Formation of porphyrins in the presence of acid-labile metalloporphyrins: A new route to mixed-metal multiporphyrin arrays

Article abstract of DOI:10.1021/ic026206d

The ability to incorporate distinct metalloporphyrins at designated sites in multiporphyrin arrays is essential for diverse applications in materials and biomimetic chemistry. The synthesis of such mixed-metal arrays via acid catalyzed reactions has largely been restricted to metalloporphyrins of stability class II (e.g., Cu, Co, Ni) or I. We describe routes for the rational synthesis of mixed-metal arrays via acid-catalyzed condensations that are compatible with metalloporphyrins of stability class III (e.g., Zn) and IV (e.g., Mg). The routes are demonstrated for p-phenylene-linked arrays. The key finding is that several mild Lewis acids [InCI₃, Sc(OTf)₃, Yb(OTf)₃, and Dy(OTf)₃], which are known to catalyze the dipyrromethane + dipyrromethane-dicarbinol condensation in CH₂Cl₂ at room temperature without acidolysis, do not demetalate zinc or magnesium porphyrins under the same conditions. Rational routes to porphyrin dyads and triads employ reaction of a (porphyrin)-dipyrromethane and a (porphyrin)-dipyrromethane-dicarbinol. The porphyrin-forming reactions (six examples) proceed in yields of 18-28%. The metalation states of the arrays prepared in this manner include Zn-free base (ZnFb), MgFb, ZnFbMg, ZnFbZn, and ZnFbFb. Studies of the catalysis process indicate that the dipyrromethane + dipyrromethane-dicarbinol condensation is catalyzed by both the Lewis acid and a Bronsted acid derived in situ from the Lewis acid. Taken together, the ability to employ otherwise acid-labile metalloporphyrins as precursors in condensation procedures should broaden the scope of accessible mixed-metal multiporphyrin arrays and motivate further studies of the application of mild Lewis acid catalysts in porphyrin chemistry.

Full text of DOI:10.1021/ic026206d

Inorg. Chem. 2003, 42, 4322−4337
Formation of Porphyrins in the Presence of Acid-Labile
Metalloporphyrins: A New Route to Mixed-Metal Multiporphyrin Arrays
Markus Speckbacher, Lianhe Yu, and Jonathan S. Lindsey*
Department of Chemistry, North Carolina State UniVersity, Raleigh, North Carolina 27695-8204
Received November 22, 2002
The ability to incorporate distinct metalloporphyrins at designated sites in multiporphyrin arrays is essential for
diverse applications in materials and biomimetic chemistry. The synthesis of such mixed-metal arrays via acid
catalyzed reactions has largely been restricted to metalloporphyrins of stability class II (e.g., Cu, Co, Ni) or I. We
describe routes for the rational synthesis of mixed-metal arrays via acid-catalyzed condensations that are compatible
with metalloporphyrins of stability class III (e.g., Zn) and IV (e.g., Mg). The routes are demonstrated for p-phenylene-
linked arrays. The key finding is that several mild Lewis acids [InCl3, Sc(OTf)3, Yb(OTf)3, and Dy(OTf)3], which are
known to catalyze the dipyrromethane + dipyrromethane−dicarbinol condensation in CH Cl2 at room temperature
2
without acidolysis, do not demetalate zinc or magnesium porphyrins under the same conditions. Rational routes to
porphyrin dyads and triads employ reaction of a (porphyrin)−dipyrromethane and a (porphyrin)−dipyrromethane−
dicarbinol. The porphyrin-forming reactions (six examples) proceed in yields of 18−28%. The metalation states of
the arrays prepared in this manner include Zn-free base (ZnFb), MgFb, ZnFbMg, ZnFbZn, and ZnFbFb. Studies of
the catalysis process indicate that the dipyrromethane + dipyrromethane−dicarbinol condensation is catalyzed by
both the Lewis acid and a Brønsted acid derived in situ from the Lewis acid. Taken together, the ability to employ
otherwise “acid-labile” metalloporphyrins as precursors in condensation procedures should broaden the scope of
accessible mixed-metal multiporphyrin arrays and motivate further studies of the application of mild Lewis acid
catalysts in porphyrin chemistry.
Introduction
enzymes.6 Attaining the desired function requires the ability
to incorporate porphyrins with particular attributes (redox
potentials, orbital composition, excited-state dynamics, con-
formational properties, metal coordination, etc.) at designated
sites in the array. The attributes of the porphyrin are largely
determined by the nature and pattern of substituents at the
perimeter of the macrocycle and the nature and oxidation
state of the central metal.
The organization of multiple porphyrins into arrays has
provided the basis for a variety of functional molecular
architectures including light-harvesting systems,1 reaction
centers,2 optoelectronic gates,3 multistate information-storage
devices,4 host-guest assemblies,5 and multielectron-redox
* To whom correspondence should be addressed. E-mail: jlindsey@
ncsu.edu.
The type of metalloporphyrins in an array has a profound
impact on the synthetic strategy that can be employed. A
major factor is that metalloporphyrins can undergo demeta-
lation upon exposure to acid. The stability of metallopor-
phyrins toward acids of various strengths is described by a
characteristic “stability class” as shown in Table 1.7 For
example, a metalloporphyrin of stability class III (e.g., Zn)
would undergo demetalation in the presence of aqueous HCl
in CH2Cl2 and also under conditions that define class II or
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4322 Inorganic Chemistry, Vol. 42, No. 14, 2003
10.1021/ic026206d CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/11/2003

Mixed-Metal Multiporphyrin Arrays
Table 1. Stability Classes of Metalloporphyrins7
recombination of undesired fragments (i.e., “scrambling”)
leads to a mixture of porphyrins, which must be expressly
avoided in any rational synthesis.13 The propensity of
dipyrromethanes toward scrambling depends on the substit-
uents on the dipyrromethanes. Under the conditions for
condensations with aldehydes, little or no scrambling is
observed with dipyrromethanes that lack any meso substit-
uents (regardless of â-substituents) or with â-unsubstituted
dipyrromethanes that bear a hindered meso substituent
(e.g., 5-mesityldipyrromethane), while extensive scrambling
typically occurs with â-unsubstituted dipyrromethanes that
bear an unhindered meso substituent (e.g., 5-phenyldipyr-
romethane).16 The choice of dipyrromethane substituents is
not necessarily innocent, as the substitution pattern plays a
large role in determining the ordering of the molecular
orbitals and thus the nature of the HOMO (a2u or a1u) of the
resulting porphyrin.17
The strategies that employ acid-catalyzed condensations
with a porphyrin precursor for preparing multiporphyrin
arrays are diverse and include the following rational ap-
proaches: (1) Condensation of a porphyrincarboxaldehyde
with pyrrole yields a star-shaped array.18-21 (2) Condensation
of a porphyrincarboxaldehyde with a dipyrromethane gives
the corresponding triad22-31 or of a multiporphyrin array
bearing one carboxaldehyde with a dipyrromethane yields
the larger array.24,29,30 In the few cases that employed
â-unsubstituted/meso-substituted dipyrromethanes,25,28,31,32
scrambling was generally reported when the meso substituent
was unhindered25,28 but not when hindered.31 (3) Condensa-
tion of a nickel porphyrin-dipyrromethane and a 1,9-
diformyldipyrromethane gives the corresponding NiFb por-
phyrin dyad.33 Of the few porphyrin-dipyrromethanes that
have been prepared, all have incorporated â-substituents.33-35
Statistical variants of most such strategies, wherein multiple
metallo-
stability class
reagenta
effect
porphyrinb
I
II
H2SO4 (neat)
H2SO4 (neat)
partial demetalation Al,c Pd, V
demetalation
Co, Cu, Ni
Fe, In, Zn
Mg, Mn, Pb
Ba, Ca, Sr
III
IV
V
HCl/H2O-CH2Cl2 demetalation
AcOH (neat)
demetalation
demetalation
H2O-CH2Cl2
a Treatment at 25 °C for 2 h. b Representative metal chelates. Each metal
is divalent unless noted otherwise. c Al(III).
I metalloporphyrins (neat H2SO4) but is stable toward the
conditions of stability class IV or V (AcOH or H2O).
Two distinct approaches have been developed for the
stepwise synthesis of multiporphyrin arrays joined by
covalent linkers: (1) coupling of intact metalloporphyrin
building blocks via Pd-mediated, Sonogashira or Glaser
reactions; (2) acid-catalyzed condensation forming one or
more new porphyrins in the process. The two approaches
are quite complementary. The former approach is compatible
with a wide variety of metalloporphyrins (stability classes
I-IV) as well as free base (Fb) porphyrins owing to the mild,
basic conditions of the Pd coupling reactions for use with
porphyrins.8,9 However, the porphyrin-porphyrin linker is
constructed in the joining process and thus is limited in scope
by the nature of the Pd-mediated coupling reaction: a
directed coupling of iodo and ethyne groups yielding an
ethyne linker (Sonogashira reaction) or an undirected dimer-
ization of ethynes yielding a butadiyne linker (Glaser
reaction). The acid-catalyzed condensation approach has
much greater latitude toward linkers because the new
porphyrin is formed around a preexisting linker. The reac-
tions have included “aldehyde + pyrrole” and “aldehyde +
dipyrromethane” condensations. However, the acid-catalysis
conditions in such reactions typically have entailed propionic
acid at reflux,10 p-toluenesulfonic acid in methanol at room
temperature,11 trifluoroacetic acid (TFA) or BF3‚O(Et)2 in
CH2Cl2 at room temperature,12,13 or trichloroacetic acid in
acetonitrile at room temperature.14 Such acid-catalysis condi-
tions restrict application of this approach to free base
porphyrins or to metalloporphyrins that are quite acid-
resistant (i.e., stability class I or II).
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Lett. 1999, 1193-1194.
In addition to the limitation on choice of metal, the use of
acid catalysis also imposes limits on the type of synthetic
components that can be employed. Dipyrromethanes are
common precursors to porphyrins15 yet are susceptible to
acidolysis. Acidolysis of dipyrromethanes followed by
(21) Shinmori, H.; Kajiwara, T.; Osuka, A. Tetrahedron Lett. 2001, 42,
3617-3620.
(22) Abdalmuhdi, I.; Chang, C. K. J. Org. Chem. 1985, 50, 411-413.
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2872.
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115, 4618-4628.
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69, 307-323.
Inorganic Chemistry, Vol. 42, No. 14, 2003 4323

