Structure and photophysics of a Schiff base-bipyridine ligand and its rhenium(I) tricarbonylchloro complex

Article abstract of DOI:10.1016/j.ica.2009.04.034

A new Schiff base-bipyridine ligand, [4-(4′-methyl)-2,2′-bipyridyl)imine]-2-hydroxybenzene, was prepared, characterized and its X-ray crystal structure obtained. The rhenium(I) tricarbonylchloro complex of this novel derivative of 2,2′-bipyridine was also

Full text of DOI:10.1016/j.ica.2009.04.034

Inorganica Chimica Acta 362 (2009) 3872–3876
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Note
Structure and photophysics of a Schiff base-bipyridine ligand and its rhenium(I)
tricarbonylchloro complex
Tenzing Doleck a, Jonathan Attard a, Frank R. Fronczek b, Amy Moskun a, Ralph Isovitsch a,
*
a Department of Chemistry, Whittier College, 13406 Philadelphia Street, Whittier, CA 90608, United States
b Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, United States
a r t i c l e i n f o
a b s t r a c t
A new Schiff base-bipyridine ligand, [4-(40-methyl)-2,20-bipyridyl)imine]-2-hydroxybenzene, was pre-
pared, characterized and its X-ray crystal structure obtained. The rhenium(I) tricarbonylchloro complex
of this novel derivative of 2,20-bipyridine was also prepared and characterized. The photophysics of both
of these compounds were explored. The absorption spectrum of the Schiff base in acetonitrile possessed
Article history:
Received 16 January 2009
Received in revised form 7 April 2009
Accepted 17 April 2009
Available online 3 May 2009
bipyridine based
p ? p
* transitions at 246 and 278 nm along with a phenolic charge transfer absorption
at 360 nm. Acetonitrile solutions of this compound were found to be luminescent at room temperature
with an emission maximum at 435 nm. The rhenium(I) metal complex prepared from the Schiff base
exhibited wavelength dependent metal-centered and ligand-centered emission at wavelengths shorter
than the analogous rhenium(I) compound prepared from 40-formyl-4-methyl-2,20-bipyridine.
Ó 2009 Elsevier B.V. All rights reserved.
Keywords:
X-ray crystal structure
Rhenium(I) tricarbonylchloro
Schiff base
Bipyridine
Photophysics
1. Introduction
nium(I) tricarbonylchloro complex of this new Schiff base was syn-
thesized and its structure and photophysics defined. The known
The 2,20-bipyridine ligand is extensively used in coordination
chemistry because it produces metal complexes of noted thermal
and kinetic stability [1]. Such metal complexes often possess useful
photophysical and electrochemical characteristics, which arise
from the interaction of the orbitals of the metal and a 2,20-bipyri-
dine ligand. Much work has focused on changing the functionality
of the 2,20-bipyridine ligand toward the end of predictably altering
the electronic properties of metal complexes that are prepared
from them [2]. The above-mentioned properties make 2,20-bipyri-
dine metal complexes versatile molecules that have been used in
areas ranging from photocatalytic CO2 reduction to metal–organic
polymers [3,4]. Of particular interest are rhenium(I) chlorotricar-
bonyl complexes possessing a 2,20-bipyridine ligand because of
their applicability to materials synthesis and biomedical sensing
[5–8].
Our research involves the synthesis of Schiff base ligands and
the preparation of photophysically interesting metal complexes
from them with the aim of applying them toward the construction
of novel materials. Toward that end, we report the synthesis, struc-
tural characterization and photophysics of a unique Schiff base
derivative of 2,20-bipyridine, [4-(40-methyl)-2,20-bipyridyl)imine]-
2-hydroxybenzene, which possesses metal binding moieties that
are positioned for formation of metal–organic oligomers. The rhe-
compounds 40-formyl-4-methyl-2,20-bipyridine and its rhenium(I)
tricarbonylchloro complex were also prepared and their photo-
physics explored for the first time. The new Schiff base and its rhe-
nium(I) tricarbonylchloro complex were found to emit more
intensely and at shorter wavelengths than 40-formyl-4-methyl-
2,20-bipyridine and its rhenium(I) tricarbonylchloro complex. Both
of the rhenium(I) tricarbonylchloro complexes discussed here emit
at longer wavelengths than the prototypical rhenium(I) tricarbon-
ylchloro diimine complex prepared from 2,20-bipyridine [9].
