The synthesis of dimeric ReI-phenylenediimine conjugates: Spectroscopic and electrochemical studies

Article abstract of DOI:10.1002/ejic.201400076

A series of dimeric Re^I(CO)₃ compounds have been prepared by means of a one-pot reaction of [Re(CO)₅X] (X = Cl, Br), pyridine-2-carboxyaldehyde, and o-, m-, or p-phenylenediamine. For the m- and p-substituted phenylenediamine reactions, phenyl-bridged bis(pyridine-2- carbaldimine) compounds of the formula [Re(CO)₃X]₂(μ- PPC) [3-7; X = Cl, Br; μ-PPC = m- or p-phenylene bis(pyridine-2-carbaldimine) ] were produced, with m-phenylenediamine forming a mixture of isomers. For reactions with o-phenylenediamine, multiple products formed (compounds 8-11), which ranged from the desired o-phenylene bis(pyridine-2-carbaldimine) adduct to 2-pyridylbenzimidazole and bridging dipyridinylquinoxaline complexes. An example of each complex was structurally elucidated by single-crystal X-ray methods. The dimeric m- and p-phenylene-bridged Re(CO)₃X compounds, including a sulfonate derivative designed to improve solubility, were investigated for possible photophysical applications through the use of UV/Vis spectroscopy and cyclic voltammetry; the study revealed coupling between the rhenium centers.

Full text of DOI:10.1002/ejic.201400076

FULL PAPER
DOI:10.1002/ejic.201400076
The Synthesis of Dimeric ReI–Phenylenediimine
Conjugates: Spectroscopic and Electrochemical Studies
Abed Hasheminasab,[a] James T. Engle,[a] Jacob Bass,[b]
Richard S. Herrick,*[b] and Christopher J. Ziegler*[a]
Dedicated to Professor Joe Templeton on the occasion of his 65th birthday
Keywords: Rhenium / Dimers / Bridging ligands / UV/Vis spectroscopy / Cyclic voltammetry
A series of dimeric ReI(CO)3 compounds have been prepared
by means of a one-pot reaction of [Re(CO)5X] (X = Cl, Br),
pyridine-2-carboxyaldehyde, and o-, m-, or p-phenylenedi-
amine. For the m- and p-substituted phenylenediamine reac-
tions, phenyl-bridged bis(pyridine-2-carbaldimine) com-
pounds of the formula [Re(CO)3X]2(μ-PPC) [3–7; X = Cl, Br; μ-
PPC = m- or p-phenylene bis(pyridine-2-carbaldimine)] were
produced, with m-phenylenediamine forming a mixture of
isomers. For reactions with o-phenylenediamine, multiple
products formed (compounds 8–11), which ranged from the
desired o-phenylene bis(pyridine-2-carbaldimine) adduct to
2-pyridylbenzimidazole and bridging dipyridinylquinoxaline
complexes. An example of each complex was structurally
elucidated by single-crystal X-ray methods. The dimeric m-
and p-phenylene-bridged Re(CO)3X compounds, including
a sulfonate derivative designed to improve solubility, were
investigated for possible photophysical applications through
the use of UV/Vis spectroscopy and cyclic voltammetry; the
study revealed coupling between the rhenium centers.
enes (DAB) has also gained significant attention.[13–17] For
several years, we have been investigating the syntheses of
Re(CO)3 compounds with DAB-type ligands by means of
metal-mediated one-pot reactions that involve Schiff base
condensation.[18–20] Relative to traditional aromatic diimine
ligands like bipyridine or phenanthroline, DAB complexes
have lower π*-orbital energies. Additionally, the absorption
properties can be tuned by means of the ligand electronic
structure to increase diffusion time and thus minimize hole–
electron recombination, which is important for various
photophysical applications.[15,21–27]
We have recently started to investigate dimeric Re(CO)3
complexes as part of our research into the synthesis of pept-
ide-based Re/Tc imaging agents. We found that Re(CO)3
dimers could be produced in one-pot reactions with amino
acids or peptides.[18,28] We found that pyridine-2-carboxyal-
dehyde, when treated with amino acids under one-pot con-
ditions, resulted in dimers with chelating diamines (pyr-
idine-2-carbaldimine; pyca) bound to the metal centers.[18]
These compounds exhibit MLCT transitions, and we hy-
pothesized that similar synthetic methods could be used to
generate binuclear Re(CO)3 compounds for photophysical
applications.
Introduction
Over the past few decades, organometallic complexes of
the general type [Re(X)(CO)3(diimine)] (X = Cl, Br) have
received much attention owing to their potential use in pho-
tophysical applications as emission sensitizers, photosensi-
tizers, photooxidants, photocatalysts, and electrocata-
lysts.[1–6] These compounds have absorptions in the visible
spectrum with long-lived excited states that frequently arise
from metal-to-ligand charge transfer (MLCT) transitions.
