Quinonoid-bridged chair-shaped dirhenium(I) metallacycles: Synthesis, characterization, and spectroelectrochemical studies

Article abstract of DOI:10.1021/ic9018794

Self-assembled, chair-shaped dirhenium(I) macrocyclic compounds featuring the two different bis-chelating quinone dianions (1, L = dhnq^2-; 2, L = dhaq^2-; H₂dhnq = 6,11-dihydroxy-5,12-naphthacenedione; H₂dhaq = 1,4-dihydroxy-9,10-anthraquinone) that interface with two fac-Re(CO)₃cores and a ditopic semirigid N-donor 1,4-bis(5,6- dimethylbenzimidazol-1-ylmethyl)naphthalene (L′ = p-NBimM) ligand coordinated to the remaining orthogonal site were prepared in high yields. Their structures were confirmed by single-crystal X-ray diffraction analysis. Electrochemical assessments, using cyclic voltammetry (CV) and UV-vis-NIR spectroelectrochemistry (SEC), revealed the existence of two well-separated, single-electron quinone ligand-centered, reversibly accessible 0,-1, and-2 redox states. Among the two singly reduced radical complexes, the symmetrically bridged quinone complex 1^?-, showed a strong absorption in the NIR regions, which was not observed for the neutral and doubly reduced states, analogous to that of the free dhnq^3?-quinone. In contrast, when 2 was reduced to 2^?-, a broad signal at 866 nm was observed, very similar to the reduced dhaq^3?-quinone. This difference in spectral behavior in the singly reduced states is likely due to the annealed benzene ring in 1 and dhnq^2-because of its symmetrical π-electron system, which is perturbed to a lesser degree compared to asymmetric 2 and dhaq^2-. Reduction to 1^?-produces a small but not negligible g factor anisotropy (Δg = 0.024) in the electron spin resonance (ESR) signal, indicative of a very small metal-centered spin (5%), but 2 ^?-shows a g value in the expected range for organic radicals (no detectable Δg). Thus, the combined investigations reveal that the singly reduced metallacycles are best described as being highly stable, noncommunicating, localized, quinonoid-centered radical complexes, [(CO) ₃Re^I(μ-L^3?-)(μ-L′)Re ^I(CO)₃]^?-.

Full text of DOI:10.1021/ic9018794

10264 Inorg. Chem. 2010, 49, 10264–10272
DOI: 10.1021/ic9018794
Quinonoid-Bridged Chair-Shaped Dirhenium(I) Metallacycles: Synthesis,
Characterization, and Spectroelectrochemical Studies
Dibyendu Bhattacharya,† Malaichamy Sathiyendiran,† Jing-Yun Wu,‡ Che-Hao Chang,† Sung-Chou Huang,‡
Yu-Ling Zeng,‡ Ching-Yao Lin,‡ P. Thanasekaran,† Bo-Chao Lin,† Chao-Ping Hsu,† Gene-Hsiang Lee,§
Shie-Ming Peng,§ and Kuang-Lieh Lu*,†
†Institute of Chemistry, Academia Sinica, Taipei 115, Taiwan, ‡Department of Applied Chemistry,
National Chi Nan University, Nantou 545, Taiwan, and §Department of Chemistry, National Taiwan University,
Taipei 107, Taiwan
Received September 22, 2009
Self-assembled, chair-shaped dirhenium(I) macrocyclic compounds featuring the two different bis-chelating quinone
dianions (1, L = dhnq2-; 2, L = dhaq2-; H2dhnq = 6,11-dihydroxy-5,12-naphthacenedione; H2dhaq = 1,4-dihydroxy-
9,10-anthraquinone) that interface with two fac-Re(CO)3 cores and a ditopic semirigid N-donor 1,4-bis(5,6-
dimethylbenzimidazol-1-ylmethyl)naphthalene (L0 = p-NBimM) ligand coordinated to the remaining orthogonal site
were prepared in high yields. Their structures were confirmed by single-crystal X-ray diffraction analysis. Electro-
chemical assessments, using cyclic voltammetry (CV) and UV-vis-NIR spectroelectrochemistry (SEC), revealed
the existence of two well-separated, single-electron quinone ligand-centered, reversibly accessible 0, -1, and -2
redox states. Among the two singly reduced radical complexes, the symmetrically bridged quinone complex 1•-
,
showed a strong absorption in the NIR regions, which was not observed for the neutral and doubly reduced states,
analogous to that of the free dhnq3•- quinone. In contrast, when 2 was reduced to 2•-, a broad signal at 866 nm was
observed, very similar to the reduced dhaq3•- quinone. This difference in spectral behavior in the singly reduced states
is likely due to the annealed benzene ring in 1 and dhnq2- because of its symmetrical π-electron system, which is
perturbed to a lesser degree compared to asymmetric 2 and dhaq2-. Reduction to 1•- produces a small but not
negligible g factor anisotropy (Δg = 0.024) in the electron spin resonance (ESR) signal, indicative of a very small
metal-centered spin (5%), but 2•- shows a g value in the expected range for organic radicals (no detectable Δg).
Thus, the combined investigations reveal that the singly reduced metallacycles are best described as being highly
stable, noncommunicating, localized, quinonoid-centered radical complexes, [(CO)3ReI(μ-L3•-)(μ-L0)ReI(CO)3]•-
.
Introduction
and their biological importance in systems ranging from
quinoproteins to photosystems.3 Depending on the relative
energies of the redox-active orbitals, dimetallic complexes with
proradical ligands can be characterized as metal-ligand
radicals, [Mn(μ-L)(μ-L0)Mnþ1] (type A) or [Mnþ0.5(μ-L)(μ-L0)-
Ligand-bridged dimetallic systems, along with selective
redox-active quinonoid ligands, have attracted considerable
interest in recent years due to the fact that the intramolecular
electronic coupling processes can be tuned in their mixed-
valence (MV) states,1 which, in turn, provides an opportunity
for understanding the energy and electron-transfer processes2
M
nþ0.5] (type B) or as a radical complex [Mn(μ-L•-)(μ-L0)Mn]
(type C).1b,4 Particularly interesting from a fundamental per-
spective are the interactions of the unpaired electron of the
ligand with either the metal or another ligand (superexchange
interactions through covalent bonds or through space electron-
ic communication), which affects the behavior of electronic
*To whom correspondence should be addressed. Fax: 886(2)27831237.
