C- and Z-shaped coordination compounds. Synthesis, structure, and spectroelectrochemistry of cis- and trans-[Re(CO)3(L)] 2-2,2′-bisbenzimidizolate with L = 4-phenylpyridine, 2,4′-bipyridine, or pyridine

Article abstract of DOI:10.1021/ic050894u

A series of C- and Z-shaped complexes of the form cis- and trans-[Re(CO)₃(L)]₂BiBzIm, where L = 4-phenylpyridine, 2,4′-bipyridine, or pyridine and BiBzIm = 2,2′-bisbenzimidizolate, have been synthesized by the reaction of [Re(CO)₄]₂BiBzIm with a slight excess of L in refluxing tetrahydrofuran. Five of the six compounds have been isolated and crystallographically and electrochemically characterized. Formation of the sixth, the cis form of the [Re(CO) ₃(4-phenylpyridine)]₂BiBzIm, is evidently inhibited by the torsional steric demands of proximal 4-phenylpyridines. The compounds are acyclic analogues of recently studied tetrarhenium molecular rectangles and are of interest, in part, because of their potential to form ligand-centered mixed-valence (LCMV) compounds upon reduction by one electron. Spectroelectrochemical measurements corroborated the formation of a LCMV version of cis-[Re(CO)₃(L)]₂BiBzIm but failed to uncover a ligand-based intervalence transition. Electrochemical measurements revealed isomer-dependent L/L electrostatic effects, resulting in greater mixed-valence ion comproportionation for C-shaped assemblies versus Z-shaped assemblies.

Full text of DOI:10.1021/ic050894u

Inorg. Chem. 2005, 44, 8707−8714
C- and Z-Shaped Coordination Compounds. Synthesis, Structure, and
Spectroelectrochemistry of cis- and
trans-[Re(CO)3(L)]2-2,2 -bisbenzimidizolate with L
2,4 -Bipyridine, or Pyridine
′
) 4-Phenylpyridine,
′
Peter H. Dinolfo, Kurt D. Benkstein, Charlotte L. Stern, and Joseph T. Hupp*
Department of Chemistry, Northwestern UniVersity, 2145 Sheridan Road,
EVanston, Illinois 60208-3113
Received June 2, 2005
A series of C- and Z-shaped complexes of the form cis- and trans-[Re(CO)3(L)]2BiBzIm, where L
) 4-phenylpyridine,
2,4′-bipyridine, or pyridine and BiBzIm ) 2,2′-bisbenzimidizolate, have been synthesized by the reaction of
[Re(CO)4]2BiBzIm with a slight excess of L in refluxing tetrahydrofuran. Five of the six compounds have been
isolated and crystallographically and electrochemically characterized. Formation of the sixth, the cis form of the
[Re(CO)3(4-phenylpyridine)]2BiBzIm, is evidently inhibited by the torsional steric demands of proximal 4-phenylpyridines.
The compounds are acyclic analogues of recently studied tetrarhenium molecular rectangles and are of interest,
in part, because of their potential to form ligand-centered mixed-valence (LCMV) compounds upon reduction by
one electron. Spectroelectrochemical measurements corroborated the formation of a LCMV version of cis-
[Re(CO)3(L)]2BiBzIm but failed to uncover a ligand-based intervalence transition. Electrochemical measurements
revealed isomer-dependent L/L electrostatic effects, resulting in greater mixed-valence ion comproportionation for
C-shaped assemblies versus Z-shaped assemblies.
Introduction
active ligands, LL, in reasonably well-defined, largely
cofacial geometries. As shown in Scheme 1, when one LL
ligand is reduced, the rectangles comprise ligand-centered
mixed-valence (LCMV) compounds.1-3 We find that elec-
tronic coupling in these systems is dominated by direct
donor-orbital/acceptor-orbital overlap, rather than metal-
mediated superexchange. In other words, the role of the
metals is limited mainly to a structural one with the
assemblies behaving electronically as two-state systems.
Depending on the identity of LL and the precise positioning
of LL pairs with respect to each other, we find that electronic
coupling can range from undetectably weak to sufficiently
strong so that full valence delocalization occurs. In most
cases but not all, one or more intervalence transfer (IT) bands
can be detected in the electronic absorption spectrum. As
with other mixed-valence compounds, if the LCMV as-
sembly’s odd electron is localized, the IT band intensity and
energy can report directly on the electronic coupling energy
and the reorganization energy for the corresponding thermal
intramolecular electron exchange reaction.11,12
We have recently reported on the use of rhenium-based
molecular rectangles1-10 as organizational motifs for the
investigation of intramolecular electron transfer. Examined
were tetrarhenium assemblies that configure pairs of redox-
* To whom correspondence should be addressed. E-mail: jthupp@
chem.northwestern.edu.
