Structurally homologous β- and meso-amidinium porphyrins

Article abstract of DOI:10.1021/ic001387+

A synthetic strategy has been developed to afford porphyrins site-derivatized with the same hydrogen-bond synthon attached directly to the macrocyclic ring. Porphyrin homologues derivatized at the β and meso positions with an amidinium group form 1:1 supramolecular complexes with benzoate acceptors and show notable differences in their excited-state properties that are dependent on the site of the salt bridge.

Full text of DOI:10.1021/ic001387+

Inorg. Chem. 2001, 40, 3643-3646
3643
construction of extended supramolecular structures.14 Despite
these advances, the utility of the directional, two-point hydrogen
bond that amidinium provides to porphyrin supramolecular
design has been heretofore limited because of the difficulty
attendant to appending the hydrogen-bond synthon directly to
the porphyrin macrocycle. Indeed, the only previous synthesis
of a porphyrin bearing an amidinium directly appended to a
porphyrin ring was elaborate and arduous.15 Our interest in
generalizing amidinium porphyrins for a variety of applications
has led us to explore new strategies to affix amidiniums to
porphyrin macrocycles. We now report a modular and facile
methodology for the synthesis of a porphyrin modified with
amidinium in the â position. In addition, we have extended this
methodology to prepare the first example of a porphyrin bearing
an amidinium group directly attached at the meso position, thus
providing a homologous pair of porphyrin building blocks with
electronically altered properties. We also describe the propensity
of the structurally homologous â- and meso-amidinium por-
phyrins to form salt bridge complexes and the effects of salt
bridge association on the excited-state properties of these novel
architectures.
Structurally Homologous â- and meso-Amidinium
Porphyrins
Chen-Yu Yeh, Scott E. Miller, Scott D. Carpenter, and
Daniel G. Nocera*
Department of Chemistry, 6-335, Massachusetts Institute of
Technology, 77 Massachusetts Avenue,
Cambridge, Massachusetts 02139
ReceiVed December 7, 2000
Introduction
Porphyrins bearing hydrogen-bond synthons provide an
efficient method for the assembly of supramolecular architec-
tures. Such systems underlie efforts directed at unraveling the
effect of hydrogen bonds on energy and electron transfer.1,2
Porphyrins featuring directional and complementary hydrogen-
bond functionality are especially pertinent to energy and
electron-transfer investigations because the donor-acceptor (D-
A) pair is formed exclusive of homodimers (i.e., D-D and A-A
pairs).3 A rigid electron/energy transfer pathway, which pos-
sesses maximum electronic communication, is created when the
hydrogen-bond synthon can be directly attached to the porphyrin
macrocycle. We have exploited these basic design elements in
the development of hydrogen-bonded networks for the study
of proton-coupled electron transfer.1,4,5 In particular, our work
has focused on porphyrins modified with the amidinium
group,6-9 which is able to associate with carboxylate to form a
salt bridge. This resultant amidinium-carboxylate interface
models the arginine-aspartate salt bridge, an important stabiliz-
ing structural element in many biological systems.10-13 The
amidinium-carboxylate salt bridge combines the dipole of an
electrostatic ion-pair interaction with a hydrogen-bonding scaf-
fold, thus allowing us to investigate how proton motion within
a hydrogen-bond interface affects the charge, energetics, and
polarity of an electron transport chain. An amidinium-car-
boxylate structural motif is not only convenient for the assembly
of donor-acceptor pairs but may also be employed in the
Experimental Section
Materials. Silica gel 60 (70-230 and 230-400 mesh, Merck) and
Merck 60 F254 silica gel (precoated sheets, 0.2 mm thick) were used
for column and analytical thin-layer chromatography, respectively.
