Self-assembly triangular and square rhenium(I) tricarbonyl complexes: A comprehensive study of their preparation, electrochemistry, photophysics, photochemistry, and host-guest properties

Article abstract of DOI:10.1021/ja001677p

A series of self-assembly macrocyclic compounds featuring fac-Re(CO)₃X (X = C1 or Br) as corners and linear bipyridyl bridging ligands have been prepared and characterized. Depending on the lengths as well as the bonding angles of the bridging ligands, the resulting geometries of these macrocyclic complexes are squares {[C1Re(CO)₃(μ-DPB)]₄ (3) and [C1Re(CO)₃(μ-AZP)]₄ (4)}, triangles {[BrRe(CO)₃(μ-BPDB)]₃ (6) and [BrRe(CO)₃(μ-BPDDB)]₃ (7)}, or a dimeric species {[C1Re(CO)₃(μ-BPET)]₂ (5)}. A general mechanism for the self-assembly processes involving soluble intermediates is proposed. The photophysical properties of these macrocyclic compounds are dominated by the characteristics of the lowest excited states which vary from metal-to-ligand charge transfer (MLCT) to ligand-localized π → π* or n → π* transitions for the different molecules. Square 3 and triangles 6 and 7 are luminescent in room-temperature solution while square 4 and dimer 5 are nonemissive. An energy transfer mechanism from the MLCT excited state to the lowest nonemissive n → π* excited state is attributed to the lack of emission in square 4. The emission from square 3 is assigned to ³MLCT character. In the cases of triangles 6 and 7, emissions from the ¹π - π* state were observed, as evidenced by their short lifetimes and structured emission bands. The large strain imposed on the triangular structures of 6 and 7 results in these molecules being photoactive. Photolysis of 6 or 7 at 313 nm is observed to break the triangular structure to form a polymeric structure. Square 4 exhibits reversible multielectron redox properties. Square 3 is also demonstrated to be a very effective host for nitro-substituted aromatic compounds.

Full text of DOI:10.1021/ja001677p

8956
J. Am. Chem. Soc. 2000, 122, 8956-8967
Self-Assembly Triangular and Square Rhenium(I) Tricarbonyl
Complexes: A Comprehensive Study of Their Preparation,
Electrochemistry, Photophysics, Photochemistry, and Host-Guest
Properties
Shih-Sheng Sun and Alistair J. Lees*
Contribution from the Department of Chemistry, State UniVersity of New York at Binghamton,
Binghamton, New York 13902-6016
ReceiVed May 15, 2000. ReVised Manuscript ReceiVed June 30, 2000
Abstract: A series of self-assembly macrocyclic compounds featuring fac-Re(CO)3X (X ) Cl or Br) as corners
and linear bipyridyl bridging ligands have been prepared and characterized. Depending on the lengths as well
as the bonding angles of the bridging ligands, the resulting geometries of these macrocyclic complexes are
squares {[ClRe(CO)3(µ-DPB)]4 (3) and [ClRe(CO)3(µ-AZP)]4 (4)}, triangles {[BrRe(CO)3(µ-BPDB)]3 (6) and
[BrRe(CO)3(µ-BPDDB)]3 (7)}, or a dimeric species {[ClRe(CO)3(µ-BPET)]2 (5)}. A general mechanism for
the self-assembly processes involving soluble intermediates is proposed. The photophysical properties of these
macrocyclic compounds are dominated by the characteristics of the lowest excited states which vary from
metal-to-ligand charge transfer (MLCT) to ligand-localized π f π* or n f π* transitions for the different
molecules. Square 3 and triangles 6 and 7 are luminescent in room-temperature solution while square 4 and
dimer 5 are nonemissive. An energy transfer mechanism from the MLCT excited state to the lowest nonemissive
n f π* excited state is attributed to the lack of emission in square 4. The emission from square 3 is assigned
3
1
to MLCT character. In the cases of triangles 6 and 7, emissions from the π-π* state were observed, as
evidenced by their short lifetimes and structured emission bands. The large strain imposed on the triangular
structures of 6 and 7 results in these molecules being photoactive. Photolysis of 6 or 7 at 313 nm is observed
to break the triangular structure to form a polymeric structure. Square 4 exhibits reversible multielectron redox
properties. Square 3 is also demonstrated to be a very effective host for nitro-substituted aromatic compounds.
Introduction
compounds offer the feasibility of host-guest interactions and
recognition phenomena based on Coulombic and/or hydrophobic
bonding, where the binding affinity is highly dependent on the
size of the cavity and the overall charge on the host molecules.16
The degree of organization achieved by nature at the
molecular level in many supramolecular assemblies formed from
different molecular components has initiated the design of
analogous synthetic assemblies during the last two decades.1
In traditional studies of supramolecular self-assembly, hydrogen
bonds are most frequently employed, as they are in all biological
systems.2 Relying on strong metal-ligand interactions, the
design and study of well-arranged metal-containing macrocycles
is one of the major current research areas in modern supramo-
lecular chemistry.3,4 Supramolecular species formed by self-
assembly of transition metals with multidentate ligands represent
an alternative to controlled processes involving biomimetic weak
interactions (hydrogen bonding, π-π stacking, van der Waals
interactions, etc.).5
(6) (a) Fujita, M.; Yazaki, J.; Ogura, K. J. Am. Chem. Soc. 1990, 112,
5645. (b) Fujita, M.; Yakazi, J.; Ogura, K. Tetrahedron Lett. 1991, 32,
5589. (c) Fujita, M.; Nagao, S.; Iida, M.; Ogura, K. J. Am. Chem. Soc.
1993, 115, 1574. (d) Fujita, M.; Sasaki, O.; Mitsuhashi, T.; Fujita, T.;
Yazaki, J.; Yamaguchi, K.; Ogura, K. Chem. Commun. 1996, 1535. (e)
Fujita, M.; Aoyagi, M.; Ogura, K. Inorg. Chim. Acta 1996, 246, 53.
(7) (a) Stang, P. J.; Cao, D. H. J. Am. Chem. Soc. 1994, 116, 4981. (b)
Stang, P. J.; Chen, K.; Arif, A. M. J. Am. Chem. Soc. 1995, 117, 6273. (c)
Olenyuk, B.; Whiteford, J. A.; Stang, P. J. J. Am. Chem. Soc. 1996, 118,
8221. (d) Whiteford, J. A.; Lu, C. V.; Stang, P. J. J. Am. Chem. Soc. 1997,
119, 2524. (e) Stang, P. J.; Cao, D. H.; Chen, K.; Gray, G. M.; Muddiman,
D. C.; Smith, R. D. J. Am. Chem. Soc. 1997, 119, 5163. (f) Manna, J.;
Kuehl, C. J.; Whiteford, J. A.; Stang, P. J.; Muddiman, D. C.; Hofstadler,
S. A.; Smith, R. D. J. Am. Chem. Soc. 1997, 119, 11611. (g) Muller, C.;
Whiteford, J. A.; Stang, P. J. J. Am. Chem. Soc. 1998, 120, 9827.
(8) (a) Stricklen, P. M.; Volcko, E. J.; Verkade, J. G. J. Am. Chem. Soc.
1983, 105, 2494. (b) Jeong, K.-S.; Cho, Y. L.; Song, J. U.; Chang, H.-Y.;
Choi, M.-G. J. Am. Chem. Soc. 1998, 120, 10982. (c) Cotton, F. A.; Daniels,
L. M.; Lin, C.; Murillo, C. A. Chem. Commun. 1999, 841. (d) Woessner,
S. M.; Helms, J. B.; Houlis, J. F.; Sullivan, B. P. Inorg. Chem. 1999, 38,
4380. (e) Sun, S.-S.; Silva, A. S.; Brinn, I. M.; Lees, A. J. Inorg. Chem.
2000, 39, 1344.
In the last 10 years, transition-metal-directed self-assembly
has been used to construct many different supramolecular
architectures such as squares,6-10 helices,11 boxes,12 interlocked
systems,13,14 and many spectacular three-dimensional struc-
tures.15 It has been well established that these macrocyclic
(1) Lehn, J.-M. Supramolecular Chemistry; VCH Publishers: New York,
1995.
(2) Lawrence, D. S.; Jiang, T.; Levett, M. Chem. ReV. 1995, 95, 2229.
(3) Caulder, D. L.; Raymond, K. N. J. Chem. Soc., Dalton Trans. 1999,
1185 and references therein.
(4) Leininger, S.; Olenyuk, B.; Stang, P. J. Chem. ReV. 2000, 100, 853
and references therein.
(9) (a) Slone, R. V.; Yoon, D. I.; Calhoun, R. M.; Hupp, J. T. J. Am.
Chem. Soc. 1995, 117, 11813. (b) Slone, R. V.; Hupp, J. T.; Stern, C. L.;
Albrecht-Schmitt, T. E. Inorg. Chem. 1996, 35, 4096. (c) Slone, R. V.;
Hupp, J. T. Inorg. Chem. 1997, 36, 5422. (d) Slone, R. V.; Benkstein, K.
D.; Be´langer, S.; Hupp, J. T.; Guzei, I. A.; Rheingold, A. L. Coord. Chem.
ReV. 1998, 171, 221. (e) Be´langer, S.; Hupp, J. T.; Stern, C. L.; Slone, R.
V.; Watson, D. F.; Carrell, T. G. J. Am. Chem. Soc. 1999, 121, 557. (f)
Be´langer, S.; Hupp, J. T. Angew. Chem., Int. Ed. 1999, 38, 2222.
(5) Conn, M. M.; Rebek, J., Jr. Chem. ReV. 1997, 97, 1647.
10.1021/ja001677p CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/02/2000

