The Re(I) coordination chemistry of a series of pyrido[2,3-b]pyrazine-derived ligands: Syntheses, characterisation and crystal structures

Article abstract of DOI:10.1016/j.poly.2009.11.013

Reaction of 2,3-diaminopyridine with one equivalent of a functionalised vicinal diketone, in ethanol, yields a series of ligands based upon the pyrido[2,3-b]pyrazine core. The ligands were characterised by ¹H, ¹³C-{¹H} NMR

Full text of DOI:10.1016/j.poly.2009.11.013

Polyhedron 29 (2010) 1088–1094
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
The Re(I) coordination chemistry of a series of pyrido[2,3-b]pyrazine-derived
ligands: Syntheses, characterisation and crystal structures
*
*
Benjamin R. Yeo, Andrew J. Hallett , Benson M. Kariuki, Simon J.A. Pope
School of Chemistry, Main Building, Cardiff University, Museum Avenue, Cardiff CF10 3AT, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of 2,3-diaminopyridine with one equivalent of a functionalised vicinal diketone, in ethanol,
yields a series of ligands based upon the pyrido[2,3-b]pyrazine core. The ligands were characterised by
1H, 13C–{1H} NMR, MS and UV–Vis spectroscopy. Reaction of the ligands with one equivalent of
{ReBr(CO)5} gave a series of Re-Ln complexes based upon the general formula fac-{ReBr(CO)3(L)} (where
L = pyrido[2,3-b]pyrazine-derived ligands, L1–L6). Solution IR studies confirmed the retention of the
facially capped, tri-carbonyl coordination geometry at rhenium, and 1H NMR studies confirmed coordina-
tion of the ligand to Re(I). EI HR MS data were obtained for each complex confirming the proposed
formulation and stoichiometry. Single crystal X-ray structures were obtained for three of the complexes
(Re-L1, Re-L2, Re-L6), with each demonstrating that the ligands coordinate to Re(I) in a bidentate manner,
via a four-membered chelate ring, which was unsymmetrical in the former two cases. The electronic
absorption spectra of the complexes showed absorption into the visible region ca. 375–500 nm, (the com-
plexes are orange-red in appearance). Following irradiation at 350–450 nm, the complexes display a solid-
state broad emission peaking between 600–700 nm. The complexes were not sufficiently luminescent in
solution to allow further investigation into the origin of this emission band, although with reference to
related 1,8-naphthyridine complexes of Re(I) it is likely to incorporate significant 3MLCT character.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 15 October 2009
Accepted 16 November 2009
Available online 23 November 2009
Keywords:
Heterocycles
Coordination chemistry
Rhenium complex
Spectroscopy
X-ray structures
1. Introduction
ous solutions with an ionic background. In contrast, materials
applications involving di-imine complexes of Re(I), such as photo-
The coordination chemistry of Re(I) with di-imine type ligands
has attracted significant effort since the discovery by Wrighton in
1974 of luminescence originating from a triplet metal(rhenium)-
to-ligand(1,10-phenanthroline) charge transfer (3MLCT) transition
[1]. This has encouraged numerous studies focusing on the synthe-
ses of poly-pyridine complexes of Re(I), their incorporation into
supramolecular architectures and their photophysical properties
[2]. Importantly, the luminescent characteristics of the generic
molecular form fac-{Re(CO)3(Lx)(Ly)}+ (where Lx = di-imine;
Ly = monodentate ligand) can be tuned via consideration of the
electronic characteristics of the coordinated ligands [2c–d,3].
Consequently, since the 3MLCT excited state is localised on the sole
di-imine ligand, this has allowed the rational design of responsive
systems that can report the presence of an analyte [4] or the local
solvent environment [5].
voltaics [7], potentially allow rarer ligand systems to be exploited:
for example, the use of 1,8-naphthyridine (when coordinated in a
bidentate fashion 1,8-naphthyridine induces a four-membered
chelate ring) with Ru(II) has shown improved photovoltaic effi-
ciencies when compared to the Grätzel prototype which incorpo-
rates 2,2-bipyridine [8]. Related recent reports have also
demonstrated that 1,8-naphthyridine derivatives can also coordi-
nate to Re(I) in a bidentate fashion, giving rise to emission from
a
3MLCT excited state [9], whilst methylation at the 2,7-positions
appears to moderately improve stability in non-polar solvents
[9]. closely related class of heterocyclic ligands are the
A
pyrido[2,3-b]pyrazines [10], which could potentially allow an anal-
ogous coordinating mode to the related 1,8-naphthyridine sys-
tems. Pyrido[2,3-b]pyrazines have been the subject of many
investigations in their own right particularly due to their biological
importance [11], including anti-tumour activity [12]. Such species
can be conveniently synthesised via the condensation of 2,3-diami-
nopyridines with a variety of functionalised dione reagents. In con-
trast, the coordination chemistry of pyrido[2,3-b]pyrazines is
extremely limited. Examples appear to be limited to pyrido[2,3-
b]pyrazine itself: complexes with Sn(II) are known [13] where liga-
tion occurred in a bidentate fashion; and bidentate coordination to
Ru(II) (in an analogous manner to 1,8-naphthyridine) has also been
More specifically, the precise physical molecular requirements
of the complex depend upon the chosen application. For example,
the use of Re(I) complexes in the fluorescence microscopy of cells
[6] obviously requires species with good kinetic stability in aque-
* Corresponding authors. Tel.: +44 (0) 29 20879316; fax: +44 (0) 29 20874030.
E-mail addresses: hallettaj@cardiff.ac.uk (A.J. Hallett), popesj@cardiff.ac.uk (S.J.A.
