Synthesis and electrochemical, photophysical, and anion binding properties of self-assembly heterometallic cyclophanes

Article abstract of DOI:10.1021/om0109096

A series of heterometallic cyclophanes have been prepared from self-assembly of cis(dppf)M(H₂O)₂(OTf)₂ (M = Pd or Pt) and BrRe(CO)₃(L)₂ with the general formula [cis-(dppf)M(μ-L)₂Re(CO)

Full text of DOI:10.1021/om0109096

Organometallics 2002, 21, 685-693
685
Syn th esis a n d Electr och em ica l, P h otop h ysica l, a n d
An ion Bin d in g P r op er ties of Self-Assem bly
Heter om eta llic Cyclop h a n es
Shih-Sheng Sun, J ason A. Anspach, Alistair J . Lees,* and Peter Y. Zavalij
Institute for Materials Research and Department of Chemistry, State University of New York
at Binghamton, Binghamton, New York 13902-6016
Received October 17, 2001
A series of heterometallic cyclophanes have been prepared from self-assembly of cis-
(dppf)M(H2O)2(OTf)2 (M ) Pd or Pt) and BrRe(CO)3(L)2 with the general formula [cis-(dppf)M-
(µ-L)2Re(CO)3Br]2(OTf)4 (4: M ) Pd, L ) 4,4′-bpy; 5: M ) Pt, L ) 4,4′-bpy; 6: M ) Pd, L )
1,2-(4′-dipyridyl)acetylene (DPA); 7: M ) Pd, L ) 1,4-(4′-dipyridyl)butadiyne (DPB); 8: M
) Pt, L ) DPB) or cis-(dppf)Pt(4-ethynylpyridine)2 and BrRe(CO)5 with the formula {[cis-
(dppf)Pt(4-ethynylpyridyl)2](fac-Re(CO)3Br)}2 (10). These complexes have been characterized
by a variety of analytical techniques including IR, NMR, elemental analysis, mass
spectrometry, and single-crystal X-ray diffraction. The photophysical properties of these
complexes have been examined in detail. Squares 4-8 exhibit room-temperature lumines-
cence from lowest-lying triplet-centered metal to ligand charge transfer (3MLCT) excited
states, albeit significantly weaker than that predicted on the basis of their excited state
energies and the energy gap law. No luminescence was observed from the neutral square
10. Pd(II)- or Pt(II)-based oxidative quenching and energy transfer pathways from the
emitting 3MLCT state to lower nonemissive excited states localized on the ferrocenyl moieties
are invoked to explain the luminescence observations. Inorganic anion binding studies for
squares 4-8 again reveal two different quenching pathways.
In tr od u ction
phine bridging ligands. Apart from their intriguing
structures, many of these metallocyclophanes or cages
are luminescent in solution. Moreover, some of them
have been found to be effective molecular hosts for
anions4a,f and certain aromatic compounds4g-j,5h,6c,d as
a consequence of Coulombic and/or hydrophobic interac-
tions between the host and guest molecules.7
The design and formation of thermodynamically
driven molecular squares through metal coordination
to ligands with predefined size and geometry have been
intensely studied in the past decade.1 The incorporation
of photoactive chromophores into the molecular entities
is particularly attractive for the development of photonic
or electronic devices because of their high sensitivity to
changes in the environment.2 Polypyridyl rhenium(I)
and ruthenium(II) complexes have drawn a lot of
attention recently; this is due to the ease of their prep-
aration, their tunable luminescent properties, and their
potential application in solar energy conversion.3 Re-
cently, Hupp and co-workers,4 several other groups,5
and ourselves6 have prepared a variety of self-assembly
metallocyclophanes or cages comprising rhenium(I)
tricarbonyl or dicarbonyl moities with pyridyl or phos-
(4) (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) Benkstein, K. D.; Hupp,
J . T.; Stern, C. L. Inorg. Chem. 1998, 37, 5404. (e) Slone, R. V.;
Benkstein, K. D.; Belanger, S. Hupp, J . T.; Guzei, I. A.; Rheingold, A.
L. Coord. Chem. Rev. 1998, 171, 221. (f) Benkstein, K. D.; Hupp, J . T.;
Stern, C. L. J . Am. Chem. Soc. 1998, 120, 12982. (g) 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. (h) Be´langer, S.; Hupp, J . T. Angew. Chem.,
Int. Ed. 1999, 38, 2222. (i) Keefe, M. H.; Slone, R. V.; Hupp, J . T.;
Czaplewski, K. F.; Snurr, R. Q.; Stern, C. L. Langmuir 2000, 16, 3964.
(j) Benkstein, K. D.; Hupp, J . T.; Stern, C. L. Angew. Chem., Int. Ed.
2000, 39, 2891.
* Corresponding author. E-mail: alees@binghamton.edu. Fax:
(+1)607-777-4478.
(5) (a) Woessner, S. M.; Helms, J . B.; Shen, Y.; Sullivan, B. P. Inorg.
Chem. 1998, 37, 5406. (b) Woessner, S. M.; Helms, J . B.; Houlis, J . F.;
Sullivan, B. P. Inorg. Chem. 1999, 38, 4380. (c) Leadbeater, N. E.;
Cruse, H. A. Inorg. Chem. Commun. 1999, 2, 93. (d) Hong, B.
Comments Inorg. Chem. 1999, 20, 177. (e) Rajendran, T.; Manimaran,
B.; Lee, F.-Y.; Lee, G.-H.; Peng, S.-M.; Wang, C. M.; Lu, K.-L. Inorg.
Chem. 2000, 39, 2016. (f) Manimaran, B.; Rajendran, T.; Lu, Y.-L.;
Lee, G.-H.; Peng, S.-M.; Lu, K.-L. Eur. J . Inorg. Chem. 2001, 633. (g)
Manimaran, B.; Rajendran, T.; Lu, Y.-L.; Lee, G.-H.; Peng, S.-M.; Lu,
K.-L. J . Chem. Soc., Dalton Trans. 2001, 515. (h) Xu, D.; Khin, K. T.;
van der Veer, W. E.; Ziller, J . W.; Hong, B. Chem. Eur. J . 2001, 7,
2425.
(6) (a) Sun, S.-S.; Lees, A. J . Inorg. Chem. 1999, 38, 4181. (b) Sun,
S.-S.; Silva, A. S.; Brinn, I. M.; Lees, A. J . Inorg. Chem. 2000, 39, 1344.
(c) Sun, S.-S.; Lees, A. J . J . Am. Chem. Soc. 2000, 122, 8956. (d) Sun,
S.-S.; Lees, A. J . Chem. Commun. 2001, 103. (e) Sun, S.-S.; Lees, A. J .
Inorg. Chem. 2001, 40, 3154.
(1) Recent reviews on transition-metal-based self-assembly sys-
tems: (a) Leininger, S.; Olenyuk, B.; Stang, P. J . Chem. Rev. 2000,
100, 853. (b) Swiegers, G. F.; Malefetse, T. J . Chem. Rev. 2000, 100,
3483. (c) Caulder, D. L.; Raymond, K. N. J . Chem. Soc., Dalton Trans.
1999, 1185. (d) Fujita, M. Chem. Soc. Rev. 1998, 27, 417. (e) Dinolfo,
P. H.; Hupp, J . T. Chem. Mater. 2001, 13, 3113.
(2) De Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson, T.; Huxley,
A. J . M.; McCoy, C. P.; Rademacher J . T.; Rice, T. E. Chem. Rev. 1997,
97, 1515, and references therein.