Solvents for synthesis were of reagent grade or better and were dried
according to standard methods.16 Spectroscopic experiments employed
dichloromethane (spectroscopic grade, Burdick & Jackson). Mass
spectral analyses were performed in the MIT Department of Chemistry
Instrumentation Facility (DCIF). Nickel(II) 5,10,15,20-tetramesitylpor-
phyrin (1) and nickel(II) 5,10,15-trimesitylporphyrin (8) were prepared
using published procedures.17,18
Nickel(II) 2-Formyl-5,10,15,20-tetramesitylporphyrin (2). A sus-
pension of 1 (420 mg, 0.5 mmol) in 300 mL of 1,2-dichloroethane
was added over a period of 10 min under a nitrogen atmosphere to a
warm solution (50-60 °C) of Vilsmeier reagent, prepared from
dimethylformamide (DMF) (19.4 mL, 250 mmol) and POCl3 (23.3 mL,
250 mmol). The solution was stirred at 70 °C under nitrogen for 3 h
and quenched with aqueous Na2CO3; the resulting solution was stirred
in air overnight. The organic layer was decanted, washed with water,
and dried over Na2SO4, and the solvent was removed. Purification by
column chromatography (silica gel, 1:1 CH2Cl2/hexanes) afforded
porphyrin 2 (335 mg, 77% yield). 1H NMR (300 MHz, CDCl3, 25
°C): δ ) 9.33 (s, 1H), 9.20 (s, 1H), 8.58-8.47 (m, 6H), 7.24 (s, 6H),
7.21 (s, 2H), 2.60 (s, 6H), 2.59 (s, 6H), 1.88 (s, 12H), 1.87 (s, 6H),
1.84 (s, 6H). ESIMS [M + H]+, m/z: calcd for C57H52N4NiO, 867.74.
Found, 867.35.
(1) Chang, C. J.; Brown, J. D. K.; Chang, M. C. Y.; Baker, E. A.; Nocera,
D. G. In Electron Transfer in Chemistry; Balzani, V., Ed.; Wiley-
VCH: Weinheim: Germany; Volume 3.2.4, pp 409-461.
(2) Sessler, J. L.; Wang, B. Springs, S. L. Brown, C. T. In ComprehensiVe
Supramolecular Chemistry, Vol. 4; Murakami, Y., Ed.; Pergamon:
Oxford 1997; Chapter 9.
(3) Turro´, C.; Chang, C. K.; Leroi, G. E.; Cukier, R. I.; Nocera, D. G. J.
Am. Chem. Soc. 1992, 114, 4013-4015.
(4) Cukier, R. I.; Nocera, D. G. Annu. ReV. Phys. Chem. 1998, 49, 337-
369.
2-Cyano-5,10,15,20-tetramesitylporphyrin (4). The diacid of free-
base 3 was obtained by using 15% H2SO4 in trifluoroacetic acid (TFA)
(30 mL) to demetalate 2 (87 mg, 0.1 mmol). Without further purification
of the diacid, hydroxylamine hydrochloride (34 mg, 0.5 mmol) was
added to a mixture of porphyrin 3 in TFA (1 mL) and formic acid (30
mL). The resulting solution was refluxed for 24 h under nitrogen and
then concentrated under reduced pressure. The residue was taken up
in CH2Cl2, washed with saturated aqueous NaHCO3 and water, dried
over Na2SO4, and taken to dryness. Column chromatography (silica
(5) Zaleski, J. M.; Turro´, C.; Mussell, R. D.; Nocera, D. G. Coord. Chem.
ReV. 1994, 132, 249-258.
(6) Roberts, J. A.; Kirby, J. P.; Nocera, D. G. J. Am. Chem. Soc. 1995,
117, 8051-8052.
(7) Kirby, J. P.; Dantzig, N. A.; Chang, C. K.; Nocera, D. G. Tetrahedron
Lett. 1995, 36, 3477-3480.
(8) Roberts, J. A.; Kirby, J. P.; Wall, S. T.; Nocera, D. G. Inorg. Chim.
Acta 1997, 263, 395-405.
(9) Kirby, J. P.; Roberts, J. A.; Nocera, D. G. J. Am. Chem. Soc. 1997,
119, 9230-9236.
(14) Papoutsakis, D.; Kirby, J. P.; Jackson, J. E.; Nocera, D. G. Chem.
Eur. J. 1999, 5, 1474-1480.