Speckbacher et al.
aldehyde or dipyrromethanes species were employed leading
to a mixture of products, have also been devised.18,20,32,36-46
Regardless of approach, the only metalloporphyrins em-
ployed in the above routes were Co,43 Cu,21,43 or Ni19,20,27,30,33,43
chelates. On the other hand, strategies that employ acid-
catalyzed condensations of non-porphyrin precursors for
preparing mixed-metal arrays generally suffer from reliance
on at least one statistical reaction.32,33,47-50 In summary, acid-
catalyzed condensation methods have enabled the rational
synthesis of M1Fb porphyrin dyads and M1FbM1 porphyrin
trimers, where M1 is from stability class II. No routes have
heretofore been developed that provide access to dyads and
triads of composition M1Fb and M1FbM3, where M1 and M3
include metals from stability class III or IV. The ability to
employ a much broader expanse of metalloporphyrins in the
acid-catalyzed condensations, particularly metals giving
photochemically active porphyrins (e.g., Zn or Mg), would
be exceptionally attractive for preparing functional multi-
porphyrin architectures.
synthesis of mixed-metal dyads owing to the directed
coupling of a boronate group and a halogen. However, the
reaction conditions require the presence of a strong base at
elevated temperature for prolonged times, and stepwise
syntheses of phenylene-linked multiporphyrin arrays larger
than dyads have not been demonstrated using the Suzuki
reaction.31 Given the limitations of the Suzuki reaction and
the acid-catalyzed condensations, new methods are needed
that enable rational syntheses of p-phenylene-linked multi-
porphyrin arrays containing diverse free base and metal-
loporphyrins (stability classes I-IV) at designated sites.
We recently developed a strategy for the rational synthesis
of porphyrins bearing up to four different meso substituents
via the reaction of a dipyrromethane and a dipyrromethane-
dicarbinol.51 For nonscrambling acid catalysis conditions, we
initially employed TFA in CH3CN at room temperature.
More recently, we found that a number of acids that were
relatively inactive toward catalysis of the pyrrole + aldehyde
condensation proved quite effective for catalyzing the
condensation of a dipyrromethane and a dipyrromethane-
dicarbinol. A subset of such acids was investigated in detail;
these acids included InCl3, Sc(OTf)3, Yb(OTf)3, and Dy-
(OTf)3 in CH2Cl2 at room temperature.52 The reactions
proceeded without detectable acidolysis of the dipyr-
romethane species and afforded higher yields of porphyrin
than obtained with TFA in CH3CN. Our initial results in
applying the new Lewis acids to reactions employing
elaborate dipyrromethanes also gave good yields without
detectable scrambling.53 The exceptional results and apparent
mildness of the acid catalysis achieved with InCl3, Sc(OTf)3,
Yb(OTf)3, and Dy(OTf)3 prompted us to investigate whether
these acids could be used in the presence of acid-labile
(stability classes III and IV) metalloporphyrins.
In this paper, we report our findings concerning the use
of the new Lewis acid catalysts in reactions of zinc and
magnesium porphyrins yielding mixed-metal multiporphyrin
arrays. The paper is divided into two parts. In part 1, we
describe the rational synthesis of the mixed-metal multipor-
phyrin dyads and triads wherein adjacent porphyrins are
joined via a p-phenylene linker. The synthesis of such arrays
required the preparation of porphyrin-dipyrromethanes and
porphyrin-dipyrromethane-dicarbinols, the joining of which
occurs in a directed manner. In part 2, we report our studies
of the catalytic conditions afforded by the new Lewis acids.
Taken together, this work provides the foundation for the
rational synthesis of mixed-metal multiporphyrin arrays
incorporating metalloporphyrins of stability classes I-IV.
Of particular interest are multiporphyrin arrays joined via
the p-phenylene linker, because arrays with such linkers
exhibit very fast excited-state energy-transfer rates between
adjacent zinc and free base porphyrins [>(10 ps)-1] while
maintaining electronic spectra that are nearly identical to
those of the monomeric components.50 p-Phenylene-linked
arrays have been prepared by acid-catalyzed condensa-
tions15,18,20,21,23-26,29-33,36-48 or by the Suzuki reaction.31,44 The
Suzuki reaction entails a Pd-mediated coupling, employs
basic conditions, and provides the basis for a rational
(33) Paolesse, R.; Pandey, R. K.; Forsyth, T. P.; Jaquinod, L.; Gerzevske,
K. R.; Nurco, D. J.; Senge, M. O.; Licoccia, S.; Boschi, T.; Smith, K.
M. J. Am. Chem. Soc. 1996, 118, 3869-3882.
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Porphyrins Phthalocyanines 1998, 2, 501-510.
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J. Chem. 1998, 22, 1327-1329.
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Angew. Chem., Int. Ed. Engl. 1991, 30, 582-584.
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Yamazaki, I.; Nishimura, Y.; Ohno, T.; Nozaki, K. J. Am. Chem. Soc.
1993, 115, 4577-4589.
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Yamazaki, I.; Mataga, N. Chem.sEur. J. 2000, 6, 33-46.
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2637-2640.
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2000, 36, 585-588.
Results and Discussion
1. Synthesis. The synthesis of the porphyrin building
blocks begins with methods that have been developed over
the past few years, including the one-flask synthesis of a
(47) Heiler, D.; McLendon, G.; Rogalskyj, P. J. Am. Chem. Soc. 1987,
109, 604-606.
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Phthalocyanines 2001, 5, 810-823.
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4324 Inorganic Chemistry, Vol. 42, No. 14, 2003

Mixed-Metal Multiporphyrin Arrays
dipyrromethane from an aldehyde and excess pyrrole,54 the
1,9-diacylation of a dipyrromethane,51 and the reduction of
a diacyldipyrromethane to the corresponding dipyrromethane-
dicarbinol.51 Each porphyrin building block (trans-AB2C
type) was prepared by condensation of a dipyrromethane +
dipyrromethane-dicarbinol followed by oxidation with
DDQ. The syntheses described herein make exclusive use
of the Lewis acids [InCl3, Sc(OTf)3, Yb(OTf)3, or Dy(OTf)3
in CH2Cl2 at room temperature]52 for catalyzing the dipyr-
romethane + dipyrromethane-dicarbinol condensation.
Synthesis of Porphyrin-Dipyrromethanes. Treatment
of dipyrromethane (1) with ethylmagnesium bromide fol-
lowed by 3,5-di-tert-butylbenzoyl chloride (2) afforded
diacyldipyrromethane 3 in 51% yield (4.70 g) (Scheme 1).
Diacyldipyrromethane 3 is a known compound but was
previously prepared in a smaller quantity (442 mg) and in
lower yield (27%).55 Compound 3 was reduced using NaBH4
in THF/methanol to give the corresponding dicarbinol.
Condensation of the latter with 5-mesityldipyrromethane (4)
in the presence of InCl3 in CH2Cl2 at room temperature for
40 min followed by oxidation with DDQ afforded porphyrin
5 in 37% yield. Porphyrin 531 has been prepared previously
in lower yield (23%) using TFA in CH3CN. Treatment of
porphyrin 5 with NBS (1.0 molar equiv) in CHCl3 in the
presence of pyridine at 0 °C56 afforded the meso-bromopor-
phyrin 6 in 93% yield. Note that no â-brominated products
Scheme 1
1
were observed upon H NMR analysis. A Suzuki coupling
of meso-bromoporphyrin 6 and 4-formylphenyl boronic acid
(2 equiv) in DMF/toluene (1:1)31 using K2CO3 (8 molar equiv
relative to 6) containing Pd(PPh3)4 (15 mol % relative to 6)
at 85 °C for 4 h gave porphyrin 7 in 92% yield.
The conditions for the standard procedure for preparing
dipyrromethanes employ the reaction of an aldehyde in
excess pyrrole (25-fold) using TFA as catalyst.54 More
recently we have found that a larger excess of pyrrole (up
to 400-fold) in conjunction with dilution in CH2Cl2 (2 or
3:1 v/v relative to pyrrole) affords a cleaner reaction and a
higher yield, at least for the few elaborate aldehydes that
have been examined.53 Treatment of porphyrin-benzalde-
hyde 7 with pyrrole (300 molar equiv) and TFA (1.1 molar
equiv) in CH2Cl2 for 2 h followed by column chromatog-
raphy afforded porphyrin-dipyrromethane 8 in 80% yield
(Scheme 1). The porphyrin-dipyrromethane gave consider-
able streaking on silica chromatography, but the impurities
were more polar and bound to the top of the silica column
and only one chromatography column was required for
purification. To our knowledge, the few prior syntheses of
porphyrin-dipyrromethanes have afforded a â-substituted
dipyrromethane,33-35 typically by reaction of a porphyrin-
carboxaldehyde with ethyl 3,4-dimethylpyrrolecarboxylate
followed by ester hydrolysis and decarboxyation.34,35 The
synthesis of 8 shows that porphyrin-dipyrromethanes lack-
ing â-substituents can be prepared in a one-flask process.
Metalation of Porphyrin-Dipyrromethanes. Metalation
of free base porphyrin 8 with Zn(OAc)2‚2H2O was performed
under standard conditions in CHCl3/methanol (Scheme 2).
The second fraction containing the desired compound was
collected, giving Zn-8 in 72% yield. The reported yield is
slightly low for Zn insertions, which can be attributed to
the propensity of the dipyrromethane moiety to cause
streaking upon chromatographic workup.
The synthesis of magnesium porphyrins can be achieved
at room temperature in high yield by two methods.57,58 The
milder of the two methods employs a homogeneous solution
(54) Littler, B. J.; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea, D.
F.; Boyle, P. D.; Lindsey, J. S. J. Org. Chem. 1999, 64, 1391-1396.
(55) Clausen, C.; Gryko, D. T.; Yasseri, A. A.; Diers, J. R.; Bocian, D. F.;
Kuhr, W. G.; Lindsey, J. S. J. Org. Chem. 2000, 65, 7371-7378.
(56) DiMagno, S. G.; Lin, V. S.-Y.; Therien, M. J. J. Org. Chem. 1993,
58, 5983-5993.
Inorganic Chemistry, Vol. 42, No. 14, 2003 4325