(3) (a) Lees, A. J . Chem. Rev. 1987, 87, 711. (b) Schanze, K. S.;
MacQueen, D. B.; Perkins, T. A.; Cabana, L. A. Coord. Chem. Rev.
1993, 122, 63. (c) Sauvage, J .-P.; Collin, J .-C.; Chambron, S.; Guillerez,
C.; Coudret, V.; Balzani, V.; Barigelletti, F.; De Cola, L.; Flamigni, L.
Chem. Rev. 1994, 94, 993. (d) Balzani, V.; J uris, A.; Venturi, M.;
Campagna, S.; Serroni, S. Chem. Rev. 1996, 96, 759.
10.1021/om0109096 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 01/24/2002

686 Organometallics, Vol. 21, No. 4, 2002
Sun et al.
the Re(I)-corner complex with 4,4′-bpy as ligand, (dppf)M(4-
phenylpyridine)2(OTf)2 (M ) Pd or Pt) were used as quenchers.
We report herein the preparation and characteriza-
tion of a series of heterometallic cyclophanes comprising
both octahedral and square planar metal components.
The successful preparation of heterometallic cyclo-
phanes requires a kinetically inert component that will
retain its structural identity, at least at room temper-
ature, and another kinetically labile component which
is necessary for the subsequent reversible self-correcting
processes before the formation of the final structure. We
have found that the incorporation of different transition
metals as corner components greatly influences, as well
as complicates, the excited state dynamics compared to
the homometallic cyclophane analogues.
Electrochemical measurements were recorded on a Princ-
eton Applied Research Model 263A potentiostat (EG&G in-
struments). The electrochemical cell consisted of a platinum
working electrode, a platinum wire counter electrode, and a
Ag/AgNO3 (0.01 M in CH3CN solution) reference electrode.
Cyclic voltammograms were obtained in deoxygenated anhy-
drous CH3CN or DMF with the electroactive material (5.0 ×
10-4 M) and 0.1 M TBAH as supporting electrolyte. Ferrocene
(Fc) was used as an internal standard for both potential
calibration and reversibility criteria. All potentials for the
complexes in the study are reported relative to Fc/Fc+. The
scan rate was 200 mV/s, and the measurements were uncor-
rected for liquid-junction potentials.
Inorganic anion binding studies for the Re(I)-based squares
were conducted in acetone solution using luminescence detec-
tion. A 0.1 mM stock solution of the square complex in acetone
was prepared, and a 2 mL portion was transferred to a 1 cm
fluorescent cell. A 10 mM solution of the anion in acetone was
made using the stock solution of the square complex, and small
aliquots were added to the cell. The changes in luminescence
intensity were monitored as a function of anion concentration.
Aromatic guest binding studies using a 1H NMR titration
procedure were conducted in acetone-d6. The square complex
(∼2 mM) was titrated by addition of the solution of the guest
molecule prepared in a stock solution of the square complex.
The observed chemical shifts (R or â protons of pyridine) were
then monitored as a function of guest concentration.
Exp er im en ta l Section
Ma ter ia ls a n d Gen er a l P r oced u r es. All reactions and
manipulations were carried out under N2 or Ar with the use
of standard inert-atmosphere and Schlenk techniques. Sol-
vents used for synthesis were dried by standard procedures
and stored under N2.8 Solvents used in luminescent and
electrochemical studies were spectroscopic and anhydrous
grades, respectively. All starting materials are commercially
available and used without further purification. The com-
pounds 4-ethynylpyridine,9 1,4-(4′-dipyridyl)butadiyne (DPB),9
(dppf)PtCl2 (dppf is 1,1′-bis(diphenylphosphino)ferrocene),10
(dppf)Pt(H2O)2(OTf)2 (OTf is trifluoromethanesulfonate),10 (dppf)-
Pd(H2O)2(OTf)2,10 and fac-BrRe(CO)3(L)2 (1, L ) 4,4′-bpy; 2, L
) DPB)6c,11 were prepared according to published procedures.
The compound 1,2-(4′-dipyridyl)acetylene (DPA) was obtained
as a generous gift from Prof. J on Zubieta at Syracuse Univer-
sity. Tetrabutylammonium hexafluorophosphate (TBAH) used
as supporting electrolyte was rigorously dried (under vacuum
at 100 °C for 24 h) prior to use. Nitrogen or argon used for the
synthesis and purging experiments was dried and deoxygen-
ated according to a previously reported method.12
Equ ip m en t a n d P r oced u r es. NMR spectra were obtained
using a Bru¨cker AM 360 spectrometer or a Bru¨cker AC 300
spectrometer. 1H NMR spectra are reported in ppm relative
to the proton resonance resulting from incomplete deuteration
of the NMR solvent. 31P NMR spectra are reported in ppm
relative to external 85% H3PO4 at 0.00 ppm. Infrared spectra
were measured on a Nicolet 20SXC Fourier transform infrared
spectrophotometer. The electrospray ionization mass spec-
trometry (ESI-MS) experiments were performed on a Hewlett-
Packard 1100 MSD electrospray mass spectrometer. Fast atom
bombardment (FAB) mass spectra were obtained on a Finni-
gan Mat 95 mass spectrometer. Elemental analyses were per-
formed by Oneida Research Service, Whitesboro, NY. UV-
vis spectra were obtained using a HP 8450A diode array spec-
trophotometer. Emission spectra and emission lifetimes 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 detailed procedures for luminescence and lifetime experi-
ments have been described in a previous paper.4c
Syn t h esis. fa c-Br R e(CO)3(DP A)2 (3). To 100 mL of
nitrogen-degassed isooctane was added 133 mg (0.33 mmol)
of BrRe(CO)5 and 236 mg (1.31 mmol) of DPA, and the
resulting mixture was stirred at 60 °C under nitrogen for 4 h.
The forming yellow precipitate was collected on the frit while
the solution was still hot and washed with isooctane (20 mL
× 3) and dried in vacuo. Yield: 90%. IR (νCO, cm-1, CH2Cl2):
2028, 1928, 1893. 1H NMR (300 MHz, DMSO-d6): 8.77 (d, 4H,
HRPy-Re, 3J H-H ) 6.6 Hz), 8.69 (d, 4 H, HRPy, 3J H-H ) 6.0 Hz),
7.75 (d, 4 H, Hâ-Re, 3J H-H ) 6.7 Hz), 7.61 (d, 4 H, HâPy, 3J H-H
) 6.0 Hz). Anal. Calcd for BrC27H16N4O3Re: C, 45.64; H, 2.27;
N, 7.88. Found: C, 45.60; H, 2.26; N, 7.50.
Gen er a l P r oced u r e for Syn th esis of th e Squ a r e Com -
plexes Cyclobis{[cis-(dppf)M](µ-4,4′-bpy)2(fa c-Re(CO)3Br )}-
(OTf)4 (M ) P d or P t).5a To a 100 mL flask containing 0.1
mmol of [M(dppf)(H2O)2](OTf)2 and 0.1 mmol of fac-BrRe(CO)3-
(4,4′-bpy)2 was added 30 mL of dried CH2Cl2, and the resulting
mixture was stirred at room temperature under nitrogen for
24 h. A copious amount of precipitate gradually appeared
during this period. The precipitate was collected on a frit,
washed with cold CH2Cl2, and dried under vacuum to afford
an analytically pure product.
