Coordination behavior of chromone Schiff bases towards the [Re VO]3+ and [ReI(CO) 3]+ cores

Article abstract of DOI:10.1080/00958972.2013.867028

Herein, we describe the coordination behavior of chromone Schiff bases towards [Re^VO]³⁺ and [Re^I(CO)3]⁺. The reaction between 2-(2-thiolphenyliminomethyl)-4H-chromen-4-one (Htch) and [Re(CO)5Cl] led to fac-[Re(C

Full text of DOI:10.1080/00958972.2013.867028

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Coordination behavior of chromone
Schiff bases towards the [ReVO]3+ and
[ReI(CO)3]+ cores
Irvin Noel Booysena, Muhammed Bilaal Ismaila & Matthew Piers
Akermana
a School of Chemistry and Physics, University of Kwazulu-Natal,
Scottsville, Pietermaritzburg, South Africa
Published online: 10 Dec 2013.
To cite this article: Irvin Noel Booysen, Muhammed Bilaal Ismail & Matthew Piers Akerman (2013)
Coordination behavior of chromone Schiff bases towards the [ReVO]3+ and [ReI(CO)3]+ cores, Journal
of Coordination Chemistry, 66:24, 4371-4386, DOI: 10.1080/00958972.2013.867028
To link to this article: http://dx.doi.org/10.1080/00958972.2013.867028
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Journal of Coordination Chemistry, 2013
Vol. 66, No. 24, 4371–4386, http://dx.doi.org/10.1080/00958972.2013.867028
Coordination behavior of chromone Schiff bases towards the
[ReVO]3+ and [ReI(CO)3]+ cores
IRVIN NOEL BOOYSEN*, MUHAMMED BILAAL ISMAIL and
MATTHEW PIERS AKERMAN*
School of Chemistry and Physics, University of Kwazulu-Natal, Scottsville, Pietermaritzburg,
South Africa
(Received 22 July 2013; accepted 7 November 2013)
Herein, we describe the coordination behavior of chromone Schiff bases towards [ReVO]3+ and
[ReI(CO)3]+. The reaction between 2-(2-thiolphenyliminomethyl)-4H-chromen-4-one (Htch) and
[Re(CO)5Cl] led to fac-[Re(CO)3(bsch)Cl] (1) (bsch = 2-benzothiazole-4H-chromen-4-one). The
square pyramidal [ReO(Hns)] (2) {H2ns = bis-[(2-phenylthiolate)iminomethyl]-methyl-1-(2-hydroxy-
phenyl)prop-2-en-1-one} and octahedral [ReO(OCH3)(PPh3)(Huch)] (3) complexes were isolated
from reactions of trans-[ReVOBr3(PPh3)2] with Htch and H3uch [(5Z)-5-((4-hydroxy-2-methoxy-2H-
chromen-3-yl)methyleneamino)-6-amino-1,3-dimethylpyrimidine-2,4(1H, 3H)-dione], respectively.
The chromone Schiff bases and their metal complexes were fully characterized via NMR-, IR- and
UV–Vis spectroscopy, single crystal XRD analysis and conductivity measurements. In addition, DFT
studies were conducted to compare selected optimized and experimental parameters of the complexes.
Keywords: Rhenium (I/V); Schiff bases; Chromone; Crystal structure; DFT studies
1. Introduction
The coordination chemistry of rhenium has been widely explored since the discovery of a
target-specific rhenium radiopharmaceutical depends on its stability, geometry and the
attachment of a biologically active molecule (BAM) to the 186/188Re radionuclide [1, 2].
Schiff bases can be readily derived from BAMs and this ligand system has shown to be
excellent chelators to rhenium, thereby stabilizing the metal in both the low and high oxida-
tion states [3, 4]. This stabilization is largely accounted to their diverse donor atoms, chela-
tion and multidenticity upon coordination [5, 6]. Subsequently, these attributes of Schiff
bases afford rhenium complexes with different structural features [8]. An example is the
rhenium(I) complex, fac-[Re(msh)(py)(CO)3] [Hmsh = 2-[(2-indol-3-yl-ethyl)iminomethyl]-
5-methylphenol], in which a stable 6-membered chelate ring formed through the deproto-
nated phenolic oxygen and imino nitrogen, while the indole moiety remained uncoordinated
[11].
Beside the influential coordination capability of Schiff bases, these organic compounds
have shown to exhibit a wide array of biological activity [12–14]. In particular, Schiff bases
derived from 3-formyl chromone and their corresponding transition metal complexes have
*Corresponding authors. Email: Booyseni@ukzn.ac.za (IN Booysen); Akermanm@ukzn.ac.za (MP Akerman)
© 2013 Taylor & Francis