Speckbacher et al.
Scheme 2
Scheme 3
of MgI2 in diethyl ether containing DIEA.58 Because of the
anticipated lability of the dipyrromethane unit toward acid,
the homogeneous method was employed. Thus, a solution
of 8 in CH2Cl2 was treated with a freshly prepared solution
of MgI2 in diethyl ether containing DIEA, which was
prepared following a known procedure58 (Scheme 2). The
standard workup including column chromatography (Al2O3,
CHCl3) afforded the desired magnesium porphyrin Mg-8 in
79% yield. As with the zinc chelate, streaking of Mg-8 on
the column was observed and is a likely source of the less
than quantitative yield.
Synthesis of Porphyrin-Diacyldipyrromethanes. Two
routes are available for the 1,9-diacylation of a dipyr-
romethane: (1) treatment of the dipyrromethane with eth-
ylmagnesium bromide followed by reaction with an acid
chloride;51 (2) alkylation of the dipyrromethane with a
benzoxathiolium tetrafluoroborate followed by hydrolysis to
unveil the acyl groups.59 Both methods presented difficulties
upon application to porphyrin-dipyrromethanes. We applied
the former route to the zinc porphyrin due to the presence
of the Grignard reagent and the latter to the free base
porphyrin owing to the use of acidic conditions (Scheme
3): (1) Treatment of Zn-8 with a solution of EtMgBr (1.0
M in THF) in anhydrous toluene followed by addition of
p-toluoyl chloride (2.5 molar equiv) afforded the resulting
monoacyl and diacyl products. Purification by successive
column chromatography gave the relatively pure porphyrin-
diacyldipyrromethane Zn-9 (∼31% yield). (2) The alkylation
procedure was performed by treating porphyrin-dipyr-
romethane 8 with 4 molar equiv of 2-(4-methylphenyl)-1,3-
benzoxathiolium tetrafluoroborate (10)60 and DBU61 (4 molar
equiv) in anhydrous THF for 1 h. Hydrolysis was performed
in situ by adding HgO (4 molar equiv) and aqueous HBF4
(8 molar equiv) for 2 h.61 Standard workup and chromatog-
raphy gave crude porphyrin-diacyldipyrromethane 9, which
was metalated using Zn(OAc)2‚2H2O in CHCl3/methanol.
Purification by column chromatography on silica gave the
relatively pure Zn-9 (∼37% overall), but a rather broadened
1H NMR spectrum was obtained. Analytical size exclusion
(57) Lindsey, J. S.; Woodford, J. N. Inorg. Chem. 1995, 34, 1063-1069.
(58) O’Shea, D. F.; Miller, M. A.; Matsueda, H.; Lindsey, J. S. Inorg. Chem.
1996, 35, 7325-7338.
(59) Lee, C.-H.; Li, F.; Iwamoto, K.; Dadok, J.; Bothner-By, A. A.; Lindsey,
J. S. Tetrahedron 1995, 51, 11645-11672.
(60) Barbero, M.; Cadamuro, S.; Degani, I.; Fochi, R.; Gatti, A.; Regondi,
V. Synthesis 1986, 1074-1076.
(61) Youngblood, W. J.; Lindsey, J. S. Unpublished data.
4326 Inorganic Chemistry, Vol. 42, No. 14, 2003

Mixed-Metal Multiporphyrin Arrays
chromatography (SEC) showed a dominant sharp peak (94%
of total based on absorption at 420 nm) accompanied by a
faster eluting small shoulder (6% of total). While later we
found that preparative SEC in conjunction with silica
chromatography could afford a product that was clean by
analytical SEC, the material of 94% purity was used in the
syntheses of the dyads and triads.
tion via column chromatography on alumina afforded MgFb-
12 in 28% yield. Note that alumina was employed rather
than silica because magnesium porphyrins are stable on
alumina but undergo demetalation on silica.
Dyad MgFb-12 also was prepared by condensation of
porphyrin Mg-8 and dipyrromethane-dicarbinol 11-diol in
CH2Cl2 using Dy(OTf)3 under the same conditions as for
the reaction employing Yb(OTf)3. However, the presence of
several byproducts resulted in a difficult workup, and dyad
MgFb-12 was obtained accompanied by slight impurities in
∼15% yield. Accordingly, Dy(OTf)3 was not used to prepare
further dyads or triads.
Synthesis of Porphyrin Triads. The successful synthesis
of ZnFb and MgFb dyads prompted us to employ the same
strategy to prepare mixed-metal triads. Thus, the condensa-
tion of porphyrin-dipyrromethane Zn-8 and porphyrin-
dipyrromethane-dicarbinol Zn-9-diol was performed in
CH2Cl2 containing Yb(OTf)3 for 50 min followed by
oxidation with DDQ (Scheme 6). Purification by column
chromatography (silica, CH2Cl2) afforded the triad ZnFbZn-
14 in 21% yield. All byproducts were more polar and
remained on top of the silica column, which is consistent
with the results of the dyad-forming experiments.
Similarly, condensation of Zn-porphyrin-dipyrromethane-
dicarbinol Zn-9-diol and Mg-porphyrin-dipyrromethane
Mg-8 was performed in CH2Cl2 using Yb(OTf)3 for 50 min,
followed by the addition of pyridine and DDQ (Scheme 7).
Column chromatography on alumina gave the triad Zn-
FbMg-14 in 23% yield.
The related triad ZnFbFb-14 was prepared via two
different routes as shown in Scheme 8. First, treatment of a
solution of ZnFbMg-14 in CH2Cl2 with silica gel63 and
stirring overnight resulted in the selectively demagnesiated
triad ZnFbFb-14. Second, condensation of Zn-porphyrin-
dipyrromethane-dicarbinol Zn-9-diol and Fb-porphyrin-
dipyrromethane 8 was performed in CH2Cl2 containing
Yb(OTf)3 followed by oxidation with DDQ, affording the
same triad ZnFbFb-14 in 18% yield. The success of this
porphyrin-forming reaction indicates that buffering of the
acid by the free base porphyrin has little adverse effect on
the reaction course. By contrast, the reactions of free base
porphyrincarboxaldehydes with pyrrole or dipyrromethane
species often require elevated concentrations of acid owing
to the buffering effect of the porphyrin.31
Chemical Characterization. Each dyad and triad was
characterized by TLC, analytical SEC, laser-desorption mass
spectrometry (LD-MS),64 absorption spectroscopy, fluores-
cence spectroscopy, and 1H NMR spectroscopy. Each puri-
fied array was relatively homogeneous by TLC and analytical
SEC, gave a strong molecule ion peak upon LD-MS analysis,
and gave a readily assignable 1H NMR spectrum. The
Synthesis of Porphyrin Dyads. The p-phenylene-linked
porphyrin dyads were prepared by the dipyrromethane-
dicarbinol + dipyrromethane condensation in CH2Cl2 using
one of the new, mild Lewis acids followed by oxidation with
DDQ. Thus, Zn-porphyrin-dipyrromethane Zn-8 was
reacted with dipyrromethane-dicarbinol 11-diol51 (2.5 mM
each) in CH2Cl2 containing ytterbium(III) trifluoromethane
sulfonate [Yb(OTf)3] at an effective concentration of 3.2 mM
(the catalyst does not dissolve completely, but we use
concentration terms for clarity about how much material is
employed). The progress of the reaction was monitored by
treatment of reaction samples with a solution of DDQ (10
mM in toluene) followed by TLC analysis. A new fast-
eluting band was observed to increase in intensity over time.
Traditional UV-vis spectroscopic analysis51 to follow the
appearance of the porphyrin Soret band cannot be used to
monitor the progress of the reaction because the starting
material contains a porphyrin species, which shows a typical
absorption pattern. After 50 min, DDQ (0.75 molar equiv/
mol of pyrrole units) was added and the reaction mixture
was stirred for 15 min. The resulting dyad ZnFb-12 was
very nonpolar compared to the starting compounds, moved
as the first tight band close to the solvent front upon column
chromatography (silica, CH2Cl2), and was readily isolated
in 23% yield (Scheme 4).
For comparison purposes, the same dyad was prepared by
the complementary reaction. Thus, reduction of porphyrin-
diacyldipyrromethane Zn-9 with NaBH4 in dry THF/
methanol (10:1) for 90 min gave the corresponding Zn-
porphyrin-dipyrromethane-dicarbinol Zn-9-diol (Scheme
4). The latter was condensed with 5-phenyldipyrromethane
(13)54 in CH2Cl2 containing Yb(OTf)3 for 50 min. Oxidation
with DDQ and purification by chromatography on silica
afforded the desired dyad ZnFb-12 in 22% yield.
The analogous dyad MgFb-12 was prepared by condensa-
tion of porphyrin Mg-8 and dipyrromethane-dicarbinol 11-
diol in CH2Cl2 containing Yb(OTf)3 (Scheme 5). In early
experiments the addition of DDQ followed by alumina
chromatography led to the isolation of the demetalated dyad
(FbFb-12). The addition of DDQ to cause oxidation of the
porphyrinogen and polypyrromethane species results in
formation of the hydroquinone product, namely DDQH2 (2,3-
dichloro-5,6-dicyano-1,4-dihydroxybenzene). The latter is
weakly acidic (pKa ) 5.14)62 and was considered to be the
likely cause of demetalation. The same condensation was
repeated and after 50 min pyridine was added to neutralize
the acid. (Pyridine was chosen rather than TEA or DIEA,
for example, because pyridine is electron-deficient and should
not be oxidized by DDQ.) Then DDQ was added. Purifica-
(63) (a) Li, J.; Diers, J. R.; Seth, J.; Yang, S. I.; Bocian, D. F.; Holten, D.;
Lindsey, J. S. J. Org. Chem. 1999, 64, 9090-9100. (b) Yang, S. I.;
Li, J.; Cho, H. S.; Kim, D.; Bocian, D. F.; Holten, D.; Lindsey, J. S.
J. Mater. Chem. 2000, 10, 283-296.
(64) (a) Fenyo, D.; Chait, B. T.; Johnson, T. E.; Lindsey, J. S. J. Porphyrins
Phthalocyanines 1997, 1, 93-99. (b) Srinivasan, N.; Haney, C. A.;
Lindsey, J. S.; Zhang, W.; Chait, B. T. J. Porphyrins Phthalocyanines
1999, 3, 283-291.
(62) Akutagawa, T.; Saito, G. Bull. Chem. Soc. Jpn. 1995, 68, 1753-1773.
Inorganic Chemistry, Vol. 42, No. 14, 2003 4327

