The synthesis of biologically relevant conjugates of Re(CO)3 using pyridine-2-carboxyaldehyde

Article abstract of DOI:10.1016/j.jorganchem.2012.10.025

The new pyridine-2-carboxaldehyde adduct, Re(CO)₃(NC ₆H₅C(O)H)Cl 1, and previously reported complex Re(CO) ₃(NC₆H₅C(O)H)Br 2 react with aniline derivatives sulfanilamide or 4-aminofluorescein in methanol giving Schiff base conjugates Re(CO)₃(pyca-R)X (pyca = pyridinecarbaldehyde imine, X = Cl, Br), 3-6. Pre-isolation of compounds 1 and 2 provides a convenient method for preparing conjugate complexes in addition to the known methods of ligand synthesis and one-pot reactions. All new compounds were completely characterized, and compound 1 and the sulfanilamide derivatives 3 and 4 were structurally elucidated by X-ray crystallography.

Full text of DOI:10.1016/j.jorganchem.2012.10.025

Journal of Organometallic Chemistry 734 (2013) 25e31
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
The synthesis of biologically relevant conjugates of Re(CO)3 using pyridine-2-
carboxyaldehyde
,
Roshinee Costa a, Kullapa Chanawanno a, James T. Engle a, Bertha Baroody b, Richard S. Herrick b
*
,
,
Christopher J. Ziegler a
*
a Department of Chemistry, University of Akron, Akron, OH 44325-3601, USA
b Department of Chemistry, College of the Holy Cross, Worcester, MA 01610, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The new pyridine-2-carboxaldehyde adduct, Re(CO)3(NC6H5C(O)H)Cl 1, and previously reported complex
Re(CO)3(NC6H5C(O)H)Br 2 react with aniline derivatives sulfanilamide or 4-aminofluorescein in meth-
anol giving Schiff base conjugates Re(CO)3(pyca-R)X (pyca ¼ pyridinecarbaldehyde imine, X ¼ Cl, Br), 3
e6. Pre-isolation of compounds 1 and 2 provides a convenient method for preparing conjugate
complexes in addition to the known methods of ligand synthesis and one-pot reactions. All new
compounds were completely characterized, and compound 1 and the sulfanilamide derivatives 3 and 4
were structurally elucidated by X-ray crystallography.
Received 22 August 2012
Received in revised form
11 October 2012
Accepted 12 October 2012
Keywords:
Rhenium
Carbonyl
Ó 2012 Elsevier B.V. All rights reserved.
Crystal structures
Sulfonamide
Fluorescein
Metal conjugate
1. Introduction
of the compounds examined for this purpose contain polypyridyl
units or dyes as ligands [11e13].
The biological chemistry of organometallic rhenium compounds
has attracted research interest in recent years [1e3]. Initially, the
goal was to examine cold analogs of technetium-99m, which is the
most commonly used nuclide in nuclear medicine with 50,000
clinical applications per day in the US [4]. The creation of a kit for the
preparation of 99mTc(CO)3(H2O)þ3 [5], opened up the possibility of
99mTc(CO)þ3 radiopharmaceuticals [6]. As technetium has no stable
nuclides, exploring the similar chemistry of its higher congener is
a convenient method of testing new targeting strategies for
researchers searching for target specific radiopharmaceuticals.
More recently, the field has been expanded by the development of
a carrier-free source of rhenium-188 (half-life of 17 h with
bꢀ ¼ 2.1 MeV) and the production of a clinically appropriate method
for the preparation of 188Re(CO)3(H2O)þ3 [7e9]. The possibility of
creating a matched pair of radiopharmaceuticalsdone for diagnosis
and one for therapydthat would target the same location is an
attractive goal for clinical medicine [10]. Similarly, d6 rhenium
compounds have been tested as in vivo luminescence agents. Many
We and others have frequently made use of Schiff base
condensation reactions to create d6 rhenium compounds with
diimine-based bioconjugates formed by the reaction of pyridine-2-
carboxaldehyde with amino acid esters [14,15], amino acids
[16], peptides [17] or amino alcohols [18]. Reactions of Re(CO)3þ
precursors and pyridine-2-carboxaldehyde with amino acid esters
yielded discrete mononuclear complexes, Re(CO)3(pyca-Xxx-OR)
Cl (pyca ¼ pyridinecarbaldehyde imine, Xxx ¼ amino acid), while
amino acids or peptides yielded C2-symmetric dimers,
[Re(CO)3(pyca-Xxx-O)]2 or complex structures with extended
hydrogen bonding networks. After the application of a strong base,
amino alcohol conjugates produced a dimetallacycle with a proton
strongly bonded between the two alkoxo oxygen atoms.