Cyclobis{[cis-(d p p f)P d ](µ-4,4′-b p y)2(fa c-R e(CO)3Br )}-
(OTf)4 (4). Yield: 87%. IR (νCO, cm-1, CH3NO2): 2027, 1924,
1897. 1H NMR (360 MHz, CD3NO2): 8.81 (d, 8H, HRPy-Re,
3J H-H ) 6.6 Hz), 8.58 (br, 8 H, HRPy-Pd), 7.93 (m, 16 H, Ho-
PhP), 7.73 (m, 8 H, Hp-PhP), 7.62 (m, 16 H, Hm-PhP), 7.42 (d,
3
3
8 H, HâPy-Re, J H-H ) 6.8 Hz) 7.38 (d, 8 H, HâPy-Pd, J H-H
)
Bimolecular Stern-Volmer quenching analyses were per-
formed by titration of the Re(I)-corner complexes with the Pd-
(II)- or Pt(II)-corner complexes. For the Re(I)-corner complexes
with DPB or DPA as ligands, the compounds (dppf)M(4-ethyl-
pyridine)2(OTf)2 (M ) Pd or Pt) were used as quenchers. For
5.8 Hz), 4.81 (s, 8 H, HR-ferr), 4.74 (s, 8 H, Hâ-ferr). 31P NMR
(CD3NO2): 36.1 (s). Anal. Calcd for C118H88N8Br2F12O18P4S4-
Fe2Pd2Re2: C, 43.71; H, 2.74; N, 3.46. Found: C, 43.91; H,
2.58; N, 3.11. ESI-MS (m/z): 1546.8 ([M + H+ - OTf-]2+, calcd
m/z 1545.9).
Cyclob is{[cis-(d p p f)P t ](µ-4,4′-b p y)2(fa c-R e(CO)3Br )}-
(OTf)4 (5). Yield: 79%. IR (νCO, cm-1, CH3NO2): 2027, 1924,
1896. 1H NMR (360 MHz, CD3NO2): 8.81 (d, 8H, HRPy-Re,
(7) Lehn, J .-M. Supramolecular Chemistry; VCH Publishers: New
York, 1995.
(8) Perrin, D. D., Armarego, W. L. F., Perri, D. R., Eds. Purification
of Laboratory Chemicals, 2nd ed.; Pergamon Press: Oxford, U.K., 1980.
(9) Ciana, L. D.; Haim, A. J . Heterocycl. Chem. 1984, 21, 607.
(10) Stang, P. J .; Olenyuk, B.; Fan, J .; Arif, A. M. Organometallics
1996, 15, 904.
3
3J H-H ) 6.8 Hz), 8.60 (d, 8 H, HRPy-Pt, J H-H ) 4.2 Hz), 7.93
(m, 16 H, Ho-PhP), 7.71 (m, 8 H, Hp-PhP), 7.59 (m, 16 H, Hm-
PhP), 7.42 (dd, 16 H, HâPy-Re, Pt), 4.80 (s, 8 H, HR-ferr), 4.70
1
(s, 8 H, Hâ-ferr). 31P NMR (CD3NO2): 5.77 (t, J Pt-P ) 3385
(11) Wrighton, M. S.; Giordano, P. J . J . Am. Chem. Soc. 1979, 101,
2888.
Hz). Anal. Calcd for C118H88N8Br2F12O18P4S4Fe2Pt2Re2: 41.44;

Self-Assembly Heterometallic Cyclophanes
Organometallics, Vol. 21, No. 4, 2002 687
H, 2.59; N, 3.28. Found: C, 41.02; H, 2.62; N, 2.99. ESI-MS
(m/z): 1560.4 ([M - 2 OTf-]2+, calcd m/z 1560.0).
frit, washed with cold ether, and dried in vacuo. Recrystalli-
zation from CH2Cl2/hexane afforded a pale yellow powder 10.
Yield: 105 mg (81%). IR (νCO, cm-1, CH2Cl2): 2023, 1918, 1885.
1H NMR (360 MHz, CD2Cl2): 8.25 (dd, 8H, HRPy), 7.76 (m, 16
H, Ho-PhP), 7.40 (m, 24 H, Hp, Hm-PhP), 6.52 (d, 4 H, HâPy,
J H-H ) 6.3 Hz), 6.47 (d, 4 H, HâPy, J H-H ) 6.6 Hz), 4.40 (s, 8
H, HR-ferr), 4.22 (s, 8 H, Hâ-ferr). 31P NMR (CD2Cl2): 15.1 (t,
J Pt-P ) 2385 Hz). FAB-MS (m/z): 2607.9 ([M + H]+, calcd m/z
2607.0), 2527.6 ([M - Br]+, calcd m/z 2527.1). Anal. Calcd for
Gen er a l P r oced u r e for Syn th esis of th e Squ a r e Com -
plexes Cyclobis{[cis-(dppf)M](µ-L)2(fa c-Re(CO)3Br )}(OTf)4
(M ) P d or P t, L ) DP A or DP B). To a 50 mL flask
containing 0.10 mmol of fac-BrRe(CO)3(L)2 and 0.10 mmol of
(dppf)M(H2O)2(OTf)2 was added 20 mL of N2-purged CH3NO2.
The resulting dark red solution was stirred at room temper-
ature for 24 h. Subsequently, 200 mL of cold ether was added
to rapidly precipitate a dark red solid. The solid was redis-
solved in 5 mL of CH3NO2, and slow diffusion of ether vapor
into the concentrated CH3NO2 solution resulted in the forma-
tion of crystalline solids. These solids rapidly collapsed to an
amorphous powder when they were filtered from the solvent.
Cyclob is{[cis-(d p p f)P d ](µ-DP A)2(fa c-R e (CO)3Br )}-
(OTf)4 (6). Yield: 42%. IR (νCO, cm-1, CH3NO2): 2028, 1925,
C
102H72N4Br2O6P4Fe2Pt2Re2: 46.98; H, 2.78; N, 2.15. Found:
C, 47.11; H, 2.79; N, 2.33.
(d p p f)ML2(OTf)2 (L ) 4-p h en ylp yr id in e, M ) P d (11),
M ) P t (12); L ) 4-eth ylp yr id in e, M ) P d (13), M ) P t
(14)). The procedures for preparing complexes 11-14 are
essentially the same; only the preparation of 11 is described
here. (dppf)Pd(H2O)2(OTf)2 (0.05 mmol) and 4-phenylpyridine
(0.1 mmol) were placed in a 25 mL flask. To the flask was
added 10 mL of CH2Cl2, and the resulting solution was stirred
at room temperature for 20 min. The solution was slowly added
into 100 mL of rapidly stirring cold hexane. The resulting fine
powder was collected on a frit and dried in vacuo to afford an
analytically pure compound.
1
3
1897. H NMR (300 MHz, CD3CN): 8.79 (d, 4 H, J H-H ) 6.5
Hz, HRPy-Re), 8.50 (bd, 4 H, HRPy-Pd), 7.78-7.55 (m, 40 H,
3
Ph), 7.50 (d, 4 H, J H-H ) 6.3 Hz, HâPy-Re), 7.38 (bd, 4 H,
HâPy-Pd), 4.68 (bs, 16 H, ferr). 31P NMR (CD3CN): 37.4 (s).
Anal. Calcd for C126H88N8Br2F12O18P4S4Fe2Pd2Re2: C, 45.33;
H, 2.66; N, 3.36. Found: C, 45.06; H, 2.79; N, 3.64. ESI-MS
(m/z): 1596.3 ([M + H+ - OTf-]2+, calcd m/z 1595.5).