Speckbacher et al.
Scheme 4
absorption spectrum of each array generally was the sum of
the spectra of the component porphyrin monomers in the
visible region, while splitting and broadening of the Soret
peak was observed in the near-UV region. The splitting of
4328 Inorganic Chemistry, Vol. 42, No. 14, 2003

Mixed-Metal Multiporphyrin Arrays
Scheme 5
Scheme 6
the Soret band was typical for p-phenylene-linked arrays,31,50
with maxima at 419 and 428 nm for ZnFb-12, at 420 and
431 nm for MgFb-12, at 419 and 435 nm for ZnFbZn-14,
at 421 and 436 nm for ZnFbMg-14, and at 420 and 434 nm
for ZnFbFb-14. The fluorescence emission spectrum of each
array containing Zn and Fb chromophores showed emission
exclusively from the Fb unit. The fluorescence emission
spectrum of arrays containing a Mg chromophore showed
the typical bands for Fb emission at 653 and 717 nm.
In no case was any sign of demetalation observed in the
isolated products. Furthermore, no bands were observed
during column chromatographic workup of the crude reaction
Inorganic Chemistry, Vol. 42, No. 14, 2003 4329

Speckbacher et al.
Scheme 7
mixtures that could be attributed to the presence of a dyad
or triad containing a demetalated porphyrin. Such demeta-
lated porphyrins would be readily detected in the dyads and
triads prepared herein. For example, TLC analysis of
ZnFbMg-14 and ZnFbFb-14 on alumina using CH2Cl2/
hexanes (2:1) gave Rf ) 0.12 and 0.80, respectively. It is
noteworthy that LD-MS could not be employed as a means
of analyzing crude samples from reactions containing
magnesium porphyrins due to demetalation of the magnesium
porphyrins during the analytical process (see Supporting
Information). However, LD-MS could be used for crude
reaction samples containing zinc or free base porphyrins. For
example, LD-MS analysis of a sample from the crude
reaction mixture for the formation of ZnFb-12 showed the
expected intense molecule ion peak for ZnFb-12 and no peak
for the corresponding demetalated species, FbFb-12.
2. Examination of Acid Catalysis Conditions. Prior to,
during the course of, and following the synthetic work
described above we performed a series of experiments that
explored the nature of the acid catalysis conditions for use
with metalloporphyrins of stability classes III and IV. The
studies were aimed initially at confirming the suitability of
the acid catalysts and then turned toward gaining a deeper
understanding of the acid catalysis process. The following
sections describe the findings from a number of these
experiments.
For stability studies we chose a series of porphyrins bearing
substituents spanning a range of steric hindrance, including
meso-tetrapentylporphyrin (H2TPnP), meso-tetraphenylpor-
phyrin (H2TPP), and meso-tetramesitylporphyrin (H2TMP)
(Chart 1). The zinc and magnesium chelate of each porphyrin
was examined. Each metalloporphyrin (2.5 mM in CH2Cl2)
was treated with each of the four different catalysts using
the concentration of the catalyst found optimal in studies of
dipyrromethane-carbinol condensations:52 InCl3 (0.32 mM);
Yb(OTf)3 (3.2 mM); Sc(OTf)3 (0.32 mM); Dy(OTf)3 (1.0
mM). Note that although the acids do not dissolve completely
in CH2Cl2, we use concentration terms nonetheless to
describe the quantity employed. The reaction mixtures were
stirred at room temperature for a specific period (1, 6 h)
prior to analysis. Samples of fixed absorption at the Q(1,0)
transition for the given metalloporphyrin (∼550 nm) were
then analyzed by fluorescence spectroscopy with illumination
of the Qx(1,0) transition for the free base porphyrin (∼515
nm). The fluorescence emission of the Fb porphyrin was
employed to determine the amount of free base porphyrin
present, using a working curve for each metalloporphyrin
(see Supporting Information). The limit of detection for each
type of porphyrin ranged from 0.10% to 1.0% depending
on the type of metal employed and the nature of the
porphyrin substituents. The results obtained are summarized
in Table 2.
Stability of Metalloporphyrins toward Acids. We first
examined the stability of a series of metalloporphyrins toward
the mild Lewis acid catalysts [InCl3, Yb(OTf)3, Sc(OTf)3,
Dy(OTf)3] that were found previously to be effective for the
dipyrromethane + dipyrromethane-dicarbinol condensation.
The amount of demetalation over the course of 1 h
generally ranged from relatively little to undetectable de-
pending on the type of acid and the type of porphyrin. In
general, the chelates of tetraphenylporphyrin were more
prone to demetalation than those of tetrapentylporphyrin or
4330 Inorganic Chemistry, Vol. 42, No. 14, 2003

Mixed-Metal Multiporphyrin Arrays
Scheme 8
Chart 1
for InCl3 and the level had increased for the sample with
Sc(OTf)3, but again no demetalation was detected for Dy-
(OTf)3 and Yb(OTf)3. No demetalation was observed for any
acid at 6 h for ZnTPnP; the only demetalation observed with
ZnTMP occurred at a trace level (0.54%) with Sc(OTf)3 at
6 h.
For MgTPP at 1 h, a significant amount of demetalation
(14%) was observed for Yb(OTf)3, minor amounts (∼3-
4%) for Sc(OTf)3 and Dy(OTf)3, but no demetalation was
observed for InCl3. The extent of demetalation increased for
the three acids over 6 h, but again no demetalation was
detected for InCl3. On the other hand, no demetalation was
observed for MgTPnP or MgTMP at the 1 h timepoint with
InCl3, Yb(OTf)3, or Dy(OTf)3; only Sc(OTf)3 gave demeta-
lation, and this occurred at the ∼1-2% level.
tetramesitylporphyrin. For ZnTPP, demetalation at 1 h was
observed only with Sc(OTf)3, and this was only at the ∼0.3%
level. At 6 h, a time period much longer than the dipyr-
romethane + dipyrromethane-dicarbinol condensation pe-
riod but long enough to accentuate any differences between
the catalysts, a small amount of demetalation was observed
Of course, for porphyrin-forming reactions employed in
the synthesis of mixed-metal arrays, demetalation at even
the 0.1-1% level is unacceptable. The disparity in extent
of demetalation among the three types of porphyrins must
Inorganic Chemistry, Vol. 42, No. 14, 2003 4331