Rhenium(I) Tricarbonyl Complexes
J. Am. Chem. Soc., Vol. 122, No. 37, 2000 8957
The cavity inside the molecule also provides a potentially
catalytic microreactor.15b,17 Additionally, incorporation of photo-
and/or redox-active centers into macrocyclic compounds is
attractive in molecular sensing technology. This latter application
provides an alternative to the detection of guest inclusion based
on photoluminescence characteristics9 or changes in redox
potential values.18 Another interesting feature of these multi-
metal chromophore supramolecules is their capability of har-
vesting light energy and undergoing a multielectron redox
process.19 These properties provide an opportunity for multi-
electron storage and subsequent multiple-electron transfer which
is a necessary characteristic for energy conversion materials and/
or electrocatalysts.20
Chart 1
Self-assembly squares have been the most well studied
molecules among these supramolecular complexes. However,
the majority of the molecular squares that have been prepared
are based on the square-planar geometry of Pt(II) or Pd(II)
complexes.6,7,10 There are only a few examples of self-assembly
molecular squares based on an octahedral geometry.8,9 Also, in
sharp contrast to the numerous examples of square structures
reported, triangular complexes are much less common,21,22
mainly due to the rarity of suitable building blocks with proper
turning angles.4 Nevertheless, triangular structures can be formed
by carefully adjusting the experimental conditions and choosing
appropriate bridging ligands.6d,e,21h,22 We report herein the
preparation and characterization of a series of fac-tricarbonyl
rhenium(I)-based self-assembly macrocyclic compounds with
various bridging ligands (Chart 1). It is demonstrated here that
the geometry of these macrocyclic compounds can be varied
among squares, triangles, or dimers by simply modifying the
bridging ligand and that their resultant electrochemical, photo-
physical, and photochemical properties and binding capabilities
toward guest molecules are very different.
Experimental Section
Materials and General Procedures. Except for those mentioned
below, all chemicals were commercially available and used without
further purification. All reactions and manipulations were carried out
under N2 or Ar with the use of standard inert-atmosphere and Schlenk
techniques. Solvents used for synthesis were dried by standard
procedures and stored under N2.23 Solvents used in luminescent and
electrochemical studies were spectroscopic and anhydrous grade,
respectively. The 4,4′-dipyridylbutadiyne (DPB),24 4,4′-azopyridine
(AZP),25 1,4-bis(4′-pyridylethynyl)-2,5-dihexyloxybenzene (BPDB),22
and 2,5-bis(4-pyridylethynyl)thiophene (BPET)26 ligands were prepared
according to published methods. The 1,4-bis(4′-pyridylethynyl)-2,5-
didodecyloxybenzene (BPDDB, 2) ligand was synthesized by following
the same procedure as reported for BPDB (1).22 Tetrabutylammonium
perchlorate (TBAP) used as supporting electrolyte in cyclic voltammetry
was dried in a vacuum oven at 100 °C for 24 h prior to use. Nitrogen
used for the synthesis and purging experiments was dried and
deoxygenated according to a previously reported method.27
(10) (a) Rauter, H.; Hillgeris, E. C.; Erxleben, A.; Lipper, B. J. Am. Chem.
Soc. 1994, 116, 616. (b) Drain, C. M.; Lehn, J.-M. J. Chem. Soc., Chem.
Commun. 1994, 2313. (c) Zhang, Y.; Wang, S.; Enright, G. D.; Breeze, S.
R. J. Am. Chem. Soc. 1998, 120, 9398. (d) Onitsuka, K.; Yamamoto, S.;
Takahashi, S. Angew. Chem., Int. Ed. Engl. 1999, 38, 174.
(11) Constable, E. C. ComprehensiVe Supramolecular Chemistry; At-
wood, J. L., Davies, J. E. D., MacNicol, D. D., Vogtle, F., Lehn, J.-M.,
Eds.; Pergamon: Oxford, 1996; Vol. 9, Chapter 6, pp 213-252 and
references therein.
Film Preparation. Thin films were prepared by first dissolving the
sample in THF. These solutions were then filtered to remove any
undissolved particles present and then spin cast on clean borosilicate
glass slides. The “wet” film was subsequently dried under vacuum
overnight before acquiring the luminescence measurements.
Equipment and Procedures. NMR spectra were obtained using a
Bru¨cker AM 360 spectrometer. Infrared spectra were measured on a
Nicolet 20SXC Fourier Transform infrared spectrophotometer. Fast
atom bombardment (FAB) mass spectra were obtained on a Finnigan
Mat 95 mass spectrometer. Elemental analysis was performed by Oneida
Research Service, Whitesboro, New York. Absorption spectra were
(12) Baxter, P. N. W.; Lehn, J.-M.; Kneisel, B. O.; Fenske, D. Chem.
Commun. 1997, 2231.
(13) (a) Fujita, M.; Ibukuro, F.; Hagihara, H.; Ogura, K. Nature 1994,
367, 720. (b) Fujita, M.; Fujita, N.; Ogura, K.; Yamaguchi, K. Nature 1999,
400, 52. (c) Fujita, M. Acc. Chem. Res. 1999, 32, 53 and references therein.
(14) (a) Sauvage, J.-P.; Dietrich-Buchecker, C. O.; Chambron, J.-C. In
ComprehensiVe Supramolecular Chemistry; Lehn, J.-M., Ed.; Pergamon
Press: Oxford, 1995; Vol. 9, Chapter 2 and references therein. (b) Stoddart,
J. F.; Raymo, F.; Amabillino, D. B. In ComprehensiVe Supramolecular
Chemistry; Lehn, J.-M., Ed.; Pergamon Press: Oxford, 1995; Vol. 9, Chapter
3 and references therein.
(15) (a) Fujita, M.; Ogura, D.; Miyazawa, M.; Oka, H.; Yamaguchi, K.;
Ogura, K. Nature 1995, 378, 469. (b) Kang, J.; Rebek, J., Jr. Nature 1997,
385, 50. (c) Takeda, N.; Umemoto, K.; Yamaguchi, K.; Fujita, M. Nature
1999, 398, 794. (d) Olenyuk, B.; Whiteford, J. A.; Fechtenkotter, A.; Stang,
P. J. Nature 1999, 398, 796. (e) Aoyagi, M.; Biradha, K.; Fujita, M. J. Am.
Chem. Soc. 1999, 121, 7457. (f) Olenyuk, B.; Levin, M. D.; Whiteford, J.
A.; Shield, J. E.; Stang, P. J. J. Am. Chem. Soc. 1999, 121, 10434.
(16) Atwood, J. L., Ed. Inclusion Phenomena and Molecular Recognition;
Plenum Press: New York, 1990.
(17) Fujita, M.; Kwon, Y. J.; Washizu, S.; Ogura, K. J. Am. Chem. Soc.
1994, 116, 1151.
(18) (a) Beer, P. D.; Szemes, F.; Balzani, V.; Sala, C. M.; Drew, M. G.
B.; Dent, S. W.; Maestri, M. J. Am. Chem. Soc. 1997, 119, 11864. (b) Beer,
P. D.; Gale, P. A.; Chen, G. Z. J. Chem. Soc., Dalton Trans. 1999, 1897
and references therein.
(19) (a) Lees, A. J. Chem. ReV. 1987, 87, 711. (b) Gamelin, D. R.;
George, M. W.; Glynn, P.; Grevels, F.-W.; Johnson, F. P. A.; Klotzbu¨cher,
W.; Morrison, S. L.; Russell, G.; Schaffner, K.; Turner, J. J. Inorg. Chem.
1994, 33, 3246. (c) Balzani, V.; Juris, A.; Venturi, M.; Campagna S.; Serroni,
S. Chem. ReV. 1996, 96, 759.
(20) (a) Astruc, D. Electron Transfer and Radical Processes in Transi-
tion-Metal Chemistry; VCH Publishers: New York, 1995. (b) Brudvig, G.
W.; Thorp, H. H.; Crabtree, R. H. Acc. Chem. Res. 1991, 24, 311.
(21) Nevertheless, the appearances of triangular structures in the literature
have rapidly increased in recent years. Some selective examples include:
(a) Ruttimann, S.; Bernardinelli, G.; Williams, A. F. Angew. Chem., Int.
Ed. Engl. 1993, 32, 392. (b) Barbera, J.; Elduque, A.; Gimenez, R.; Oro,
L. A.; Serrano, J. L. Angew. Chem., Int. Ed. Engl. 1996, 35, 2832. (c) Lee,
S. B.; Hwang, S.; Chung, D. S.; Yun, H.; Hong, J.-I. Tetrahedron Lett.
1998, 39, 873. (d) Hall, J. R.; Loeb, S. J.; Shimizu, G. K. H.; Yap, G. P.
A. Angew. Chem., Int. Ed. Engl. 1998, 37, 121. (e) McQuillan, F. S.;
Berridge, T. E.; Chen, H.; Hamor, T. A.; Jones, C. J. Inorg. Chem. 1998,
37, 4959. (f) Thompson, A.; Rettig, S. J.; Dolphin, D. Chem. Commun.
1999, 631. (g) Lai, S.-W.; Chan, M. C.-W.; Peng, S.-M.; Che, C.-M. Angew.
Chem., Int. Ed. Engl. 1999, 38, 669. (h) Cotton, F. A.; Daniels, L. M.; Lin,
C.; Murillo, C. A. J. Am. Chem. Soc. 1999, 121, 4538. (i) Habereder, T.;
Warchhold, M.; No¨th, H.; Severin, K. Angew. Chem., Int. Ed. Engl. 1999,
38, 3225. (j) Schnebeck, R.-D.; Freisinger, E.; Glahe´, F.; Lippert, B. J.
Am. Chem. Soc. 2000, 122, 1381.
(22) Sun, S.-S.; Lees, A. J. Inorg. Chem. 1999, 38, 4181.
(23) Perrin, D. D.; Armarego, W. L. F.; Perri, D. R., Eds. Purification
of Laboratory Chemicals, 2nd ed.; Pergamon Press: Oxford, U.K., 1980.
(24) Ciana, L. D.; Haim, A. J. Heterocycl. Chem. 1984, 21, 607.
(25) Launay, J.-P.; Tourrel-Pagis, M.; Lipskier, J.-F.; Marvaud, V.;
Joachim, C. Inorg. Chem. 1991, 30, 1033.
(26) (a) Neenan, T. X.; Whitesides, G. M. J. Org. Chem. 1988, 53, 2489.
(b) Wu, I.-Y.; Lin, J. T.; Luo, J.; Li, C.-S.; Tsai, C.; Wen, Y. S.; Hsu,
C.-C.; Yeh, F.-F.; Liou, S. Organometallics 1998, 17, 2188.
(27) Schadt, M. J.; Lees, A. J. Inorg. Chem. 1986, 25, 672.

