Extreme electronic modulation of the cofacial porphyrin structural motif

Article abstract of DOI:10.1021/ja012489h

The synthesis, electrochemistry, and optical spectroscopy of an extensive series of cofacial bis-[(porphinato)zinc(II) compounds are reported. These species were synthesized using sequential palladium-catalyzed cross-coupling and cobalt-mediated [2+2+2] c

Full text of DOI:10.1021/ja012489h

Published on Web 03/30/2002
Extreme Electronic Modulation of the Cofacial Porphyrin
Structural Motif
James T. Fletcher and Michael J. Therien*
Contribution from the Department of Chemistry, UniVersity of PennsylVania,
231 South 34th Street, Philadelphia, PennsylVania 19104-6323
Received November 6, 2001
Abstract: The synthesis, electrochemistry, and optical spectroscopy of an extensive series of cofacial bis-
[(porphinato)zinc(II)] compounds are reported. These species were synthesized using sequential palladium-
catalyzed cross-coupling and cobalt-mediated [2+2+2] cycloaddition reactions. This modular methodology
enables facile control of the nature of macrocycle-to-macrocycle connectivity and allows unprecedented
modulation of the redox properties of face-to-face porphyrin species. We report the synthesis of 5,6-bis-
[(5′,5′′-10′,20′-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]indane (1), 5,6-bis[(2′-5′,10′,15′,-
20′-tetraphenylporphinato)zinc(II)]indane (2), 5-([2′-5′,10′,15′,20′-tetraphenylporphinato]zinc(II))-6-[(5′′-
10′′,20′′-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]indane (3), 5-([2′-5′,10′,15′,20′-tetrakis-
(trifluoromethyl)porphinato]zinc(II))-6-[(5′′-10′′,20′′-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)-
zinc(II)]indane (4), 5-(2′-5′,10′,15′,20′-[tetrakis(trifluoromethyl)porphinato]zinc(II))-6-[(2′′-5′′,10′′,15′′,20′′-tetra-
phenylporphinato)zinc(II)]indane (5), 5,6-bis([2′-5′,15′-diphenyl-10′,20′-(trifluoromethyl)porphinato]zinc(II))-
indane (6), and 5,6-bis([2′-5′,10′,15′,20′-tetrakis(trifluoromethyl)porphinato]zinc(II))indane (7); 4-7 define
the first examples of cofacial bis[(porphinato)metal] compounds in which σ-electron-withdrawing perfluoroalkyl
groups serve as macrocycle substituents, while 2, 6, and 7 constitute the first such structures that possess
a â-to-â linkage topology. Cyclic voltammetric studies show that the electrochemically determined HOMO
and LUMO energy levels of these cofacial bis(porphinato) complexes can be lowered by 780 and 945 mV,
respectively, relative to the archetypal members of this class of compounds; importantly, these orbital energy
levels can be modulated over well-defined increments throughout these wide potentiometric domains.
Analyses of these cofacial bis[(porphinato)metal] potentiometric data, in terms of the absolute and relative
frontier orbital energies of their constituent [porphinato]zinc(II) building blocks, as well as the nature of
macrocycle-to-macrocycle connectivity, provide predictive electronic structural models that rationalize the
redox behavior of these species.
Introduction
of these species, demanding syntheses coupled with limited tools
to modulate the electronic properties of these systems have
impeded the syntheses of more elaborate versions of the
structural motif.16-22
Though mastered by Nature, the design of biomimetic
catalysts satisfying the thermodynamic and kinetic requirements
necessary for small-molecule multielectron transformations
remains challenging. Pioneering studies involving cofacial
metallodiporphyrins have provided valuable mechanistic insight
regarding a variety of small-molecule reductive and oxidative
transformations that require respectively pairwise delivery or
removal of electrons.1-15 Despite the established catalytic utility
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Chem. 1993, 32, 2562-2569.
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9
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J. AM. CHEM. SOC. 2002, 124, 4298-4311
10.1021/ja012489h CCC: $22.00 © 2002 American Chemical Society

Cofacial Prophyrin Structural Motif
A R T I C L E S
1
electrochemical cell. H and 19F NMR experiments were performed
respectively on 250- and 200-MHz Bru¨ker instruments. MALDI-TOF
mass spectroscopic data were obtained with a Perceptive Voyager DE
instrument in the Laboratories of Dr. Virgil Percec (Department of
Chemistry, University of Pennsylvania). Samples were prepared as
micromolar solutions in THF, and dithranol (Aldrich) was utilized as
the matrix.
We recently established a new synthesis of 1,2-phenylene-
bridged cofacial porphyrins23 that exploits ethyne-bridged
multiporphyrin precursors, and sequential Pd-catalyzed cross-
coupling24-31 and metal-templated [2+2+2] cycloaddition
reactions.32-34 We report herein the syntheses, optical spec-
troscopy, and electrochemistry of an extensive series of face-
to-face bis[(porphinato)zinc(II)] compounds in which the nature
of macrocycle-macrocycle connectivity, as well as the redox
properties of their (porphinato)metal components, are varied
systematically. Potentiometric data show that these engineering
strategies enable unprecedented modulation of cofacial bis-
[(porphinato)metal] electronic structure; complimentary elec-
tronic structure analyses define simple models that rationalize
the redox behavior of these systems in terms of the absolute
and relative energy levels of the frontier orbitals of their
respective (porphinato)zinc(II) building blocks.
General Procedure for the Preparation of Ethyne-Bridged Bis-
[(porphinato)zinc(II)] Complexes.30,31 A 50-mL Schlenk tube was
charged with a 2- or 5-ethynylporphyrin compound (1 equiv), a meso-
or â-bromoporphyrin complex (1.2 equiv), Pd2(dba)3 (0.15 equiv), and
AsPh3 (1.2 equiv).39 These reagents were dissolved in 5:1 THF/TEA
and stirred for 3-26 h at 40 °C. Following evaporation of the solvent,
the residue was purified via chromatography.
Bis[(5,5′-10,20-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphi-
nato)zinc(II)]ethyne (12). Reagents: (5-bromo-10,20-bis[4-(3-meth-
oxy-3-methylbutoxy)phenyl]porphinato)zinc(II) (53 mg, 0.068 mmol),
(5-ethynyl-10,20-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)-
zinc(II) (45 mg, 0.062 mmol), Pd2(dba)3 (9 mg, 0.0093 mmol), AsPh3
(23 mg, 0.074 mmol), THF (10 mL), and triethylamine (2 mL). Reaction
time, 3 h. Chromatographic purification: silica gel, 1:1 hexanes/THF,
followed by SX-1 biobeads, THF. The dark green band was isolated,
giving desired product 12 (0.083 g, 94% based on 45 mg of the
porphyrinic starting material). 1H NMR (250 MHz, 50:1 CDCl3/
pyridine-d5): δ 10.45 (d, J ) 4.53 Hz, 4H), 10.04 (s, 2H), 9.24 (d, J
) 4.43 Hz, 4H), 9.15 (d, J ) 4.43 Hz, 4H), 8.98 (d, J ) 4.35 Hz, 4H),
8.16 (d, J ) 8.43 Hz, 8H), 7.31 (d, J ) 8.53 Hz, 8H), 4.38 (t, J ) 7.26
Hz, 8H), 3.33 (s, 12H), 2.22 (t, J ) 6.98 Hz, 8H), 1.37 (s, 24H). Visible
(THF): 403 (5.08), 411 (5.08), 430 (5.00), 478 (5.46), 548 (4.21), 565
(4.18), 701 (4.69) nm. MS (MALDI-TOF) m/z: 1535 (calcd for
C90H86N8O8Zn2 1535).
Experimental Section
Inert atmosphere manipulations were carried out under nitrogen
prepurified by passage through an O2 scrubbing tower (Schweizerhall
R3-11 catalyst) and a drying tower (Linde 3-Å molecular sieves). Air-
sensitive solids were handled in a Braun 150-M glovebox. Standard
Schlenk techniques were employed to manipulate air-sensitive solutions.
A syringe pump was utilized to control reproducibly the time-dependent
concentration of reagents in all metal-templated cycloaddition reactions.
Unless otherwise noted, all solvents utilized in this work were
obtained from Fisher Scientific (HPLC grade) and distilled under
nitrogen. Tetrahydrofuran and toluene were distilled from Na/ben-
zophenone, and triethylamine was distilled from CaH2; dioxane
(anhydrous) was used as received from Aldrich. Pd2dba3, AsPh3, and
Co2(CO)8 were obtained from Strem. 1,6-Heptadiyne, TBAF, and
ethynylbenzene were obtained from Aldrich. Trimethylsilylacetylene
and triisopropylsilylacetylene were obtained from GFS Chemicals.
Halogenated derivatives35 of [5,10,15,20-tetrakis(trifluoromethyl)por-
phinato]zinc(II)36,37 and [5,15-bis(trifluoromethyl)-10,20-diphenylpor-
phinato]zinc(II)38 species were prepared similarly to methods reported
previously; synthetic details are described in the Supporting Information.
Chromatographic purification (Silica 60, 230-400 mesh, EM
Science) of all compounds was performed on the benchtop. Chemical
Bis[(2-5,10,15,20-tetraphenylporphinato)zinc(II)]ethyne (13). This
compound was synthesized by methods reported previously.31
([2-5,10,15,20-Tetraphenylporphinato]zinc(II))-[5′-10′20′-bis-
[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]ethyne (14).
Reagents: (2-ethynyl-5,10,15,20-tetraphenylporphinato)zinc(II) (45 mg,
0.0641 mmol), (5-bromo-10,20-bis[4-(3-methoxy-3-methylbutoxy)-
phenyl]porphinato)zinc(II) (55 mg, 0.0705 mmol), Pd2(dba)3 (9 mg,
0.0096 mmol), AsPh3 (24 mg, 0.0769 mmol), THF (10 mL), and
triethylamine (2 mL). Reaction time, 26 h. Chromatographic purifica-
tion: silica gel, 7:3 hexanes/THF. The green band was isolated, giving
desired product 14 (51 mg, 57% based on 45 mg of the porphyrinic
1
shifts for H NMR spectra are relative to residual protium (CDCl3, δ
7.24 ppm), while those for 19F NMR spectra are referenced to
fluorotrichloromethane (δ ) 0.00 ppm).
1
starting material). H NMR (250 MHz, 50:1 CDCl3/pyridine-d5): δ
10.04 (s, 1H), 9.61 (d, J ) 4.53 Hz, 2H), 9.50 (s, 1H), 9.24 (d, J )
4.43 Hz, 2H), 8.97 (d, J ) 4.34 Hz, 2H), 8.91 (d, J ) 4.43 Hz, 2H),
8.88 (s, 2H), 8.83 (s, 2H), 8.77 (d, J ) 4.63 Hz, 1H), 8.72 (d, J ) 4.53
Hz, 1H), 8.27 (m, 4H), 8.18 (m, 4H), 8.13 (d, J ) 8.18 Hz, 4H), 7.67
(m, 9H), 7.27 (d, J ) 8.18 Hz, 4H), 6.90 (t, J ) 7.45 Hz, 2H), 5.67 (t,
J ) 7.48 Hz, 1H), 4.35 (t, J ) 7.05 Hz, 4H), 3.31 (s, 6H), 2.19 (t, J
) 7.00 Hz, 4H), 1.34 (s, 12H). Visible (THF): 433 (5.37), 447 (5.46),
566 (4.41), 613 (4.34) nm. MS (MALDI-TOF) m/z: 1452 (calcd for
C90H70N8O4Zn2 1454).