E-mail: lu@chem.sinica.edu.tw.
(1) (a) Das, A. K.; Sarkar, B.; Fiedler, J.; Zalis, S.; Hartenbach, I.;
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r
pubs.acs.org/IC
Published on Web 10/14/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10265
coupling in the mixed-valence state.5,6 It is noteworthy that
few examples of coordination compounds feature ligands
rather than metal ions as electron donors and acceptors.
Tetra- or hexa-Re(I) complexes with either bipyridyl or
triazine ligand-centered MV systems are prime candidates
for realizing direct through space electronic communication.5
Electron transfer via superexchange may be the dominant
mechanism, provided the energy of the lowest unoccupied
molecular orbital of the bridging ligand is close to that of the
donor and acceptor moieties, since the comparable energies
of the bridge and metal redox centers would act to reduce the
activation barrier to electron tunneling.6,7 Therefore, dinu-
clear ruthenium(II) complexes bridged by o- and p-quinonoid
molecules in combination with different coligands and mixed-
valence systems have been scrutinized in detail.1a,b,7 However,
the subtleties of quinonoid-rhenium(I) behavior, which un-
derlie the importance of these compounds, have yet to be
explored.
produced via the reduction of neutral dirhenium(I) metalla-
cycles show the following features: (1) quinone ligand-centered;
two well-separated, single-electron, reversibly accessible 0,
-1, -2 redox states. The benziimidazolate ligand appears to
be redox innocent within the solvent window: (2) small-to-
negligible g-factor anisotropy in the ESR signal, signifying
a e 5% metal-centered spin in the singly reduced states, and
(3) highly stable, noncommunicating, localized, quinonoid-
centered radical complexes, as evidenced by combined SEC
and ESR data. The electronic characteristics of the radical-
anionic and dianionic states of such biologically relevant
quinonoid ligands in the forms of free and Re(I) complexes
are described for the first time.
Experimental Section
Materials. All starting materials and products were stable toward
moisture and air. Commercial grade reagents, Re2(CO)10, 6,11-
dihydroxy-5,12-naphthacenedione (H2-dhnq), and 1,4-dihydroxy-
9,10-anthraquinone (H2-dhaq), were used as received. The solvents
used in this study were of spectroscopic grade.
Herein, we report on the synthesis (in a one step assembly)
of the chair-shaped, neutral dirhenium(I) metallacycles, [(CO)3-
ReI(μ-L)(μ-L0)ReI(CO)3], (1, L = dhnq2-; 2, L = dhaq2-
;
Physical Measurements and Procedures. Elemental analyses
for carbon, hydrogen, and nitrogen were recorded with a Perkin-
Elmer 2400 CHN elemental analyzer. Infrared spectra were
measured on a Perkin-Elmer PARAGON 1000 FT-IR spectrom-
H2dhnq = 6,11-dihydroxy-5,12-naphthacenedione; H2dhaq =
1,4-dihydroxy-9,10-anthraquinone) and a ditopic semirigid
N-donor ligand, 1,4-bis(5,6-dimethylbenzimidazol-1-ylmethyl)-
naphthalene (L0=p-NBimM). The ligand valence state distri-
bution in 1n and 2n, where n=0, -1, and -2, was assessed
via CV and UV-vis-NIR SEC. Type C quinone-containing
1
eter. H NMR spectra were recorded on Bruker AC 300 and
AMX-400 FT-NMR spectrometers. FAB-MS data were ob-
tained using a JEOL JMS-700 double focusing mass spectrom-
eter. The electronic absorption spectra were obtained on a
Hewlett-Packard 8453 spectrophotometer at room temperature
in a 1 cm quartz cell. Fluorescence spectra were recorded on a
Hitachi F4500 spectrometer. Excited-state lifetimes were mea-
sured as described previously.5d Cyclic voltammograms (CV) and
differential pulse voltammograms (DPV) were measured in an-
hydrous DMSO/0.1 M Bu4NClO4 as a supporting electrolyte
at 298 K. CVs were carried out at a 100 mV/s scan rate, using a
three-electrode configuration (Pt working electrodes, Pt wire
counter electrode, and saturated calomel electrode (SCE) refer-
ence electrode) and a CHI Electrochemical Workstation 611A
under deaerated conditions. UV-vis-NIR spectroelectrochem-
istry was performed with an airtight, optically transparent thin-
layer electrochemical cell (OTTLE) constructed with a 100 mesh
platinum gauze working electrode in a 1 mm quartz cell.8 UV-
vis-NIR spectra were recorded with a JASCO V-570 UV-vis-
NIR spectrophotometer. Both electrolysis cells were assembled
under an inert atmosphere in a glovebox equipped with a drytrain
to exclude moisture and oxygen.
radical complexes, [(CO)3ReI(μ-L3•-)(μ-L0)ReI(CO)3]•-
,
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Crystallographic Determination. Suitable single crystals of
1 C7H8 and 2 C7H8 with dimensions of 0.30 ꢀ 0.30 ꢀ 0.15
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Coropceanu, V.; Bredas, J. L.; Hupp, J. T. Inorg. Chem. 2005, 44, 5789. (d)
3
3
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mm and 0.25 ꢀ 0.20 ꢀ 0.15 mm, respectively, were selected for
indexing and the collection of intensity data. Measurements
were performed using graphite-monochromatized Mo KR ra-
diation (λ = 0.71073 A) on a Nonius Kappa CCD diffracto-
meter. Intensity data were collected at 150(2) K within the
limits 1.59° < θ < 27.49° for 1 C7H8 and 1.38° < θ < 27.50°
3
for 2 C7H8. The structures were solved by direct methods and
3
refined on F2 by full-matrix least-squares calculations using
SHELX-97 program packages.9 Because of the disordered
syndrome of guest molecules, the carbon atoms of the toluene
molecules in 2 C7H8 were refined isotropically. Other non-
3
hydrogen atoms were refined anisotropically, and all hydrogen
atoms were assigned by geometrical calculation and refined as
riding models. Details of the structure determinations are given
(8) (a) Lin, C.-Y.; McGlashen, M. L.; Hu, S.; Shim, Y. K.; Smith, K. M.;
Spiro, T. G. Inorg. Chim. Acta 1996, 252, 179. (b) Krejcik, M.; Danek, M.;
Hartl, F. J. Electroanal. Chem. Interfacial Electrochem. 1991, 317, 179.