4372
I.N. Booysen et al.
displayed optimal anti-thymidine phosphorylase inhibitory-, antibacterial-, antioxidant- and
antifungal activity as well DNA binding capabilities. In addition, the chromone Schiff base,
7-methoxychromone-3-carbaldehyde-(3,4-dimethyl)-pyrrole hydrazone has shown selective
chemosensing capability for the toxic Al3+ species in the presence of other metal cations
[15].
From a coordination chemistry perspective, chromone Schiff bases typically form
coordination bonds through the imino nitrogen and the keto/enol oxygen as well as donor
atoms on neighboring (with respect to the Schiff base group) aliphatic or aromatic moieties
[16–21]. A typical example is the neutral tridentate chelator 6-hydroxychromone-3-carbalde-
hydethiosemicarbazone, which coordinates to the Ni2+ metal atom through the classical
N
imino,Oketonic donor set as well as the thiocarbonyl sulfur atom of the thiosemicarbazone
moiety [22].
In this account, we explored the coordination behavior of Schiff bases incorporating a chro-
mone moiety towards the fac-[ReI(CO)3]+ and [ReVO]3+ cores. Three rhenium compounds,
fac-[ReI(CO)3(bsch)Cl] (1) (bsch = 2-benzothiazole-4H-chromen-4-one), [ReVO(Hns)] (2)
{H2ns=bis-[(2-phenylthiolate)iminomethyl]-methyl-1-(2-hydroxyphenyl)prop-2-en-1-one} and
[ReVO(OCH3)(PPh3)(Huch)] (3) [Huch = (5Z)-5-((4-hydroxy-2-methoxy-2H-chromen-3-yl)
methyleneamino)-6-amino-1,3-dimethylpyrimidine-2,4-(1H,3H)-dione], were isolated. The
2-(2-thiolphenyliminomethyl)-4H-chromen-4-one (Htch) Schiff base was reacted with [ReI
(CO)5Cl] and trans-[ReVOBr3(PPh3)2] to form 1 and 2, respectively. The ligand cyclized to
afford the bidentate heterocyclic chelator, bsch (in 1), and transformed to form a N2S2
tetradentate chelator (in 2). In 3, free H3uch did not transform or cyclize but rather coordinated
as the Huch dianionic tridentate chelator.

Chromone Schiff bases
4373
2. Experimental
2.1. Materials and methods
Pentacarbonylrhenium(I)chloride, ammonium perrhenate, 4-oxo-4H-chromene-3-carbaldehyde,
2-aminothiophenol and 5,6-diamino-1,3-dimethyl uracil were obtained from Sigma Aldrich.
All solvents were obtained from Merck SA. The rhenium(V) metal precursor, trans-[Re-
OBr3(PPh3)2], was synthesized as previously reported [23]. The chemicals were used without
any further purification. Ultrapure water was produced from an Elga Purelab Ultra system.
The infrared spectra were recorded on a Perkin–Elmer Spectrum 100 from 4000–650 cm−1.
1H NMR spectra were obtained using a Bruker Avance 400 MHz spectrometer. All NMR
spectra were recorded in DMSO-d6 except for 1, which was recorded in deuterated
dichloromethane (CD2Cl2). UV–Vis spectra were recorded using a Perkin–Elmer Lambda
25. Melting points were determined using a Stuart SMP3 melting point apparatus. The
conductivity measurements were determined at 295 K on a Radiometer R21M127 CDM
230 conductivity and pH meter.
2.2. 3-{(Z)-[(2-Sulfanylphenyl)imino]methyl}-4H-chromen-4-one (Htch)
2-Aminobenzenethiol (0.154 cm3, 1.44 mM) and 3-formylchromone (0.250 g, 1.44 mM)
were refluxed in 25 cm3 of MeOH. After 3 h, the solution was reduced to 10 cm3 and the
yellow precipitate was filtered. The precipitate was washed with cold MeOH and petroleum
ether. Yield = 63%. M.P. 224.4 – 228.6 °C; IR (νmax/cm–1): ν(S–H) 3309, ν(C=O) 1618,
1
ν(C=N) 1595, v(C=C) 1456; H NMR (295K/ppm): (figure S1, see online supplemental
material at http://dx.doi.org/10.1080/00958972.2013.867028) 8.24 (s, 1H, H1), 8.09
(d, 1H, H8), 7.83 (t, 1H, H9), 7.68 (d, 1H, H5), 7.51 (t, 1H, H3), 7.01 (d, 1H, H2), 6.89 (t,
1H, H10), 6.68-6.71 (m, 2H, SH, H6), 6.61 (t, 1H, H4), 6.40 (d, 1H, H11). UV–Vis
(DMF, λmax (ε, M–1cm–1)): 296 nm (9056), 313 nm (11513).
2.3. (5Z)-5-((4-Hydroxy-2-methoxy-2H-chromen-3-yl)methyleneamino)-6-amino-1,3-dime
thylpyrimidine-2,4(1H, 3H)-dione (H3uch)
1,3-Dimethyl-5,6-diamino uracil (0.244 g, 1.44 mM) and 3-formylchromone (0.250 g,
1.44 mM) were dissolved in MeOH (35 cm3) and stirred for 3 h under reflux. The fluffy
yellow precipitate was filtered and washed with cold methanol and petroleum ether. Yield =
66%. M.P. 282.7 – 284.9 °C; IR (νmax/cm–1): ν(O–H) 3434, ν(N–H) 3309, ν(C=O) 1683,
1
ν(C=N) 1613, v(C=C) 1583; H NMR (295K/ppm): (see figure S2) 9.78 (s, 1H, H1), 9.28
(s, 1H, H6), 8.13 (d, 1H, H5), 7.82 (t, 1H, H4), 7.70 (d, 1H, H2), 7.52 (t, 1H, H3), 7.41
(br, s, 2H, NH2), 6.89 (s, 2H, NH2), 3.87 (s, 3H, C(8)H3), 3.76 (s, 3H, C(7)H3), 3.31 (br, s,
1H, OH), 3.18 (s, 3H, OC(10)H3). UV–Vis (DMF, λmax (ε, M–1cm–1)): 294 nm (3004),
358 nm (2632).
2.4. fac-[Re(CO)3(bsch)Cl] (1)
Htch (0.069 g, 0.246 mM) was added to [Re(CO)5Cl] (0.1 g, 0.246 mM) in 20 cm3 of
toluene. The mixture was heated at reflux temperature under an inert N2 atmosphere for
4 h. A yellow precipitate was filtered and dried overnight. It was then dissolved in CH2Cl2
and layered with n-hexane from which yellow parallelogram-shaped crystals formed.