2. Results and discussion
2.1. Synthesis and characterization of the ligands
Compound 1, 40-formyl-4-methyl-2,20-bipyridine, which is pre-
pared by SeO2 oxidation of 4,40-dimethyl-2,20-bipyridine, is a ver-
satile synthon for numerous bipyridine derivatives [10,11].
Scheme 1 illustrates the condensation reaction between 1 and 2-
aminophenol performed under anhydrous conditions that resulted
in the formation of 2. Slow evaporation of the condensation reac-
tion mixture gave analytically pure 2 as an orange–yellow micro-
crystalline solid in 53% yield.
The spectra of 2 had the expected characteristics of a hydroxyi-
mine-bipyridine type compound. The IR spectrum of 2 featured a
band at 1597 cmꢀ1 attributable to the C@N stretching mode of
the imine. Proton resonances for HC@N and OH were observed in
* Corresponding author. Tel.: +1 5629074200x4499; fax: +1 5624644591.
E-mail address: risovits@whittier.edu (R. Isovitsch).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.04.034

T. Doleck et al. / Inorganica Chimica Acta 362 (2009) 3872–3876
3873
2.3. Synthesis and characterization of the metal complexes
N
N
The rhenium(I) tricarbonylchloro compound 4 was prepared by
combining equimolar portions of 2 and Re(CO)5Cl in benzene [14].
Compound 4, which readily precipitated from its reaction mixture,
was filtered from the hot reaction mixture and then washed with
several portions of hot benzene to remove residual 2. Compound
4 was isolated as a mustard yellow solid in 64% yield. Complex 3,
which is the rhenium(I) tricarbonylchloro compound of 1, was pre-
pared in the manner of Gholamkhass and coworkers [2].
2-aminophenol
N
OH
N
4 Å molecular sieves
anhydrous methanol
reflux
N
H
H
O
2
1
Scheme 1.
The IR spectrum of 4 had the three carbonyl bands that are
characteristic of rhenium(I) tricarbonylchloro bipyridine metal
complexes at 2020, 1913 and 1887 cmꢀ1. The 1H NMR spectrum
of 4 showed a general downfield shift of proton resonances when
compared to 2. The identification of 4 is supported by an elemental
analysis and a HRESI-MS which had a prominent [MꢀCl]+ peak at
m/z = 560.0611 (calculated mass for [MꢀCl]+ m/z = 560.0620). The
low solubility of the metal complex prohibited the acquisition of
an acceptable 13C NMR spectrum. Spectra of 4 corresponded to
those found in the literature [2].
the 1H NMR spectrum, while the HC@N carbon resonance was ob-
served at 157.4 ppm in the 13C NMR spectrum [12]. The elemental
analysis of 2 gave acceptable C, H and N values. The structure of 2 is
also supported by the HRESI-MS, which has a [M+H]+ peak at m/
z = 290.1278 (calculated mass for [M+H]+ m/z = 290.1293). These
key indicators support the formation of 2 as a pure compound.
2.2. X-ray crystal structure of 2
2.4. Photophysical characterization
The structure of 2 was determined by X-ray diffraction and is
shown in Fig. 1. A crystal of the required quality was grown by
the slow evaporation of the reaction mixture. Selected bond
lengths and angles are listed in the caption of Fig. 1; crystallo-
graphic data are presented in Section 4.
Compound 2 crystallized in the orthorhombic space group Pbca.
The bipyridine rings were found to adopt an essentially planar,
transoid arrangement due to repulsion between hydrogen atoms
and steric repulsion between groups at the 4 and 40-positions.
The phenyl ring was found to be twisted (C7–N1–C2–C1
ꢀ134.38° (10)), out of the plane of the bipyridine rings. The bond
lengths comprising the imine linkage between the bipyridine moi-
ety and the phenyl ring, C10–C7 and N1–C2, were found to be be-
tween those of a single and double bond. Overall bond distances
and angles were found to correspond well with those of related
compounds [13].
Table 1 summarizes acetonitrile solution absorption and emis-
sion data for compounds 1–5. The absorption spectrum of 1 pos-
sesses bands characteristic of bipyridine
p ?
p* transitions at
279
and
308 nm
[15].
Dilute
acetonitrile
solutions
(ꢁ1.1 ꢂ 10ꢀ5 M) of 1 excited at 279 nm were weakly emissive at
room temperature with a broad, featureless maximum at 465 nm.