(10) Crane, B. R.; Siegel, L. M.; Getzoff, E. D. Science 1995, 270, 59-
(15) Deng, Y.; Roberts, J. A.; Peng, S. M.; Chang, C. K.; Nocera, D. G.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2124-2127.
(16) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinmann: Oxford, 1996.
(17) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 54, 828-836.
(18) Shultz, D. A.; Gwaltney, K. P.; Lee, H. J. Org. Chem. 1998, 63, 4034-
4038.
67.
(11) Howell, E. H.; Villafranca, J. E.; Warren, M. S.; Oatley, S. J.; Kraut,
J. Science 1986, 231, 1123-1128.
(12) Ramirez, B. E.; Malmstro¨m, B. G.; Winkler, J. R.; Gray, H. B. Proc.
Natl. Acad. Sci. U.S.A. 1995, 92, 11949-11951.
(13) Brzezinski, P. Biochemistry 1996, 35, 5611-5615.
10.1021/ic001387+ CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/19/2001

3644 Inorganic Chemistry, Vol. 40, No. 14, 2001
Notes
gel, 1:1 CH2Cl2/hexanes) afforded porphyrin 4 (66 mg, 82% yield). 1H
NMR (300 MHz, CDCl3, 25 °C): δ ) 9.26 (s, 1H), 8.79 (d, J ) 5.4
Hz, 2H), 8.72 (d, J ) 5.4 Hz, 2H), 8.57 (s, 2H), 7.35 (s, 2H), 7.31 (s,
6H), 2.68 (s, 3H), 2.65 (s, 9H), 1.89 (s, 12H), 1.88 (s, 6H), 1.86 (s,
6H). ESIMS [M + H]+, m/z: calcd for C57H53N5, 808.06. Found,
808.43. Nickel(II) was inserted into porphyrin 4 using standard
1.86 (s, 12H), 1.85 (s, 6H). ESIMS [M + H]+, m/z: calcd for C48H42N4-
NiO, 749.57. Found, 749.27.
Nickel(II) 5-Cyano-10,15,20-trimesitylporphyrin (12). Nickel(II)
porphyrin 9 (69 mg, 0.1 mmol) was demetalated by TFA (20 mL)
containing 15% H2SO4 to give the diacid of free-base 10. The diacid,
which was not further purified, was dissolved in a mixture of formic
acid (20 mL) and TFA (0.5 mL); hydroxylamine hydrochloride (34
mg, 0.5 mmol) was added to this solution. The mixture was refluxed
for 24 h under a nitrogen atmosphere and concentrated under reduced
pressure. The residue was taken up in CH2Cl2, washed with saturated
aqueous NaHCO3 and then water, dried over Na2SO4, and taken to
dryness. Column chromatography (silica gel, 1:1 CH2Cl2/hexanes) gave
pure 11 (54 mg, 75% yield). Nickel(II) was inserted using standard
methods to yield 12.19 1H NMR (300 MHz, CDCl3, 25 °C): δ ) 9.44
(d, J ) 4.8 Hz, 2H), 8.78 (d, J ) 4.8 Hz, 2H), 8.60 (d, J ) 4.8 Hz,
2H), 8.57 (d, J ) 4.8 Hz, 2H), 7.27 (s, 4H), 7.23 (s, 2H), 2.62 (s, 6H),
2.60 (s, 3H), 1.85 (s, 18H). ESIMS [M + H]+, m/z: calcd for C48H41N5-
Ni, 746.57. Found, 746.26.
1
methods19 to yield 5. H NMR (300 MHz, CDCl3, 25 °C): δ ) 9.08
(s, 1H), 8.60-8.48 (m, 6H), 7.25 (s, 2H), 7.21 (s, 6H), 2.60 (s, 3H),
2.58 (s, 9H), 1.84 (s, 18H), 1.82 (s, 6H). ESIMS [M + H]+, m/z: calcd
for C57H51N5Ni, 864.74. Found, 864.05.