4374
I.N. Booysen et al.
Yield = 71%, m.p. = 301.7 – 304.6 °C. IR (νmax/cm−1): ν(C≡O) 2013, 1924, 1880 (vs),
ν(C=O) 1636 (m), ν(C–O–C) 1620, 1609 (m), ν(C–N)Heterocyclic 1570 (m), ν(C–S) 760 (s);
1H NMR (295 K/ppm): 9.05 (s, 1H, H12), 8.71 (d, 1H, H8), 8.51 (d, 1H, H5), 8.09 – 8.00
(m, 2H, H6, H73), 7.89 – 7.67 (m, 4H, H14, H15, H16, H17). UV–Vis (DMF, (λmax (ε,
M–1cm–1)): 258 nm (1130); 291 nm (1030); 317 nm (766); 381 nm (sh, 362). Conductivity
(DMF, 10–3 M): 0.596 Ohm–1cm–2Ml–1.
2.5. [ReO(Hns)] (2)
The title compound was afforded from reaction of Htch (0.059 g, 0.0207 mM) with trans-
[ReOBr3(PPh3)2] (0.1 g, 0.0103 mM) in 20 cm3 of refluxing MeOH for 3.5 h. The resultant
reaction mixture was allowed to cool to room temperature. A maroon precipitate was fil-
tered, dried under vacuum and then dissolved in CH2Cl2 and layered with n-hexane. From
this solution reddish brown parallelogram-shaped crystals were isolated which were suitable
for single crystal X-ray analysis. Yield = 52%, m.p. = >350 °C; IR (νmax/cm−1): ν(O–H)
3062 (w), ν(C=O) 1621, 1585, 1574 (m), ν(C=N) 1558 (m), ν(Re=O) 986 (m),
1
ν(C–S) 749, 761 (vs); H NMR (295K/ppm): 10.49 (br, s, 1H, OH), 9.28 (s, 2H, H7, H8),
7.83 – 7.71 (m, 4H, H18, H19, H20, H21), 7.51 – 7.42 (m, 2H, H2, H14), 7.32 (t, 2H, H3,
H4), 7.20 (t, 2H, H12, H13), 7.10 – 7.00 (m, 2H, H5, H11). (see figure S3) UV–Vis (DMF,
(λmax (ε, M–1cm–1))): 252 nm (sh, 1320); 268 nm (sh, 1060); 281 nm (1040); 310 nm
(1080); 440 nm (193). Conductivity (DMF, 10-3 M): 0.571 Ohm–1cm–2M–1.
2.6. [ReO(OCH3)(PPh3)(Huch)] (3)
Equimolar ratios of H3uch (0.037 g, 0.0103 mM) and trans-[ReOBr3(PPh3)2] (0.1 g,
0.0103 mM) were dissolved in 20 cm3 of MeOH and the resultant reaction mixture was
refluxed for 3 h. The mother liquor was halved and both aliquots were diluted with a further
10 cm3 of MeOH. Brown platelet crystals formed via slow evaporation of the mother liquor.
Yield = 58%, m.p. = >350 °C. IR (νmax/cm–1): ν(N-H) 3039(w), ν(C=O) (sh) 1722, 1683(s),
ν(C=N) 1616, 1558, 1570(s), ν(C–O–C) 1513, 1463(s), ν(O–CH3) 1186(m), ν(Re=O) 1093
1
(m); H NMR (295K/ppm): 9.79 (s, 1H, H7), 9.33 (br, s, 1H, N(1)H), 9.30 (s, 1H, H9),
8.98 (br, s, 3H, C(3)H3), 8.51 (br, s, 3H, C(1)H3), 8.13 (d, 1H, H14), 7.81 (t, 2H, H12,
H13), 7.74 (d, 1H, H11), 7.57–7.28 (m, 15H, PPh3), 4.17 (br, s, 3H, C(17)H3), 1.20 (s, 3H,
C(18)H3). 31P NMR (295K/ppm) 25.67. UV–Vis (DMF, (λmax (ε, M–1cm–1)): 240 nm
(3590); 280 nm (2970); 344 nm (446); 401 nm (230); 473 nm (56); 766 nm (4.45). Con-
ductivity (DMF, 10–3 M): 1.548 Ohm–1cm–2M–1.
2.7. X-ray diffraction
X-ray data were recorded on a Bruker Apex Duo equipped with an Oxford Instruments
Cryojet operating at 100(2) K and an Incoatec microsource operating at 30 W power. Crys-
tal and structure refinement data are given in table 1. Selected bond lengths and angles are
given in tables 2 and 3. The data were collected with Mo Kα (λ = 0.71073 Å) radiation at a
crystal-to-detector distance of 50 mm. The following conditions were used for the data col-
lection: omega and phi scans with exposures taken at 30 W X-ray power and 0.50° frame
widths using APEX2 [23]. The data were reduced with SAINT [24] using outlier rejection,