(9) Sheldrick, G. M. SHELX-97 (including SHELXS and SHELXL);
€
€
University of Gottingen: Gottingen, Germany, 1997.

10266 Inorganic Chemistry, Vol. 49, No. 22, 2010
Bhattacharya et al.
Table 1. Crystallographic Data of 1 C7H8 and 2 C7H8
effects were modeled in the integral equation formalism14 for the
polarizable continuum model15 (IEF-PCM) in the DMSO (di-
electric ε = 46.7).12 The lowest 55 singlet excited states were cal-
culated using TDDFT/IEF-PCM. DMSO was chosen as the
solvent in TDDFT calculations in order to be consistent with
the experiment. The output contained information (Table S4 in
the Supporting Information) for the excited-state energies and
oscillator strengths ( f ) and a list of the excitations that give rise to
each excited state, the orbitals involved, and the wave function
coefficients of the excitations.
Finally, to depict the detail of the frontier molecular orbitals,
the stereocontour graphs of some related frontier molecular
orbitals of the complexes for the ground states and the singly
reduced states were drawn with the GView 3.09 program based
on the computational results. For orbital contributions, the
molecular orbital compositions were analyzed using the AOMIX
program.16
3
3
1 C7H8
3
2 C7H8
3
formula
Mr
cryst syst
space group
a (A)
C61H44N4O10Re2
1365.40
triclinic
C57H42N4O10Re2
1315.35
orthorhombic
Pnma
13.2905(2)
24.1141(4)
15.0699(3)
90
P1
13.4838(2)
13.8023(2)
16.2869(3)
96.2148(11)
111.0004(9)
108.1030(9)
2606.03(7)
2
b (A)
c (A)
R (deg)
β (deg)
γ (deg)
V (A3)
Z
90
90
4829.73(14)
4
150(2)
0.71073
1.809
5.076
2568
1.036
0.0285/0.0715
0.0443/0.0777
1.602/-1.067
T (K)
λ (A)
150(2)
0.71073
1.740
4.707
1336
1.055
0.0357/0.0913
0.0473/0.0969
2.098/-1.972
Dcalc (g cm-3
)
μ (mm-1
F(000)
)
Synthesis of 1,4-Bis(5,6-dimethylbenzimidazol-1-ylmethyl)-
naphthalene (p-NBimM) Ligand (L0). KOH (0.095 g, 1.75 mmol)
was slowly added to 20 mL of a THF solution of benzimidazole
(0.51 g, 3.45 mmol), with stirring, whereupon a white milky
suspension appeared to form. Stirring was continued at room
temperature for 4 h. After that, 20 mL of a THF solution of 1,4-
bis(bromomethyl)naphthalene (0.53 g, 1.7 mmol) was added
dropwise, and the reaction mixture was stirred continuously
overnight. The solvent was removed under reduced pressure; the
reaction mixture was washed thoroughly with water (15 mL ꢀ 3)
to remove trace base and extracted with dichloromethane
(15 mL ꢀ 3). The combined organic layer was again washed
with water (20 mL) and dried over Na2CO3. The crude product
was dissolved in hot acetone, followed by the slow addition
of petroleum ether (bp 40-60 °C) and then cooling to room
temperature to afford a white microcrystalline solid. Yield:
goodness-of-fit
R1a/wR2b [I > 2σ(I)]
R1a/wR2b (all data)
largest residuals (e A-3
)
P
P
P
P
a R1= ||Fo| - |Fc||/ |Fo|. b wR2={ [w(Fo2 -Fc ) ]/ [w(Fo2)2]}1/2
.
2 2
in Table 1; selected bond distances and angles for the two
structures are provided in the Supporting Information, Table S1.
Computational Section. The singlet ground-state geometries
of complexes 1 and 2 were optimized in the gas phase using the
B3LYP10 functional and the LANL2DZ11 for the Re core
potentials and basis set in the Gaussian 03 program package.12
The initial geometry was obtained from the crystal structures for
1 and 2. The 6-31G* basis set13 was used for all other atoms.
Selected parameters of the optimized geometries of 1 and 2 are
listed in the Supporting Information, Table S2. Dihedral angles
between the aryl groups are also listed in the Supporting
Information, Table S3.
The singly reduced geometries of 1 and 2 were calculated
using the unrestricted B3LYP functional (UB3LYP) in the gas
phase. Ground-state B3LYP optimized geometries were used
for geometry optimization in singly reduced states.
To estimate the theoretical values for vertical excitations from
the ground state, computations using the TDDFT in the gas
phase, the B3LYP hybrid functional, and the 6-31G* basis set
were used in the first step. In the second step, the dielectric solvent
1
0.37 g, 24%. H NMR (400 MHz, DMSO-d6, 25 °C, relative
to SiMe4): δ 8.25 (dd, 2H, J = 3.3 Hz, H11, H14), 8.17 (s, 2H, H2),
7.65 (dd, 2H, J = 3.3 Hz, H12, H13), 7.42 (s, 2H, H4), 7.22 (s, 2H,
H7), 6.89 (s, 2H, H9, H10), 5.91 (s, 4H, H8), 2.28 (s, 6H, CH3,
p-NBimM), 2.22 (s, 6H, CH3, p-NBimM). Anal. Calcd for
C30H28N4: C, 81.05; H, 6.35; N, 12.60. Found: C, 80.77; H,
6.13; N, 12.06%. FAB-MS: m/z 444.57 [Mþ].
Synthesis of [{Re(CO)3}2(μ-dhnq)(μ-p-NBimM)] (1). A mix-
ture of Re2(CO)10 (221 mg, 0.33 mmol), p-NBimM (171 mg, 0.32
mmol), and H2-dhnq (120 mg, 0.34 mmol) in toluene in a Teflon
flask was placed in a steel bomb. The bomb was placed in an
oven maintained at 160 °C for 72 h and then cooled to 25 °C.
