Oxorhenium(V) complexes with naphtholate-oxazoline ligands in the catalytic epoxidation of olefins

Article abstract of DOI:10.1016/j.poly.2014.03.024

Four oxorhenium(V) complexes 3a-d equipped with naphtholate-oxazoline based ligands 2a-d have been prepared and characterized by NMR, IR and mass spectroscopy as well as elemental analysis. Ligands 2a-d were prepared in a two-step procedure from commercially available starting materials. Ligands 2a-b and 2c-d are regioisomers to each other regarding the position of the -OH group and oxazoline moiety on the naphthol ring. Reaction of 2a-d with (NBu ₄)[ReOCl₄] in methanol under reflux gave oxorhenium(V) complexes 3a-d of the type [ReOCl(L)₂] as green solids in acceptable to good yields. The molecular structures of complexes 3b and 3d have been determined via single crystal X-ray diffraction analysis and both display a distorted octahedral geometry. Complexes 3a-d are active catalysts for the epoxidation of cyclooctene with tert-butylhydroperoxide (TBHP) showing moderate conversions between 41% and 65%.

Full text of DOI:10.1016/j.poly.2014.03.024

Polyhedron 75 (2014) 141–145
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Oxorhenium(V) complexes with naphtholate-oxazoline ligands
in the catalytic epoxidation of olefins
Belina Terfassa a,b, Jörg A. Schachner a, Pedro Traar a, Ferdinand Belaj a, Nadia C. Mösch Zanetti a,
⇑
a Institute of Chemistry, Karl-Franzens-University Graz, Stremayrgasse 16, 8010 Graz, Austria
b Department of Chemistry, Addis Ababa University, Addis Ababa, Queen Elizabeth II St., Ethiopia
a r t i c l e i n f o
a b s t r a c t
Article history:
Four oxorhenium(V) complexes 3a–d equipped with naphtholate-oxazoline based ligands 2a–d have
been prepared and characterized by NMR, IR and mass spectroscopy as well as elemental analysis.
Ligands 2a–d were prepared in a two-step procedure from commercially available starting materials.
Ligands 2a–b and 2c–d are regioisomers to each other regarding the position of the –OH group and
oxazoline moiety on the naphthol ring. Reaction of 2a–d with (NBu4)[ReOCl4] in methanol under reflux
gave oxorhenium(V) complexes 3a–d of the type [ReOCl(L)2] as green solids in acceptable to good yields.
The molecular structures of complexes 3b and 3d have been determined via single crystal X-ray
diffraction analysis and both display a distorted octahedral geometry. Complexes 3a–d are active
catalysts for the epoxidation of cyclooctene with tert-butylhydroperoxide (TBHP) showing moderate
conversions between 41% and 65%.
Received 9 January 2014
Accepted 13 March 2014
Available online 25 March 2014
Keywords:
Rhenium complexes
Epoxidation
Homogeneous catalysis.
Bidentate ligands
TBHP
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
interest in rhenium-catalyzed epoxidations prompted us to further
investigate unexplored oxazoline ligands and their oxorhenium(V)
The remarkable ability of oxorhenium(V) complexes to catalyze
various oxygen atom transfer (OAT) reactions has spurred intensive
research over the last few years. Espenson and coworkers described
the OAT reactivity of oxorhenium(V) complexes from various
pyridine-N-oxides to phosphines [1]. Abu-Omar and coworkers
reported an oxorhenium(V) complex employing the phenolate-
oxazoline ligand 2-(4,5-dihydrooxazol-2-yl)phenol (Hhoz) which
is capable of reducing kinetically stable perchlorate anions to
chloride ions via OAT to sulphides [2,3]. Finally, OAT to olefinic
substrates resulting in the formation of epoxides employing rhe-
nium catalysts has developed into an important field of research,
especially since the advent of the oxorhenium(VII) catalyst methyl-
trioxorhenium (MTO) [4–6]. MTO is a highly active catalyst for ole-
fin epoxidation, however, with certain substrates unwanted,
hydrolytic ring opening of the formed epoxide occurs due to the
Brønstedt acidity of MTO [7]. Oxorhenium(V) complexes are usually
less prone towards hydrolysis, thereby avoiding the unselective
hydrolysis of epoxides. For this reason several oxorhenium(V) com-
plexes have been investigated as epoxidation catalysts by us and
other groups over the last years [8–14]. The success of oxazoline
ligands as stabilizing motifs in catalysis as well as our ongoing
chemistry. Within this paper we would like to report on the synthe-
sis and characterization of four naphtholate-oxazoline ligands 2a–d
(Fig. 1) as well as the oxorhenium(V) complexes 3a–d thereof.