Pope).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.11.013

B.R. Yeo et al. / Polyhedron 29 (2010) 1088–1094
1089
reported [14]. The coordination chemistry of substituted pyr-
ido[2,3]pyrazines is confined to dipyridophenazine analogues
[15] which induce coordination via the bipyridine-like chelating
units to both Os(II) and Re(I). Monodentate coordination of a 2,3-
dipyrrolyl derivative has been reported with Ni(II) and exploited
in the design of colorimetric chemosensing of fluoride [16], and a
bridging motif within a Rh(II) dimer has recently been highlighted
[17].
Therefore the aim of this work was to synthesise a range of
functionalised ligands based upon the pyrido[2,3-b]pyrazine core
and investigate their coordination chemistry with Re(I) together
with their spectroscopic properties.
tone from the crude material. In all cases, using this general ap-
proach, gave the pure ligands in excellent yields (78–97%).
The corresponding rhenium complexes (Scheme 1), fac-{Re-
Br(CO)3(Ln)} were synthesised by reacting one equivalent of penta-
carbonylbromorhenium with the ligand, in toluene, except in the
case of L5 where the product did not appear to be stable under such
condition: chloroform was used as an alternative [9]. The reaction
mixtures typically turned red-brown in appearance within an hour,
but were left for 12–24 h to allow reaction completion. The com-
plexes were isolated as air stable powders, which were typically
orange-to-brown in appearance. The complexes were character-
ised by 1H NMR, 13C–{1H} NMR (where sufficiently soluble), IR
and UV–Vis spectroscopies, ES LR MS and EI HR MS, and for se-
lected examples via single crystal X-ray diffraction.
The solution state (chloroform) IR studies showed that each
complex possessed three carbonyl stretching frequencies m(CO) be-
2. Results and discussion
tween 2040 and 1900 cmꢀ1, consistent with an approximate local
Cs symmetry at Re(I). 1H NMR spectra were obtained for each com-
plex confirming the presence of the ligand in all cases. Since the
pyrido[2,3-b]pyrazine core induces asymmetry (each proton envi-
ronment is chemically unique) in the ligands it was not possible to
determine the binding mode of the ligand in terms of chelating
bidentate or monodentate. However, each of the spectra showed
a single set of proton resonances that were shifted in comparison
to the free ligands. As an example, the aromatic proton resonances
of L1 showed a general downfield shift upon coordination to Re(I).
The two unique methyl resonances also showed very minor, but
observable downfield shifts of 0.03–0.05 ppm. 13C–{1H} NMR spec-
tra were obtained for those complexes with sufficient solubility,
but only allowed ligand-based resonances to be observed: carbonyl
resonances were not. For each of the complexes low resolution
electrospray mass spectra were easily obtained and showed
2.1. Synthesis and characterisation of the ligands and corresponding
Re(I) complexes
Although most of the ligands included here have been synthes-
ised previously (L1 [18], L2 [19], L3 [20], L4 [21], L5 [22]) these re-
ports do not uniformly include data such as 1H and 13C–{1H}
NMR, mass spectrometry or UV–Vis absorption spectroscopy,
which we include herein for clarity and convenience (however,
note that due to solubility limitations it was not possible to obtain
13C–{1H} NMR spectra for L5 and L6). Our general synthetic meth-
odology for the ligands L1–L6 involved reacting 2,3-diaminopyri-
dine with one equivalent of an appropriate vicinal diketone in
ethanol, with a small aliquot of acetic acid. Purification was
achieved by the use of activated charcoal and occasionally column
chromatography (typically silica using CH2Cl2 to MeOH solvent
gradient from 10:0 to 9:1) was required to remove unreacted dike-
N
N
N
N
N
N
N
N
N
L3
L2
L1
OMe
OMe
N
N
N
N
N
N
N
N
N
L6
L5
L4
Br
N
N
N
N
OC
OC
=
L1 - L6
where
Re
CO
Scheme 1. The ligands L1–L6 and the Re(I) complexes synthesised in this study.

1090
B.R. Yeo et al. / Polyhedron 29 (2010) 1088–1094
Table 2
clusters of peaks with the correct isotopic distributions, corre-
sponding to the species [Re(MeCN)(CO)3(Ln)]+ which are presum-
ably formed via exchange of bromide for acetonitrile. In contrast,
EI HR MS routinely revealed a cluster of peaks accurately corre-
sponding to the [M]+ parent ion. However, in one example (Re-
L2) some additional peaks were also observed corresponding to
[M+NH4]+, and the higher molecular weight species [2M+NH4]+
and [Re(CO)3(L)2]+. Since the 1H NMR spectrum of this complex re-
vealed only one set of ligand-based resonances (i.e. one species in
solution) and the crystal structure revealed the 1:1 (Re:L2) com-
plex adduct, these additional peaks in the high-resolution mass
spectrum are therefore attributed to fragmentation and recombi-
nation processes following ionisation.
Selected bond lengths and bond angles.
Re-L1
Re-L2
C22H13BrN3O3Re
Re-L6
C12H9BrN3O3Re.CHCl3
C20H9BrN3O3ReꢁCHCl3
Bond distances (Å)
Re1–N1
Re1–N2
2.247(6) Re1–N1
2.213(6) Re1–N2
2.209(6) Re1–N1
2.252(6) Re1–N2
2.224(15)
2.227(15)
Bond angles (°)
N1–Re1–N2 59.9(2)
N1–Re1–N2 60.1(2)
N1–Re1–N2 61.3(6)
2.3. Electronic and luminescence spectroscopy
The absorption properties of the ligands and complexes were
assessed in chloroform solution (approximately 10ꢀ4 M; Fig. 2).