These metal Schiff base compounds have typically been made in
one-pot reactions [16], as the iminopyridine ligands are not easy to
isolate and use in further reactions. With the report on the
synthesis of 2, a bidentate adduct of pyridine-2-carboxaldehyde,
we began looking to explore this reagent to expand the scope of
amines used to create d6 rhenium Schiff base complexes. We chose
to focus on examining the reactivity of 2 and its chloride analog
with substituted anilines, inspired by the work of Miguel and co-
workers [19].
* Corresponding authors.
E-mail address: rherrick@holycross.edu (R.S. Herrick).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2012.10.025

26
R. Costa et al. / Journal of Organometallic Chemistry 734 (2013) 25e31
There has been a significant amount of work on the synthesis
2. Results and discussion
and characterization of N-phenyl-2-pyridylimino Re(CO)3
complexes. Since the early 1980s, both the chloride and the
bromide variants of the unsubstituted N-phenyl-2-pyridylimino
compounds have been prepared, typically by first separately
synthesizing the pyridyl imine followed by chelation to the metal
ion [20e22]. Alternatively, one-pot methods can be used; we
employed such a method to generate Re(CO)3 complexes incorpo-
rating pyca-amino acid conjugates. Using the older method of pre-
generating the pyridyl imine ligand, a variety of functional groups
can be introduced on the phenyl ring, including alkyl groups
[21,23,24], hydroxides [25], halides [26,27], additional phenyls
[23,28], and crown ethers [29,30]. Miguel used compound 2 to not
only generate the phenyl species, but also analogs with carboxylic
acids and alkynes substituted on the phenyl ring [19,31].
For this study, we chose to prepare complexes prepared from
sulfanilamide and 4-aminofluorescein. Sulfonamide compounds
have found use as inhibitors of carbonic anhydrase, and thus have
been used as therapeutics for glaucoma, hypertension, and for use
as diuretics [32]. The discovery of isozymes of carbonic anhydrase
that are overexpressed in certain tumors has also encouraged
researchers to create new sulfonamide compounds with applica-
2.1. Syntheses
The synthesis of compound 2 using an extended reflux of the
aldehyde and one equivalent of Re(CO)5Br in hexane was first re-
ported by Miguel and co-workers [15]. We found that the synthesis
also works well in toluene, producing pure product that can be
isolated and stored as a reagent for later reactions. We also
prepared the new compound 1 by this same method. Both
complexes were prepared in yields of greater than 80%. Compounds
3e6 were prepared as orange solids by methanol reflux of the
appropriate aldehyde complex with one equivalent of the amine.
Sulfonamide derivatives 3 and 4 were obtained as pure compounds
by slow evaporation of the reaction solution. Fluorescein conjugate
compounds 5 and 6 were insoluble. After precipitating from the
solution during the reaction, they were washed and dried. Single
crystals of 1, 3 and 4 were grown using either diffusion of hexane
into a CH2Cl2 solution in the case of 1 or by slow evaporation of
a methanol solution in the cases of 3 and 4.
Preparations of all reported compounds are insensitive to the
presence of atmospheric oxygen and water vapor. No special
precautions were taken in these syntheses.
tions such as targeting by luminescence or g-emission [33,34]. The
publication of the crystal structure of carbonic anhydrase with the
metal complex, CpRRe(CO)3 (R ¼ arylsulfonamide) bound to the
zinc ion at the active site highlights the interest in this area [35,36].
Although 4-aminofluorescein is not fluorescent, the parent fluo-
rescein molecule finds heavy use in cellular imaging, and the
development of Re(CO)þ3 based luminescent molecules has been
a goal of many research groups [11e13]. We were curious whether
incorporating the exocyclic amine of 4-aminofluorescein into
a metal-bound diimine ligand would return the fluorescent prop-
erties of this chromophore.
