Novel ruthenium(II) and (III) compounds with multidentate Schiff base chelates bearing biologically significant moieties

Article abstract of DOI:10.1016/j.poly.2014.05.021

In this account, we report the isolation of novel ruthenium(II/III) compounds from the respective reactions of the metal precursor, trans-[RuCl ₂(PPh₃)₃] with multidentate Schiff base ligands bearing biologically significa

Full text of DOI:10.1016/j.poly.2014.05.021

Polyhedron 79 (2014) 250–257
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Novel ruthenium(II) and (III) compounds with multidentate Schiff base
chelates bearing biologically significant moieties
⇑
Irvin Noel Booysen , Sanam Maikoo, Matthew Piers Akerman, Bheki Xulu
School of Chemistry and Physics, University of KwaZulu-Natal, Private Bag X01, Scottsville 3209, Pietermaritzburg, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
In this account, we report the isolation of novel ruthenium(II/III) compounds from the respective reactions
of the metal precursor, trans-[RuCl2(PPh3)3] with multidentate Schiff base ligands bearing biologically
significant moieties (viz. antipyrine, uracil or chromone). From these coordination reactions of trans-
[RuCl2(PPh3)3] with 4-((pyridine-2yl)methyleneimino)antipyrine (pap), 4-((pyridine-2-ylimino)
methylene)-chromone (pch), 2,6-bis-((6-amino-1,3-dimethyluracilimino)methylene)pyridine (H4ucp)
and 2,6-bis-((antipyrine-imino)methylene)pyridine (bpap); the complex salt, trans-[RuIICl(pap)(PPh3)2]
BF4 and the ruthenium(II/III) complexes: trans-P, cis-Cl-[RuIII(pch)Cl2(PPh3)2], [RuII(H3ucp)Cl(PPh3)] and
cis-[RuIICl2(bpap)(PPh3)] were formed, respectively. These ruthenium compounds were characterized via
multinuclear NMR, IR, UV–Vis, emission and ESR spectroscopy, conductivity measurements as well as
structural elucidations were confirmed via single crystal X-ray diffraction. The redox properties of metallic
compounds were investigated by voltammetric analysis. In addition, DFT studies of [RuII(H3ucp)Cl(PPh3)]
were conducted to provide insight into its intrinsic solid state structural features.
Received 13 February 2014
Accepted 11 May 2014
Available online 20 May 2014
Keywords:
Ruthenium(II/III)
Uracil
Chromone
Antipyrine
Crystal structure
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
anti-oxidant activities as well as DNA binding capability [11,12].
An example is the organometallic ruthenium(II) complex,
The widespread interest in ruthenium Schiff base compounds is
due to their diverse applications rendered through their unique
properties such as photoluminescence, catalytic and biological
properties [1–5]. Schiff bases are known to stabilize ruthenium in
the oxidation states +II and +III which is emphasized by the metal
center’s preferential coordination affinity towards neutral nitrogen
donor atoms, like imino and pyridyl nitrogens [6,7]. Furthermore,
the utilization of multidentate Schiff base ligands provides addi-
tional thermodynamic stability through the formation of chelate
rings [8]. A typical example is the isolation of the ‘3+3’ paramag-
netic ruthenium(III) compounds, [Ru(LR)2]ClO4 from the 1:2 molar
ratio reactions between [RuII(DMSO)4Cl2] and 4-R-2-((2-(pyridin-
2-yl)hydrazono)methyl)phenol (HLR; R = H, Cl, Br, Me and OMe) [9].
The discovery of the potent anti-metastatic cancer activity of
NAMI A, trans-[RuCl4(DMSO)(Im)](ImH) {ImH = imidazole}, has
led to concerted efforts in the design of new ruthenium
compounds with Schiff base ligands containing various biologically
significant moieties [10]. In particular, Schiff bases derived from
4-aminoantipyrine and their ruthenium complexes have shown
an array of biological activities including antibacterial and
[Ru(oap)(CO)(B)Cl]
(Hoap = 4-((2-hydroxybenzylidene)imino)-
aminoantipyrine) containing the monoanionic OketoNiminoOhydroxyl
tridentate oap chelator which showed optimal DNA cleavage activ-
ity [13]. Recently, we have explored the coordination susceptibility
of potentially tridentate Schiff bases (formed from 5,6-diamino-
1,3-dimethyluracil (H2ddd) and respective 2-substituted aromatic
aldehydes) towards the trans-[RuII(PPh3)]2+ core [14]. The motiva-
tion behind the use of H2ddd is its biological relevance as a nucle-
otide base derivative as well as the fact that uracil-derivatives are
well established chemotherapeutic drugs (e.g. uracil-mustard)
[15].