The absorption spectrum of compound 2, shown in Fig. 3, had
bipyridine
p ? p
* absorptions at 246 and 278 nm along with a un-
ique absorption at 360 nm, which was assigned to a phenolic
charge transfer transition on the hydroxyimino moiety of 2
[15,16]. Acetonitrile solution and solid state room temperature
emission spectra of 2 are also shown in Fig. 3. Excitation of dilute
acetonitrile solutions (ꢁ4.6 ꢂ 10ꢀ4 M) of 2 at 360 nm resulted in
emission with a maximum at 435 nm and poorly resolved fine
structure at longer wavelengths [17]. This fine structure is more
clearly resolved in the solid state emission spectrum of 2.
Prominent features in the absorption spectrum of compound 4
in acetonitrile, which is shown in Fig. 4, include bipyridine
p ?
p*
transitions at 258 and 295 nm, and an MLCT observed at 400 nm.
The absorption bands of 4 are characteristic of similar compounds
with the only difference being that its MLCT is more intense [9,15].
Table 1
Acetonitrile solution absorption and emission data for compounds 1-5.
Compound
Absorption wavelength kmax nm
(e)
Emission wavelength kmax nm
(kex nm)
a
1
225 (22 300)
279 (10 700)
308 (6220)
465 (279)
2
3
4
246 (22 800)
278 (20 900)
360 (11 000)
435 (360)
449b
466b
246 (21 100)
310 (11 100)
402 (4050)
550 (254)
550, 730c (400)
730c (450)
258 (22 600)
295 (18 800)
400 (15 600)
500, 655 (400)
630 (345)
655 (258)
5d
370 (3420)
622
a
Extinction coefficient units Mꢀ1 cmꢀ1
Vibrational structure maxima.
An approximated value at the less sensitive red end of the R928P photomulti-
.
Fig. 1. Molecular structure of 2. Selected bond lengths (Å) and angles (°): C18–C15
1.5026(15), C15–C16 1.3947(15), C17–N3 1.3467(13), C13–C12 1.4951(14), C7–C10
1.4751(14), C7–N1 1.2794(13), N1–C2 1.4178(12), C2–C1 1.4093(14), C1–O1
1.3558(12), C1–C2–N1 117.61(9), N1–C7–C10 120.56(9), C2–N1–C7–C10
ꢀ178.39(10), C7–N1–C2–C1 ꢀ134.38(10).
b
c
plier tube.
d
See Ref. [9].

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T. Doleck et al. / Inorganica Chimica Acta 362 (2009) 3872–3876
H
O
H
N
N
N
N
Cl(OC)3Re
Cl(OC)3Re
Cl(OC)3Re
N
N
N
HO
4
5
3
Fig. 2. The rhenium(I) tricarbonylchloro complexes prepared from 1 and 2 and Re(CO)3(bpy)(Cl), compound 5.
4000
3500
30000
3000
2500
20000
2000
1500
10000
1000
500
0
0
300
400
500
600
λ (nm)
Fig. 3. Electronic absorption and emission spectra of compound 2. (———) absorption spectrum in acetonitrile, (ꢃꢃꢃꢃ) room temperature emission spectrum in acetonitrile
(kexc = 360 nm), (—) room temperature solid state emission spectrum (kexc = 360 nm).
30000
λexc = 345 nm
30000
25000
20000
20000
15000
λexc = 400 nm
10000
10000
5000
λexc = 258 nm
0
0
300
400
500
600
700
λ (nm)
Fig. 4. Electronic absorption and emission spectra of compound 4 in acetonitrile. (—) Absorption spectrum, (
) room temperature excitation spectrum (kobs = 625 nm),
(
) room temperature emission spectrum (kexc = 258 nm), (......) room temperature emission spectrum (kexc = 400 nm), (
) room temperature emission spectrum
(kexc = 345 nm).
Excitation of compound 4 in its different absorption bands lead to
the observation of dual metal-centered and ligand-centered emis-
sion bands as shown in Fig. 4. Such dual emission is not common,
but examples have been documented [18–20]. For example, excita-
tion at 400 nm led to the observation of emission with maxima at
500 nm (ligand-centered) and 655 nm (metal-centered). The me-
tal-centered emission is similar to other Re(I) tricarbonylchloro dii-
mine complexes [9,21].