Interest in these compounds is driven by their attractive fea-
tures, including a stable low-spin d6 rhenium metal center
and the ability to tune their photophysical properties by
varying the diimine ligand, the sixth ligand (such as the
halide), and solvent.[7,8]
The chemistry and properties of ReI complexes contain-
ing bidentate heterocyclic diimine ligands, such as 2,2Ј-bi-
pyridine, 1,10-phenanthroline, and their derivatives, have
been extensively investigated.[9–12] In addition to these tradi-
tional ligands, the chemistry of substituted 1,4-diazabutadi-
[a] Department of Chemistry, University of Akron,
OH 44325-3601, USA
E-mail: ziegler@uakron.edu
There have been many examples of bridged Re(CO)3
complexes, and it has been shown that the identity of the
bridging unit affects the degree of coupling between the two
coordination complexes. Connick and co-workers published
a study on binuclear Re(CO)3 compounds, in which the
http://www.uakron.edu/chemistry/faculty-staff/ziegler.dot
[b] Department of Chemistry, College of the Holy Cross,
Box C, Worcester, MA 01610-2395, USA
http://academics.holycross.edu/chemistry/faculty-staff/herrick
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201400076.
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Figure 1. Compounds reported in this work.
bridging unit is a cyclophane linked to two chelating units pound 5 with a pendant sulfonate that had enhanced solu-
constructed from pyridine-2-carbaldimines.[29] These types bility. As controls to better understand the effects of dimer-
of bridged complexes exhibit metal–metal interactions upon ization, we also generated monomeric compounds 1 and 2;
reduction, as demonstrated by cyclic voltammetry. Ad- to the best of our knowledge, 1 is a new compound, whereas
ditionally, the bridging unit not only affects the MLCT in- 2 was previously synthesized.[41–45] Additionally, the binu-
teraction but also alters the metal–metal interactions and clear compound 3 has been previously generated but was
intramolecular electron transfer depending on the type, size, not structurally elucidated.[29] Analogous reactions with o-
aromaticity, and conjugation of the bridge.[30–37] In ad- phenylenediamine resulted in multiple compounds, includ-
dition, heterodentate bridging ligands have been found to ing the desired o-PPC adduct (8) as well as 2-pyridylbenz-
be useful in designing binuclear and trinuclear com- imidazole (11), bridging dipyridinylquinoxaline (9), or
plexes.[38–40] Included in the study by Connick et al. was benzimidazole (10) complexes. For the desired p- and m-
one complex in which two pyridine-2-carbaldimine Re- PPC complexes, we recorded UV-visible absorption spectra
(CO)3 centers are bridged by a para-substituted benzene and performed cyclic voltammetry experiments to confirm
ring.[29] We hypothesized that the degree of coupling be- that the bridging phenylene unit does mediate coupling be-
tween Re(CO)3 centers might depend on the position of tween the Re(CO)3X coordination complexes. In the cyclic
substitution on the phenylene ring. Having experience with voltammetry experiments, we observed coupled two-elec-
the preparation and characterization of d6 rhenium pyr- tron redox behavior, and the reversibility was mediated by
idine-2-carbaldimine compounds,[18–20] we decided to ex- the identity of the halide.
plore this area further.
Herein, we report a series of binuclear ReI complexes
Results and Discussion
bridged by a phenylene bis(pyridine-2-carbaldimine) (PPC)
ligand constructed from an o-, m-, or p-phenylenediamine
and pyridine-2-carbaldehyde by using one-pot conditions
(Figure 1). The para-substituted version of this ligand has
Syntheses and Structures
As previously demonstrated in our work[18–20] and in the
been referred to as p-phenylenebis(picolinaldimine) by Con- work of Miguel et al.,[46–48] chelating aldehydes readily react
nick and co-workers,[29] and could alternatively be de- with primary amines in the presence of [Re(CO)5X] to af-
scribed as p-phenylenebis(2-pyridinylmethyleneamine). In ford the corresponding Schiff base compounds (Scheme 2).
our procedures, pyridine-2-carbaldehyde and phenylenedi- For example, mononuclear ReI complexes 1 and 2 were syn-
amines were mixed in the presence of [Re(CO)5X] (X = Cl, thesized by using this method for use as controls in this
Br) to form dimeric Schiff base complexes with bridging study. Compound 2 has been previously generated,[41–44] but
PPC ligands (compounds 3–7). We also prepared com- we observed a new crystal form. Compound 1 has neither
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been synthesized nor structurally elucidated prior to this
study, but similar compounds have been gener-
ated.[13–17,44,49,50] In prior work, the Schiff bases were syn-
thesized first, typically through mixing the aniline or phen-
ylene diamine with pyridine-2-carbaldehyde in dry meth-
anol for 20 min to afford a pale yellow solution that con-
tained the Schiff base product. Generally, these Schiff bases
are sensitive to hydrolysis, which makes them prone to de-
gradation. We found that one-pot conditions that contained
the phenylenediamine, [Re(CO)5X], and pyridine-2-carbal-
dehyde produce the desired metal-bound Schiff base com-
plexes in reasonable yields (Scheme 1). The reactions were
readily monitored through the formation of a red solution
owing to the formation of products with metal-to-ligand
charge-transfer bands.