11 (purple): Yield: 85%. 1H NMR (360 MHz, acetone-d6):
8.86 (bd, 4 H, HRPy), 8.04-7.99 (m, 8 H, Ho-PhP), 7.74-7.70
(m, 4 H, Hp-PhP), 7.65-61 (m, 8 H, Hm-PhP), 7.57 (d, 4 H,
3J H-H ) 6.6 Hz, HâPy), 7.45-7.41 (m, 10 H, Ph), 4.89 (s, 4 H,
HR-ferr), 4.79 (s, 4 H, Hâ-ferr). 31P NMR (acetone-d6): 34.2 (s).
Anal. Calcd for C58H46N2O6F6P2FeS2Pd: C, 54.88; H, 3.65; N,
2.21. Found: C, 54.95; H, 3.79; N, 2.51.
Cyclob is{[cis-(d p p f)P d ](µ-DP B)2(fa c-R e (CO)3Br )}-
(OTf)4 (7). Yield: 78%. IR (νCO, cm-1, CH3NO2): 2028, 1927,
1
3
1898. H NMR (360 MHz, CD3CN): 8.77 (d, 4 H, J H-H ) 6.7
3
Hz, HRPy-Re), 8.46 (d, 4 H, J H-H ) 5.3 Hz, HRPy-Pd), 7.71-
3
7.50 (m, 40 H, Ph), 7.51 (d, 4 H, J H-H ) 6.8 Hz, HâPy-Re),
7.32 (bd, 4 H, HâPy-Pd), 4.67 (bs, 16 H, ferr). 31P NMR (CD3-
CN): 36.9 (s). Anal. Calcd for C134H88N8Br2O18F12P4Fe2S4Pd2-
Re2: C, 46.85; H, 2.58; N, 3.26. Found: C, 46.81; H, 2.54; N,
3.05. ESI-MS (m/z): 1602.4 ([M + H+ - OTf- - Br]2+, calcd
m/z 1602.5).
12 (yellow): Yield: 82%. 1H NMR (300 MHz, acetone-d6):
3
8.90 (d, 4 H, J H-H ) 4.1, HRPy), 8.03-7.97 (m, 8 H, Ho-PhP),
7.69-7.62 (m, 4 H, Hp-PhP), 7.61-7.55 (m, 12 H, Hm-PhP, Hâ-
Py), 7.47-7.42 (m, 10 H, Ph), 4.87 (s, 4 H, HR-ferr), 4.75 (s, 4
1
H, Hâ-ferr). 31P NMR (acetone-d6): 5.35 (t, J Pt-P ) 3401 Hz).
C y c lo b is {[ci s-(d p p f)P t ](µ-D P B )2(fa c-R e (C O )3B r )}-
(OTf)4 (8). The preparation of square 8 was performed in CH3-
NO2 solution at 40 °C. Yield: 81%. IR (νCO, cm-1, CH3NO2):
C
58H46N2O6F6P2FeS2Pt: C, 51.30; H, 3.41; N, 2.06. Found: C,
51.12; H, 3.64; N, 2.51.
13 (purple): Yield: 95%. 1H NMR (300 MHz, acetone-d6):
8.63 (dd, 4H, 3J H-H ) 6.6, 4J P-H ) 3.3, HRPy), 7.98-7.91 (m, 8
H, Ho-PhP), 7.73-7.70 (m, 4 H, Hp-PhP), 7.65-7.59 (m, 8 H,
1
2028, 1926, 1896. H NMR (360 MHz, CD3NO2): 8.80 (d, 4 H,
3
3J H-H ) 7.2 Hz, HRPy-Re), 8.47 (d, 4 H, J H-H ) 4.5 Hz, HR-
Py-Pt), 7.96-7.91 (m, 16 H, Ho-PhP), 7.74 (m, 8 H, Hp-PhP),
3
3
Hm-PhP), 6.99 (d, 4 H, HâPy, J H-H ) 5.4 Hz), 4.84 (s, 4 H,
7.65-7.62 (m, 16 H, Hm-PhP), 7.52 (d, 4 H, J H-H ) 6.8 Hz,
3
HâPy-Re), 7.25 (d, 4 H, 3J H-H ) 5.9 Hz, HâPy-Pt), 4.80 (s, 8 H,
HR-ferr), 4.70 (s, 8 H, Hâ-ferr). 31P NMR (CD3NO2): 5.84 (t,
1J Pt-P ) 3412 Hz). Anal. Calcd for C134H88N8Br2O18F12P4Fe2S4-
Pt2Re2: C, 44.55; H, 2.46; N, 3.10. Found: C, 44.11; H, 1.98;
N, 2.62. ESI-MS (m/z): 1656.5 ([M - 2 OTf-]2+, calcd m/z
1656.0).
HR-ferr), 4.77 (s, 4 H, Hâ-ferr), 2.48 (q, 4 H, -CH2CH3, J H-H
3
) 7.8 Hz), 1.03 (t, 6 H, -CH2CH3, J H-H ) 7.6 Hz). 31P NMR
(acetone-d6): 33.8 (s). C50H46N2O6F6P2FeS2Pd: C, 51.19; H,
3.95; N, 2.39. Found: C, 51.63; H, 4.03; N, 2.65.
14 (yellow): Yield: 92%. 1H NMR (300 MHz, acetone-d6):
4
8.67 (dd, 4H, J P-H ) 3.8, HRPy), 7.97-7.91 (m, 8 H, Ho-PhP),
7.71-7.67 (m, 4 H, Hp-PhP), 7.62-7.57 (m, 8 H, Hm-PhP), 7.02
cis-(d p p f)P t(4-eth yn ylp yr id yl)2 (9). To a solution con-
taining 113 mg (1.1 mmol) of 4-ethynylpyridine in 80 mL of
THF at -70 °C was added 0.7 mL of t-BuLi (1.7 M in pentane),
and the resulting mixture was stirred for 1 h at -70 °C. To
this solution was added 385 mg (0.5 mmol) of Pt(dppf)Cl2 in
50 mL of THF, and the mixture was allowed to warm to room
temperature by itself and stirred for 16 h. The solvent was
removed under vacuum, and the residue was extracted with
CH2Cl2 (4 × 50 mL). The CH2Cl2 solution was reduced in
volume to 5 mL. Diethyl ether was then slowly added to
precipitate the pale yellow solid. The solid was collected and
dried in vacuo. Yield: 260 mg (56%). 1H NMR (360 MHz,
CDCl3): 8.18 (d, 4H, HRPy, J H-H ) 6.0 Hz), 7.78 (m, 8 H, Ho-
PhP), 7.78 (m, 4 H, Hp-PhP), 7.37 (m, 8 H, Hm-PhP), 6.61 (d,
4 H, HâPy, J H-H ) 6.0 Hz), 4.34 (s, 4 H, HR-ferr), 4.18 (s, 4 H,
Hâ-ferr). Anal. Calcd for C48H36N2P2FePt: C, 60.45; H, 3.80;
N, 2.94. Found: C, 60.89; H, 3.91; N, 2.59.
3
(d, 4 H, HâPy, J H-H ) 5.9 Hz), 4.82 (s, 4 H, HR-ferr), 4.73 (s,
4 H, Hâ-ferr), 2.50 (q, 4 H, -CH2CH3, 3J H-H ) 7.9 Hz), 1.04 (t,
3
6 H, -CH2CH3, J H-H ) 7.5 Hz). 31P NMR (acetone-d6): 5.24
(t, 1J Pt-P ) 3398 Hz). C50H46N2O6F6P2FeS2Pt: C, 47.59; H, 3.67;
N, 2.22. Found: C, 47.93; H, 3.44; N, 2.38.