(1) Dinolfo, P. H.; Williams, M. E.; Stern, C. L.; Hupp, J. T. J. Am. Chem.
Soc. 2004, 126, 12989-13001.
(2) Dinolfo, P. H.; Hupp, J. T. J. Am. Chem. Soc. 2004, 126, 16814-
16819.
(3) Dinolfo, P. H.; Lee, S. J.; Coropceanu, V.; Bre´das, J.-L.; Hupp, J. T.
Inorg. Chem. 2005, 44, 5789-5797.
(4) Hartmann, H.; Berger, S.; Winter, R.; Fiedler, J.; Kaim, W. Inorg.
Chem. 2000, 39, 4977-4980.
(5) Woessner, S. M.; Helms, J. B.; Shen, Y.; Sullivan, B. P. Inorg. Chem.
1998, 37, 5406-5407.
(6) Benkstein, K. D.; Hupp, J. T.; Stern, C. L. Inorg. Chem. 1998, 37,
5404-5405.
(7) Benkstein, K. D.; Hupp, J. T.; Stern, C. L. J. Am. Chem. Soc. 1998,
120, 12982-12983.
(8) Thanasekaran, P.; Liao, R. T.; Liu, Y. H.; Rajendran, T.; Rajagopal,
S.; Lu, K. L. Coord. Chem. ReV. 2005, 249, 1085-1110.
(9) Benkstein, K. D.; Stern, C. L.; Splan, K. E.; Johnson, R. C.; Walters,
K. A.; Vanhelmont, F. W. M.; Hupp, J. T. Eur. J. Inorg. Chem. 2002,
2002, 2818-2822.
(10) Benkstein, K. D.; Hupp, J. T.; Stern, C. L. Angew. Chem., Int. Ed.
2000, 39, 2891-2893.
(11) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391-444.
(12) Hush, N. S. Coord. Chem. ReV. 1985, 64, 135-157.
10.1021/ic050894u CCC: $30.25
Published on Web 10/27/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 24, 2005 8707

Dinolfo et al.
Scheme 1
Chart 1
isomer-dependent electrostatic interactions. Unfortunately,
however, the reduced assemblies proved insufficiently stable
for L/L IT bands to be detected in the visible or near-infrared
(NIR) spectrum.
Experimental Section
General Methods. Infrared spectra were recorded on a Bio-Rad
FTS-40 Fourier transform infrared spectrometer. All NMR spectra
were recorded on either a Varian Mercury 400-MHz spectrometer
or a Varian INOVA 500-MHz spectrometer, and the chemical shifts
were referenced to the standard solvent shift. Low resolution mass
spectrometry-fast atom bombardment (LRMS-FAB) and high
resolution mass spectrometry-FAB (HRMS-FAB) were obtained
in the Mass Spectrometry Laboratory of the School of Chemical
Sciences, University of Illinois. UV-visible absorption spectra were
recorded with a Varian Cary 5000 spectrophotometer.
Materials. All commercial reagents were of ACS grade and used
without further purification. Tetrahydrofuran (THF) and methylene
chloride were purified using a two-column solid-state purification
system (Glasscontour System, Joerg Meyer, Irvine, CA). 4-PhPy
and 2,4′-bpy were purchased from Aldrich, and pyridine was
obtained from Fisher Scientific. Whatman preparatory TLC plates
(150-Å silica, 1000-µm layer thickness) were used for the separation
of cis and trans isomers. 4 was synthesized by previous literature
preparations.10
trans-[Re(CO)3(4-PhPy)]2BiBzIm (1trans). 4 (65 mg, 0.078
mmol) and 4-PhPy (28 mg, 0.180 mmol) were combined with 80
mL of THF under N2. The mixture was heated to reflux under N2
for 24 h. The removal of ∼80% of the solvent and the addition of
hexanes gave 74 mg of 1trans, a light yellow powder after filtration
(87% yield). IR confirmed the trans form as the only isomer
isolated. 1H NMR (400 MHz, acetone-d6): 8.42 (d, 4H), 7.88
(q, 4H), 7.66 (q, 4H), 7.62 (d, 4H), and 7.50 (m, 10H). IR (THF,
cm-1): 2020 (CtO), 1915 (CtO), and 1904 (CtO). LRMS-
FAB (m/z): [M]+ calcd for Re2C42H26O6N6, 1082.1; found, 1082.4.