Furthermore complexes 3a–d were tested as potential catalysts in
the epoxidation of olefins (see Schemes 1–3).
2. Results and discussion
Ligands 2a–b (Fig. 1) were synthesized by a two step reaction in
analogy to published procedures [15,16] varying in the R-groups
adjacent to the nitrogen as well as in the position of the substitu-
ents on the naphthole ring. Starting from commercially available
1-hydroxy-2-naphthoic acid and the respective aminoalcohol the
amides 1a–b were obtained by a Steglich condensation. The cycli-
zation to obtain the respective oxazoline ligands 2a–b was
achieved with thionyl chloride [17]. Compounds 2a–b represent
bidentate O,N-coordinating ligands, where the hydroxyl group is
easily deprotonated by bases like NEt3, whereby they become
mono-anionic.
Since the amides 1c–d are not directly accessible from 2-hydro-
xy-3-naphthoic acid, which is in contrast to amides 1a–b, the
methyl ester was first synthesized in situ, which could then be
converted to the amides 1c–d via a Steglich condensation [18].
⇑
Corresponding author. Tel.: +43 316 380 5286; fax: +43 316 380 9835.
E-mail address: nadia.moesch@uni-graz.at (N.C. Mösch Zanetti).
http://dx.doi.org/10.1016/j.poly.2014.03.024
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

142
B. Terfassa et al. / Polyhedron 75 (2014) 141–145
O
R
O
3a-d
1 mol%
R
OH
O
3 equiv.TBHP
OH
N
N
CHCl3, 50 °C
R
R
Scheme 3. Epoxidation of cyclooctene.
2a
2c
R = H
R = H
2d R = CH3
2b R = CH3
Table 1
Selected IR bond stretching frequencies of ligands 2a–d and complexes 3a–d.
Fig. 1. Ligands 2a–d used to synthesize oxorhenium(V) complexes.
Bond stretching frequencies, )
(cmÀ1
m
2a
2b
2c
2d
The synthesis of 1c had been previously mentioned, but without
analytical data [19].
mC@N
1626
3a
1620
3b
1651
3c
1650
3d
The oxorhenium(V) complexes 3a–d were obtained by refluxing
a mixture of (NBu4)[ReOCl4] and two equivalents of the corre-
sponding ligand 2a–d in the presence of triethylamine in metha-
nol. Upon cooling and concentration of the deep green reaction
mixture the respective complexes started to precipitate in analyt-
ical pure form as a green–brownish solid.
mC@N
mRe@O
1596
961
1594
956
1610
966
1599
952
Interestingly the complexes showed quite different solubilities.
Complexes 3a and 3c (with R = H) are barely soluble in polar sol-
vents like methanol, ethanol, acetone, dichloromethane (DCM),
chloroform, THF or dimethylsulfoxide (DMSO) whereas 3b and
3d are quite soluble in these solvents. This is due to the four
methyl groups in complexes 3b and 3d. 1H NMR spectra of 3a–d
display two different sets of ligand signals indicating the formation
of asymmetrically substituted Re(V) complexes. Similar to previ-
ously published disubstituted complexes, an asymmetrical coordi-
nation of the two bidentate ligands is expected, where one ligand
coordinates the Re atom in an all-equatorial fashion and the second
ligand in an equatorial-axial fashion [3,12,20,21]. This proposed
structure was confirmed by single crystal X-ray diffraction analysis
of complex 3b and 3d respectively (vide infra). The formation of
complexes 3a–d was also apparent by IR spectroscopy. Upon
coordination of the respective ligand 2a–d a shift to lower wave
numbers of the mC@N band of the oxazoline moiety is observed as
well as the appearance of the diagnostic mRe@O band between 900
and 1000 cmÀ1 (Table 1) [2,12].
complexes 3a–d. Single crystals of 3b and 3d suitable for X-ray
crystallography could be grown by slow evaporation of a concen-
trated solution of the respective complex in acetone (3b) or meth-
anol (3d). Both 3b and 3d adopt an asymmetric conformation in
the solid state, where one ligand coordinates in an equatorial fash-
ion and the other ligand in an equatorial-axial fashion. Further-
more, both structures show some disorder, with atoms occupying
two different orientations. This disorder however does not change
the overall geometry of the complexes. Since both structures are
quite similar, only the structure of 3b will be discussed in detail.