Electronic spectra were recorded on an OLIS UV/visible/NIR
spectrophotometry system that is based on the optics of a Carey 14
spectrophotometer. Cyclic voltammetric measurements were carried out
with a PAR 273 electrochemical analyzer and a single-compartment
(23) Fletcher, J. T.; Therien, M. J. J. Am. Chem. Soc. 2000, 122, 12393-12394.
(24) Heck, R. F. Acc. Chem. Res. 1979, 12, 146-151.
(25) Kumada, M. Pure Appl. Chem. 1980, 52, 669-678.
(26) Negishi, E.-I.; Luo, F. T.; Frisbee, R.; Matsushita, H. Heterocycles 1982,
18, 117-122.
(27) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508-524.
(28) DiMagno, S. G.; Lin, V. S.-Y.; Therien, M. J. J. Am. Chem. Soc. 1993,
115, 2513-2515.
([2-5,10,15,20-Tetrakis(trifluoromethyl)porphinato]zinc(II))-
[(5′-10′20′-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)-
zinc(II)]ethyne (15). Reagents: (5-ethynyl-10,20-bis[4-(3-methoxy-
3-methylbutoxy)phenyl]porphinato)zinc(II) (40 mg, 0.055 mmol),
[2-bromo-5,10,15,20-tetrakis(trifluoromethyl)porphinato]zinc(II) (48
mg, 0.066 mmol), Pd2(dba)3 (8 mg, 0.0083 mmol), AsPh3 (20 mg, 0.066
mmol), THF (5 mL), and triethylamine (1 mL). Reaction time, 20 h.
Chromatographic purification: silica gel, 3:1 hexanes/THF, followed
by biobeads SX-1, THF. The dark green band was isolated, giving
desired product 15 (65 mg, 86% based on 40 mg of the porphyrinic
(29) DiMagno, S. G.; Lin, V. S.-Y.; Therien, M. J. J. Org. Chem. 1993, 58,
5983-5993.
(30) Lin, V. S.-Y.; DiMagno, S. G.; Therien, M. J. Science 1994, 264, 1105-
1111.
(31) Lin, V. S.-Y.; Therien, M. J. Chem. Eur. J. 1995, 1, 645-651.
(32) Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1984, 23, 539-644.
(33) Schore, N. E. Chem. ReV. 1988, 88, 1081-1119.
(34) Lautens, M.; Klute, W.; Tam, W. Chem. ReV. 1996, 96, 49-92.
(35) Tse, M. K.; Zhou, Z.; Mak, T. C. W.; Chan, K. S. Tetrahedron 2000, 56,
7779-7783.
(36) DiMagno, S. G.; Williams, R. A.; Therien, M. J. J. Org Chem. 1994, 59,
6943-6948.
1
starting material). H NMR (250 MHz, 50:1 CDCl3/pyridine-d5): δ
(37) Goll, J. G.; Moore, K. T.; Ghosh, A.; Therien, M. J. J. Am. Chem. Soc.
1996, 118, 8344-8354.
(38) Nishino, N.; Wagner, R. W.; Lindsey, J. S. J. Org. Chem. 1996, 61, 7534-
7544.
(39) Wagner, R. W.; Johnson, T. E.; Li, F.; Lindsey, J. S. J. Org. Chem. 1995,
60, 5266-5273.
9
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A R T I C L E S
Fletcher and Therien
10.22 (q, J ) 2.70 Hz, 1H), 10,21 (d, J ) 4.58 Hz, 2H), 10.09 (s, 1H),
9.65 (m, 6H), 9.26 (d, J ) 4.53 Hz, 2H), 9.15 (d, J ) 4.55 Hz, 2H),
8.99 (d, J ) 4.43 Hz, 2H), 8.15 (d, J ) 8.53 Hz, 4H), 7.30 (d, J )
8.63 Hz, 4H), 4.39 (t, J ) 7.13 Hz, 4H), 3.34 (s, 6H), 2.23 (t, J ) 7.14
Hz, 4H), 1.38 (s, 12H). 19F NMR (200 MHz, 50:1 CDCl3/pyridine-d5):
δ -33.74 (s, 3F), -36.42 (s, 3F), -36.74 (s, 3F), -36.83 (s, 3F).
Visible (THF): 432 (5.24), 565 (4.16), 612 (4.13), 700 (4.12) nm. MS
(MALDI-TOF) m/z: 1423 (calcd for C70H50F12N8O4Zn2 1423).
([2-5,10,15,20-Tetrakis(trifluoromethyl)porphinato]zinc(II))-
[(2′-5′,10′,15′,20′-tetraphenylporphinato)zinc(II)]ethyne (16). Re-
agents: (2-ethynyl-5,10,15,20-tetraphenylporphinato)zinc(II) (48 mg,
0.0684mmol),[2-bromo-5,10,15,20-tetrakis(trifluoromethyl)porphinato]-
zinc(II) (50 mg, 0.0684 mmol), Pd2(dba)3 (9 mg, 0.0103 mmol), AsPh3
(25 mg, 0.0821 mmol), THF (10 mL), and triethylamine (2 mL).
Reaction time, 17 h. Chromatographic purification: silica gel, 4:1
hexanes/THF. The purple-green band was isolated, giving desired
product 16 (47 mg, 51% based on 48 mg of the porphyrinic starting
methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II) (0.300 g, 0.384
mmol), ethynylbenzene (0.21 mL, 1.92 mmol), Pd2(dba)3 (0.053 g,
0.058 mmol), AsPh3 (0.141 g, 0.461 mmol), THF (15 mL), and
triethylamine (3 mL). Reaction time, 14 h. Chromatographic purifica-
tion: silica gel, 3:1 hexanes/THF. The dark green band was isolated,
giving desired product 19 (0.270 g, 88% based on 300 mg of the
(bromoporphinato)zinc(II) starting material). 1H NMR (250 MHz, 50:1
CDCl3/pyridine-d5): δ 10.04 (s, 1H), 9.77 (d, J ) 4.45 Hz, 2H), 9.22
(d, J ) 4.43 Hz, 2H), 8.98 (d, J ) 4.68 Hz, 2H), 8.93 (d, J ) 4.48 Hz,
2H), 8.07 (d, J ) 8.40 Hz, 4H), 8.00 (m, 2H), 7.49 (m, 2H), 7.43 (m,
1H), 7.26 (d, J ) 8.40 Hz, 4H), 4.36 (t, J ) 7.16 Hz, 4H), 3.32 (s,
6H), 2.20 (t, J ) 7.14 Hz, 4H), 1.36 (s, 12H). Visible (THF): 435
(5.57), 529 (3.55), 567 (4.19), 615 (4.26) nm. MS (MALDI-TOF) m/z:
856 (calcd for C52H49N4O4Zn 856).
[5,10,15,20-Tetraphenyl-2-(phenylethynyl)porphinato]zinc(II) (20).
Reagents: (2-ethynyl-5,10,15,20-tetraphenylporphinato)zinc(II) (68 mg,
0.097 mmol), iodobenzene (0.11 mL, 0.97 mmol), Pd2(dba)3 (13 mg,
0.015 mmol), AsPh3 (0.036 g, 0.116 mmol), THF (5 mL), and
triethylamine (1 mL). Reaction time, 20 h. Chromatographic purifica-
tion: silica gel, 4:1 hexanes/THF. The purple-green band was isolated,
giving desired product 20 (59 mg, 78% based on 68 mg of the
porphyrinic starting material). 1H NMR (250 MHz, 50:1 CDCl3/
pyridine-d5): δ 9.14 (s, 1H), 8.81 (s, 2H), 8.80 (s, 2H), 8.76 (d, J )
4.67 Hz, 1H), 8.67 (d, J ) 4.68 Hz, 1H), 8.15 (m, 8H), 7.67 (m, 14H),
7.35 (m, 3H). Visible (THF): 432, 524, 564, 600 nm. MS (MALDI-
TOF) m/z: 776 (calcd for C52H32N4Zn 776).
1
material). H NMR (250 MHz, 50:1 CDCl3/pyridine-d5): δ 9.65 (m,
7H), 9.38 (q, J ) 2.94 Hz, 1H), 8.93 (d, J ) 4.79 Hz, 1H), 8.86 (d, J
) 4.78 Hz, 1H), 8.83 (s, 2H), 8.80 (d, J ) 4.84 Hz, 1H), 8.75 (d, J )
4.64 Hz, 1H), 8.36 (m, 4H), 8.19 (m, 4H), 7.85 (m, 3H), 7.73 (m, 9H).
19F NMR (200 MHz, 50:1 CDCl3/pyridine-d5): δ -34.24 (s, 3F),
-36.51 (s, 3F), -36.83 (s, 3F), -37.10 (s, 3F). Visible (THF): 431
(5.39), 565 (4.37), 645 (4.47) nm. MS (MALDI-TOF) m/z: 1343 (calcd
for C70H34F12N8Zn2 1342).
Bis([2-5,15-(trifluoromethyl)-10,20-diphenylporphinato]zinc-
(II))ethyne (17). Reagents: [2-bromo-5,15-bis(trifluoromethyl)-10,20-
diphenylporphinato]zinc(II) (42 mg, 0.0612 mmol), [2-ethynyl-5,15-
bis(trifluoromethyl)-10,20-diphenylporphinato]zinc(II) (49 mg, 0.0661
mmol), Pd2dba3 (8 mg, 0.0092 mmol), AsPh3 (15 mg, 0.0735 mmol),
THF (5 mL), and triethylamine (1 mL). Reaction time, 19 h.
Chromatographic purification: silica gel, 1:1 hexanes/toluene. The final
green band was isolated, giving desired product 17 (37 mg, 45% based
on 42 mg of the (bromoporphinato)zinc(II) starting material). 1H NMR
(250 MHz, 50:1 CDCl3/pyridine-d5): δ 9.70-9.40 (m, 6H), 8.90-8.70
(m, 8H), 8.30-7.90 (m, 8H), 7.90-7.40 (m, 12H). 19F NMR (200 MHz,
50:1 CDCl3/pyridine-d5): δ -32.45 (s, 3F), -35.74 (s, 3F). Visible
(THF): 427 (5.21), 573 (4.17), 628 (sh) (4.54), 646 (4.66) nm. MS
(MALDI-TOF) m/z: 1342 (calcd for C70H34F12N8Zn2 1342).
[5,15-Bis(trifluoromethyl)-10,20-diphenyl-2-(phenylethynyl)por-
phinato]zinc(II) (21). Reagents: [2-bromo-5,15-bis(trifluoromethyl)-
10,20-diphenylporphinato]zinc(II) (23 mg, 0.031 mmol), ethynylben-
zene (34 µL, 0.31 mmol), Pd2dba3 (4 mg, 0.0047 mmol), AsPh3 (11
mg, 0.0372 mmol), THF (5 mL), and triethylamine (1 mL). Reaction
time, 17 h. Chromatographic purification: silica gel, 1:1 hexanes/
toluene. The green band was isolated, giving desired product 21 (18
mg, 76% based on 23 mg of the (bromoporphinato)zinc(II) starting
1
material). H NMR (250 MHz, 50:1 CDCl3/pyridine-d5): δ 9.53 (m,
3H), 8.86 (s, 1H), 8.80 (d, J ) 4.98 Hz, 1H), 8.77 (d, J ) 4.71 Hz,
1H), 8.75 (d, J ) 4.0.82 Hz, 1H), 8.07 (m, 4H), 7.78 (m, 2H), 7.70
(m, 6H), 7.40 (m, 3H). 19F NMR (200 MHz, 50:1 CDCl3/pyridine-d5):
δ -32.39 (s, 3F), -35.77 (s, 3F). Visible (THF): 430 (5.13), 571 (3.89),
615 (4.30) nm. MS (MALDI-TOF) m/z: 760 (calcd for C42H22F6N4Zn
760).