T. Doleck et al. / Inorganica Chimica Acta 362 (2009) 3872–3876
3875
10000
7500
5000
2500
0
30000
25000
20000
15000
10000
5000
0
300
400
500
600
700
λ (nm)
Fig. 5. Electronic absorption and emission spectra of compound 4. (———) absorption spectrum in propionitrile (—), room temperature emission spectrum in propionitrile
(kexc = 350 nm), (ꢃꢃꢃꢃ) emission spectrum in propionitrile glass at 77 K (kexc = 350 nm).
Excitation scans in both emission bands of 4 revealed a highly
emissive absorption feature at 345 nm. This feature dominates
excitation spectral probing below 550 nm and is not directly ob-
served in the absorption spectrum of 4. It resembles the phenolic
charge transfer observed at 360 nm in the absorption spectrum
of 2, which can be seen in Fig. 3. Excitation of 4 at 345 nm pro-
duced intense metal-centered emission at 630 nm that was
approximately five times more intense than emission that resulted
from excitation into any other absorption band.
The low temperature emission properties of 4 were investigated
in propionitrile glass. Fig. 5 shows the absorption and emission
spectra of 4 in propionitrile solution at room temperature and
the emission spectrum in propionitrile glass at 77 K. The absorp-
tion spectrum in is essentially the same as in acetonitrile with
maxima at 260, 298 and 404 nm. Excitation of a dilute solution
(ꢁ3.0 ꢂ 10ꢀ4 M) of 4 at 350 nm resulted in the observation of the
dual ligand- and metal-centered emission observed when 4 is dis-
solved in acetonitrile. The dual emission was shifted to shorter
wavelengths and was relatively more intense as compared to 4
in acetonitrile, with maxima at 482 and 598 nm, respectively. In
a proprionitrile glass at 77 K, the emission spectrum of 4 had one
shifted maximum at 612 nm that we attribute to the ligand-cen-
tered emission [18].
ized. Acetonitrile solutions of these compounds were found to
luminescent at room temperature, with 4 exhibiting both ligand-
centered and metal-centered emission that was excitation
wavelength dependent. The excitation spectrum of compound 4,
observing in both of its emission bands, revealed a phenolic
ligand-centered charge transfer absorption that, when irradiated,
was very efficient in inducing metal-centered luminescence. The
known compounds 1 and 3 were prepared and their photochemis-
try explored for the first time. Acetonitrile solution of both were
weakly luminescent at room temperature. Compound 3 exhibited
similar excitation wavelength dependent dual ligand-centered
and metal-centered luminescence that compound 4 did. The room
temperature emission of 1 and 3 is comparatively weaker and at
longer wavelengths than that of 2 and 4. The rhenium(I) tricarbon-
ylchloro compounds that were discussed in this work emit at long-
er wavelengths than compound 5 [9] (Fig. 2).
4. Experimental
4.1. General methods
All manipulations were carried out under an atmosphere of ar-
gon using standard techniques. Reagents and anhydrous solvents
were used as received. Compound 3 was prepared utilizing the lit-
erature procedure. Melting points were determined in open capil-
laries and are uncorrected. 1H and 13C NMR spectra were acquired
on a Joel ECX 300 MHz spectrometer using TMS as the internal
standard. IR spectra were recorded as KCl disks on a JASCO 460
FT-IR. Elemental analyses were performed by M–H–W Laboratories
of Tucson Arizona. Mass spectrometry was provided by the Wash-
ington University Mass Spectrometry Resource with support from
the NIH National Center for Research Resources (Grant No.
P41RR0954).
Acetonitrile solutions of compound 3 had absorptions similar to
those of 4 in that they exhibited absorptions at 246 and 310 nm,
which were assigned as bipyridine
p ?
p* transitions. The MLCT
was observed at 402 nm. Dilute acetontitrile solutions (ꢁ1.6 ꢂ
10ꢀ4 M) of 3 exhibited dual luminescence with ligand-centered
and metal-centered transitions. Excitation of 3 at 254 nm produced
ligand-centered emission at 550 nm, while excitation in the
red-edge of the MLCT produced metal-centered emission at
approximately 730 nm. Excitation of 3 at 400 nm, the center of
the MLCT absorption, resulted in dual ligand-centered and metal-
centered emission at 550 nm and approximately 730 nm,
respectively.