Figure 2. The structures of monomeric complexes 1 and 2 with
35% thermal ellipsoids. Hydrogen atoms have been omitted for
clarity.
Scheme 1. Synthesis of monomeric complexes 1 and 2.
Compounds 1 and 2 were soluble in hot solvents such as
methanol and THF. Crystals suitable for X-ray diffraction
structural elucidation were collected through slow cooling
and solvent evaporation. The structures of these com-
pounds, shown in Figure 2, reveal the expected bidentate
coordination mode of the pyridineimine and diazabutadi-
ene ligands on 1 and 2, respectively, as well as the standard
facial Re(CO)3 unit.
For the bridged p- and m-PPC compounds, we were able
to isolate the corresponding dimeric adducts 3–7
(Schemes 2 and 3). All four compounds showed limited sol-
ubility in most solvents, with the exception of the sulfonate-
modified compound 5, which exhibited increased solubility.
We were able to structurally elucidate the p-phenylenediam-
ine compound 5 and both m-phenylene compounds 6 and
7, but we were unable to isolate suitable crystals of 3 or 4.
The structures of 5–7 are shown in Figures 3 and 4.
For compound 5, the structural features of the ReI cen-
ters are in good agreement with previously elucidated
Re(CO)3(diimine) compounds and monomeric species such
as 2.[43–45] Owing to the presence of the sulfonate group,
free rotation of the adjacent C–N bond is hindered, but the
opposite C–N bond [bound to the Re(1) center] is free to
rotate. For compound 5, one can consider the formation of
two possible isomers in the dimer structure, which results
from the presence of chlorides either on the same side or
on opposite sides of the phenylenediimine plane. However,
Scheme 2. Synthesis of binuclear complexes 3, 4, 6, and 7 from the
reaction of pyca and m- or p-phenylenediamine.
1
in the H NMR spectra of 3 and 4, we observed only one
imine proton resonance, and in the elucidated structure of
5, we observed only one of the two isomers. It is important Scheme 3. Synthesis of complex 5.
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the desired dimeric compounds. Instead, we observed the
formation of mixtures of different compounds, as shown by
the reactions in Scheme 4. We were able to isolate crystals
of four different products (Figure 5), but we were not able
to successfully control these one-pot reactions to exclusively
produce the desired dimeric compounds and we were not
able to purify the compounds from these mixtures. Three
of the four compounds were obtained from reactions with
[Re(CO)5Cl] and one with [Re(CO)5Br]. The primary prod-
uct of these reactions is (2Ј-pyridyl)benzimidazole com-
plexes of Re(CO)3, such as compound 10. The formation of
these compounds is not surprising, as o-phenylenediamine
will react with pyridine-2-carbaldehyde to form the free (2Ј-
pyridyl)benzimidazole. Upon treating o-phenylenediamine
with pyridine-2-carbaldehyde in the absence of metal, we
never observed the formation of the desired free o-PPC li-
gand, in contrast to prior reports.[51–53] We also observed a
second (2Ј-pyridyl)benzimidazole complex that resulted
from the addition of a second equivalent of pyridine-2-
carbaldehyde to the exterior nitrogen site on the ligand,
compound 11. We did observe the formation of a small
amount of the desired compound 8. However, we addition-
ally observed that this compound could ring close to form
a dimeric Re(CO)3X compound with a bridging dipyridinyl-
quinoxaline unit (9). Our observations on these compounds
parallel those seen by Chattopadhyay and co-workers on
a series of similar reactions with [Ru(bpy)2] (bpy = 2,2Ј-
bipyridyl) reagents.[54]
Figure 3. The structures of binuclear complex 7 with 35% thermal
ellipsoids. Hydrogen atoms have been omitted for clarity. Com-
pound 6 has an identical structure but shows some disorder along
the Cl(2)–Re(2)–CO axis (see the Supporting Information).
Figure 4. The structure of sulfonate-modified binuclear complex 5
with 35% thermal ellipsoids. Hydrogen atoms have been omitted
for clarity.
to note that the appearance of one resonance in the NMR
spectra might be due to the similarity in chemical shift for
the imine proton in both isomers, and the observation of a
single isomer in the crystal structure does not rule out the
existence of the alternate isomer.
Compounds 6 and 7 also exhibit the expected structural
features around the two rhenium atoms in these dimers.
Isomerization is clearly observed in the NMR spectra and
X-ray structures of 6 and 7, with the halides observed on
both the same side and on opposite sides of the phenylene-
diimine plane (Figure 3). In the crystal structures, these two
compounds exhibit a chiral twist around a C2 axis that is
coincident with that of the phenylenediimine unit. We ob-
serve both enantiomers in each crystal structure in addition
to the two diastereomers that result from the orientation of
the halide atoms. We believe that the two twist enantiomers
might be able to interconvert through rotation about the C–
N bonds of the phenylenediimine units.