Speckbacher et al.
Table 2. Stability of Metalloporphyrins toward the Four Acid Catalysts
extent of metalloporphyrin demetalation with a given catalystc (%)
Yb(OTf)3 Sc(OTf)3 Dy(OTf)3
1 h 1 h
limit of detection
(LOD)b (%)
InCl3
entry
metalloporphyrina
1 h
6 h
6 h
1 h
6 h
6 h
1
2
3
4
5
6
ZnTMP
ZnTPP
ZnTPnP
MgTMP
MgTPP
MgTPnP
0.10
0.10
0.32
1.0
0.32
0.32
<LOD
<LOD
<LOD
<LOD
<LOD
<LOD
<LOD
0.54
<LOD
<LOD
<LOD
1.0
<LOD
<LOD
<LOD
<LOD
14.0
<LOD
<LOD
<LOD
1.5
60
0.5
<LOD
0.35
<LOD
1.1
3.9
2.4
0.54
3.0
<LOD
7.7
5.5
3.5
<LOD
<LOD
<LOD
<LOD
2.7
<LOD
<LOD
<LOD
<LOD
7.3
<LOD
<LOD
<LOD
a Each metalloporphyrin was employed in a 2.5 mM solution in CH2Cl2 at room temperature. b Limit of detection in the fluorescence assay of the
corresponding free base porphyrin derived by demetalation. c The amount of each catalyst employed is described in concentration terms (regardless of
solubility) and is as follows: InCl3, 0.32 mM; Yb(OTf)3, 3.2 mM; Sc(OTf)3, 0.32 mM; Dy(OTf)3, 1.0 mM.
2H+ + M-porphyrin f H2-porphyrin + M2+ (1)
stem from differences in electronic properties and steric
hindrance. In general, the rate of demetalation increases with
If the Lewis acid acted exclusively as a Lewis acid, a source
of protons would not be available to participate in the
demetalation. However, Lewis acids are known to engage
in Lewis-acid-promoted formation of Brønsted acids, thereby
providing a source of protons.66 To assess the Brønsted
acidity of the four catalysts, we treated solutions of H2TPP
(2.5 mM) in CH2Cl2 with each of the catalysts InCl3 (0.32
mM), Yb(OTf)3 (3.2 mM), Sc(OTf)3 (0.32 mM), and Dy-
(OTf)3 (1.0 mM) at room temperature. After 1 h, a sample
from each reaction mixture (in CH2Cl2) was removed and
then diluted in CH2Cl2 to give a solution for spectroscopic
measurement (∼3 µM total porphyrin species). The resulting
spectra showed the presence of two bands, the Soret band
and the band at ∼440 nm characteristic of H4TPP2+.67 The
spectrum obtained with Yb(OTf)3 is shown in Figure 1, along
with that of H4TPP2+ formed by protonation of H2TPP with
TFA. The percentage of H4TPP2+ present in these samples
was as follows: InCl3 (16%); Yb(OTf)3 (47%); Sc(OTf)3
(42%); Dy(OTf)3 (<1%). In the case of Yb(OTf)3, the
spectrum also was recorded in a very thin film cell without
dilution, giving a 39% yield of H4TPP2+, thereby indicating
that the observed protonation is not a dilution artifact. The
observation of the characteristic absorption spectrum of H4-
TPP2+ under the conditions of the porphyrin-forming reaction
implies that the presence of the Lewis acid results in the
formation of a Brønsted acid (that protonates H2TPP to give
H4TPP2+). The source of protons is likely from H2O, which
is present in reagent-grade CH2Cl2 at a concentration of ∼8-
10 mM.68
the electron-richness of the porphyrin ring, as the mechanism
of acid-induced demetalation entails protonation of the core
nitrogen atoms.65 The rate of demetalation generally de-
creases with increasing steric bulk protecting the metal site.65
In the synthetic work described herein, each porphyrin was
substituted with one mesityl group, two 3,5-di-tert-butylphe-
nyl groups, and one p-phenylene group. The propensity
toward demetalation of such metalloporphyrins would be
expected to lie between that of the MTPP and MTMP
benchmarks. To the extent that the acid-stability studies
provide a realistic model of the conditions to which the
metalloporphyrins are exposed during porphyrin formation,
we should have observed some demetalated components in
the Yb(OTf)3-catalyzed syntheses using magnesium porphy-
rin building blocks.
The demetalation studies exposed the metalloporphyrin to
the acid catalyst in CH2Cl2. In the porphyrin-forming
reactions, other species are present that can interact with the
acid catalyst, such as the dipyrromethane-dicarbinol. To
examine whether the presence of a dipyrromethane-di-
carbinol diminished the extent of demetalation, a solution
of MgTPP (2.5 mM) and the dipyrromethane-dicarbinol 11-
diol (2.5 mM) in CH2Cl2 was treated with Yb(OTf)3 (3.2
mM). After 1 h, the demetalation of MgTPP was only 0.8%,
17-fold less than the 14% observed when MgTPP alone was
treated with Yb(OTf)3. On the other hand, the same experi-
ment using 5-phenyldipyrromethane (13, 2.5 mM) (which
should not bind Yb(OTf)3) rather than 11-diol resulted in
13% demetalation of MgTPP. These results demonstrate that,
under the porphyrin-forming conditions, the catalyst Yb-
(OTf)3 likely causes less demetalation of the metallopor-
phyrin than observed in the more demanding control
experiments performed herein.
The presence of the Lewis acid and a Brønsted acid
derived from the Lewis acid raised the question as to the
nature of the catalysis that promotes the condensation of the
dipyrromethane + dipyrromethane-dicarbinol. The hindered
base 2,6-di-tert-butylpyridine,69 which can act as a proton
sponge but not bind to bulky Lewis acids, has been employed
to discriminate Lewis and Brønsted acid catalysis. We
examined the extent of demetalation of MgTPP in the
Lewis Acids and Brønsted Acids. The observation of
demetalation due to the presence of a Lewis acid prompted
us to consider the mechanism of demetalation. The reaction
for the demetalation of a metalloporphyrin requires a source
of protons, shown as follows:
(66) Barrett, A. G. M.; Braddock, D. C.; Henschke, J. P.; Walker, E. R. J.
Chem. Soc., Perkin Trans. 1 1999, 873-878.
(67) Ohno, O.; Kaizu, Y.; Kobayashi, H. J. Chem. Phys. 1985, 82, 1779-
1787.
(65) Hambright, P. In The Porphyrin Handbook; Kadish, K. M., Smith, K.
M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000, Vol.
3, pp 129-210.
(68) Li, F.; Yang, K.; Tyhonas, J. S.; MacCrum, K. A.; Lindsey, J. S.
Tetrahedron 1997, 53, 12339-12360.
(69) Brown, H. C.; Kanner, B. J. Am. Chem. Soc. 1966, 88, 986-992.
4332 Inorganic Chemistry, Vol. 42, No. 14, 2003

Mixed-Metal Multiporphyrin Arrays
Figure 2. The yield of porphyrin as a function of time for dipyr-
romethane-dicarbinol + dipyrromethane condensation under catalysis by
Yb(OTf)3 in CH2Cl2. (The concentration of each reactant is 2.5 mM, and
the concentration of the catalyst is 3.2 mM.) Each data point is the average
from duplicate samples. Key: DTBP ) 2,6-di-tert-butylpyridine.
Scheme 9
Figure 1. Top panel: Absorption spectra of H2TPP and H4TPP2+ (formed
by acidification with TFA) in CH2Cl2 at room temperature. Bottom panel:
Absorption spectrum of H2TPP (2.5 mM) in CH2Cl2 after 1 h of treatment
with Yb(OTf)3 (3.2 mM) at room temperature, followed by dilution in CH2-
Cl2 to give a ∼3 µM solution of total porphyrin species.
presence of Yb(OTf)3 (3.2 mM) and various amounts of 2,6-
di-tert-butylpyridine. In the absence of any 2,6-di-tert-
butylpyridine, the extent of demetalation was 14% after 1 h
in CH2Cl2. In the presence of 2,6-di-tert-butylpyridine (3.2
mM), only 0.44% demetalation was observed, while with
10 times as much 2,6-di-tert-butylpyridine (32 mM) the
observable demetalation was below the limits of detection
(<0.32%) after 1 h at room temperature. Thus, demetalation
did not occur to a measurable extent in the presence of the
mild Lewis acid and an agent that selectively neutralized
any Brønsted acids.
Next, we examined whether the porphyrin-forming reac-
tion would proceed with exclusive Lewis acid catalysis or
whether the observed catalysis in the presence of the Lewis
acids actually emanated from the Lewis-acid-derived Brøn-
sted acid. A solution of dipyrromethane-dicarbinol 11-diol
(2.5 mM) and 5-phenyldipyrromethane (13, 2.5 mM) in CH2-
Cl2 was treated with Yb(OTf)3 and various amounts of 2,6-
di-tert-butylpyridine (3.2 or 32 mM) at room temperature
(Scheme 9). The yield of the porphyrin over time was
recorded by removing an aliquot and oxidizing the aliquot
with DDQ, followed by UV-vis spectroscopy. The results
are shown in Figure 2. In the absence of 2,6-di-tert-
butylpyridine, the reaction was fast with near-maximal yield
observed at the earliest time point (5 min). In the presence
of 1 equiv of 2,6-di-tert-butylpyridine (relative to Yb(OTf)3),
the reaction was much slower and required 30-60 min to
reach the same yield as without the base. The same slow
reaction was observed with 10 equiv of 2,6-di-tert-butylpy-
ridine. LD-MS analysis of each crude reaction mixture
showed no scrambled porphyrin product. The isolated yield
of porphyrin from each of the above reactions was consistent
with the yield observed spectroscopically: 28% (in the
absence of 2,6-di-tert-butylpyridine); 22% (in the presence
of 3.2 mM 2,6-di-tert-butylpyridine); 26% (in the presence
of 32 mM 2,6-di-tert-butylpyridine).
Inorganic Chemistry, Vol. 42, No. 14, 2003 4333