8958 J. Am. Chem. Soc., Vol. 122, No. 37, 2000
Sun and Lees
obtained using a HP 8450A diode array spectrophotometer interfaced
to an IBM computer.
Electrochemical measurements were recorded on a Princeton Applied
Research Model 263A potentiostat (EG&G instruments). The electro-
chemical cell consisted of a platinum working electrode, a platinum
wire counter electrode, and a Ag/AgNO3 (0.1 M in CH3CN solution)
reference electrode. Cyclic voltammograms were obtained in deoxy-
genated anhydrous solvent with the electroactive material (1.0 × 10-3
M) and 0.1 M TBAP as supporting electrolyte. Ferrocene (Fc) was
used as an internal standard for both potential calibration and revers-
ibility criteria. All potentials for the complexes in the study are reported
relative to Fc/Fc+. The scan rate was 250 mV/s, unless otherwise noted,
and the measurements were uncorrected for liquid-junction potentials.
General Procedure for Synthesis of the Square Complexes, [XRe-
(CO)3(µ-L)]4. To a flask (250 mL) containing a 1:1 molar ratio of
XRe(CO)5 and bridging ligand (L) was added 80 mL of toluene/THF
(1:1 v/v) and the solution was heated at 70 °C for 48 h. The solution
volume was then reduced to half under reduced pressure. Subsequently,
the precipitate was collected and dried in vacuo to afford an analytical
pure product with a yield typically higher than 90%.
Emission and excitation spectra were recorded in deoxygenated
solvent solution at 293 K with an SLM 48000S lifetime fluorescence
spectrophotometer equipped with a red-sensitive Hamamatsu R928
photomultiplier tube. The emission spectra were collected on samples
with OD ∼ 0.1 at the excitation wavelength. Low-temperature emission
spectra were performed using an Oxford Instruments DN1740K liquid-
nitrogen cryostat equipped with an external Oxford Instruments Model
3120 temperature controller. In all emission experiments, the sample
solutions were filtered through 0.22-µm Millipore filters prior to
measurement. UV-visible spectra were checked before and after
irradiation to monitor possible sample degradation. Emission maxima
were reproducible to within 2 nm. Luminescence quantum yields (Φem
)
for metal complexes and organic ligands were calculated relative to
[Ru(bpy)3]2+ in air-equilibrated aqueous solution (Φem ) 0.028) and
perylene in deoxygenated EtOH solution (Φem ) 0.89), respectively.28
Luminescence quantum yields were taken as the average of three
separate determinations and were reproducible to within 10%.
Film luminescence measurements were obtained by placing the
sample film at a 22.5° angle to the incident beam. The film glass slide
and nitro-substituted aromatic compound were placed in a sealed 50-
mL vial separated by cotton. The luminescence spectra were obtained
before and after exposing the films to the vapor of nitro-substituted
aromatic compounds for 3 mins.
Room-temperature luminescence lifetimes were recorded on a SLM
48000S phase-modulation lifetime fluorescence spectrophotometer. The
excitation light passed through a monochromator and was then intensity
modulated at a different frequency by a Debye-Sears ultrasonic
modulator. The sample and reference solutions were placed in a two-
chamber turret. The emission intensities of each at the observation
wavelength were approximately balanced by neutral density filters. The
phase shift and modulation of each sample was measured alternately
25 times. The results were averaged and analyzed by an interfaced IBM
computer. For a single-exponential decay, the phase (φ) and modulation
(m) are related to the lifetime (τ) according to the following equa-
tions: tan φω ) ωτ and mω ) (1 + ω2τ2)-1/2, where ω is the angular
modulation frequency (2π times the modulation frequency in hertz).
The errors for the fitted lifetimes are estimated to be within 10%.
Low-temperature luminescence lifetimes and solution lifetimes longer
than 50 ns were obtained using a Molectron Pulsed Nd:YAG laser (λex
) 355 nm) system as the excitation source and a Tektronix 7612A
digitizer was used for decay data acquisition. The excited-state decay
was then fitted by a least-squares method according to the equation
I(t) ) I0 exp(-t/τ), where I(t) is the emission intensity at time t after
the laser pulse and I0 is the initial intensity at t ) 0. Single exponential
decays were observed in each case and the obtained lifetimes were
found to be reproducible to within (5%.
Square 3 (X ) Cl, L ) DPB): IR (DMF, υCO, cm-1) 2021, 1928,
1
3
1892. H NMR (DMSO-d6) 8.73 (d, JH-H ) 6.5 Hz, 16 H, PyHR),
7.77 (d, JH-H ) 6.5 Hz, 16 H, PyHâ). 13C NMR (acetone-d6) 196.5,
3
192.1, 155.7, 155.6, 132.2, 129.4, 81.4, 79.9. MS (FAB, m-NBA as
matrix) m/z 2002.4 (M - Cl-) (calculated m/z 2003 for M - Cl-).
Anal. Calcd for Cl4C68H32N8O12Re4: C, 40.00; H, 1.58; N, 5.49.
Found: C, 39.86; H, 1.44; N, 5.61.
Square 4 (X ) Cl, L ) AZP): IR (DMF, υCO, cm-1) 2023, 1922,
1
3
1895. H NMR (DMSO-d6) 9.01 (d, JH-H ) 6.0 Hz, 16 H, PyHR),
3
7.97 (d, JH-H ) 6.1 Hz, 16 H, PyHâ). The solubility of 4 is too low
in organic solvent to obtain the 13C spectrum. MS (FAB) m/z 1961.1
(M + H+) (calculated m/z 1960.9 for M + H+). Anal. Calcd for
Cl4C52H32N16O12Re4: C, 31.84; H, 1.65; N, 11.43. Found: C, 31.49;
H, 1.78; N, 11.23.
[ClRe(CO)3(µ-BPET)]2 (5): The procedure for preparing the dimer
5 is similar to the preparation of the square complexes. The yellow
solid collected from the toluene/THF mixture was subjected to column
chromatography on silica gel eluting with CH2Cl2/THF (4:1 v/v). The
second major yellow band was collected and the volume was reduced
to ∼5 mL. This solution was slowly added to a rapidly stirring cold
pentane solution (250 mL). The resulting fine yellow powder was
collected and dried in vacuo. Yield 71%. IR (CH2Cl2, υCO, cm-1) 2027,
1
3
1926, 1891. H NMR (acetone-d6): 8.83 (d, JH-H ) 6.8 Hz, PyHR),
7.65 (d, 3JH-H ) 6.8 Hz, PyHâ), 7.57 (s, Hthiophene). 13C NMR (acetone-
d6) 197.1, 193.0, 154.8, 136.1, 134.0, 128.2, 125.5, 92.2, 90.3. MS
(FAB, m-NBA as matrix) m/z 1148.9 (M - Cl-) (calculated m/z 1149
for M - Cl-). Anal. Calcd for Cl2C42H20N4O6S2Re2: C, 42.57; H, 1.70;
N, 4.73. Found: C, 42.94; H, 1.66; N, 4.15. Alternatively, 5 can be
prepared as a single product with 86% isolated yield by prolonging
the reaction period to 1 week.
General Procedure for Synthesis of the Triangular Complexes,
[BrRe(CO)3(µ-L)]3. To a flask (250 mL) containing 1:1 molar ratio
of BrRe(CO)5 and bridging ligand (L) was added benzene (80 mL)
and the resulting solution was heated at 70 °C for 48 h. The solvent
was reduced to half and hexane (100 mL) was added to force further
precipitation. The precipitate was collected and dried in vacuo to afford
an analytically pure product with a yield typically higher than 90%.
Triangle 6 (L ) BPDB): IR (CH2Cl2, υCO, cm-1) 2027, 1928, 1893.
1H NMR (CDCl3) 8.77 (d, JH-H ) 6.6 Hz, 12H, PyHR), 7.34 (d, 3JH-H
) 6.5 Hz, 12 H, PyHâ), 7.00 (s, 6 H, Ph), 4.01 (t, 3JH-H ) 6.5 Hz, 12
Photolysis experiments were carried out with a 200-W medium-
pressure Hg arc lamp with Ealing Corp. interference filters (10-nm band-
pass) to isolate the excitation wavelength. Sample solutions prepared
in 1,2-dichloroethane (1,2-DCE) were filtered through 0.22-µm Mil-
lipore filters immediately before use and then deoxygenated prior to
photolysis. Throughout photolysis the solutions were rapidly stirred to
ensure sample homogeneity and a uniform optical density in the light
path. All reactions were also performed in the dark to assess the extent
of thermal processes, which were found to be negligible under our
experimental conditions.
Molecular weights of photolysis products were estimated via gel
permeation chromatography (GPC). A Waters 510 dual reciprocating
piston pump with 100-µL heads delivered the deoxygenated toluene
mobile phase at a flow rate of 1.00 mL/min. The pump was controlled
by a Waters Pump Control Module run by Waters Millennium
chromatography software. All samples were prepared in toluene. Data
points were collected every 0.25 s, and molecular weights were
calculated relative to polystyrene standards using Wyatt Technology
Corporation’s ASTRA chromatography software, version 4.20, on a
PC.
3
H, -OCH2C5H11), 1.81 (t, JH-H ) 6.6 Hz, 12 H, -OCH2CH2C4H9),
1.50-1.33 (m, 36 H, -OC2H4C3H6CH3), 0.87 (m, 18 H, -OC5H10CH3).
13C NMR (CDCl3) 195.1, 154.4, 154.3, 134.3, 127.5, 117.0, 113.9, 95.1,
91.7, 69.8, 31.7, 29.3, 25.8, 22.8, 14.2. MS (FAB, m-NBA as matrix)
m/z 2494.2 (M + H+) (calculated m/z 2493), 2412.1 (M - Br-)
(calculated m/z 2412). Anal. Calcd for Br3C105H108N6O15Re3: C, 50.59;
H, 4.37; N, 3.37. Found: C, 50.11; H, 4.79; N, 3.03.
Triangle 7 (L ) BPDDB): IR (CH2Cl2, υCO, cm-1) 2027, 1926,
1
3
1893. H NMR (CDCl3) 8.81 (d, JH-H ) 6.2 Hz, 12 H, PyHR), 7.37
3
3
(d, JH-H ) 6.4 Hz, 12 H, PyHâ), 7.03 (s, 6 H, Ph), 4.03 (t, JH-H
)
3
6.4 Hz, 12 H, -OCH2C11H23), 1.85 (q, JH-H ) 7.4 Hz, 12 H,
-OCH2CH2C10H21), 1.51 (q, 3JH-H ) 7.0 Hz, 12 H, -OC2H4CH2C9H19),
(28) (a) Nakamaru, K. Bull. Chem. Soc. Jpn. 1982, 55, 2697. (b) Walters,
K. A.; Ley, K. D.; Schanze, K. S. Chem. Commun. 1998, 1115.
3
1.25 (m, 96 H, -OC3H6C8H16CH3), 0.87 (t, JH-H ) 6.8 Hz, 18 H,