Each of the ligands absorbed throughout the 250–350 nm range; li-
gands with extended conjugation (L5 and L6) absorbed towards
2.2. Single crystal X-ray diffraction studies
Single crystals, which were deemed suitable for X-ray diffrac-
tion studies were obtained by vapour diffusion of diethyl ether into
a concentrated chloroform solution of the complexes Re-L1, Re-L2
and Re-L6. The parameters associated with the data collections are
shown in Table 1; selected bond lengths and bond angles are pre-
sented in Table 2. Each of the structures confirmed the proposed
binding mode of the ligand (Fig. 1); that is in a bidentate, chelating
fashion via the pyrido-nitrogen and the pyrazine nitrogen with an
acute \N–Re–N bite angle of ca. 60°. The Re–N, Re–Br and Re–C
distances are all typical of related examples in the literature,
particularly that of the Re(I) complex of 2,7-dimethyl-1,8-naph-
thyridine (c.f average Re–N 2.21; N–Re–N angle of 59.7°) [9]. How-
ever, subtle differences were evident within the Re–N interactions.
In the cases of Re-L1 and Re-L2 the four-membered chelate rings
were asymmetric reflecting the relative donating ability of the N-
donors. In the former the electron rich methyl substituents appear
to enhance the donating ability of the pyrazine. However, in the
case of Re-L2 where the substituents are less electron donating
and more sterically demanding, the trend was reversed, with the
Re–N interaction from the pyrazine unit weaker than that of the
pyridine.
400 nm. These features are assigned to overlapping
p ?
p* transi-
tions, as observed for related functionalised pyrido[2,3-b]pyrazines
[22]. For the complexes each of the spectra revealed transitions be-
tween 240 and 450 nm. The higher energy transitions are associ-
ated with intra-ligands transitions, primarily 1p p* in nature and
–
red-shifted in comparison to the free ligands. The broad, lowest en-
ergy transitions can be assigned as an 1MLCT absorption, in line
with previous reports into other di-imine complexes of Re(I)
[1,23] and consistent with the more closely related 1,8-naphthyri-
dine complexes [8,9], although it is possible that there are also li-
gand-centred components at these wavelengths since these may
be expected to red-shift upon coordination to Re(I). In this series
of compounds the precise positioning of the lowest energy absorp-
tion appears to vary according to the coordinated ligand. It is nota-
ble that for ligands with similar steric requirements (e.g. L3 versus
L4) modulation of the remote aryl substituents (i.e. methyl versus
methoxy) induced subtle changes in the energetic positioning of
this lowest energy band.
Firstly, for comparison, solid-state luminescence measurements
were conducted upon the free ligands. Each ligand was found to be
Table 1
Parameters associated with the single crystal diffraction data collection.
Re-L1
Re-L2
Re-L6
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C12H9BrN3O3ReꢁCHCl3
628.70
150(2)
C22H13BrN3O3Re
633.46
150(2)
0.71073
monoclinic
P2(1)/n
C20H9BrN3O3ReꢁCHCl3
724.78
150(2)
0.71073
0.71073
triclinic
triclinic
ꢀ
ꢀ
P1
P1
8.2920(2)
9.1180(2)
12.6520(3)
99.4440(10)
99.3960(10)
99.9500(10)
911.16(4)
2
2.292
588
0.40 ꢂ 0.35 ꢂ 0.30
2.31–27.47
5525
9.1773(4)
16.8130(8)
13.7372(5)
90.00
108.355(2)
90.00
2011.78(15)
4
8.8137(4)
10.9748(5)
13.6102(6)
113.079(2)
90.233(3)
112.151(2)
1103.78(9)
2
2.181
684
0.08 ꢂ 0.05 ꢂ 0.05
1.65–26.48
6562
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
Volume (Å3)
Z
Density (calculated) (Mg/m3)
F(0 0 0)
2.091
1200
Crystal size (mm3)
h Range for data collection (°)
All reflections
0.20 ꢂ 0.20 ꢂ 0.15
3.20–27.49
8160
Independent reflections
Observed reflections
Goodness-of-fit (GOF) on F
Rint
3985
3816
1.053
0.0479
4586
3599
1.048
0.0370
4424
3968
1.075
0.0293
Final R indices [I > 2
R indices (all data)
r(I)]
R1 = 0.0520, wR2 = 0.1436
R1 = 0.0537, wR2 = 0.1456
R1 = 0.0488, wR2 = 0.1118
R1 = 0.0685, wR2 = 0.1218
R1 = 0.0899, wR2 = 0.2389
R1 = 0.0975, wR2 = 0.2435

B.R. Yeo et al. / Polyhedron 29 (2010) 1088–1094
1091
Fig. 1. Structural representations obtained for Re-L1 (top left), Re-L2 (top right) and Re-L6 (bottom) based upon the fac-{ReBr(CO)3} core. Solvents of crystallisation and
hydrogen atoms are omitted for clarity.
Table 3
25000
20000
15000
10000
5000
0
0000
0000
0000
0000
0000
Steady state emission maxima of the complexes (kex = 400–450 nm) and correspond-
ing ligands (in parentheses) in the solid state.
Re-L1
Re-L2
Re-L3
Re-L4
Re-L5
Re-L6
Complex
kem (nm)
Re-L1
Re-L2
Re-L3
Re-L4
Re-L5
Re-L6
596 (450)
668 (457)
631 (457)
625 (480)
630 (499)
604 (475)
0
450
550
650
750
wavelength / nm
naphthyridine and more generally comparable with other di-imine
complexes of Re(I) [4,23]. Additional attempts to measure the
luminescence properties of the complexes in chloroform solution
gave only weak emission features characteristic of ligand-centred
processes, with no evidence for emission at longer wavelengths
>500 nm, which could be associated with an 3MLCT origin.
Although the lability of the ligands was not elucidated from the
1H NMR data (no observable signals corresponding to free ligand)
further reactivity studies showed that in the presence of one equiv-
alent of 2,20-bipyridine, in chloroform (60 °C, 24 h), ligand ex-
change occurred at Re-Ln resulting in liberation of the free
pyrido[2,3-b]pyrazine ligand. Again, this is in accordance with
our previous work on related 1,8-naphthyridine complexes of
Re(I) and therefore suggests that the visible luminescence arising
from the solid samples of the complexes is probably 3MLCT in nat-
ure, although in these cases additional ligand-centred contribu-
tions cannot be ruled out.