Good quality, dark-green single crystals of 1 were obtained. The
crystals were separated by filtration, washed with toluene and
hexane, and dried in the air. Yield: 74% (198 mg, 0.15 mmol).
1H NMR (400 MHz, DMSO-d6, 25 °C, relative to SiMe4): δ 8.61
(s, 2H, H2, p-NBimM), 8.38 (m, 4H, Ha, dhnq), 8.10 (m, 2H, H14,
p-NBimM), 7.89 (s, 2H, H4, p-NBimM), 7.73 (m, 4H, Hb, dhnq),
7.71 (s, 2H, H7, p-NBimM), 7.70 (m, 2H, H13, p-NBimM), 7.23
(m, 5H, toluene), 7.15 (s, 2H, H9, p-NBimM), 5.93 (s, 4H,
H8(-CH2-), p-NBimM), 2.29 (s, 3H, CH3, toluene), 2.25 (s, 6H,
CH3, p-NBimM), 1.36 (s, 6H, CH3, p-NBimM). Anal. Calcd for
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M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci,
B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada,
M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian,
H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski,
J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg,
J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.;
Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.;
Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.;
Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill,
P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision E.01; Gaussian, Inc.: Wallingford, CT, 2003.
C54H36N4O10Re2 (C7H8): C, 53.66; H, 3.25; N, 4.10. Found: C,
3
53.37; H, 3.32; N, 4.07%. FAB-MS: m/z 1272.15 [Mþ].
Synthesis of [{Re(CO)3}2(μ-dhaq)(μ-p-NBimM)] (2). A mix-
ture of Re2(CO)10 (219 mg, 0.34 mmol), p-NBimM (170 mg, 0.34
mmol), and H2-dhaq (118 mg, 0.35 mmol) in toluene in a Teflon
flask was placed in a steel bomb. The bomb was placed in an
oven maintained at 160 °C for 48 h and then cooled to 25 °C.
Good quality, dark-green single crystals of 2 were obtained. The
crystals were separated by filtration, washed with toluene and
hexane, and dried in the air. Yield: 66% (219 mg, 0.18 mmol).
(13) (a) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys.
1998, 109, 8218. (b) Bauernschmitt, R.; Ahlrichs, R. Chem. Phys. Lett. 1996,
256, 454. (c) Casida, M. E.; Jamorski, C.; Casida, K. C.; Salahub, D. R. J. Chem.
Phys. 1998, 108, 4439.
(15) Tomasi, J.; Menucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999.
(16) (a) Gorelsky, S. I. AOMIX. http://www.sg-chem.net (accessed Sep
2010). (b) Gorelsky, S. I.; Lever, A. B. P. J. Organomet. Chem. 2001, 187, 635.
(14) (a) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151. (b) Tomasi,
ꢀ
J.; Mennucci, B.; Cances, E. J. Mol. Struct. THEOCHEM 1999, 464, 211.

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10267
Scheme 1. Self-Assembly of Metallacycles 1 and 2
1H NMR (400 MHz, DMSO-d6, 25 °C, relative to SiMe4): δ 8.65
(s, 2H, H2, p-NBimM), 8.30 (d, 2H, Hb dhaq), 8.17 (m, 2H, H14,
p-NBimM), 7.82 (s, 2H, H4, p-NBimM), 7.76 (m, 6H, Ha, Hc,
dhaq; H13, p-NBimM), 7.22 (m, 2H, H7, p-NBimM), 7.18 (m,
5H, toluene), 7.02 (s, 2H, H9, p-NBimM), 6.08-5.96 (s, 4H,
H8(-CH2-), p-NBimM), 2.29 (s, 3H, CH3, toluene), 2.25 (s,
6H, CH3, p-NBimM), 2.05 (s, 6H, CH3, p-NBimM). Anal. Calcd
for C50H34N4O10Re2: C, 49.09; H, 2.80; N, 4.58. Found: C,
48.97; H, 2.79; N, 4.47%. FAB-MS: m/z 1224.14 [Mþ].
X-Ray Structural Details and Computed Geometries.
ORTEP and perspective diagrams of complexes 1 and 2
are shown in Figures 1 and 2 and in the Supporting
Information, Figure S2, along with pertinent crystallo-
graphic data, and selected bond distances and angles can
be found in Tables 1 and S1 (Supporting Information),
respectively. Single-crystal X-ray diffraction analysis re-
vealed that the p-NBimM ligand in 1 and 2 adopts a
syn conformation and serves as a molecular clip to bind the
two fac-Re(CO)3 cores, which are paired by a dianionic
doubly chelating quinone-based ligand (dhnq2- for 1 and
dhaq2- for 2), using the adjacent phenolate and quinone
oxygens, resulting in an octahedrally coordinated C3NO2
environment around the rhenium centers. Dirhenium(I)
Results and Discussion
Synthesis and Characterization. The synthesis of dirhe-
nium(I) metallacycles [{Re(CO)3}2(μ-L)(μ-L0)] (1, L =
dhnq2-; 2, L = dhaq2-; H2dhnq = 6,11-dihydroxy-5,12-
naphthacenedione; H2dhaq = 1,4-dihydroxy-9,10-anthra-
quinone; L0 =1,4-bis(5,6-dimethylbenzimidazol-1-ylmethyl)-
naphthalene, p-NBimM) via the orthogonal bonding ap-
proach is shown in Scheme 1.5d,17 The synthetic approach
involved the simultaneous introduction of a bis-chelating,
quinone-based moiety to interface with two fac-Re(CO)3
cores and a ditopic semirigid N-donor ligand, p-NBimM,
to the remaining orthogonal site, leading to the formation
of chair-shaped metallacycles in high yields. The products
are air- and moisture-stable and are soluble in polar
organic solvents. Compared to the free ligands, 1H NMR
signals of the quinone moieties were shifted downfield
metallacycles 1 and 2 have Re Re distances of 8.608 and
3 3 3
8.622 A, respectively. Of note, the dihedral angle between
the two benzimidazole groups of the p-NBimM ligand in
1 is smaller than that in 2. As a result, 1 has a “closed”
chairlike structure, while 2 is a widely “opened up” metall-
acycle (Figure S3 and Table S3 in the Supporting In-
formation). In addition, the shortest nonbonding distance
between naphthalene-C of the p-NBimM ligand and the
dhnq/dhaq plane is 4.01 A for 1 and 4.60 A for 2 (Figure S3
in the Supporting Information).