Rhenium(I) Tricarbonyl Complexes
J. Am. Chem. Soc., Vol. 122, No. 37, 2000 8959
Scheme 1
protons in 3 and 4 were shifted downfield relative to their free
ligands, DPB and AZP, due to the dative bonding of the nitrogen
lone pair to the Re(I) centers. FAB+ mass spectral measurements
yielded peaks at m/z 2002.4 (3 - Cl-) for 3 and m/z 1961.1 (4
+ H+) for 4. All these data support the square structures for 3
and 4.
The roughly 120° bonding angle in BPET, however, prevents
the formation of a square geometry. According to Stang’s
molecular library4c and from our molecular modeling results,30
both trimeric and dimeric structures have comparable energies
and are capable of forming through assembling of BPET and
ClRe(CO)5. Indeed, the 1H NMR spectrum from the crude
product exhibited two species in solution with a 4:1 ratio.
Repeated recrystallization failed to separate these two species.
Column chromatography on silica gel, first eluting with CH2-
-OC11H22CH3). 13C NMR (CDCl3) 195.0, 192.0, 154.4, 154.3, 134.3,
127.5, 117.0, 113.9, 95.1, 91.7, 69.8, 32.1, 29.8, 29.54, 29.49, 29.4,
26.1, 22.9, 14.3. MS (FAB, m-NBA as matrix) m/z 2997.1 (M + H+)
(calculated m/z 2996). Anal. Calcd for Br3C141H180N6O15Re3: C, 56.50;
H, 6.05; N, 2.80. Found: C, 56.29; H, 5.75; N, 2.78.
General Procedure for Synthesis of BrRe(CO)3(L)2. To a two-
neck flask (100 mL) containing 101.5 mg (0.25 mmol) of BrRe(CO)5
and L (1 mmol) was added nitrogen-degassed isooctane (30 mL) and
the resulting mixture was refluxed under nitrogen for 6 h. The precipitate
was collected on the frit while the solution was still hot and then washed
with hot isooctane (20 mL × 5) and dried in vacuo. Typically, no further
purification is needed.
1
Cl2, isolated a very small amount of yellow product. The H
NMR spectrum showed very complicated peak patterns which
are not able to confirm the exact structure. The second deep
yellow band was eluted with a 1:4 (v/v) ratio of THF/CH2Cl2.
Corner 8 {fac-BrRe(CO)3(DPB)2}: Yield 92%. IR (υCO, cm-1, CH2-
1
1
Cl2) 2028, 1929, 1894. H NMR (DMSO-d6) 8.77 (d, 4H, HRPy-Re,
The H NMR spectrum showed two downfield shifted clean
3
3JH-H ) 6.5 Hz), 8.68 (d, 4 H, HRPy, JH-H ) 5.8 Hz), 7.76 (d, 4 H,
doublets originating from pyridyl R and â protons and a singlet
3
3
Hâ-Re, JH-H ) 6.6 Hz), 7.62 (d, 4 H, HâPy, JH-H ) 5.8 Hz). 13C
NMR (DMSO-d6) 195.6, 191.0, 154.4, 150.1, 130.9, 128.6, 127.5,
126.1, 82.4, 79.6, 79.2, 75.8. Anal. Calcd for C31H16N4BrO3Re: C,
49.08; H, 2.13; N, 7.39. Found: C, 49.23; H, 2.21; N, 7.29.
originating from the thiophene’s protons. This compound
1
corresponds to the major species observed in the H NMR
spectrum of the crude product. Elemental analysis established
the 1:1 ratio between Re(I) and BPET for 5.
Corner 9 {fac-BrRe(CO)3(AZP)2}: Yield 98%. IR (υCO, cm-1, CH2-
Cl2) 2028, 1930, 1895. 1H NMR (acetone-d6): 9.21 (d, 4H, HRPy-Re,
FAB+ mass spectral measurement for 5 confirmed its dimeric
structure. Several reports from the literature have shown that
the dimeric structure can be self-assembled from a labile cis
transition metal component and a conformational flexible
bridging ligand,6c,e,31 while the formation of cyclic trimer
(pseudohexagonal macrocycle) requires a more rigid bridging
ligand and more precise coordinating direction toward the metal
components.32 The formation of a slightly more thermodynami-
cally stable cyclic dimer over the corresponding cyclic trimer
implies that, in this case, entropy effects play a more important
role in the self-assembly process.
On the other hand, only unidentified oligomers were formed
when refluxing BPDB and BrRe(CO)5 in a 1:1 ratio of toluene/
THF. Instead, 6 was prepared by refluxing BPDB and BrRe-
(CO)5 in neat benzene with 90% isolated yield. The fact that 6
exhibits both light and thermal decomposition in THF solution
may contribute to the inappropriateness of using THF as the
solvent for synthesis of 6. The incorporation of the long alkoxyl
chains into the bridging ligand substantially increases the
solubility of compound 6 in most moderately polar organic
solvents. However, attempts to grow a single crystal of 6 from
a variety of solvents or solvent combinations were still unsuc-
cessful.
3JH-H ) 6.8 Hz), 8.90 (d, 4 H, HRPy, JH-H ) 6.3 Hz), 7.98 (d, 4 H,
3
3
3
Hâ-Re, JH-H ) 6.8 Hz), 7.83 (d, 4 H, HâPy, JH-H ) 6.2 Hz). 13C
NMR (DMSO-d6) 195.7, 191.2, 156.9, 156.6, 156.1, 151.8, 118.6, 116.0
Anal. Calcd for C23H16N8BrO3Re: C, 38.43; H, 2.23; N, 15.60.
Found: C, 38.26; H, 2.09; N, 15.74.
Results and Discussion
Synthesis, Characterization, and General Properties. The
bridging ligands DPB, AZP, and BPET were synthesized by
published methods. Ligands BPDB and BPDDB were synthe-
sized in good yield by the Pd-catalyzed Sonogashira coupling
reaction29 between 1 or 2 and 4-bromopyridine in the presence
of 6 mol % PdCl2(PPh3)2 and 12 mol % CuI in n-propylamine
(Scheme 1). The corner complexes, 8 and 9, were prepared in
refluxing isooctane solution containing BrRe(CO)5 and excess
ligand.9b Attempts to prepare fac-BrRe(CO)3(BPDB)2 or fac-
BrRe(CO)3(BPDDB)2 in refluxing isooctane were unsuccessful.
The crude products included a triangle and several unidentified
products. This result is noteworthy and it will be discussed in
more detail later. The self-assembly complexes 3-7 were
prepared in essentially quantitative yield by reaction between
XRe(CO)5 (X ) Cl or Br) and bridging ligands in refluxing
toluene/THF mixture or neat benzene (Schemes 2 and 3).
The characterization of the molecular structures was achieved
by a variety of analytical techniques including infrared spec-
troscopy, NMR spectroscopy, FAB+ mass spectrometry, and
elemental analysis. Infrared carbonyl stretching frequencies of
these complexes exhibit tricarbonyl stretching patterns in the
1890-2030 cm-1 region that are typical for fac-Re(LL)(CO)3X
complexes.19 Elemental analysis confirms the 1:1 stoichiometry
between Re(I) and bridging ligand. 1H NMR spectroscopic data
of 3 and 4 show simply two sets of doublets originating from
R and â protons in the pyridine rings and this confirms the
symmetric structures. As expected, both R- and â-pyridyl
Interestingly, all spectroscopic data (NMR, IR) and elemental
analysis for complex 6 are consistent with a square structure.
However, the mass spectrum yielded a peak at 2494.2 (M +
H+), which corresponds to the triangular structure, and no peaks
corresponding to square or higher order oligomers were found
in the spectrum. The formation of a molecular triangle instead
(30) Based on the CAChe Work System 3.8, Oxford Molecular, Oxford,
1995. We realized that the energy minimization from molecular mechanics
provides by no means the global energy minimized structure because the
calculated structure highly depends on the local geometry from the starting
model. However, the optimized geometry still can provide a good estimation
of the molecular shape and size.
(31) (a) McMorran, D. A.; Steel, P. J. Angew. Chem., Int. Ed. Engl. 1998,
37, 3295. (b) Fan, J.; Whiteford, J. A.; Olenyuk, B.; Levin, M. D.; Stang,
P. J.; Fleischer, E. B. J. Am. Chem. Soc. 1999, 121, 2741. (c) Schmitz, M.;
Leininger, S.; Fan, J.; Arif, A. M.; Stang, P. J. Organometallics 1999, 18,
4817.
(29) (a) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
4467. (b) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N.
Synthesis 1980, 627. (c) Ziessel, R.; Suffert, J.; Youinou, M.-T. J. Org.
Chem. 1996, 61, 6535. (d) Grosshenny, V.; Romero, F. M.; Ziessel, R. J.
Org. Chem. 1997, 62, 1491.
(32) Leininger, S.; Schmitz, M.; Stang, P. J. Org. Lett. 1999, 1, 1921.