Speckbacher et al.
amylenes) were used as received. The CHCl3 used was stabilized
with ethanol unless noted otherwise.
In summary, the presence of 2,6-di-tert-butylpyridine with
the mild Lewis acid (1) greatly suppresses demetalation of
a class IV metalloporphyrin and (2) does not diminish the
yield of porphyrin obtained. One interpretation of these
results is that the reaction in the presence of the 2,6-di-tert-
butylpyridine derives predominantly from Lewis acid ca-
talysis, while that in the absence of 2,6-di-tert-butylpyridine
stems from a combination of Lewis acid catalysis and Lewis-
acid-derived Brønsted acid catalysis. These observations
suggest that further studies of the use of mild Lewis acid
catalysts, in conjunction with bases that neutralize Brønsted
acids, may lead to further improvements in catalysis for use
with otherwise acid-labile metalloporphyrins.
Chromatography. Adsorption column chromatography was
performed using flash silica gel (Baker, 60-200 mesh). Neutral
alumina (80-200 mesh) was obtained from Fisher Scientific.
Analytical SEC70 was performed to assess the purity of multipor-
phyrin arrays.
Noncommercial Compounds. Dipyrromethanes 1, 4, and 13,54
acylating agents 2-(4-methylphenyl)-1,3-benzoxathiolium tetrafluo-
roborate (10)60 and 3,5-di-tert-butylbenzoyl chloride (2),71 and 1,9-
bis(4-methylbenzoyl)-5-phenyldipyrromethane (11)51 were obtained
by literature procedures. The benchmark porphyrins for the acid
catalysis studies H2TPnP,12 MgTPP,57 H2TMP,72 MgTMP,57 and
ZnTMP73 were prepared according to literature procedures. Por-
phyrins H2TPP and ZnTPP are available from Aldrich; the syntheses
of ZnTPnP and MgTPnP are described below.
Conclusions
1,9-Bis(3,5-di-tert-butylbenzoyl)dipyrromethane (3). From a
known procedure55 at 5.4 times the scale, reaction of dipyrromethane
1 (2.31 g, 15.8 mmol) and acid chloride 2 (10.0 g, 39.6 mmol)
afforded a white powder (4.70 g, 51%) with analytical data
consistent with those reported in the literature.55
The condensation of a dipyrromethane and a dipyr-
romethane-dicarbinol can be catalyzed with very mild acids.
The reaction catalyzed by Yb(OTf)3 proceeds without
detectable acidolysis of the dipyrromethane species and
without significant demetalation of zinc or magnesium
tetraarylporphyrins. The catalytic conditions were used in
the synthesis of p-phenylene-linked porphyrin dyads and
triads containing zinc, magnesium, and free base porphyrins.
The syntheses were carried out in a rational way owing to
the inherent asymmetry of the joining reactions. The joining
reactions include the following: porphyrin-dipyrromethane
+ dipyrromethane-dicarbinol; porphyrin-dipyrromethane-
dicarbinol + dipyrromethane; porphyrin-dipyrromethane +
porphyrin-dipyrromethane-dicarbinol. The porphyrin-form-
ing reactions proceed in yields of 18-28%. A current
limitation of this approach entails the methodology for
diacylation of porphyrin-dipyrromethanes. Regardless, the
acid-catalyzed condensation of a dipyrromethane and a
dipyrromethane-dicarbinol is a valuable complement to the
use of Pd-mediated coupling reactions with intact porphyrin
building blocks. The former approach opens a pathway to
multiporphyrin arrays comprised of free base porphyrins and
diverse metalloporphyrins without apparent restrictions on
the nature of the linker that joins adjacent porphyrins. Further
studies of mild Lewis acid catalysts are warranted to explore
the scope of application to elaborate substrates that incor-
porate diverse acid-labile components, including other met-
alloporphyrins of stability classes III and IV.
5,15-Bis(3,5-di-tert-butylphenyl)-10-mesitylporphyrin (5). From
a standard procedure,51 3 (1.74 g, 3.00 mmol) was reacted with
NaBH4 (2.28 g, 60.0 mmol) in dry THF/methanol (240 mL, 10:1)
for 90 min. After reduction was complete, the reaction was
quenched with aqueous NH4Cl (150 mL). CH2Cl2 was added, and
the organic phase was collected, washed with brine, and dried (K2-
CO3). Removal of the solvent afforded the dipyrromethane-
dicarbinol as a colorless foam. The latter was condensed with 4
(0.790 g, 3.00 mmol) in CH2Cl2 (1200 mL) containing InCl3 (86
mg, 0.39 mmol) for 40 min followed by oxidation with DDQ (2.04
g, 9.00 mmol). Filtration through a pad of silica and removal of
the solvent gave a purple solid (883 mg, 37%): the analytical data
were consistent with those reported in the literature for the synthesis
employing other acid catalysis.31
5-Bromo-10,20-bis(3,5-di-tert-butylphenyl)-15-mesitylporphy-
rin (6). From a standard procedure,56 a stirred solution of porphyrin
5 (427 mg, 0.530 mmol) in CHCl3 (130 mL) was cooled in an ice
bath for 15 min. Then NBS (94 mg, 0.53 mmol) was added. After
12 min, TLC analysis showed complete consumption of starting
material. The reaction was quenched with acetone (20 mL), and
the crude product was purified by column chromatography [silica,
CH2Cl2/hexanes (1:1)]. Removal of the solvent afforded a purple
solid (438 mg, 93%): 1H NMR δ -2.64 (s, 2H), 1.54 (s, 36H),
1.85 (s, 6H), 2.61 (s, 3H), 7.26 (s, 2H), 7.80-7.81 (m, 2H), 8.06
(m, 4H), 8.64 (d, J ) 4.8 Hz, 2H), 8.80 (d, J ) 4.4 Hz, 2H), 8.92
(d, J ) 4.4 Hz, 2H), 9.67 (d, J ) 4.8 Hz, 2H); LD-MS obsd m/e
882.2; FAB-MS obsd m/e 882.4228, calcd m/e 882.4236 (C57H63N4-
Br).
Experimental Section
Synthesis. General Methods. All 1H NMR spectra (400 MHz)
were collected in CDCl3 unless noted otherwise. Absorption and
fluorescence spectra were collected in chloroform at room tem-
perature unless specified otherwise. Mass spectra of porphyrins were
obtained via laser desorption mass spectrometry (LD-MS) without
a matrix64 or by high-resolution fast atom bombardment mass
spectrometry (FAB-MS) using a matrix of nitrobenzyl alcohol and
poly(ethylene glycol). The acid catalysts were used as received from
Aldrich.
Solvents. THF was distilled from sodium benzophenone ketyl
as required. Toluene was distilled from CaH2. Anhydrous methanol
(Aldrich), anhydrous DMF (Aldrich), CH2Cl2 (ACS), CHCl3
(Fisher, certified ACS grade, stabilized with 0.8% ethanol), and
CHCl3 (Aldrich, 99.8%, certified ACS grade, stabilized with
5,15-Bis(3,5-di-tert-butylphenyl)-10-(4-formylphenyl)-20-mesi-
tylporphyrin via Suzuki Coupling (7). From a standard proce-
dure,31 porphyrin 6 (0.50 g, 0.57 mmol), 4-formylphenylboronic
acid (0.17 g, 1.1 mmol), anhydrous K2CO3 (0.63 g, 4.5 mmol),
and Pd(PPh3)4 (98 mg, 85 µmol, 15%) were weighed into a 100
(70) Li, J.; Ambroise, A.; Yang, S. I.; Diers, J. R.; Seth, J.; Wack, C. R.;
Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Am. Chem. Soc. 1999,
121, 8927-8940.
(71) Korshunov, M. A.; Bodnaryuk, F. N.; Fershtut, E. V. Zh. Org. Khim.
1967, 3, 140-143.
(72) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828-836.
(73) (a) Badger, G. M.; Jones, R. A.; Laslett, R. L. Aust. J. Chem. 1964,
17, 1028-1035. (b) Yang, S. I.; Seth, J.; Strachan, J.-P.; Gentemann,
S.; Kim, D.; Holten, D.; Lindsey, J. S.; Bocian, D. F. J. Porphyrins
Phthalocyanines 1999, 3, 117-147.
4334 Inorganic Chemistry, Vol. 42, No. 14, 2003