A comparison of selected bond lengths and bond angles
with optimized geometries is presented in Table S2 of the
Supporting Information. The bond lengths and angles
obtained by the DFT calculations for the two complexes
in the ground state were very similar (difference in bond
length is less than 0.06 A) to the experimentally deter-
mined structure. In the series, the experimental Re-N
and Re-O bond lengths were 2.19 and 2.09 A for 1,
respectively. The results of a DFT study indicate that the
bond distances for Re-N and Re-O in compound 1 are
2.25 and 2.13 A, respectively. The DFT bond length for 2
is very similar to that of 1, as shown in Table S2.
Molecular Orbitals. On the basis of the DFT/B3LYP/
6-311G* theoretical framework, the ground-state electron-
ic structure was calculated to determine the energies and
additional single-point energy calculations for composi-
tions of the molecular orbitals (MOs) using the optimized
geometries of 1 and 2. The assignment of the type of each
MO was made on the basis of its composition and by
1
(atom numbering for H NMR assignments are in the
Supporting Information, Figure S1).18 Infrared carbonyl
stretching frequencies of these complexes exhibit tricarbon-
yl stretching patterns in the 1880-2050 cm-1 range.5,19
The FAB mass spectra of 1 and 2 showed molecular ion
peaks at m/z 1272.15 and 1224.14, respectively. The struc-
tures of complexes 1 and 2 were further confirmed by
single-crystal X-ray diffraction analysis.
(17) (a) Sathiyendiran, M.; Wu, J.-Y.; Velayudham, M.; Lee, G.-H.; Peng,
S.-M.; Lu, K.-L. Chem. Commun. 2009, 3795. (b) Liao, R.-T.; Yang, W.-C.;
Thanasekaran, P.; Tsai, C.-C.; Sathiyendiran, M.; Liu, Y.-H.; Rajendran, T.; Lin,
H.-M.; Tseng, T.-W.; Lu, K.-L. Chem. Commun. 2008, 3175. (c) Sathiyendiran,
M.; Liao, R.-T.; Thanasekaran, P.; Luo, T.-T.; Venkataramanan, N. S.; Lee, G.-H.;
Peng, S.-M.; Lu, K.-L. Inorg. Chem. 2006, 45, 10052.
(18) Su, C. Y.; Cai, Y. P.; Chen, C. L.; Smith, M. D.; Kaim, W.; zur Loye,
H. C. J. Am. Chem. Soc. 2003, 125, 8595.
ꢀ ꢁ
´
(19) Blanco-Rodrıguez, A. M.; Towrie, M.; Collin, J.-P.; Zalis, S.; Vlcek,
A., Jr. Dalton Trans. 2009, 3941.

10268 Inorganic Chemistry, Vol. 49, No. 22, 2010
Bhattacharya et al.
respectively, can be attributed to intense MLCTþLLCT
transitions [394 nm, HOMO-6 f LUMO (66%); 422 nm,
HOMO-5 f LUMO (66%)] for 1 and 2, (Figure S5 and
Table S4 in the Supporting Information). The peak posi-
tions are red-shifted in the theoretical spectra; nevertheless,
the trends observed for the relative transition energies be-
tween 1 and 2 are consistent. For example, the transition
energy for the first excited state of 1 is higher (2.26 eV) than
thatof 2 (2.11eV). A similar trend isobservedin the experi-
mental absorption band (605 nm in 1 and 640 nm in 2).
The free p-NBimM ligand exhibited fluorescence at
333 and 390(sh) nm with a lifetime of 5.8 ns, whereas the
quinones emitted in the 537-565 nm regions with a lifetime
of >2 ns (Figure S7 and Table S5 in the Supporting
Information) in deoxygenated THF. Upon excitation at
the MLCT position in a THF solution at 298 K under
nitrogen, complexes 1 and 2 displayed emissions at 467, 534,
570(sh), and 644 nm and 475, 540, and 554(sh) nm, respec-
tively (Figure S8 in the Supporting Information).5d,22 The
emissions observed at ∼470 and 535 nm are tentatively
Figure 1. (a) Slightly off front view and (b) side view of the molecular
structure of 1 (ORTEP plot with 30% thermal ellipsoids). Hydrogen
atoms are omitted for clarity.
1
assigned to the LLCT transition and intraligand π-π*
transition of quinone ligands, respectively, whereas the
weak emission of 1 observed at 644 nm can be attributed
to a Re f p-NBimM metal-to-ligand charge transfer transi-
tion (Table S5 in the Supporting Information).23 These
assignments were further confirmed by recording the emis-
sion spectra of 1 and 2 in THF at 77 K (Figure S9 in the
Supporting Information). Low temperature emission spec-
tra of 1 and 2 clearly show a well-defined Re f p-NBimM
metal-to-ligand charge transfer transition at 615 nm, which
can be attributed to a Re-N coordinated MLCT transi-
tion.24 To confirm the presence of multiple excited states,
the excitation spectra of 1 and 2 were recorded, and the
uncorrected excitation spectra are shown in Figure S8 in the
Supporting Information. The excitation spectra of 1showed
that the emission at 467 nm is due to a LLCT transition, and
the emission at 534 nm is assigned to the intraligand 1π-π*
transition of the quinone ligand. Similar behavior was
observed in the case of the excitation spectra of 2. The
emission decays had lifetimes (τ) of 12.1 and 14.3 ns for 1
and 2, respectively (Figure S10 in the Supporting In-
formation), which can be attributed to the fluorescence
lifetime of an admixture of singlet π-π* of p-NBimM and
a quinone moiety.
Electrochemistry. The disodium salts of quinones and
metallacycles 1 and 2 were investigated by cyclic voltam-
metry (DMSO, 298 K, [Bu4N][ClO4] (0.1 M) as the
supporting electrolyte, vs SCE.