250
350
450
wavelength / nm
550
650
Fig. 2. Main: UV–Vis absorption spectra (in chloroform) for the complexes. Inset:
representative emission spectrum for Re-L1 (solid; kex = 400 nm).
emissive in the green region, with emission maxima between 450
and 500 nm following excitation at 350 nm (summarized in paren-
theses within Table 3), and thus the Stokes shifts associated with
the free ligands were typically around 5000 cmꢀ1
.
Previous studies into 1,8-naphthyridine complexes of Re(I) have
shown that despite such species possessing poor 3MLCT emissivity
in solution, which is probably due to rapid fluxionality of the ligand
[9], they retain useful emission in the solid state [8]. Similarly, in
this study, following excitation at 350–400 nm, the complexes
did not demonstrate any 3MLCT emission at room temperature in
chloroform solution. However, in the solid state each complex
showed a typically broad transition in the range 590–670 nm (Ta-
ble 3; Fig. 2, inset), consistent with related complexes with 1,8-
3. Conclusions
This paper describes the synthesis of a range of 2,3-substituted
pyrido[2,3-b]pyrazines together with their coordination chemistry

1092
B.R. Yeo et al. / Polyhedron 29 (2010) 1088–1094
with Re(I). In all cases the ligands have been shown to bind in a
bidentate fashion; these are the first examples of coordination
chemistry involving this series of ligands. The complexes possess
ligand-centred and MLCT absorption bands in the visible region,
and are emissive in the solid state.
128.7, 128.4, 128.2, 127.3, 127.0, 124.2 ppm. EI MS found m/z
284.1, 322.1, 347.1, 567.2 and 589.2; calculated m/z 284.1, 322.2,
347.1, 567.2 and 589.2 for [M+H]+, [M+K]+, [M+MeCN+Na]+,
[2M+H]+ and [2 M+Na]+, respectively. UV–Vis (CHCl3): kmax
(e/
dm3 molꢀ1 cmꢀ1) = 245 (16 300), 351 (13 800) nm.
L3: prepared similarly to L2 from 2,3-diaminopyridine (0.120 g,
1.10 mmol) and 4,40-dimethylbenzil (0.263 g, 1.10 mmol). Yield =
0.298 g (87%). 1H NMR (250 MHz, CDCl3) dH = 9.04 (1H, d,
4. Experimental
3
3JHH = 4.2 Hz), 8.39 (1H, d, JHH = 8.4 Hz), 7.58 (1H, dd, JHH = 8.3
All reactions were performed with the use of vacuum line and
Schlenk techniques. Reagents were commercial grade and were
used without further purification. Pentacarbonylbromorhenium
was synthesised according to the literature [24]. 1H and 13C–{1H}
NMR spectra and were run on NMR-FT Bruker 400 and 250 MHz
spectrometers and were recorded in CDCl3. 1H and 13C–{1H} NMR
chemical shifts (d) were determined relative to internal TMS and
are given in ppm. Infrared spectra were run on a Perkin–Elmer
FT-1600 spectrophotometer as CHCl3 solutions. UV–Vis studies
were performed on a Jasco V-570 spectrophotometer. Low-resolu-
tion mass spectra were obtained using a Waters LCT Premier XE. EI
HR MS data was obtained by the staff of the EPSRC National Mass
Spectrometry Service, at Swansea University.
3
3
and 4.2 Hz), 7.47 (2H, d, JHH = 8.2 Hz), 7.38 (2H, d, JHH = 8.2 Hz),
7.07 (4H, m), 2.32 (3H, s), 2.30 (3H, s) ppm. 13C–{1H} NMR
(101 MHz, CDCl3) dC = 156.3, 154.8, 153.7, 139.6, 139.4, 138.0,
136.0, 135.9, 135.4, 130.2, 129.7, 129.2, 128.9, 125.0, 124.2 ppm.
ES MS found m/z 312.4; calculated m/z 312.3 for [M+H]+. UV–Vis
(CHCl3): kmax (e
/dm3 molꢀ1 cmꢀ1) = 244 (13 700), 363 (8700) nm.
L4: Prepared similarly from 2,3-diaminopyridine (0.150 g,
1.37 mmol) and anisil (0.390 g, 1.44 mmol). Yield = 0.458 g (97%).
3
1H NMR (250 MHz, CDCl3) dH = 8.98 (1H, d, JHH = 4.2 Hz), 8.34
(1H, d, 3JHH = 8.3 Hz), 7.45–7.57 (3H, m), 7.41 (2H, d, 3JHH = 6.8 Hz),
6.70–6.81 (4H, m), 3.70 (3H, s), 3.69 (3H, s) ppm. 13C–{1H} NMR
(101 MHz, CDCl3) dC = 160.77, 160.6, 155.8, 154.2, 153.5, 149.8,
137.8, 135.9, 131.8, 131.3, 131.1, 130.7, 124.8, 113.9, 113.6, 55.4,
55.3 ppm. AP MS found m/z 344.1; calculated m/z 344.3 for
4.1. Crystallography
[M+H]+. UV–Vis (CHCl3): kmax /dm3 molꢀ1 cmꢀ1) = 250 (26 000),
(e
385 (14 500) nm.
4.1.1. Data collection and processing
L5: Prepared similarly from 2,3-diaminopyridine (0.132 g,
1.21 mmol) and 9,10-phenanthrenequinone (0.264 g, 1.27 mmol).
