
chemistry of a mixture of the neutral and π-cation radical
metallooctaethylporphyrins; the equilibrium position of such
H2(OEP )] ; the log(KMV) value is a respectable 3.2. In this
case the mixing experiments could be successfully performed
•
+
only by reaction of [Zn(OEP )] with H2OEP and not the
reverse. It is not immediately clear to us why this is so, since
it is easier to oxidize Zn(OEP) than H2OEP; the difference in
the first ring oxidation potential is at least 180 mV.29 Thus the
most likely explanation for the reactivity difference would
appear to be mechanistic, rather than thermodynamic, in origin.
•/2
+
reactions largely favors the mixed-valence species [M(OEP )]2 .
We have denoted the comproportionation constants as KMV
throughout this paper.
Values of KMV have been determined for a number of
different systems in which the metal ion and/or the counterion
were varied. The magnitudes of KMV are moderately large and
range from ∼1600 to ∼20 000. A complete listing of deter-
mined KMV values is given in Table 2. There are a number of
features to be noted. For the Ni, Cu, and Pd systems, where
the metal ions are expected to remain four coordinate, the
formation constants are independent of the choice of the
counterion, hexachloroantimonate or perchlorate ion. For the
zinc systems, where the zinc ion has an affinity for a possible
axial ligand, there is a dependence on the counterion. Thus
the differences in KMV for the two zinc systems can be attributed
to whether or not the counterion ligates the central zinc atom.
We have attempted to keep all systems anhydrous; however,
we should note that in the zinc and vanadyl systems, the
presence of water could have an effect on the equilibrium
constants. Both metals are known to form aquated π-cation
•
+
We find that in methylene chloride solution all [M(OEP )]
π-cation radicals exhibit a near-IR absorption band whose
intensity is strongly concentration dependent. The behavior of
this band is that expected for the dimeric species of a monomer-
dimer equilibrium:
KD
•
+
•
2+
2
[M(OEP )] y\ z [M(OEP )]
2
The concentration dependence of the near-IR spectra of
•
+
[
M(OEP )] species can be readily used to obtain values for
the dimerization formation constants KD, which are listed in
Table 3. Although the thermodynamics of the dimerization of
•
30
[
Zn(OEP )]Br was studied two decades ago, to our knowledge
no other quantitative measurements have been made for any
porphyrin radical derivatives. However, temperature-dependent
19
electronic spectral and EPR studies have shown that vanadyl,
1
9,26
radicals
and there are apparent differences in axial ligand
31
32
magnesium, and copper derivatives undergo strongly coupled
dimerization reactions in solution. In our experience this
dimerization is the norm. The data in Table 3 show that
palladium species have even larger dimerization constants than
the zinc derivatives. This may be related to the strong tendency
for even neutral Pd(OEP) to aggregate and to have limited
solubility.
affinity between the neutral and π-cation porphyrins.
Finally it is to be noted that the mixed-valence π-cation
radical formation constants show variation with the central metal
ion. The solid-state structures,1
6,17
and the likely solution
structures, have as their most important inter-ring attractive
interaction a π-π interaction; the metal‚‚‚metal distances are
larger than that expected for a significant metal-metal bond
interaction. The strengths of these π-π links are surely related,
inter alia, to the energies of the HOMO and LUMO orbitals,
which are known27 to be metal dependent. Thus a modest
dependence on metal ion identity in the various derivatives is
reasonably expected.
In all cases the mixed-valence π-cation formation constants
KMV) are significantly larger (by a factor of 10 to 100) than
(
the analogous dimerization constants KD. Such differences are,
of course, consistent with the observation of the importance of
the comproportionation pathway over disproportionation in
reactions of the π-cation radicals. The observation further
suggests significant stabilization from the delocalization of a
Values of KMV for a limited number of mixed-valence hetero
systems (in which the two metals are different) have also been
determined. For the Ni/Cu systems, the two formation constants
are equalsindependent of whether the reacting π-cation radical
(28) The absorbance values from the mixed metal reactions are clearly
incompatible with the presence of only the homonuclear dimers in any
proportions.