Chromone Schiff bases
Table 1. Crystal data and structure refinement data.
4375
1·CH2Cl2
2
3
Chemical formula
Formula weight
C20H11Cl3NO5ReS
669.93
C22H15O3ReS2
605.71
C36H30N4O7PReBr
851.85
Temperature (K)
Crystal system
Space group
100(2)
Monoclinic
P21/c
100(2)
Monoclinic
P121/c1
100(2)
Triclinic
P-1
Unit cell dimensions (Å, °)
a = 10.8720(4)
b = 13.9219(5)
c = 14.3374(5)
α = 90.00°
β = 103.687(2)°
γ = 90.00°
0.40 × 0.15 × 0.10
2108.47(13)
4
a = 11.3071(5)
b = 13.9167(6)
c = 12.9198(5)
α = 90.00°
β = 106.785(2)°
γ = 90.00°
0.22 × 0.18 × 0.10
1946.41(11)
4
a = 9.7726(4)
b = 12.8281(5)
c = 14.3678(6)
α = 69.801(2)°
β = 86.957(2)°
γ = 83.384(2)°
0.18 × 0.10 × 0.06
1679.00(14)
2
Crystal size (mm)
V (Å3)
Z
Density (Calcd) (Mg/m3)
Absorption coefficient (mm–1
F (000)
2.11
6.277
1279.7
2.07
6.486
1167.7
1.68
3.723
839.8
)
θ Range for data collection (°)
1.9; 26.4
3.6; 28.3
5; 27.0
Index ranges
–13 ≤ h ≤ 13
–14 ≤ k < 17
–17 ≤ l ≤ 17
17401
3965
4237
4237/0/275
1.077
0.017; 0.039
0.015
–13 ≤ h ≤ 15
–15 ≤ k < 18
–17 ≤ l ≤ 13
13794
4452
4798
4798/0/278
1.261
0.015; 0.045
0.019
–7 ≤ h ≤ 11
–14 ≤ k < 16
–18 ≤ l ≤ 17
16569
6143
6620
6620/0/445
1.142
0.101; 0.095
0.015
Reflections measured
Observed reflections [I>2σ(I)]
Independent reflections
Data/restraints/parameters
Goodness of fit on F2
Observed R, wR2
Rint
Table 2. Selected bond lengths [Å] and angles [°] for 1.
Experimental
Optimized
Re–C1
Re–C2
Re–C3
Re–N
Re–O4
Re–Cl1
C10–N
C10–S
1.923(3)
1.900(3)
1.922(3)
2.210(2)
2.147(2)
2.4605(7)
1.314(4)
1.738(3)
1.730(3)
1.399(4)
1.469(4)
1.362(4)
1.256(3)
1.377(4)
176.68(8)
175.0(1)
173.0(1)
82.24(8)
111.5(2)
127.0(3)
1.916
1.905
1.922
2.266
2.211
2.510
1.312
1.762
1.743
1.400
1.466
1.362
1.242
1.382
175.95
173.95
172.03
81.03
111.90
126.81
C9–S
C4–N
C10–C11
C11–C12
C19–O4
C16–C17
C1–Re–Cl1
C2–Re–O4
C3–Re–N
O4–Re–N
C10–N–C4
N–C4–C5

4376
I.N. Booysen et al.
Table 3. Selected bond lengths [Å] and angles [°] for 2 and 3.
2
3
Experimental
Optimized
Experimental
Optimized
Re–O1
Re–N1
Re–N2
Re–S1
Re–S2
C7–N1
C9–N2
C7–C8
1.684(2)
2.080(2)
2.083(2)
2.2848(7)
2.2892(6)
1.335(3)
1.331(3)
1.399(3)
1.402(3)
1.238(4)
1.358(4)
108.65(8)
108.07(7)
105.29(8)
108.99(7)
87.10(8)
82.31(6)
82.48(6)
124.1(2)
142.35(6)
146.64(6)
121.0(2)
120.0(2)
1.686
2.104
2.098
2.314
2.318
1.329
1.333
1.396
1.401
Re–O1
Re–O2
Re–O3
Re–N1
Re–N4
Re–P
N4–C7
C8–C9
C8–C16
C15–C16
C16–O3
C9–O5
C17–O5
O1–Re–O2
O3–Re–N1
N4–Re–P
N1–Re–N4
N4–Re–O3
C5–N4–C7
C18–O2–Re
C17–O5–C9
–
1.697(4)
1.918(4)
2.109(4)
2.000(5)
2.087(5)
2.457(2)
1.305(9)
1.51(1)
1.394(8)
1.472(9)
1.300(8)
1.403(8)
1.43(1)
168.3(2)
168.3(2)
170.1(1)
79.3(2)
91.9(2)
124.4(5)
140.0(4)
114.5(6)
–
1.703
1.941
2.165
2.013
2.080
2.553
1.320
1.507
1.403
1.473
1.288
1.402
1.425
166.83
168.40
171.68
79.37
90.73
123.54
146.43
114.41
–
C8–C9
C16–O2
C22–O3
O1–Re–N1
O1–Re–S1
O1–Re–N2
O1–Re–S2
N1–Re–N2
N1–Re–S1
N2–Re–S2
C7–C8–C9
N1–Re–S2
N2–Re–S1
Re–N1–C7
Re–N2–C9
1.240
1.338
106.88
109.72
108.45
108.97
83.31
81.96
82.00
124.19
144.15
141.84
121.27
119.92
–
–
–
scan speed scaling, as well as standard Lorentz and polarization correction factors. A
SADABS semi-empirical multi-scan absorption correction [25] was applied to the data.
Direct methods, SHELXS-97 [26] and WinGX [27] were used to solve all three structures.
All non-hydrogen atoms were located in the difference density map and refined anisotropi-
cally with SHELXL-97 [26]. All hydrogens were included as idealized contributors in the
least squares process. Their positions were calculated using a standard riding model with
C–Haromatic distances of 0.93 Å and Uiso = 1.2 Ueq. The CH2Cl2 solvate C–H bonds of 1,
the phenolic O–H bonds of 2 and amido N–H bond of 3 were located in the difference
density map and refined isotropically.
2.8. Computational details
Computational calculations were conducted with Gaussian 09W [28]. Geometry optimiza-
tions of the rhenium complexes were achieved through DFT calculations using the B3LYP
functional, with an accompanying hybrid basis set viz. the 6-311G++ (d, p) basis set was
applied to all the C, H, N, O, and S (for 1 and 2)/P (for 3) atoms and the LANL2DZ basis
set applied to the metal centre [11, 29]. The crystal structures were used as starting struc-
tures but the CH2Cl2 solvent (for 1) of recrystallization was removed prior to any calcula-
tions. Good agreement was found between the optimized and geometrical parameters (refer
tables 2 and 3) with the minor deviations due to the fact that gas phase optimized structures
do not account for hydrogen bonding interactions or any short distance contacts. Using the
optimized structures of the metal complexes, frequency calculations confirmed that the
respective structures are at global minima on the potential energy surfaces and from these,
reliable vibrational energies were attained [30].