Cr ysta llogr a p h y. Single crystals of square 5 suitable for
X-ray crystallography were grown by slow diffusion of ether
vapor into a concentrated CH3NO2 solution of square 5. The
crystal was measured on a Siemens Smart CCD single-crystal
diffractometer.13a The crystal structure was determined by the
direct method and refined by a least-squares procedure using
SHELXS97 and SHELXL97 crystallographic packages.13b Ex-
perimental and crystallographic parameters are shown in
Table 1.
Resu lts a n d Discu ssion
Cyclob is{[cis-(d p p f)P t (4-e t h yn ylp yr id yl)2](fa c-R e -
(CO)3Br )} (10). To a 100 mL flask containing 95.3 mg (0.1
mmol) of [cis-Pt(dppf)(4-ethynylpyridine)2] and 40.6 mg (0.1
mmol) of BrRe(CO)5 was added 30 mL of THF, and the
resulting mixture was refluxed for 40 h. A copious amount of
yellow precipitate gradually appeared during this period.
Subsequently, 50 mL of hexane was added to the solution to
force further precipitation. The precipitate was collected on a
Syn t h et ic St r a t egy a n d Gen er a l P r op er t ies.
Scheme 1 represents the concepts leading to the syn-
(12) Dunwoody, N.; Sun, S.-S.; Lees, A. J . Inorg. Chem. 2000, 39,
4442.
(13) (a) SMART and SAINT; Bruker AXS Inc.: Madison, WI, 1999.
(b) Sheldrick, G. M. SHELXS97 and SHELXL97; University of
Go¨ttingen: Go¨ttingen, Germany, 1997.

688 Organometallics, Vol. 21, No. 4, 2002
Sun et al.
Ta ble 1. Exp er im en ta l a n d Cr ysta llogr a p h ic
resultant precipitate comprises several undesirable
products that were very difficult to purify; these are
presumably various oligomeric or polymeric species. In
the case of square 8, a slightly higher temperature was
required to ensure the completion of the reactions.
The synthetic route performed for square 10 is
depicted in Scheme 3. The Pt(II)-acetylide corner 9 was
prepared from the nucleophilic reaction between the
deprotonated 4-ethynylpyridyl carbanion15 and cis-(dp-
pf)PtCl2. The pale yellow Pt(II)-acetylide corner was
subsequently refluxed with BrRe(CO)5 in THF for 40 h
to afford, after workup, the neutral square 10 in good
yield.
P a r a m eter s for Squ a r e 5‚2CH3NO2
formula
space group
C120H94Br2F12Fe2N10O22P4Pt2Re2S4
P1
a
b
c
10.5791(3) Å
16.8007(3) Å
19.7839(5) Å
91.743(1)°
94.729(1)°
95.635(1)°
3484.78(15) Å3
1
R
â
γ
V
Z
fw
3542.28
calcd density
F(000)
abs coeff, µ
radiation
1.688 g/cm3
1720
4.692 mm-1
Mo KR
Ch a r a cter iza tion of Squ a r es 4-8. Infrared spectra
for squares 4-8 comprise three carbonyl stretching
bands that are characteristic of the facial geometry
associated with Re(I) centers.16 No significant differ-
ences in the carbonyl stretching frequencies were ob-
monochromator
measurement temp
scan
no. of measured reflns
R equivalents
R σ
no. of unique reflns
no. of observed reflns
observed criterion
min h, k, l
graphite
153 K
ω
22 019
0.0273
0.0643
15 185
11 882
1
served among the various square complexes. H NMR
spectra of these square complexes show the same
patterns as the individual components, but the peaks
are slightly shifted in their positions. The pyridyl
protons of the complexes are downfield shifted com-
pared to those of free ligands due to the dative
bonding character of the metal-nitrogen bonding. In the
cases of squares 4-8, the R and â protons of the
pyridines coordinated to Re(I) are more downfield
shifted (0.13-0.46 ppm for R protons and 0.04-0.23
ppm for â protons) than the ones coordinated to Pd(II)
or Pt(II). This is due to the stronger electron-withdraw-
ing nature of BrReI(CO)3 compared to (dppf)Pd(II) and
(dppf)Pt(II). More significantly, the 31P NMR spectra
show a single signal for each of these square complexes,
indicative of the highly symmetrical structures. For the
>2σ(I)
-11, -18, -25
13, 21, 26
3, 56.5°
max h, k, l
min., max. 2θ
data collection software
structure refinement
software
Bru¨ker SMART
Bru¨ker SHELXTL
no. of refined params
RF
wRF2
792
0.060
0.142
1.069
GOF
Sch em e 1
1
Pt(II)-containing squares, the J Pt-P coupling constants
are observed in the range of 3385-3461 Hz, and these
values are comparable to other previously reported
(dppf)Pt-containing complexes.10,17 The ESI-MS data for
squares 4-8 are strongly supportive of the formation
of the square structures with detection of the parent
ion in each instance. The obtained crystal structure of
5 unambiguously confirms the square geometry (vide
infra).
Ch a r a cter iza tion of Squ a r e 10. The infrared spec-
trum of square 10 exhibits a typical facial tricarbonyl
stretching pattern, similar to those observed from
squares 4-8. The square structure is established on the
basis of the FAB-MS data, which show the parent ion
cluster at m/z 2607.9 (M + H)+. It is interesting to note
that the pyridyl protons in the 1H NMR spectrum of
square 10 recorded in CD2Cl2 display four sets of
doublets with equal integrated areas, although only a
triplet due to the Pt-P coupling is observed in the 31P
NMR spectrum. The splitting of the pyridyl protons
implies that the two pyridine rings experience slightly
different magnetic environments. Two possible reasons
can explain this peak splitting, viz., the restricted
thesis of the heterometallic cyclophanes in this work.14
The general strategy for preparation of the heterome-
tallic cyclophanes relies on the successful synthesis of
the corner component which comprises a thermody-
namically inert metal center at room temperature and
two uncoordinated donor sites arranged in a cis-
position.4a Subsequent coordination of the donor ligands
to another labile metal component, followed by a ther-
modynamically driven self-assembly process, generates
the expected heterometallic cyclophanes. Squares 4-8
were prepared according to Scheme 2. By simply mixing
equivalent amounts of (dppf)M(H2O)2(OTf)2 (M ) Pd or
Pt) and fac-BrRe(CO)3(L)2 (L ) 4,4′-bpy, DPB, or DPA)
in CH2Cl2 or CH3NO2 solution for 24 h, the correspond-
ing squares were isolated with relatively high yields.
1H NMR results showed that the reactions proceeded
essentially quantitatively, although typical isolated
yields were ca. 80% after collection of the extremely fine
powders of the products. Square 6 was particularly
difficult to collect on a frit, and it was isolated in only
42% yield. The preparation of squares 6, 7, and 8
required the use of CH3NO2 as the reaction solvent.
When the reactions were performed in CH2Cl2, the
(15) Whiteford, J . A.; Lu, C. V.; Stang, P. J . J . Am. Chem. Soc. 1997,
119, 2524.
(16) (a) Wrighton, M. S.; Giordano, P. J . J . Am. Chem. Soc. 1979,
101, 2888. (b) Worl, L. A.; Duesing, R.; Chen, P.; Ciana, L. D.; Meyer,
T. J . J . Chem. Soc., Dalton Trans. 1991, 849.
(17) (a) Longato, B.; Pilloni, G.; Valle, G.; Corain, B. Inorg. Chem.