Yield = 0.275 g (81%). 1H NMR (250 MHz, CDCl3) dH = 9.35 (1H, d,
Diffraction data for Re-L1, Re-L2, Re-L3 were collected on a Non-
ius KappaCCD using graphite-monochromated Mo K
(k = 0.71073 Å) at 150 K. Software package APEX 2 (v2.1) was used
for the data integration, scaling and absorption correction.
a radiation
3JHH = 5.1 Hz), 8.99–9.12 (2H, m), 8.91 (1H, d, JHH = 8.2 Hz), 8.38–
8.47 (2H, m), 8.23 (1H, dd, JHH = 8.2 and 5.1 Hz), 7.68–7.79 (2H,
m), 7.51–7.64 (2H, m) ppm. EI MS found m/z 282.1 and 563.2; cal-
culated m/z 282.1 and 563.2 for [M+H]+ and [2M+H]+, respectively.
3
4.1.2. Structure analysis and refinement
The structure was solved by direct methods using SHELXS-97 and
was completed by iterative cycles of
UV–Vis (CHCl3): kmax
(14 800) nm.
(e
/dm3 molꢀ1 cmꢀ1) = 254 (32 900), 386
DF-syntheses and full-matrix
L6: Prepared similarly from 2,3-diaminopyridine (0.200 g,
1.83 mmol) and acenaphthenequinone (0.350 g, 1.92 mmol).
Yield = 0.369 g (78%). 1H NMR (250 MHz, CDCl3) dH = 8.92 (1H, d,
3JHH = 4.3 Hz), 8.26 (1H, d, 3JHH = 8.3 Hz), 8.06 (1H, d, 3JHH = 7.0 Hz),
least squares refinement. All non-H atoms were refined anisotrop-
ically and difference Fourier syntheses were employed in position-
ing idealised hydrogen atoms and were allowed to ride on their
parent C-atoms. All refinements were against F2 and used SHELX
-
3
97 [25].
7.86 (1H, d, JHH = 7.0 Hz), 7.61–7.72 (2H, m), 7.32–7.49 (3H, m)
ppm. EI MS found m/z 256.0; calculated m/z 256.1 for [M+H]+.
4.2. Synthesis of the ligands
4.3. Synthesis of the complexes
L1: 2,3-diaminopyridine (0.209 g, 1.92 mmol) and 2,3-butanedi-
one (0.2 ml, 2.28 mmol) were heated at reflux in a mixture of EtOH
(10 ml) and AcOH (0.5 ml) for 16 h. The solvent and unreacted 2,3-
butanedione were removed in vacuo and the crude product dis-
solved in CHCl3 (20 ml), which was then heated with activated
charcoal for 30 min. The mixture was filtered and the solvent re-
moved in vacuo to give the product as an off-white powder.
Yield = 0.278 g (91%). 1H NMR (250 MHz, CDCl3) dH = 8.97 (1H, br
4.3.1. fac-[Re(CO)3(L1)Br]
[Re(CO)5Br] (0.050 g, 0.12 mmol) and L1 (0.020 g, 0.13 mmol)
were heated at reflux in toluene (10 ml) for 16 h. The solvent
was removed in vacuo and the crude product dissolved in the min-
imum of CHCl3. The product was precipitated by the slow addition
of petroleum ether to give an orange/red powder. Yield = 0.045 g
(73%). 1H NMR (250 MHz, CDCl3) dH = 8.86 (1H, d, JHH = 4.3 Hz),
3
3
3
s), 8.25 (1H, d, JHH = 8.3 Hz), 7.55 (1H, dd, JHH = 8.3 and 4.2 Hz),
8.50 (1H, d, JHH = 8.2 Hz), 7.76 (1H, dd, JHH = 8.2 and 4.3 Hz),
2.73 (3H, s), 2.69 (3H, s) ppm. 13C–{1H} NMR (101 MHz, CDCl3)
dC = 157.3, 155.0, 152.3, 150.2, 137.3, 135.9, 124.3, 23.1,
23.0 ppm. EI MS found m/z = 160.1, 201.1 and 319.2; calculated
m/z 160.1, 201.1 and 319.2 for [M+H]+, [M+MeCN+H]+ and
2.79 (3H, s), 2.72 (3H, s) ppm. 13C–{1H} NMR (101 MHz, CDCl3)
dC = 159.2, 158.1, 151.7, 138.6, 137.7, 127.5, 24.0, 22.2 ppm. IR
(CHCl3, cmꢀ1
) m(CO) = 2034(s), 1938(m), 1911(m). ES MS found
m/z 471; calculated m/z 470 for [MꢀBr+MeCN]+. EI HR MS found
[2M+H]+, respectively. UV–Vis (CHCl3): kmax /dm3 molꢀ1 cmꢀ1) =
(e
m/z 506.9355; calculated m/z 506.9351 for [C12H9O3N3Br185Re]+.
243 (33 300), 314 (10 500) nm.
UV–Vis (CHCl3): kmax (e
/dm3 molꢀ1 cmꢀ1) = 241 (7400), 292
L2: prepared similarly from 2,3-diaminopyridine (0.150 g,
1.37 mmol) and benzil (0.303 g, 1.44 mmol). The product was puri-
fied by column chromatography (silica/CH2Cl2). Excess benzil was
removed by eluting with CH2Cl2. The ligand L2 was then eluted
with CH2Cl2/MeOH (9:1). Yield = 0.325 g (84%). 1H NMR (250
(12 100), 438 (1200) nm.
