Structurally homologous β- and meso-alkynyl amidinium porphyrins

Article abstract of DOI:10.1021/ic700838s

Alkynylamidinium groups have been introduced at the β and meso positions of a nickel(II) porphyrin (PNi(II)) framework. The modification permits the distance between the amidinium-amidine acid-base group and porphyrin to be increased while effectively maintaining π conjugation between the porphyrin macrocycle and the acid-base functionality. Use of an ethynyl spacer as a linker (i) extends the amidinium functionality away from the sterically bulky mesityl groups of the porphyrin, allowing it to be nearly planar with respect to the porphyrin ring, and (ii) draws the π-orbital character of the porphyrin out toward the amidinium functionality, thereby engendering sensitivity of the electronic properties of the porphyrin macrocycle to the protonation state of the amidinium. The barrier for rotation of the amidinium group, as calculated by time-dependent density functional theory (TDDFT), is ～8.5kT (5 kcal/mol) for both porphyrins. Analysis of UV-visible absorption profiles for the β- and meso-alkynylamidinium PNi(II) upon deprotonation enables accurate determination of the amidinium acidity constants for the ground state (pK_a(β) = 7.03 ± 0.1, pK_a(meso) = 7.74 ± 0.1 in CH₃CN) and excited state (pK_a *(β) = 6.89 ± 0.1, pK_a*(meso) = 8.37 ± 0.1 in CH₃CN) porphyrins. Whereas pK_a* < pK _a for the β-alkynylamidinium porphyrin, pK_a* > pK_a for the meso-alkynylamidinium porphyrin, indicating that β-alkynylamidinium PNi(II) is a photoacid and meso-alkynylamidinium PNi(II) is a photobase. These divergent behaviors are supported by analysis of the frontier molecular orbitals of the homologous pair with TDDFT.

Full text of DOI:10.1021/ic700838s

Inorg. Chem. 2007, 46, 8668−8675
Structurally Homologous
â- and meso-Alkynyl Amidinium Porphyrins
Joel Rosenthal, Elizabeth R. Young, and Daniel G. Nocera*
Department of Chemistry, 6-335, Massachusetts Institute of Technology,
77 Massachusetts AVenue, Cambridge, Massachusetts 02139-4307
Received May 1, 2007
Alkynylamidinium groups have been introduced at the
framework. The modification permits the distance between the amidinium
to be increased while effectively maintaining conjugation between the porphyrin macrocycle and the acid
â
and meso positions of a nickel(II) porphyrin (PNi(II))
amidine acid base group and porphyrin
base
−
−
π
−
functionality. Use of an ethynyl spacer as a linker (i) extends the amidinium functionality away from the sterically
bulky mesityl groups of the porphyrin, allowing it to be nearly planar with respect to the porphyrin ring, and (ii)
draws the
of the electronic properties of the porphyrin macrocycle to the protonation state of the amidinium. The barrier for
rotation of the amidinium group, as calculated by time-dependent density functional theory (TDDFT), is 8.5kT (5
kcal/mol) for both porphyrins. Analysis of UV visible absorption profiles for the - and meso-alkynylamidinium
PNi(II) upon deprotonation enables accurate determination of the amidinium acidity constants for the ground state
π-orbital character of the porphyrin out toward the amidinium functionality, thereby engendering sensitivity
∼
−
â
(pKa(
â
)
)
±
7.03
0.1 in CH3CN) porphyrins. Whereas pKa* < pKa for the
-alkynylamidinium PNi(II) is a photoacid and meso-alkynylamidinium
±
0.1, pKa(meso)
)
7.74
±
0.1 in CH3CN) and excited state (pKa*(
â) ) 6.89 ± 0.1, pKa*(meso)
)
8.37
â
-alkynylamidinium porphyrin, pKa* > pKa for the
meso-alkynylamidinium porphyrin, indicating that
â
PNi(II) is a photobase. These divergent behaviors are supported by analysis of the frontier molecular orbitals of the
homologous pair with TDDFT.
Introduction
amidinium-carboxylate interfaces.8 The amidinium-carboxy-
late salt-bridge combines the dipole of an electrostatic ion
pair interaction within a hydrogen-bonding scaffold and
allows for the investigation of the manner in which proton
motion within a hydrogen-bond interface affects the charge,
energetics, and electronic coupling of electron transport.1,7-10
The charge shift resulting from the motion of the proton can
couple to the charge shift of the electron transfer through
the polarization of the surrounding environment.11-15 It is
this mechanism that distinguishes electron-transfer (ET) from
PCET.
Supramolecular porphyrin assemblies constructed from
donor (D) and acceptor (A) termini bridged by a hydrogen-
bond network ([H+]) may be used for the study of photo-
induced proton-coupled electron transfer (PCET).1 Initial
D-[H+]-A assemblies employed dicarboxylic acid dimers2,3
and Watson-Crick base pairs as [H+].4 Subsequent studies
showed that the [H+] interface exerts its greatest influence
on the PCET event when it is asymmetric and capable of
supporting proton transfer.5-7 For these reasons, we have
focused on photoinduced charge transport mediated by
The kinetics for the individual proton-transfer (PT) and
ET steps of the photoinduced PCET event may be probed
by transient spectroscopy as long as the PT and ET are
* To whom correspondence should be addressed. E-mail: nocera@
mit.edu.
(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, 2001; Vol. 3.2.4, pp 409-461.
(2) Turro´, C.; Chang, C. K.; Leroi, G. E.; Cukier, R. I.; Nocera, D. G. J.
Am. Chem. Soc. 1992, 114, 4013-4015.