The cyclic voltammogram (CV) of the dianionic qui-
nones reveals up to two possible electron additions (Table 3
and Figure S11 in the Supporting Information), which are
assigned to the [L]2- f [L]3•- and [L]3•- f [L]4- reduc-
tions (Scheme 2).1f,7g The clip benzimidazolate ligand
contributes no redoxpropertieswithinthe solventwindow,
as evidenced by a CV investigation. Because the complexes
Figure 2. (a) Front view and (b) side view of the molecular structure of
2 (ORTEP plot with 50% thermal ellipsoids). Hydrogen atoms are
omitted for clarity.
visual inspection of its three-dimensional representation.
Selected frontier molecular orbitals and their relative
energies were computed and are shown in Figure 3. Com-
plexes 1 and 2 have HOMO-LUMO gaps of 2.54 and
2.36 eV, respectively. Although the trend for the calculated
LUMO energies of 1 and 2 is inconsistent with the ob-
served reduction potential, the first singlet-singlet transi-
tion of 2 is found to be more red-shifted than that of 1,
which is in line with the trend observed in the electronic
absorption spectra (vide infra). Quinones are noninnocent
and thus provide high-lying, filled π orbitals.20 Thus, the
HOMO of 1 contained 76% dhnq2- ligand. The HOMO
of 2 is quite similar to that of 1, with 73% of its population
as the dhaq ligand. The LUMO of complexes 1 and 2
contained g90% π* orbitals of the quinone. The calcu-
lated molecular orbital isosurface plots in the SOMOs of
radical anions, 1•- and 2•-, essentially LUMOs of 1 and 2,
were spread over the π* orbital of the quinone ring (Figure
S4, in the Supporting Information).
Photophysical Properties. The UV-vis absorption spec-
tra were recorded in DMSO solutions at 298 K (Figure 4,
and Table 2). TDDFT calculations with a solvation model
(Table S4 in the Supporting Information) indicated that
the main transitions of complexes 1 and 2 in the 550-
640 nm range are probably excitation of the quinonoid
ligand with some contributions from metal-ligand charge
transfer (MLCT) transitions.21 The experimentally deter-
mined absorption bands at 387 nm in 1 and 413 nm in 2,
(22) (a) DeArmond, M. K.; Carlin, C. M. Coord. Chem. Rev. 1981, 36,
325. (b) Song, L.-Q.; Feng, J.; Wang, X.-S.; Yu, J.-H.; Hou, Y.-J.; Xie, P.-H.;
Zhang, B.-W.; Xiang, J.-F.; Ai, X.-C.; Zhang, J.-P. Inorg. Chem. 2003, 42, 3393.
(c) Tyson, D. S.; Luman, C. R.; Zhou, X.; Castellano, F. N. Inorg. Chem. 2001,
40, 4063.
(23) Glazer, E. C.; Magde, D.; Tor, Y. J. Am. Chem. Soc. 2005, 127, 4190.
(24) Thanasekaran, P.; Liao, R. T.; Liu, Y. H.; Rajendran, T.; Rajagopal,
S.; Lu, K. L. Coord. Chem. Rev. 2005, 249, 1085.
(20) Pierpont, C. G.; Francesconi, L. C.; Hendrickson, D. N. Inorg.
Chem. 1978, 17, 3470.
(21) (a) Lynch, M. W.; Hendrickson, D. N.; Fitzgerald, B. J.; Pierpont,
C. G. J. Am. Chem. Soc. 1984, 106, 2041. (b) Buchanan, R. M.; Pierpont, C. G.
J. Am. Chem. Soc. 1980, 102, 4951.

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10269
Figure 3. Frontier molecular orbitals for the most relevant terms of compounds 1 (left) and 2 (right, isodensity value = 0.02) calculated using B3LYP
functionals andLANL2DZECP, with the IEF-PCM solventmodel. MOenergiesare in electrovolts. AOMIXbreakdownsofrelevant MOsintoconstituent
components are in parentheses.
then gains a second electron to likely become a dianion at a
half-wave potential of -1.22 V (Figure 5a). The half-wave
potentials for the two consecutive reductions of 2 were
-0.88 and -1.36 V vs SCE, respectively (Figure 5b). The
current ratio is good, and the plot of currents vs ν1/2 is used
toidentify that the reaction is diffusion controlled. The first
and second quinone reductions for the 1 and 2 assemblies
were cathodically shifted compared to the redox potentials
of the free dianionic quinone analogs.25 The differences
are inconsistent with the expectation of electrostatic sta-
bilization of the negatively charged, reduced forms of the
ligands by the coordinated rhenium cations.26 This shift-
ing is more evident in 1 compared to that of 2. Moreover,
the Re(I/0) reduction was not observed within the solvent
window (þ0.7 to -2.8 V). The large differences in potentials
Figure 4. Electronic absorption spectra of complexes 1 (black) and 2
(blue) in DMSO solution (10-4 M) at 298 K.
between the two successive reduction processes point
contain one quinone moiety, two successive one-electron
ligand-based reduction waves would be expected. Figure 5
shows CV data for 1 and 2. As the potential was scanned
from zero to the negative region, 1 accepts an electron to
become a radical anion at a half-wave potential of -0.70 V
(25) Peover, M. E. J. Chem. Soc. 1962, 4540.
(26) (a) Sun, S.-S.; Lees, A. J. J. Am. Chem. Soc. 2000, 122, 8956. (b)
Gosztola, D.; Niemczyk, M. P.; Svec, W.; Lukas, A. S.; Wasielewski, M. R.
J. Phys. Chem. A 2000, 104, 6545. (c) O'Reilly, J. E.; Elving, P. J. J. Am. Chem.
€
Soc. 1972, 94, 7941. (d) Knodler, A.; Wanner, M.; Fiedler, J.; Kaim, W. J. Chem.
Soc., Dalton Trans. 2002, 3079.

10270 Inorganic Chemistry, Vol. 49, No. 22, 2010
Bhattacharya et al.