2.2. Description of crystal structures
The structures of compounds 1, 3 and 4 were elucidated by
single crystal X-ray diffraction. The molecular structures are shown
in Figs. 1 and 2. Crystal data and experimental details are listed in
Table 1. Table 2 lists some of the key bond lengths and angles in
these complexes. All of the complexes have slightly distorted
octahedral geometries, with the three carbonyls arranged in a facial
configuration as expected for d6 Re(CO)3 compounds.
In this report we present the reaction of sulfanilamide and 4-
aminofluorescein with Re(CO)3(C5H4C(O)H)X; X ¼ Cl, Br (1 and
previously prepared 2, respectively) to form new compounds 3e6
(see Scheme 1 for numbering of compounds). The structures of
both sulfanilamide products, 3 and 4, as well as 1, were elucidated
by X-ray crystallography. Compounds 5 and 6 each regained fluo-
rescence intensity upon incorporation of the exocyclic amine into
the rhenium-bound diimine. The new pyridine aldehyde complex 1
and all of the new pyridyl imine compounds were completely
characterized.
Compound 1 is nearly isostructural to 2. The aldehyde and
pyridine units form a planar bidentate chelate to the metal ion with
ꢀ
ꢀ
a ReeN distance of 2.177(5) A and a ReeO distance of 2.172(4) A.
The carbonyls adopt the expected facial geometry and exhibit
standard bond lengths and angles for Re(CO)3 compounds. The Ree
ꢀ
Cl bond length (2.4792(17) A) is typical for the Re(CO)3Cl
compounds, such as those that we previously observed in the
similarly structured oxime and dioxime complexes [37]. The CeO
ꢀ
bond length of the aldehyde (1.241(7) A) is slightly lengthened
versus that seen in free pyridine aldehydes, indicative of back
Scheme 1. General reaction scheme and numbering system for compounds 1e6.

R. Costa et al. / Journal of Organometallic Chemistry 734 (2013) 25e31
27
interaction with the sulfonamide nitrogen with a O/N distance of
ꢀ
w2.85 A. In addition, there is a second hydrogen bond observed
between the sulfonamide NH2 group and the rhenium-bound
chloride of a neighboring compound. This second interaction has an
ꢀ
N/Cl distance of w3.26 A. For compound 4, the solvent methanol
was modeled as a diffuse contribution to the difference map
without assigning specific atom positions using the SQUEEZE
program, so we were not able to discern any solid state hydrogen
bonding interactions between 4 and solvent. However, due to the
alternate packing structure in 4, no hydrogen bonding between the
sulfonamide NH2 group and the metal-bound bromide is observed.
We speculate that the differences in packing between the two
structures (and the observation of two different space groups for
essentially isostructural molecules) may result from the presence of
the NHeCl hydrogen bond in 3 and the absence of an analogous
NHeBr interaction in 4.
2.3. Spectroscopic characterization
Fig. 1. The structure of compound 1 with 35% thermal ellipsoids. Hydrogen atoms have
been omitted for clarity.
IR spectra of the new compounds displayed two metal carbonyl
bands between about 2010 and 1840 cmꢀ1. This is consistent with
vibrations analogous to a1 and e modes resulting from a pseudo-C3v
symmetry for the compounds. Compound 1 also exhibits a carbonyl
stretch from the metal-bound aldehyde unit at 1662 cmꢀ1. This
value is significantly altered from that of free pyridine-2-
carboxyaldehyde, which appears at 1730 cmꢀ1 [38].
bonding to the carbonyl oxygen. Miguel and co-workers see a
ꢀ
similar CeO bond length of 1.223(8)
A
for the bromide
derivative [15].
Complexes 3 and 4 were each characterized by single crystal X-
ray crystallography; the molecular structures both of these
compounds are shown in Fig. 2. Compounds 3 and 4 maintain
a planar bidentate ligand, a facial set of carbonyls and a ReeX bond
perpendicular to the pyridyl imine chelate. The bond distances and
angles around the rhenium metal centers in these two compounds
are as expected for pyridyl imine Re(CO)3 compounds. The imine
The NMR spectra of each compound were consistent with the
observed structures for these complexes. The spectrum of
compound 1 provided important information confirming its iden-
tity. The 1H NMR spectrum showed a singlet at 9.98 ppm that is
diagnostic of the aldehyde proton, and pyridine protons show up in
expected locations. The 13C NMR spectrum showed a peak at
192.6 ppmdappearing in the middle of the metal carbonyl
resonancesddue to the aldehyde carbon atom. As expected, the 1H
ꢀ
bonds in 3 and 4 are longer (1.284(10) and 1.303(9) A, respectively)
than the corresponding aldehyde carbonyl bonds in 1 and 2, due to
the increased
p back bonding to the diimine unit. The two
and 13C spectra of 1 closely match the spectra of 2. The 1H and 13
C
compounds also exhibit different orientations of the phenyl ring of
the sulfanilamide group; in compound 4, the plane of the ring
phenyl ring is out of the plane of the pyridyl imine chelate by w41ꢁ,
whereas in compound 3 the two planes are more orthogonal, with
an angle of w76ꢁ. Compounds 5 and 6 did not produce single
crystals despite numerous attempts and were characterized by
spectroscopic methods. All CO bond lengths fall within the normal
range as expected for Re(CO)3 compounds.