(3) de Rege, P. J. F.; Williams, S. A.; Therien, M. J. Science 1995, 269,
1409-1413.
(4) Sessler, J. L.; Wang, B.; Springs, S. L.; Brown, C. T. In ComprehensiVe
Supramolecular Chemistry; Murakami, Y., Ed.; Pergamon Press:
Oxford, 1996; Vol. 4, pp 311-336 and references therein.
(5) Roberts, J. A.; Kirby, J. P.; Nocera, D. G. J. Am. Chem. Soc. 1995,
117, 8051-8052.
(6) Roberts, J. A.; Kirby, J. P.; Wall, S. T.; Nocera, D. G. Inorg. Chim.
Acta 1997, 263, 395-405.
(7) Kirby, J. P.; Roberts, J. A.; Nocera, D. G. J. Am. Chem. Soc. 1997,
119, 9230-9236.
(8) Damrauer, N. H.; Hodgkiss, J. M.; Rosenthal, J.; Nocera, D. G. J.
Phys. Chem. B 2004, 108, 6315-6321.
(9) Chang, C. J.; Chang, M. C. Y.; Damrauer, N. H.; Nocera, D. G.
Biophys. Biochim. Acta 2004, 1655, 13-28.
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8668 Inorganic Chemistry, Vol. 46, No. 21, 2007
10.1021/ic700838s CCC: $37.00
© 2007 American Chemical Society
Published on Web 09/14/2007

Structurally Homologous â- and meso-Alkynyl Amidium Porphyrins
Scheme 1
Nickel(II) 2-Bromo-5,10,15,20-tetramesitylporphyrin (2). 21H,-
23H-5,10,15,20-Trimesitylporphyrin (300 mg, 0.383 mmol) was
dissolved in 350 mL of CH2Cl2 and MeOH (9:1 vol/vol) in a 1-L
round-bottom flask. The solution was stirred and heated to reflux
under air, and 89 mg of NBS (0.50 mmol) was added. After 2 h,
the reaction mixture was cooled to room temperature and concen-
trated in vacuo. The crude material was purified by chromatography
on silica using hexanes and CH2Cl2 (5:1) as the eluent to provide
the freebase 2-bromo-5,10,15,20-tetramesitylporphyrin product. The
freebase porphyrin was dissolved in 25 mL of DMF, and a large
excess of Ni(OAc)2‚(H2O)4 was added to the solution. The resulting
mixture was heated at reflux under a nitrogen atmosphere for 45
min. The resulting dark red solution was then cooled to room
temperature, diluted with 200 mL of water, and extracted twice
with 50 mL of CH2Cl2. The organic extracts were combined, washed
several times with water, dried over Na2SO4, and then concentrated
under reduced pressure. Purification on silica using hexanes and
CH2Cl2 (3:1) as the eluent delivered 112 mg of the title compound
in 32% yield. 1H NMR (300 MHz, CDCl3, 25 °C): δ 8.69 (s, 1H),
8.51 (m, 4H), 8.47 (d, J ) 3.3 Hz, 2H), 7.20 (br s, 6H), 7.16 (br
s, 2H), 2.56 (s, 9H), 2.55 (s, 3H), 1.82 (s, 12H), 1.77 (s, 6H), 126
(s, 6H). HRESIMS [M]+, m/z: Calcd for C56H51BrN4Ni, 916.2645.
Found, 916.2605.
spectroscopically distinguished. This task is not particularly
onerous for ET but does present challenges for PT. An
observable absorption shift upon deprotonation requires
strong ground- and excited-state communication between
protonic functionality and redox chromophore.16 Such com-
munication is mitigated by a saturated bridge or by canting
of the amidinium out of the porphyrin plane.17-19 In order
to circumvent these issues, we now describe the preparation
of a homologous set of meso- and â-alkynylamidinium
porphyrin derivatives. As schematically shown in Scheme
1, the protonic functionality is extended away from the
macrocycle and, at the same time, electronic communication
is preserved between protonic interface and the porphyrin
owing to the cylindrical symmetry of an alkyne spacer. The
syntheses of the meso- and â-alkynylamidinium porphyrin
derivatives are modular and facile; both have been obtained
in just four steps. The benefit of an interposed alkynyl spacer
between the amidine and porphyrin is revealed by the strong
wavelength dependence of the porphyrin’s electronic proper-
ties on the amidine/amidinium protonation state. As will be
show herein, communication between the porphyrin and
amidinium-amidine acid-base functionality may be probed
in both the porphyrin ground and excited electronic states.
Nickel(II) 2-(Trimethylsilyl)ethynyl-5,10,15,20-tetramesi-
tylporphyrin (3). Porphyrin 2 (100 mg, 0.109 mmol), PdCl2(PPh3)2
(25 mg, 0.035 mmol) and CuI (5 mg, 0.025 mmol) were combined
in a 50-mL Schlenk flask. The flask was evacuated and backfilled
with nitrogen. Dry DMF and NEt3 (15 mL, 1:1) were added to the
mixture. The solution was heated to 110 °C under nitrogen, and
500 µL of (trimethylsilyl)acetylene was added to the stirred solution.