In complex 3b the two oxazoline N atoms N13 and N33 are trans
to each other, and the two naphtholate O atoms O21 and O41
are in a cis orientation (Fig. 2). Consequently the remaining oxo
(O1) and chloro (Cl1) ligands are also in cis configuration to each
other, completing the distorted, octahedral coordination geometry
around the Re atom. The rhenium atom as well as the oxo and
chloro ligands are disordered resulting in a slight displacement
(Re2, O2, Cl2, not labeled in Fig. 2) with a lower occupation factor
of 0.4005(10). Nevertheless, the overall geometry of both disor-
dered structures remains the same, with one naphtholate oxygen
atom occupying a position trans to the terminal oxo ligand.
Mass spectroscopy as well as elemental analysis are also
consistent with the proposed formulas of oxorhenium(V)
1. MeOH, H2SO4
2. H2NC(R)2CH2OH,
DCC, DMAP (cat.)
OH
OH
OH
SOCl2
O
O
N
R
R
R
OH
HN
O
R
1c R = H
1d
2c R = H
2d
R = CH3
R = CH3
OH
Scheme 1. Synthesis of ligands 2c–d via amides 1c–d.
R
R
Re
R
O
R
O
+ NEt3
Δ, MeOH
Cl
N
O
R
OH
N
R
N
O
(NBu4)[ReOCl4] + 2
O
- 2 HNEt3Cl
O
2a-d
3a-d
Scheme 2. Synthesis of oxorhenium(V) complexes 3a–d.

B. Terfassa et al. / Polyhedron 75 (2014) 141–145
143
Fig. 2. Molecular views of 3b (left) and 3d (right) showing the atomic numbering scheme. Probability ellipsoids are drawn at the 50% level; H atoms are omitted for clarity.
Bonds to disordered atoms with lower occupation factors are drawn with dashed lines.
Table 2
3. Epoxidation of cyclooctene
Comparison of selected bond distances and angles of 3b and 3d.
Bond distances (Å)
3b
Bond-distances (Å)
3d
Complexes 3a–d were tested as catalysts for the epoxidation of
cyclooctene with tert-butylhydroperoxide (TBHP) as oxidant. The
best conditions were found with 1 mol% of catalyst and 3 equiv.
of TBHP in CHCl3 at 50 °C. Under these conditions catalyst 3a
reached 49%, catalyst 3b 54%, catalyst 3c 65% and catalyst 3d
41% conversion of cyclooctene to cyclooctene oxide selectively.
Experiments were carried out for eight hours, after which no more
conversion to cyclooctene oxide could be detected. Due to their
low solubilities, complexes 3a and 3c did not fully dissolve ini-
tially, in contrast to 3b and 3d. We believe that the initial lower
conversion rates of 3a and 3c after 1 h are mainly due to this lower
solubility. At longer reaction times, also complexes 3a and 3c dis-
solved. At the end of the reaction time of 8 h, homogeneous, almost
colorless solutions were observed. The discoloration indicates the
formation of colorless perrhenate(VII) species in solution, which
was also observed for other oxorhenium(V) catalysts after a similar
time span [11–14,21,24]. This points to concomitant catalyst
decomposition over the reaction time and explains the lack of
catalysis after longer reaction times. No induction period for all
four complexes could be observed. Due to their pronounced differ-
ence in solubility, it is difficult to observe an isomeric effect be-
tween complexes 3a and 3b or 3c and 3d respectively.
Experiments in acetonitrile or methanol only gave small con-
versions to epoxide, which were mainly due to the uncatalyzed
reaction with TBHP. Test reactions without 3a–d confirmed this
finding. Also, if the amount of catalyst loading or oxidant is de-
creased, the conversion to product drops significantly. Experiments
using hydrogenperoxide also resulted in low conversion (>5%). De-
tails of the catalytic reaction are given in the Section 4 (see
Table 3).