4.3.2. fac-[Re(CO)3(L2)Br]
Prepared similarly from [Re(CO)5Br] (0.040 g, 0.10 mmol) and L2
(0.028 g, 0.10 mmol) in chloroform. Yield = 0.048 g (77%). 1H NMR
3
3
MHz, CDCl3) dH = 9.12 (1H, d, JHH = 4.2 Hz), 8.46 (1H, d,
(250 MHz, CDCl3) dH = 8.99 (1H, d, JHH = 4.0 Hz), 8.66 (1H, d,
3JHH = 8.4 Hz), 7.66 (1H, dd, JHH = 8.4 and 4.2 Hz), 7.42–7.59 (4H,
m), 7.21–7.37 (6H, m) ppm. 13C–{1H} NMR (101 MHz, CDCl3)
dC = 155.1, 153.5, 152.9, 148.6, 137.3, 137.0, 136.9, 135.0, 129.2,
3JHH = 8.6 Hz), 7.86 (1H, dd, JHH = 8.6 and 4.0 Hz), 7.59 (2H, d,
JHH = 8.5 Hz), 7.50 (1H, t, JHH = 7.4 Hz), 7.28–7.44 (7H, m) ppm. IR
(CHCl3, cmꢀ1
) m(CO) = 2035(s), 1937(m), 1908(m). ES MS found

B.R. Yeo et al. / Polyhedron 29 (2010) 1088–1094
1093
m/z 612; calculated m/z 611 for [MꢀBr+MeCN]+. EI HR MS found
Acknowledgements
m/z 649.0001; calculated m/z 649.0008 for [C22H17O3N4Br185Re].
UV–Vis (CHCl3): kmax
(e
/dm3 molꢀ1 cmꢀ1) = 242 (18 300), 382
The EPSRC are thanked for support together with Cardiff Uni-
versity. The staff of the EPSRC National Mass Spectrometry Service
are also thanked for their contributions.
(12 100) nm.
4.3.3. fac-[Re(CO)3(L3)Br]
References
Prepared similarly from [Re(CO)5Br] (0.035 g, 0.09 mmol) and L3
(0.030 g, 0.10 mmol) in toluene. Yield = 0.046 g (80%). 1H NMR
[1] M.S. Wrighton, D.L. Morse, J. Am. Chem. Soc. 96 (1974) 998.
[2] (a) A.J. Lees, Chem. Rev. 87 (1987) 711;
3
(250 MHz, CDCl3) dH = 8.92 (1H, d, JHH = 3.8 Hz), 8.59 (1H, d,
(b) C.R.K. Glasson, L.F. Lindoy, G.V. Meehan, A. Coleman, C. Brennan, J.G. Vos,
M.T. Pryce, Coord. Chem. Rev. 252 (2008) 940;
(c) D.J. Stufkens, A. Vlcek, Coord. Chem. Rev. 177 (1998) 127;
(d) D.R. Striplin, G.A. Crosby, Coord. Chem. Rev. 211 (2001) 163;
(e) D. Beck, J. Brewer, J. Lee, D. McGraw, B.A. DeGraff, J.N. Demas, Coord. Chem.
Rev. 251 (2007) 546;
(f) M.H. Keefe, K.D. Benkstein, J.T. Hupp, Coord. Chem. Rev. 205 (2000) 201;
(g) A.J. Hallett, P. Christian, J.E. Jones, S.J.A. Pope, Chem. Commun. (2009)
4278;
(h) K.S. Schanze, D.B. Macqueen, T.A. Perkins, L.A. Cabana, Coord. Chem. Rev.
122 (1993) 63.
3JHH = 8.6 Hz), 7.79 (1H, dd, JHH = 8.6 and 3.8 Hz), 7.48 (2H, d,
3
JHH = 8.2 Hz), 7.22–7.32 (4H, m), 7.09 (2H, d, JHH = 8.2 Hz), 2.36
(3H, s), 2.32 (3H, s) ppm. IR (CHCl3, cmꢀ1
) m(CO) = 2034(s),
1928(m), 1913(m). ES MS found m/z 623; calculated m/z 622.0
for [MꢀBr+MeCN]+. EI HR MS found m/z 658.9976, calculated m/z
658.9977 for [C24H17O3N3Br185Re]+. UV–Vis (CHCl3): kmax
(e/
dm3 molꢀ1 cmꢀ1) = 244 (22 800), 395 (9300) nm.
[3] (a) L. Sacksteder, A.P. Zipp, E.A. Brown, J. Streich, J.N. Demas, B.A. DeGraff,
Inorg. Chem. 29 (1990) 4335;
4.3.4. fac-[Re(CO)3(L4)Br]
(b) J.V. Caspar, B.P. Sullivan, T.J. Meyer, Inorg. Chem. 23 (1984) 2104;
(c) A.P. Zipp, L. Sacksteder, J. Streich, A. Cook, J.N. Demas, B.A. DeGraff, Inorg.
Chem. 32 (1993) 5629;
(d) L.A. Worl, R. Duesing, P. Chen, L.D. Ciana, T.J. Meyer, J. Chem. Soc., Dalton
Trans. (1991) 849.
Prepared similarly from [Re(CO)5Br] (0.053 g, 0.13 mmol) and L4
(0.047 g, 0.14 mmol) in toluene. Yield = 0.061 g (68%). 1H NMR
3
(250 MHz, CDCl3) dH = 8.86 (1H, d, JHH = 4.6 Hz), 8.55 (1H, d,
3JHH = 8.6 Hz), 7.76 (1H, dd, JHH = 8.6 and 4.7 Hz), 7.58 (2H, d,
3JHH = 8.7 Hz), 7.35 (2H, d, 3JHH = 8.8 Hz), 6.92 (2H, d, 3JHH = 8.8 Hz),
[4] (a) Selected examples: P.D. Beer, V. Timoshenko, M. Maestri, P. Passaniti, V.
Balzani, B. Balzani, Chem. Commun. (1999) 1755;
3
6.79 (2H, d, JHH = 8.8 Hz), 3.80 (3H, s), 3.77 (3H, s) ppm. 13C–{1H}
(b) M.P. Coogan, V. Fernandez-Moreira, B.M. Kariuki, S.J.A. Pope, F.L. Thorp-
Greenwood, Angew. Chem., Int. Ed. 48 (2009) 4965;
(c) J.B. Cooper, M.G.B. Drew, P.D. Beer, J. Chem. Soc., Dalton Trans. (2000)
2721;
(d) J.N. Demas, B.A. DeGraff, Anal. Chem. 63 (1991) 829A;
(e) C.-C. Ko, L.-X. Wu, K. Man-Chung Wong, N. Zhu, V.W.-W. Yam, Chem. Eur. J.