After 40 h, the reaction was cooled to room temperature and the
solvent was removed under reduced pressure. The crude material
was dissolved in CH2Cl2 and washed twice with water (75 mL),
and the organic layer was dried over Na2SO4. The crude material
was purified on silica using hexanes and CH2Cl2 (7:1). The title
compound was obtained in 72% yield (74 mg). 1H NMR (300 MHz,
CDCl3, 25 °C): δ 8.68 (s, 1H), 8.49 (m, 4H), 8.47 (d, J ) 3.3 Hz,
2H), 7.19 (br s, 6H), 7.16 (br s, 2H), 2.56 (s, 9H), 2.55 (s, 3H),
1.82 (s, 12H), 1.77 (s, 6H), 1.56 (s, 6H), 0.72 (s, 9H). HRESIMS
[M + H]+, m/z: Calcd for C61H60N4NiSi, 935.4013. Found,
935.4002.
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, respec-
tively. Solvents for synthesis were of reagent grade or better and
were dried according to standard methods.20 Spectroscopic experi-
ments employed CH3CN (Spectroscopic Grade), which was also
dried according to standard methods and stored under vacuum. Mass
spectral analyses were performed in the MIT Department of
Chemistry Instrumentation Facility (DCIF) or at the University of
Illinois Mass Spectrometry Laboratory. 21H,23H-5,10,15,20-Tet-
ramesityl porphyrin (1),21 21H,23H-5,10,15-trimesityl porphyrin
(6),22 phenyl cyanate,23 and chloromethylaluminum amide24 were
prepared using published procedures.
(11) Cukier, R. I.; Nocera, D. G. Annu. ReV. Phys. Chem. 1998, 49, 337-
369.
Nickel(II) 2-(Cyano)ethynyl-5,10,15,20-tetramesitylporphyrin
(4). The trimethylsilyl-protected porphyrin (3) (70 mg, 0.075 mmol)
was dissolved in 15 mL of THF and CH3OH (3:1), and 100 mg
(0.1 mol) of K2CO3 was added to the stirred solution. The resulting
mixture was stirred for 18 h under air. To the mixture, 25 mL of
saturated NH4Cl and 20 mL of CH2Cl2 were added. The organic
layer was separated, washed twice with 20 mL of saturated NH4-
Cl, and dried over Na2SO4. The resulting solution was filtered
(12) Cukier, R. I. Biochim. Biophys. Acta 2004, 1655, 37-44.
(13) Hammes-Schiffer, S. In Electron Transfer in Chemistry; V. Balzani,
Ed.; Wiley-VCH: Weinheim, Germany, 2001, Vol. 1.1.5, p 189.
(14) Hammes-Schiffer, S. Acc. Chem. Res. 2001, 34, 273-281.
(15) Hammes-Schiffer, S.; Iordanova, N. Biochim. Biophys. Acta 2004,
1655, 29-36.
(16) Rosenthal, J.; Hodgkiss, J. M.; Young, E. R.; Nocera, D. G. J. Am.
Chem. Soc. 2006, 128, 10474-10483.
(17) Yeh, C.-Y.; Miller, S. E.; Carpenter, S. D.; Nocera, D. G. Inorg. Chem.
2001, 40, 3643-3646.
(18) Deng, Y.; Roberts, J. A.; Peng, S.-M.; Chang, C. K.; Nocera, D. G.
Angew. Chem., Int. Ed. 1997, 36, 2124-2127.
(22) Shultz, D. A.; Gwaltney, K. P.; Lee, H. J. Org. Chem. 1998, 63, 4034-
(19) Kirby, J. P.; van Dantzig, N. A.; Chang, C. K.; Nocera, D. G.
Tetrahedron Lett. 1995, 36, 3477-3480.
4038.
(23) Moss, R. A.; Chu, G.; Sauers, R. R. J. Am. Chem. Soc. 2005, 127,
(20) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th Ed.; Butterworth-Heinmann: Oxford, 1996.
(21) Lindsey, J. S.; Wagner, R. W. J. Org. Chem. 1989, 34, 828-836.
2408-2409.
(24) Levin, J. I.; Turos, E.; Weinreb, S. M. Synth. Commun. 1982, 12,
989-993.
Inorganic Chemistry, Vol. 46, No. 21, 2007 8669

Rosenthal et al.
1
through a short plug of silica and concentrated under reduced
pressure. The resultant material was placed in a 50 mL Schlenk
tube and dissolved in 10 mL of THF. The purple solution was
cooled to -78 °C under a nitrogen atmosphere and 1.75 mL of 1.6
M n-BuLi in hexane (2.8 mmol) was added via syringe. The
resultant solution was stirred for 90 min, PhOCN (1.0 mL) was
added, and the reaction was allowed to warm to room temperature
over the course of several hours. To the resulting green solution
was added 50 mL of 1 M NaOH, and the biphasic mixture was
extracted twice with 100 mL of CH2Cl2. The combined organic
extracts were washed twice each with brine and water and then
dried over Na2SO4 and concentrated under reduced pressure. The
desired product was purified by chromatography on silica using
hexanes and CH2Cl2 (4:1) as the eluent to generate 42 mg of the
reaction generated 119 mg (63% yield) of the title compound. H
NMR (300 MHz, CDCl3, 25 °C): δ 9.31 (d, J ) 2.1 Hz, 2H), 8.69
(d, J ) 2.1 Hz, 2H), 8.69 (d, J ) 2.3 Hz, 2H), 8.52 (d, J ) 2.3 Hz,
2H), 7.23 (s, 4H), 7.20 (s, 2H), 2.59 (s, 6H), 2.56 (s, 3H), 1.82 (s,
6H), 1.57 (s, 12H). HRESIMS [M + Na]+, m/z: Calcd for
C50H42N5NiNa, 792.2608. Found, 792.2614.