1988, 27, 956. (b) Granifo, J .; Vargas, M. E.; Garland, M. T.; Baggio,
R. Inorg. Chim. Acta 2000, 305, 143. (c) Leininger, S.; Fan, J .; Schmitz,
M.; Stang, P. J . Prog. Natl. Acad. Soc. 2000, 97, 1380.
(14) Bassani, D. M.; Lehn, J .-M.; Fromm, K.; Fenske, D. Angew.
Chem., Int. Ed. 1998, 37, 2364.

Self-Assembly Heterometallic Cyclophanes
Organometallics, Vol. 21, No. 4, 2002 689
Sch em e 2
Sch em e 3
rotation of the pyridyl rings at room temperature or the
coexistence of stereoisomers with respect to the relative
positions between the Br and CO ligands. The former
possibility is excluded on the basis of variable-temper-
ature (263-310 K) 1H NMR experiments in CD2Cl2,
which showed no sign of coalescence of these peaks. In
addition, the crystal structure of square 5 exhibits
disorder associated with the Br and CO ligands, and
interestingly, there is also the reported observation of
two singlets from the nominally equivalent pyrazine
protons in the 1H spectrum of square [ClRe(CO)3(µ-
pyrazine)]4 by Hupp et al.4b Thus, the coexistence of
stereoisomers is understood to be the cause of the two
types of pyridyl protons for square 10.
Cr ysta l Str u ctu r e of Squ a r e 5. The single crystals
of square 5 were grown by slow diffusion of ether vapor
into a concentrated solution of 5 in CH3NO2. The
ORTEP diagram of 5 is shown in Figure 1. Selected
bond distances and angles as well as torsion angles are
given in Tables 2 and 3, respectively. The coordination
geometry around the Re center is facial with three car-
bonyl groups, two nitrogen pyridine donors, and one

690 Organometallics, Vol. 21, No. 4, 2002
Sun et al.
Despite the collection of the X-ray data at low tem-
perature, the solvent in this structure is still disordered.
From difference Fourier analysis, it was possible to
localize four CF3SO3 anions and two CH3NO2 molecules.
However, the displacement parameters of these species
were high, so the geometry was fixed throughout the
refinement. More solvent peaks were also found but
were not identified due to very high thermal displace-
ment parameters; therefore, these were refined as
separate carbon atoms (see Supporting Information).
The crystal packing pattern for 5 indicates the formation
of extended channels with CH3NO2 and unidentified
solvent molecules filled inside the channels. It is also
noticeable that the triflate anions are not located inside
the square but are located between the squares.
Electr och em istr y. Cyclic voltammetric experiments
were performed in CH3CN or DMF solution at ambient
temperature, and the data are presented in Table 4. It
can be seen that two oxidation potentials are observed
for all the square complexes except for square 10. On
the basis of the oxidation potentials of the Re(I)-based
corners and (dppf)Pt(H2O)2(OTf)2, the first reversible
oxidation process is assigned to Fe2+/3+ and the second
irreversible oxidation process is assigned to Re1+/2+. The
Re1+/2+ oxidation potential in square 10 was not ob-
served due to the limit of the DMF window in our
experimental conditions. The reduction processes ob-
served are assigned to the successive addition of elec-
trons to the π* orbitals localized in the bridging ligand.
In general, the redox potentials between the Pd(II) and
Pt(II) squares containing the same bridging ligand do
not vary too much, and the reduction potentials shift
to a less cathodic position when the bridging ligand is
more conjugated, i.e., DPB > DPA > bpy.
P h otop h ysica l P r op er ties. The obtained photo-
physical data, including absorption, emission spectra,
lifetimes, and emission quantum yields, of all the square
complexes studied are summarized in Table 5. It is
noted that the absorption spectra of the squares 4-8
and square 10 are very similar to other previously
reported Re(I) diimine complexes.4a,b,6a,c,e,18a In general,
there are two main absorption features in the UV and
near UV region for the squares and their Re-corner
complexes. The higher energy feature is assigned to a
ligand localized IL (π-π*) absorption, and the lower
energy absorption is attributable to a Re (dπ) to ligand
(π*) charge transfer (MLCT) transition.11 A very weak
absorption band centered around 530 nm is assigned
to a d-d transition localized on the dppf moiety.21 Fig-
ure 2 compares the absorption spectra of square 7 and
its corresponding Re(I)-corner 2 and Pd-corner 13 com-
plexes in CH2Cl2 solution. Typically, the MLCT bands
of the square complexes are slightly red shifted and the
extinction coefficients are roughly twice those of their
corresponding Re(I) corner complexes, as anticipated.
F igu r e 1. ORTEP diagram of square 5. Ellipsoids are
shown at 50% probability level. The counterions and
solvent molecules are not shown for clarity.
Ta ble 2. Selected Bon d Dista n ces (Å) a n d Bon d
An gles (d eg) for Squ a r e 5‚2CH3NO2
Re-N(1)
Pt-N(2)
Pt-P(1)
2.235(7)
2.095(7)
2.284(2)
Re-N(3)
Pt-N(4)
Pt-P(2)
2.213(7)
2.119(7)
2.289(2)
N(1)-Re-N(3)
N(2)-Pt-P(2)
P(1)-Pt-P(2)
84.3(2)
89.9(2)
99.04(7)
N(2)-Pt-N(4)
N(4)-Pt-P(1)
83.6(3)
88.2(2)
Ta ble 3. Selected Tor sion An gles (d eg) for Squ a r e
5‚2CH3NO2
C(40)
C(42)
C(40)
C(42)
C(50)
C(52)
C(50)
C(52)
C(41)
C(41)
C(41)
C(41)
C(51)
C(51)
C(51)
C(51)
C(46)
C(46)
C(46)
C(46)
C(56)
C(56)
C(56)
C(56)
C(45)
C(45)
C(47)
C(47)
C(55)
C(55)
C(57)
C(57)
28.9(13)
-156.0(9)
-144.9(9)
30.2(12)
35.8(12)
-145.8(8)
-143.9(9)
34.5(12)
bromine atom. The average Re-N bond distance is ∼2.2
Å, which is comparable to related structures reported
in the literature.4b,g,i,18 The average Pt-N bond is ∼2.1
Å, and this is also close to that determined for related
Pt(II)-pyridine complexes.4g,19 The Pt-P bonds and
P-Pt-P angles are comparable to reported crystal
structures containing (dppf)Pt units.20 It is found that
the stacking effect between the phenyl ring of diphenyl-
phosphinoferrocene and the neighboring pyridyl ring
causes a slight distortion from an ideal square structure;
the N-Pt-N angle is 83.6° and the N-Re-N angle is
84.3°. A similar stacking effect has also been observed
in other reported square crystal structures.4a,19 Also, the
bridging 4,4′-bpy ligands are not flat, and the dihedral
angles between its rings are 32°. The dimensions of the
cavity, as defined by the distances between Re(I) and
Pt(II), are 11.28 × 11.38 Å. It should also be pointed
out that the pair of trans Br and CO groups is statisti-
cally disordered in a ratio 1:2. Indeed, this halide/CO
disorder has also been observed in other crystal struc-
tures of square complexes containing fac-rhenium tri-
carbonyl halide moieties.4b
Square complexes 4-8, as well as their Re(I)-corner
complexes, exhibit luminescence in room-temperature
solution, whereas square 10 does not have detectable
luminescence in room-temperature CH2Cl2 solution. The
striking feature of these emitting squares, compared to
their corresponding Re(I)-corner complexes, is that the
(18) (a) Lin, J . T.; Sun, S.-S.; Wu, J . J .; Liaw, Y.-C.; Lin, K.-J . J .