HRMS-FAB (m/z): [M]+ calcd for Re2C42H26O6N6, 1084.1029;
found, 1084.1026.
cis- and trans-[Re(CO)3(2,4′-bpy)]2BiBzIm (2cis and 2trans).
4 (75 mg, 0.09 mmol) and 2,4′-bpy (30 mg, 0.19 mmol) were
combined with 80 mL of THF under N2. The mixture was heated
to reflux under N2 for 24 h. The removal of ∼80% of the solvent
and the addition of hexanes gave 93 mg of 2cis and 2trans, a light
yellow powder after filtration (94% yield, combined). Preparatory
TLC separation using a 100:1 CH2Cl2/MeOH mixture gave 2trans
and 2cis as yellow crystalline powders after recrystallization. 2trans.
Yield ) 21 mg. 1H NMR (400 MHz, CD2Cl2): 8.63 (d, 2H), 8.36
With these observations in mind, we reasoned that
analogous acyclic C- and Z-shaped compounds might also
display LCMV behavior. Examples are the dirhenium 2,2′-
bisbenzimidazolate (BiBzIm) compounds, 1cis and 1trans,
shown in Chart 1. We further reasoned that, given the
comparative unimportance of superexchange coupling in the
LCMV rectangles, large differences in intramolecular elec-
tronic coupling might be encountered for C-shaped versus
Z-shaped assemblies. Although not investigated here, we also
reasoned that open-ended C-shaped assemblies, in contrast
to BiBzIm-based rectangles, might be sufficiently pliable to
accommodate planar guest molecules, such as LL/LL inter-
calants, potentially permitting noncovalent “through matter”
electronic coupling behavior to be investigated.13-18
Outlined here are the synthesis, X-ray structural charac-
terization, electrochemical behavior, and electronic spectral
properties of an almost complete series of cis- and trans-
[Re(CO)3(L)]2BiBzIm compounds, where L ) 4-phenyl-
pyridine (4-PhPy), 2,4′-bipyridine (2,4′-bpy), or pyridine. In
two of the three cases, the syntheses yielded isolatable
quantities of both isomers, with separation obtained by thin-
layer chromatography (TLC). In some cases, transiently
reversible stepwise reduction of L/L pairs within a given
assembly was observed, implying the formation of LCMV
species. The measurements provide evidence for the expected
(13) Troisi, A.; Ratner, M. A.; Zimmt, M. B. J. Am. Chem. Soc. 2004,
126, 2215-2224.
(14) Zimmt, M. B.; Waldeck, D. H. J. Phys. Chem. A 2003, 107, 3580-
3597.
(15) Kaplan, R.; Napper, A. M.; Waldeck, D. H.; Zimmt, M. B. J. Phys.
Chem. A 2002, 106, 1917-1925.
(16) Kaplan, R. W.; Napper, A. M.; Waldeck, D. H.; Zimmt, M. B. J. Am.
Chem. Soc. 2000, 122, 12039-12040.
(17) Napper, A. M.; Read, I.; Kaplan, R.; Zimmt, M. B.; Waldeck, D. H.
J. Phys. Chem. A 2002, 106, 5288-5296.
(18) Kumar, K.; Lin, Z.; Waldeck, D. H.; Zimmt, M. B. J. Am. Chem.
Soc. 1996, 118, 243-244.
8708 Inorganic Chemistry, Vol. 44, No. 24, 2005

C- and Z-Shaped Coordination Compounds
Table 1. Crystallographic Data for 1trans, 2trans, 2cis, 3trans, 3cis, and 4
1trans + 1.5 × THF
compound
+ hexane
2trans and 2cis
3trans
3cis
4
empirical formula
formula weight
crystal color, habit
crystal dimensions
(mm)
Re3C75H65N9O10.5
1818.96
yellow, plate
0.93 × 0.54 × 0.50
Re3C60H36N12O9
1627.61
colorless, prismatic
0.08 × 0.09 × 0.22
Re2C30H18N6O6
930.92
yellow, plate
0.30 × 0.28 × 0.10
Re2C30H18N6O6
930.92
colorless, polyhedron
0.05 × 0.08 × 0.08
Re2C22H8N4O8
828.74
colorless, columnar
0.52 × 0.23 × 0.18
crystal system
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/n (No. 14)
12.244(I)
27.632(1)
22.478(1)
95.050(1)
7575.64(4)
4
monoclinic
P21/n (No. 14)
10.986(2)
19.522(3)
25.768(4)
97.593(2)
5477.8(15)
4
monoclinic
P21/n (No. 14)
9.079(2)
16.653(3)
9.459(2)
91.161(3)
1429.8(5)
2
tetragonal
I4h2d (No. 122)
14.697(18)
tetragonal
I41/a (No. 88)
21.362(2)
26.136(5)
9.784(2)
â (deg)
V (Å3)
5645.3(12)
8
4464.5(13)
8
Z
T (K)
153
1.538
4.843
0.961
0.042
0.096
153
1.974
6.684
1.064
0.033
0.071
153
2.162
8.514
1.041
0.022
0.052
153
153
1.23
10.894
1.198
0.021
0.046
F (calcd, g/cm3)
2.191
8.626
1.063
0.029
0.057
µ (mm-1
)
goodness of fit on F2
R (F)a
R
w (F2)b
2
2
2
a R(F) ) ∑||Fo| - |Fc||/∑|Fo|. b Rw(F2) ) [∑w(Fo - Fc )2]/[∑w(Fo )2]1/2
.