NMR spectra of compounds 3 and 4 displayed peaks due to the
amide and phenyl groups in the regions expected. Spectra for each
compound show peaks for the pyridine and imine functional
groups in the expected regions. As typically observed for similar
metal complexes, the protons closest to the metal are shifted
downfield more than their neighbors.
As the structures of compounds 5 and 6 could not be elucidated
by X-ray crystallography, NMR spectra were carefully analyzed and
mass spectrometry was employed to verify their identities. In
In compound 3, we were able to refine on a solvent methanol in
the crystal structure. This methanol engages in a hydrogen bonding
Fig. 2. The structures of 3 (left) and 4 (right) with 35% thermal ellipsoids. Hydrogen atoms have been omitted for clarity.

28
R. Costa et al. / Journal of Organometallic Chemistry 734 (2013) 25e31
Table 1
ꢀ
X-ray data collection and structure parameters for compounds 1, 3, and 4. Measurements were made at 100(2)K using Mo Ka radiation (
l
¼ 0.71073 A).
Compound
1
3
4
Empirical formula
Formula weight
Crystal system
Space group
Unit cell dimensions
C9H5ClNO4Re
412.79
Monoclinic
P2(1)/c
C16H10ClN3O6ReS
593.98
C15H11BrN3O5ReS
611.44
Monoclinic
C2/c
Rhombohedral
R-3
ꢀ
ꢀ
ꢀ
a ¼ 10.533(5) A
a ¼ 25.649(13) A
a ¼ 36.910(5) A
a
¼ 90ꢁ
a
¼ 90ꢁ
a
¼ 90ꢁ
ꢀ
ꢀ
ꢀ
b ¼ 8.880(4) A
b ¼ 10.666(6) A
b ¼ 36.910(5) A
b
¼ 91.130(7)ꢁ
b
¼ 91.639(6)ꢁ
b
¼ 90ꢁ
ꢀ
ꢀ
ꢀ
c ¼ 11.506(5) A
c ¼ 14.356(8) A
c ¼ 8.0828(10) A
g
¼ 90ꢁ
g
¼ 90ꢁ
g
¼ 120ꢁ
3
ꢀ
3
ꢀ
3
ꢀ
Volume
Z
Density (calculated)
Absorption coefficient
F(000)
Crystal size
Theta range for data collection
Index ranges
1076.0(8) A
4
3926(4) A
8
9536(2) A
18
2.548 mg/m3
2.010 mg/m3
1.916 mg/m3
11.537 mmꢀ1
6.471 mmꢀ1
7.745 mmꢀ1
760
2264
5184
0.17 ꢂ 0.14 ꢂ 0.09 mm3
0.10 ꢂ 0.10 ꢂ 0.03 mm3
0.30 ꢂ 0.08 ꢂ 0.04 mm3
1.93ꢁe26.99ꢁ
1.59ꢁe27.57ꢁ
1.91ꢁe27.52ꢁ
ꢀ13 <¼ h <¼ 13,
ꢀ11 <¼ k <¼ 11,
ꢀ14 <¼ l <¼ 14
8689
ꢀ32 <¼ h <¼ 33,
ꢀ13 <¼ k <¼ 13,
ꢀ18 <¼ l <¼ 18
15,759
ꢀ47 <¼ h <¼ 47,
ꢀ47 <¼ k <¼ 47,
ꢀ10 <¼ l <¼ 10
26,228
Reflections collected
Independent reflections
Completeness to theta
Absorption correction
Max. and min. transmission
Refinement method
2352 [R(int) ¼ 0.0582]
4418 [R(int) ¼ 0.0755]
4851 [R(int) ¼ 0.0550]
99.9%
97.1%
99.1%
SADABS
SADABS
SADABS
0.4233 and 0.2445
Full-matrix least-squares on F2
2352/0/145
0.8295 and 0.5639
Full-matrix least-squares on F2
4418/0/256
0.7470 and 0.2047
Full-matrix least-squares on F2
4851/0/236
Data/restraints/parameters
Goodness-of-fit on F2
0.91
1.001
0.923
Final R indices [I > 2sigma(I)]
R indices (all data)
Largest diff. peak and hole
R1 ¼ 0.0294, wR2 ¼ 0.0574
R1 ¼ 0.0461, wR2 ¼ 0.1012
R1 ¼ 0.0402, wR2 ¼ 0.1189
R1 ¼ 0.0395, wR2 ¼ 0.0585
R1 ¼ 0.0614, wR2 ¼ 0.1107
R1 ¼ 0.0496, wR2 ¼ 0.1274
ꢀꢀ3
ꢀꢀ3
ꢀꢀ3
2.231 and ꢀ0.935 e A
1.863 and ꢀ2.189 e A
1.653 and ꢀ1.334 e A