Complexes 3a–d are catalytically active in the epoxidation of
cyclooctene to cyclooctene oxide. All catalysts show good selectiv-
ity towards the epoxide, as no other side products, for example the
ring-opened diol, are observed. The overall catalytic activity as well
as catalyst stability is comparable to other oxorhenium(V) com-
plexes [11–14,21,24].
Re1–O1
1.672(3)
Re1–O1
1.673(10)
1.975(5)
2.003(5)
2.091(6)
2.137(6)
2.4361(18)
Re1–O21
Re1–O41
Re1–N13
Re1–N33
Re1–Cl1
1.9691(15)
2.0790(18)
2.0921(17)
2.1245(18)
2.3845(11)
Re1–O22
Re1–O42
Re1–N13
Re1–N33
Re1–Cl1
Bond angles (°)
3b
Bond angles (°)
3d
O1–Re1–O41
N13–Re1–N33
O21–Re1–Cl1
O21–Re1–N13
O1–Re1–O21
O1–Re1–Cl1
178.38(14)
163.58(7)
167.09(5)
88.18(6)
97.18(14)
93.21(13)
O1–Re1–O42
N13–Re1–N33
O22–Re1–Cl1
O22–Re1–N13
O1–Re1–O22
O1–Re1–Cl1
169.8(7)
164.5(2)
172.65(12)
87.4(2)
100.0(7)
85.1(7)
All bond lengths and angles in 3b as well as 3d fall within the
expected range for an octahedral oxorhenium(V) complex (Table 2)
[3,12–14,20,22,23]. The Re@O bond distance Re1–O1 is 1.672(3) Å
for 3b, which lies within the typical range of such bonds of 1.688–
1.694 Å. The two axial ligands O1 and O41 show almost perfect lin-
ear arrangement at 178.38(14)°. The deviation from an ideal octa-
hedron is displayed by the angle of the trans oriented N atoms N13
and N33 of 163.58(7)°, which also reflects the small bite angel of
the bidentate ligand 2b. As mentioned above, the structural
parameters of 3b and 3d are very similar to each other, with one
exception. The bond distance from Re to the naphtholate oxygen
trans to the terminal oxo ligand is significant shorter in 3d
(Re1–O42 = 2.003(5) Å) compared to 3b (Re1–O41 = 2.0790(18) Å)
(Table 2). This might be attributed to the difference in pKa of the
oxygen atom and the reduced steric pressure in 3d.
Table 3
Results of epoxidation of cyclooctene catalyzed by 3a–d.
t (h)
Epoxide (%)
3a
3b
3c
3d
4. Conclusions
1
2
8
16
30
54
27
42
49
10
21
41
27
47
65
Within this paper we present the synthesis and characterization
of four novel oxorhenium(V) complexes 3a–d equipped with the
bidentate monoanionic naphtholate-oxazoline ligands 2a–d. All
Cond.: 1 mol% 3a–d, 3 equiv. TBHP, CHCl3, 50 °C.

144
B. Terfassa et al. / Polyhedron 75 (2014) 141–145
analytical data is consistent with the formation of asymmetrically
disubstituted complexes. Complexes 3b and 3d were additionally
characterized by single crystal X-ray diffraction analysis. Further-
more the complexes 3a–d were tested in the catalytic epoxidation
of cyclooctene where they showed to be active catalysts compara-
ble to other oxorhenium(V) complexes.
ATR frequencies (cmÀ1): 1651 ( C@N). MS (EI, m/z): 213.1 [M+].
m
Anal. Calc. for C13H11NO2 (213.23): C, 73.23; H, 5.20; N, 6.57.
Found: C, 72.34; H, 5.11; N, 6.41%.