In this account, we report the formation of ruthenium(II/III)
compounds with multidentate Schiff bases containing antipyrine,
uracil or chromone moieties. The ruthenium compounds, trans-
[RuIICl(pap)(PPh3)2]BF4 (1), trans-P, cis-Cl-[RuIII(pch)Cl2(PPh3)2]
(2), [RuII(H3ucp)Cl(PPh3)] (3) and cis-[RuIICl2(bpap)(PPh3)] (4) were
isolated from the coordination reactions of trans-[RuCl2(PPh3)3]
with the Schiff bases: 4-((pyridine-2yl)methyleneimino)antipyrine
(pap), 4-((pyridine-2ylimino)methylene)-chromone (pch), 2,6-bis-
((6-amino-1,3-dimethyluracilimino)methylene)pyridine (H4ucp)
and 2,6-bis-((antipyrine-imino)methylene)pyridine (bpap) ligands,
respectively. Our selection of the chromone moiety is based on its
⇑
Corresponding author. Tel.: +27 797378626; fax: +27 332605009.
E-mail address: Booyseni@ukzn.ac.za (I.N. Booysen).
biological relevance as
a
secondary metabolite and that
http://dx.doi.org/10.1016/j.poly.2014.05.021
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

I.N. Booysen et al. / Polyhedron 79 (2014) 250–257
251
documented chromone Schiff base transition metal complexes
have shown to exhibit excellent anti-tumor behavior [16,17].
analysis. The ruthenium metal complexes were made up in 2 mM
solutions in DCM along with tetrabutylammoniumhexafluorophos-
phate (0.1 M) as a supporting electrolyte. Between each measure-
ment, the GCWE surface was polished with a slurry of ultrapure
water and alumina on a Buehler felt pad followed by rinsing with
excess ultrapure water and ultra-sonication in absolute ethanol.
2.2. 4-((Pyridine-2-yl)methyleneimino)antipyrine (pap)
Equimolar amounts of 4-aminoantipyrine (1.00 g; 4.27 mmol)
and 2-pyridylaldehyde (0.458 g; 4.27 mmol) were heated at reflux
temperature for 3 h in 30 cm3 of methanol. As the resultant bright
yellow solution was allowed to cool to room-temperature, yellow
needles were grown. These crystals were filtered, washed with
cold methanol, anhydrous petroleum ether and dried under vac-
uum. Yield = 93%, M.P. = 159–163 °C. IR
(m m(C@O)
max/cmꢀ1):
1904 (vs),
m
(C@N) 1623 (s); 1H NMR (295 K/ppm, see Fig. S1):
9.58 (s, 1H, H5), 8.62 (d, 1H, H1), 8.18 (d, 1H, H4), 7.87 (t, 1H,
H3), 7.54 (t, 2H, H2, H8), 7.43–7.39 (m, 4H, H6, H7, H9, H10), 3.23
(s, 3H, NCH3), 2.09 (s, 3H, CH3). 13C NMR (295 K/ppm): 206.91,
159.76, 156.45, 154.81, 152.89, 149.91, 137.05, 134.85, 129.66,
127.68, 125.56, 124.73, 119.79, 115.99, 35.52, 31.14. UV–Vis
(DMF, (kmax
(e,
Mꢀ1 cmꢀ1))): 242 nm (sh, 124100), 257 nm
(128100), 342 nm (161400). Calc. for C17H16N4O: C, 69.85; H,
5.52; N, 19.17. Found: C, 69.59; H, 5.03; N, 18.70%.
2. Experimental
2.3. 4-((Pyridine-2-ylimino)methylene)-chromone (pch)
2.1. Materials and methods
The condensation reaction of 3-formylchromone (0.925 g;
5.31 mmol) and pyridin-2-amine (0.510 g; 5.42 mmol) was con-
ducted in methanol (30 cm3) under reflux (for 3 h) in the presence
of three drops of piperidine. The orange solution was allowed
to cool to room temperature and a mustard-colored precipitate
was filtered and washed with cold methanol as well as petroleum
trans-[RuCl2(PPh3)3], 3-formylchromone, 2-pyridylaldehyde,
2,6-dicarboxylaldehydepyridine, 4-aminoantipyrine, 2-aminopyri-
dine, 5,6-diamino-1,3-dimethyluracil, ammonium tetrafluorobo-
rate and electrochemical analysis grade tetrabutylammonium
hexafluorophosphate were obtained from Sigma Aldrich. The syn-
thetic procedure of the diimine ligand, 2,6-bis-((antipyrine-
imino)methylene)pyridine (bpap) was adopted from a literature
method [18]. All solvents were obtained from Merck SA. Analytical
reagent grade toluene was dried over sodium wire while the other
solvents and chemicals were used without any further purification.