Bis([2-5,10,15,20-tetrakis(trifluoromethyl)porphinato]zinc(II))-
ethyne (18). Reagents: [2-bromo-5,10,15,20-tetrakis(trifluoromethyl)-
porphinato]zinc(II) (61 mg, 0.0840 mmol), [2-ethynyl-5,10,15,20-
tetrakis(trifluoromethyl)porphinato]zinc(II) (48 mg, 0.0700 mmol),
Pd2dba3 (10 mg, 0.0105 mmol), AsPh3 (26 mg, 0.0840 mmol), THF
(10 mL), and triethylamine (2 mL). Reaction time, 17 h. Chromato-
graphic purification: silica gel, 1:1 hexanes/toluene. The final green
band was isolated, giving desired product 18 (67 mg, 73% based on
[5,10,15,20-Tetrakis(trifluoromethyl)-2-(phenylethynyl)porphi-
nato]zinc(II) (22). Reagents: [2-bromo-5,10,15,20-tetrakis(trifluoro-
methyl)porphinato]zinc(II) (45 mg, 0.0621 mmol), ethynylbenzene (37
mL, 0.336 mmol), Pd2dba3 (9 mg, 0.0101 mmol), AsPh3 (25 mg, 0.0806
mol), THF (5 mL), and triethylamine (1 mL). Reaction time, 20 h.
Chromatographic purification: silica gel, 1:1 hexanes/toluene. The green
band was isolated, giving desired product 22 (41 mg, 89% based on
1
48 mg of the porphyrinic starting material). H NMR (50:1 CDCl3/
1
45 mg of the (bromoporphinato)zinc(II) starting material). H NMR
pyridine-d5): δ 10.09 (q, J ) 2.78 Hz, 2H), 9.78 (m, 2H), 9.66 (m,
10H). 19F NMR (200 MHz, 50:1 CDCl3/pyridine-d5): δ -33.71 (s,
3F), -36.51 (s, 3F), -36.80 (s, 3F), -36.95 (s, 3F). Visible (THF):
425 (5.20), 572 (4.20), 620 (sh) (4.31), 655 (4.68) nm. MS (MALDI-
TOF) m/z: 1311 (calcd for C50H14F24N8Zn2 1310).
General Procedure for the Preparation of (Phenylethynylpor-
phinato)zinc(II) Complexes.40,41 A 50-mL Schlenk tube was charged
with ethynylbenzene (5-20 equiv), a meso- or â-bromoporphyrin (1
equiv), Pd2(dba)3 (0.15 equiv), and AsPh3 (1.2 equiv). These reagents
were dissolved in 5:1 THF/TEA and stirred for 14-20 h at 40 °C.
Following evaporation of the solvent, the residue was purified via
chromatography.
(50:1 CDCl3/pyridine-d5): δ 9.69 (q, J ) 2.86 Hz, 1H), 9.58 (m, 5H),
7.85 (m, 2H), 7.46 (m, 2H), 7.10 (m, 1H). 19F NMR (200 MHz, 50:1
CDCl3/pyridine-d5): δ -33.79 (s, 3F), -36.55 (s, 3F), -36.84 (s, 3F),
-37.12 (s, 3F); visible (THF) 424 (5.03), 574 (3.98), 624 (4.36) nm.
MS (MALDI-TOF) m/z: 743 (calc for C32H12F12N4Zn 744).
Standard Procedure for the Syntheses of Cofacial Bis[(porphi-
nato)zinc(II)] Compounds.23,42 A 50-mL Schlenk tube was charged
with an ethyne-bridged bis[(porphinato)zinc(II)] compound (1 equiv)
and Co2(CO)8 (1 equiv). These reagents were dissolved in 5:1 toluene/
dioxane and heated to 100 °C; dropwise addition of 5 mL of a toluene
solution containing 1,6-heptadiyne (20 equiv) and Co2(CO)8 (1 equiv)
over a 17-h period followed. After the addition was complete, the
solution was evaporated to dryness and the residue purified by
chromatography.
(10,20-Bis[4-(3-methoxy-3-methylbutoxy)phenyl]-5-(phenyleth-
ynyl)porphinato)zinc(II) (19). Reagents: (5-bromo-10,20-bis[4-(3-
5,6-Bis[(5′,5′′-10′,20′-bis[4-(3-methoxy-3-methylbutoxy)phenyl]-
porphinato)zinc(II)]indane (1). Reagents: 12 (50 mg, 35 µmol), Co2-
(40) LeCours, S. M.; Guan, H.-W.; DiMagno, S. G.; Wang, C. H.; Therien, M.
J. J. Am. Chem. Soc. 1996, 118, 1497-1503.
(41) LeCours, S. M.; DiMagno, S. G.; Therien, M. J. J. Am. Chem. Soc. 1996,
118, 11854-11864.
(42) Fletcher, J. T.; Therien, M. J. Inorg. Chem. 2002, 41, 331-341.
9
4300 J. AM. CHEM. SOC. VOL. 124, NO. 16, 2002

Cofacial Prophyrin Structural Motif
A R T I C L E S
(CO)8 (12 mg, 35 µmol), dioxane, (2.5 mL), and toluene (10 mL).
Added dropwise: 1,6-heptadiyne (40 µl, 350 µmol) and Co2(CO)8 (12
mg, 35 µmol). Chromatographic purification: silica gel, 3:2 hexanes/
THF. The red band was isolated, giving desired product 1 (50 mg,
Visible (THF): 418 (5.43), 555 (4.26), 612 (4.22) nm. MS (MALDI-
TOF) m/z: 1514 (calcd for C77H58F12N8O4Zn2 1514). The final green-
brown band was also isolated, giving pure recovered starting material
15 (14 mg, 55%).
1
94% based on 50 mg of starting material 12). H NMR (250 MHz,
5-(2′-5′,10′,15′,20′-[Tetrakis(trifluoromethyl)porphinato]zinc-
(II))-6-[(2′′-5′′,10′′,15′′,20′′-tetraphenylporphinato)zinc(II)]in-
dane (5). Reagents: 16 (35 mg, 26 µmol), Co2(CO)8 (9 mg, 26 µmol),
dioxane (1 mL), and toluene (5 mL). Added dropwise: 1,6-heptadiyne
(30 µL, 260 µmol) and Co2(CO)8 (9 mg, 26 µmol). Chromatographic
purification: silica gel, 4:1 hexanes/THF. The first green band was
isolated, giving desired product 5 (13 mg, 35% based on 35 mg of
starting material 16). 1H NMR (250 MHz, 50:1 CDCl3/pyridine-d5): δ
9.74 (m, 1H), 9.63 (m, 1H), 9.57 (m, 2H), 9.37 (m, 1H), 9.13 (m, 1H),
9.04 (m, 1H), 8.93 (s, 1H), 8.90 (d, J ) 4.71 Hz, 1H), 8.88 (d, J )
4.68 Hz, 1H), 8.86 (d, J ) 4.66 Hz, 1H), 8.82 (d, J ) 4.60 Hz, 1H),
8.74 (d, J ) 4.60 Hz, 1H), 8.65 (d, J ) 4.66 Hz, 1H), 8.44 (m, 1H),
8.30 (m, 1H), 8.23 (m, 4H), 8.16 (m, 4H), 7.70 (m, 8H), 7.10 (m, 2H),
6.64 (m, 2H), 2.83 (m, 4H), 2.08 (m, 2H). 19F NMR (200 MHz, 50:1
CDCl3/pyridine-d5): δ -32.79 (s, 3F), -36.07 (s, 3F), -36.36 (s, 3F),
-38.19 (s, 3F). Visible (THF): 426 (5.36), 562 (4.83), 612 (4.22) nm.
MS (MALDI-TOF) m/z: 1436 (calcd for C77H42F12N8Zn2 1434). A
slower brown-green band, following the product band, was also isolated,
giving pure recovered starting material 16 (21 mg, 60%).
5,6-Bis([2′-5′,15′-(trifluoromethyl)-10′,20′-diphenylporphinato]-
zinc(II))indane (6). Reagents: 17 (22 mg, 16.7 µmol), Co2(CO)8 (6.5
mg, 18.6 µmol), dioxane (1 mL), and toluene (4 mL). Added
dropwise: 1,6-heptadiyne (25 µL, 186 µmol) and Co2(CO)8 (6.5 mg,
18.6 µmol). Chromatographic purification: silica gel, 100:100:1
hexanes/toluene/THF. The green band was isolated, giving desired
product 6 (12 mg, 45% based on 22 mg of starting material 17).1H
NMR (250 MHz, 50:1 CDCl3/pyridine-d5): δ 9.67 (m, 2H), 9.49 (m,
2H), 9.42 (m, 2H), 8.95-8.60 (m, 8H), 8.42 (s, 2H), 8.10-7.40 (m,
18H), 6.48 (m, 2H), 3.05 (m, 4H), 2.15 (m, 2H). 19F NMR (200 MHz,
50:1 CDCl3/pyridine-d5): δ -31.00 (s, 3F), -35.68 (s, 3F). Visible
(THF): 419 (5.30), 564 (4.12), 610 (4.41) nm. MS (MALDI-TOF)
m/z: 1434 (calcd for C77H42F12N8Zn2 1434).
pyridine-d5): δ 10.31 (d, J ) 4.65 Hz, 4H), 10.11 (s, 2H), 9.29 (d, J
) 4.50 Hz, 4H), 9.13 (d, J ) 4.68 Hz, 4H), 8.95 (d, J ) 4.48 Hz, 4H),
8.64 (s, 2H), 8.02 (d, J ) 8.28 Hz, 4H), 7.66 (d, J ) 8.25 Hz, 4H),
7.32 (d, J ) 8.33 Hz, 4H), 7.25 (d, J ) 8.45 Hz, 4H), 4.43 (t, J ) 7.06
Hz, 8H), 3.41 (t, J ) 6.88 Hz, 4H), 3.30 (s, 12H), 2.40 (m, 2H), 2.26
(t, J ) 6.98 Hz, 8H), 1.36 (s, 24H). Visible (THF): 411 (5.54), 434
(4.74), 558 (4.32), 589 (3.72), 599 (3.71) nm. MS (MALDI-TOF) m/z:
1627 (calcd for C97H94N8O8Zn2 1627).
5,6-Bis[(2′-5′,10′,15′,20′-tetraphenylporphinato)zinc(II)]in-
dane (2). Reagents: 13 (25 mg, 18.1 µmol), Co2(CO)8 (6 mg, 18.1
µmol), dioxane (1 mL), and toluene (4 mL). Added dropwise: 1,6-
heptadiyne (41 µL, 362 µmol) and Co2(CO)8 (6 mg, 18.1 µmol).