Chromone Schiff bases
4377
Figure 1. An ORTEP view of 1·CH2Cl2 showing 50% probability displacement ellipsoids and the atom labelling.
3. Results and discussion
3.1. Synthesis, spectral characterization and DFT studies of the rhenium complexes
In the formation of fac-[ReI(CO)3(bsch)Cl] (1) (figure 1), the Schiff base Htch underwent
cyclization between the thiol sulfur and the imino carbon (C10) to afford the neutral
bidentate chelator, bsch. In a previous study, we observed a similar phenomenon whereby
the Schiff base ligand, 2-(2-aminophenyliminomethyl)-4H-chromen-4-one (H2pch) cyclized
in a similar manner upon coordination to the fac-[Re(CO)3Cl] moiety, to afford the
heterocyclic, 2-benzimidazole-4H-chromen-4-one (bzch) chelator [31]. However, the Schiff
base Htch transformed upon reacting with the more acidic rhenium(V) precursor,
trans-[ReVOBr3(PPh3)2], which led to formation of the square pyramidal oxorhenium(V)
complex, [ReO(Hns)] (2) (figure 2).
Upon reacting H3uch with trans-[ReVOBr3(PPh3)2], [ReVO(OCH3)(PPh3)(Huch)] (3)
(figure 3) was isolated, where Huch acted as a dianionic tridentate chelator through the
anionic enol oxygen O3, neutral imino nitrogen N4, and singly deprotonated amido N1
nitrogen. It was no surprise that a methoxy group covalently linked to the 2-position (i.e. C9)
of the chromone moiety. A mechanism has previously been proposed for the nucleophilic
addition of an ethoxy group to the same carbon position of 3-formylchromone [17, 18].

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I.N. Booysen et al.
Figure 2. An ORTEP view of 2 showing 50% probability displacement ellipsoids and the atom labelling.
All the compounds exhibit good solubility in most polar solvents but moderate to non-
solubility in non-polar solvents. These rhenium compounds have significantly low molar
1
conductivity values due to their charge neutrality. The H NMR spectra show well-resolved
peaks due to the diamagnetic nature for the d2 (for 2 and 3) systems. Despite increasing the
1
number of scans and using deuterated chloroform or dimethylsulfoxide, no informative H
NMR spectrum could be attained for the diamagnetic, d6 system (complex 1). Nevertheless,
1
an interpretable H NMR spectrum of 1 could be attained in deuterated dichloromethane. In
comparison to the spectrum of the free ligand (i.e. Htch), the spectrum of 1 indicates there
are no signals found for the protons associated with the mercapto and Schiff base imino
groups. Also, selected aromatic peaks of the bsch chelator coalesce into two multiplets
which integrates to two (between 8.09 – 8.00 ppm) and four (between 7.89 – 7.67 ppm)
protons, respectively. The remaining signals of 1 appear downfield as two doublets, which
are assigned to the aromatic protons on the benzothiazole moiety, and a sharp singlet, due
to the proton (H12) of the chromone moiety. For 2, the hydrogens of the bridging phenyl
ring occur as a four proton multiplet and the proton of the phenolic oxygen appears as a
broad singlet (at 10.49 ppm). The remaining proton signals emphasize symmetry within the
molecule, e.g. only one sharp signal appears for the Schiff base hydrogens which integrates
1
for two (see figure S3). A distinctive feature of the H NMR spectrum of 3 is the sharp sin-
glet at 4.17 ppm which is assigned to the three protons of the methoxy group covalently
bonded to the chromone moiety. The methine proton was found as a singlet at 9.79 ppm

Chromone Schiff bases
4379
Figure 3. An ORTEP view of 3 showing 50% probability displacement ellipsoids and the atom labelling.
and is found at a similar position relative to the imine proton of the free ligand H3uch
(at 9.78 ppm). Furthermore, the proton of the singly deprotonated amino group is found at
9.33 ppm and this signal integrates to one proton. This signal is a clear indication that
1
coordination has occurred since in the free-ligand’s H NMR spectrum the signal of the
amino proton’s signal appears as a broad singlet at 7.41 ppm (figure S2). This was also
accompanied by the disappearance of the hydroxyl proton of the chromone group.
The IR spectrum of 1 is dominated by the intense bands of the tricarbonyl co-ligands
(2013, 1924 and 1880 cm–1), while a strong vibrational band is found at 760 cm–1 due to
ν(C–S). All other bands associated with the chromone and heterocyclic moieties are found
as medium intensity bands between 1636 and 1570 cm–1 (figure S4). In the IR spectrum of
2, the hydroxyl vibration appears as a weak intensity band at 3062 cm–1. No ether vibra-
tional bands were found for 2 (compared to the ν(C–O–C) bands at 1620 and 1609 cm–1 for
1) due to transformation of the chromone. The three medium intensity ketonic vibrational
bands at 1621, 1585 and 1574 cm–1 are further evidence that the ligand has transformed.
The two bands due to v(C=N) overlap and appear as a broad vibrational band (1558 cm–1),
while the two C–S vibrations appear individually as sharp and strong bands at 749 and
761 cm–1. The vibrational peaks at 986 (for 2) and 917 (for 3) cm–1 are found in the
890 – 1020 cm–1 range [32] and are ascribed to rhenium(V)-oxo stretching vibrations. The
optimized and experimental (see figures S4 and S5) IR spectra of 1 showed distinctive