Mixed-Metal Multiporphyrin Arrays
mL Schlenk flask. The flask was pump-purged with argon three
times. Then toluene/DMF (57 mL, 1:1) was added and the reaction
mixture was heated to 85 °C and stirred for 4 h. TLC analysis
showed complete consumption of the starting material. The solvent
was removed under vacuum, and the crude product was purified
by column chromatography (silica, CHCl3) affording a purple solid
(474 mg, 92%): analytical data were consistent with those reported
in the literature.31
suspension of 10 (628 mg, 2.00 mmol) in anhydrous THF (9.7 mL)
was degassed with a stream of Ar for 5 min. Then DBU (300 µL,
2.00 mmol) was added and the reaction mixture rapidly became
clear and homogeneous. A solution of porphyrin 8 (511 mg, 0.50
mmol) in anhydrous THF (10 mL) was added via syringe. After
the sample was stirred for 1 h, TLC analysis showed complete
alkylation. Then HgO (435 mg, 2.00 mmol) and aqueous HBF4
(523 µL, 4.00 mmol) were added and stirring was continued for
further 2 h. The reaction was quenched with a saturated aqueous
solution of NH4Cl (150 mL). CH2Cl2 was added, and the organic
phase was collected, washed with NaHCO3 and brine, and dried
(Na2SO4). After removal of the solvent the crude product was
purified by column chromatography [silica, CHCl3/ethyl acetate (10:
1)]. Some nonpolar byproducts could be separated as they appear
in a slightly purple colored forerun. The product fraction came off
as a very tight and concentrated band, affording a purple powder
5,15-Bis(3,5-di-tert-butylphenyl)-10-[4-(dipyrromethane-5-yl)-
phenyl]-20-mesitylporphyrin (8). Following a modified proce-
dure53 for dipyrromethane synthesis,54 a solution of porphyrin 7
(227 mg, 0.250 mmol) and pyrrole (5.0 g, 75 mmol) in CH2Cl2
(12 mL) was treated with TFA (22 µL, 0.28 mmol) at room
temperature. TLC analysis [silica, CHCl3/TEA (99:1)] showed
complete consumption of the starting porphyrin-benzaldehyde after
2 h. The reaction mixture was neutralized with TEA. The solvent
and the excess pyrrole were removed under reduced pressure.
Chromatography [silica, CHCl3/TEA (99:1)] afforded the desired
product as a purple solid (205 mg, 80%): 1H NMR δ -2.65 (br s,
2H), 1.53 (s, 36H), 1.86 (s, 6H), 2.62 (s, 3H), 5.84 (s, 1H), 6.18
(br s, 2H), 6.29-6.32 (m, 2H), 6.85-6.87 (m, 2H), 7.27 (s, 2H),
7.60 (d, J ) 8.1 Hz, 2H), 7.78-7.79 (m, 2H), 8.08-8.09 (m, 4H),
8.18 (d, J ) 8.1 Hz, 2H), 8.22 (br s, 2H), 8.69 (d, J ) 4.8 Hz, 2H),
8.83-8.88 (m, 6H); LD-MS obsd m/e 1024.8; FAB-MS obsd m/e
1024.6115, calcd m/e 1024.6131 (C72H76N6); λabs (toluene) 421,
516, 550, 593, 649 nm; λem (λex ) 515 nm, toluene) 652, 720 nm.
[5,15-Bis(3,5-di-tert-butylphenyl)-10-[4-(dipyrromethan-5-yl)-
phenyl]-20-mesitylporphinato]zinc(II) (Zn-8). A solution of 8
(0.30 g, 0.29 mmol) in CHCl3 (32 mL) was treated with a solution
of Zn(OAc)2‚2H2O (0.32 g, 1.5 mmol) in methanol (5 mL). The
reaction mixture was stirred overnight at room temperature, leaving
only a small amount of free base species upon analysis by TLC
and UV-visible spectroscopy. Purification by column chromatog-
raphy (silica, CHCl3) afforded two fractions owing to the significant
difference in polarities of the free base porphyrin and the zinc
chelate. The fraction containing the zinc chelate was concentrated,
affording a purple solid (229 mg, 72%): 1H NMR δ 1.53 (s, 36H),
1.86 (s, 6H), 2.62 (s, 3H), 5.84 (s, 1H), 6.17-6.19 (m, 2H), 6.29-
6.31 (m, 2H), 6.86-6.87 (m, 2H), 7.28 (s, 2H), 7.60 (d, J ) 8.4
Hz, 2H), 7.79-7.80 (m, 2H), 8.10-8.11 (m, 4H), 8.18 (d, J ) 8.0
Hz, 2H), 8.22 (br s, 2H), 8.79 (d, J ) 4.8 Hz, 2H), 8.95-8.99 (m,
6H); LD-MS obsd m/e 1085.8, FAB-MS obsd m/e 1086.5219, calcd
1
(252 mg, ∼40%). The H NMR spectrum was broadened though
the LD-MS spectrum showed the desired product as the dominant
peak: LD-MS obsd m/e 1258.3; FAB-MS obsd m/e 1260.6940,
calcd m/e 1260.6969 (C88H88N6O2); λabs 422, 518, 553, 593, 649
nm; λem (λex ) 518 nm) 651, 717 nm. The crude material obtained
in this manner from several runs was pooled and metalated. In one
run, a sample of the crude free base porphyrin (368 mg, 0.290
mmol) was dissolved in CHCl3 (32 mL) and then a solution of
Zn(OAc)2‚2H2O (0.320 g, 1.46 mmol) in methanol (5 mL) was
added. The reaction mixture was stirred at room temperature. After
2 h, TLC and UV-visible spectroscopic analysis showed complete
metalation. Purification by column chromatography (silica, CHCl3)
gave a purple solid (356 mg, 92%; 37% overall). The Rf of Zn-9
(silica, CHCl3) is much higher than for the corresponding free base
porphyrin 9, implying the zinc chelate is less polar than the free
base compound. Analytical SEC showed this material to be 94%
pure: 1H NMR δ 1.59 (s, 36H), 1.91 (s, 6H), 2.43 (s, 6H), 2.67 (s,
3H), 6.12 (s, 1H), 6.30-6.33 (m, 2H), 6.71-6.75 (m, 2H), 6.81
(s, 2H), 7.24-7.32 (m, 6H), 7.60-7.62 (m, 2H), 7.85-7.95 (m,
6H), 8.17 (s, 4H), 8.32-8.33 (m, 2H), 8.85-8.86 (m, 2H), 9.01-
9.07 (m, 4H), 11.69 (br s, 2H); LD-MS obsd m/e 1325.2, FAB-
MS obsd m/e 1322.6044, calcd m/e 1322.6104 (C88H86N6O2Zn);
λabs 423, 551, 593 nm; λem (λex ) 551 nm) 602, 647 nm.
Route B. Following a standard procedure,51 a solution of Zn-8
(340 mg, 0.310 mmol) in anhydrous toluene (6.5 mL) was treated
with a solution of EtMgBr (1.0 M solution in THF, 1.56 mL, 1.56
mmol) for 30 min. Then p-toluoyl chloride (100 µL, 0.780 mmol)
was added and the mixture was stirred for an additional 45 min at
room temperature. Then the reaction was quenched with saturated
aqueous NH4Cl (25 mL). Ethyl acetate was added, and the organic
phase was washed with water and brine and then dried (Na2SO4).
The solvent was removed and the crude product was purified by
column chromatography (silica, CH2Cl2) to remove some nonpolar
byproducts. Then the eluant was changed [CH2Cl2/ethyl acetate (20:
1)] to obtain the monoacylated product. The desired diacylated
compound was recovered by using the solvent mixture CH2Cl2/
ethyl acetate (10:1). A final column (silica, CHCl3) afforded a purple
powder (128 mg, 31%) of comparable purity to that of the 94%
pure material obtained from route A.
Dyad ZnFb-12. Following a common procedure,51 reduction of
11 (57 mg, 0.13 mmol) with NaBH4 (95 mg, 2.5 mmol) in
anhydrous THF/methanol (10 mL, 10:1) and standard workup gave
the dipyrromethane-dicarbinol 11-diol as a yellow foam. The latter
was then condensed with porphyrin-dipyrromethane Zn-8 (136
mg, 0.130 mmol) in CH2Cl2 (50 mL) containing Yb(OTf)3 (100
mg, 0.160 mmol) for 50 min followed by oxidation with DDQ (85
mg, 0.38 mmol). Filtration through a pad of silica and removal of
m/e 1086.5266 (C72H74N6Zn); λabs 424, 551, 592 nm; λem (λex
)
551 nm) 602, 649 nm.
[5,15-Bis(3,5-di-tert-butylphenyl)-10-[4-(dipyrromethan-5-yl)-
phenyl]-20-mesitylporphinato]magnesium(II) (Mg-8). Following
a general procedure,58 a solution of “ethereal MgI2-DIEA” reagent
(28 mL) was added via syringe to a stirred solution of porphyrin 8
(234 mg, 0.230 mmol) in CH2Cl2 (75 mL) under argon. After 10
min, TLC and UV-visible absorption spectral analysis showed
complete metalation. Workup by extraction with CHCl3, washing
the organic phase with brine, and purification by column chroma-
tography (alumina, CHCl3) afforded a purple solid (189 mg,
79%): 1H NMR δ 1.55 (s, 36H), 1.87 (s, 6H), 2.63 (s, 3H), 5.81
(s, 1H), 6.17-6.19 (m, 2H), 6.28-6.30 (m, 2H), 6.83-6.85 (m,
2H), 7.27-7.28 (m, 2H), 7.57 (d, J ) 7.6 Hz, 2H), 7.77-7.78 (m,
2H), 8.13 (s, 4H), 8.19-8.21 (m, 4H), 8.72 (d, J ) 4.4 Hz, 2H),
8.88-8.93 (m, 6H); LD-MS obsd m/e 1044.6, FAB-MS obsd m/e
1046.5857, calcd m/e 1046.5825 (C72H74N6Mg); λabs 408 (sh), 430,
566, 607 nm; λem (λex ) 566 nm) 612, 666 nm.
[5,15-Bis(3,5-di-tert-butylphenyl)-10-[4-(1,9-bis(4-methylben-
zoyl)dipyrromethan-5-yl)phenyl]-20-mesitylporphinato]zinc-
(II) (Zn-9). Route A. Following a dialkylation procedure,59,61
a
Inorganic Chemistry, Vol. 42, No. 14, 2003 4335