addition to the expected peaks for the pyridine and imine groups in
both the 1H and 13C NMR spectra, the compounds showed the
appropriate number of peaks in proper locations, consistent with
a modified 4-aminofluorescein moiety. In addition, 13C NMR data
established the formation of compound 5 and 6 as observed by the
conversion of the aldehyde (C(H)]O) to an imine via the reaction
with 4-fluoresceinamine, as shown in Fig. 3. The aldehyde carbon
atom shifts from 192.6 ppm (top spectrum) in 1 to 159.7 ppm in 5.
Compound 6 shows a similar resonance for its imine carbon at
160.2 ppm. Comparison of the actual sample isotope pattern
recorded for the mass spectra for 5 and 6 tothe simulated (expected)
isotope pattern (Fig. 4) confirms the identity of these compounds.
The UVevisible spectra for 5 and 6 were measured, and as ex-
pected the spectra are dominated by fluorescein-based transitions.
The spectra of both compounds along with that of 4-
aminoflurescein are shown in Fig. 5. We determined the extinc-
tion coefficients for the primary absorption peak to be
Table 2
Selected bond lengths (A) and bond angles ( ) for 1, 3, and 4.
ꢁ
ꢀ
1
3
4
Bond lengths
Re(1)eC(1)
Re(1)eC(2)
Re(1)eC(3)
Re(1)eN(1)
Re(1)eCl(1)
Re(1)eBr(1)
Re(1)eO(4)
Re(1)eN(2)
C(9)eO(4)
1.896(7)
1.903(6)
1.908(7)
2.177(5)
2.4792(17)
2.172(4)
1.241(7)
1.916(8)
1.909(8)
1.935(8)
2.187(6)
2.499(2)
1.926(7)
1.922(6)
1.918(6)
2.170(5)
2.6294(7)
2.187(6)
1.303(9)
2.161(6)
C(9)eN(2)
1.278(10)
Bond angles
C(1)eRe(1)eC(3)
C(1)eRe(1)eC(2)
C(3)eRe(1)eC(2)
C(3)eRe(1)eO(4)
C(2)eRe(1)eO(4)
C(3)eRe(1)eN(2)
C(2)eRe(1)eN(2)
C(2)eRe(1)eN(1)
C(1)eRe(1)eN(1)
O(4)eRe(1)eN(1)
N(2)eRe(1)eN(1)
C(1)eRe(1)eCl(1)
C(3)eRe(1)eCl(1)
O(4)eRe(1)eCl(1)
N(1)eRe(1)eCl(1)
N(2)eRe(1)eCl(1)
C(1)eRe(1)eBr(1)
C(3)eRe(1)eBr(1)
N(1)eRe(1)eBr(1)
N(2)eRe(1)eBr(1)
90.1(3)
89.7(2)
87.5(2)
97.6(2)
91.52(19)
90.1(3)
88.1(3)
88.9(3)
89.1(3)
86.9(3)
90.3(3)
7.96
ꢂ
104 Mꢀ1 cmꢀ1 at 500 nm for compound
5 and
6.54 ꢂ 104 Mꢀ1 cmꢀ1 at 497 nm for compound 6. These molar
absorptivities are in the same range as that seen in unmodified 5-
aminoflurescein. Compounds 5 and 6 each fluoresce when the
absorption maxima at 500 nm and 497 nm, respectively, are irra-
diated. Fig. 6 shows the excitation and emission spectra of the two
fluorescein conjugate complexes. The emission resembles that of
normal fluorescein in both profile and Stokes shift. When compared
to normal fluorescein, which has a quantum yield of 0.79 [39], the
quantum yield of emission is significantly reduced, measuring
w0.038 for both compounds 5 and 6. The reduction in quantum
yield can be directly attributed to the heavy atom effect due to the
close proximity of the rhenium and the halide. However, the
emission of these two compounds is increased relative to that of 4-
aminofluorescein, which is non-emissive. The formation of the