4.2. Photochemical measurements
3. Conclusions
Emission and absorption spectra were recorded at room tem-
perature in spectrophotometric grade acetonitrile and propionitrile
utilizing a Horiba Jobin Yvon FluoroMax-4 fluorometer and a Hew-
lett Packard 8453 diode array spectrometer. All solutions were
The novel Schiff base compound 2 and its rhenium(I) tricarbon-
ylchloro compound 4 were prepared and structurally character-

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T. Doleck et al. / Inorganica Chimica Acta 362 (2009) 3872–3876
degassed with argon prior to luminescence measurements. All
emission spectra were corrected for detector response utilizing a
correction curve supplied by the fluorometer manufacturer.
Emission maxima observed at greater than 700 nm are at the less
sensitive, red end of the R928P photomulitplier tube that is in
our fluorometer and are thus approximated.
independent and 3841 had I > 2r(I). Structure solution was by di-
rect methods, and all H atoms were visible in difference maps.
The OH hydrogen atom position was refined, while others were
placed in calculated positions, with a torsional parameter refined
for the methyl group. Crystal data: C18H15N3O, M = 289.33, ortho-
rhombic,
a = 16.9331(14),
b = 7.5584(5),
c = 22.447(2) Å,
V = 2872.9(4) Å3, T = 90 K, space group Pbca, Z = 8, Rint = 0.026, R1/
wR2 = 0.045/0.137, Goodness-of-fit on F2 = 1.06.
4.3. Synthesis of 2
40-Formyl-4-methyl-2,20-bipyridine
1
(0.189 g, 0.959 mmol)
Acknowledgements
and 2-aminophenol (0.105 g, 0.959 mmol) were added to anhy-
drous methanol (25 mL) with anhydrous 4 Å molecular sieves.
The reaction was refluxed overnight (under a drying tower). The
reaction mixture was filtered hot and allowed to slowly evaporate
which gave 2 [0.145 g (53%)] as yellow–orange microcrystals. m.p.
179 °C. Anal. Calc. for C18H15N3O: C, 74.71; H, 5.24; N, 14.53.
Found: C, 74.72; H, 5.14; N 14.51%. IR (KBr disk) 3401, 3063,
The authors would like to thank the Fletcher Jones Foundation
for funds that allowed the purchase of the fluorometer. Whittier
College is acknowledged for the funds that supported this research
and provided summer support for Tenzing Doleck. Dr. Andrew
Maverick is acknowledged for helpful discussion.
1597 cmꢀ1
.
1H NMR (300 MHz, acetone-d6): 2.45 (s, 3H), 6.94 (m,
References
2H), 7.26 (m, 2H), 7.51 (m, 1H), 8.06 (dd, 1H), 8.27 (s, 1H), 8.33
(m, 1H), 8.56 (d, 1H), 8.77 (d, 1H), 8.90 (m, 1H), 8.99 (s, 1H)
ppm. 13C NMR (75 MHz, CD2Cl2): 21.0, 115.3, 116.2, 120.2, 120.3,
121.3, 121.9, 125.1, 130.2, 134.7, 143.50, 148.4, 149.1, 149.9,
153.0, 155.2, 155.4, 157.4 ppm. ESI-MS [M+H]+ m/z = 290.1278.
Calc. mass for [M+H]+ m/z = 290.1293.
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4.4. Synthesis of 4
0.050 g (0.173 mmol) of 2 was added to 0.057 g (0.157 mmol) of
Re(CO)5Cl in 15 mL benzene. The mixture was refluxed for 1 h. The
precipitate that formed was collected by vacuum filtration and
washed with hot benzene several times to give analytically pure
3 [0.057 g (64%)] as a mustard yellow powder. m.p. dec. >260 °C.
Anal. Calc. for C21H15N3O4ClRe: C, 42.38; H, 2.52; N, 7.06. Found:
C, 42.20; H, 2.75; N, 6.83%. IR (KCl disk) 2020, 1913, 1887 cmꢀ1
.
1H NMR (300 MHz, acetone-d6): 2.62 (s, 3H), 6.97 (m, 2H), 7.28
(m, 1H), 7.52 (dd, 1H), 7.62 (d, 1H), 8.23 (dd, 1H), 8.32 (s, 1H),
8.62 (s, 1H), 8.92 (d, 1H), 9.08 (s, 1H), 9.17 (d, 1H), 9.21 (s, 1H).
ESI-MS [MꢀCl]+ m/z = 560.0611. Calc. mass for [MꢀCl]+ m/
z = 560.0620.
4.5. Structure determination for 2
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