Synthesis of 2d. A suspension of 1d (2.0 g, 7.72 mmol, 1 equiv.)
in dichloromethane (70 mL) was cooled to 0 °C. Thionyl chloride
(1.56 g, 13.12 mmol, 1.5 equiv.) was added slowly under Ar over
a period of 15 min. The solution was allowed to slowly warm up
to room temperature and stirred overnight after which the solution
was concentrated to a small volume. The crude product of 2dÁHCl
was precipitated with ether, subjected to aqueous work-up with
saturated NaHCO3 and dried in vacuo to yield 1.28 g (5.32 mmol,
69%) of 2b as light-brown flakes. 1H NMR (300 MHz, DMSO-d6) d
11.96 (s, 1H), 8.32 (s, 1H), 7.94 (dd, J = 8.3, 1.1 Hz, 1H), 7.76 (dd,
J = 8.3, 1.1 Hz, 1H), 7.50 (ddd, J = 8.3, 6.8, 1.1 Hz, 1H), 7.34 (m,
2H), 4.23 (s, 2H), 1.38 (s, 6H). 13C NMR (75 MHz, DMSO-d6) d
163.10, 155.17, 136.59, 129.76, 129.24, 128.85, 127.05, 126.48,
124.11, 113.00, 110.64, 78.33, 67.66, 28.45. Selected ATR frequen-
4.1. Experimental
4.1. General remarks
Unless otherwise specified, all experiments were performed un-
der atmospheric conditions with standard laboratory equipment at
the Institute of Chemistry, University of Graz. The metal precursor
(NBu4)[ReOCl4] was prepared according to a published procedure.
[25] Amides 1c–d as well as ligands 2a–b were synthesized accord-
ing to a published procedure. [19,16] All other chemicals and
solvents were purchased from commercial sources and used as
received. 1-Hydroxy-2-naphthoic acid was purchased from
Sigma–Aldrich (>97.0%). 1H and 13C NMR spectra were recorded
on a Bruker Optics Instrument. Chemical shifts are reported in
parts per million (ppm) and referenced to residual protons or car-
bons in the deuterated solvent. Electron impact mass spectroscopy
measurements (EI-MS) were recorded with an Agilent 5973 MSD
mass spectrometer with Direct Probe. Gas chromatography mass
spectroscopy measurements (GC–MS) have been performed with
a model Agilent 7890 A (column type Agilent 19091 J-433),
coupled to a mass spectrometer type Agilent 5975 C. Samples for
infrared spectroscopy were measured on a Bruker Optics ALPHA
ATR FT-IR Spectrometer. IR bands are reported with wave number
(cmÀ1). Elemental analyses were measured on a Heraeus Vario
Elementar automatic analyzer at the Technische Universität Graz,
Department of Inorganic Chemistry.
cies (cmÀ1): 1650 ( C@N). MS (EI, m/z): 241.1 [M+]. Anal. Calc. for
m
C15H15NO2 (241.29): C, 74.76; H, 6.27; N, 5.81. Found: C, 73.58;
H, 6.23; N, 5.58%.
Synthesis of 3a. To a green solution of (NBu4)[ReOCl4] (0.586 g,
1 mmol) in methanol (25 mL), 2a (0.426 g, 2 mmol) was added
with stirring. Triethylamine (0.202 g, 2 mmol) was slowly added
to the mixture which resulted in a deep green precipitate. The mix-
ture was refluxed for 3 h, cooled to room temperature and concen-
trated to ꢀ5 mL of solvent to enforce precipitation of 3a. After
filtration, washing with small amounts of methanol and drying un-
der vacuo analytically pure 3a was obtained (0.540 g, 0.82 mmol,
82%). 1H NMR (300 MHz, DMSO-d6) d 3.72 (q, J = 10.5 Hz, 2H),
4.16 (dt, J = 25.0, 9.0 Hz, 2H), 4.71 (m, 2H), 5.10 (m, 2H), 7.15 (t,
J = 7.6 Hz, 1H), 7.31 (m, 1H), 7.43 (m, 4H), 7.69 (m, 2H), 7.83 (m,
2H), 7.97 (m, 1H), 8.86 (d, J = 8.0 Hz, 2H). Due to the low solubility
of 3a no 13C NMR spectrum could be obtained. Selected ATR IR fre-
quencies (cmÀ1): 1599 (
mC@N), 959 (mRe@O). EI-MS (m/z): = 662.3
[M+], 627.4 [M+À Cl]. Anal. Calc. for C26H20ClN2O5Re (662.11): C,
47.16; H, 3.04; N, 4.23. Found: C, 46.29; H, 2.99; N 3.99%.