Schiff base and metal coordination of the amine nitrogen of 4-
96.7(3)
97.2(3)
94.2(3)
98.1(3)
91.7(2)
100.8(2)
94.3(2)
97.5(2)
92.4(2)
97.4(2)
74.91(17)
74.9(2)
92.4(2)
93.1(2)
74.8(2)
95.40(18)
94.31(18)
83.17(11)
85.15(12)
83.76(16)
82.08(17)
93.74(19)
92.39(19)
82.77(14)
85.19(14)

R. Costa et al. / Journal of Organometallic Chemistry 734 (2013) 25e31
29
Intens.
[%]
Intens.
[%]
743.0
60
743.0
100
80
40
60
40
20
741.0
741.0
741
20
0
744.0 745.0
745.0
745
744.0
744
742.0
742
742.0
742
746.0
746
746.0
746
747.0
747
0
740
743
747 m/z
740
741
743
744
745
m/z
Intens.
2000
Intens
.
x105
4
787.0
787.0
1500
1000
500
0
3
2
789.0
789.0
785.0
788.0
785.0
788.0
1
0
790.0
790.0
790
786.0
786
786.0
786
785
787
788
789
790 m/z
785
787
788
789
m/z
Fig. 3. Top: predicted (left) and observed (right) ESI MS of 5. Bottom: predicted (left) and observed (right) ESI MS of 6.
aminofluorescein restores the fluorescence; the restoration of
fluorescence upon metal binding of exocyclic amines is a well-
known property of fluorescein-based dyes [40e43].
conjugate complexes. The metal-bound aldehyde readily reacts
with amines such as anilines to afford imines. In the current study,
we have successfully generated conjugates incorporating sulfanil-
amide and 4-aminofluorescein, producing the corresponding
Re(CO)3(pyca-R)X compound with the pyridyl imine covalently
linked to the desired biological molecule. Our work continues with
these compounds and similar biologically relevant conjugates of
Re(CO)3.
2.4. Conclusions
We have found that pyridine aldehyde-bound rhenium(I)
carbonyls can be used to readily generate biologically relevant
Fig. 4. 13C NMR spectra for 1 (top), 5 (middle) and 6 (bottom). The arrow shows the shift of the aldehyde carbon resonance for 1 relative to the imine carbon resonances for 5 and 6.

30
R. Costa et al. / Journal of Organometallic Chemistry 734 (2013) 25e31
on py). 13C NMR (d6-DMSO,
d ppm) 196.3, 193.7, 192.6, 152.30, 150.3,
137.7, 128.4, 121.68. ESI MS (positive ion) [M ꢀ Cl]þ: 377.9 m/z Calc.
377.35 m/z; CHN Anal. Calc. for ReC9H5O4NCl: C, 26.19; H, 1.22; N,
3.39. Found: C, 25.87; H, 0.99; N, 3.20. IR (CO stretch, cmꢀ1):
2023(s), 1892(s), 1662(m).
3.3. Synthesis of 3 and 4
Preparation of 3: 1 (0.050 g, 0.12 mmol) was mixed with one
equivalent of sulfanilamide (0.020 g, 0.12 mmol) and 10 mL
methanol was then added to the flask. The mixture was heated to
reflux. During the 6 h reflux, the red solution turned to a dark
orangeered color. The solution volume was decreased by slow
evaporation and an orange solid was collected by filtration. The
product was washed with diethyl ether and dried. Crystals suitable
for characterization by X-ray diffraction were formed by slow
evaporation of methanol.
Fig. 5. UVevisible absorption spectra for 5, 6, fluorescein, and 4-aminofluorescein.