Nickel(II) 5-(Amidinium)ethynyl-10,15,20-tetramesitylpor-
phyrin Chloride (10). Starting with 50 mg of 9 (0.065 mmol), the
meso-alkynyl porphyrin was prepared in a manner identical to that
used for â-homolog 5. The reaction generated 35 mg (67% yield)
of the title compound. 1H NMR (300 MHz, CDCl3, 25 °C): δ 10.09
(br s, 4H), 9.52 (br s, 4H), 8.65 (br s, 4H), 8.47 (d, J ) 5.8 Hz,
4H), 8.41 (d, J ) 5.8 Hz, 4H), 7.17 (s, 2H) 7.09 (s, 4H), 2.55
(s, 3H), 2.46 (s, 6H) 1.79 (s, 6H) 1.72 (s, 12H). HRESIMS
[M - Cl]+, m/z: Calcd for C50H45N6Ni, 787.3054. Found, 787.3032.
Physical Measurements. 1H NMR spectra were recorded at
25 °C in the MIT DCIF on a Varian XL-500, Unity 300 or Mercury
300 spectrometer. All chemical shifts are reported using the standard
δ notation in ppm; positive chemical shifts are to a higher frequency
from the given reference. Absorption spectra were obtained using
a Spectral Instruments 440 Series spectrophotometer.
1
title compound (63% yield). H NMR (300 MHz, CDCl3, 25 °C):
δ 8.79 (s, 1H), 8.58 (m, 4H), 8.53 (d, J ) 3.3 Hz, 2H), 7.24 (br s,
6H), 7.21 (br s, 2H), 2.71 (s, 9H), 2.63 (s, 3H), 1.89 (s, 12H), 1.81
(s, 6H), 1.68 (s, 6H). HRESIMS [M + H]+, m/z: Calcd for
C59H52N5Ni, 888.3571. Found, 889.3536.
Nickel(II) 2-(Amidinium)ethynyl-5,10,15,20-tetramesitylpor-
phyrin chloride (5). To a toluene solution (10 mL) of 4 (25 mg,
0.028 mmol) was added chloromethylaluminum amide (4.0 mL,
1.2 M in toluene) under a nitrogen atmosphere. The reaction solution
was heated at 100 °C under nitrogen for 5 days. The resulting
solution was then cooled to room temperature, poured slowly onto
a slurry of silica gel (15 g) in CHCl3 (30 mL), and stirred under
air for 10 min. The slurry was filtered over sintered glass, and the
filtercake was washed with CH2Cl2/CH3OH (3:1) until all porphyrin
product had been eluted. The filtrate was concentrated under
reduced pressure, and the residue purified by column chromatog-
raphy on silica using CH2Cl2 and CH3OH (10:1) as the eluent to
yield 16 mg (59% yield) of the title compound. 1H NMR (300 MHz,
CDCl3, 25 °C): δ 10.12 (br s, 4H), 8.72 (s, 1H), 8.57-8.46 (m,
6H), 7.22 (s, 6H), 7.11 (s, 2H), 2.53 (s, 12H), 1.82 (s, 12H), 1.79
(s, 6H), 1.73 (s, 6H). HRESIMS [M - Cl]+, m/z: Calcd for
C59H55N6Ni, 905.3836. Found, 905.3812.
Absorption spectroscopy on samples of PNi(II) â- and meso-
alkynylamidinium chloride was performed in a high-vacuum cell
comprised of a 1-cm path length clear fused-quartz cuvette (Starna
cells) connected to a 10-cm3 solvent reservoir via a graded seal.
High-vacuum Teflon valves were used to seal the cell from the
environment and the cuvette from the solvent reservoir.
Steady-state spectroscopic experiments on the Ni(II) porphyrins
were performed at concentrations of 9 (meso) and 16 (â) µM in
acetonitrile to give an optical density of ∼0.85 (meso) and ∼0.48
(â) AU at the Soret absorption band and ∼0.07 (meso) and ∼0.03
(â) AU at the Q10 absorption band. Sample preparation was
performed under high vacuum. An aliquot of Ni(II) porphyrin (2.7
× 10-8 (meso) and 4.6 × 10-8 (â) mol) was added to the cuvette,
and the cell was evacuated under high vacuum (10-5 Torr) to
remove the transferring solvent. Three milliliters of dry acetonitrile
were vacuum-transferred to the solvent reservoir where it was
subject to three freeze-pump-thaw cycles. The cell was sealed to
the environment and removed from the high-vacuum manifold. A
titration with 4-dimethylaminopyridine (DMAP) was performed to
monitor the spectral shifts associated with deprotonation of the
amidinium functionality. Prior to each DMAP addition, acetonitrile
was vacuum-transferred to the cuvette with a dry ice/acetone bath
and sealed from the solvent reservoir and environment. This
procedure ensured that the solvent volume remained constant
throughout the course of the experiment. A stock solution of DMAP
was added to the open solvent reservoir, while preserving the
vacuum in the cuvette, and the transferring solvent was removed
with a high-vacuum manifold (10-5 Torr). The solvent reservoir
was then sealed from the environment and opened to the cuvette
compartment to introduce each new DMAP addition.