Table 2. Absorption Maximaa from UV-vis-NIR Data in Various Oxidation States From OTTLE Spectroelectrochemistryb
n
compound
Na2dhnq
0
-1
-2
267 (1513), 383 (1188), 410 (922),
456 (211), 485 (338), 517 (303), 563 (60.7)
278 (909), 331 (sh) (272), 471 (715.8)
287 (1917), 383 (1188), 535 (715), 729 (72.3),
944 (49.1), 1079 (66.7), 1255 (72.3)
308 (525), 389 (586.5), 496 (2170), 681 (181.7),
764 (227), 854 (184)
295 (1672), 377 (533), 488 (sh) (416),
526 (724), 716 (83.6), 798 (77.5)
336 (sh) (322.3), 477 (2071.4),
563 (346.6)
Na2dhaq
1
275 (7456), 387 (1458.6), 518 (sh) (395.8),
559 (763.7), 605 (1011.4)
292 (7395.6), 392 (898.7), 436 (sh) (787), 524 (sh) 305 (6918.7), 529 (1765.8), 768 (620),
(638.6), 582 (1905.8), 804 (247), 1012 (191.5),
1170 (433), 1397 (601.5)
853 (620)
2
275 (4519.4), 413 (1317.6), 587 (479.1),
640 (621.8)
274 (6418.2), 393 (sh) (835.3), 537 (1263.5),
866 (461.4)
280 (6244.4), 472 (1248.3), 609 (589)
a Absorption maxima λmax in nm, molar absorption coefficient ε ꢀ 10-3 (in parentheses) in M-1 cm-1
.
b From measurement in 0.1 M Bu4NClO4/
DMSO.
Table 3. Electrochemical Data from Cyclic Voltammetrya,b
Na2dhnq Na2dhaq p-NBimM
Scheme 2. Redox Chart (1, L = dhnq; 2, L = dhaq; L0 = p-NBimM)
1
2
E1/2 (red1) -0.55 (70) -0.71 (30)
E1/2 (red2) -1.11 (60) -1.18 (50)
-
-
-0.70 (70) -0.88 (60)
-1.22 (45) -1.36 (60)
a Potential in V vs SCE are listed, with peak potential differences in
parentheses, scan rate 100 mV s-1 b In 0.1 mM Bu4NClO4/DMSO.
.
to a large thermodynamic stability for the intermediate
radical anions, 1•- and 2•-. It is much easier to reduce
the dhnq2- moiety in 1 than reduce the dhaq2- moiety in 2
(Figure 5).
UV-vis-NIR Spectro-Electrochemistry. The redox re-
actions of 1 and 2 were investigated via UV-vis-NIR
SEC measurements using an optically transparent thin-
layer electrochemical (OTTLE) technique8 to establish
the likely sites of electron transfer: the accessible reduc-
tions for the states with n = 0, -1, and -2 (Figures 6 and
7 and Table 2). With knowledge of the redox potentials of
1, 2, Na2dhnq, and Na2dhaq, it is now possible to gen-
erate certain reduced states of these compounds electro-
chemically and to identify their characteristic signatures
in the corresponding UV-vis-NIR spectra. Figures 6
and 7 illustrate the development of the absorption bands
as a function of time of 1 and 2, respectively.
Among the two singly reduced radical complexes, the
symmetrically bridged quinone complex 1 shows a strong
absorption in the NIR region, which is absent in the
neutral and doubly reduced states. The behavior of 1 in
the singly and doubly reduced states is analogous to that
of Na2dhnq quinone.
The electronic spectrum of 1 changed substantially upon
reduction to 1•-, particularly in the lower-energy region. A
reduction from the 1 f 1•- state (Figure 6a) caused the
MLCT absorption band at λmax = 387 nm (black curve) to
progressively red-shift with a constant drop in intensity,
and the ILCT band (λmax = 559 and 605 nm) merged with
a new, higher intensity band at λmax = 582 nm (Figure 6a,
blue curve). This is probably a consequence of the sequen-
tial addition of electrons to the empty π* orbitals of the
ligand. After reduction, the energy of these orbitals is
increased, and the MLCT transition is shifted to a higher
energy.5d,6c This is accompanied by the appearance of
several new absorption bands at 804, 1012, 1170, and
1397 nm, which are characteristics of the radical anion
form of Na2dhnq (Figure S13, in the Supporting Infor-
mation). The π* MO of the quinone ring is considerably
stabilized through the 4-fold coordination of the two
σ-electrophilic fac-Re(CO)3 moieties, and thus all of the
peaks are red-shifted in 1•- compared to [dhnq]3•- 5d,6c
.
Interestingly, 1•- exhibited an intense NIR band at 1397
nm (ε = 601.5 M-1 cm-1), which were absent in both the
neutral and dianionic forms. The intense lowest-energy
absorption was an internal π (SOMO) f π* ILCT transi-
tion of the quinone radical anion. The presence of two
isosbestic points at λ = 433 and 630 nm indicates a clean
conversion from 1 f 1•- without the formation of inter-
mediates or byproducts.
Upon successive reduction, i.e., 1 f 1•- f 12-, the dπ
Re(I) f π* ligand MLCT transition completely disap-
peared, and an ILCT band in the visible region was blue-
shifted to 529 nm from 582 nm (Figure 6b). Two new
peaks are generated at 767 and 853 nm in 12- and are
probably due to the ILCT of the quinone, which is also
observed in [dhnq]4- (Figure S13, right in the Supporting
Information). The large blue-shift from the NIR to the
visible range upon double reduction is due to the sub-
sequent addition of an electron and an increase in the
π* MO energy of the quinone ring.5d Furthermore, the
electrochemical measurements indicate that conversion
between the 1 f 1•- and the 1•- f 12- states is nearly
94% and 99% reversible, respectively. The number of
electrons transferred during this reaction was further
established using absorbance-applied potential titrations
(Figure S12 in the Supporting Information).
In contrast, the one electron reduction of 2 to the 2•-
state did not dramatically alter the electronic spectrum
(Figure 7). The ILCT transitions are blue-shifted from
587 and 640 to 537 nm and increase in intensity. Similar to
1 f 1•-, the MLCT peak is progressively red-shifted with
a constant drop in intensity. One broad signal at 866 nm
upon the reduction of 2 f 2•- occurred. This behavior

Article
Inorganic Chemistry, Vol. 49, No. 22, 2010 10271
Figure 5. Cyclic voltammograms (black, solid line) and differential pulse voltammograms (blue, broken line) (showing reduction potential only) of1 (left)
and 2 (right) in DMSO (1 mM) in 0.1 M [Bu4N][ClO4] with a scan rate of 100 mV/s vs Ag/AgCl.