Chromatographic purification: silica gel, 1:1 hexanes/toluene. The
purple band was isolated, giving desired product 2 (12 mg, 45% based
1
on 25 mg of starting material 13). H NMR (250 MHz, 50:1 CDCl3/
pyridine-d5): δ 8.46-8.92 (m, 14H), 6.80-8.42 (m, 42 H), 2.96 (d, J
) 7.2 Hz, 2H), 2.83 (t, J ) 7.2 Hz, 2H), 2.10 (m, 2H). Visible (THF):
424 (5.44), 561 (4.34), 599 (3.92) nm. MS (MALDI-TOF) m/z: 1468
(calcd for C97H62N8Zn2 1466).
5-([2′-5′,10′,15′,20′-Tetraphenylporphinato]zinc(II))-6-[(5′′-
10′′,20′′-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc-
(II)]indane (3). Reagents: 14 (25 mg, 17.8 µmol), Co2(CO)8 (6 mg,
17.9 µmol), dioxane (1 mL), and toluene (5 mL). Added dropwise:
1,6-heptadiyne (41 µL, 357 µmol) and Co2(CO)8 (6 mg, 17.9 µmol).
Chromatographic purification: silica gel, 4:1 hexanes/THF. The first
green band was isolated, giving desired product 3 (18 mg, 67% based
1
on 25 mg of starting material 14). H NMR (250 MHz, 50:1 CDCl3/
pyridine-d5): δ 9.86 (s, 1H), 9.52 (d, J ) 4.61 Hz, 1H), 9.23 (d, J )
4.51 Hz, 1H), 9.06 (d, J ) 4.37 Hz, 1H), 8.97 (d, J ) 4.38 Hz, 1H),
8.96 (d, J ) 4.25 Hz, 1H), 8.92 (d, J ) 4.57 Hz, 1H), 8.75 (m, 2H),
8.61 (m, 6H), 8.49 (d, J ) 4.70 Hz, 1H), 8.32 (d, J ) 4.68 Hz, 1H),
8.09 (m, 1H), 7.98 (m, 3H), 7.86 (m, 3H), 7.58 (m, 16H), 7.25 (m,
2H), 7.14 (m, 2H), 7.03 (m, 1H), 6.87 (m, 1H), 6.36 (m, 1H), 4.54 (m,
1H), 4.46 (t, J ) 7.18 Hz, 2H), 4.17 (t, J ) 7.15 Hz, 2H), 3.37 (s, 3H),
3.23 (s, 3H), 3.04 (t, J ) 6.80 Hz, 4H), 2.28 (t, J ) 7.09 Hz, 4H), 2.07
(t, J ) 7.12 Hz, 2H), 1.42 (s, 6H), 1.26 (s, 6H). Visible (THF): 418
(5.45), 438 (5.09), 555 (4.31), 595 (3.76) nm. MS (MALDI-TOF) m/z:
1546 (calcd for C97H78N8O4Zn21546). The final green band was also
collected, which corresponded to pure starting material 14 (8 mg, 32%).
5,6-Bis([2′-5′,10′,15′,20′-tetrakis(trifluoromethyl)porphinato]-
zinc(II))indane (7). Reagents: 18 (16 mg, 12.2 µmol), dioxane (1 mL),
and toluene (4 mL). Added dropwise: 1,6-heptadiyne (28 µL, 243
µmol) and Co2(CO)8 (8 mg, 24.3 µmol). Chromatographic purifica-
tion: silica gel, 1:1 hexanes/toluene. The green band was isolated,
giving desired product 7 (6 mg, 29% based on 16 mg of starting material
1
18). H NMR (250 MHz, 50:1 CDCl3/pyridine-d5): δ 9.65 (m, 2H),
9.53 (m, 4H), 9.12 (dq, J1 ) 2.22 Hz, J2 ) 3.01 Hz, 2H), 8.67 (dq, J1
) 2.60 Hz, J2 ) 2.63 Hz, 2H), 8.48 (m, 2H), 8.35 (s, 2H), 8.18 (dq, J1
) 2.50 Hz, J2 ) 2.67 Hz, 2H), 3.45 (m, 4H), 2.51 (m, 2H). 19F NMR
(200 MHz, 50:1 CDCl3/pyridine-d5): δ -34.29 (s, 3F), -36.23 (s, 3F),
-36.45 (s, 3F), -37.16 (s, 3F). Visible (THF): 399 (5.18), 570 (3.94),
608 (4.23) nm. MS (MALDI-TOF) m/z: 1400 (calcd for C57H22F24N8-
Zn2 1402). The final green band was also isolated, giving pure recovered
starting material 18 (14 mg, 63%).
Standard Procedure for Metal-Templated Cycloaddition Reac-
tions Involving (Phenylethynylporphinato)zinc(II) Substrates.23A 50-
mL Schlenk tube was charged with a (phenylethynylporphinato)zinc(II)
compound (1 equiv) and Co2(CO)8 (1 equiv). These reagents were
dissolved in 5:1 toluene/dioxane and heated to 100 °C; dropwise
addition of 5 mL of a toluene solution containing 1,6-heptadiyne (10
eq) over a 90-min period followed. After the addition was complete,
the solution was evaporated to dryness and the residue purified by
chromatography.
5-([2′-5′,10′,15′,20′-Tetrakis(trifluoromethyl)porphinato]zinc-
(II))-6-[(5′′-10′′,20′′-bis[4-(3-methoxy-3-methylbutoxy)phenyl]por-
phinato)zinc(II)]indane (4). Reagents: 15 (25 mg, 18.2 µmol),
Co2(CO)8 (6 mg, 18.2 µmol), dioxane (1 mL), and toluene (5 mL).
Added dropwise: 1,6-heptadiyne (21 µL, 182 µmol) and Co2(CO)8 (6
mg, 18.2 µmol). Chromatographic purification: silica gel, 4:1 hexanes/
THF. The first green band was isolated, giving desired product 4 (11
mg, 41% based on 25 mg of starting material 15).1H NMR (250 MHz,
50:1 CDCl3/pyridine-d5): δ 9.95 (s, 1H), 9.52 (s, 2H), 9.41 (s, 3H),
9.27 (d, J ) 4.53 Hz, 1H), 9.19 (m, 1H), 9.16 (d, J ) 4.73 Hz, 1H),
9.10 (d, J ) 4.43 Hz, 1H), 9.05 (d, J ) 4.45 Hz, 1H), 9.04 (d, J )
4.48 Hz, 1H), 8.92 (m, 1H), 8.81 (d, J ) 4.55 Hz, 1H) 8.79 (m, 1H),
8.76 (d, J ) 4.43 Hz, 1H), 8.42 (s, 1H), 8.38 (dd, J1 ) 8.27 Hz, J2 )
2.05 Hz, 1H), 8.07 (dd, J1 ) 8.27 Hz, J2 ) 1.96 Hz, 1H), 7.87 (dd, J1
) 8.27 Hz, J2 ) 2.00 Hz, 1H), 7.81 (dd, J1 ) 8.53 Hz, J2 ) 2.00 Hz,
1H), 7.43 (dd, J1 ) 8.49 Hz, J2 ) 2.20 Hz, 1H), 7.28 (dd, J1 ) 11.41
Hz, J2 ) 3.16 Hz, 1H), 7.15 (dd, J1 ) 10.8 Hz, J2 ) 2.45 Hz, 1H),
7.09 (dd, J1 ) 10.8 Hz, J2 ) 2.45 Hz, 1H), 4.41 (t, J ) 7.13 Hz, 2H),
4.28 (t, J ) 7.18 Hz, 2H), 3.45 (m, 4H), 3.33 (s, 3H), 3.27 (s, 3H),
2.48 (m, 2H), 2.24 (t, J ) 7.15 Hz, 2H), 2.14 (t, J ) 7.15 Hz, 2H),
1.38 (s, 6H), 1.31 (s, 6H). 19F NMR (200 MHz, 50:1 CDCl3/pyridine-
d5): δ -31.50 (s, 3F), -36.51 (s, 3F), -36.72 (s, 3F), -38.36 (s, 3F).
5-Phenyl-6-[(5′-10′,20′-bis[4-(3-methoxy-3-methylbutoxy)phenyl]-
porphinato)zinc(II)]indane (8). Reagents: 19 (40 mg, 46.6 µmol),
Co2(CO)8 (16 mg, 46.6 µmol), dioxane (1 mL), and toluene (4 mL).
Added dropwise: 1,6-heptadiyne (53 µL, 466 µmol). Chromatographic
purification: silica gel, 4:1 hexanes/THF. The purple band was isolated,
giving desired product 8 (44 mg, 99% based on 40 mg of starting
9
J. AM. CHEM. SOC. VOL. 124, NO. 16, 2002 4301

A R T I C L E S
Fletcher and Therien
Scheme 1a
1
material 19). H NMR (250 MHz, 50:1 CDCl3/pyridine-d5): δ 10.11
(s, 1H), 9.21 (d, J ) 4.48 Hz, 2H), 8.93 (d, J ) 4.43 Hz, 2H), 8.87 (d,
J ) 4.60 Hz, 2H), 8.81 (d, J ) 4.55 Hz, 2H), 8.03 (m, 4H), 7.81 (s,
1H), 7.57 (s, 1H), 7.20 (m, 4H), 7.08 (m, 2H), 6.24 (m, 3H), 4.32 (t,
J ) 7.37 Hz, 4H), 3.30 (s, 6H), 3.24 (t, J ) 7.34 Hz, 2H), 3.07 (t, J
) 7.32 Hz, 2H), 2.30 (m, 2H), 2.18 (t, J ) 7.14 Hz, 4H), 1.34 (s,
12H). Visible (THF): 422 (5.56), 552 (4.21), 592 (3.57) nm. MS
(MALDI-TOF) m/z: 948 (calcd for C59H56N4O4Zn 948).
5-Phenyl-6-[(2′-5′,10′,15′,20′-tetraphenylporphinato)zinc(II)]in-
dane (9). Reagents: 20 (59 mg, 75.8 µmol), Co2(CO)8 (26 mg, 75.8
µmol), dioxane (2 mL), and toluene (8 mL). Added dropwise: 1,6-
heptadiyne (86 µL, 758 µmol). Chromatographic purification: silica
gel, 4:1 hexanes/THF. The purple band was isolated, giving desired
1
product 9 (64 mg, 97% based on 59 mg of starting material 20). H
NMR (250 MHz, 50:1 CDCl3/pyridine-d5): δ 8.83 (s, 2H), 8.81 (s,
1H), 8.80 (s, 1H), 8.72 (d, J ) 4.71 Hz, 1H), 8.58 (d, J ) 4.69 Hz,
1H), 8.45 (s, 1H), 8.15 (m, 6H), 7.90-7.10 (m,18H), 7.06 (m, 2H),
6.75 (m, 1H), 6.65 (m, 2H), 2.93 (t, J ) 7.25 Hz, 2H), 2.78 (t, J )
7.33 Hz, 2H), 2.10 (m, 2H). Visible (THF): 427 (5.55), 558 (4.16),
597 (3.92) nm. MS (MALDI-TOF) m/z: 868 (calcd for C59H40N4Zn
868).
a Reagents and conditions: (a) N-bromosuccinimide (1.1 equiv), metha-
nol, reflux, 1.5 h (42%); (b) trimethylsilylacetylene (10 equiv), Pd(PPh3)2Cl2
(10 mol %), CuI (10 mol %), THF/TEA 5:2, 40 °C, 17 h (75%); (c) (i)
TBAF (2 equiv), THF, 10 min, (ii) NaHCO3 (aq) (96%); (d) 1 (1.2 equiv),
Pd2(dba)3 (15 mol %), AsPh3 (1.2 equiv), THF/TEA 5:1, 40 °C, 17 h (73%).
stabilized uniformly by nearly 700 mV36,37 relative to their
corresponding (5,10,15,20-tetraphenylporphinato)zinc(II) bench-
marks.