A Benesi-Hildebrand plot was developed for the spectral shift
of the Soret band (1/∆ASoret vs 1/[DMAP]) of PNi(II) upon its
titration with DMAP. The Soret band of meso-substituted Ni(II)
porphyrin is observed at 422 nm with a measured extinction
coefficient of 94 000 M-1 cm-1, while the Soret band of the
â-substituted PNi(II) is observed at 432 nm with an extinction
coefficient of 32 500 M-1 cm-1. Titration with DMAP yields strong
isosbestic points for both the meso- and â-substituted PNi(II)
systems. Because an isosbestic point occurs near the Soret peak of
the amidinium, as opposed to that of the amidine porphyrin, the
absorption changes associated with the Soret band of the latter
(∆ASoret) were used in the Benesi-Hildebrand analysis. A 2-nm
Nickel(II) 5-Bromo-10,15,20-tetramesitylporphyrin (7). N-
Bromosuccinimide (NBS, 134 mg, 0.752 mmol) was added in one
portion to a solution of 21H,23H-5,10,15-trimesitylporphyrin (500
mg, 0.752 mmol) in chloroform (400 mL), and the reaction was
stirred at room temperature under air for 30 min. The solvent was
removed, and the residue was purified by column chromatography
(silica gel, 1:1 hexanes/dichloromethane) to provide the freebase
5-bromo-10,15,20-trimesitylporphyrin product. The porphyrin was
metalated with Ni(OAc)2‚(H2O)4 in a manner identical to that
described for 2. The title compound was purified on silica, using
hexanes and CH2Cl2 (3:1) as the eluent, to deliver 523 mg (87%
yield). 1H NMR (300 MHz, CDCl3, 25 °C): δ 9.64 (d, J ) 4.5 Hz,
2H), 8.77 (d, J ) 4.5 Hz, 2H), 8.57 (s, 4H), 7.23 (s, 6H), 2.61 (s,
6H), 2.68 (s, 3H), 1.84 (s, 6H), 1.89 (s, 12H). HRESIMS [M +
H]+, m/z: Calcd for C47H42BrN4Ni, 799.1946. Found, 799.1938.
Nickel(II) 5-(Trimethylsilyl)ethynyl-10,15,20-tetramesitylpor-
phyrin (8). Starting with 300 mg of 7 (0.375 mmol), the meso-
alkynyl porphyrin was prepared in a manner identical to that used
for â-homolog 3. The reaction generated 247 mg (81% yield) of
1
the title compound. H NMR (300 MHz, CDCl3, 25 °C): δ 9.54
(d, J ) 2.6 Hz, 2H), 8.67 (d, J ) 2.6 Hz, 2H), 8.53 (br s, 4H), 7.24
(s, 4H), 7.21 (s, 2H), 2.61 (s, 6H) 2.58 (s, 3H), 1.86 (s, 6H), 1.32
(s, 12H), 0.59 (s, 9H). HRESIMS [M + H]+, m/z: Calcd for
C52H50N4NiSi, 817.3231. Found, 817.3217.
Nickel(II) 5-(Cyano)ethynyl-10,15,20-tetramesitylporphyrin
(9). Starting with 200 mg of 8, the meso-alkynyl porphyrin was
prepared in a manner identical to that used for â-homolog 4. The
8670 Inorganic Chemistry, Vol. 46, No. 21, 2007

Structurally Homologous â- and meso-Alkynyl Amidium Porphyrins
Scheme 2
blue-shift is observed in the meso-substituted porphyrin spectra to
a final Soret peak position at 420 nm, while the â-substituted
porphyrin Soret undergoes a 3-nm red-shift to 435 nm. An acidity
constant, Ka, for meso- and â-substituted Ni(II)P is obtained from
the ratio of the y intercept and slope of linearly fit data.25
porphyrins. Calculations were performed in which the plane of the
amidinium functionality was rotated with respect to the porphyrin
plane. The final coordinates of an initial DFT geometry optimization
were utilized as the input coordinates. The dihedral angle (θdihedral),
defined by the N-CamH+ bond of the amidinium and the Câ-CR
or Cmeso-CR and on the porphyrin ring, was rotated by intervals of
10° and restrained. Default convergence criteria were used for
rotated calculations.
Computations. Density functional theory (DFT) and time-
dependent density functional theory (TDDFT) calculations were
performed using the Amsterdam Density Functional (ADF2006.01,
TDDFT; ADF2004.01, DFT) program26-28 on a home-built Linux
cluster comprising 60 Intel processors organized in groups of 12
running in parallel. The generalized gradient approximation was
used as implemented in ADF by the Becke exchange functional,29
and the PW91c correlation functional.30 Convergence criteria
tolerance for DFT was altered from default and set as follows.
Energy, 1 × 10-4 Hartrees; gradients, 1 × 10-3 Hartrees;
coordinates, 1 × 10-3 Å; bond angles, 0.5°. Relativistic corrections
were included using the scalar zero-order regular approximation
(ZORA).31,32 A basis set of triple-ú Slater-type functions augmented
by a polarization set (TZP) was used for Zn and N atoms and
double-ú with polarization (DZP) was used for C and H. Relativistic
frozen core approximation was used to treat shells up to 2p for Zn
and 1s for C and N as core shells. Orbitals were visualized using
the Molekel v.4.2 Linux-mesa software.33,34
Results and Discussion
Alkynyl amidinium functionalities were fused to either the
â (5) or meso (10) positions of the mesityl porphyrin
macrocycle following the synthetic protocol outlined in
Scheme 2. 1 was the starting material of choice for the
synthesis of the â-alkynyl amidinium derivative, owing to
the superior solubilizing properties of the mesityl groups on
the porphyrin periphery. Bromination of 1 using NBS gives
2,35 which may be Sonagashira coupled to trimethylsily-
lacetylene to cleanly generate alkynyl porphyrin 3. Following
deprotection of the alkynyl group using tetrabutylammonium
fluoride, lithiation and reaction with phenyl cyanate provides
â-alkynyl cyano porphyrin 4. The nitrile is transformed to
the corresponding amidinium 5 using an excess of Weinreb’s
amide transfer reagent36 under harsh conditions (100 °C, 5
days). meso-Alkynyl amidinium porphyrin 10 is prepared
along similar lines to the â-homolog, using 6 as the starting
material. The synthesis of 5 and 10 in just four steps each
represents a dramatic improvement over previously prepared
amidinium porphyrin derivatives, which are prepared via
arduous 26- and 12-step synthetic routes.16,37
TDDFT was utilized for calculations on the protonated (ami-
dinium) and deprotonated (amidine) functionalities of both Ni(II)
(25) Connors, K. A. In Binding Constants: A Measurement of Molecular
Complex Stability; Wiley: New York, 1987.