(d, 4H), 7.87 (q, 4H), 7.75 (t, 2H), 7.72 (d, 4H), 7.65 (d, 2H), 7.43
(q, 4H), and 7.32 (d, 2H). IR (THF, cm-1): 2020 (CtO), 1916
(CtO), and 1904 (CtO). LRMS-FAB (m/z): [M]+ calcd for
Re2C40H24O6N8, 1086.1; found, 1086.4. HRMS-FAB (m/z): [M]+
calcd for Re2C40H24O6N8, 1086.0934; found, 1086.0933. 2cis.
Yield ) 19 mg. 1H NMR (400 MHz, CD2Cl2): 8.30 (d, 2H), 8.07
(d, 4H), 7.89 (q, 4H), 7.45 (q, 4H), 7.39 (t, 2H), 7.33 (d, 4H), 7.16
(d, 2H), and 7.07 (t, 2H). IR (THF, cm-1): 2027 (CtO), 2019
(CtO), 1918 (CtO), and 1903 (CtO). LRMS-FAB (m/z): [M]+
calcd for Re2C40H24O6N8, 1086.1; found, 1086.5. HRMS-FAB
(m/z): [M]+ calcd for Re2C40H24O6N8, 1086.0934; found, 1086.0933.
cis- and trans-[Re(CO)3(pyridine)]2BiBzIm (3cis and 3trans).
4 (150 mg, 0.181 mmol) and pyridine (0.030 mL, 30 mg, 0.380
mmol) were combined with 175 mL of THF under N2. The mixture
was heated to reflux under N2 for 24 h. The removal of ∼80% of
the solvent and the addition of hexanes gave 154 mg of 3cis and
3trans, a light yellow powder after filtration (91% yield, combined).
Preparatory TLC separation using a 50:50 THF/hexanes mixture
gave 3trans and 3cis as yellow crystalline powders after recrys-
Figure 1. Carbonyl stretching region of the IR spectra for 4 (dashed-
dotted black line), a reaction mixture of 2 (solid green line), 2trans (long
dashed blue line), and 2cis (short dashed red line) in THF.
1
tallization. 3trans. Yield ) 66 mg. H NMR (400 MHz, CDCl3):
8.00 (d, 4H), 7.83 (q, 4H), 7.41 (q, 4H), 7.34 (t, 2H), and 6.72 (t,
4H). IR (CH2Cl2, cm-1): 2021 (CtO), 1914 (CtO), and 1902
(CtO). LRMS-FAB (m/z): [M]+ calcd for Re2C30H18O6N6, 930.9;
found, 931.1. HRMS-FAB (m/z): [M]+ calcd for Re2C30H18O6N6,
The structures were solved by direct methods, and refinement
was done using the SHELXT crystallographic software package.20
The refinements of all structures were relatively straightforward.
For all structures, non-hydrogen atoms were refined anisotropically
and hydrogen atoms were added in idealized positions. The structure
for compound 1trans contained two disordered THF molecules and
one disordered hexane molecule of which only one of the THF
solvent molecules was refined anisotropically; the remaining solvent
molecules were refined isotropically. Crystal descriptions and sizes
along with structure refinement data are shown in Table 1. X-ray
crystallographic files (CIF) are included in Supporting Information.
Oak Ridge thermal-ellipsoid plot (ORTEP) drawings of 1trans,
2trans, 2cis, 3trans, 3cis, and 4 are shown in Figure 1. Table 2
contains Re-N bond distances.