5-Phenyl-6-([2′-5′,15′-bis(trifluoromethyl)-10′,20′-diphenylpor-
phinato]zinc(II))indane (10). Reagents: 21 (30 mg, 39.4 µmol), Co2-
(CO)8 (13 mg, 26.8 µmol), dioxane (1 mL), and toluene (5 mL). Added
dropwise: 1,6-heptadiyne (45 µL, 394 µmol). Chromatographic
purification: silica gel, 1:1 hexanes/toluene. The green band was
isolated, giving desired product 10 (26 mg, 77% based on 30 mg of
starting material 21). 1H NMR (250 MHz, 50:1 CDCl3/pyridine-d5): δ
9.54 (m, 3H), 8.85 (d, J ) 4.83 Hz, 1H), 8.83 (d, J ) 4.79 Hz, 1H),
8.79 (d, J ) 4.98 Hz, 1H), 8.39 (m, 1H), 8.02 (s, 1H), 7.88-7.42 (m,
13H), 7.10 (m, 3H), 3.12 (m, 2H), 2.98 (m, 2H), 2.20 (m,2H). 19F NMR
(200 MHz, 50:1 CDCl3/pyridine-d5): δ -31.22 (s, 3F), -35.65 (s, 3F).
Visible (THF): 424 (5.24), 566 (3.94), 608 (4.24) nm. MS (MALDI-
TOF) m/z: 853 (calcd for C49H30F6N4Zn 852).
Macrocycle-to-macrocycle ethyne-bridged bis[(porphinato)-
metal] compounds30,31 serve as key precursors to cofacial
porphyrin compounds synthesized via metal-templated cycload-
dition reactions. The synthesis of the extremely electron-poor
ethyne-linked dimeric (porphinato)zinc(II) species, bis[([(2,2′-
5,10,15,20-trifluoromethyl]porphinato)zinc(II)]ethyne (18), is
depicted in Scheme 1. [(5,10,15,20-Trifluoromethyl)porphinato]-
zinc(II)37 can be readily brominated with N-bromosuccinim-
ide,28,29,35,43,44 producing [(2-bromo-5,10,15,20-trifluoromethyl)-
porphinato]zinc(II); palladium-catalyzed cross-coupling with
trimethylsilylacetylene gives the ethyne-elaborated macrocycle
[5,10,15,20-(trifluoromethyl)-2-(trimethylsilylethynyl)porphinato]-
zinc(II). Deprotection with TBAF yields [(2-ethynyl-5,10,15,-
20-trifluoromethyl)porphinato]zinc(II), which can be cross-
coupled with [(2-bromo-5,10,15,20-trifluoromethyl)porphinato]-
zinc(II) to afford 18. Compound 18 serves as the first example
of an electron-deficient ethyne-bridged bis[(porphinato)metal]
species; notably these reactions demonstrate that perfluoroalkyl-
substituted porphyrin macrocycles can be both suitably deriva-
tized and coupled in yields comparable to that established
previously for their electron-rich counterparts.28-31,40,41
5-Phenyl-6-([2′-5′,10′,15′,20′-tetrakis(trifluoromethyl)porphinato]-
zinc(II))indane (11). Reagents: 22 (20 mg, 26.8 µmol), Co2(CO)8 (9
mg, 26.8 µmol), dioxane (1 mL), and toluene (4 mL). Added
dropwise: 1,6-heptadiyne (31 µL, 268 µmol). Chromatographic
purification: silica gel, 1:1 hexanes/toluene. The green band was
isolated, giving desired product 11 (20 mg, 89% based on 20 mg of
starting material 22). 1H NMR (250 MHz, 50:1 CDCl3/pyridine-d5): δ
9.61 (m, 5H), 9.40 (dq, J1 ) 2.51 Hz, J2 ) 2.70 Hz, 1H), 9.01 (q, J )
3.07 Hz, 1H), 7.72 (s, 1H), 7.51 (s, 1H), 7.07 (m, 2H), 6.52 (m, 3H),
3.15 (m, 4H), 2.27 (m, 2H). 19F NMR (200 MHz, 50:1 CDCl3/pyridine-
d5): δ -33.18 (s, 3F), -36.15 (s, 3F), -36.57 (s, 3F), -37.21 (s, 3F).
Visible (THF): 420 (5.10), 570 (3.96), 612 (4.22) nm. MS (MALDI-
TOF) m/z: 837 (calcd for C39H20F12N4Zn 836).
As such, ethyne-bridged precursor compounds bis[(5,5′-10,-
20-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc-
(II)]ethyne (12), bis[(2,2′-5,10,15,20-tetraphenylporphinato)-
zinc(II)]ethyne (13), ([2-5,10,15,20-tetraphenylporphinato]-
zinc(II))-[5′-10′20′-bis[4-(3-methoxy-3-methylbutoxy)phen-
yl]porphinato)zinc(II)]ethyne (14), ([2-5,10,15,20-tetrakis(tri-
fluoromethyl)porphinato]zinc(II))-[(5′-10′20′-bis[4-(3-meth-
oxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]ethyne (15), ([2-
5,10,15,20-tetrakis(trifluoromethyl)porphinato]zinc(II))-[(2′-5′,-
10′,15′,20′-tetraphenylporphinato)zinc(II)]ethyne (16), and bis-
([2-5,15-(trifluoromethyl)-10,20-diphenylporphinato]zinc-
(II))ethyne (17) were all fabricated in a straightforward fash-
ion.30,31 Ethyne-bridged, bis[(porphinato)zinc(II)] species 12-
18 were converted to their corresponding 1,2-phenylene-bridged
bis[(porphinato)metal compounds (Scheme 2) via a Co-tem-
plated cycloaddition with 1,6-heptadiyne; note that in these
species (1-7), both macrocycle electronic structure and the
Results and Discussion
Synthesis. Recently, we reported a synthetic route that utilizes
cobalt-mediated cycloaddition of ethyne-elaborated porphyrin
substrates to give 5,6-indanyl-bridged cofacial porphyrin struc-
tures.23 In this study, we exploit the modular nature of this
synthetic method, varying the electronic properties of the
porphyrin units and the topological connectivity between these
species, to establish the extent to which the overall electronic
properties of cofacial (porphinato)metal complexes can be
regulated.
A series of monomeric aryl- and perfluoroalkyl-functionalized
(porphinato)zinc(II) units were employed as building blocks in
these syntheses. The electronic properties of these two broad
classes of (porphinato)zinc(II) compounds vary markedly; note,
for example, that [5,10,15,20-tetrakis(perfluoroalkyl)porphinato]-
zinc(II) complexes possess HOMO and LUMO levels that are
9
4302 J. AM. CHEM. SOC. VOL. 124, NO. 16, 2002

Cofacial Prophyrin Structural Motif
A R T I C L E S
Scheme 2a
transformations are sensitive to both alkyne steric32-34,42,45-48
and electronic45,49 properties; hence, the range of yields observed
for this series is not surprising, given that reaction time was
held essentially constant. Interestingly, in most cases, more than
90% of the porphyrin reactant was accounted for either as
product or recovered starting material (see Experimental Sec-
tion); as deleterious side reactions appear lacking, it is likely
that each of these transformations can be optimized individually
to give similarly high yields as that observed for the 12-to-1
conversion.
While 1,2-phenylene-bridged bis[(porphinato)metal] com-
pounds analogous to meso-meso bridged 1 and meso-â
bridged 5-([2′-5′,10′,15′,20′-tetraphenylporphinato]zinc(II))-
6-[(5′′-10′′,20′′-bis[4-(3-methoxy-3-methylbutoxy)phenyl]por-
phinato)zinc(II)]indane (3) have been reported,23 5,6-bis[(2′-
5′,10′,15′,20′-tetraphenylporphinato)zinc(II)]indane (2) and
5,6-bis([2′-5′,10′,15′,20′-tetrakis(trifluoromethyl)porphinato]-
zinc(II))indane (7) serve as archetypal examples of â-â bridged
cofacial porphyrin structures. Products 5-([2′-5′,10′,15′,20′-
tetrakis(trifluoromethyl)porphinato]zinc(II))-6-[(5′′-10′′,20′′-
bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]-
indane (4), 5-(2′-5′,10′,15′,20′-[tetrakis(trifluoromethyl)por-
phinato]zinc(II))-6-[(2′′-5′′,10′′,15′′,20′′-tetraphenylporphin-
ato)zinc(II)]indane (5), 5,6-bis([2′-5′,15′-(trifluoromethyl)-
10′,20′-diphenylporphinato]zinc(II))indane (6), and 7 are the first
examples of cofacial bis[(porphinato)metal] complexes that
possess perfluoroalkyl-substituted macrocycles; furthermore,
these species highlight that this synthetic strategy enables
extensive regulation of both the extent of macrocycle-macro-
cycle electronic asymmetry and the energetics of the cofacial
(porphinato)metal frontier orbitals (vide infra).
The nature of interporphyryl π-π interactions will play a
pivotal role in determining the electronic properties of cofacial
bis[(porphinato)metal] complexes and will be determined by
the indane-to-porphyrin macrocycle linkage topology (Figure
1). Unlike 1,2-phenylene-bridged cofacial porphyrin structures
with meso-meso connectivity,19,23,42,50 two possible structural
conformations can be manifest in meso-â and â-â bridged
systems. In the meso-â bridging motif highlighted in 3 and 4
(Figure 1A), the two enantiomers possess equivalent π-cofacial
electronic interactions. In contrast, disparate π-π interactions
are evident for the two â-â bridged atropisomers possible for
1
2, 5, 6, and 7 (Figure 1B). Both H and 19F NMR analyses of
each of these â-â bridged bis[(porphinato)zinc(II)] complexes
confirm that only one atropisomer exists in solution at room
temperature (see Experimental Section), presumably due to the
fact that the eclipsed Z isomer possesses augmented steric strain
relative to the E configuration (Figure 1B).
aGeneral reaction conditions employed: (i) Co2(CO)8 (1 equiv), toluene/
dioxane 5:1, 100 °C, 10 min; (ii) 1,6-heptadiyne (20 equiv), Co2(CO)8 (1
equiv), 5 mL of toluene, added 17 h dropwise.
(43) Samuels, E.; Shuttleworth, R.; Stevens, T. S. J. Chem. Soc. C 1968, 145-
147.
(44) Callot, H. J. Bull. Soc. Chim. Fr. 1974, 1492.
(45) Dickson, R. S.; Fraser, P. J. AdV. Organomet. Chem. 1974, 12, 323-377.
(46) Hillard, R. L., III; Vollhardt, K. P. C. J. Am. Chem. Soc. 1977, 99, 4058-
4069.
(47) Berris, B. C.; Hovakeemian, G. H.; Lai, Y.-H.; Mestdagh, H.; Vollhardt,
K. P. C. J. Am. Chem. Soc. 1985, 107, 5670-5687.
(48) Chiusoli, G. P.; Costa, M.; Zhou, Z. Gazz. Chim. Ital. 1992, 122, 441-
449.
(49) It is known that Co2(CO)8 forms more stable complexes with alkynes
containing electron-withdrawing substituents (see ref 45); a less reactive
cobalt intermediate is likely to be at least partly responsible for the decreased
product yields that are apparent in cycloaddition reactions involving
perfluoroalkyl-substituted porphyrin species.