8960 J. Am. Chem. Soc., Vol. 122, No. 37, 2000
Sun and Lees
Scheme 2
Scheme 3
of a square indicates that the entropy term plays an important
role in the self-assembly process, because the square structure
is more stable in terms of enthalpy whereas entropy effects favor
the triangular structure. The long hexoxyl groups in BPDB are
believed to be responsible for this more favorable entropy
contribution in the self-assembly process. Previously, Fujita et
al. have observed the concentration-dependent equilibrium
between the molecular square and triangle in the self-assembly
process between (en)Pd(NO3)2 and DPB.6d Cotton et al. were
also able to isolate the triangular or square complexes by simply
changing the stoichiometry between the starting materials.21h
However, we did not isolate any trace amount of the triangular
structure from the assembly of ClRe(CO)5 and DPB. The lack
of a triangular structure is likely due to the poor solubility of 3
in a THF/toluene mixture and immediate precipitation from
solution once the square is formed. In fact, Hupp and co-workers
have isolated two different crystallographic forms, planar and
folded, of [ClRe(CO)3(µ-4,4′-bpy)]4.9e The folded form is
apparently more stable when the bridging ligand gets longer,
as evident from molecular modeling. The larger BPDB ligand
apparently makes the entropy effects more influential than the
enthalpy effects. Also, the larger strain that exists in the
triangular structure may be responsible for the increased thermal
and photochemical decomposition of 6 in THF solution and the
different photochemical behavior of the triangle 6 from the other
square molecules, 3 and 4.
Mechanism for Formation of the Self-Assembly Struc-
tures. Refluxing equimolar XRe(CO)5 and bis-pyridyl ligand
in a THF/toluene mixture is the protocol reaction condition for
generating self-assembly square complexes.9d The labile solvento
intermediate, XRe(CO)3(THF)2, forms first and then the ther-
modynamically most stable product precipitates from the
solution.9d In this mechanism, the equilibrium-shifting precipita-
tion of the square complexes plays an important role. The
typically low solubility of square complexes (and other pre-
square structures) in THF requires more soluble solvento
complexes to keep the intermediates (dimers, trimers, etc.) in
solution so that the thermodynamically structure-correcting
processes can occur and the square complexes immediately
precipitate from solution. There were either no cyclic structures
formed or the products included many unidentified mixtures
when we tried to prepare 3-5 in neat toluene or benzene as
solvents, presumably due to the inability of toluene or benzene
to form the solvento intermediates. It should also be noted that
Hupp et al. observed no formation of square complexes in
toluene and this further supports this mechanism.9d
On the other hand, we have found that triangle 6 does form
in refluxing benzene (a noncoordinating solvent). The reason

Rhenium(I) Tricarbonyl Complexes
J. Am. Chem. Soc., Vol. 122, No. 37, 2000 8961
Ered (∆Ep, mV), V
Table 1. Redox Potentialsa
compds
solvent
Eox (∆Ep, mV), V
[ClRe(CO)3(µ-DPB)]4
[ClRe(CO)3(µ-AZP)]4
[ClRe(CO)3(µ-BPET)]2
[BrRe(CO)3(µ-BPDB)]3
[BrRe(CO)3(µ-BPDDB)]3
BrRe(CO)3(DPB)2
BrRe(CO)3(AZP)2
DPB
AZP
BPET
BPDB
BPDDB
THF
DMF
CH2Cl2
DMF
DMF
CH2Cl2
DMF
THF
b
-1.70 (i)
1.32 (115)
1.03 (i), 1.29 (i)c
1.06 (i)
1.07 (i)
1.09 (i)
-0.72 (80), -1.16 (135), -1.45 (i), -1.78 (85)
-1.66 (i)
-1.79 (i)
-1.73 (i)
-1.69 (i), -1.97 (i)
-0.68 (84), -1.58 (i)
>-2.00
1.18 (i)
DMF
-1.33 (90), -1.94 (i)
>-2.00
>-2.00
>-2.00
CH2Cl2
CH2Cl2
CH2Cl2
1.27 (i)c
a Analyses were performed in 1 mM deoxygenated solutions containing 0.1 M TBAP; the scan rate is 250 mV/s. All potentials are in volts vs
[Fe(C5H5)2]+/0 (0.11 V with a peak separation of 85 mV in DMF, 0.23 V with a peak separation of 130 mV in CH2Cl2, and 0.19 V with a peak
separation of 118 mV in THF); (i) ) irreversible process. b The oxidation potential is out of the solvent window. c Oxidation on thiophene.
is probably because of the better solubility of triangle 6 in
comparison to simple Re(I) squares. Thus, the structure-
correcting processes can still proceed in solution until the most
thermodynamically stable product forms. To further test our
hypothesis, complex 7 was prepared in the same procedure as
triangle 6 by employing the bridging ligand, BPDDB, which
contains dodecoxyl chains to increase the solubility in organic
solvents. Again, the product formed in benzene was the self-
assembly triangle without other oligomers. Besides, the failure
to synthesize the corresponding corner complexes in refluxing
isooctane even in the presence of a 10-fold excess of free BPDB
or BPDDB also supports the presence of soluble kinetic products
in hot isooctane, which can continue to self-assemble to the
most thermodynamically stable product. Therefore, the more
general mechanism of the self-assembly process between XRe-
(CO)5 and bridging ligand can be described as follows: The
self-assembly process can proceed as long as the intermediates
are soluble in the solVent or mixture of solVents so that the
Figure 1. Cyclic voltammogram of square 4 in DMF/0.1 M Bu4NClO4
at a scan rate of 250 mV/s.
thermodynamically structure-error correcting process can
continue until the most thermodynamically stable product forms.
Electrochemical Properties. Redox potentials obtained from
comparable to literature-reported values for a variety of poly-
cyclic voltammograms of macrocycles 3-7 and their corner
pyridyl rhenium tricarbonyl compounds.33b,35 These results can
complexes and bridging ligands are presented in Table 1. All
be ascribed to the similar electronic environment that exists
ligands showed reduction potentials higher than -2.0 V except
around the metal centers. Except for square 4, all redox
AZP. These relatively low reduction potentials are ascribed to
potentials of the macrocyclic compounds studied here are
the addition of a pair of electrons to the lowest π* orbital
irreversible or only quasireversible at extremely high scan rates
localized on the azo bond. Except for square 4, all macrocyclic
(>10 V/s). The metal-centered oxidation produces a labile 17-
complexes exhibit one or two reduction waves corresponding
electron ReII species36 that undergoes facile CO substitution.37
to the reduction of bridging ligands and they vary depending
Square 4 exhibits a reversible multielectron redox process
on the properties of different ligands. These reduction potentials
(see Figure 1) that deserves further discussion. Due to its low-
are much lower than those observed for the free ligands. The
lying π* LUMO of azo bond character, the AZP ligand is a
complexation of ligand to the metal usually stabilizes the π*
stronger π acceptor compared to the other bridging ligands
level in the ligand and lowers the energy of the π* orbital.33 In
studied here and this property is also reflected in the higher
the case of complex 5, two oxidation waves have been observed.
oxidation potential (Eox) for square 4. Indeed, the Eox for square
The first oxidation potential is assigned to the formation of a
4 is 30 to 260 mV more positive than Eox for the other
cation radical localized on the thienyl moieties,34 and the second
macrocyclic compounds. The electrochemically quasireversible
oxidation potential is assigned to the oxidation localized on the
oxidation wave is indicative of a four-electron charge-transfer
metal centers. Due to the above-noted solubility limitation, cyclic
process and is assigned to a metal center oxidation (Re1+/2+).38,39
voltammetry for square 3 can only be performed in THF solution
In contrast to the simple anodic electrochemical process, the
and the oxidation potential is out of the THF window (∼1.0
cathodic electrochemical processes are more complicated. The
V). In general, the oxidation potentials for all macrocyclic
first reversible reduction wave and second electrochemically
compounds and their corresponding corners do not vary much
(35) Worl, L. A.; Duesing, R.; Chen, P.; Ciana, L. D.; Meyer, T. J. J.
Chem. Soc., Dalton Trans. 1991, 849.
and fall in the range between 1.03 and 1.32 V, which are
(33) (a) Zulu, M. M.; Lees, A. J. Inorg. Chem. 1988, 27, 1139. (b) Lin,
J. T.; Sun, S.-S.; Wu, J. J.; Liaw, Y. C.; Lin, K. J. J. Organomet. Chem.
1996, 517, 217.
(34) (a) Thackeray, J. W.; White, H. S.; Wrighton, M. S. J. Phys. Chem.
1985, 89, 5133. (b) Zhu, S. S.; Swager, T. M. J. Am. Chem. Soc. 1997,
119, 12568. (c) Graf, D. D.; Mann, K. R. Inorg. Chem. 1997, 36, 150.
(36) (a) Hershberger, J. W.; Klinger, R. J.; Kochi, J. K. J. Am. Chem.
Soc. 1983, 105, 61. (b) Kochi, J. K. J. Organomet. Chem. 1986, 300, 139.
(c) Tyler, D. R. Prog. Inorg. Chem. 1988, 36, 125.
(37) (a) Grob, R.; Kaim, W. Inorg. Chem. 1986, 25, 498. (b) Kaim, W.;
Kramer, H. E. A.; Vogler, C.; Rieker, J. J. Organomet. Chem. 1989, 367,
107.