10 (2004) 766;
(f) M.J. Li, W.M. Kwok, W.H. Lam, C.H. Tao, V.W.W. Yam, D.L. Phillips,
Organometallics 28 (2009) 1620;
(g) M.W. Louie, H.W. Liu, M.H.C. Lam, T.C. Lau, K.K.W. Lo, Organometallics 28
(2009) 4297;
NMR (101 MHz, CDCl3) dC = 162.5, 161.7, 158.8, 156.4, 152.5, 151.5,
137.8, 134.1, 132.1, 131.8, 129.2, 127.7, 126.0, 114.7, 114.1, 55.6,
55.5 ppm. IR (CHCl3, cmꢀ1
) m(CO) = 2034(s), 1936(m), 1906(m).
ES MS found m/z 655; calculated m/z 654 for [MꢀBr+MeCN]+. EI
HR MS found m/z 690.9872; calculated m/z 690.9876 for
[C24H17O5N3Br185Re]+. UV–Vis (CHCl3): kmax /dm3 molꢀ1 cmꢀ1) =
(e
267 (17 500), 431 (7100) nm.
(h) H.D. Stoeffler, N.B. Thornton, S.L. Temkin, K.S. Schanze, J. Am. Chem. Soc.
117 (1995) 7119;
(i) J.D. Lewis, R.N. Perutz, J.N. Moore, J. Phys. Chem. A 108 (2004) 9037.
[5] (a) M.P. Coogan, V. Fernández-Moreira, J.B. Hess, S.J.A. Pope, C. Williams, New
J. Chem. 33 (2009) 1094;
4.3.5. fac-[Re(CO)3(L5)Br]
Prepared similarly from [Re(CO)5Br] (0.040 g, 0.10 mmol) and
L5 (0.029 g, 0.10 mmol) in toluene. Yield = 0.041 g (66%). Due to
poor solubility it was not possible to obtain either 1H or 13C–{1H}
NMR spectra of [Re(CO)3(L5)Br] in CDCl3 or d6-dmso IR (CHCl3,
(b) L. Sacksteder, M. Lee, J.N. Demas, B.A. DeGraff, J. Am. Chem. Soc. 115 (1993)
8230;
(c) X.Q. Guo, F.N. Castellano, L. Li, H. Szmacinski, J.R. Lakowicz, J. Sipior, Anal.
Biochem. 254 (1997) 179.
cmꢀ1
) m(CO) = 2034(s), 1937(m), 1910(m). ES MS found m/z 591;
[6] (a) A.J. Amoroso, M.P. Coogan, J.E. Dunne, V. Fernández-Moreira, J.B. Hess, A.J.
Hayes, D. Lloyd, C. Millet, S.J.A. Pope, C. Williams, Chem. Commun. (2007)
3066;
calculated m/z 590 for [MꢀBr+MeCN]+. EI HR MS found m/z
628.9507; calculated m/z 628.9508 for [C22H11O3N3Br185Re]+. UV–
(b) A.J. Amoroso, M.P. Coogan, V. Fernández-Moreira, A.J. Hayes, D. Lloyd, C.
Millet, S.J.A. Pope, New J. Chem. 32 (2008) 1097;
(c) K.K.W. Lo, M.W. Louie, K.S. Sze, J.S.Y. Lau, Inorg. Chem. 47 (2008) 602;
(d) L.A. Mullice, R.H. Laye, L.P. Harding, N.J. Buurma, S.J.A. Pope, New J. Chem.
32 (2008) 2140;
Vis (CHCl3): kmax (e
/dm3 molꢀ1 cmꢀ1) = 248 (14 400), 314 (4600),
411 (4200) nm.
(e) C.S.K. Mak, H.L. Wong, Q.Y. Leung, W.Y. Tam, W.K. Chan, A.B. Djurisic, J.
Organomet. Chem. 694 (2009) 2770;
(f) F. He, Y.H. Zhou, S.J. Liu, L.L. Tian, H. Xu, H.Y. Zhang, B. Yang, Q.F. Dong, W.J.
Tian, Y.G. Ma, J.C. Shen, Chem. Commun. (2008) 3912;
(g) H.L. Wong, L.S.M. Lam, K.W. Cheng, K.Y.K. Man, W.K. Chan, C.Y. Kwong, A.B.
Djurisic, Appl. Phys. Lett. 84 (2004) 2557.
4.3.6. fac-[Re(CO)3(L6)Br]
Prepared similarly from [Re(CO)5Br] (0.037 g, 0.09 mmol) and L6
(0.027 g, 0.11 mmol) in toluene. Yield = 0.039 g (71%). Due to poor
solubility it was not possible to obtain either 1H or 13C–{1H} NMR
spectra of [Re(CO)3(L6)Br] in CDCl3 or d6-dmso. IR (CHCl3, cmꢀ1
)
[7] (a) A. Kukrek, D. Wang, Y. Hou, R. Zong, R. Thummel, Inorg. Chem. 45 (2006)
10131;
(b) A.E.M. Boelrijk, T.X. Neenan, J. Reedijk, J. Chem. Soc., Dalton Trans. (1997)
4561.
[8] U. Monkowius, Y.N. Svartsov, T. Fischer, M. Zabel, H. Yersin, Inorg. Chem.
Commun. 10 (2007) 1473.
m
(CO) = 2023(s), 1934(m), 1913(m). ES MS found m/z 567; calcu-
lated m/z 566 for [MꢀBr+MeCN]+. EI HR MS found m/z 605.9352;
calculated m/z 605.9351 for [C20H9O3N3Br185Re]+. UV–Vis (CHCl3):
kmax (e
/dm3 molꢀ1 cmꢀ1) = 240 (6000), 321 (10 200) nm.