1
932.0403; found, 932.0404. 3cis. Yield ) 45 mg. H NMR (400
MHz, CDCl3): 8.26 (d, 4H), 7.82 (q, 4H), 7.59 (t, 2H), 7.39 (q,
4H), and 7.06 (t, 4H). IR (CH2Cl2, cm-1): 2028 (CtO), 2021
(CtO), 1917 (CtO), and 1901 (CtO). LRMS-FAB (m/z): [M]+
calcd for Re2C30H18O6N6, 930.9; found, 931.0. HRMS-FAB
(m/z): [M]+ calcd for Re2C30H18O6N6, 932.0403; found, 932.0404.
Crystal Structure Determinations. Crystals of 1trans, 2trans,
2cis, 3trans, and 3cis were grown from the slow diffusion of hexane
into saturated THF solutions. For 2trans and 2cis, the unseparated
reaction product was used, resulting in isomer cocrystallization.
Crystals of 4 were obtained from the slow evaporation of a saturated
CH2Cl2 solution. The crystals were mounted on glass fibers using
oil and immediately placed in a cold nitrogen stream at 153 K.
Data collection was done using a Bruker Smart 1000 charge-coupled
device diffractometer with Mo KR radiation (λ ) 0.710 69 Å). Data
were processed using the SMART and SAINT programs.19 An
analytical integration absorption correction was applied to all data,
and the data for 3trans were corrected for secondary extinction.
(19) SMART, version 5.054; data collection; Bruker Analytical X-ray
Instruments, Inc.: Madison, WI, 2000. SAINT-Plus, version 6.02A;
data processing software for the SMART System; Bruker Analytical
X-ray Instruments, Inc.: Madison, WI, 2000.
(20) Sheldrick, G. M. SHELXTL, DOS/Windows/NT version 5.10; Bruker
Analytical X-ray Instruments, Inc.: Madison, WI, 1997.
Inorganic Chemistry, Vol. 44, No. 24, 2005 8709

Dinolfo et al.
Scheme 2
Electrochemical Measurements. All cyclic voltammetric and
by a carbonyl, results in the weakening of the opposing Ms
CO bond.21-24
differential pulse voltammetric experiments were performed and
analyzed using a CHI900 (CH Instruments, Austin, TX) potentiostat.
Electrolyte solutions [0.1 M tetrabutylammonium hexafluorophos-
phate (TBAPF6; >99%, Fluka)] were prepared with anhydrous THF
and were nitrogen degassed prior to use. A Pt wire was used as the
counter electrode, and 2-mm-diameter Pt or Au macro disk
electrodes were used as the working electrodes. A silver wire was
used as a pseudo-reference electrode with ferrocene (Aldrich,
purified by sublimation) added as an internal reference at the end
of each experiment. All experiments were run under a nitrogen
atmosphere. UV/visible/NIR spectroelectrochemical (SEC) experi-
ments were carried out as previously described.1
In THF as the solvent, both forms of 2 and 3 were obtained
with the ratio of cis to trans varying between 1:1 and 2:3,
depending on the concentrations used.25 Under the mild
reaction conditions used here, only the trans isomer of 1 was
isolated. Presumably, the steric demands of the partially
twisted 4-PhPy ligand disfavor the formation of the cis
complex enough to force the reaction primarily toward trans
ligation. Although 1cis could not be isolated, evidence for
its formation as a minority product (ca. 5% relative to 1trans)
was found in the IR spectra of concentrated reaction solutions
(not shown). The usefulness of the carbonyl spectrum as a
diagnostic is illustrated in Figure 1. Shown in Figure 1 are
spectra for 4, a reaction mixture of 2, and isolated 2trans
and 2cis in THF. The trans complex (C2h symmetry) shows
three CtO stretches, while the cis complex (C2V) displays
four.
Crystal Structures. Table 1 contains crystal information
and refinement data for 1trans, 2trans/2cis, 3trans, 3cis,
and 4. Table 2 lists selected ResN bond lengths from the
five structures. ORTEP drawings for the X-ray crystal
structures of 1trans, 2trans, 2cis, 3trans, 3cis, and 4 are
shown in Figure 2. 4 crystallizes in the I41/a space group
with no appreciable distortions of the BiBzIm ligand. The
Re atoms exist in a slightly distorted octahedral coordina-
tion: NsResCO bonding angles range between 86.2 and
91.7°, while the NsResN angle is 77.82(9)°. The ResRe
distance is 5.72 Å.