Organomet. Chem. 1996, 517, 217. (b) Be´langer, S.; Hupp, J . T.; Stern,
C. L. Acta Crystallogr. 1998, C54, 1596.
(19) Stang, P. J .; Cao, D. H.; Saito, S.; Arif, A. M. J . Am. Chem.
Soc. 1995, 117, 6273.
(20) Bandoli, G.; Dolmella, A. Coord. Chem. Rev. 2000, 209, 161.
(21) (a) Geoffroy G. L.; Wrighton, M. S. Organometallic Photochem-
istry; Academic Press: New York, 1979. (b) Lee, E. J .; Wrighton, M.
S. J . Am. Chem. Soc. 1991, 113, 8562.

Self-Assembly Heterometallic Cyclophanes
Organometallics, Vol. 21, No. 4, 2002 691
Ta ble 4. Red ox P oten tia lsa
compound
Eox, V
Ered, V
BrRe(CO)3(bpy)2
BrRe(CO)3(DPB)2
BrRe(CO)3(DPA)2
0.96
1.10 (i)
1.07 (i)
-1.95, -2.10
-1.80 (i), -2.18 (i)
-1.88 (i)
[(dppf)Pd(µ-bpy)2Re(CO)3Br]2(OTf)4
[(dppf)Pt(µ-bpy)2Re(CO)3Br]2(OTf)4
[(dppf)Pd(µ-DPA)2Re(CO)3Br]2(OTf)4
[(dppf)Pd(µ-DPB)2Re(CO)3Br]2(OTf)4
[(dppf)Pt(µ-DPB)2Re(CO)3Br]2(OTf)4
[(dppf)Pt(4-ethynylpyridine)2Re(CO)3Br]2
(dppf)Pd(4-Etpy)2(OTf)2
0.75, 1.10 (i)
0.76, 1.10 (i)
0.76, 1.08 (i)
0.76, 1.06 (i)
0.74, 1.12 (i)
0.39
0.78
0.80
-2.18 (i), -2.41
-2.09 (i), -2.41
-1.92 (i), -2.38 (i)
-1.83 (i), -2.20 (i)
-1.77 (i), -2.16 (i)
-1.56 (i)
b
(dppf)Pt(OH2)2(OTf)2
a
Analyses were performed in 0.5 mM deoxygenated CH3CN solutions containing 0.1 M TBAH, scan rate is 200 mV/s. All potentials in
volts vs [Fe(C5H5)2]+/0 (0.12 V with peak separation 80 mV in DMF and 0.17 V with peak separation 100 mV in CH3CN); (i) ) irreversible
process and the reported values are peak positions. bDMF solution.
Ta ble 5. Electr on ic Absor p tion a n d Em ission Da ta a t 293 Ka
absorption spectra
emissionb
compd
λ
max, nm (10-3ꢀ, M-1 cm-1
)
λ
max, nm
τ, nsc
Φem
0.016
0.0040
0.0083
1
2
3
4
5
6
7
8
10
11
12
13
14
248 (35.5), 320 (14.0)
585
612
592
605
612
610
628
632
d
d
d
d
d
1200
100
160
9.4
251 (47.5), 262 (46.4), 288 (28.4), 302 (31.1), 322 (36.0), 345 (31.5)
281 (41.7), 296 (38.3), 326 (20.0)
254 (96.0), 337 (32.4), 516 (2.33)
252 (116), 343 (28.0)
289 (143), 344 (57.1), 510 (3.52)
262 (100), 308 (84.5), 327 (94.4), 349 (77.2), 502 (3.99)
259 (116), 311 (77.7), 328 (87.9), 347 (73.1)
277 (56.0), 294 (61.5), 323 (81.9)
282 (60.3), 337 (10.0), 502 (2.52)
281 (53.4)
5.82 × 10-4
3.65 × 10-4
4.80 × 10-4
4.80 × 10-4
4.02 × 10-4
8.4
14.6
11.7
9.2
255 (28.7), 288 (21.3), 343 (5.40), 502 (1.16)
258 (21.2)
a
b
c
CH2Cl2 solution. λex ) 360 nm. The errors on the obtained lifetimes for square complexes are estimated to be 20% due to the low
emission quantum yields. dNo detectable emission in room-temperature solution.
Ta ble 6. Bim olecu la r Qu en ch in g Con sta n ts
Der ived fr om Ster n -Volm er An a lysis betw een th e
Re(I)- a n d P d (II)- or P t(II)-Cor n er Com p lexes in
CH2Cl2 Solu tion a t 293 K
complex/quencher
kq, M-1 s-1
BrRe(CO)3(bpy)2/(dppf)Pd(4-phpy)2(OTf)2
BrRe(CO)3(bpy)2/(dppf)Pt(4-phpy)2(OTf)2
BrRe(CO)3(DPB)2/(dppf)Pd(4-Etpy)2(OTf)2
BrRe(CO)3(DPB)2/(dppf)Pt(4-Etpy)2(OTf)2
BrRe(CO)3(DPA)2/(dppf)Pd(4-Etpy)2(OTf)2
1.2 × 1010
5.9 × 109
6.9 × 109
1.1 × 109
7.8 × 109
state as the energy separation between these levels is
reduced; this consequently results in an increase in the
nonradiative decay rates. However, the lifetimes deter-
mined for squares 4-8 are apparently much shorter
than predicted on the basis of the energy gap law when
one compares them to other fac-tricarbonylrhenium
complexes.4a,22 Two structurally similar square com-
plexes of the formula cyclobis{[cis-(dppp)M](µ-4,4′-bpy)2-
[Re(CO)3Cl]}(OTf)4 (M ) Pd or Pt and dppp is 1,3-
diphenylphosphinopropane) have been determined to
exhibit analogous photophysical properties, and the
short lifetimes in these square complexes have been
rationalized by invoking intramolecular quenching from
the Pd(II)- or Pt(II)-corners.4a,e
Table 6 summarizes the bimolecular quenching con-
stants (kq) determined from Stern-Volmer quenching
studies between the individual Re(I)-corner and the
Pd(II)- or Pt(II)-corner molecular components. It can be
seen that the obtained quenching constants range from
9.2 × 108 to 1.2 × 1010 M-1 s-1. Figure 3 shows the
Stern-Volmer plot observed for the corner complexes
3 and 13, confirming that there is efficient bimolecular
F igu r e 2. Electronic absorption spectra of square 7 (thick
line), Re(I) corner 2 (dashed line), and Pd(II) corner 13
(solid line) in CH2Cl2 solution.
squares exhibit band maxima at longer emission wave-
lengths (416-754 cm-1), shorter lifetimes (7-143-fold),
and lower emission quantum yields (8-44-fold) than
their corresponding corner complexes. Typically, the
luminescent quantum yields of fac-tricarbonylrhenium-
(I) complexes are governed by the rate of nonradiative
3
decay from the emitting MLCT state, which is largely
determined by the energy gap law.22 According to this
law, the lowest energy emitting excited state is sub-
jected to greater vibrational overlap with the ground
(22) Caspar, J .; Meyer, T. J . J . Phys. Chem. 1983, 87, 952. According
to the data reported in this reference, an 1100 cm-1 decrease in E00
will cause a roughly 5-fold increase in the nonradiative rate constant.
Hence, the lifetimes of the square complexes in our work exhibit values
of only about 20% of that predicted by the energy gap law.

692 Organometallics, Vol. 21, No. 4, 2002
Sun et al.
F igu r e 4. Binding isotherm depicting change of emission
n
of square 4 upon addition of Bu4BF4 in acetone solution.