4380
I.N. Booysen et al.
similarities. For example in 1, intense vibrations associated with the fac-[Re(CO)3]+ core are
computed at 2101, 2020 and 1995 cm−1 (figure S5), while a medium intensity band is
computed at 1659 cm−1 which constitute the main vibrations as induced by the heterocyclic
and chromone moieties. However, for 2 the calculated [ReVO]3+ core vibrates at a much
higher frequency (observed at 1021 cm−1) than what was experimentally (at 986 cm−1)
observed (figure S6). This is largely ascribed to the fact that the gas phase frequency calcula-
tion of 2 does not account for weak inter- and intramolecular interactions as observed within
the crystal structure. This was also observed in the deviation of the calculated (observed at
3411 cm–1) and experimental hydroxyl group vibrational bands (observed at 3062 cm−1),
where the aforementioned is also influenced by the classical hydrogen-bonding between the
ketonic oxygen and the hydroxyl group.
Multiple vibrations were found in the IR spectrum of 3, resulting in considerable overlap-
ping of the vibrational bands, whereby many bands coalesced into broad bands or into
bands with poor resolution. However, a weak intensity band at 3039 cm−1 was clearly
observed and is assigned to ν(N–H). A frequency calculation of the optimized structure
provided insight into the interpretation of the experimental IR spectrum of 3 (figure 4).
Similar ketonic vibrational frequencies of the uracil moiety were found between the
experimental (a shoulder at 1772 cm−1 and a strong vibration at 1683 cm−1) and optimized
molecules (1741 and 1696 cm−1). Similarly, the ν(C=N) vibrational bands appear as three
peaks in both the calculated (1588, 1633 and 1642 cm−1) and experimental (1570, 1588
and 1616 cm−1) spectra. The peaks between 1450 and 1550 cm−1 in the calculated spectrum
are largely due to the vibrations associated with the chromone moiety [i.e. ν(C–O–C)], and
similarly in the corresponding region of the experimental spectrum, two bands (1463 and
1513 cm−1) were found. Medium intensity bands were experimentally observed for the
Figure 4. Experimental (red) and calculated (blue) IR spectra of 3. Note that the calculated spectrum is not plotted
to scale (see http://dx.doi.org/10.1080/00958972.2013.867028 for color version).

Chromone Schiff bases
4381
metal to oxo (1093 cm−1) and metal to methoxy (1186 cm−1) bonds which were assigned
based on the calculated vibrations at 1137 [Re(V)=O] and 1176 cm−1 [Re(V)–OCH3].
These rhenium to oxygen vibrational bands were similar to other compounds with Re(V)=O
and Re(V)-OCH3 bonds [33]. The close correlation between the optimized and geometrical
parameters of the respective complexes is further emphasized by the low calculated RMSD
values (figure 5). The larger RMSD value of 3 is mainly attributed to the flexible degree of
rotation of the P-C single bonds within the PPh3 co-ligands.
The electronic spectra of 1 and 2 exhibit similar electron transitions in comparison to the
electronic spectrum of the free ligand (Htch) (see figure S7). Therefore, the electronic tran-
sitions within the UV region (but below 375 nm) were assigned to π–π* transitions (258,
291, 317 nm for 1 and 252, 268, 281, 310 nm for 2). The similarities are due to the highly
delocalized nature of the free ligand, Htch, and chelators, bsch (for 1) and Hns (for 2). A
ligand-to-metal charge transfer band (LMCT) is observed at 381 nm for 1 while for 2, a
metal-to-ligand charge transfer (MLCT) band at 440 nm is observed. Indicative to 1 and 2,
3 showed an array of π-π* transitions (240, 280 and 344 nm) in addition to two MLCT
bands at 401 and 473 nm. No d-d transition was observed for 1 due to its low-spin d6 elec-
tron configuration whereas the oxorhenium(V) complex 3 had a distinctive d-d transition
(but with a low extinction coefficient) due to its d2 configuration. No metal-based electronic
transition could be observed for the other oxorhenium(V) compound 2.
3.2. Crystal structure of 1·CH2Cl2
Compound 1 exhibits a distorted octahedral geometry which is largely due to the six-mem-
bered chelate [Re, O4, C19, C11, C10, N] ring which affords a constrained bite angle of
82.24(8)° [O4–Re–N] (figure 1). This phenomenon induces larger deviations from linearity
within the equatorial angles (N–Re–C3 = 173.0(1)° and O4–Re–C2 = 175.0(1)°) compared
to the axial angle [C1–Re–Cl1] of 176.68(8)°.
Evaluating the bonds within the coordination sphere, the Re–C2 bond (1.900(3) Å) has a
significantly shorter bond distance than the other metal carbon bonds (Re–C1 = 1.923(3) Å
and Re–C3 = 1.922(3) Å). This is accounted to the weaker trans-influence of the ketonic
oxygen O4 compared to the N donor atom and Cl1 co-ligand. All the other coordination
Figure 5. Overlay structures of the optimized and crystal structures for the respective complexes with their
corresponding RMSD values. The crystal structures are shown in blue while the optimized structures are given in
yellow (see http://dx.doi.org/10.1080/00958972.2013.867028 for color version).