Scheme 4. Observed products from the reaction between pyca and
m-phenylenediamine.
One characteristic resonance for these compounds is the
The reactions of o-phenylenediamine with pyridine-2- imine-bound hydrogen atom, which appears in the de-
carbaldehyde and [Re(CO)5X] did not exclusively produce shielded region of δ = 9.0–9.5 ppm. As discussed above, for
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Figure 5. The structures of complexes 8, 9, 10, and 11 with 35% thermal ellipsoids. Hydrogen atoms have been omitted for clarity.
Compound 11 shows a half-occupied solvent water molecule hydrogen-bound to the free pyridine nitrogen atom.
compounds that exhibit isomerism, we can readily observe solvent dependency of these MLCT bands can be found in
the presence of two structurally similar isomers in the the Supporting Information. The electronic structure of the
NMR spectra. The IR spectra of compounds 1–7 show diimine does affect the magnitude of the extinction coeffi-
either two or three distinct carbonyl stretching frequencies cient for these MLCT transitions, as can be seen between
between 2030 and 1860 cm–1. The pseudo-C3v symmetry diazabutadiene compound 1 (ε = 3.34ϫ103 m–1 cm–1) and
that results from a facial arrangement of carbonyls at the pyca complex 2 (ε = 5.55ϫ103 m–1 cm–1). The dimeric com-
Re centers will produce stretches that correspond to the a1 plexes exhibit more intense MLCT absorptions than the
and e vibrational modes; however, deviations from C3v corresponding monomeric compounds, with the highest
cause the e mode to split in some cases.
values for the p-phenylenediamine-bridged dimers (3–5) rel-
Re(CO)3 diimine complexes with pendant aromatic rings ative to the m-phenylenediamine variants (6, 7).
show two types of electronic absorption in the UV-visible
spectrum. First, there are higher-energy absorptions be-
tween 300 and 350 nm that comprise intraligand bands that
could result from πǞπ* transitions. These bands are not
Cyclic Voltammetry
sensitive to solvent changes, which is indicative of πǞπ*
The redox potentials for solutions of compounds 1–7 in
character. Additionally, there are lower-energy bands lo- dimethylformamide (DMF) were determined using 0.1 m
cated between 400–450 nm that result from metal-to-ligand tetrabutylammonium hexafluorophosphate (TBAPF6) as
charge transfer (MLCT) transitions. These bands exhibit the electrolyte. Scan rates between 0.10 to 0.50 Vs–1 were
solvatochromism, and increasing the dielectric constant of examined, whereas the best sensitivity was observed at
the solvent will shift the position to the lower wavelengths. 10.0 μAV–1. Generally, ReI–carbonyls show irreversible oxi-
This solvatochromism was examined by Connick et al.[29] dations near approximately 1.5 V; however, the reduction
in their previous report on 3. Experiments that show the potentials are readily observable and reversible in solution
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in DMF. The cyclic voltammograms for the cathodic region responds to the degree of coupling between the coordina-
for compounds 1–7 are shown in Figure 6, and Table 1 lists tion complexes. These values, shown in Table 1, are much
the observed reduction potentials along with other param- larger than the noninteracting statistical value of 4. Al-
eters. For the monomeric compounds 1 and 2, we observe though the measurements of Kc using potentials have been
very different reduction chemistry. Compound 1 shows shown to be unpredictable and dependent on the anion,[56]
multiple reductions and some irreversible behavior owing we observe higher values than what is seen for the corre-
to the redox activity of the diazabutadiene unit.[30–32] Com- sponding cyclophane compounds developed by Connick et
pound 2 shows much cleaner redox behavior, with a single al.,[29] as would be expected for systems with direct π over-
reduction that can be assigned to the rhenium pyridine im- lap. Additionally, we also see a slight increase when com-
ine complex. For the dimeric compounds 3–7, we observe paring the para- to the meta-substituted complexes, as
two well-defined separated reversible reductions. For the p- would be expected from resonance arguments. Lastly, we
phenylenediamine-bridged compounds, the reductions are do observe differences between the bromide and chloride
separated by 188 and 178 mV, respectively, for 3 and 4. We variants as well, with the bromide analogues exhibiting
observe slightly smaller separations for 6 and 7 with values smaller Kc values. This difference can be interpreted in two
of 175 and 144 mV. Additionally, we observed differences ways: Either the identity of the halide affects the coupling
in reversibility between the chloride analogues (3 and 6) between the coordination complexes, or the anion has an
and the corresponding bromide compounds (4 and 7). The effect on the reduction processes, as noted in other mixed-
chlorides show significantly greater reversibility than the valence metal systems by D’Alessandro and Keene et al.[56]
bromides, as measured by the ipa/ipc for the two waves taken However, the identity of the halide has a small effect over-
together. For the sulfate-modified complex 5, we observe all, and might also influence the redox chemistry by means
only one reduction in contrast to the two redox events in of kinetic processes.
compounds 3–7. The overall negative shift of the reduction,
and the lack of a corresponding second wave, results from
the presence of the anionic group in the bridging ligand.