Nickel(II) 2-Amidinium-5,10,15,20-tetramesitylporphyrin Chlo-
ride (6). To a toluene solution (300 mL) of nickel(II) porphyrin 5 (400
mg, 0.462 mmol) was added chloromethylaluminum amide (19.3 mL,
1.2 M in toluene) under a nitrogen atmosphere. The reaction solution
was heated at 90 °C under nitrogen for 7 d. The resulting mixture was
cooled to room temperature, poured slowly onto silica gel (80 g) in
CHCl3 (200 mL), and stirred for 10 min. The slurry was filtered, and
the filtercake that was collected was washed with a mixture of CHCl3
and methanol until the filtrate was colorless. The solvent was removed.
Purification by column chromatography (silica gel, 10:1 CHCl3/CH3-
OH) followed by recrystallization from CH2Cl2 and methanol gave
amidinium porphyrin 6 (288 mg, 68% yield). 1H NMR (300 MHz, 3%
CD3OD in CDCl3, 25 °C): δ ) 8.78 (s, 1H), 8.54-8.42 (m, 6H), 7.18
(s, 6H), 7.14 (s, 2H), 2.54 (s, 12H), 1.80 (s, 12H), 1.79 (s, 6H), 1.78
(s, 6H). HRFABMS (M+), m/z: calcd for C48H45N6Zn, 881.3842.
Found, 881.3863.
Nickel(II) 5-Amidinium-10,15,20-trimesitylporphyrin Chloride
(13). To a solution of 12 (69 mg, 0.1 mmol) in toluene (50 mL) was
added chloromethylaluminum amide (5.0 mL, 1.0 M in toluene) under
a nitrogen atmosphere. The resulting solution was heated at 80 °C under
nitrogen for 7 d. The mixture was cooled to room temperature, poured
slowly into silica gel (20 g) in CHCl3 (50 mL), and stirred for 10 min.
The slurry was filtered, and the resulting filtercake was washed with a
mixture of CHCl3 and methanol until the filtrate was no longer colored.
The solvent was removed, and purification by column chromatography
(silica gel, 10:1 CHCl3/CH3OH) followed by recrystallization from
dichloromethane and methanol afforded amidinium porphyrin 13 (56
Zinc(II) 2-Amidinium-5,10,15,20-tetramesitylporphyrin Chloride
(7). Concentrated H2SO4 (5 mL) was added to 6 (92 mg, 0.1 mmol).
The mixture was stirred at room temperature for 5 min, and ice was
then added. The suspension was neutralized with solid Na2CO3 and
filtered to give the crude free base porphyrin, which was not further
purified. ZnCl2 (136 mg, 1.0 mmol) was added to a solution of the
free-base porphyrin in DMF (20 mL). The mixture was heated at 60
°C for 30 min and concentrated under vacuum, and water was added.
The resulting precipitate was collected by filtration and washed with
water. The solid, which was purified by column chromatography (silica
gel, 10:1 CHCl3/CH3OH), was dissolved in 20% methanol in CHCl3.
The solution was cooled to 0 °C after which 5 mL of a 0.1 N methanolic
KOH solution was added. The reaction was stirred at 0 °C for 5 min
and concentrated under reduced pressure; addition of 10 mL of a 1 N
aqueous KOH solution resulted in the formation of a suspension. The
solid collected from filtration was taken up in 20% methanol in CHCl3
(20 mL) and added to 30 mL of a 1 N aqueous NaCl solution containing
HCl (adjusted to pH 1-2). The organic solvent was removed, and the
mixture was filtered and washed with water. Purification by preparative
TLC (silica gel, 10:1 CHCl3/CH3OH) afforded 7 (37 mg, 40% yield).
1H NMR (300 MHz, 5:1 CD3OD/CDCl3, 25 °C): δ ) 8.80 (s, 1H),
8.60-8.50 (m, 6H), 7.22 (s, 6H), 7.15 (s, 2H), 2.58 (s, 12H), 1.82 (s,
6H), 1.81 (s, 6H), 1.78 (s, 12H). HRFABMS (M+), m/z: calcd for
C48H45N6Zn, 887.3780. Found, 887.3752. The hexafluorophosphate salt
of 7 was prepared from metathetical exchange with TlPF6 in methanol.