3. Experimental
Compound 3: Yield 56 mg, 82%. 1H NMR (d6-DMSO,
d ppm): 9.41
3.1. Materials and methods
(s, 1H, N]CH), 9.10 (d, J ¼ 5.5 Hz, 1H, H on py), 8.39 (d, J ¼ 3.5 Hz,
1H, H on py), 8.03 (m, 2H, H on C6H4), 7.88 (m, 2H, H on py), 7.72 (d,
J ¼ 9.0 Hz, 2H, H on C6H4), 7.51 (s, 2H, NH2). 13C NMR (d6-DMSO,
All reagents were purchased from Strem, Acros Organics or
SigmaeAldrich and used as received. NMR spectroscopy was per-
formed with Varian VXR 300 MHz and Varian 500 MHz NMR
instruments. Elemental analyses were carried out at the School of
Chemical Sciences Microanalytical Laboratory at the University of
Illinois at Urbana-Champaign. 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. IR spectra were recorded on a Nicolet Series II Magna-
IR 750 spectrometer. UVevisible spectroscopy was carried out on
a Hitachi U-3010 spectrometer. Fluorescence measurements were
made on a Horiba Jobin Yvon FluoroMax-4 spectrofluorometer.
Fluorescence quantum yield measurements were made relative to
normal fluorescein as a standard [44].
d
ppm) 197.9, 197.3, 187.7, 171.9, 155.3, 153.7, 153.1, 144.7, 141.1, 131.3,
130.7, 127.6, 123.4, 112.9, 105.0. ESI MS (positive ion) [M þ Na]þ:
589.9 m/z Calc. 589.9 m/z; CHN Anal. Calc. for ReC15H11O5N3SCl: C,
31.78; H, 1.96; N, 7.41. Found: C, 31.74; H, 1.99; N, 7.29. IR (CO
stretch, cmꢀ1): 2021(m), 1895(s).
Preparation of 4: This compound was synthesized via an iden-
tical procedure as 3. Crystals for structural analysis were grown by
slow evaporation from methanol.
Compound 4: Yield 57 mg, 85%. 1H NMR (d6-DMSO,
d ppm): 9.39
(s,1H, N]CH), 9.13 (d, J ¼ 5.5 Hz,1H, H on py), 8.39 (m, 1H, H on py),
8.03 (d, J ¼ 7 Hz, 2H, H on C6H4), 7.87 (m, 2H, H on py), 7.73 (d,
J ¼ 5 Hz, 2H, H on C6H4), 7.51 (s, 2H, NH2). 13C NMR (d6-DMSO,
d
ppm) 197.4, 196.9, 187.1, 171.8, 155.3, 153.9, 153.21, 144.7, 141.0,
3.2. Synthesis of 1 and 2
131.4, 130.6, 127,8, 127.6, 123.4, 112.9 ESI MS (positive ion):
[M þ Na]þ: 633.9 m/z Calc. 634.4 m/z; CHN Anal. Calc. for
ReC15H11O5N3SBr: C, 29.46; H, 1.81; N, 6.87. Found: C, 29.01; H, 1.64;
N, 6.66. IR (CO stretch, cmꢀ1): 2023(m), 1897(s).
Compound 1 and previously reported 2 [15] were prepared by
similar methods. No special precautions were taken as the
synthetic procedure and the resulting products are not air or
moisture sensitive. In a typical reaction, Re(CO)5Cl (0.200 g,
0.552 mmol) was mixed with one equivalent of pyridine-2-
carboxaldehyde (0.0590 g, 0.552 mmol) and 15 mL of toluene.
The mixture was then heated to reflux for 24 h, slowly causing the
solution to turn orangeered. Upon completion of the reaction, a red
product was collected by filtration. The product was washed with
diethyl ether and dried. Crystals suitable for X-ray diffraction were
grown by vapor diffusion of pentane into a CH2Cl2 solution of the
compound.
3.4. Synthesis of 5 and 6
Preparation of 5: Compound 1 (50 mg, 0.12 mmol) was mixed
with one equivalent of 4-aminofluorescein (42 mg, 0.12 mmol) and
25 mL methanol was added to the flask. The mixture was heated to
reflux. During the 6 h reflux, an orange precipitate formed which
was collected by filtration. The product was washed with cold
methanol and dried.