Conclusion
We have investigated the synthesis of a series of phenyl-
ene-bridged dimer species that incorporate the Re(CO)3X
unit (X = Cl, Br) by using one-pot methods for their syn-
thesis. We are continuing our work in this area by investiga-
ting alternate bridging units as well as incorporating
Re(CO)3X into phenylene-bridged polymeric systems.
Experimental Section
Materials and Methods: Reagents were purchased from Strem,
Acros Organics, or Sigma–Aldrich and used as received without
further purification. Syntheses were performed to limit exposure to
air or water. For CV experiments, THF, acetonitrile (MeCN),
DMF, DMSO, chloroform, dichloromethane, and benzene were
dried and deoxygenated by alumina and copper columns in the
Pure Solve solvent system (Innovative Technologies, Inc.); they
were stored over molecular sieves. Sodium 2,5-diaminobenzenesulf-
onate was prepared by mixing an aqueous solution of 2,5-diami-
Figure 6. Cyclic voltammograms of monomers and dimers in 0.1 m
nobenzenesulfonic acid (1 equiv.) with sodium hydrogen carbonate
TBAPF6/DMF at 0.25 V and 10.0 μAV–1 sensitivity versus AgCl.
(1 equiv.) for 1 h. The solvent water was then evaporated and a
brown solid was dried under vacuum for two days.
NMR spectra were recorded with Varian Mercury 300 and
500 MHz instruments. Chemical shifts were reported with respect
Table 1. Reduction potential data.
to residual solvent peaks as internal standard (1H: [D6]DMSO, δ =
2.50 ppm; 13C: [D6]DMSO, δ = 39.7 ppm). Infrared spectra were
collected with a Nexus 870 FTIR instrument. Electronic absorption
spectra were recorded with a Hitachi U-2000 UV-visible spectro-
photometer. Elemental analyses were performed by Atlantic Micro-
lab of Norcross, GA. Mass spectrometric analyses were carried out
at the Mass Spectrometry and Proteomics Facility at the Ohio State
University in Columbus, OH or at The University of Akron in
Akron, OH.
0Ј
0Ј
E1 [V]
E2 [V]
ΔE
[mV]
ipa/ipc
Kc
(ΔEp [mV])
(ΔEp [mV])
2
3
4
5
6
7
–0.918(73)
–0.790(82)
–0.768(61)
–1.06(76)
–0.811(70)
–0.816(21)
–
–
0.92
0.92
0.79
0.93
0.89
0.72
–
–0.978(70)
–0.946(70)
–
–0.986(73)
–0.960(61)
188
178
–
175
144
1507
1021
–
909
272
For the four compounds that show two waves, we can
X-ray Data Collection and Structure Determination: X-ray intensity
calculate a conproportionation constant (Kc),[55] which cor- data for compounds 1, 2, 6, 7, 8, and 10 were measured at 100 K
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Table 2. Crystallographic data for 1–6.
1
2
5
6
Empirical formula
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C17H12ClN2O3Re
C15H10ClN2O3Re
487.90
NaC24H13Cl2N4O9Re2S
976.74
C24H14Cl2N4O6Re2
897.69
monoclinic
P21/c
14.0136(14)
11.8934(12)
16.0657(16)
90
104.0500(10)
90
513.95
orthorhombic
Pnma
10.3518(17)
19.220(3)
8.2014(14)
90
90
90
1631.8(5)
4
2.092
triclinic
triclinic
¯
¯
P1
P1
7.9120(19)
7.9120(19)
16.322(4)
99.325(4)
91.969(4)
93.705(4)
1529.5(6)
4
9.4398(3)
12.4025(4)
16.2635(6)
100.487(2)
99.660(2)
102.676(2)
1783.30(10)
2
α [°]
β [°]
γ [°]
V [Å3]
2597.6(5)
4
2.295
Z
Dcalcd. [Mgm–3]
2.119
8.132
1.819
15.415
μ [mm–1]
7.628
9.565
F(000)
976
12941
1936
920
12772
6694
0.935
0.0542
0.0993
0.1046
0.1174
832
20463
5458
1.025
0.0296
0.0728
0.0332
0.0746
1672
21944
5963
0.753
0.0393
0.1018
0.0529
0.1168
Reflections collected
Independent reflections
GoF on F2
R1 [on Fo2, IϾ2σ(I)]
wR2 [on Fo2, IϾ2σ(I)]
R1 (all data)
wR2 (all data)
1.448
0.0309
0.0598
0.0415
0.0624
Table 3. Crystallographic data for 7–11.