8962 J. Am. Chem. Soc., Vol. 122, No. 37, 2000
Sun and Lees
Table 2. Electronic Absorption and Emission Data at 293 K
emission
absorption spectra
compds
λ
max, nm (10-3E, M-1 cm-1
)
λ
max, nm
τ, ns
Φem
3a
256 (227), 298 (141), 322 (151), 360 (160)
272 (243), 391 (123)
383 (107), 422 (46.8)
329 (118), 406 (145)
338, 420
243 (86.4), 261 (71.6), 329 (113), 412 (142)
259, 277, 335, 414
250 (64.1), 287 (37.8), 301 (42.4), 320 (46.0), 342 (38.3)
282 (56.8), 355 (22.9)
244 (18.9), 253 (17.8), 272 (8.1), 284 (11.9), 302 (15.2), 323 (13.2)
287 (19.9), 458 (0.24)
248 (14.6), 266 (13.2), 351 (47.7), 372 (sh, 36.2)
308 (12.0), 318 (11.5), 377 (9.5)
635
e
e
499, 476
534
512, 470
536
600
e
39
8.5 × 10-4
4b
5c
6d
0.36
2.4
0.37
2.9
0.032
0.012
0.011
11f
7d
12g
8a
110
9b
DPBa
AZPc
BPETc
BPDBd
BPDDBd
e
e
404, 384
450 (sh), 427
446, 426
0.26
2.5
2.6
0.26
0.59
0.30
308 (12.6), 318 (13.7), 340 (sh, 7.9), 388 (10.1)
a THF solution, λex ) 400 nm. b DMF solution. c CH2Cl2 solution, λex ) 360 nm. d 1,2-DCE solution, λex ) 360 nm. e No detectable emission
in room-temperature solution. f Photolysis of [BrRe(CO)3(µ-BPDB)]3 in 1,2-DCE at 313 nm after 20 h. g Photolysis of [BrRe(CO)3(µ-BPDDB)]3 in
1,2-DCE at 313 nm after 14 h.
quasireversible (∆Ep ) 135 mV vs 85 mV for Fc/Fc+ in DMF),
a
but chemically reversible (ip /ipc ) 1) wave are assigned to the
addition of four electrons successively to the lowest π* orbital
localized on the azo bond.38 The last reversible two-electron
reduction wave is assigned to the addition of two electrons to
the lowest π* orbital localized on the pyridyl rings. The third
irreversible reduction wave is more uncertain in nature. We
tentatively assigned this peak to the reduction of the metal center
(Re1+/0).39 The capability of multiple charge storage and transfer
within a single molecule is the prerequisite for molecules being
used as energy conversion materials and/or electrocatalysts.20
This multiple redox behavior in square 4 warrants further
investigation of the charge-transfer capabilities and its potential
applications.40
Figure 2. Electronic absorption and emission spectra of 7.5 × 10-6
Photophysical Properties. Table 2 summarizes the photo-
physical data of complexes 3-7 as well as their corresponding
corner complexes. The electronic absorption spectra of all the
compounds exhibit two main features that are assigned as
bridging-ligand-localized π f π* and metal-to-bridging-ligand
charge transfer (MLCT) transitions. In square 3, the lowest-
energy band is solely of MLCT character, while in compounds
4-7, the π f π* and MLCT bands are substantially overlap-
ping. Square 4 exhibits an additional low-energy n f π*
transition that originates from the trans-AZP ligand which
substantially overlaps with the MLCT band. Significantly, we
find that the character of the lowest energy bands has a profound
influence on the observed photophysical and photochemical
properties.
Figure 2 compares the absorption spectra of square 3, corner
8, and free ligand DPB, in THF solution. It is very clear from
the vibronic structure of the high-energy bands below 320 nm
that these bands originate from DPB-localized π f π* transi-
tions. The lowest energy bands in square 3 and corner 8 are
assigned to MLCT transitions. The MLCT band in square 3
red shifts 1385 cm-1 compared to that of the corner 8. A similar
M [ClRe(CO)3(µ-DPB)]4 ()), ClRe(CO)3(DPB)2 (-), and DPB (‚‚‚)
in THF at 293 K.
red shift has also been observed from the protonated corner of
fac-ClRe(CO)3(4,4′-bpy)2 by Giordano and Wrighton.41a This
result can be ascribed to either the fact that there is a more
extended conjugation in square 3 than in corner 8 or the
influence of the electrostatically stabilizing negative charge in
the DPB π* orbital by Re(I).41
Square 3 exhibits luminescence at room temperature, which
is independent of the excitation wavelength (340-400 nm), with
an emission lifetime of 39 ns in THF solution. The wavelength-
independent quantum yield and the good congruence between
the absorption and excitation spectra of square 3 indicate that
the internal conversion from higher excited states to the lowest
3MLCT state is very efficient and that luminescence originates
3
solely from the lowest MLCT state. The assignment of the
lowest emitting level as MLCT in character is based on the
position and shape of the emission band and the emission
lifetime which are consistent with those previously reported for
similar complexes assigned as MLCT emitters.41-43 As observed
in the absorption spectra, the emission maxima of square 3 shifts
to lower energy compared to the emission maxima of its corner
8. The emission lifetime of square 3 is shorter than its corner
8; this is attributed to an enhanced nonradiative decay rate for
(38) (a) Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. C. J. Am.
Chem. Soc. 1978, 100, 4248. (b) Stor, G. J.; Hartl, F.; van Outersterp, J.
W. M.; Stufkens, D. J. Organometallics 1995, 14, 1115. (c) Kelso, L. S.;
Reitsma, D. A.; Keene, F. R. Inorg. Chem. 1996, 35, 5144. (d) Otsuki, J.;
Tsujino, M.; Iizaki, T.; Araki, K.; Seno, M.; Takatera, K.; Watanabe, T. J.
Am. Chem. Soc. 1997, 119, 7895.
(39) (a) Caspar, J. V.; Sullivan, B. P.; Meyer, T. J. Inorg. Chem. 1984,
23, 2101. (b) Sullivan, B. P.; Bolinger, C. M.; Conrad, D.; Vining, W. J.;
Meyer, T. J. J. Chem. Soc., Chem. Commun. 1985, 1414. (c) Moya, S. A.;
Guerrero, J.; Pastene, R.; Schmidt, R.; Sariego, R.; Sartori, R.; Sanz-
Aparocop, J.; Fomseca, I.; Martinez-Ripoll, M. Inorg. Chem. 1994, 33, 2341.
(40) Sun, S.-S.; Lees, A. J. Work in progress.
(41) (a) Giordano, P. J.; Wrighton, M. S. J. Am. Chem. Soc. 1979, 101,
2888. (b) Callahan, R. W.; Brown, G. M.; Meyer, T. J. Inorg. Chem. 1980,
19, 2797. (c) Sun, S.-S.; Robson, E.; Dunwoody, N.; Silva, A. S.; Brinn, I.
M.; Lees, A. J. Chem. Commun. 2000, 201.
(42) Caspar, J. V.; Meyer, T. J. J. Phys. Chem. 1983, 87, 952.
(43) Baba, A. I.; Shaw, J. R.; Simon, J. A.; Thummel, R. P.; Schmehl,
R. H. Coord. Chem. ReV. 1998, 171, 43.