Ultrapure water was produced from an ElgaPurelab Ultra system.
The infrared spectra were recorded in the solid state on a Perkin-
Elmer Spectrum 100 in the 4000–650 cmꢀ1 range. The NMR spectra
were obtained using either a Bruker Avance 400 or 500 MHz spec-
trometers. All NMR spectra were recorded in DMSO-d6 except for
the 13C NMR spectra of diimine complexes 3 and 4 which were
recorded in deuterated chloroform. The X-band EPR spectrum was
recorded using a Bruker EMX Ultra X spectrometer. UV–Vis spectra
were recorded using a Perkin Elmer Lambda 25. The extinction
ether. Yield = 95%, M.P. = 115–118 °C. IR
(m m(C@O)
max/cmꢀ1):
1647 (s), (C@N) 1611, 1603, 1590 (s),
m
m
(C–O–C) 1557; 1H NMR
(295 K/ppm): 11.77 (d, 1H, H1), 8.40–8.34 (m, 4H, H4, H5, H6,
H10), 7.89–7.70 (m, 2H, H2, H3), 7.59–7.51 (m, 2H, H8, H9), 7.36
(d, 1H, H7). 13C NMR (295 K/ppm): 206.93, 181.24, 175.29,
156.60, 148.74, 143.10, 139.67, 137.29, 123.17, 112.66, 110.73,
109.72, 105.19, 101.67. UV–Vis (DMF, (kmax
244 nm (3200), 267 nm (1997), 384 nm (7400). Calc. for
15H10N2O2: C, 71.99; H, 4.03; N, 11.19. Found: C, 71.53; H, 4.55;
(e,
Mꢀ1 cmꢀ1))):
C
N, 10.73%.
coefficients (e
) are given in dm3 molꢀ1 cmꢀ1. Melting points were
determined using a Stuart SMP3 melting point apparatus. The con-
ductivity measurements were determined at 295 K on a Radiome-
ter R21M127 CDM 230 conductivity and pH meter.
Cyclic voltammetry measurements were done using an Autolab
potentiostat equipped with a three electrode system: a glassy car-
bon working electrode (GCWE), a pseudo Ag|AgCl reference elec-
trode and an auxiliary Pt counter electrode. The AutolabNova 1.7
software was utilized for the operation of the potentiostat and data
2.4. 2,6-bis-((6-Amino-1,3-dimethyluracilimino)methylene)pyridine
(H4ucp)
A reaction mixture of 5,6-diamino-1,3-dimethyluracil (1.25 g;
7.40 mmol) and pyridine-2,6-dicarbaldehyde (0.503 g; 3.72 mmol)
was heated at reflux in methanol (40 cm3) in the presence of a cata-

252
I.N. Booysen et al. / Polyhedron 79 (2014) 250–257
lyst, piperidine (3 drops). After 3 h, the bright orange solution was
allowed to cool to room temperature and the yellow precipitate
which formed was collected (via filtration), washed with cold meth-
anol as well as petroleum ether. Yield = 95%, M.P. = 234–237 °C.
under reflux for 3 h. A maroon precipitate was filtered by gravity,
dissolved in dichloromethane and the resultant solution was lay-
ered with hexane. After several days of slow diffusion red, cubic
crystals, which were ideal for X-ray analysis, were attained.