(29) Measurement in butyronitrile: Fuhrhop, J.-H.; Kadish, K. M.; Davis,
D. G. J. Am. Chem. Soc. 1973, 95, 5140. A measurement in methylene
chloride gave a difference slightly larger: K. Brancato-Buentello, unpub-
lished observations.
•
+
• +
is [Ni(OEP )] or [Cu(OEP )] . The resulting spectroscopic
properties of the two Ni/Cu mixed systems are also indistin-
guishable from each other. Moreover, the final spectroscopic
properties of the mixed Ni/Cu systems are distinct from either
the purely nickel or purely copper system. We can thus
(30) Fuhrhop, J. H.; Wasser, P.; Riesner, D.; Mauzerall, D. J. Am. Chem.
Soc. 1972, 94, 7996.
(31) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J.
(
26) Song, H.; Orosz, R. D.; Reed, C. A.; Scheidt, W. R. Inorg. Chem.
990, 29, 4274.
27) Zerner, M.; Gouterman, M.; Koboyashi, H. Theor. Chim. Acta 1966,
, 363.
Am. Chem. Soc. 1970, 92, 3451.
1
(32) (a) Mengersen, C.; Subramanian, J.; Fuhrhop, J.-H. Mol. Phys. 1976,
32, 893. (b) Browett, W. R.; Stillman, M. J. Inorg. Chim. Acta 1981, 49,
69.
(
6

2844 J. Am. Chem. Soc., Vol. 119, No. 12, 1997
Brancato-Buentello et al.
single radical electron over two porphyrin rings in the mixed-
valence π-cations.
We have further explored the differences in these two
reactions by determining the values of ∆H and ∆S, which were
evaluated by the van’t Hoff method. The necessary use of
methylene chloride as solvent limits the temperature range
available for the study, but the data appear adequate to define
trends. Values of ∆H and ∆S are listed in Table 4. Two gen-
eralizations are apparent. The reaction enthalpies are favorable
for both reactions. The formation of π-cation radical dimers is
the more enthalpically favored process as might be expected;
however, the reaction enthalpies differ by less than a factor of
two. The reaction entropies are all negative as might be
expected for dimerization reactions. The formation of π-cation
radical dimers leads to reaction entropies that are much more
negative than those for the formation of the mixed-valence
π-cation radicals. Indeed the reaction entropies differ by factors
near 5. This must result from a much larger solvent reorganiza-
tion that occurs in the π-cation radical dimerization reaction.
Importantly, the overall greater thermodynamic stability of the
mixed-valence π-cation radicals can be directly attributed to
the reaction entropy differences.
Figure 2. Simplified bonding scheme for formation of binuclear (a)
π-cation and (b) mixed-valence π-cation radical species.
orbital of a monomeric metallooctaethylporphyrin π-cation
radical is, in D4h symmetry, an a1u orbital. Two such orbitals,
one from each monomer, can combine to form a new “bonding”
and a new “antibonding” molecular orbital that are delocalized
over both porphyrin rings in the resultant dimer. Such a simple
bonding picture is consistent, for example, with the known solid-
Electronic Spectra. Two species of prime interest in this
study, the mixed-valence π-cation radical octaethylporphyrin
•
/2
+
species, [M(OEP )]2 , and the π-cation radical octaethylpor-
•
2+
26
state diamagnetism of the [Zn(OEP )]2 dimer. In this simple
model, the assignment of the near-IR dimerization band is that
of excitation from the bonding to the antibonding molecular
orbital. Knowledge of the excitation energy then allows an
estimate of the enthalpy of formation: it must be nominally
less than half the excitation energy. This expectation is
qualitatively met, with the spectroscopic transition energies for
the nickel, copper, and zinc systems at 30-31 kcal/mol and
with the measured enthalpies for the nickel and copper systems
approximately one-third as large. A similar pattern is seen in
the vanadyl radical system, where the red-shifted transition has
an energy of 21 kcal/mol and the measured dimerization
enthalpy is 6 kcal/mol. The zinc system, where the enthalpy
was measured in a different (more polar) solvent system, has a
value a bit greater than half that of the measured excitation
energy.