7
8
9
10
11
Empirical formula
Formula weight
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C24H14Br2N4O6Re2 C24H14Cl2N4O6Re2 C24H12Cl2N4O6Re2 C17H15ClN3O4ReS C21H14BrN4O3.5Re
986.61
897.69
orthorhombic
Pbca
895.68
579.03
orthorhombic
Pbca
644.47
monoclinic
Cc
triclinic
triclinic
¯
¯
P1
P1
8.067(3)
12.008(5)
14.269(6)
102.066(5)
91.049(5)
95.591(5)
1344.2(9)
2
17.991(4)
11.987(2)
23.220(5)
90
90
90
5007.4(18)
8
2.382
7.7720(3)
11.8847(5)
14.6527(6)
70.690(2)
89.783(2)
79.473(2)
1253.46(9)
2
14.220(6)
10.837(5)
25.327(11)
90
90
90
3903(3)
8
1.967
21.9528(11)
12.7721(7)
8.7439(5)
90
109.201(3)
90
2315.3(2)
4
1.849
α [°]
β [°]
γ [°]
V [Å3]
Z
Dcalcd. [Mgm–3]
2.438
2.373
μ [mm–1]
12.016
908
9.924
3344
20.992
832
6.498
2216
7.004
1224
F(000)
Reflections collected
Indep. reflections
GoF on F2
R1 [on Fo2, IϾ2σ(I)]
wR2 [on Fo2, IϾ2σ(I)]
R1 (all data)
wR2 (all data)
11107
5796
0.969
0.0438
0.0926
0.0737
0.1079
28686
4437
0.887
0.0384
0.1096
0.0596
0.1300
13425
3787
0.913
0.0225
0.0581
0.0251
0.0605
4334
4334
1.952
0.0729
0.0998
0.0879
0.1016
9466
4303
0.992
0.0334
0.0777
0.0387
0.0801
with a CCD-based X-ray diffractometer system equipped with a
Mo-target X-ray tube (Mo-Kα radiation, λ = 0.71073 Å) operated
at 2000 W power. Data for 5, 9, and 11 were collected with a CCD-
based diffractometer with dual Cu/Mo ImuS microfocus optics
(Cu-Kα radiation, λ = 1.54178 Å). Crystals were mounted on a cry-
oloop using Paratone oil and placed under a steam of nitrogen at
100 K. The detector was placed at a distance of 5.009 cm from
the crystal. Crystal data and structure refinement parameters are
summarized in Tables 2 and 3.
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Cyclic Voltammetry: Cyclic voltammograms were obtained with a
standard three-electrode cell and a BAS 100B electrochemical ana-
lyzer from Bioanalytical Systems and were recorded at 298 K under
the following conditions: 10–3 m samples in dried DMF in the pres-
ence of 0.1 m TBAPF6 as a supporting electrolyte, Ag/Ag+ refer-
ence electrode, 0.79 mm2 gold working electrode, and platinum wire
auxiliary electrode. The working electrode was polished first with
3 μm fine diamond, then 0.05 μm alumina. The electrode was
rinsed with ethanol and deionized water after each polishing and
wiped with a Kimwipe. The nonaqueous Ag/Ag+ reference elec-
trode was used to avoid an excess amount of junction potential and
CCDC-979430 (for 1), -979431 (for 2), -979432 (for 5), -979433 (for
6), -979434 (for 7), -979435 (for 8), -979436 (for 9), -979437 (for
10), and -979438 (for 11) contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
Eur. J. Inorg. Chem. 2014, 2643–2652
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FULL PAPER
water contamination. It was prepared by soaking the silver wire in
the degassed DMF solution of 0.01m AgClO4/0.1m TBAPF6. To
compare with previous studies, the approximated formal potentials
for reversible redox couples [E°Ј = (Epc + Epa)/2] were all referenced
against Ag/AgCl. ΔEp = Epa – Epc, ΔE = (E1°Ј – E2°Ј), and peak
currents were calculated by use of an extrapolated baseline cur-
rent.[57,58] Potentials were converted to Ag/AgCl reference electrode
based on standard conversion factors reported by Amatore and
Kochi et al. [Equation (1)][59,60] and compared with previous data
reported by Connelly and Geiger.[61] The linear dependence be-
tween the square root of the scan rate (ν1/2) and both the cathodic
(ipc) and anodic (ipa) currents were examined between 0.1 to
0.7 Vs–1, and the formal potentials are independent of scan rate.
At a 0.25 Vs–1 sweep rate, the Fc/Fc+ occurs at (0.414Ϯ0.005) V
(ΔEp = 83 mV; ipa/ipc = 0.99).
Compound 4: Yield 97 mg (78%). 1H NMR (300 MHz, [D6]-
DMSO): δ = 9.44 (s, 2 H, N=CH), 9.13 (d, J = 3.0 Hz, 2 H, H on
py), 8.40 (m, J = 7.2 Hz, 4 H, H on py), 7.87 (m, 2 H, on py), 7.80
(d, J = 0.9 Hz, 4 H, on phenyl) ppm. 13C NMR (75 MHz, [D6]-
DMSO): δ = 197.5, 196.9, 187.2, 171.3, 155.4, 153.8, 151.1, 151.1,
141.0, 131.3, 130.4, 124.0 ppm. C24H14Br2N4O6Re2 (986.61): calcd.