IR (m
max/cmꢀ1):
m(C@O) 1678 (s),
m
(C@N) 1596 (s); 1H NMR
Yield = 73%, M.P. = 271–274 °C. IR (
m m(C@O) 1686 (s),
max/cmꢀ1):
(295 K/ppm, see Fig. S2): 9.70 (s, 2H, H4, H8), 8.33 (d, 2H, H5, H7),
7.78 (t, 1H, H6), 7.46 (br, s, 4H, N(1)H2, N(11)H2), 3.41 (s, 6H, C3H3,
C9H3), 3.19 (s, 6H, C2H3, C10H3). 13C NMR (295 K/ppm): 214.30,
206.92, 157.65, 157.11, 154.41, 150.89, 150.24, 150.13, 29.90,
m(C@N) 1618 (s),
m
(Ru-PPh3) 696 (m); 1H NMR (295 K/ppm): 8.59
(s, 1H, H7), 8.31 (s, 1H, H13), 7.85 (br, s, 2H, N3H2), 7.46–7.21 (m,
15H, PPh3), 7.17 (d, 1H, H9), 7.03 (d, 1H, H11), 6.83 (t, 1H, H10),
5.77 (s, 1H, N7H) 3.55 (s, 3H, C3H3), 3.03 (s, 6H, C1H3, C16H3),
2.79 (s, 3H, C18H3); 31P NMR (295 K/ppm): 36.89. 13C NMR
(295 K/ppm): 206.57, 137.63, 133.44, 132.40, 132.09, 132.06,
131.77, 131.67, 128.75, 128.63, 128.51, 128.27, 127.96, 127.71,
127.48, 125.01, 124.77, 124.53, 29.88, 29.83, 29.63, 29.44. UV–Vis
28.91. UV–Vis (DMF, (kmax
(e,
Mꢀ1 cmꢀ1))): 233 nm (7900),
278 nm (sh, 4900), 285 nm (5000), 368 nm (8800), 451 nm (600),
588 nm (600). Calc. for C19H21N9O4: C, 51.93; H, 4.82; N, 28.69.
Found: C, 51.45; H, 4.50; N, 28.27%
(DMF, (kmax (e
, Mꢀ1 cmꢀ1))): 267 nm (sh, 20500); 387 nm (7100);
419 nm (sh, 6800); 474 (sh, 4800); 556 nm (sh, 3800). Conductivity
(DMF, 10ꢀ3 M): 11.17 ohmꢀ1 cm2 molꢀ1. Calc. for C38H35Cl3N9O4-
PRu: C, 49.60; H, 3.83; N, 13.70. Found: C, 50.07; H, 4.63; N, 13.32%.
2.8. cis-[RuIICl2(bpap)(PPh3)] (4)
The title compound was formed from the 1:1 molar ratio reac-
tion of bpap (0.0527 g; 104
(0.100 g, 104
mol) in (20 cm3) toluene after 6 h of reflux. A dark
brown precipitate was filtered and recrystallized via the slow dif-
fusion of a dichloromethane and n-hexane [1:1 ( )] solution
which resulted in the formation of brown XRD quality parallelo-
grams. Yield = 63%, M.P. = 323–327 °C. IR ( (C@O)
max/cmꢀ1):
1667, 1655, 1638 (s), (C@N) 1591 (s),
(Ru-PPh3) 695 (vs); 1H
lmol) and trans-[RuCl2(PPh3)3]
l
II
.
2.5. trans-[Ru Cl(pap)(PPh3)2]BF4 (1)
v:v
Equimolar amounts of pap (0.0305 g, 104 lmol) and trans-
[RuCl2(PPh3)3] (0.100 g, 104
lmol) were refluxed for 12 h in anhy-
m
m
drous toluene (30 cm3). A dark-blue precipitate was collected by
filtration, washed with anhydrous di-ethyl ether and dried under
vacuum. This precipitate along with an excess of NH4BF4 was dis-
solved in a CH2Cl2:EtOH (2:1, v:v) solvent mixture and from the
slow evaporation of the resultant solution, dark blue cubic-shaped
crystals suitable for X-ray crystallographic analysis were obtained.
m
m
NMR (295 K/ppm): 8.21–8.02 (m, 4H, H12, H14, H15, H16, H18),
7.60–6.62 (m, 25H, PPh3, H2, H3, H4, H5, H6, H25, H26, H27, H28,
H29), 3.21 (s, 6H, C7H3, C9H3), 2.92 (s, 6H, C21H3, C22H3); 31P
NMR (295 K/ppm): 31.22. 13C NMR (295 K/ppm): 206.58, 137.63,
128.74, 128.51, 128.27, 127.95, 127.71, 125.04, 124.78, 124.51,
Yield = 65%, M.P. = 250–255 °C. IR (
m
max/cmꢀ1):
m(C@O) 1902 (vs),
124.09, 29.87, 29.57. UV–Vis (DMF, (kmax (e
, Mꢀ1 cmꢀ1))): 244 nm
m
(C@N) 1621 (s); 1H NMR (295 K/ppm): 8.49 (d, 1H, H1), 8.11 (s,
(sh, 26400); 279 nm (16400); 418 nm (5000); 498 nm (sh,
2800); 599 nm (sh, 1400). Conductivity (DMF, 10ꢀ3 M):
15.39 ohmꢀ1 cm2 molꢀ1. Calc. for C49H44Cl8N7O2PRu: C, 49.93; H,
3.76; N, 8.32. Found: C, 49.47; H, 3.39; N, 8.72%.