1H NMR (300 MHz, 5:1 CD3OD/CDCl3, 25 °C): δ ) 8.78 (s, 1H),
8.54-8.42 (m, 6H), 7.16 (s, 6H), 7.09 (s, 2H), 2.52 (s, 9H), 2.50 (s,
3H), 1.77 (s, 6H), 1.76 (s, 6H), 1.74 (s, 6H), 1.73 (s, 6H).
1
mg, 70% yield). H NMR (300 MHz, CDCl3, 25 °C): δ ) 9.21 (d, J
) 4.8 Hz, 2H), 8.77 (d, J ) 4.8 Hz, 2H), 8.61 (d, J ) 4.8 Hz, 2H),
8.58 (d, J ) 4.8 Hz, 2H), 7.22 (s, 6H), 2.58 (s, 9H), 1.81 (s, 6H), 1.79
(s, 12H). HRFABMS (M+), m/z: calcd for C48H45N6Zn, 763.3059.
Found, 763.3079.
Zinc(II) 5-Amidinium-10,15,20-trimesitylporphyrin Chloride (14).
Concentrated H2SO4 (5 mL) was added to 13 (80 mg, 0.1 mmol). After
stirring the mixture at room temperature for 5 min, ice was added. The
suspension was neutralized with solid Na2CO3 and filtered to give the
crude free-base porphyrin. Without further purification, the free-base
porphyrin was dissolved in DMF (20 mL) and ZnCl2 (136 mg, 1.0
mmol) was added. The mixture was heated at 60 °C for 30 min and
then concentrated under vacuum. The precipitate that was formed upon
water addition to the concentrate was collected by filtration and washed
with water. The solid was purified by column chromatography (silica
gel, 10:1 CHCl3/CH3OH), reisolated, and dissolved in 20% methanol
in CHCl3. The solution was cooled to 0 °C to which 5 mL of a 0.1 N
methanolic KOH solution was added. The reaction was stirred at 0 °C
for 5 min and concentrated under reduced pressure, and an additional
10 mL of a 1 N aqueous KOH solution was added to the concentrate
to produce a suspension, which was filtered. The solid was taken up in
20% methanol in CHCl3 (20 mL) and added to 30 mL of a 1 N aqueous
NaCl solution containing HCl (adjusted to pH 1-2). The organic solvent
was removed, and the mixture was then filtered and washed with water.
Purification by preparative TLC (silica gel, 10:1 CHCl3/CH3OH)
1
afforded 14 (28 mg, 35% yield). H NMR (300 MHz, 1:20 CD3OD/
Nickel(II) 5-Formyl-10,15,20-trimesitylporphyrin (9). Porphyrin
8 (400 mg, 0.69 mmol), which was suspended in 1,2-dichloroethane
(100 mL), was added over a 10 min period under a nitrogen atmosphere
to a warm solution (50-60 °C) of a Vilsmeier reagent, prepared from
DMF (2.7 mL, 34.5 mmol) and POCl3 (2.7 mL, 34.5 mmol). The
solution was stirred at 70 °C under nitrogen for 3 h, quenched with
aqueous NaOAc, and stirred in air overnight. The organic layer was
separated, washed with water, and dried over Na2SO4, and the solvent
layer was removed. Purification by column chromatography (silica gel,
CDCl3, 25 °C): δ ) 9.26 (br s, 2H), 8.80 (br s, 2H), 8.64 (d, J ) 4.8
Hz, 2H), 8.60 (d, J ) 4.8 Hz, 2H), 7.25 (s, 4H), 7.23 (s, 4H), 2.60 (s,
6H), 2.59 (s, 3H), 1.81 (s, 6H), 1.78 (s, 12H). HRFABMS (M+), m/z:
calcd for C48H45N6Zn, 769.2997. Found, 769.3240.
Physical Measurements. 1H NMR spectra were collected at the MIT
DCIF on either a Varian XL-300, Unity 300, or Mercury 300
spectrometer. Spectra were recorded at 25 °C. All chemical shifts are
reported using the standard δ notation in parts-per-million; positive
chemical shifts are to higher frequency from the given reference.