C 29.21, H 1.43, N 5.67; found C 30.02, H 1.35, N 5.80. IR (CO
stretch): ν = 2016, 1894 cm–1. MS (ESI): calcd. for C H N O -
˜
24 14
4
6
Re2Br2Na [M + Na+]: 1008.82; found 1008.82. UV/Vis (DMF):
λmax (ε, m–1 cm–1) = 342 (1.61ϫ104), 412 nm (6.81ϫ103).
Synthesis of [Re(CO)3Cl(p-PPC-SO3Na)Re(CO)3Cl] (5): Sodium
2,5-diaminobenzenesulfonate (DABS) was prepared by dissolving
2,5-diaminobenzenesulfonic acid in deionized water and adjusting
the pH of the solution to 7.0–7.5 by adding small portions of a
sodium hydrogen carbonate solution. The solution was heated,
then water was removed to yield a brown solid, which was dried at
90 °C under vacuum overnight. DABS (0.0300 g, 142 mmol) was
then dissolved in a 10 mL mixture of MeOH/2-propanol (1:3) and
stirred for 10 min. Pyridine-2-carbaldehyde (0.0302 g, 0.282 mmol)
was added to the mixture while stirring. Then, [Re(CO)5Cl]
(0.100 g, 0.282 mmol) was added and the mixture heated to reflux
for 8 h. The solvent was then removed by rotary evaporation.
CH2Cl2 was added to the solid, and the mixture was filtered. The
red solid was dried under vacuum. Crystals suitable for X-ray dif-
fraction were obtained by slow evaporation of a methanol solution,
yield 40.0 mg (59%). 1H NMR (500 MHz, [D6]DMSO): δ = 9.48
(2 H, N=CH), 9.08 (d, J = 4.0 Hz, 2 H, H on py), 8.43–8.37 (m, 4
H, H on py), 8.06 (s, 1 H, on phenyl), 7.95 (d, J = 8.5 Hz, 1 H on
phenyl), 7.86 (s, 2 H on py), 7.74 (d, J = 8.5 Hz, 1 H on phenyl)
ppm. 13C NMR (75 MHz [D6]DMSO): δ = 198.4, 198.0, 196.9,
187.9, 187.2, 173.7, 171.4, 155.5, 155.3, 153.5, 149.2, 147.5, 141.1,
140.9, 140.8, 131.2, 130.9, 130.5, 130.4, 125.5, 124.1, 122.0 ppm.
C24H13Cl2N4NaO9Re2S (999.74): calcd. C 28.83, H 1.31, N 5.60;
E(NHE) = E(SCE) + 0.24 = E(Ag/AgCl) + 0.28 =
E(Ag/AgClO4) + 0.66 (1)
[Re(CO)3(dab-Ph)Cl] (1): Compounds similar to 1 have been pre-
pared.[13–17,44,62,63] Aniline (0.026 g, 0.282 mmol) was mixed with
glyoxal (0.0081 g, 0.141 mmol) in methanol for 20 min. [Re-
(CO)5Cl] (0.050 g, 0.141 mmol) was added and the mixture was
heated to reflux for 2 h. Compound 1 was obtained as a solid by
evaporation of the reaction solvent, washed with diethyl ether, and
dried. Crystals suitable for X-ray diffraction were also produced by
slow evaporation of the reaction solvent, yield 62 mg (85%). 1H
NMR (300 MHz, [D6]DMSO): δ = 9.00 (s, 2 H, N=CH), 7.58 (m,
10 H, H on phenyl) ppm. 13C NMR (75 MHz, [D6]DMSO): δ =
197.2, 185.3, 168.5, 150.7, 130.2, 130.1, 122.7 ppm.
C17H12ClN2O3Re (513.95): calcd. C 39.73, H 2.35, N 5.45; found
C 39.71, H 2.22, N 5.61. IR (CO stretch): ν = 2019, 1909 cm–1. MS
˜
(ESI): calcd. for ReC17H12N2O3ClNa [M + Na+]: 536.99; found
536.98. UV/Vis (acetone): λmax (ε, m–1 cm–1) = 360 (8.3ϫ103),
474 nm (3.34ϫ103).
found C 30.71, H 1.87, N 5.63. IR (CO stretch): ν = 2022, 1894
˜
cm–1. MS (ESI): calcd. for Re2C24H13O9N4SCl2 [M – Na–]: 976.88;
found 976.55. UV/Vis (THF): λmax (ε, m–1 cm–1) = 317 (8.93ϫ103),
435 nm (1.26ϫ104⁾.
[Re(CO)3(pyca-Ph)Cl] (2): This previously reported com-
pound[43–45] can be prepared by using one-pot conditions. Aniline
(0.0131 g, 0.141 mmol) was mixed with pyridine-2-carboxyaldehyde
(0.0151 g, 0.141 mmol) in methanol for 20 min. [Re(CO)5Cl]
(0.050 g, 0.141 mmol) was then added and the mixture was heated
to reflux for 4 h. The solution was evaporated slowly to afford light
red crystals, which were suitable for X-ray diffraction. We obtained
a crystal structure of 2 in a new crystal form, yield 55 mg (80%).