4382
I.N. Booysen et al.
sphere bonds within 1 were comparable with similar rhenium(I) compounds as found in the
literature. For example, the metal-ketonic oxygen (Re–O4 = 2.147(2) Å) and chloride
(Re–Cl1 = 2.4605(7) Å) bond lengths were similar to the bonds of the benzimidazole ana-
logue, fac-[ReI(CO)3Cl(bzch)] {Re-Oketonic = 2.137(2) Å and Re–Cl1 = 2.492(6) Å} [31].
The Re-N distance of 2.210(2) Å was in the range of rhenium(I) compounds with coordi-
nating benzothiazole moieties [34, 35]. For example, in fac-[ReI(Habt)(CO)3Br], where the
2-aminophenyl-1H-benzothiazole (Habt) coordinated in an analogues manner (i.e. as a
neutral bidentate chelator within the equatorial plane), a rhenium imidazolium bond distance
of 2.227(2) Å was observed [36].
Within the benzothiazole moiety, the bond orders of the C–N bonds are readily
distinguishable based on their distances (C10–N = 1.314(4) Å and C4–N = 1.399(4) Å). In
contrast to the C–N bond distance, the C–S single bonds (C9–S = 1.730(3) Å and C10–S =
1.738(3) Å) are nearly equidistant. It was found that the C11-C12 (1.362(4) Å) double bond
has a similar bond length as the delocalized C–C double bonds (e.g. C16–C17 = 1.377(4) Å)
within the phenyl ring, showing unequivocally that it is a localized double bond.
The chromone and benzothiazole moieties are bridged by a carbon-carbon (C10-C11 =
1.469(4) Å) single bond which allows the moieties to form a dihedral angle of 7.95°. In
turn, these moieties are found considerably out of the mean equatorial plane by 34.72° (for
benzothiazole) and 36.84° (for chromone). The crystal lattice is stabilized by a dichloro-
methane (CH2Cl2) solvent molecule which interacts via non-classical hydrogen-bonds with
a molecule of 1. Further stabilization is afforded through intermolecular π-π* bonding
between the bsch chelators of adjacent molecules with a distance of 3.729 Å between two
opposing conjugated phenyl rings. Each unit cell consists of four molecules of 1 with their
corresponding dichloromethane solvent molecule of recrystallization. Furthermore, the bsch
chelator of each molecule aligns with the [b]-axis (see figure S8).
Facial tricarbonyl rhenium(I) complexes with benzothiazole-based chelators are well doc-
umented [34–38]. A typical example is fac-[Re(CO)3(bt)2Cl], which was isolated from the
2 : 1 molar ratio reaction of benzothiazole (bt) with [Re(CO)5Cl] in anhydrous methanol.
The two bt chelators coordinated as two neutral monodentate chelators [37]. Recently, the
isolation of a unique dinuclear rhenium(I) compound, (μ-dbt)2[Re(CO)3]2, [Hdbt = bis-{1,2-
(benzothiazol-2-ylidene)}thiourea] was reported. The complex contains two fac-[Re(CO)3]+
moieties and two bridging anionic tridentate chelators dbt which coordinates individually
in a bidentate manner to one metal center but also monodentate to the other metal
center [38].
3.3. Crystal structure of 2
Compound 2 exhibits a square-pyramidal geometry with the rhenium-oxo core occupying
the apical position relative to the tetradentate N2S2 chelator (Hns) in the equatorial plane
(figure 2). However, the structure is highly distorted as the Re atom lies 0.664 Å out of the
mean equatorial plane [N1, N2, S1, S2], which affords large deviations from the idealized
90° for the angles: O1–Re–N1 = 108.65(8)°, O1–Re–N2 = 105.29(8)°, O1–Re–S1 =
108.07(7)° and O1–Re–S2 = 108.99(7)°. Another contributive factor to the distortion is the
influence of the five- and six-membered chelate rings which affords constrained bite angles
(N1–Re–S1 = 82.31(6)°, N1–Re–N2 = 87.10(8)° and N2–Re–S2 = 82.48(6)°) which are
smaller than the idealized angle (90°).

Chromone Schiff bases
4383
The Re–N1–C7 (121.0(2)°) and Re–N2–C9 (120.0(2)°) angles emphasize the sp2-hybrid-
ization of the N1 and N2 atoms. The hybridization of the nitrogens is further supported by
the Schiff base bond distances (C7–N1 = 1.335(3) Å and C9–N2 = 1.331(3) Å), which were
comparable to other chelating Schiff base moieties coordinated to the oxorhenium(V) core
[39]. Interestingly, the bridging carbon to carbon bond distances (C7–C8 = 1.399(3) Å and
C8–C9 = 1.402(3) Å) have similar bond distances compared to those found within the phe-
nyl rings (e.g. C5–C6 = 1.401(3) Å). Of further interest is that these bridging carbons afford
a bond angle of C7–C8–C9 = 124.1(2)°, which is close to the bond distances and angles
found in the para-diazabenzenesulfonic complexes, [M(oyl)-N=N-C6H4SO3H] {M = Ni or
CuH, oyl = 1,1,2,8,9,9-hexamethyl-4,6-dioxa-5-hydro-3,7,10,14-tetraazacyclotetradecane-
2,7,10,12-tetraene}, containing an analogous aliphatic sp2 NCCCN bridged moiety [40].
This implies that the bridging carbon to carbon bonds are delocalized double bonds and
that the C7–C8–C9 bridge acts as a delocalized anionic moiety. This affirms that the Hns
chelator acts as a tri-anionic tetradentate chelator.
The phenolic bond distance (C22–O3 = 1.358(4) Å) is much longer than the ketonic bond
distance (C16–O2 = 1.238(4) Å), which indicates that the latter is a double bond. In addi-
tion, one intramolecular classical hydrogen-bond is observed between the above-mentioned
moieties (O3–H11···O2 = 1.87(4) Å). Despite the highly delocalized nature of the Hns che-
lator, no intermolecular π–π stacking was observed.
Although oxorhenium(V) complexes adopting square pyramidal geometries are common
in literature [41–43], rhenium complexes with a delocalized monoanionic carbon bridged
system could not be found.
A neutral square pyramidal complex [ReO(dne)],
H3dne = N′-(2-aminobenzyl)-N-[(2-aminophenyl)methylidene]ethane-1,2-diamine and its
complex salt derivative, [ReO(ane)]PF6 were formed from the reactions of the symmetrical
diimine ligand N,N’-bis-(2-aminophenyl)methylidene]ethane-1,2-diamine (H2ane) with
trans-[ReVO2(py)4]Cl and cis-[ReO2I(PPh3)2], respectively. The chelator dne {in [ReO
(dne)]} acts as a triamido imine in which one imine group is reduced upon coordination
whereas for [ReO(ane)]PF6 the ane chelator acts as a dimino diamido moiety [44].
Furthermore, several square pyramidal oxorhenium(V) complexes with
a ‘3+1’
mixed-ligand coordination behavior have been reported. The motivation behind this design
is that it limits the formation of multiple isomers with different biological activity, which is
commonly found for the tetradentate chelated oxorhenium(V) complexes. An additional
advantage is that a biologically active molecule can be covalently conjugated to either the
tri- or mono-organic ligands within the coordination sphere or directly coordinated to the
metal center [45–47].
3.4. Crystal structure of 3
The six-coordinate complex, [ReO(OCH3)(PPh3)(Huch)], was isolated as XRD quality
brown plates, grown from slow evaporation of the mother liquor. Two crystallographically
dependent molecules of 3 occupy the asymmetric unit cell in the space group P-1,
i.e. Z = 2 (see figure S9). The rhenium metal atom is in the center of a distorted octahedron
with the dianionic tridentate chelator, Huch, and the PPh3 co-ligand occupying the
equatorial plane, while the axial is formed by the metal oxo (Re=O) moiety and the
methoxy [O2–C18H3] co-ligand (figure 3). The octahedral distortion is brought about by
five- and six-membered chelate rings with bite angles of 79.3(2)° [N1–Re–N4] and 91.9(2)°
[O3–Re–N4], which induce deviations from linearity for the angles: O1–Re–O2 = 168.3
(2)°, N4–Re–P = 170.1(1)° and O3–Re–N1 =168.3(2)°.