(26) ADF2004.01, SCM, Theoretical Chemistry; Vrije Universiteit: Am-
sterdam, The Netherlands, http://www.scm.com.
(27) te Velde, G.; Bickelhaupt, F. M.; van Gisbergen, S. J. A.; Fonseca
Guerra, C.; Baerends, E. J.; Snijders, J. G.; Ziegler, T. J. Comput.
Chem. 2001, 22, 931-967.
(28) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor.
Chem. Acc. 1998, 99, 391- 403.
(29) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100.
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M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671-6687.
(31) Van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem Phys. 1993,
99, 4597-4610.
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8943-8953.
(33) Flu¨kiger, P.; Lu¨thi, H. P.; Portmann, S.; Weber, J. Molekel V.4.2/3;
Swiss Center for Scientific Computing: Manno, Switzerland, 2000-
2002.
Electronic coupling between the amidinium functionality
and porphyrin macrocycle is signified by the pH dependence
of the electronic absorption spectra of both the â- and meso-
(35) Zhou, X.; Tse, M. N.; Wan, T. S. M.; Chan, K. S. J. Org. Chem.
1996, 61, 3590-3593.
(36) Levin, J. I.; Turos, E.; Weinreb, S. M. Synth. Commun. 1982, 12,
989-993.
(37) Deng, Y.; Roberts, J. A.; Peng, S. M.; Chang, C. K.; Nocera, D. G.
Angew. Chem., Int. Ed. Engl. 1997, 36, 2124-2127.
(34) Portmann, S.; Lu¨thi, H. P. Chimia 2000, 54, 766-769.
Inorganic Chemistry, Vol. 46, No. 21, 2007 8671

Rosenthal et al.
Figure 1. Spectral changes associated with deprotonation of (a) amidinium porphyrin 5 and (b) amidinium porphyrin 10 in CH3CN.
Figure 2. Frontier molecular orbitals that contribute to the observed electronic transitions for both the amidinium and amidine forms of amidinium porphyrin
5, which is a photoacid. The energies shown are calculated for the visualized orbitals by TDDFT.
alkynyl amidinium porphyrins. Figure 1A shows an experi-
mentally observable perturbation of the UV-vis absorbance
profile of 5 upon deprotonation of the amidinium group. A
bathochromic shift is observed for the Soret band from 432
to 435 nm upon deprotonation of the amidinium. Well-
anchored isosbestic points are maintained at 425, 564, and
602 nm between the spectra of the protonated (amidinium)
and deprotonated (amidine) forms of the proton interface.
The red-shift in the Soret band is accompanied by similar
spectral shifts in the Q-band region. The absorption spectrum
of the meso-alkynyl amidinium 10 also displays altera-
tion upon deprotonation of the amidinium functionality.
However, as shown in Figure 1B, a hypsochromic shift is
observed upon conversion of amidinium to amidine. The
maximum of the Soret band shifts from 422 to 420 nm
while maintaining well-anchored isosbestic points at 428,
550, and 568 nm. Similar modulation of the absorption
profile is observed in the Q-band region of 10 upon
deprotonation.
The electronic coupling responsible for spectral shifts
associated with the protonation state of the amidine is
reflected by geometry-optimized TDDFT and DFT calcula-
tions for the porphyrin molecular orbitals of 5 and 10. Two
factors dominate the coupling: the electronic structure of
the frontier molecular orbitals of the porphyrin ring at the
meso- and â-positions and the twist angle of the acid-base
group with regard to the porphyrin ring. The results of these
TDDFT and DFT calculations offer qualitative insight toward
the experimental behavior of 5 and 10, as shown in Figures
2-4. Although these computations do not incorporate a
dielectric solvent model, they provide a qualitative picture
of the orbital and energetic perturbations associated with
amidinium deprotonation of porphyrins 5 and 10. Such
comparisons have previously been shown to be useful.16
8672 Inorganic Chemistry, Vol. 46, No. 21, 2007

Structurally Homologous â- and meso-Alkynyl Amidium Porphyrins
Figure 3. Frontier molecular orbitals that contribute to the observed electronic transitions for both the amidinium and amidine forms of amidinium porphyrin
10, which is a photobase. The energies shown are calculated for the visualized orbitals by TDDFT.
frontier orbitals of 5 and 10 are shown in Figures 2 and 3,
respectively, for both the alkynylamidinium acid (left) and
the corresponding conjugate alkynylamidine base (right).