UV/Vis (acetone): λmax (ε, m–1 cm–1) = 393 (5.55ϫ103), 414 nm
(5.00ϫ103).
Syntheses of [Re(CO)3X(m-PPC)Re(CO)3X] (6, X = Cl; 7, X = Br):
The procedure for the synthesis of 6 is representative of both com-
pounds.
Compound 6: [Re(CO)5Cl] (0.050 g, 0.141 mmol), m-phenylenedi-
amine (0.00762 g, 0.0704 mmol), and pyridine-2-carbaldehyde
(0.0151 g, 0.141 mmol) were mixed in methanol and heated to re-
flux for 4 h. The solution was evaporated slowly to yield crystals,
which were washed with diethyl ether and hexane, yield 33.3 mg
1
(53%). H NMR (300 MHz, [D6]DMSO): δ = 9.44 (2 H, N=CH),
Syntheses of [Re(CO)3X(p-PPC)Re(CO)3X] (3, X = Cl; 4, X = Br):
The procedure for the synthesis of 3 is representative of both com-
pounds.
9.10 (d, J = 5.1 Hz, 2 H, H on py), 8.40 (d, J = 6.0 Hz, 4 H, H on
py), 7.88 (m, 2 H, on py), 7.81–7.66 (two sets of m, 4 H on phenyl)
ppm. 13C NMR (75 MHz [D6]DMSO): δ = 197.5, 196.8, 187.1,
170.9, 154.8, 153.2, 151.0, 140.6, 130.9, 130.2, 122.8, 116.3,
116.0 ppm. C24H14Cl2N4O6Re2 (897.70): calcd. C 32.11, H 1.57, N
Compound 3: [Re(CO)5Cl] (0.050 g, 0.141 mmol), p-phenylenedi-
amine (0.00762 g, 0.0704 mmol), and pyridine-2-carbaldehyde
(0.0151 g, 0.141 mmol) were heated to reflux in methanol (25 mL)
for 4 h. The resultant solid product was filtered and washed with
diethyl ether for characterization, yield 54 mg (84%). 1H NMR
(300 MHz, [D6]DMSO): δ = 9.47 (s, 2 H, N=CH), 9.10 (d, J =
5.1 Hz, 2 H, H on py), 8.40 (d, J = 4.2 Hz, 4 H, H on py), 7.88
(dd, J = 3.0 Hz, 2 H, on py), 7.79 (s, 4 H, on phenyl) ppm. 13C
NMR (75 MHz, [D6]DMSO): δ = 197.5, 196.8, 187.4, 170.8, 154.9,
153.1, 150.4, 140.6, 130.7, 130.1, 123.5 ppm. C24H14Cl2N4O6Re2
(897.70): calcd. C 32.11, H 1.57, N 6.24; found C 32.34, H 1.73, N
6.24; found C 32.29, H 1.48, N 6.54. IR (CO stretch): ν = 2022,
˜
1903, 1879 cm–1. MS (ESI): calcd. for Re2C24H14O6N4Cl2Na [M +
Na+]: 920.93; found 920.93. UV/Vis (DMF): λmax (ε, m–1 cm–1) =
300 (2.89ϫ104), 410 nm (8.64ϫ103).
Compound 7: Yield 64.2 mg (51%). 1H NMR (300 MHz, [D6]-
DMSO): δ = 9.42 (s, 2 H, N=CH), 9.13 (d, J = 5.4 Hz, 2 H, H on
py), 8.40 (m, 4 H, H on py), 7.86 (m, 2 H, on py), 7.83–7.71 (m, 4
H on phenyl) ppm. 13C NMR (75 MHz, [D6]DMSO): δ = 197.5,
196.8, 187.0, 171.3, 155.2, 153.9, 151.5, 141.0, 131.4, 131.3, 130.5,
6.32. IR (CO stretch): ν = 2016, 1886 cm–1. MS (ESI): calcd. for
˜
Re2C24H14O6N4Cl2Na [M + Na+]: 920.93; found 920.94. UV/Vis 123.3, 117.1 ppm. C24H14Br2N4O6Re2 (986.61): calcd. C 29.11, H
(DMF): λmax (ε, m–1 cm–1) = 341 (1.80ϫ104), 390 nm (8.93ϫ103).
1.43, N 5.67; found C 29.82, H 1.33, N 5.89. IR (CO stretch): ν =
˜
Eur. J. Inorg. Chem. 2014, 2643–2652
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2020, 1910 cm–1. MS (ESI): calcd. for C24H14N4O6Re2Br2Na [M +
Na+]: 1008.82; found 1008.82. UV/Vis (DMF): λmax (ε, m–1 cm–1) =
303 (2.17ϫ104), 420 nm (6.38ϫ103).
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Acknowledgments
R. S. H. would like to thank the Petroleum Research Fund (grant
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