Speckbacher et al.
the solvent gave a purple solid (44 mg, 23%): 1H NMR δ -2.61
(s, 2H), 1.58 (s, 36H), 1.91 (s, 6H), 2.65 (s, 3H), 2.75 (s, 6H), 7.25
(s, 2H), 7.31 (s, 2H), 7.61 (d, J ) 7.2 Hz, 4H), 7.76-7.80 (m,
3H), 7.84-7.85 (m, 2H), 8.18-8.20 (m, 4H), 8.26 (d, J ) 7.2 Hz,
4H), 8.60-8.65 (m, 4H), 8.84 (d, J ) 4.4 Hz, 2H), 8.87 (d, J )
4.8 Hz, 2H), 8.93 (d, J ) 4.8 Hz, 2H), 9.02 (d, J ) 4.4 Hz, 2H),
9.06 (d, J ) 4.4 Hz, 2H), 9.18 (d, J ) 4.4 Hz, 2H), 9.31 (d, J )
4.4 Hz, 2H), 9.40 (d, J ) 4.4 Hz, 2H); LD-MS obsd m/e 1509.6,
FAB-MS obsd m/e 1506.6750, calcd m/e 1506.6893 (C103H94N8-
Zn); λabs 419, 428, 516, 551, 590, 647 nm; λem (λex ) 551 nm)
654, 717 nm.
Dyad ZnFb-12 via Zn-9-diol. Following a standard procedure,51
Zn-9 (102 mg, 0.0800 mmol) was reacted with NaBH4 (58 mg,
1.5 mmol) in dry THF/methanol (6.2 mL, 10:1) for 90 min. After
reduction was complete, the reaction was quenched with aqueous
NH4Cl (40 mL). CH2Cl2 was added, and the organic phase was
collected, washed with brine, and dried (K2CO3). Removal of the
solvent afforded the porphyrin-dipyrromethane-dicarbinol Zn-
9-diol as a purple solid. The latter was condensed with 13 (17 mg,
0.080 mmol) in CH2Cl2 (31 mL) containing Yb(OTf)3 (62 mg, 0.10
mmol) for 50 min followed by oxidation with DDQ (52 mg, 0.23
mmol). Purification by column chromatography (silica, CH2Cl2)
and precipitation with methanol gave a purple solid (25 mg, 22%):
the analytical data were consistent with those from the previous
experiment.
8.90-8.92 (m, 4H), 9.07-9.08 (m, 8H), 9.32-9.33 (m, 8H); LD-
MS obsd m/e 2375.4, calcd average mass m/e 2371.1 (C160H154N12-
Zn); λabs 419, 435, 518, 553, 593, 648 nm; λem (λex ) 553 nm)
654, 717 nm.
Triad ZnFbMg-14. Following a standard procedure,51 Zn-9 (165
mg, 0.124 mmol) was reacted with NaBH4 (233 mg, 6.20 mmol)
in dry THF/methanol (9.9 mL, 10:1) for 90 min. After reduction
was complete, the reaction was quenched with aqueous NH4Cl (60
mL). CH2Cl2 was added, and the organic phase was collected,
washed with brine, and dried (K2CO3). Removal of the solvent
afforded the porphyrin-dipyrromethane-dicarbinol Zn-9-diol as
a purple solid. The latter was condensed with Mg-8 (130 mg, 0.124
mmol) in CH2Cl2 (50 mL) containing Yb(OTf)3 (99 mg, 0.16 mmol)
for 50 min. Then pyridine (4 mL) was added followed by DDQ
(83 mg, 0.37 mmol). Purification by column chromatography
(Al2O3, CH2Cl2) and precipitation with methanol gave a purple solid
(65 mg, 23%): 1H NMR (CDCl3/THF-d8) δ -2.42 (s, 2H), 1.62
(s, 72H), 1.93 (s, 6H), 1.95 (s, 6H), 2.67 (s, 6H), 2.81 (s, overlapped
with water signal of THF, 6H), 7.32 (s, 4H), 7.69 (d, J ) 8.0 Hz,
4H), 7.85-7.87 (m, 4H), 8.18-8.21 (m, overlapped with signal of
pyridine ligand, 8H), 8.29 (d, J ) 8.0 Hz, 4H), 8.66-8.72 (m, 8H),
8.74 (d, J ) 4.4 Hz, 2H), 8.78 (d, J ) 4.4 Hz, 2H), 8.92 (d, J )
4.4 Hz, 2H), 8.96 (d, J ) 4.4 Hz, 2H), 9.08 (d, J ) 4.4 Hz, 2H),
9.12-9.15 (m, 6H), 9.34 (d, J ) 4.4 Hz, 2H), 9.38-9.41 (m, 6H);
LD-MS obsd m/e 2333.8, calcd average mass m/e 2331.2 (C160H154
-
Dyad MgFb-12. Following a common procedure,51 reduction
of 11 (82 mg, 0.18 mmol) with NaBH4 (137 mg, 3.60 mmol) in
anhydrous THF/methanol (15 mL, 10:1) and standard workup gave
the dipyrromethane-dicarbinol 11-diol as a yellow foam. The latter
was then condensed with porphyrin-dipyrromethane Mg-8 (189
mg, 0.180 mmol) in CH2Cl2 (72 mL) containing Yb(OTf)3 (144
mg, 0.230 mmol) for 50 min. Then pyridine (5 mL) was added
followed by DDQ (122 mg, 0.540 mmol). Purification by column
chromatography (alumina, CH2Cl2) afforded the desired dyad in
the first fraction, which moved as a tight band close to the solvent
front. Precipitation with methanol gave a purple solid (75 mg, 28%).
A second preparation at smaller scale (0.079 mmol of Mg-8)
afforded a 27% yield (31 mg): 1H NMR (CDCl3/THF-d8) δ -2.61
(s, 2H), 1.58 (s, 36H), 1.92 (s, 6H), 2.64 (s, 3H), 2.75 (s, overlapped
with water signal of THF, 6H), 7.30 (s, 2H), 7.45-7.46 (m, 2H),
7.60 (d, J ) 6.4 Hz, 4H), 7.79-7.84 (m, 5H), 8.17 (s, 4H), 8.25
(d, J ) 6.4 Hz, 4H), 8.59-8.65 (m, 4H), 8.75-8.77 (m, 2H), 8.87-
8.95 (m, 6H), 9.06-9.10 (m, 4H), 9.32-9.33 (m, 4H); LD-MS
obsd m/e 1465.4, FAB-MS obsd m/e 1466.7490, calcd m/e
1466.7452 (C103H94N8Mg); λabs 420, 431, 517, 565, 606, 647 nm;
λem (λex ) 566 nm) 653, 718 nm.
Triad ZnFbZn-14. Following a standard procedure,51 Zn-9 (155
mg, 0.117 mmol) was reacted with NaBH4 (220 mg, 5.85 mmol)
in dry THF/methanol (9.4 mL, 10:1). After reduction was complete,
the reaction was quenched with aqueous NH4Cl (50 mL). CH2Cl2
was added, and the organic phase was collected, washed with brine,
and dried (K2CO3). Removal of the solvent afforded the porphyrin-
dipyrromethane-dicarbinol Zn-9-diol as a purple solid. The latter
was condensed with Zn-8 (127 mg, 0.117 mmol) in CH2Cl2 (47
mL) containing Yb(OTf)3 (94 mg, 0.15 mmol) for 50 min followed
by oxidation with DDQ (79 mg, 0.35 mmol). Purification by column
chromatography (silica, CH2Cl2) afforded one main fraction, which
moved as a tight band close to the solvent front. Precipitation with
methanol gave a purple solid (56 mg, 21%): 1H NMR (CDCl3/
THF-d8) δ -2.47 (s, 2H), 1.56 (s, 72H), 1.89 (s, 12H), 2.62 (s,
6H), 2.76 (s, overlapped with water signal of THF, 6H), 7.45-
7.46 (m, 4H), 7.64 (d, J ) 6.4 Hz, 4H), 7.81 (s, 4H), 8.15 (s, 8H),
8.23 (d, J ) 6.4 Hz, 4H), 8.61-8.65 (m, 8H), 8.72-8.74 (m, 4H),
MgN12Zn); λabs 421, 436, 519, 558, 605, 649 nm; λem (λex ) 558
nm) 654, 717 nm.
Triad ZnFbFb-14. Following a standard procedure,51 Zn-9 (123
mg, 0.093 mmol) was reacted with NaBH4 (175 mg, 4.50 mmol)
in dry THF/methanol (7.5 mL, 10:1) for 90 min. After reduction
was complete, the reaction was quenched with aqueous NH4Cl (40
mL). CH2Cl2 was added, and the organic phase was collected,
washed with brine, and dried (K2CO3). Removal of the solvent
afforded the porphyrin-dipyrromethane-dicarbinol Zn-9-diol as
a purple solid. The latter was condensed with 8 (95 mg, 0.093
mmol) in CH2Cl2 (37 mL) containing Yb(OTf)3 (74 mg, 0.12 mmol)
for 50 min followed by oxidation with DDQ (62 mg, 0.27 mmol).
Purification by column chromatography (silica, CH2Cl2) afforded
one main fraction, which moved as a tight band close to the solvent
front, followed by another, streaking fraction. The first band was
isolated. Precipitation with methanol gave a purple solid (38 mg,
18%): 1H NMR (CDCl3) δ -2.47 (br s, 2H), -2.46 (br s, 2H),
1.59 (s, 72H), 1.91-1.92 (m, 12H), 2.65 (s, 6H), 2.79 (s, 6H), 7.32
(s, 4H), 7.67 (d, J ) 8.0 Hz, 4H), 7.85-7.86 (m, 4H), 8.19-8.21
(m, 8H), 8.27 (d, J ) 8.0 Hz, 4H), 8.66 (s, 8H), 8.76 (d, J ) 4.4
Hz, 2H), 8.85 (d, J ) 5.2 Hz, 2H), 8.93 (d, J ) 4.8 Hz, 2H), 9.03
(d, J ) 4.8 Hz, 2H), 9.09 (d, J ) 4.8 Hz, 2H), 9.13 (d, J ) 4.4 Hz,
4H), 9.20 (d, J ) 4.4 Hz, 2H), 9.31-9.38 (m, 6H), 9.43 (d, J )
5.2 Hz, 2H); LD-MS obsd m/e 2304.5, calcd average mass m/e
2309.2 (C160H156N12Zn); λabs 420, 434, 518, 554, 592, 649 nm; λem
(λex ) 554 nm) 654, 719 nm.
Triad ZnFbFb-14 via Selective Demetalation of ZnFbMg-
14. A sample of ZnFbMg-14 (0.040 g, 0.017 mmol) was dissolved
in 50 mL of CH2Cl2. Then silica gel (5 g) was added and the
resulting mixture was stirred overnight at room temperature. The
silica gel was removed by filtration. TLC analysis showed quantita-
tive transformation of the Mg chelate to the Fb porphyrin. No further
purification was necessary. The solvent was removed, and the
desired triad ZnFbFb-14 was obtained as a purple solid (36 mg,
92%): the analytical data were consistent with those from the
experiment described above.
(meso-Tetrapentylporphinato)zinc(II) (ZnTPnP). A sample of
H2TPnP (236 mg, 0.40 mmol) in CHCl3 (30 mL) was treated with
4336 Inorganic Chemistry, Vol. 42, No. 14, 2003

Mixed-Metal Multiporphyrin Arrays
a solution of Zn(OAc)2‚2H2O (439 mg, 2.0 mmol) in MeOH (2
mL) overnight at room temperature. Standard workup and chro-
matography (silica, CH2Cl2) afforded a purple solid (241 mg,
92%): 1H NMR δ 1.01 (t, J ) 7.6 Hz, 12H), 1.54-1.59 (m, 8H),
1.79-1.82 (m, 8H), 2.46 (br s, 8H), 4.71 (br s, 8H), 9.28 (s, 8H);
LD-MS obsd m/e 651.1, FAB-MS obsd m/e 652.3508, calcd m/e
652.3483 (C40H52N4Zn); λabs (toluene) 424, 558, 597 nm; λem (λex
) 558 nm, toluene) 602, 657 nm.
612.4042 (C40H52N4Mg); λabs (toluene) 408, 428, 575, 615 nm; λem
(λex ) 575 nm, toluene) 618, 678 nm.
Acknowledgment. This research was supported by the
NIH (Grant GM36238). Mass spectra were obtained at the
Mass Spectrometry Laboratory for Biotechnology. Partial
funding for the Facility was obtained from the North Carolina
Biotechnology Center and the National Science Foundation.
(meso-Tetrapentylporphinato)magnesium(II) (MgTPnP). Fol-
lowing a published procedure,57 a sample of H2TPnP (236 mg, 0.40
mmol) in CHCl3 (30 mL, stabilized with amylenes) containing
DIEA (1.39 mL, 8.0 mmol) was treated with MgI2 (1.11 g, 4.0
mmol) for 10 min at room temperature. Standard workup and
chromatography (alumina, CH2Cl2) afforded a purple solid (218
mg, 89%): 1H NMR δ 1.03 (t, J ) 7.6 Hz, 12H), 1.61 (br s, 8H),
1.87 (br s, 8H), 2.58 (br s, 8H), 4.99 (br s, 8H), 9.50 (s, 8H); LD-
MS obsd m/e 610.9, FAB-MS obsd m/e 612.4060, calcd m/e
Supporting Information Available: Description of the met-
alloporphyrin acid-stability experiments, absorption spectra of H2-
1
TPP upon exposure to various Lewis acids in CH2Cl2, H NMR
and LD-MS (or MALDI-MS) of all new compounds, and analytical
SEC traces for all arrays. This material is available free of charge
via the Internet at http://pubs.acs.org.
IC026206D
Inorganic Chemistry, Vol. 42, No. 14, 2003 4337