Rhenium(I) Tricarbonyl Complexes
J. Am. Chem. Soc., Vol. 122, No. 37, 2000 8963
Scheme 4
square 3 due to the lower energy of the excited state with a
more effective vibronic overlap.41,42
blue shiftes with increasing H+ equivalent and reaches the final
spectrum with a band maxima at 362 nm after the addition of
8 equiv of H+ (see Supporting Information). This result cannot
be explained by a simple protonation reaction on the central
azo groups. If this is the case, the electron-withdrawing
characteristic of protonated azo group will lower the π* level
and a red shift should be observed.
In pH 7 aqueous solution, 4,4′-azopyridine is very easily
reduced to 1,2-bis(4-pyridyl)hydrazine (BPH) with E° near 0
V vs SCE.48 In acidic DMSO solution, the reduction process is
expected to be even easier. Consequently, we suggest that the
spectral changes are accompanied by a proton-induced reduction
on the AZP ligand (see Scheme 5).25 The electron-donating
amine group on BPH has a higher π* level than the azo group
on AZP and, thus, the blue shift in the absorption spectra is
observed upon H+ titration.
The absorption spectrum of the cyclic dimer 5 features a broad
band centered at 383 nm and a shoulder at 422 nm. The low-
energy shoulder is assigned to be MLCT in character and the
band at 383 nm is assigned to a ligand localized π f π*
transition. In contrast to its free ligand (BPET), which exhibited
strong fluorescence in room-temperature CH2Cl2 solution, there
is no luminescence detected from 5 under the same conditions.
The lack of emission is also attributed to the efficient vibrational
relaxation on the basis of the energy gap law.42
The detailed photophysical data of 6 and 7 are summarized
in Table 2. In general, the photophysical properties of triangles
6 and 7 are very similar. The following discussion will mainly
focus on triangle 6, but will point out any notable differences
in the physical properties of 7. The absorption spectrum of 6
differs significantly in comparison to its corresponding free
bridging ligand, BPDB. The spectrum of 6 exhibits two broad
bands and both bands are red-shifted compared to the absorption
bands of BPDB. The intensity ratio of the lower-energy band
In contrast, neither square 4 nor its corner 9 have any
detectable luminescence at room temperature under the same
experimental conditions. This is attributed to effective intramo-
lecular energy transfer from the initially formed 1,3MLCT excited
states to the lowest nonemitting n f π* state.44 It is well-known
that compounds containing the azo group exist in both cis and
trans isomers and they are readily susceptible to photoisomer-
ization.45 Indeed, photolysis of corner 9 at 366 nm resulted in
trans-AZP to cis-AZP isomerization. However, photolysis of 4
at 366 or 313 nm resulted in slow decomposition and there was
no sign of isomerization. This result is in contrast to the result
obtained from photolysis studies of cyclobis[(dppf)Pd(µ-AZP)2-
Re(CO)3Br](OTf)4 (dppf is 1,1′-diphenylphosphinoferrocene and
OTf is CF3SO3-) at 366 nm; here the isomerization of the
bridging ligand induced the conversion from the tetranuclear
species to the dinuclear species (see Scheme 4).46 These
observations are associated with the different lability of Pd(II)
and Re(I) in solution. The trans-cis isomerization process
generates a large strain on the square structure and causes the
breaking of metal-nitrogen bonds. At room temperature, the
more labile Pd(II) metal center can more readily undergo the
trans-cis conformational change and form the dimeric structure,
while the more inert Re(I) is unable to recombine the Re-N
bonds and decomposition occurs immediately after ligand
isomerization.
The lone pairs on the azo group in the AZP ligand are
susceptible for protonation in acidic environment.47 UV-visible
spectrophotometric titration of square 4 by HCl was performed
in DMSO solution. The MLCT band that is centered at 386 nm
(44) Yam, V. W.-W.; Lau, V. C.-Y.; Wu, L.-X. J. Chem. Soc., Dalton
Trans. 1998, 1461.
(45) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis
Horwood: Chihester, U.K., 1991.
(46) Sun, S.-S.; Lees, A. J. Manuscript in preparation.
(47) Gerson, F.; Heilbronner, E. HelV. Chim. Acta 1962, 45, 51.
(48) Haladjian, J.; Pilard, R.; Bianco, P.; Asso, L.; Galy, J. P.; Vidal, R.
Electrochim. Acta 1985, 30, 695.

8964 J. Am. Chem. Soc., Vol. 122, No. 37, 2000
Sun and Lees
Scheme 5
vs the higher-energy band is also larger in 6 than the corre-
sponding ratio in BPDB. The lowest-energy band is assigned
to a mixture of MLCT and π f π* excited states. The room-
temperature emission spectrum of 6 in 1,2-DCE exhibits
structured features with maxima at 499 and 476 nm. It should
be noted that the luminescence is not dependent on the excitation
wavelength in the 340-400 nm range and that there is a single-
exponential decay from these two bands. The low-temperature
(77 K) emission spectrum of 6 in 2-methyltetrahydrofuran
(MeTHF) displays more distinct structure with bands at 463,
488, 520, and 549 nm (1188 cm-1 average spacing). These
observations illustrate that the dual emission bands are actually
vibronic structure from the π-π* excited state. The very small
blue shift on going from the solution to a rigid glass also
supports the assignment of the emission from a ligand-based π
f π* excited state.19a,49
Figure 3. Emission spectrum of [BrRe(CO)3(µ-BPDB)]3 in 77 K
2-MeTHF glass. The inset shows the lowest-energy band.
Careful scrutiny of the photophysical data of triangle 6 and
its free ligand, BPDB, reveals several interesting points. The
extremely short lifetime and relatively high-energy emission
from 6 indicate that the emission is apparently BPDB-localized
1π-π* fluorescence. Fluorescence from a 1π-π* state is
typically not observed in transition metal complexes.19a How-
1
ever, the fast radiative decay rate (kr ∼ 108 s-1) from the π-
π* excited state localized on BPDB is apparently still able to
compete with other internal conversion processes. In addition,
BPDB exhibits a longer lifetime than 6 and the quantum yield
of BPDB is almost 20 times larger than that in 6. On this basis,
it can be concluded that the Re(CO)3Br moieties in 6 act as
energy quenchers. Consequently, the relatively low fluorescence
quantum yield and short lifetime in room-temperature solution
1
are due to the intramolecular energy transfer from the π-π*
3
3
Figure 4. Absorption spectra of [BrRe(CO)3(µ-BPDB)]3 in 1,2-DCE
at 293 K as a function of photolysis time (λex ) 313 nm). t ) 0, 1, 2,
3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 h.
state to the MLCT or π-π* states.50 Considering the large
spin-orbital coupling exerted from Re(I) atoms,19a it is unusual
3
that there is no phosphorescence observed from either the -
MLCT or 3π-π* excited states in triangle 6. If we treat complex
6 as a cyclic trimer, the folding motions of the hexoxyl chains
in 6 are believed to be responsible for the rapid vibrational
relaxation from the 3MLCT excited state. This is further
supported by the higher quantum yield of BPDB and its
corresponding triangle in comparison to BPDDB and its
corresponding triangle. In fact, in addition to the vibronic
structures observed at high-energy positions, a very weak
emission centered at 602 nm with a lifetime 163 µs was also
observed in a 77 K glass (see Figure 3). This structureless band
is too low in energy to be one of the vibronic features from the
π-π* excited state. Thus, it is reasonable to assign this band
as a 3MLCT transition on the basis of the absence of vibrational
structure, although some incorporation of 3π-π* character
cannot be completely ruled out.43
Photochemical Properties. The highly strained triangular
structures of 6 and 7 suggest that the thermodynamical stability
of 6 and 7 may be altered by external stimuli such as light.
Photolysis into the π-π* transition at 313 nm resulted in both
absorption bands slowly decreasing and red shifting. The
emissions are also red-shifted upon photolysis. The two absorp-
tion bands are moved from 329 and 406 nm to 338 and 420
nm, respectively. The relative intensity of these two absorption
bands also changes during photolysis. Figures 4 and 5 present
the progression of absorption spectra of 6 and the corresponding
luminescence spectra in deoxygenated 1,2-DCE solution as a
function of photolysis time. The appearance of an isosbestic
point at 436 nm in the absorption data indicates a clean
photochemical conversion from triangle 6 to a final product.
The luminescence spectrum is also red-shifted with bands at
499 and 476 nm moving to 534 nm. There is no further shifting
(49) Juris, A.; Campagna, S.; Bidd, I.; Lehn, J.-M.; Ziessel, R. Inorg.
Chem. 1988, 27, 4007.
(50) (a) Ley, K. D.; Li, Y.; Johnson, J. V.; Powell, D. H.; Schanze, K.
S. Chem. Commun. 1999, 1749. (b) Ley, K. D.; Schanze, K. S. Coord.
Chem. ReV. 1998, 171, 287. (c) Ley, K. D.; Whittle, C. E.; Bartberger, M.
D.; Schanze, K. S. J. Am. Chem. Soc. 1997, 119, 3423.