4.2. Single crystal X-ray diffraction analysis
Synthesis of 3b. To a green solution of (NBu4)[ReOCl4] (0.586 g,
1 mmol) in methanol (20 mL) 2b (0.482 g, 2 mmol) was added with
stirring. Triethylamine (0.202 g, 2 mmol) was slowly added to the
mixture which resulted in a deep green solution. The mixture
was refluxed for 3 h, cooled to room temperature and concentrated
to ꢀ2 mL of solvent to enforce precipitation of 3b. The product ob-
tained was filtered, washed with small amounts of methanol and
dried in vacuo to yield 0.52 g of 3b (0.72 mmol, 72%) as a yellowish
green powder. Dark brown prisms of single crystals suitable for
X-ray diffraction were obtained by slow evaporation of a saturated
solution of 3b in acetone. 1H NMR (300 MHz, CDCl3) d 1.78 (s, 3H),
2.00 (s, 3H), 2.01 (s, 3H), 2.07 (s, 3H), 4.49 (d, J = 8.3 Hz, 1H), 4.75
(m, 2H), 4.83 (d, J = 8.3 Hz, 1H), 7.11 (d, J = 9.0 Hz, 1H), 7.34 (m,
4H), 7.50 (ddd, J = 8.1, 6.8, 1.3 Hz, 1H), 7.62 (m, 1H), 7.69 (m,
1H), 7.80 (d, J = 8.8 Hz, 1H), 7.91 (d, J = 9.0 Hz, 1H), 7.96 (m, 1H),
8.36 (m, 1H). 13C NMR (75 MHz, CDCl3) d 26.55, 27.43, 28.10,
28.46, 72.87, 78.52, 79.37, 81.87, 102.42, 103.31, 117.28, 119.06,
124.52, 124.78, 125.01, 125.14, 125.51, 125.72, 126.65, 126.84,
127.21, 129.46, 129.70, 137.25, 137.80, 164.31, 166.34, 170.71,
All the measurements were performed using graphite-monochro-
matized Mo K radiation at 100 K. 3b: C30H28ClN2O5Re, Mr 718.19,
a
monoclinic, space group P 21/c, a = 16.9287(7) Å, b = 12.8952(5) Å,
c = 12.4258(5) Å,
dcalc = 1.806 g cmÀ3
collected Hmax = 30.00°), from which 7693 were unique
(Rint = 0.0224), with 6529 having I > 2
b = 103.114(2)°,
l
V = 2641.80(18) Å3,
Z = 4,
,
= 4.746 mmÀ1. A total of 23693 reflections were
(
r(I). For 392 parameters final R
indices of R1 = 0.0212 and wR2 = 0.0466 (GOF = 1.069) were obtained.
3d: C30H28ClN2O5Re, Mr 718.19, monoclinic, space group P 21/c,
a = 18.8284(8) Å, b = 7.4928(3) Å, c = 20.0590(9) Å, b = 103.414(1)°,
V = 2752.7(2) Å3, Z = 4, dcalc = 1.733 g cmÀ3 = 4.554 mmÀ1. A total
, l
of 21271 reflections were collected (Hmax = 30.0°), from which 7989
were unique (Rint = 0.0166), with 7108 having I > 2r(I). For 399 param-
eters final R indices of R1 = 0.0465 and wR2 = 0.1626 (GOF = 1.587)
were obtained. For full details on data collection and refinement refer
to the Supporting information.
Synthesis of 2c. A suspension of 1c (1.73 g, 7.5 mmol, 1 equiv.)
in dichloromethane (70 mL) was cooled to 0 °C. Thionyl chloride
(1.78 g, 15 mmol, 2 equiv.) was added slowly under Ar over a per-
iod of 15 min. The solution was allowed to slowly warm up to
room temperature and stirred overnight after which the solution
was concentrated to a small volume. The crude product of 2cÁHCl
was precipitated with ether, subjected to aqueous work-up with
saturated NaHCO3 and dried in vacuo to yield 1.53 g (7.03 mmol,
93%) of 2b as white flakes. 1H NMR (300 MHz, DMSO-d6) d 12.00
(s, 1H), 8.32 (s, 1H), 7.94 (dd, J = 8.3, 1.1 Hz, 1H), 7.76 (dd, J = 8.3,
1.1 Hz, 1H), 7.50 (ddd, J = 8.3, 6.8, 1.1 Hz, 1H), 7.33 (m, 2H), 4.51
(t, J = 9.5 Hz, 2H), 4.11 (t, J = 9.5 Hz, 2H). 13C NMR (75 MHz,
177.23, 1C obscured. Selected IR frequencies (cmÀ1): 1617 m(C@N)
,
956
m
(Re@O). EI-MS (m/z): 718.1 [M+], 683.1 [M+]ÀCl. Anal. Calc.