Pt(II)-acetylide system is expected to provide a better
orbital overlap between the (dppf)Pt(II) and 4-ethy-
nylpyridine moieties (compared to (dppf)Pt(II) and bi-
pyridyl ligands in squares 4-8). Both of these aspects
will enhance the efficiency of intramolecular energy and
electron transfer in square 10.
F igu r e 3. Stern-Volmer plot of the bimolecular emission
quenching between the Re(I) corner 3 and the Pd(II) corner
13 complexes in CH2Cl2 solution at 293 K.
Sch em e 4
An ion Bin d in g Stu d ied by Lu m in escen ce. Anion
sensing chemistry has become one of the most intriguing
fields of supramolecular chemistry owing to its funda-
mental role in many chemical and biological processes.23
The well-defined structures and positive charges as-
sociated with the squares studied here render them as
potentially effective hosts for anionic species. The fact
that several of the squares are also luminescent is an
additional feature that may facilitate their development
as sensor molecules.
quenching between the Re(I) and Pd(II) or Pt(II) corners.
This plot is representative for all of our quenching data.
In our square complexes, however, the lifetime data
(vide supra) illustrate an even more effective quenching
process taking place, which is probably the result of the
incorporation of the ferrocenyl moieties in the Pd or Pt
corners. It seems unlikely, though, that the increased
quenching efficiency is solely due to oxidative electron
transfer from the bridging ligand radical anion π*
orbital to the Pd(II) or Pt(II) corners. Given the fact that
dppf is a better electron donor than dppp, this should
provide a stronger electrostatic stabilization of the
positively charged metal center (Pd or Pt) and, thus,
decrease the exothermicity of the oxidative quenching
process. Alternatively, we note that there is a partial
overlap between the absorption spectrum of (dppf)M-
(4-phenylpyridine)2(OTf)2 (M ) Pd or Pt) or (dppf)M(4-
ethylpyridine)2(OTf)2 (M ) Pd or Pt) and the emission
spectra of both the square and Re(I)-corner complexes.
This observation strongly suggests that the quenching
is also influenced by intramolecular energy transfer
Binding studies between squares 4-8 and different
inorganic anions were carried out in acetone solution.
Among the anions we investigated, viz., ClO4-, OAc-,
OTf-, PF6-, and BF4-, only the latter two induced
significant changes in the luminescence intensities of
squares 4-8 upon addition. The binding behaviors of
the BF4- and PF6- anions in acetone solution are quite
unusual. Figure 4 shows a representative binding curve
between the BF4- anion and square 4. The luminescence
-
intensity first decreases when a small amount of BF4
is added to 0.1 mM square 4 in acetone solution. When
-
the amount of added BF4 apparently rises over a
critical concentration, the luminescence intensity then
starts to increase and finally reaches a plateau. Similar
-
results were also observed for the PF6 anion as well
as for the other luminescent squares 5-8. The unusual
-
changes of the emission intensity upon addition of BF4
or PF6- also indicate that these are not just simple anion
exchange processes.
The results strongly suggest that two different quench-
ing pathways are responsible for the decay of the excited
state and that these pathways compete with one an-
other. It should be noted that the emission position and
band shape do not change with the anion binding, which
suggests an indirect electronic perturbation on the
3
from the MLCT excited state to lower ferrocene-based
metal-center (1,3MC) states.21 A qualitative energy
diagram portraying the photophysical processes occur-
ring in the square molecules is depicted in Scheme 4.
As noted above, square 10 does not display detectable
emission at room temperature, and this is also at-
tributed to the quenching effect exerted from the
(dppf)Pt(II) moieties. In this case, there is a relatively
short distance between the Re(I) chromophores and
(dppf)Pt(II). Additionally, the σ-bonding character of the
-
Re(I) chromophores. The initial binding of BF4 to the
square complex is expected to cause a red shift of the
ferrocenyl LF band and, thus, increase the probability
of energy transfer from the 3MLCT state to 1,3LF states.
(23) Schmidtchen, F. P.; Berger, M. Chem. Rev. 1997, 97, 1609.

Self-Assembly Heterometallic Cyclophanes
Organometallics, Vol. 21, No. 4, 2002 693
The result is a decrease in the luminescence intensity.
When more BF4- is added into the solution, the excess
anions start to stabilize the positive charges on Pd(II)
and reduce the oxidative quenching probability, and
thus, the luminescence intensity is increased.
The photophysical properties have been shown to be
predominantly influenced by the electronic properties
of the bridging ligands. No luminescence was observed
from neutral square 10, although squares 4-8 that are
bridged by rigid or semirigid ligands such as 4,4′-bpy,
DPA, and DPB exhibit room-temperature luminescence,
albeit weaker than predicted by energy gap law consid-
erations. The low luminescence quantum yields are as-
cribed to two different quenching pathways, specifically
Pd(II)- or Pt(II)-based oxidative quenching and energy
The physical basis of why only BF4- and PF6- induce
the change in the luminescence of the square complexes
deserves further investigation with other systems.
Presumably this relates to the anion encapsulation
inside the square cavity and the electrostatic induced
ion-pair interaction between the square and anion. The
involvement of the solvent’s dielectric property and the
polarizability of the anions are also expected to play
significant roles in these host-guest interactions.
Ar om a t ic Gu est Bin d in g St u d ied b y 1H NMR
Titr a tion . The aromatic guest binding studies were
conducted in acetone-d6 for squares 4-8 and CD3CN for
3
transfer from the emitting MLCT states to lower non-
emissive excited levels localized on the ferrocenyl moi-
eties. Unusual anion binding behavior has been deter-
mined, and this is found to be also influenced by these
two different quenching mechanisms. 1H NMR titration
experiments carried out for several aromatic guest mole-
cules indicate that there are not any strong π-π asso-
ciation interactions with these squares.
1
square 10. No significant H NMR shifts were observed
(less than 0.01 ppm for pyridyl protons) for each of these
squares when they were mixed with the guest molecules
including 1,3,5-trimethoxybenzene, 2-naphthalenesulfon-
ic acid (sodium salt), and 2,6-naphthalenedisulfonic acid
(disodium salt). These results suggest that there are
little or negligible π-π or hydrophobic interactions
between the guest molecule and the square host.
Ack n ow led gm en t. We are grateful to the Division
of Chemical Sciences, Office of Basic Energy Sciences,
Office of Science, U.S. Department of Energy (Grant
DE-FG02-89ER14039), for support of this research. We
also thank Prof. Omowunmi A. Sadik for access to
electrochemical equipment, Dr. J u¨rgen Schulte for help-
ing acquire 31P NMR spectra, and Prof. J on Zubieta at
Syracuse University for supplying the DPA ligand. The
Department of Chemistry of Syracuse University is also
acknowledged for access to the X-ray facility for collect-
ing the single-crystal data.
Con clu sion s
Two series of heterometallic cyclophanes have been
prepared and characterized from self-assembly of cis-
(dppf)M(H2O)2(OTf)2 (M ) Pd or Pt) and BrRe(CO)3(L)2
(L ) bis-monodentate pyridyl ligands) or cis-(dppf)Pt-
(4-ethynylpyridine)2 and BrRe(CO)5. The single-crystal
structure of square 5 establishes the dimensions to be
11.28 × 11.38 Å on the edges, based on the Re-Pt
interatomic distances. The crystal stacking diagram
shows extended channels, which are filled with solvent
molecules in the solid state.
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
data, in CIF format, and a figure showing the crystal packing
diagram (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
OM0109096