Absorption spectra were obtained using either a Cary-17 spectropho-
tometer modified by On-Line Instrument Systems (OLIS) to include
computer control or a Spectral Instruments 440 Series spectrophotom-
eter.
1
1:1 CH2Cl2/hexanes) afforded porphyrin 9 (378 mg, 73% yield). H
NMR (300 MHz, CDCl3, 25 °C): δ ) 12.10 (s, 1H), 9.80 (d, J ) 5.4
Hz, 2H), 8.69 (d, J ) 5.4 Hz, 2H), 8.46 (d, J ) 5.4 Hz, 2H), 8.41 (d,
J ) 5.4 Hz, 2H), 7.22 (s, 4H), 7.18 (s, 2H), 2.59 (s, 6H), 2.56 (s, 3H),
Luminescence lifetime measurements were obtained with a Hamamat-
su C4334-0 Streak Camera, which was the detector on a previously
(19) Adler, A. D.; Longo, F. R.; Kampas, F.; Kim, J. J. Inorg. Nucl. Chem.
1970, 32, 2443-2445.

Notes
Inorganic Chemistry, Vol. 40, No. 14, 2001 3645
Scheme 1. Synthetic Routes for â- and meso-Amidinium
Porphyrinsa
1
Figure 1. Selected H NMR spectra of 13 (2.34 mM) in the absence
of and in the presence of 0, 1.08, 2.50, 5.29, 11.05, 26.02, and 56.84
mM (bottom to top) tetrabutylammonium benzoate in DMSO-d6. The
spectral range captures the amidinium protons internal (NHax) and
external (NHeq) to the salt bridge interface. The arrow identifies the
1H resonances of the â-pyrrole protons.
insertion affords the amidinium porphyrin 7 after workup. The
synthesis of the â-amidinium porphyrin 7 from 1 in just six
steps represents a significant improvement over our previous
synthesis of a â-amidinium porphyrin, which encompassed an
arduous 26-step procedure.15 The modular nature of the approach
reported herein is highlighted by our successful preparation of
the first porphyrin with an amidinium group directly attached
to a meso position of the porphyrin superstructure, the analogous
meso-amidinium porphyrin 14, from nickel(II) 5,10,15-tri-
mesitylporphyrin (8). To the best of our knowledge, 7 and 14
represent the first homologous porphyrin pair that features a
hydrogen-bond donor group directly attached to the â and meso
positions of a porphyrin ring.
a (a) Vilsmeier reagent, 1,2-dichloroethane; (b) 15% H2SO4 in TFA;
(c) NH2OH‚HCl, HCO2H, and TFA (30:1); (d) Ni(OAc)2‚4H2O, DMF;
(e) AlCl(CH3)(NH2), toluene; (f) 1. H2SO4; 2. ZnCl2, DMF.
described subpicosecond laser system.20 Luminescence measurements
were performed on solutions of porphyrin at concentrations of ∼2 ×
10-5 M, giving an optical density of 0.2-0.3 at the Q10 band (λexc
)
550 nm). Samples for lifetime measurements were contained within a
cell equipped with a solvent reservoir and a 1 cm clear fused-quartz
cell (Starna Cells, Inc.). The two chambers were isolated from each
other by a high-vacuum Teflon valve and from the environment with
a second high-vacuum Teflon valve. Samples were subject to at least
three freeze-pump-thaw cycles (10-3 Torr).
The two-point hydrogen bond of the amidinium-carboxylate
interface features two favorable secondary interactions,24 sup-
ported by electrostatic stabilization of proximate and opposite
charges within the salt bridge. To assess the stability of the salt
bridge for the homologous â- and meso-amidinium porphyrin
1
systems, we undertook H NMR measurements of the nickel-
Results and Discussion
(II) forms of the porphyrins in the presence of benzoates.