•
2+
phyrin dimers, [M(OEP )]2 , exhibit broad bands in the near-
IR portion of the electromagnetic spectrum. Positions of the
near-IR maxima have been given in Table 1: the “before
reaction” column corresponds to the spectrum of π-cation radical
dimers and the “after reaction” column to the spectrum of
mixed-valence π-cation radical complexes. Owing to the
broadness of the near-IR bands the reported maxima are
probably accurate to no better than ∼5 nm. The other species
involved in the equilibria described above, viz., neutral metal-
looctaethylporphyrins and monomeric π-cation radicals, do not
absorb significantly in the near-IR region.
Representative spectra (four systems) for the near-IR region
are shown in Figure 1. The bands represented by the dashed
lines (the “before reaction” spectra) are from the absorption of
•
2+
the dimeric π-cation radical species, [M(OEP )]2 . This band
occurs in the near-IR region at 900-960 nm except for the
vanadyl derivative, which has a significantly red-shifted band.
In all cases the apparent extinction coefficient of this band is
strongly concentration dependent. As noted earlier, the con-
centration dependence of the band can be quantitatively
explained by a dimerization reaction in which the dimer absorbs
much more strongly than the monomer. The position of the
near-IR bands is similar to the band assigned by Fuhrhop et
The electronic spectra of the new, related mixed-valence
•
/2
+
species, [M(OEP )]2 (represented by the solid lines in Figure
), also have distinct near-IR bands. The near-IR band is
1
frequently shifted to the red relative to the band observed for
the related dimeric π-cation radical species. A significant
spectral red shift is displayed in the copper, nickel, palladium,
and mixed copper/nickel systems. This is illustrated in Figure
1
a for the copper system. Also apparent in the displayed
3
0
31
al. and Fajer et al. as resulting from dimerization in the [Zn-
(
spectrum is the strong tailing of the near-IR band farther into
the infrared. This tailing is especially prominent in the nickel
and copper cases, where the tailing continues into the infrared.
The tailing even leads to anomalously high backgrounds in the
•
+
• +
OEP )] and [Mg(OEP )] radical systems, respectively. It is
important to note that the solution dimerization of the metal-
looctaethylporphyrin π-cation radicals and the concomitant
appearance of a near-IR band appears to be much more prevalent
than has been previously recognized. Interestingly however,
•
/2
+
IR spectra of [M(OEP )]2 , M ) Cu or Ni (KBr pellets, high
-
1
backgrounds down to ∼1600 cm ). The position of the near-
IR band in the mixed-valence zinc systems, however, shows
relatively small shifts compared to the band position observed
for the radical cation dimers. Moreover, as can be seen in
Figures 1c and 1d, there is a strong anion dependence on the
intensity of the bands in the different species, which may reflect
differences in axial ligation. The near-IR maxima of the
π-cation and mixed-valence π-cation radical vanadyl systems
also show only small differences, but both are quite red-shifted
compared to the zinc systems (cf. Figure 1b). As can be seen
from the displayed spectra in Figures 1b-d, these latter
compounds do not have strongly tailed near-IR bands. The most
•
+
the π-cation radical of the parent free base [H2 (OEP )] shows
neither a near-IR band nor the expected concentration-dependent
blue-shifted Soret band. Both of these spectral features ac-
company π-cation radical dimer formation. Thus, there is no
spectral evidence for the formation of free base radical dimers.
The simplest electronic structure model that can be used to
understand dimerization of π-cation radicals can be schemati-
cally depicted as shown in Figure 2.3 The half-filled molecular
3
(33) More detailed MO bonding models for lanthanide double-deckers
that encompass these combinations of the a1u orbitals have been given
previously. See refs 15e,g,h.