1trans crystallizes in the P21/n space group with two
molecules comprising the asymmetric unit (only one of which
is shown in Figure 2). The structure also contains disordered
THF and hexane molecules. Notably, there is a slight
distortion of the molecule away from an orthogonal ideal
such that it adopts a “Z” geometry featuring ligand NsRes
Re angles of about 80°. Evident in the structure are the
above-mentioned pyridine/phenyl torsional displacements (ca.
30°) that are likely responsible for the preferential formation
of the trans isomer. ResRe distances are 5.74 and 5.72 Å.
As shown by the packing diagram in Figure 3, 2trans
and 2cis cocrystallize with the 2,4′-bpy ligands of 2trans
Table 2. Re-N Bond Distances for 1trans, 2trans, 2cis, 3trans, 3cis,
and 4
bond lengths (Å)
LL pyridyl N-Re
imidazolyl N-Re
1trans
Re1-N9
2.212(2)
2.195(2)
2.180(2)
2.220(4)
2.2099(4)
2.203(3)
2.223(2)
2.205(5)
Re1-N4
2.223(2)
2.220(2)
2.206(2)
2.212(2)
2.216(2)
2.206(2)
2.239(4)
2.2179(4)
2.2139(3)
2.2299(3)
2.203(3)
2.214(3)
2.210(2)
2.226(2)
2.196(5)
2.220(5)
2.208(3)
2.209(3)
2.22(1)
Re1-N5
Re2-N2
Re2-N3
Re3-N6
Re3-N7
Re3-N7
Re3-N8
Re1-N1
Re1-N3
Re2-N2
Re2-N4
Re1-N1
Re1-N2
Re1-N1
Re1-N2
Re1-N1
Re1-N2
Re2-N1
Re3-N8
Re3-N9
Re1-N5
Re2-N6
Re1-N3
Re1-N3
2trans
2cis
3trans
3cis
4
average
2.21(1)
Results and Discussion
Synthesis and Characterization. The general preparative
procedure for 1-3 is shown in Scheme 2. A slight excess
of a pyridine-based ligand is combined with 4 in refluxing
THF under N2 for approximately 24 h. The reactions are
monitored by IR for the disappearance of the carbonyl
stretches of 4 at 2111, 2017, 1988, and 1946 cm-1 and the
appearance of carbonyl stretches for the products. Note that
the as-prepared assemblies are neutral, with the dianionic
BiBzIm ligand formally charge balancing the pair of Re(I)
centers. The reactivity of 4 is a good example of the classic
trans labilization of group VII complexes by carbonyl
ligandssthe effect, arising from competitive π back bonding
(21) Abel, E. W.; Wilkinson, G. J. Chem. Soc. 1959, 1501-1505.
(22) Wojcicki, A.; Basolo, F. J. Am. Chem. Soc. 1961, 83, 525-528.
(23) Angelici, R. J.; Basolo, F. J. Am. Chem. Soc. 1962, 84, 2495-2499.
(24) Angelici, R. J.; Basolo, F. Inorg. Chem. 1963, 2, 728-731.
(25) In refluxing toluene, however, only 3trans is formed when 4 and
pyridine are combined.
8710 Inorganic Chemistry, Vol. 44, No. 24, 2005

C- and Z-Shaped Coordination Compounds
Figure 2. ORTEP renderings of 1trans, 2trans, 2cis, 3trans, 3cis, and 4 shown at the 50% probability level. Rhenium atoms are shown in green, nitrogen
in blue, oxygen in red, and carbon in black. Hydrogen atoms have been removed for clarity. Only one of the two isomers for 1trans is shown.
lying across the 2,4′-bpy ligands of 2cis in a manner that
facilitates good van der Waals contact. The compounds
crystallize in the P21/n space group, where the asymmetric
unit contains one half of a molecule of 2trans and a full
molecule of 2cis. The absence of a proton in one of the meta
positions on the second pyridyl ring allows the two rings of
2,4′-bpy to lie nearly planar, likely facilitating cis ligation.
The torsion angle between the pyridine rings averages 6°
for 2trans and 11° for 2cis. Similar to 1trans, the structure
of 2trans distorts toward a “Z” shape, with 2,4′-bpy
NsResRe angles of approximately 84°. Slightly larger
distortions are seen for 2cis, where the 2,4′-bpy ligands are
bowed toward each other, resulting in 2,4′-bpy NsResRe
angles of 78.2 and 80.4°. The ResRe distances in 2trans
and 3cis are 5.78 and 5.74 Å, respectively.