Rhenium(I) Tricarbonyl Complexes
J. Am. Chem. Soc., Vol. 122, No. 37, 2000 8965
Chart 3
complexes have been studied for their molecular recognition
capabilities toward small aromatic molecules,6a-c,7a,b,52 inorganic
anions,9a,18 or porphyrin molecules,9c-g based on their different
cavity sizes and intermolecular forces such as hydrogen bonding
and hydrophobic or electrostatic interactions. Hupp et al. have
elegantly demonstrated that different sized rhenium tricarbonyl
based square complexes can effectively select different sized
guest molecules by electrochemical transport experiments.9e-g,52
In fact, these square complexes are packed with infinite channels
in their solid states.9b,e,53 Such properties render these macro-
cyclic molecules potential hosts for certain aromatic molecules
in the solid state as well as in solution.
Figure 5. Normalized emission spectra of [BrRe(CO)3(µ-BPDB)]3 in
1,2-DCE at 293 K as a function of photolysis time (λex ) 313 nm). t
) 0, 1, 2, 3, 4, 5, 6, 14 h.
1
Host-Guest Interactions Studied by H NMR. Several
Chart 2
aromatic compounds have been studied for their potential
chemistry in solution. For all the aromatic compounds tested
here which include toluene, p-methoxybenzene, 1,3,5-trimethoxy-
benzene, naphthalene, and several nitro-substituted aromatic
compounds, none of them showed any significant binding
interactions with triangles 6 and 7 because the peak shifts in
the 1H NMR spectral peaks are all smaller than 0.01 ppm. These
results are not too unexpected as the energy minimized
molecular structures for triangles 6 and 7 derived from CAChe
MM2 molecular modeling showed very limited internal voids
due to the long alkoxyl groups penetrating through the voids
(see Supporting Information).
of the emission spectrum after 14 h of photolysis. Similar
absorption spectral changes with an isosbestic point at 448 nm
are also observed in triangle 7 when photolyzed at 313 nm.
The lifetime measured is 2.4 ns for the photolysis product from
6 after 20 h of irradiation and 2.9 ns for the photolysis product
from 7 after 14 h of photolysis, which are both very close to
the lifetimes of their corresponding free ligands. The infrared
spectrum in 1,2-DCE for the photolysis product from 6 after
14 h of irradiation exhibits υCO bands at 2025, 1925, and 1892
cm-1 compared to 2025, 1924, and 1892 cm-1 before photolysis.
Also, the infrared spectrum in 1,2-DCE for the photolysis
product from 7 after 6 h of irradiation exhibits υCO bands at
2025, 1928, and 1892 cm-1 compared to 2025, 1925, and 1892
cm-1 before photolysis. As expected, the infrared spectra do
not show too much difference before and after photolysis
because the local environments around Re(I) are quite similar
in these triangular and polymeric structures.
The features of the final absorption spectrum after photolysis
closely resemble the absorption spectra of (diimine)ClRe(CO)3
chromophores containing polymeric systems studied by Schanze
et al. (Chart 2).50 The GPC data showed that the molecular
weights (Mn) of photolysis products from 6 and 7 are 12300
(polydispersity ) 3.6) and 15600 (polydispersity ) 4.7),
respectively. In light of all the above results, we tentatively
assigned the photolyzed products from 6 or 7 to zigzag
polymeric structures (see Scheme 6). The red shifts in the
absorption and emission spectra are believed to occur because
the breaking of the triangle releases the steric strain associated
with the structure and this increases the conjugation of the
aromatic system.
Host-Guest Interactions Studied by Luminescence.
Whereas typical aromatic compounds encapsulated inside the
metallo-cyclophane structures are not expected to result in
significant changes in the luminescence properties, nitro-
substituted aromatic compounds may be capable of serving as
electron acceptors from the excited state and, thus, able to
quench the luminescence. A series of nitro-substituted aromatic
compounds (see Chart 3) have been used to test for quenching
effects. In THF solution, the luminescence of both square 3 and
its corner 8 is effectively quenched by these nitro-substituted
aromatic compounds with quenching constants ranged from 4.94
× 108 to 9.12 × 109 M-1 s-1 56
.
In solid films, the luminescence spectral bands from both
square 3 and corner 8 are blue shifted compared to their
corresponding luminescence in solution. This phenomenon is
(51) (a) Chemosensors of Ion and Molecule Recognition; Desvergne, J.
P., Czarnik, A. W., Eds.; Kluwer Academic Publishers: Boston, 1997. (b)
de Silva, A. P.; Gunaratne, H. Q.; Gunnlaugsson, N. T.; Huxley, A. J. M.;
McCoy, C. P.; Rademacher, J. T.; Rice, T. E. Chem. ReV. 1997, 97, 1515
and references therein. (c) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc.
1998, 120, 5321, 11864.
(52) Keefe, M. H.; Slone, R. V.; Hupp, J. T.; Czaplewski, K. F.; Snurr,
R. Q.; Stern, C. L. Langmuir, 2000, 16, 3964.
(53) Be´langer, S.; Hupp, J. T.; Stern, C. L. Acta Crystallogr. 1998, C54,
1596.
(54) Handbook of Physical Properties of Organic Chemicals; Howard,
P. H., Meylan, W. M., Eds.; CRC Press: Boca Raton, FL, 1997.
(55) Handbook Series in Organic Electrochemistry; Meites, L., et al.,
Eds.; CRC Press: Boca Raton, FL, 1978; Vol. 1.
(56) See Supporting Information for their quenching constants derived
from Stern-Volmer analysis.
Host-Guest Interactions. Luminescent compounds with
internal cavities have been shown to have potential applications
as chemosensory devices in recent years.51 Many macrocyclic

8966 J. Am. Chem. Soc., Vol. 122, No. 37, 2000
Sun and Lees
Scheme 6
Figure 7. The percent quenching of the luminescence of square 3 film
after exposure to vapor of different quenchers at 180 s.
However, in these solid films, more factors need to be
considered to rationalize the luminescence quenching, namely,
∆G°, vapor pressure (VP) of the quenchers, and the binding
constant (Kb). Figure 6 compares the luminescence response of
films of square 3 and corner 8 to the vapor of 2,3-dinitrotoluene
(DNT) at 180 s. Clearly, only square 3 displays significant
luminescence quenching after exposure to the vapor of DNT in
the solid film. The main factor that results in the substantially
different quenching effect between the square 3 and corner 8
in the solid films is the difference in binding strength toward
the quencher. The quenching effect appearing in the film of
square 3 is apparently due to the porosity that exists in the film,
which provides cavities for binding the quencher molecules.9e,52
Figure 7 summarizes the quenching effects by different quench-
ers. In general, higher vapor pressure from the quenchers that
can bring greater numbers of binding molecules toward the
square and more nitro groups in the quenchers that are able to
provide a greater electronic interaction between the film and
the quenchers will result in larger quenching effects.51c
Figure 6. Luminescence response of films of square 3 (top) and corner
8 (bottom) upon exposure to DNT vapor for 180 s. The thick and thin
lines represent the luminescence before and after exposure to DNT
vapor, respectively.
recognized as a luminescence rigidochromic effect.57 In solution,
the quenching constants are mainly determined by the electron-
transfer rate from 3 or 8 to the quenchers and are, thus,
dependent on the free energy change (∆G°). For an oxidative
electron-transfer reaction, the ∆G° can be approximated ac-
cording to ∆G° ) Eox - E0-0 - Ered, where Eox, E0-0, and Ered
are oxidation potentials of 3 or 8, the lowest triplet 0-0 excited-
state energy of 3 or 8, and the reduction potentials of the
quenchers, respectively. Given the higher E0-0 of corner 8 than
E0-0 of square 3, the quenching constants in THF solution for
corner 8 are generally larger than those for square 3.56
Conclusion
We have demonstrated here that different geometric macro-
cyclic supramolecules can be prepared, in a predictable way,
by assembling suitable polypyridyl bridging ligands and fac-
XRe(CO)3 corners. The electrochemical, photophysical, and
photochemical properties of these macrocyclic supramolecules
are highly dependent on the nature of the bridging ligand. A
more generalized mechanism for the self-assembly process is
(57) (a) Wrighton, M. S.; Morse, D. L. J. Am. Chem. Soc. 1974, 96,
998. (b) Lees, A. J. Comm. Inorg. Chem. 1995, 17, 319 and references
therein.

Rhenium(I) Tricarbonyl Complexes
J. Am. Chem. Soc., Vol. 122, No. 37, 2000 8967
proposed which involves intermediates which are soluble in
solution so that the structure-error correcting processes can
proceed until the most thermodynamically stable product
appears.
89ER14039), for support of this research. We also thank Prof.
Omowunmi A. Sadik for access to electrochemical equipment
and Prof. Jiann T. Lin at the Institute of Chemistry, Academia
Sinica at Taipei, Taiwan for help in obtaining mass spectra of
complexes 3 and 6.
A series of guest inclusion studies have revealed that these
macrocyclic compounds are efficient hosts for nitro-substituted
aromatic molecules. The incorporation of luminescent chromo-
phores into the host molecules provides a more sensitive
spectroscopic detection technique than the conventional NMR
method for monitoring the host-guest interactions. Through
current binding studies in solution as well as in the solid state,
we have demonstrated that these macrocyclic compounds can
be effectively used in sensor applications by detecting the
luminescence.
Supporting Information Available: A table listing solution
quenching constants of square 3 and corner 8 obtained from
Stern-Volmer analysis with different nitro-substituted aromatic
compounds, a figure showing the spectrophotometric titration
of square 4 by HCl in DMSO solution, and a figure showing
the space-filling structures derived from CAChe MM2 molecular
modeling for macrocyclic compounds 3-7 (PDF). This material
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We are grateful to the Division of
Chemical Science, Office of Basic Energy Sciences, Office of
Energy Research, U.S. Department of Energy (Grant DE-FG02-
JA001677P