Metalloporphyrin Mixed-Valence π-Cation Radicals
J. Am. Chem. Soc., Vol. 119, No. 12, 1997 2845
•
Figure 3. Spectral sum (350-700 nm) of equimolar solutions of M(OEP) and [M(OEP )]Y held in the two compartments of a mixing cuvette
(
“before reaction” spectra (- - -)). “After reaction” spectra (s) are obtained after the two solutions are mixed; these spectra are identical with that
•
/2
-6
-5
obtained from pure [M(OEP )]
2
Y. (a) M ) Cu, Y ) SbCl
6
, concentration ) 9.37 × 10 M (Soret), 7.5 × 10 M (vis); (b) M ) VO, Y ) SbCl
6
,
-
6
-5
-5
-4
concentration ) 9.37 × 10 M (Soret), 7.5 × 10 M (vis); (c) M ) Zn, Y ) SbCl
6
, concentration ) 6.25 × 10 M (Soret), 2.5 × 10 M (vis);
-
6
-5
(
d) M ) Zn, Y ) ClO
4
, concentration ) 9.37 × 10 M (Soret), 7.5 × 10 M (vis).
reasonable spectral assignment of these near-IR bands is one
related to that given earlier for the π-cation dimers (Figure 2).
The mixed-valence π-cation radicals have one additional
electron which must be placed in the “antibonding” combination;
the near-IR transition is thus the one between the second-highest
occupied molecular orbital (SHOMO) and the singly occupied
HOMO.
The near-IR bands of the mixed-valence π-cation radicals
resemble the new near-IR band that appears when bacterial
photosynthetic reaction centers, e.g., those from Rhodopseudomo-
IR bands in analogous lanthanide double-deckers are only
coincidently similar to that of P . The mixed-valence π-cation
+
radical near-IR band is unlikely to be an optical hole transition
between the two porphyrin rings, i.e., the classic intervalence
transfer band. A lower energy band has been recently found
+
40
by difference spectroscopy for P in the mid-IR; this appears
to be the intervalence transfer band. While the current study
does not attempt to present an unequivocal assignment of the
near-IR bands, it clearly shows that geometrically unconstrained
dimers can display near-IR bands. The mixed-valence π-cation
radical systems provide a new basis for the exploration of effects
of inter-ring geometry and metal variation on the spectroscopic
properties.
6
a,34-37
nas Viridis or Rhodobacter sphaeroides, are photooxidized
+
and form the oxidized special pair, P . A common assignment
of the near-IR transitions would seem probable. The assignment
+
38
of this near-IR band for P remains a matter of current interest.
The spectral features in the UV-vis region of all the π-cation
radicals investigated here are similar to those reported by
Stocker et al.3 present strong evidence for the band arising from
an electronic transition between states completely delocalized
over the two rings on the vibrational and electronic time scales.
9
3
0
•
2+
Fuhrhop et al. for the [Zn(OEP )]2 dimer. In the dimer
spectra, the Soret band is weaker, blue-shifted, and broadened
compared to the neutral precursors while the R and â bands of
the π-cation radicals are red-shifted as well as substantially
broadened. There is, of course, a concentration dependence in
these spectra consistent with the dimerization process.
+
The delocalized states of P would be consistent with a SHOMO
3
8
f HOMO transition. However, Reimers and Hush favor
another assignment based on calculations of expected intensities.
They further note that their calculations suggest that the near-
The UV-vis spectra of the mixed-valence π-cation radicals,
on the other hand, display Soret absorptions and R,â bands that
show only very small changes in band position compared to
the equivalent neutral porphyrin. These mixed-valence bands
are broadened relative to the bands displayed by the neutral
(40) (a) Breton, J.; Nabedryk, E.; Parson, W. W. Biochemistry 1992,
31, 7503. (b) Parson, W. W.; Nabedryk, E.; Breton, J. In The Photosynthetic
Bacterial Reaction Center II: Structure, Spectroscopy, and Dynamics;
Breton, J., Vermeglio, A., Eds.; Plenum Press: New York, 1992; p 79.
(
34) Reed, D. J. Biol. Chem. 1969, 244, 4936.
(35) Parson, W. W.; Cogdell, R. J. Biochim. Biophys. Acta 1975, 416,
1
05.