3trans crystallizes in the P21/n space group with distor-
tions similar to those of 1trans and 2trans (the pyridine Ns
ResRe angle is 83.5°). The ResRe distance is 5.73 Å. 3cis
crystallizes in the I4h2d space group and features slightly larger
distortions (inward bowing of pyridine ligands) than those
found for 2cis, where the pyridine ligands bow in toward
each other. The pyridine NsResRe angle is 75.4°. The Res
Re distance is the shortest of the six molecules at 5.69 Å.
Table 2 contains ResN bond distances for the six
molecules. Both the imidazolyl NsRe and the pyridyl Ns
Re bond distances [averaging 2.22(1) and 2.21(1) Å,
Inorganic Chemistry, Vol. 44, No. 24, 2005 8711

Dinolfo et al.
Figure 4. UV-visible absorption spectra obtained in CH2Cl2. Panel a
shows 4 (solid black line) and 1trans (long dashed blue line). Panel b shows
2trans (long dashed blue line) and 2cis (short dashed green line). Panel c
shows 3trans (long dashed blue line) and 3cis (short dashed green line).
the reduction potentials for 4, 1trans, 2trans, 2cis, 3trans,
and 3cis as well as the reduction potentials for the free
ligands.
Figure 3. Packing diagram for 2trans/2cis (stick figure renderings). For
clarity, 2trans is shown in blue and 2cis is shown in green. The top diagram
shows a side view and the bottom shows a top view of the packing
orientation along the a axis. For each molecule of 2trans, two molecules
of 2cis are interspaced on either side overlapping the 2,4′-bpy ligands and
forming columns along the a axis.
Visible-region SEC measurements with 2cis (Figure 6)
show the formation of the characteristic absorption spectrum
of the 2,4′-bpy radical anion, establishing the 2,4′-bpy ligand
as the site of reduction.26 On the basis of comparative free-
ligand reduction potentials, we tentatively ascribe the initial
reductions of 1trans, 3trans, and 3cis to pyridyl ligands as
well, but we cannot rule out reduction of Re(I) to Re(0),
especially where electrochemical irreversibility is encoun-
tered. (In these coordination environments, Re(0) represents
a 19-electron species and would be expected to display
instability and electrochemical irreversibility.)27 The positive
shifts in putative ligand reduction potentials in comparison
to free-ligand reductions are understandable on the basis of
electrostatic stabilization of the coordinated ligand anion by
the cationic metal center. At the same time, the reduction
potentials are still considerably more negative than those
found for related molecular rectangles, where two metal
cations coordinate each ligand. As noted above, the differ-
ences in the ease of reduction of the coordinated L in C and
respectively] are consistent with those found for molecular
rectangles synthesized using the [Re(CO)3]2BiBzIm bridge
unit.1,9
Electronic Absorption. Figure 4 contains electronic
absorption spectra for 4, 1trans, 2trans, 2cis, 3trans, and
3cis in CH2Cl2 as the solvent. The spectra are dominated by
high energy transitions (<400 nm) associated with the
BiBzIm bridging ligand. Notably absent are the distinct
visible-region metal-to-ligand charge-transfer (MLCT) transi-
tions (ReI f Lπ*) obtained for analogous molecular
rectangles.1,10 The difunctional ligands of the rectangles are,
of course, ligated by two, rather than one, rhenium cations,
resulting in lower Lπ* orbital energies and accounting for
the shift of MLCT bands into the visible region.
Electrochemistry. Cyclic voltammograms (CVs) of 1trans,
2trans, and 2cis in THF display chemically reversible
reduction waves, while CVs of 4, 3trans, and 3cis show
multiple irreversible reduction waves. Figure 5 shows a CV
for 4, 2trans, and 2cis. CVs for 1trans, 3trans, and 3cis
are included in Supporting Information. Table 3 summarizes
(26) Yang, L.; Wimmer, F. L.; Wimmer, S.; Zhao, J. X.; Braterman, P. S.
J. Organomet. Chem. 1996, 525, 1-8.
(27) Paolucci, F.; Marcaccio, M.; Paradisi, C.; Roffia, S.; Bignozzi, C. A.;
Amatore, C. J. Phys. Chem. B 1998, 102, 4759-4769.