1H, H5), 7.59 (t, 1H, H3), 7.24–7.16 (m, 34H, H2, H4, 2 x PPh3),
7.07–7.02 (m, 3H, H6, H8, H10); 6.88 (t, 2H, H7, H9); 2.98 (s, 3H,
N-CH3), 2.16 (s, 3H, CH3); 31P NMR (295 K/ppm): 34.23. 13C NMR
(295 K/ppm): 206.42, 137.31, 133.36, 129.99, 129.81, 128.86,
128.16, 127.84, 125.27, 124.47, 124.12, 30.64, 20.89. UV–Vis
(DMF, (kmax
(e
,
Mꢀ1 cmꢀ1))): 281 nm (sh, 145200); 357 nm
2.9. X-ray diffraction
(11200); 359 nm (sh, 9400); 598 (4200). Conductivity (DCM,
10ꢀ3 M): 127.09 ohmꢀ1 cm2 molꢀ1. Calc. for C49H40Cl2N4P2BF4Ru:
C, 58.52; H, 4.01; N, 5.57. Found: C, 58.48; H, 4.22; N, 5.32%.
The X-ray data for all the metal complexes 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. For 1, a low resolution structure could only be
attained, see Fig. S4. Crystal and structure refinement data for
the metal complexes 2, 3 and 4 are given in Table 1. Selected bond
lengths and angles are given in Tables 2–4. In all three cases the
data were collected with Mo Ka (k = 0.71073 Å) radiation at a crys-
tal-to-detector distance of 50 mm. The following conditions were
used for the data collection: omega and phi scans with exposures
2.6. trans-P, cis-Cl-[RuIII(pch)Cl2(PPh3)2] (2)
A 1:1 molar reaction mixture of pch (0.0261 g; 104
lmol) and
trans-[RuCl2(PPh3)3] (0.100 g, 104 mol) was heated at refluxed in
l
ethanol (30 cm3) for 3 h. A blue precipitate was collected by filtra-
tion and washed with anhydrous di-ethyl ether. This precipitate
was dissolved in chloroform and layered with hexane and after
several days, XRD quality blue parallelograms were attained
using the slow diffusion method. Yield = 74%, M.P. = 247–253 °C.
taken at 30 W X-ray power and 0.50° frame widths using APEX
2
[19]. The data were reduced with the programme SAINT [20] using
outlier rejection, scan speed scaling, as well as standard Lorentz
and polarization correction factors. A SADABS semi-empirical multi-
scan absorption correction was applied to the data [21]. Direct
methods, SHELXS-97 [22] and WinGX [23] were used to solve all
three structures. All non-hydrogen atoms were located in the dif-
ference density map and refined anisotropically with SHELXL-97
[22]. All hydrogen atoms 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 amido N–H bond of 2 was located in the differ-
ence density map and refined isotropically.
IR (
1508 (s),
271 nm (9900); 322 nm (4900); 386 nm (3800); 478 nm (sh, 800);
636 (300). Conductivity (DCM, 10ꢀ3 M): 78.74 ohmꢀ1 cm2 molꢀ1
m
max/cmꢀ1):
[Ru-(PPh3)2] 693 (s). UV–Vis (DMF, (kmax
m
(C@O) 1639 (m),
m
(C@N) 1613 (m),
m(C–O–C)
m
(e
, Mꢀ1 cmꢀ1))):
.
Calc. for C53H41Cl8N2O2P2Ru: C, 53.74; H, 3.49; N, 2.36. Found: C,
53.93; H, 3.22; N, 2.01%.
2.7. [RuII(H3ucp)Cl(PPh3)] (3)
A mixture of H4ucp (0.0458 g; 0.104 mmol) and trans-[RuCl2
(PPh3)3] (0.100 g; 0.104 mmol) in methanol (20 cm3) was heated

I.N. Booysen et al. / Polyhedron 79 (2014) 250–257
253
Table 1
Crystal data and structure refinement data.