36) (a) Fajer, J.; Brune, D. C.; Davis, M. S.; Forman, A.; Spaulding, L.
D. Proc. Natl. Acad. Sci. U.S.A. 1975, 72, 4956.
37) Dutton, P. L.; Kaufmann, K. J.; Chance, B.; Rentzepis, P. M. FEBS
Lett. 1975, 60, 275.
(
(
(
(
38) Reimers, J. R.; Hush, N. S. J. Am. Chem. Soc. 1995, 117, 1302.
39) Stocker, J. W.; Hug, S.; Boxer, G. S. Biochim. Biophys. Acta 1993,
1
144, 325.

2846 J. Am. Chem. Soc., Vol. 119, No. 12, 1997
Brancato-Buentello et al.
porphyrin. In addition, the mixed-valence π-cation radical
spectra show shoulders and additional weaker bands that are
similar to those shown by the π-cation radical derivatives. Thus,
the observed spectrum of any of the mixed-valence π-cation
radicals is close to the sum of the spectra of the two reaction
components. The near superposition of the spectral sum of the
two reacting components (neutral and π-cation radical) and the
mixed-valence π-cation radical products is shown in Figures
region. All of these singly oxidized double-decker derivatives
do display a new near-IR band. It is tempting to conclude that
the spectral differences in the two model complex systems
originate in the structural differences and, in particular, the
distinctly different inter-ring orientations known to exist in the
solid state. A more complete understanding of the differing
spectral changes in the two model systems may come from
characterization of their excited state photophysical properties
that are currently in progress.
3
a-d.
It is interesting to compare the spectral changes seen in these
Summary. This study clearly demonstrates the solution
stability of partiallysbut preciselysoxidized metalloporphyrin
species that are formally related to the photooxidized special
pair of bacterial photosynthesis. The solution reactions of
equimolar quantities of neutral and one-electron oxidized
metalloporphyrins to yield the novel, mixed-valence, nonco-
valently-linked, supramolecular products demonstrate the stabil-
ity that arises from delocalization of a one-electron hole over
two porphyrin rings. These mixed-valence π-cation radical
complexes display new electronic absorption bands in the near-
IR which are not observed in either the neutral M(OEP) or the
new mixed-valence π-cation radicals with related systems. The
spectral differences between neutral M(OEP) and the corre-
sponding mixed-valence π-cation radicals are similar to the
spectral differences observed between the reduced and photo-
oxidized states of reaction centers.6
e,34,41
This is in distinct
contrast to the spectral changes observed between the neutral
and singly-oxidized lanthanide, actinide, and early transition
1
5,42
metal double-decker reaction center model complexes.
In
the double-decker MP2 RC model systems, the spectral changes
+
upon oxidation to MP2 are significant broadening and blue
shifts in the Soret region and only weak absorptions in the visible
•
+
[
M(OEP )] π-cation radical derivatives.
(41) (a) Vermeglio, A.; Paillotin, G. Biochim. Biophys. Acta 1982, 681,
3
4
1
2. (b) Clayton, R. K.; Clayton, B. J. Biochim. Biophys. Acta 1978, 501,
78. (c) Parson, W. W.; Cogdell, R. J. Biochim. Biophys. Acta 1975, 416,
Acknowledgment. We thank the National Institutes of
05.
Health for support of this research under Grant No. GM-38401.
We thank Dr. Robert Warren for helpful discussions on
determination of preliminary formation constants.
(42) (a) Buchler, J. W.; De Cian, A.; Elschner, S.; Fischer, J.; Hammer-
schmitt, P.; Weiss, R. Chem. Ber. 1992, 125, 107. (b) Buchler, J. W.;
H u¨ ttermann, J.; L o¨ ffler, J. Bull. Chem. Soc. Jpn. 1988, 61, 71. (c) Buchler,
J. W.; De Cian, A.; Fischer, J.; Kihn-Botulinski, M.; Weiss, R. Inorg. Chem.
1
988, 27, 339.
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