Molecular orbitals lacking significant porphyrin π-orbital
character are omitted for clarity, as they are not responsible
for the porphyrin transitions observed in the absorption
profiles of Figure 1. Considering the electronic distribution
about the porphyrin ring, it has been shown that HOMOs of
a2u and a1u symmetries result in maximal orbital overlap with
alkyne groups in meso- and â-positions, respectively.44-46
In the systems described here, these orbitals are the parents
of the HOMO-3 and HOMO-4 orbitals shown for 5 in Figure
2 and HOMO-4 and HOMO-3 orbitals for 10 shown in
Figure 3. These orbitals, which are the highest occupied
porphyrin-based MOs, provide the linchpin for electronic
coupling of the alkynyl group to the porphyrin ring. The
energy gap is affected not only by the sensitivity of the
aforementioned porphyrin-based HOMO levels on protona-
tion state but dependent on the LUMOs as well. The
LUMO+2 and LUMO+1 of protonated and deprotonated 5
(Figure 2) and the LUMO, LUMO+3 and the LUMO+1,
LUMO+3 of the protonated and deprotonated 10 (Figure 3)
show orbital density on both the porphyrin macrocycle and
Figure 4. Relative potential energies of (A) â-alkynylamidinium porphyrin
(5) and (B) meso-alkynylamidinium porphyrin (10) are shown as the dihedral
angle defining the plane of the porphyrin relative to the amidinium is rotated
and restrained. The LUMO+3 of 5 and the LUMO of 10, the two lowest
lying unoccupied orbitals with significant porphyrin π-orbital and substituent
density are shown at θ (relative to θi) ) 0°, 30°, 60°, 90°, 120°, 165°,
180°.
DFT calculations may reasonably predict electronic transi-
tion wavelengths for optical absorptions in porphyrins with
â and meso substituents.38-43 The calculated porphyrin
(38) Walsh, P. J.; Gordon, K. C.; Officer, D. L.; Campbell, W. M.
Theochem. 2006, 759, 17-24.
(39) Liao, M-S.; Scheiner, S. Chem. Phys. Lett. 2003, 367, 199-206.
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13-26.
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J. S.; Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1997, 119, 11191-
11201.
(45) Yang, S. I.; Seth. J.; Balasubramanian, T.; Kim, D.; Lindsey, J. S.;
Holten, D.; Bocian, D. F. J. Am. Chem. Soc. 1999, 121, 4008-4018.
(46) Holten, D.; Bocian, D. F.; Lindsey, J. S. Acc. Chem. Res. 2003, 35,
57-69.
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4748-4754.
(42) van Gisberg, S. J. A.; Rosa, A.; Ricciarde, G.; Baerends, E. J. J. Chem.
Phys. 1999, 111, 2499-2506.
(43) Sundholm, D. Phys. Chem. Chem. Phys 2000, 2, 2275-2281.
Inorganic Chemistry, Vol. 46, No. 21, 2007 8673

Rosenthal et al.
barrier height for rotation of the alkynylamidinium func-
tionality: 8.42 kT (4.99 kcal/mol) for â-alkynylamidinium
and 8.54 kT (5.05 kcal/mol) for meso-alkynylamidinium.
The spectral signatures accompanying deprotonation of the
amidinium functionalities of alkynyl porphyrins 5 and 10
provide observables that can be used to determine the pKa
of the amidinium in the porphyrin in its ground and excited
electronic states. Titration of 5 in CH3CN with the base
DMAP produces the spectral series shown in Figure 1A. A
Benesi-Hildebrand plot of the spectral shifts (see Figure
S3 in the Supporting Information) yields a deprotonation
constant (Ka′), from which the pKa of 5 may be determined
according to the relation
Figure 5. Generalized Jablonski diagrams of (a) molecular species that
behave as photoacids and (b) molecular species that behave as photobases.
pKa(PNi(II)) ) pKa(DMAP) - log(Ka′)
(2)
The pKa of DMAP in CH3CN is 12.33,49 yielding a pKa )
7.03 ( 0.10 for the ground state of â-alkynyl amidinium
porphyrin 5. In its excited state ([AH+]*), 5 is predicted to
be a stronger acid than in its ground state since the absorption
spectrum of the conjugate base displays a bathochromic shift
relative to that of the conjugate acid (hν1 > hν2).50-53 This
follows from the simplified Jablonski diagram shown in
Figure 5. The thermodynamic relationship between the
ground and excited states for a proton-containing species
(AH+) and its conjugate base (A) predict that its excited state
([AH+]*) is a stronger acid than the ground state when a
bathochromic shift accompanies acid deprotonation (hν1 >
hν2). Put another way, the free energy change associated with
deprotonation of the molecule in the excited state (∆Ga*) is
less than that for deprotonation in the ground state (∆Ga).
The excited-state acidity constant, pKa*, is formalized by
the Fo¨rster equation,53,54
the alkynylamidinium functionality. This electronic redis-
tribution is responsible for the spectral sensitivity of the
porphyrin to the amidinium protonation state.
The calculations shown in Figure 4 predict a θdihedral
)
14° for â-alkynylamidinium porphyrin 5 and a θdihedral ) 12°
for meso-alkynylamidinium porphyrin 10. These relatively
small cant angles contrast sharply with the roughly 75° cant
angle calculated for previously studied porphyrin-amidinium
compounds in which the amidinium was directly linked to
the porphyrin.17,19 The steric clashing between the amidinium
and hydrogens of the porphyrin ring that plagues a direct
linkage is avoided by poising the amidinium at the end of
the ethynyl linker of 5 and 10. The electronic structure of
the alkynyl spacer is cylindrically symmetric and hence the
electronic communication between the amidinium and por-
phyrin macrocycle should be perturbed minimally with
rotation. Systematic variation of θdihedral to span angles
between 0° and 180° confirms the intuition that conjugation
persists upon amidinium rotation. Molecular orbitals and
relative orbital energies for rotational calculations on 5 and
10 are presented in Figures S1 and S2 in the Supporting
Information.
pKa* ) ∆Ga*/2.3RT ) pKa - (hν1 - hν2)/2.3RT (3)
For eq 3, hν1 and hν2 represent the energies of the 0-0
electronic transitions for the conjugate acid and base,
respectively. Analysis of the electronic absorption spectra
associated with deprotonation of 5 leads yields a pKa* )
6.89 ( 0.10 in CH3CN at 23 °C. These results demonstrate
that the acidity of 5 is greater in its excited-state as compared
to its ground state (pKa ) 7.03 ( 0.10).