Compound 5: Yield 81 mg, 91%. 1H NMR (d6-DMSO,
d ppm):
Compound 1: Yield 132 mg, 58%. 1H NMR (d6-DMSO,
d ppm):
10.23 (br s, 2H OH), 9.47 (s,1H, N]CeH), 9.10 (d, J ¼ 5.6 Hz,1H, H on
py), 8.39 (m, 2H, H on Ar), 8.14 (d, 2.0 Hz, 1H, H on Ar), 7.89 (m, 1H H
on py), 7.72 (m,1H, H on py), 7.51 (d, J ¼ 8.1 Hz,1H, H on Ar), 6.70 (m,
2H, H on Ar), 6.64 (d, J ¼ 8.1 Hz, 1H, H on Ar), 6.63 (m, 3H, H on Ar).
9.98 (s, 1H, (CHO)), 8.83 (d, J ¼ 4.2 Hz, 1H, H on py), 8.06 (t,
J ¼ 7.5 Hz,1H, H on py), 7.94 (d, J ¼ 7.5 Hz,1H, H on py), 7.71(m,1H, H
13C NMR (d6-DMSO,
d ppm) 197.8, 197.4, 187.8, 172.7, 168.1, 160.2,
155.3, 153.7, 152.8, 152.3, 141.2, 131.4, 130.8, 130.6, 129.5, 129.4,
127.9, 126.0, 118.2, 113.2, 109.6, 102.8, 83.6. ESI MS (positive ion)
(M
þ
H): 743.0 m/z Calc. 743.1 m/z; CHN Anal. Calc. for
ReC29H16O8N2Cl$0.5H2O: C, 46.37; H, 2.18; N, 3.73. Found: C, 46.16;
H, 1.96; N, 3.70. IR (CO stretch, cmꢀ1): 2017(m), 1920(s), 1862(s).
Preparation of 6: This compound was synthesized via an iden-
tical procedure as 5.
Compound 6: Yield 76 mg, 89%. 1H NMR (d6-DMSO,
d ppm):
10.19 (br s, 2H OH), 9.47 (s, 1H, N]CeH), 9.12 (d, J ¼ 5.6 Hz, 1H, H on
py), 8.38 (m, 2H, H on Ar), 8.14 (d, 2.0 Hz,1H, H on Ar), 8.00 (m, 1H H
on py), 7.85 (m, 1H, H on py), 7.53 (d, J ¼ 8.1 Hz, 1H, H on Ar), 6.67
(m, 2H, H on Ar), 6.63 (d, J ¼ 8.1 Hz, 1H, H on Ar), 6.60 (m, 3H, H
Fig. 6. Excitation and emission spectra for 5, 6, and fluorescein.

R. Costa et al. / Journal of Organometallic Chemistry 734 (2013) 25e31
31
on Ar). 13C NMR (d6-DMSO,
d ppm) 196. 9, 196.10, 186.7, 172.2, 167.7,
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159.7, 154.8, 153.4, 152.3, 152.0, 151.8, 140.6, 131.0, 130.2, 129.1,
128.9, 127.4, 125.5, 117.8, 112.8, 109.0, 102.4, 83.6. ESI MS (positive
ion) (M þ H): 787.0 m/z Calc. 787.5 m/z; CHN Anal. Calc. for
ReC29H16O8N2Br$0.5H2O: C, 43.78; H, 2.15; N, 3.52. Found: C, 43.75;
H, 1.84; N, 3.46; IR (CO stretch, cmꢀ1): 2018(m), 1927(s), 1868(s).
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3.5. X-ray structure determination of 1, 3 and 4
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FLEX) on a Bruker SMART APEX CCD-based X-ray diffractometer
ꢀ
system equipped with a Mo-target X-ray tube (
l
¼ 0.71073 A)
operated at 2000 W power. The crystals were mounted on a cry-
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nitrogen at 100 K. The detector was placed at a distance of 5.009 cm
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Acknowledgments
RSH would like to thank the Research Corporation (CC6663/
6616) and the Petroleum Research Fund (51085-UR3) for financial
support of this research. CJZ acknowledges the National Institutes
of Health (R15 GM083322) for funds used in this work.
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Appendix A. Supplementary material
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[38] AIST: Integrated Spectral Database System of Organic Compounds. (Data were
obtained from the National Institute of Advanced Industrial Science and
Technology (Japan).)
CCDC 895955e895957 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
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