(porphinato)metal-(porphinato)metal linkage topology are var-
ied systematically.
The [2+2+2] cycloaddition protocol was optimized for
substrate 12, which was converted to 5,6-bis[(5′,5′′-10′,20′-
bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]-
indane (1) in 94% yield; these reaction conditions were
employed for all ethyne-bridged porphyrin substrates in this
study. It is well known that such metal-mediated cycloaddition
(50) Shimazaki, Y.; Takesue, H.; Chishiro, T.; Tani, F.; Naruta, Y. Chem. Lett.
2001, 538-539.
9
J. AM. CHEM. SOC. VOL. 124, NO. 16, 2002 4303

A R T I C L E S
Fletcher and Therien
zinc(II) building block, is a spectral hallmark of cofacial
bis[porphyrin] compounds having modest porphyrin-porphyrin
interplanar separations.51-53 The λmax of the Soret band of
meso-meso bridged dimer 1 is shifted hypsochromically 634
cm-1 in comparison to benchmark 8. In contrast, â-â bridged
dimer 2 is blue-shifted by only 166 cm-1 versus benchmark 9.
Note, however, that electron-poor â-â bridged 7 displays strong
exciton coupling, with the λmax of its Soret band hypsochromi-
cally shifted by 1253 cm-1 relative to 11. These data indicate
that the relative degree of exciton coupling between porphyrin
units is not dependent solely upon macrocycle-macrocycle
relative orientation; the nature of the macrocycle frontier orbitals
clearly plays a role as well (vide infra). The general electronic
spectral features peculiar to cofacial [(porphinato)zinc(II)]
complexes possessing either asymmetric connectivity or elec-
tronically disparate macrocycle units have been discussed
previously.23
Figure 1. Illustration of the possible meso-â enantiomers and â-â
atropisomers in cofacial bis[(porphinato)zinc(II)] complexes featuring these
linkage topologies.
Electrochemical Studies. A large number of dimeric por-
phyrin and (porphinato)zinc compounds bridged cofacially by
rigid aryl spacers have been reported,16-22 and their electronic
properties relative to simple porphyrinic monomers have been
discussed.54 Each of these previously reported systems were
connected via the macrocycle meso positions, and the observed
frontier orbital destabilization evident in the cofacial bis-
[(porphinato)metal] complex relative to the corresponding
monomeric chromophore was attributed to electrostatic repul-
sions between the π-systems of the closely stacked porphyrin
macrocycles.54 Due to the symmetry of the porphyrin macro-
cycle’s frontier molecular orbitals, differences in both macro-
cycle-macrocycle connectivity and electronic structure should
have a profound impact on the nature of electronic interactions
in such π-stacked structures. Potentiometric studies of the series
of cofacial structures reported herein enable quantitative as-
sessment of the extent to which these factors impact the frontier
orbital energy levels of face-to-face bis[(porphinato)metal]
compounds.
Scheme 3a
Structural studies evince that the cofacial orientation of 1,2-
phenylene-bridged bis[(porphinato)metal] compounds is de-
pendent strongly upon the solvent system used to obtain X-ray-
quality crystals.19,23,42,50 When noncoordinating solvents are
used, such structures adopt largely coplanar conformations; in
contrast, when coordinating solvents are utilized, X-ray crystal-
lographic studies show that cofacial bis[(porphinato)metal]
species express open conformations possessing large dihedral
angles between the porphyrin least-squares planes. Because
coordinating solvents were utilized in our electrochemical
studies, we consider only the open cofacial conformation in the
analyses of these data.
In a 1,2-phenylene-bridged bis[(porphinato)zinc(II)] structure,
a number of sub-van der Waals contacts are enforced, assuming
a typical dihedral angle of 60° between macrocycle least-squares
planes, as demonstrated by both computational studies and X-ray
crystallographic analyses.23,42,55 For example, computational
modeling (Supporting Information) indicates the closest of these
cofacial contacts for a meso-meso bridged structure are at the
aGeneral reaction conditions employed: (i) Co2(CO)8 (1 equiv), toluene/
dioxane 5:1, 100 °C, 10 min; (ii) 1,6-heptadiyne (10 equiv), 5 mL of toluene,
added 90 min dropwise.
To facilitate the analyses of spectroscopic and potentiometric
data for these cofacial bis[(porphinato)zinc(II)] complexes, an
appropriate set of monomeric standards was synthesized. 5-(6-
Phenyl)indanyl-derivatized porphyrin monomers (Scheme 3)
were constructed via palladium-catalyzed cross-coupling of
ethynylbenzene with meso- or â-brominated porphyrin precur-
sors, followed by cycloaddition with 1,6-heptadiyne; these
compounds (8-11, Scheme 3) were utilized as analytical
benchmarks in lieu of simple 5,15- or 5,10,15,20-derivatized
porphyrins.
Electronic Spectroscopy. Figure 2 depicts the electronic
absorption spectra of cofacial bis[(porphinato)zinc(II)] com-
plexes 1-7; absorption and emission data for these species are
summarized in Table 1. A strongly allowed exciton band, blue-
shifted with respect the Soret band of the simple (porphinato)-
(51) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. Pure Appl. Chem. 1965, 11,
371-392.
(52) Chang, C. K. AdV. Chem. Ser. 1979, No. 173, 162-177.
(53) Hunter, C. A.; Sanders, J. K. M.; Stone, A. J. Chem. Phys. 1989, 133,
395-404.
(54) Le Mest, Y.; L′Her, M.; Hendricks, N. H.; Kim, K.; Collman, J. P. Inorg.
Chem. 1992, 31, 835-847.
9
4304 J. AM. CHEM. SOC. VOL. 124, NO. 16, 2002

Cofacial Prophyrin Structural Motif
A R T I C L E S
Figure 2. Electronic absorption spectra of 5,6-bis[(porphinato)zinc(II)]indane complexes 1-7 and 5-phenyl-6-[(porphinato)zinc(II)]indane compounds 8-11
recorded in THF solvent (T ) 23 °C).
Table 1. Prominent Absorption and Emission Bands of
5,6-Bis[(porphinato)zinc(II)]indane and
5-Phenyl-6-[(porphinato)Zinc(II)]indane Complexes, Recorded in
C5′, C5-C3′; â-â, C2-C4′, C4-C2′). Note also that each
cofacial bis(porphyrin) linkage topology shows additional
macrocycle-macrocycle atom-atom contacts at the van der
Waals separation distance of ∼3.4 Å (meso-meso, C4-C4′,
C6-C6′; meso-â, C4-C4′, C6-C2′; â-â, C3-C3′). Other
porphyryl atomic positions manifest atom-atom interplanar
separations exceeding π-contact distances. For example, at a
60° dihedral angle between the macrocycle least-squares planes,
the average N1-N1′ and N2-N2′ separations for the meso-
meso linked structure are ∼4.9 Å; thus, our first-order analysis
of the electronic interactions that fix the frontier orbital energy
levels of 5,6-indanyl-bridged cofacial (porphinato)zinc(II) struc-
tures focus on these positions of close contact between the two
macrocycle planes.56
Cyclic voltammetric data for the 5,6-bis[(porphinato)zinc-
(II)]indane and 5-phenyl-6-[(porphinato)zinc(II)]indane com-
plexes are presented in Table 2; exemplary cyclic voltammetric
responses for these species are shown in the Supporting
Information. It is evident from the comparison of the electron-
rich bis[(porphinato)zinc(II)] species 1, 2, and 3 that macro-
cycle-macrocycle connectivity influences frontier orbital energy
levels. Compound 1 (Figure 3A) displays oxidative and reduc-
tive cyclic voltammetric processes consistent with those reported
THF Solvent
electronic absorption
B-band region Q-band region
compd λ (nm) ν (cm-1
fluorescent emission
-1
-1
)
log (ꢀ) λ (nm) ν (cm
)
log (ꢀ) λ (nm)
ν (cm )
1
411 24 331 5.54 558 17 921 4.32
434 23 041 4.74 589 16 978 3.72
599 16 695 3.71
611
657
16 367
15 521
2
3
424 23 585 5.44 561 17 825 4.34
599 16 694 3.92
418 23 923 5.45 555 18 018 4.31
438 22 831 5.09 595 16 807 3.76
418 23 923 5.43 555 18 018 4.26
612 16 340 4.22
617
662
612
655
639
684
629
16 207
15 106
16 340
15 267
15 649
14 620
15 898
4
5
426 23 474 5.36 562 17 794 4.83
612 16 340 4.22
6
419 23 866 5.13 564 17 730 4.12
610 16 393 4.41
625
633
16 000
15 798
7
399 25 063 5.18 570 17 544 3.94
608 16 447 4.23
8
422 23 697 5.56 552 18 116 4.21
592 16 892 3.57
601
649
611
655
635
16 639
15 408
16 367
15 267
15 748
9
427 23 419 5.55 558 17 921 4.16
597 16 750 3.92
10
11
424 23 585 5.24 566 17 668 3.94
608 16 447 4.24
420 23 810 5.10 570 17 544 3.96
612 16 340 4.22
635
15 748
(55) Cofacial bis[(porphinato)zinc(II)] species were modeled using the CaChe
molecular mechanics package; the porphyrin macrocycles were restricted
to planar conformations in which the interporphyryl dihedral angle was
fixed at 60° (see Supporting Information). The interporphyryl separations
predicted by these structural models were within (0.1 Å of the distances
manifest in the X-ray crystallographic structures of open meso-meso linked,
1,2-phenylene bridged cofacial porphyrin compounds.23,42,50
C3, C5, and C7 positions of the macrocycles, which possess
average interplanar atom-atom separations (C3-C3′, C5-C5′,
C7-C7′) of less than 3.0 Å. Similar sub-van der Waals contacts
are evident in the meso-â, and â-â structures (meso-â, C3-
(56) For a complete listing of interporphyryl atom-atom contact distances used
in this analysis, see the Supporting Information.
9
J. AM. CHEM. SOC. VOL. 124, NO. 16, 2002 4305

A R T I C L E S
Fletcher and Therien
Figure 3. Potentiometrically determined relative frontier molecular orbital energy levels for the following: (A) 5-phenyl-6-[(5′-10′,20′-bis[4-(3-methoxy-
3-methylbutoxy)phenyl]porphinato)zinc(II)]indane (8) and 5,6-bis[(5′,5′′-10′,20′-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]indane (1);
(B) 5-phenyl-6-[(2′-5′,10′,15′,20′-tetraphenylporphinato)zinc(II)]indane (9) and 5,6-bis[(2′-5′,10′,15′,20′-tetraphenylporphinato)zinc(II)]indane (2); and (C)
5-phenyl-6-([2′-5′,10′,15′,20′-tetrakis(trifluoromethyl)porphinato]zinc(II))indane (11) and 5,6-bis([2′-5′,10′,15′,20′-tetrakis(trifluoromethyl)porphinato]zinc-
(II))indane (7).
Table 2. Comparative Cyclic Voltammetric Data of the
monomer have been commonly attributed to electrostatically
5,6-Bis[(porphinato)zinc(II)]indane Complexes and
driven destabilization of the HOMO in the neutral bis[(porphi-
nato)zinc(II)] complex and the fact that the cation radical state
of the dimer gains added stability due to electronic delocaliza-
tion.54 Consistent with previously reported electrochemical
analyses,54 1’s initial cathodic redox process is shifted substan-
tially (-230 mV) to more negative potential relative to reference
monomer 8.