A sense for the barrier of rotation can be gleaned from
the calculated potential energy of the system upon rotation
of amidinium as shown in Figure 4.47,48 The potential energy
change relative to the geometry optimized calculation in
terms of kT varies as
Using eq 2 and the Ka′ determined from the Benesi-
Hildebrand plot obtained for 10 (Figure S4 in the Supporting
Information), the ground-state pKa for meso-alkynyl ami-
dinium porphyrin 10 is calculated to be 7.74 ( 0.10, which
is similar to that of 5. However, in contrast to 5, 10 is
photobasic. In its excited state ([AH+]*), 10 is more basic
∆U(θ) ) U(θ) - U(θi)
(1)
where U(θ) ) A sin [ω(θ - θc)] and θi is the initial cant
angle for the system, θi ) 14.4 for â-alkynylamidinium and
θi ) 12.7 for meso-alkynylamidinium. A is the amplitude of
the sine curve, ω is related to the period of oscillation, and
θc is the offset angle. Fitted values for â-alkynylamidinium
are A ) 4.21 kT, ω ) 2.11, and θc ) 50.7° and meso-
alkynylamidinium are A ) 4.27 kT, ω ) 2.42, and θc )
54.7° Two times the fitted amplitude (2A) represents the
(49) Izutsu, K. Acid-Base Dissociation Constants in Dipolar Aprotic
SolVents; Blackwell Scientific: Cambridge, MA, 1990.
(50) Ireland, J. F.; Wyatt, P. A. H. AdV. Phys. Org. Chem. 1976, 12, 131-
160.
(51) Shizuka, H. Acc. Chem. Res. 1985, 18, 141-147.
(52) Arnaut, L. G.; Formosinho, S. J. J. Photochem. Photobiol. A 1993,
75, 1-20.
(53) Tolbert, L. M.; Solntsev, K. M. Acc. Chem. Res. 2002, 35, 19-27
and references therein.
(47) Okuno, Y.; Kamikado, T.; Yokohama, S.; Mashiko, S. Theochem.
2002, 594, 55-60.
(48) Crossely, M. J.; Field, L. D.; Forster, A. J.; Harding, M. M.; Sternhell,
Sever. J. Am. Chem. Soc. 1987, 109, 341-348.
(54) Weller, A. Z. Elektrochem. 1952, 56, 662-668.
8674 Inorganic Chemistry, Vol. 46, No. 21, 2007

Structurally Homologous â- and meso-Alkynyl Amidium Porphyrins
than in its ground state, as indicated by the hypsochromic
shift of the conjugate base relative to that of the conjugate
acid (hν1 < hν2) since ∆Ga* is greater than ∆Ga (Figure
5B).50-52 Following eq 3, the excited-state pKa of 10 is
determined to be 8.37 ( 0.10.
porphyrin 10. As a result, ∆Ga < ∆Ga* and porphyrin 10 is
predicted to be a photobase, in agreement with the observed
absorption shifts.
In summary, amidiniums substituted at the â and meso
positions of porphyrins maintain electronic communication
with the macrocycle via ethynyl spacers. A facile and
modular synthesis of the alkynyl amidinium homologs
permits the arduous synthesis accompanying direct linkage
of amidiniums directly to the porphyrin to be avoided. Both
congeners display similar ground-state acidity constants but
exhibit opposite trends in terms of relative acidity upon
photoexcitation. â-Alkynylamidinium porphyrin (5) is pho-
toacidic, whereas the meso-alkynylamidinium porphyrin (10)
is photobasic. These disparate properties invite the possibility
of probing the role of excited-state acidity in promoting
PCET reactivity through hydrogen bonding interfaces.
The disparate behavior of the acidity properties of 5 and
10 in their ground and excited states is reflected in the
energetics arising from the â- and meso-alkynylamidinium
substitution pattern. The energy differences calculated by
TDDFT between HOMOs of the protonated and deprotonated
forms of 5 in Figure 2 is ∆Ga ) 2.22 eV, whereas the
LUMOs energy difference is ∆Ga* ) 2.06 eV, respectively.
Given that ∆Ga > ∆Ga*, porphyrin 5 is predicted to be a
photoacid in accordance with the experimentally observed
bathochromic shift observed upon deprotonation of 5. The
same TDDFT calculations were carried out for the meso-
alkynylamidinium porphyrin (10), the results of which are
shown in Figure 3. Unlike the â-amidinium derivative,
porphyrin 10 shows the opposite trend for the relative values
of ∆Ga and ∆Ga*. A third low-lying LUMO orbital with
significant alkynylamidinium and porphyrin π-orbital density
is evident in the protonated meso-alkynylamidinium molec-
ular orbitals resulting in an increased ∆Ga* value. Values
of ∆Ga ) 2.46 eV and ∆Ga* ) 2.91 eV are found for
Acknowledgment. J.R. thanks the Fannie and John Hertz
Foundation for a predoctoral fellowship. This work was
supported by the National Institutes of Health GM GM47274.
Supporting Information Available: Molecular orbitals and
UV-vis data. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC700838S
Inorganic Chemistry, Vol. 46, No. 21, 2007 8675