for C30H28ClN2O5Re (718.21): C, 50.17; H; 3.93; N, 3.90. Found: C,
50.47; H, 4.00; N, 3.83%.
Synthesis of 3c. To a green solution of (NBu4)[ReOCl4] (0.586 g,
1 mmol) in methanol (25 mL), 2c (0.426 g, 2 mmol) was added
with stirring. Triethylamine (0.202 g, 2 mmol) was slowly added
to the mixture which resulted in a deep green precipitate. The mix-
ture was refluxed for 3 h, cooled to RT and concentrated to ꢀ5 mL
of solvent to enforce precipitation of 3c. After filtration, washing
with small amounts of methanol and drying under vacuo analyti-
cally pure 3c was obtained (0.477 g, 0.73 mmol, 73%).
DMSO-d6)
d 165.05, 154.70, 136.12, 129.33, 128.81, 128.41,
126.57, 126.01, 123.66, 112.44, 110.21, 67.05, 53.29. Selected

B. Terfassa et al. / Polyhedron 75 (2014) 141–145
145
1H NMR (300 MHz, DMSO-d6) d 8.67 (s, 1H), 8.29 (s, 1H), 8.03 (d,
J = 8.3 Hz, 1H), 7.90 (m, 2H), 7.63 (m, 3H), 7.36 (m, 3H), 7.01 (s, 1H),
4.96 (m, 4H), 4.15 (m, 4H). Due to the low solubility of 3c no 13C
NMR spectrum could be obtained. Selected ATR IR frequencies
charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or
from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2014.03.024.
(cmÀ1): 1610 ( Re@O). EI-MS (m/z): = 662.3 [M+], 627.4
mC@N), 966 (m
[M+ÀCl]. Anal. Calc. for C26H20ClN2O5Re (662.11): C, 47.16; H,
3.04; N, 4.23. Found: C, 46.77; H, 2.89; N, 4.39%.
Synthesis of 3d. To a green solution of (NBu4)[ReOCl4] (0.586 g,
1 mmol) in methanol (20 mL) 2d (0.482 g, 2 mmol) was added with
stirring. Triethylamine (0.202 g, 2 mmol) was slowly added to the
mixture which resulted in a deep green solution. The mixture
was refluxed for 3 h, cooled to RT and concentrated to ꢀ2 mL of
solvent to enforce precipitation of 3d. The product obtained was
filtered, washed with small amounts of methanol and dried in
vacuo to yield 0.256 g of 3d (0.256 mmol, 25%) as a deep green
powder. 1H NMR (300 MHz, CDCl3) d 8.66 (s, 1H), 8.32 (s, 1H),
7.78 (m, 1H), 7.74 (m, 1H), 7.60 (m, 2H), 7.43 (m, 1H), 7.32 (m,
2H), 7.24 (m, 1H), 7.17 (s, 1H), 7.10 (s, 1H), 4.73 (dd, J = 8.3,
2.6 Hz, 2H), 4.65 (dd, J = 8.3, 5.1 Hz, 2H), 2.08 (s, 3H), 1.98 (s, 6H),
1.72 (s, 3H). 13C NMR (75 MHz, CDCl3) d 172.62, 170.02, 166.50,
159.63, 138.35, 138.26, 133.78, 133.66, 129.79, 129.25, 129.11,
127.57, 126.58, 126.24, 125.97, 124.03, 123.22, 114.61, 113.68,
112.64, 82.27, 79.65, 79.43, 73.44, 27.55, 27.11, 26.98, 26.06, 2C
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Appendix A. Supplementary data
CCDC 935186 and 942214 contains the supplementary crystal-
lographic data for 3b and 3d. These data can be obtained free of