5-Phenyl-6-[(porphinato)zinc(II)]indane Benchmarksa
E1/2 (mV)
+
+
2+
2+
compd
ZnP/ZnP
ZnP /ZnP
ZnP/ZnP
ZnP-/ZnP
1
2
3
4
5
6
7
8
9
620, 750
725, 820
735, 805
710
1070, 1250
1110b
1080, 1250
990
-1720b
-1445b
-1740b
1420b
1415b
-1070, -1650
-980, -1455
-1040, -1290
-775, -890
-1490
740
995
Both â-â and meso-â bridged dimers 2 and 3, which also
feature exclusively meso-aryl substituents, differ markedly from
1 with respect to the degree that their frontier orbitals are
perturbed relative to reference (porphinato)zinc(II) complexes.
The initial step of 2’s first oxidative process (Figure 3B) is
shifted only modestly (15 mV) relative to reference 9, while
the second anodic step is strongly stabilized by 80 mV (Table
2). In further contrast to the cyclic voltammetric data obtained
for 1, the potential of 2’s first reductive process is unchanged
versus that determined for reference monomer 7. Meso-â
bridged dimer 3 (Figure 4A) displays similar trends in its first
oxidation process: the potential of 3’s initial anodic step differs
little from the one-electron (1e) oxidation potentials determined
for 8 and 9, while its second anodic step is stabilized on the
order of 100 mV. Notably, in contrast to â-â bridged 2, 3’s
first reductive process is destabilized relative to its monomeric
1085,b 1250b
1210,b 1420b
705
740
1010
1045
-1445
10
11
1090b
1320b
-985
-780
a Experimental conditions: solvent, benzonitrile; [porphyrin] ) 1-2 mM;
[TBAClO4] ) 0.1 M; scan rate, 100-1000 V/s; reference electrode, Ag
wire. E1/2 values reported are relative to SCE; the ferrocene/ferrocenium
redox couple (0.43 vs SCE, benzonitrile) was used as the internal standard.
b Signifies a 2e redox step. See Figures 3 and 4.
for meso-meso bridged cofacial (porphinato)zinc(II) dimers.19,54
Note that the initial anodic step of the first oxidative process of
1 is destabilized by 85 mV relative to monomeric 8, while the
second step of the first oxidative process is stabilized by 45
mV relative to this benchmark (Table 2). These observed
potentiometric shifts with respect to the (porphinato)zinc(II)
9
4306 J. AM. CHEM. SOC. VOL. 124, NO. 16, 2002

Cofacial Prophyrin Structural Motif
A R T I C L E S
Figure 4. Potentiometrically determined relative frontier molecular orbital energy levels for the following: (A) 5-phenyl-6-[(5′-10′,20′-bis[4-(3-methoxy-
3-methylbutoxy)phenyl]porphinato)zinc(II)]indane (8), 5-phenyl-6-[(2′-5′,10′,15′,20′-tetraphenylporphinato)zinc(II)]indane (9), and 5-([2′-5′,10′,15′,20′-
tetraphenylporphinato]zinc(II))-6-[(5′′-10′′,20′′-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]indane (3); and (B) 5-phenyl-6-[(5′-10′,20′-
bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]indane (8), 5-phenyl-6-([2′-5′,10′,15′,20′-tetrakis(trifluoromethyl)porphinato]zinc(II))indane (11),
and 5-([2′-5′,10′,15′,20′-tetrakis(trifluoromethyl)porphinato]zinc(II))-6-[(5′′-10′′,20′′-bis[4-(3-methoxy-3-methylbutoxy)phenyl]porphinato)zinc(II)]indane
(4).
(porphinato)zinc(II) benchmarks to a degree similar (∼-275
mV) to that evinced for meso-meso bridged 1 (Table 2).
The relative differences between the respective frontier orbital
energies of 1, 2, and 3 can be explained qualitatively using a
simple through-space interaction model that considers the
relevant interactions of the classic, macrocycle-localized Gou-
terman four orbitals (Figure 5).57 Each of the (porphinato)zinc-
(II) units of face-to-face porphyrin compounds 1, 2, and 3
possess HOMOs with a2u symmetry; this orbital has significant
electron density at its meso and N pyrrolyl positions. Qualitative
evaluation of the extent of out-of-phase orbital interactions
between macrocycle a2u HOMOs in 1, 2, and 3 as a function of
bridging connectivity provides a rationale for the observed
relative energies of the frontier orbitals of these complexes.
Figure 6 identifies the nature of the highest lying filled-
filled interaction as a function of macrocycle-macrocycle
connectivity and the symmetries of the HOMOs of the constitu-
Figure 5. (Porphinato)metal frontier orbitals described by Gouterman.57
ent (porphinato)zinc(II) units of the cofacial porphyrin complex.
The red and blue coding denotes opposite phases of the porphyrin p orbital
wave functions, and circle size reflects approximate relative atom-centered
electron densities.
The extent of wave function overlap of the HOMOs of the two
(porphinato)zinc(II) units in the regions of conformational space
that feature sub-van der Waals contacts between porphyrin
macrocycles (assuming a 60° dihedral angle between the
porphyrin least-squares plane)55 are highlighted in Figure 6, with
solid red/blue denoting out-of-phase orbital overlap and solid
purple indicating in-phase orbital overlap. As depicted in Figure
6A, the meso-meso linked, 5,6-indanyl-bridged cofacial bis-
(57) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New
York, 1978; Vol. 3, pp 1-165.
9
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A R T I C L E S
Fletcher and Therien
Each of these bis[(porphinato)zinc(II)] compounds possesses
strongly stabilized second steps of their first oxidative processes,
a commonly observed characteristic of cofacial porphyrin
systems.19,54 It is interesting to note that the degree of stabiliza-
tion (meso-meso (130 mV) > meso-â (95 mV) > â-â (70
mV)) observed for the second steps of the first oxidative process
for 1, 2, and 3 coincides with the extent of (porphinato)zinc-
(porphinato)zinc HOMO-HOMO wave function overlap (Sup-
porting Information) for a coplanar bis[(porphinato)zinc(II)]
structure. This analysis is consistent with the postulate that
stabilization of the second anodic step of the first oxidative
process for cofacial porphyrin compounds derives from sub-
stantial electronic delocalization made possible in electrochemi-
+
cally generated (PZn)2 species.54
Disparate frontier orbital energies of the constituent (porphi-
nato)zinc(II) units play a key role in determining the observed
anodic cyclic voltammetric responses for electronically asym-
metric bis[(porphinato)zinc(II)] complexes 4 and 5 (Table 2,
Figure 4). The initial anodic steps of the first oxidative processes
for the respective meso-â and â-â connected structures 4 and
5 are unperturbed relative to their electron-rich monomeric
components, despite the fact that for meso-â bridged 4 there
exists out-of-phase overlap of the HOMOs of the constituent
(porphinato)zinc(II) moieties in regions of space that feature
sub-van der Waals porphyrin-porphyrin contacts (Figure 6C).
This effect has its genesis in the fact that the HOMO energy
levels of the (porphinato)zinc(II) units in these cofacial structures
differ by more than 600 mV (Figure 4B). Notably, electronic
delocalization effects are clearly apparent in the third oxidation
for 4 and 5, with the anodic process associated primarily with
the tetrakis(perfluoroalkyl)porphyrin component stabilized ∼100
mV (E1/2 (PZn)22+/3+ ) 1420 mV (4); E1/2 (PZn)22+/3+ ) 1415
mV (5)) relative to the initial oxidation of benchmark 11 (E1/2
(PZn)2+/3+ ) 1320 mV; see Table 2 and Figure 4B).
It is interesting to compare the redox profile of â-â
connected dimer 7 (Figure 3C), composed of (porphinato)zinc-
(II) units having a1u HOMOs36,37,59 with dimer 2 (Figure 3B),
which possesses (porphinato)zinc(II) components with a2u
HOMOs and an equivalent macrocycle-macrocycle linkage
topology. Unlike 2, 7 shows strong destabilization of its HOMO
level with respect to its monomeric benchmark complex 11. In
the region of sub-van der Waals macrocycle-macrocycle
contact (Figure 6D) electron-rich â-â linked cofacial porphyrin
dimer 2 shows minimal overlap of the building block (porphi-
nato)zinc(II) HOMO wave functions; in contrast, electron-poor
â-â linked dimer 7 shows significant out-of-phase (porphinato)-
zinc-(porphinato)zinc a1u-a1u overlap in this region of space
(Figure 6F), giving rise to substantial electronic interactions that
result in destabilization of the HOMO of the cofacial bis-
[(porphinato)zinc(II)] complex.
The relative energy levels of the lowest unoccupied molecular
orbitals of these cofacial bis[(porphinato)metal] systems can be
rationalized in a manner analogous to that used to explain
differences in the observed anodic electrochemistry of these
species (Figure 7). Note that, in contrast to the case for the filled
states, the extent of LUMO destabilization in the bis[(porphi-
nato)zinc(II)] complex will correlate with the extent of in-phase
Figure 6. Qualitative depiction of (porphinato)zinc(II)-(porphinato)zinc-
(II) HOMO-HOMO interactions occurring within van der Waals contact
in cofacial porphyrin systems possessing idealized 60° dihedral angles
between porphyrin least-squares planes.55 The label B indicates the
connectivity to the 5,6-indanyl bridge. Color coding: red/blue, out-of-phase
orbital overlap; purple, in-phase orbital overlap.
[(porphinato)zinc(II)] 1 system enforces significant a2u-a2u out-
of-phase orbital overlap; note that, at the meso carbon atoms
connected to the 5-and 6-indane positions, this unfavorable
electronic interaction occurs at a distance substantially less than
van der Waals contact and is thus likely a primary determinant
of the extensive destabilization of the compound 1 HOMO
relative to that of monomeric 8. In contrast, there is little sub-
van der Waals out-of-phase orbital overlap at the bridging â
positions of dimer 2 (Figure 6D); as a result, this face-to-face
porphyrin compound does not display significant destabilization
of its HOMO level with respect to benchmark 9. Similarly, due
to minimal overlap of the (porphinato)zinc(II) a2u HOMOs
connected via a meso-â linkage topology (Figure 6B), 3 also
evinces no measurable destabilization of its HOMO energy level
(Figure 4A) relative to that determined for its (porphinato)zinc-
(II) building blocks. Related connectivity-dependent electronic
interactions have been elucidated for various classes of conju-
gated linear porphyrin arrays.30,31,58
(58) 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.
(59) Moore, K. T.; Fletcher, J. T.; Therien, M. J. J. Am. Chem. Soc. 1999, 121,
5196-5209.