2ꢁ2CHCl3
3ꢁCH2Cl2
4ꢁCHCl3
Chemical formula
Formula weight
Temperature (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
C53H41Cl8N2O2P2Ru
1184.49
100(2)
monoclinic
P21/c
C38H35Cl3N9O4PRu
922.16
100(2)
C49H44Cl8N7O2PRu
1178.55
100(2)
triclinic
triclinic
ꢀ
ꢀ
P1
P1
15.4971(10)
16.9324(12)
20.7864(13)
90
111.254(2)
90
0.31 ꢂ 0.10 ꢂ 0.02
5083.42
4
12.3357(5)
12.7812(7)
15.8388(6)
68.056(2)
75.651(2)
79.853(2)
0.20 ꢂ 0.12 ꢂ 0.08
2234.32
13.0192(6)
15.9070(8)
16.3224(8)
102.418(2)
107.355(2)
104.596(2)
0.28 ꢂ 0.15 ꢂ 0.10
2964.18
b (Å)
c (Å)
a
(°)
b (°)
(°)
c
Crystal size (mm)
V (Å3)
Z
2
2
Dcalc (Mg mꢀ3
)
1.548
0.836
2396
1.368
0.613
936
1.320
0.693
1196
Absorption coefficient (mmꢀ1
F(000)
)
h range for data collection (°)
Index ranges
1.41; 26.01
ꢀ18 6 h 6 19
ꢀ20 6 k < 20
ꢀ25 6 l 6 12
46039
1.41; 26.06
ꢀ15 6 h 6 15
ꢀ15 6 k < 10
ꢀ19 6 l 6 19
21876
1.39; 26.04
ꢀ16 6 h 6 15
ꢀ19 6 k < 19
ꢀ20 6 l 6 20
38865
Reflections measured
Observed reflections [I > 2
r
(I)]
9818
8487
11388
Independent reflections
7974
7451
9986
Data/restraints/parameters
Goodness-of-fit on F2
Observed R, wR2
Rint
7974/0/613
1.035
0.0557; 0.1534
0.049
7451/0/521
1.059
0.0478; 0.1369
0.022
9986/0/617
1.073
0.0494; 0.1252
0.025
Table 3
Table 2
Selected bond lengths [Å] and bond angles [°] for 3.
Selected bond lengths [Å] and bond angles [°] for 2.
Experimental
Optimized
Experimental
Ru–N5
Ru–Cl
Ru–N7
Ru–N4
Ru–N6
N4–C7
N6–C13
N4–C5
N6–C14
Ru–P
N4–Ru–N5
N5–Ru–N6
N6–Ru–N7
N4–Ru–N6
N5–Ru–N7
Cl–Ru–P
1.954(4)
2.476(3)
2.143(4)
2.189(3)
2.001(3)
1.314(5)
1.324(5)
1.420(5)
1.388(5)
2.294(1)
76.4(1)
81.2(1)
79.1(1)
157.4(1)
159.9(1)
172.96(3)
1.9863
2.5027
2.1799
2.3055
2.0196
1.3010
1.3225
1.4115
1.3676
2.3683
75.30
80.88
78.54
155.50
159.05
173.55
Ru–Cl1
Ru–Cl2
Ru–N2
Ru–C15
Ru–P1
Ru–P2
N2–C6
C8–O2
C15–O1
C14–O1
C7–C15
C7–C8
2.452(1)
2.575(1)
2.151(4)
1.933(5)
2.393(1)
2.391(1)
1.338(6)
1.235(7)
1.370(5)
1.383(6)
1.433(7)
1.455(7)
1.481(8)
78.6(2)
C8–C9
N2–Ru–C15
Cl1–Ru–Cl2
P1–Ru–P2
C6–N2–C5
88.55(4)
176.31(4)
116.3(4)
metal complex, the lack of any negative eigen values in the fre-
quency calculations confirmed that the structure is at a global min-
imum on the potential energy surface [26].
2.10. Computational details
Computational calculations were conducted with GAUSSIAN 09W
[24]. Geometry optimization of the ruthenium complex 3 was
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, Cl and P atoms and the
LANL2DZ basis set, which makes use of effective core potentials,
applied to the metal center [25]. Prior to the calculation, the sol-
vent molecule of recrystallization was removed from the crystal
structure and the resultant structure was used as the starting con-
former. Good agreement was found between the optimized and
geometrical parameters (refer to Table 3) with the minor devia-
tions attributed to the fact that gas phase optimized structure do
not account for non-classical hydrogen bonding interactions or
any short distance contacts. Using the optimized structure of the
3. Results and discussion
3.1. Synthesis and spectral characterization
The novel ruthenium compounds 1–4 were isolated from the
equimolar coordination reactions of trans-[RuCl2(PPh3)3] with
pap, pch, H4ucp and bpap, respectively (see Fig. S1, 4–6). Both
pap (for 1) and bpap (for 4) ligands acts as neutral tridentate che-
lators which coordinates through their respective NpyNiminoOketone
(in 1) and (Nimino)2Npy (in 2) donor sets. The monoanionic H3ucp
(for 3) ligand displaces all the equatorial co-ligands of the metal
precursor by coordinating through the bridging pyridyl nitrogen,
the imino nitrogens and the singly deprotonated amido nitrogen.