8712 Inorganic Chemistry, Vol. 44, No. 24, 2005

C- and Z-Shaped Coordination Compounds
Figure 6. UV/visible SEC measurement of 2cis during the reduction to
the 1- state. The new absorption bands at approximately 400 and 560 nm
are indicative of the 2,4′-bpy radical anion. The elapsed time for the series
of SEC measurements shown was approximately 2 h. Measurements were
also made in the extended NIR region (not shown) but did not reveal any
new absorption bands.
voltammetry (DPV) scans (see the example in Figure 5)
display the broadened peaks and flattened tops expected for
closely spaced pairs of reductions. Fits of the scans as
overlapping Gaussians yielded formal potential differences,
∆Ef, of 100 and 90 mV, respectively, for the first two
reductions of 1trans and 2trans, respectively. In contrast,
the 0/1- and 1-/2- reductions for 2cis are separated by
170 mV. For the corresponding 4,4′-bipyridine-based rect-
angle, they are separated by 230 mV. These translate into
LCMV comproportionation constants of 50, 30, 750, and
7700, respectively, for 1trans, 2trans, 2cis, and the 4,4′-
bipyridine-based rectangle. The differences are ascribable to
variations in the electrostatic interactions between reduced
pairs of bipyridyl ligands, the interactions being substantially
greater for the proximal pairs found in the cis assembly and
the rectangle.
Finally, returning to spectral measurements, attempts to
observe a red or NIR intervalence transition for 2cis via either
SEC or chemical reduction with 5% sodium amalgam were
unsuccessful, even at a lower temperature (0 °C). The band
presumably exists, but the reduced compound lacks the
stability needed over the extended electrolysis times required
for the NIR SEC experiments.
Figure 5. Top graph: CV of 4 at a potential scan rate of 200 mV/s. Middle
graph: CV (solid black line) and DPV (dashed-dotted black line) of 2trans.
Bottom graph: CV (solid black line) of 2cis. All scans were performed in
THF with 0.1 M TBAPF6 as the supporting electrolyte and referenced
against the ferrocene/ferrocenium couple.
Table 3. Reduction Potentials of 1trans, 2trans, 2cis, 3trans, 3cis,
and 4 (V vs Fc/Fc+)a
0/1-
1-/2-
0/1-
Conclusions
compound
E1/2
E1/2
ligand
E1/2
1trans
2trans
2cis
3trans
3cis
-2.32b
-2.07b
-2.01
-2.42b
-2.16b
-2.18
-2.54c
4-PhPyd
-2.62f
-2.50
Molecules of the form cis- and trans-[Re(CO)3(L)]2BiBzIm,
where L ) 4-PhPy (trans only), 2,4′-bpy, and pyridine, have
been synthesized. X-ray crystallography is consistent with
the proposed structures. In the case where L ) 4-PhPy, the
trans isomer is observed in greater than 20:1 excess over
the cis isomer, likely reflecting the torsional steric demands
of proximal phenylpyridine ligands. The compounds are
acyclic analogues of recently studied tetrarhenium molecular
rectangles and are of interest because of their potential to
form LCMV compounds upon reduction by one electron.
SEC measurements corroborated the formation of LCMV
versions of cis-[Re(CO)3(L)]2(2,2′-bisbenzimidizolate) but
failed to uncover a ligand-based intervalence transition as a
result of the instability of the reduced species. Electrochemi-
cal measurements revealed isomer-dependent L/L electro-
static effects, resulting in greater mixed-valence ion com-
proportionation for C- versus Z-shaped assemblies.
2,4′-bpye
-2.39c
-2.49c
-1.69c
Pyd
-3.12f
4
-2.07c
a In THF with 0.1 M TBAPF6 used as the supporting electrolyte. b 0/1-
and 1-/2- waves strongly overlap in CV measurements. Values are
estimated from Gaussian deconvolutions of the DPV scan. c Irreversible
reduction wave. Value shown for the peak cathodic wave. d Value taken
from J. Chem. Soc. A 1968, 381-182. e Value taken from ref 26. f Converted
to Fc0/+ reference by subtracting 0.36 from the reported value referenced
to Ag/AgCl.
Z versus the ease of reduction of rectangle assemblies are
reflected in the absence or presence, respectively, of visible-
region MLCT absorption bands.
The more careful inspection of reduction waves for 1trans
and 2trans indicates that they likely comprise two strongly
overlapping single-electron reductions. Differential pulse
Inorganic Chemistry, Vol. 44, No. 24, 2005 8713

Dinolfo et al.
Supporting Information Available: X-ray crystallographic files
for 1trans, 2trans/2cis, 3trans, 3cis, and 4 (CIF) and electrochemi-
cal data for 1trans, 3trans, and 3cis. This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank Baoqing Ma for assistance
in the crystallographic refinement of 1trans. We gratefully
acknowledge the Office of Science, U.S. Department of
Energy (Grant DE-FG02-87ER13808), for financial support
of our research.
IC050894U
8714 Inorganic Chemistry, Vol. 44, No. 24, 2005