Figure 1 displays the 1H NMR spectral changes resulting from
the association of meso-amidinium porphyrin 13 to 3,5-
dinitrobenzoate in d6-DMSO. Signatures of the salt bridge are
the concentration-dependent downfield shift of the amidinium
protons involved in hydrogen bonding and an insensitivity of
the chemical shift for the amidinium protons external to the
salt bridge. Such behavior has been observed previously for the
complexation of bicyclic guanidinium to carboxylate25 and
amidinium to phosphate26 and carboxylate.6-9 The singlet at
10.20 ppm, corresponding to the internal amidinium proton
(NHax), shifts downfield upon the addition of 3,5-dinitroben-
zoate, whereas the chemical shift of the external amidinium
proton (NHeq) does not change significantly during the titration
(<0.2 ppm). A nonlinear least-squares fit27 of these chemical
shift data for the hydrogen-bonded amidinium protons vs the
Amidinium functionalities were directly attached at either the
â (7) or meso (14) positions of the mesityl porphyrin macrocycle
according to the synthetic procedures outlined in Scheme 1. For
the â-amidinium system, the readily available nickel(II) 5,10,-
15,20-tetramesitylporphyrin (1) was chosen as the starting
material due to the solubilizing properties of the ancillary methyl
groups. Formylation of 1 proceeds smoothly by the Vilsmeier
reaction to deliver 2.21 Demetalation with 15% H2SO4 in TFA
and reaction with hydroxylamine hydrochloride in refluxing
formic acid containing TFA gives nitrile 4.22 After nickel
insertion, the nitrile is transformed to amidinium 6 by treatment
with an excess of Weinreb’s amide transfer reagent23 under harsh
conditions (90 °C, 7 d). It is noteworthy that the amide transfer
reaction is not observed when the zinc(II) complex or free base
of the nitrile is used. Demetalation of nickel followed by zinc
(20) Chang, C. J.; Deng, Y.; Heyduk, A. F.; Chang, C. K.; Nocera, D. G.
Inorg. Chem. 2000, 39, 959-966.
(21) Morgan, A. R.; Tertel, N. C. J. Org. Chem. 1986, 51, 1347-1350.
(22) Olah, G. A.; Keumi, T. Synthesis 1979, 112-113.
(23) Levin, J. I.; Turos, E.; Weinreb, S. M. Synth. Commun. 1982, 12,
989-993.
(24) Pranata, J.; Wierschke, S. G.; Jorgensen, W. L. J. Am. Chem. Soc.
1991, 113, 2810-2819.
(25) Mu¨ller, G.; Riede, J.; Schmidtchen, P. Angew. Chem., Int. Ed. Engl.
1988, 27, 1516-1518.
(26) Go¨bel, M. W.; Bats, J. W.; Du¨rner, G. Angew. Chem., Int. Ed. Engl.
1992, 31, 207-209.

3646 Inorganic Chemistry, Vol. 40, No. 14, 2001
Notes
carboxylate concentration yields a binding constant of KA
)
benzoate to solutions of the respective porphyrins. Whereas the
fluorescence maxima and decay rate of meso-substituted por-
phyrin 14 remain essentially unchanged upon salt-bridge forma-
tion (14‚benzoate: λem,max(Q0,0) ) 611 nm and (Q0,1) ) 670 nm,
τobs ) 2.64 ns), the photophysics of 7 are considerably perturbed
236 ( 5 M-1. As has been observed for benzamidinium-
benzoate28 and other primary two-point hydrogen-bond sys-
tems,29,30 a significantly higher binding constant (KA ) 6860
( 420 M-1) is observed when the 3,5-dinitrobenzoate is replaced
by benzoate owing to the increased basicity of the carboxylate
guest. â-Amidinium porphyrin 6 exhibits similar behavior to
its meso-amidinium counterpart and gives comparable binding
constants with benzoate (KA ) 2450 ( 130 M-1) and 3,5-
dinitrobenzoate (KA ) 161 ( 4 M-1). These results establish
the high stability of the amidinium-carboxylate interface, which
persists in solution even when the dielectric constant of the
solvent is high.