254
I.N. Booysen et al. / Polyhedron 79 (2014) 250–257
Table 4
the H3ucp chelator is a reflection of the asymmetry within the
coordination sphere of 3. Similarly, the up-field shifts of the aro-
matic signals of 3 relative to those of the free-ligand affirm coordi-
nation of the H3ucp tetradentate chelator. Another distinctive
comparative feature is the disappearance of the broad singlet of
the amino groups (at 7.46 ppm in H4ucp) and the emergence of
two singlets (in 3) assigned to the amino (at 7.85 ppm) and amido
Selected bond lengths [Å] and bond angles [°] for 4.
Experimental
Ru–N3
Ru–N5
Ru–Cl1
Ru–Cl2
Ru–P
Ru–N4
C12–N3
C18–N5
2.125(3)
2.105(3)
2.4448(9)
2.4420(8)
2.3155(8)
1.945(3)
1.313(4)
1.311(5)
1.410(5)
1.409(5)
78.4(1)
(at 5.77 ppm) protons, respectively. In addition,
a multiplet
observed between 7.46 and 7.21 ppm integrates to 15 protons,
unequivocally confirmed the presence of one coordinated triphen-
ylphosphine co-ligand. For 4, two multiplets were attained
accounting for the bridging imino and pyridyl protons (between
8.21 and 8.02 ppm) as well as the phenyl protons (between 7.60
and 6.62 ppm) of the aminoantipyrine moieties. 31P NMR spectros-
copy confirmed the presence of the phosphorous atoms at 34.23
(for 1), 36.89 (for 3) and 31.22 (for 4) ppm. The fact that the
PPh3 co-ligands of 1 resonate at the same frequency is evidence
that they reside in similar chemical environments.
C10–N3
C19–N5
N3–Ru–N4
N4–Ru–N5
N3–Ru–N5
N4–Ru–Cl1
P–Ru–Cl2
C10–N3–C12
C18–N5–C19
78.2(1)
156.6(1)
174.07(9)
176.99(3)
115.4(3)
114.8(3)
The presence of the paramagnetic ruthenium(III) center in com-
plex 2 was confirmed via room temperature solution (in DCM) ESR
spectroscopy, see Fig. S17. The deviation (between 3300 and
4000 G) from the typical rhombic ESR spectrum reflects distortion
of the octahedral geometry of 2 [29,30]. In fact, the g-value
(2.0951) for 2 is similar to that attained in the poorly resolved solu-
tion ESR spectrum of the ruthenium(III) complex, trans-
[RuCl(bzp)(PPh3)2] (Hbzp = N-(2-hydroxybenzylidene)-benzimid-
azole) with a g-value of 2.1101 [30].
Interestingly for 2, the monoanionic pch chelator affords a con-
strained five-membered chelate ring through its formation of the
ruthenium-carbene and the ruthenium-imino coordination bonds.
Thus, in contrast to compounds 1, 3 and 4 which illustrates typical
coordination affinity for pyridyl atoms, no pyridyl coordination
bond occurred for 2.
All the metal complexes dissolve readily in chlorinated solvents
but exhibit partial solubility in other polar solvents such as meth-
anol, ethanol and acetonitrile. Complexes 1, 2 and 4 are non-elec-
trolytes in dichloromethane while the high molar conductivity
value of 3 is typical of a 1:1 complex salt [27]. No significant differ-
ences are observed between the IR spectra of the aminoantipyrine-
derived Schiff bases (pap and bpap) and their corresponding metal
complexes 1 and 4 as both contains neutral chelators (see Figs. S5
The highly delocalized nature of the Schiff base chelators of
compounds 1–4 are emphasized by the several common intrali-
gand
p–
p⁄ electronic transitions observed within the overlay
UV–Vis spectra of the free-ligands and their respective metal com-
plexes, see Figs. S9–S12. These electronic transitions are found
below 400 nm for 1, 2 and 4 while the spectrum of 3 is only dom-
inated by ligand-based electronic transitions. At more red-shifted
wavelengths, metal-to-ligand charge transfer transitions were
observed at 359 and 598 nm for 1, 478 nm for 2 and 418, 498
and 599 nm for 4. As expected, a d–d electronic transition was
and S8), e.g.