[9] M. Andrews, R.H. Laye, S.J.A. Pope, Transition Met. Chem. 34 (2009) 493.
[10] (a) I.A. Kaye, J. Med. Chem. 7 (1964) 240;
(b) M. Israel, A.R. Day, J. Org. Chem. 24 (1959) 1455.
[11] (a) C. Temple, G.A. Rener, W.R. Waud, P.E. Noker, J. Med. Chem. 35 (1992)
3686;
Supplementary data
(b) A. Cugola, D. Donati, M. Guarneri, F. Micheli, A. Missio, A. Pecunioso, A.
Reggiani, G. Tarzia, V. Zanirato, Bioorg. Med. Chem. Lett. 6 (1996) 2749;
(c) C. Temple, J.D. Rose, R.D. Elliott, J.A. Montgomery, J. Med. Chem. 11 (1968)
1216.
CCDC 751351, 751352 and 751353 contain the supplementary
crystallographic data for (Re-L1), (Re-L2) and (Re-L6), respectively.
These data can be obtained free of charge via http://www.ccdc.ca-
m.ac.uk/conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
[12] I.Y. Postovskii, V.N. Carushin, G.A. Mokrushina, S.K. Kotovskaya, A.S. Barybin,
O.N. Chupakhin, A.I. Chernyshev, Pharm. Chem. J. 16 (1982) 824.
[13] (a) A.J. Crowe, P.J. Smith, G. Atassi, Inorg. Chim. Acta 93 (1984) 179;
(b) A.J. Crowe, P.J. Smith, J. Organomet. Chem. 224 (1982) 223.
[14] R.J. Staniewicz, R.F. Sympson, D.G. Hendricker, Inorg. Chem. 16 (1977) 2167.

1094
B.R. Yeo et al. / Polyhedron 29 (2010) 1088–1094
[15] (a) M.I.J. Polson, S.L. Howell, A.H. Flood, A.K. Burrell, A.G. Blackman, K.C.
Gordon, Polyhedron 23 (2004) 1427;
8 (2007) 389;
(c) Z. Zhao, D.D. Wisnoski, S.E. Wolkenberg, W.H. Leister, Y. Wang, C.W.
Lindsley, Tetrahedron Lett. 45 (2004) 4873;
(d) M. Makosza, J. Golinski, S. Ostrowski, A. Rykowski, A.B. Sahasrabudhe,
Chem. Ber. 124 (1991) 577;
(e) J. Armand, K. Chekir, J. Pinson, J. Heterocycl. Chem. 17 (1980) 1237;
(f) A. Nagel, H.C. Van der Plas, G. Geurtsen, A. Van Veldhuizen, J. Heterocycl.
Chem. 16 (1979) 301;
(g) Y. Ding, J.-L. Girardet, K.L. Smith, G. Larson, B. Prigaro, V.C.H. Lai, W. Zhong,
J.Z. Wu, Bioorg. Med. Chem. Lett. 15 (2005) 675.
(b) L.C. Abbott, C.J. Arnold, T.Q. Ye, K.C. Gordon, R.N. Perutz, R.E. Hester, J.N.
Moore, J. Phys. Chem. A 102 (1998) 1252.
[16] T. Ghosh, B.G. Maiya, A. Samanta, Dalton Trans. (2006) 795.
[17] G. Tresoldi, S.L. Schiavo, F. Nicolo, P. Cardiano, P. Piraino, Inorg. Chim. Acta 344
(2003) 190.
[18] (a) S. Veeraraghavan, F.D. Popp, J. Heterocycl. Chem. 19 (1982) 425;
(b) Y. Blache, A. Cueiffier, O. Chavignon, A. Elhakmaoui, H. Viols, J.C. Teulade,
J.C. Milhavet, G. Dauphin, J.P. Chapat, Heterocycles 38 (1994) 283;
(c) S. Kumari, P. Ghai, A. Sexsena, Indian J. Chem. Sect. B 31B (1992) 92;
(d) M. Sako, Sci. Synth. 16 (2004) 1269;
[20] A. Ohta, H. Takakashi, N. Miyata, H. Hirono, T. Nishio, E. Uchino, K. Yamada, Y.
Aoyagi, Y. Suwabe, M. Fujitake, T. Suzuki, K. Okamoto, Biol. Pharm. Bull. 20
(1997) 1076.
(e) A.A.H. Saeed, Can. J. Spectrosc. 31 (1986) 40;
(f) J.M. Cosmao, N. Collignon, G. Queguiner, Can. J. Chem. 60 (1982) 2785;
(g) J. Armand, K. Chekir, J. Pinson, Can. J. Chem. 56 (1978) 1804;
(h) V. Petrow, J. Saper, J. Chem. Soc. (1948) 1389;
[21] Z. Kucybala, A. Kosobucka, Polym. Bull. 56 (2006) 321.
[22] K.R.J. Thomas, J.T. Lin, Y.T. Tao, C.H. Chuen, J. Mater. Chem. 12 (2002) 3516.
[23] F.L. Thorp-Greenwood, M.P. Coogan, R.H. Laye, A.J. Hallett, S.J.A. Pope, J.
Organomet. Chem. 694 (2009) 1400.
(i) I.A. Kaye, J. Med. Chem. 7 (1964) 240.
[19] (a) J.S. Yadav, B.V.S. Reddy, K. Premalatha, K.S. Shankar, Synthesis (2008)
3787;
[24] S.P. Schmidt, W.C. Trocler, Inorg. Synth. 28 (1990) 160.
[25] SHELXL-PC Package, Bruker Analytical X-ray Systems, Madison, WI, 1998.
(b) H.R. Darabi, S. Mohandessi, K. Aghapoor, F. Mohsenzadeh, Catal. Commun.