Figure 6. UV-vis-NIR spectra of(a) 1 (black, left) and 1•- (blue, left) and (b) 1•- (blue, right) and12- (olive, right) under increasingcathodic potential in
0.1 M TBAP in DMSO at 25 °C. Concentration: [1] = 2.77 ꢀ 10-4 M.
Figure 7. UV-vis-NIR spectra of (a) 2 (black, left) and 2•- (blue, left) and (b) 2•- (blue, right) and 22- (olive, right) under increasing cathodic potential
in 0.1 M TBAP in DMSO at 25 °C. Concentration: [2] = 2.37 ꢀ 10-4 M.
is characteristic of the [dhaq]3•- (Figure S14, in the
Supporting Information). A further one electron reduc-
tion of 2•- caused an ILCT band in the visible region to be
further blue-shifted to 472 nm from 537 nm and the ILCT
band of the quinone was shifted to 609 nm from 866 nm,
as was also observed in the successive reduction of 1.
Electrochemical measurements indicate that the conver-
sion between the 2 f 2•- and the 2•- f 22- states is nearly
97% and 95% reversible, respectively.
The difference in spectral behavior in the singly reduced
states of quinones and also in the metallacycles is prob-
ably due to the annealed benzene ring in 1 and dhnq2-
because of its symmetric π-electron system, which is
perturbed to a lesser degree compared to the asymmetric
quinonoid-bridged 2 and the free quinone, dhaq2-. Thus,
combinations arising from this special situation are sum-
marized in Scheme 2 (L = dhnq2- in 1 and dhaq2- in 2).
Furthermore, the spin distribution in terms of the valence-
bond description, which represents the contribution of all
the possible resonance structures of the radical anion of
quinone, is shown in Scheme 3.
Electron Spin Resonance. The identities of singly re-
duced species, generated via the use of cobaltocene (Cp2Co)
as a reducing agent in DMSO at room temperature, were
further characterized in the form of glassy frozen solutions
by X-band ESR spectroscopy. The rhombic ESR signal

10272 Inorganic Chemistry, Vol. 49, No. 22, 2010
Bhattacharya et al.
Figure 8. EPR spectra [X-band, DMSO, at 77 K] (concentration: 10-2 M; microwave frequency 9.6127, microwave power 40 mW, modulation amplitude
7.75 G, modulation frequency 100 kHz) of 1•- (g1 = 2.012, g2 = 2.006, g3 = 1.9875, left) and 2•- (g = 2.0008, right).
Scheme 3. Spin Distribution in Terms of Valence-Bond Description Representing the Contribution of All of the Resonance Structures
in 1•- was observed (g1 = 2.012, g2 = 2.006, g3 = 1.9875,
gavg=2.0018, Δ=g1 - g3=0.024 at 77 K; Figure 8, left).27
In contrast to 1•-, a free radical signature in the EPR
spectrum was observed for 2•-; i.e., the g value of 2•-
(giso= 2.0008) was consistent with the absence of any con-
tribution by metal orbitals (the same as the free-electron
value) to the SOMO of 2•- (Figure 8, right).28 The electron
spin density showed a typical pattern for a semiquinone-
like structure on each ring (Figure S15 in the Supporting
Information).29 The spin density analysis is consistent
with the observed Δg value, in that the SOMO for 1•- and
of a metal-based spin in the singly reduced states further
rules out the possibility of any redox redistribution.
Summary and Conclusions
In summary, two new chair-shaped dirhenium(I) metalla-
cycles, based on quinonoid ligands, were designed and pre-
pared in high yields. Electrochemical assessments using CV
and UV-vis-NIR SEC of these new complexes revealed
that they were quinone ligand-centered; two well-separated,
single-electron, reversibly accessible 0, -1, and -2 redox
states, while the benzimidazolate ligand appears to be redox
innocent within the solvent window. The combined investi-
gations reveal that the singly reduced metallacycles are best
described as highly stable, noncommunication, localized,
quinonoid-centered radical complexes, [(CO)3ReI(μ-L3•-)-
(μ-L0)ReI(CO)3]•-, with less compatible metal-based spin
characteristics, via redox redistribution.
2
•- was distributed mainly on the quinonoid ligand with
a 5% contribution from the dπ-Re orbital in 1•- and
even less in 2•-.
Judging from the ESR spectra, the alternatives to the
redox redistributed form, [(CO)3Re0(μ-L•-)(μ-L0)Re0-
(CO)3]•-, are less compatible with the absence of Re-
quinone orbital mixing and thus superexchange inter-
actions in the singly reduced metallacycles. The absence
Acknowledgment. We thank Academia Sinica and the
National Science Council of Taiwan for financial support.
(27) (a) Berger, S.; Klein, A.; Kaim, W. Inorg. Chem. 1998, 37, 5664. (b)
Klein, A.; Vogler, C.; Kaim, W. Organometallics 1996, 15, 236.
(28) (a) Ho, T.-I.; Creber, K. A. M.; Wan, J. K. S. J. Am. Chem. Soc. 1981,
103, 6524. (b) Creber, K. A. M.; Wan, J. K. S. J. Am. Chem. Soc. 1981, 103,
2101.
(29) (a) Mattar, S. M. J. Phys. Chem. B 2004, 108, 9449. (b) Mattar, S. M.;
Stephens, A. D. Chem. Phys. Lett. 1999, 306, 249. (c) Morley, J. O. J. Phys.
Chem. 1995, 99, 5956.
Supporting Information Available: X-ray crystallographic
files (CIF) of 1 and 2, atom numbering for H NMR assign-
ments, photophysical data of 1 and 2, electrochemical data of
ligands, absorbance-applied potential titration plots, geometry
optimized Cartesian coordinates (Tables S6-S9). This material
is available free of charge via the Internet at http://pubs.acs.org.
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