(7‚benzoate: λem,max(Q0,0) ) 604 and (Q0,1) ) 653 nm, τobs
)
2.04 ns). These results suggest that the amidinium group is more
strongly coupled to the porphyrin π-system when attached at
the â position than at the meso-position. This differential
perturbation of electronic structure by site substitution about
the porphyrin periphery has been observed for covalent net-
works;32 the results reported here show similar behavior in a
noncovalent networks, thus providing a convenient means to
tune the electronic properties of hydrogen-bonded amidinium
porphyrin assemblies.
Soret (B) and Q absorption bands for the zinc(II) amidinium
porphyrins 7 and 14 are characteristic of a standard four orbital
model for porphyrin spectra.31 The absorption spectra for the
compounds are similar. For 7: λabs,max(B) ) 429 nm (ꢀ ) 2.57
× 105 M-1 cm-1); λabs,max(Q1,0) ) 556 nm (ꢀ ) 7.76 × 103
M-1 cm-1); λabs,max(Q0,0) ) 594 nm (ꢀ ) 3.94 × 103 M-1 cm-1).
For 14: λabs,max(B) ) 424 nm (ꢀ ) 3.05 × 105 M-1 cm-1);
In summary, the bottleneck created by laborious synthesis
of porphyrin macrocycles directly appended with amidinium
has been circumvented with a facile and efficient synthesis. The
new methodology affords both â- and meso-substituted archi-
tectures. The congeners readily form supramolecular assemblies
by their association to carboxylates with high binding constants.
The â- and meso-substituted amidinium porphyrins are spec-
troscopically similar, but emission data indicate that amidinium
interacts more strongly with the porphyrin ring when attached
at the â position as opposed to the meso position. This
observation invites the possibility of investigating the role of
hydrogen-bond-mediated coupling on the rates of proton-coupled
electron transfer. More generally, comparative studies of â- and
meso-substituted amidinium porphyrins should advance our
understanding of the spectroscopic and electron/energy transfer
properties of hydrogen-bonded porphyrin supramolecules.
λ
λ
λ
abs,max(Q1,0) ) 565 nm (ꢀ ) 1.66 × 104 M-1 cm-1);
abs,max(Q0,1, sh) ) 596 nm (ꢀ ) 4.88 × 103 M-1 cm-1);
abs,max(Q0,0) ) 611 nm (ꢀ ) 5.92 × 103 M-1 cm-1). Excitation
into the absorption manifold of 7 and 14 results in intense
luminescence with features that are typical of Q(0,0) and Q(0,1)
transitions (λem,max(Q0,0) ) 611 and 615 nm; λem,max(Q0,1) ) 655
and 670 nm for 7 and 14 in CH2Cl2, respectively). Time-resolved
measurements reveal a monophasic and exponential lumines-
cence decay; lifetimes are typical of a singlet excited state
(τobs(7) ) 1.80 ns, τobs(14) ) 2.58 ns). The same luminescence
experiments were carried out on the salt-bridge complexes,
formed by the addition of 10 equiv of tetrabutylammonium
(27) Wilcox, C. S. In Frontiers in Supramolecular Organic Chemistry and
Photochemistry; Schneider, J.-J., Durr, H., Eds.; VCH: Weinheim,
1991.
(28) Krechl, J.; Smrckova, S.; Kuthan, J. Collect. Czech. Chem. Commun.
1990, 55, 460-468.
(29) Haushalter, K. A.; Lau, J.; Roberts, J. D. J. Am. Chem. Soc. 1997,
118, 88891-8896.
(30) Wilcox, C. S.; Kim, E.; Romano, D.; Kuo, L. H.; Burt, A. L.; Curran,
D. P. Tetrahedron 1995, 51, 621-634.
(31) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic
Press: New York, 1978; Vol. A, Part A, Physical Chemistry.
Acknowledgment. This work was supported by a grant from
the National Institutes of Health (GM 47274). We thank
Christopher J. Chang for helpful discussions, and Zhi-Heng Loh
for determining extinction coefficients.
IC001387+
(32) Deng, Y.; Chang, C.-K.; Nocera, D. G. Angew. Chem., Int. Ed. 2000,
39, 1066-1068.