9
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Cofacial Prophyrin Structural Motif
A R T I C L E S
Note that, for meso-meso bridged cofacial porphyrin com-
plexes, significant in-phase (porphinato)metal-(porphinato)-
metal wave function overlap exists in the region of sub-van der
Waals contact, regardless of whether the primary electronic
interaction in the LUMO of the face-to-face structure involves
parallel (Figure 7A) or orthogonal (Figure 7B) components of
the eg sets of the (porphinato)zinc(II) units (Figure 5). Similarly,
meso-â bridged structures also display significant in-phase
overlap of the constituent (porphinato)zinc(II)-based LUMO
wave functions in the region of van der Waals contact (Figure
7C,D). As such, meso-â bridged 3, which is composed of
electronically similar meso-aryl (porphinato)zinc(II) units, dis-
plays a similarly large LUMO destabilization as was observed
for meso-meso bridged 1. It is interesting to note that the
insightful Hunter-Sanders electrostatic model,63 which captures
the essence of most essential trends with respect to the electronic
structure of π-stacked aromatics, predicts that repulsive elec-
trostatic interactions should be considerably less for the meso-â
linkage topology with respect to that manifest by a meso-meso
bridge at equivalent interplanar separations; the fact that this
trend is not manifest in the potentiometric data of 1 and 3 further
underscores that the relative phase relationships of the frontier
orbitals of the component aromatic units is an additional factor
that need also be considered in such theoretical analyses.
In contrast, electronically symmetric â-â bridged 2 and 7
each display no destabilization of their LUMO energy levels
(Figure 3), suggesting that insignificant overlap of the (porphi-
nato)zinc(II) monomer-based eg symmetric LUMO wave func-
tions is manifest in the region of sub-van der Waals contact.
Examination of the possible modes of interaction between the
orthogonal and parallel eg wave function components shows that
two combinations (Figure 7F and G) result in partial wave
function overlap in the region of sub-van der Waals contact,
while one combination of (porphinato)zinc(II)-localized or-
thogonal eg wave functions (Figure 7E) results in a cofacial bis-
[(porphinato)zinc(II)] LUMO having no electronic interaction
between the porphyrin units. Potentiometric data are thus
consistent with the orbital interaction shown in Figure 7E being
relevant to the description of the radical anion states of 2 and
7.
Figure 7. Qualitative depiction of (porphinato)zinc(II)-(porphinato)zinc-
(II) LUMO-LUMO interactions occurring within van der Waals contact
in cofacial porphyrin systems possessing idealized 60° dihedral angles
between porphyrin least-squares planes.55 The label B indicates the
connectivity to the 5,6-indanyl bridge. Color coding: see Figure 6.
Notably, the LUMOs for both cofacial bis[(porphinato)zinc-
(II)] complexes 4 and 5, which feature significant macrocycle-
macrocycle electronic asymmetry, show similar degrees of
destabilization with respect to the potentiometrically determined
LUMO energy level of the [5,10,15,20-tetrakis(perfluoroalkyl)-
porphinato]zinc(II) benchmark 11. The cathodic potentiometric
data obtained for meso-â linked 4 (Figure 4, Table 2) follow
the trend expected for this macrocycle-macrocycle linkage
topology (Figure 7C,D) with the measured E1/2 (PZn)-/0 value
destabilized 290 mV relative to the E1/2 (PZn)-/0 potential
(Figure 7, Table 2). Interestingly, â-â linked 5 displays an E1/2
(PZn)-/0 value (Table 2) destabilized by 200 mV relative to
11; this potentiometric behavior contrasts that elucidated for
â-â bridged 2 and 7 (vide supra), in which the E1/2 (PZn)-/0
values are unperturbed relative to their respective standards 9
macrocycle-macrocycle LUMO-LUMO interactions; such
orbital overlap effects further augmentation of the already large
porphyrin-porphyrin repulsive interactions, causing uniform
destabilization of all occupied orbital energy levels.
The strong destabilization of the LUMO energy observed for
meso-meso bridged 1 is consistent with that delineated for other
cofacial porphyrin structures having meso-meso connectivity.54
While cathodic potentiometric data for stacked aromatic struc-
tures are sparse, it is important to note that the few literature
examples of π-cofacial arene one-electron reduction potentials
follow the trend delineated for meso-meso bridged cofacial
bis(porphyrins). Particularly relevant in this regard is the body
of data that shows that flavin moieties involved in π-stacking
interactions with other aromatics display flavin-/0 potentials that
decrease precipitously with increasing electron-releasing char-
acter of the π-stacked arene; this behavior has been observed
both in flavoenzymes60,61 and in simple model compounds.62
(61) Pellett, J. D.; Becker, D. F.; Saenger, A. K.; Fuchs, J. A.; Stankovich, M.
T. Biochemistry 2001, 40, 7720-7728.
(62) Breinlinger, E. C.; Rotello, V. M. J. Am. Chem. Soc. 1997, 119, 1165-
1166.
(63) Hunter, C. A.; Saunders: J. K. M. J. Am. Chem. Soc. 1990, 112, 5525-
5534.
(64) Chang, C. K. J. Heterocycl. Chem. 1977, 14, 1285-1288.
(65) Eaton, S. S.; Eaton, G. R.; K., C. C. J. Am. Chem. Soc. 1985, 107, 3177-
3184.
(60) Zhou, Z.; Swenson, R. P. Biochemistry 1996, 35, 15980-15988.
9
J. AM. CHEM. SOC. VOL. 124, NO. 16, 2002 4309

A R T I C L E S
Fletcher and Therien
Figure 8. Classic examples of the electron-rich cofacial bis[(porphinato)-
zinc(II)] architecture.1,2,4,5,16,20,52,64,65 Potentiometric data for these species54
are used as benchmarks in Figure 9.
and 11 (Figure 6, Table 2). While the origin of this behavior is
an open question, it likely derives from the strong electronic
asymmetry inherent to 4 and 5. As noted above, cyclic
voltammetric data argue that the orbital interactions displayed
in Figure 7E describe the essential characteristics of the LUMO
for electronically symmetric cofacial bis[(porphinato)zinc(II)]
compounds 2 and 7. Because the HOMO and LUMO energy
levels of [5,10,15,20-tetrakis(perfluoroalkyl)porphinato]zinc(II)
complexes are lowered uniformly by ∼0.7 eV with respect to
the analogous orbitals of (5,10,15,20-tetraphenylporphinato)-
zinc(II) species,37 significant charge resonance interactions in
5 may enforce a more coplanar ground-state structure than is
manifest by 2 and 7. While X-ray structural data to support
this hypothesis are lacking, evidence bolstering this assertion
can be gleaned from the cyclic voltammetric data obtained for
these species: 2 and 7 show peak-to-peak potentiometric
separations (∆Ep values) for the initial cathodic steps of their
respective first reductive processes of 145 mV, while the
analogous redox process for 5 displays a ∆Ep value of 230 mV
(Supporting Information), signaling a larger structural reorga-
nization upon forming the radical anion state than accompanies
the analogous reaction in the former species. Likewise, as charge
resonance will also drive enhanced configuration interaction in
5 relative to 2 and 7, the (porphinato)zinc(II)-localized
LUMO-LUMO interactions displayed in Figure 7E-G may
all contribute to description of 5’s radical anion state and, thus,
play a supplementary role in effecting net destabilization of 5’s
Figure 9. Redox potentials of the first oxidation (Eox(11) or Eox(1)) and
reduction (Ered(11) or Ered (1)) processes of cofacial bis[(porphinato)zinc-
(II)] complexes vs SCE. The values for compounds FTF4, DPB, and DPA
(Figure 8) are taken from ref 54.
Analyses of these cofacial bis[(porphinato)metal] potentiometric
data, in terms of the absolute and relative frontier orbital energies
of their constituent [porphinato]zinc(II) building blocks, as well
as the nature of macrocycle-to-macrocycle connectivity, provide
predictive electronic structural models that rationalize the redox
behavior of these species. Importantly, this comprehensive data
set emphasizes the importance of the relative phase relationships
of the macrocycle-localized frontier orbitals in determining the
electronic properties of face-to-face bis(porphyrin) compounds.
As such, this investigation suggests a key refinement to the
highly successful Hunter-Sanders model of arene-arene π-π
interactions, which predicts the electronic structural properties
of stacked aromatic assemblies through analysis of π-system
charge distributions.
It is noteworthy that cofacial porphyrin constructs have shown
utility as ligands for a wide range of transition-metal-catalyzed
multielectron redox transformations of small-molecule sub-
strates;12 well-studied examples include multielectron oxy-
gen1-6,8,10 and nitrogen9 reduction reactions, as well as water,7,13
hydrogen,11,15 and hydrocarbon15 oxidations. Perhaps the most
remarkable aspect of these reactions is that the initial examples
of each were all delineated with bimetallic catalysts possessing
ligand frameworks having similar electronic structure.
A necessary consequence of utilizing similarly electron-rich
porphyrin macrocycles in cofacial bis[(porphinato)metal)] cata-
lysts is the fact that these systems often operate at substantial
overpotential with respect to the thermodynamic potential of
the small-molecule multielectron redox conversion of interest.12
As illustrated in Figure 9, through systematic augmentation of
the number of σ-electron-withdrawing perfluoroalkyl substitu-
ents in the bis[(porphinato)metal] ligand framework, the cathodic
and anodic potential domains can be tuned incrementally through
approximate 1-V windows. Such potentiometric engineering
defines an important tool to exploit in the development of new
cofacial porphyrin redox catalysts that operate closer to the
substrate thermodynamic potential. Finally, given that this report
(i) defines archetypal examples of highly electronically asym-
metric bis[(porphinato)metal] structures and (ii) shows that
macrocycle-macrocycle lateral shift can be modulated via the
E
1/2 PZn-/0 value relative to the E1/2 PZn-/0 potential determined
for 11.
Conclusion
This work demonstrates that sequential Pd-catalyzed cross-
coupling and metal-templated [2+2+2] cycloaddition reactions
define a powerful approach to modulate systematically the
electronic structure of the cofacial bis[(porphinato)metal] struc-
tural motif; this unprecedented potentiometric engineering of
face-to-face porphyrin structures derives from the fact that this
synthetic method is amenable to porphyrinic building blocks
of widely varying electronic structure and provides for the first
time a straightforward approach to control the topological
connectivity between the conjugated macrocycles. Structural
highlights of this extensive series of new cofacial bis(porphyrin)
species include the first examples of such complexes that feature
â-to-â bridging motifs, as well as macrocycles that possess
σ-electron-withdrawing perfluoroalkyl substituents.
Cyclic voltammetric studies show that the electrochemically
determined HOMO and LUMO energy levels of these cofacial
bis(porphinato)zinc(II) complexes can be lowered by 780 and
945 mV, respectively, relative to the archetypal members of
this class of compounds (Figures 8 and 9);54 importantly, these
orbital energy levels can be modulated over well-defined
increments throughout these wide potentiometric domains.
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Cofacial Prophyrin Structural Motif
A R T I C L E S
nature of the porphyrin-porphyrin topological connectivity,
entirely new opportunities may evolve for the development of
catalysts to effect heterolytic bond activation and subsequent
multielectron redox transformations of dipolar small molecules.
Supporting Information Available: Characterization data and
detailed syntheses of halogenated and ethynyl-substituted tri-
fluoromethylporphyrin precursor compounds, exemplary cyclic
voltammetric responses for 2, 6, 7, and 9-11, computed cofacial
porphyrin structures and tabulated data used to approximate
interplanar separations, and qualitative depictions of HOMO and
LUMO interactions occurring at van der Waals contact in
cofacial porphyrin systems possessing fully coplanar orienta-
tions. This material is available free of charge via the Internet
at http://pubs.acs.org. See any current masthead page for
ordering information and Web access instructions.
Acknowledgment. This work was supported in part by the
MRSEC Program of the National Science Foundation (DMR00-
79909). M.J.T. acknowledges the Camille and Henry Dreyfus
Foundations for a research fellowship. J.T.F. thanks the
International Precious Metals Institute (IPMI) for a graduate
student research award.
JA012489H
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