4384
I.N. Booysen et al.
The enol form of the ligand in 3 is confirmed from the longer C–O distance (C16–O3 =
1.300(8) Å) compared to the ketone bond distances in 1 (C19–O4 = 1.256(3) Å) and 2
(C16–O2 = 1.238(4) Å). The presence of the enol tautomer is further supported by the sig-
nificantly different C–C (C8–C16 = 1.394(8) Å and C15–C16 = 1.472(9) Å) bonds within
the chromone moiety, which is indicative of variable bond orders. Similar rhenium(V)-oxo
bond lengths were found for 2 (Re–O = 1.684(2) Å) and 3 (Re–O1 = 1.697(4) Å). The
C7–N4–C5 bond angle of 124.4(5)° deviates from the ideal 120° expected for a bond angle
containing a bridging sp2-hybridized nitrogen, which is largely ascribed to the influence of
the five-membered chelate ring. However, the Re–N4 bond distance of 2.087(5) Å is well
within the expected range (2.03(1) – 2.15(1) Å) for rhenium(V) compounds containing
Schiff base chelates [48–50]. The other rhenium to nitrogen coordination bond, Re–N1
(2.000(5) Å), is close to the lower end of the literature range (1.91(1) – 2.01(1) Å) found for
rhenium(V)-amido bonds [51–55]. As expected, the rhenium methoxy (Re–O2 = 1.918(4) Å)
bond length in 3 is much longer than the rhenium oxo bonds (for 2 and 3) but similar
to other Re(V)–O–CH3 bond distances typically observed for oxorhenium(V) compounds
[56, 57]. In addition, the Re-P bond distance of 2.457(2) Å is almost identical to that
found in the p-tolylimido (p-NC6H4CH3) rhenium(V) complex salt, [Re(p-NC6H4CH3)
(hpb)2(PPh3)]Cl, Hhpb = 2-(2-hydroxyphenyl)-1H-benzimidazole {Re–P = 2.460(2) Å}
[58].
4. Conclusion
The coordination behavior of the chromone derived Schiff bases towards the fac-
[ReI(CO)3]+ and [ReVO]3+ cores resulted in formation of metal complexes with unique
structural features. For 1, fac-[ReI(CO)3(bsch)Cl], the bsch chelator, shared the same
proposed pathway of formation and coordination mode as the analogous heterocyclic, 2-
benzimidazole-4H-chromen-4-one (bzch) chelator found in a previously isolated rhenium(I)
compound, fac-[Re(CO)3(bzch)Cl]. Similarly, in 3, [ReO(OCH3)(PPh3)(Huch)], the Huch
chelator, coordinated as a dianionic tridentate moiety through the singly deprotonated uracil
amido nitrogen and the classical chromone Schiff base NiminoOenol donor set which is
common to other tridentate chromone Schiff bases coordinated to transition metal
complexes. For 2, the manner in which Htch transformed was not previously observed for
this class of ligands. The resultant complexes were spectroscopically characterized and their
structures confirmed by single crystal X-ray diffraction. Optimized and experimental
geometrical parameters of the complexes are in reasonable agreement. Furthermore, the
computed vibrational properties of the complexes have similarities to that of the respective
experimental IR spectra. Future prospects entail anticancer-activity testing against various
cell-lines and the compounds that exhibit optimal cytotoxicity may proceed to radiopharma-
ceutical studies.
Supplementary material
CCDC 940580, 940578 and 940578 contains the supplementary crystallographic data for
1·CH2Cl2, 2 and 3. These data can be obtained free of charge at www.ccdc.cam.ac.uk/
conts/retrieving.html [or from the Cambridge Crystallographic Data Centre (CCDC), 12
Union Road, Cambridge CB2 1EZ, UK; Fax: +44(0)1223-336033; or E-mail: depos-
it@ccdc.cam.ac.uk].

Chromone Schiff bases
4385
Acknowledgements
We are grateful to the University of KwaZulu-Natal and the National Research Foundation of
South Africa for financial support.
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