(C@O) 1902 cmꢀ1 and
2, the ketonic and ether bonds [
1508 cmꢀ1] vibrates at higher frequencies compared to those of
the free-ligand, pch (C–O–C)
(C@O) 1647 cmꢀ1 and
m
(C@O) 1904 cmꢀ1
(C@N) 1621 cmꢀ1 for 1. In contrast, for
(C@O) 1639 cmꢀ1 and
(C–O–C)
, m
(C@N) 1623 cmꢀ1 for pap and
m
m
m
m
observed for the paramagnetic ruthenium(III) complex
2 at
636 nm in comparison to none observed for the low-spin d6 ruthe-
nium(II) compounds 1, 3 and 4.
[
m
m
1557 cmꢀ1] due to the difference in the stereoelectronic properties
between the monoanionic coordinated (in 2) and neutral uncoordi-
nated chromone moieties (in the free ligand, pch), see Fig. S6. In
addition, the three strong imino bond vibrations (at 1611, 1603
and 1590 cmꢀ1) of the free pch ligand coalesce into one medium-
intensity vibration (at 1613 cmꢀ1 for 2) upon coordination. Fur-
3.2. Electrochemistry
The redox properties of the metallic compounds were probed
using cyclic voltammetry, refer to Table 5, Figs. 1 and S18. All the
attained cyclic voltammograms (CVs) were diffusion controlled at
incrementing scan rates, e.g. see the overlay CVs of 2 in Fig. S19
as an example. These CVs are also classified as one-electron redox
processes as their peak current ratios approach one. In addition, all
the CVs exhibits quasi-reversible behavior since their respective
peak to peak separations (refer to Table 4) are different from that
thermore, the
the free H4ucp ligand were found at lower frequencies in compar-
ison to its metal complex 3 [ (C@N) at
(C@O) at 1686 cmꢀ1 and
1618 cmꢀ1], see Fig. S7. The characteristic Ru–PPh3 stretches of
m
(C@O/N) [1678 and 1596 cmꢀ1, respectively] of
m
t
the compounds 2–4 were observed at 693, 696 and 695 cmꢀ1
,
respectively but the trans axial-[Ru(PPh3)2]2+ moiety of 1 was iden-
tical to the ligand-based vibration at 693 cmꢀ1 [28].
The 1H NMR spectrum of 1 is dominated by a multiplet in the
7.24–7.16 ppm region due to the protons of the triphenylphos-
phine co-ligands along with the H2 and H4 pyridyl protons at
7.22 and 7.16 ppm. The imino proton resonates (at 8.11 ppm) at
a considerably lower chemical shift than the analogous signal (at
9.58 ppm) of the free-ligand, pap. The remaining aromatic protons
of the pap chelator (of 1) appears as a doublet, triplet, multiplet
and triplet which integrates to one, one, three and two protons,
respectively while the methyl singlets are found further up-field
at 2.98 ppm (for NCH3) and 2.16 ppm (for CH3). Indicative to 1,
the coordination through the imino nitrogens in 3 results in the
Schiff base proton signals shifting to lower frequencies (at 8.59
and 8.31 ppm) compared to the equivalent signal found in the free
ligand, H4ucp (at 9.70 ppm). The splitting of the imino protons for
of the standard, ferrocene [
ingly, complexes 3 [ Ep = 50 mV] and 4 [
ter electron transfer kinetics than observed for 1 [
and 2 [ Ep = 120 mV]. These redox behaviors are due to the more
D
Ep = 80 mV at 100 mV/s]. Interest-
Ep = 90 mV] showed fas-
Ep = 100 mV]
D
D
D
D
delocalized diimine chelators of 3 and 4 which promote faster elec-
tron transfer kinetics in comparison to the mono-imine chelators
Table 5
Selected CV parameters (at 100 mV/s) for 1, 2, 3 and 4.
Complex
1
2
3
4
E
D
(V)
E (mV)
0.68
100
0.56
120
0.37
50
0.32
90
½

I.N. Booysen et al. / Polyhedron 79 (2014) 250–257
255
Fig. 1. Overlay CVs of compounds 1 (at 50 mV/s), 2 (at 300 mV/s), 4 (at 300 mV/s) and ferrocene (at 100 mV/s).
of 1 and 2. This phenomenon is further supported by the fact that 3
[E = 0.37 V versus Ag|AgCl] and 4 [E = 0.32 V versus Ag|AgCl]
½
½
have lower halfwave-potentials (E ) compared to 1 [E = 0.68 V
½
½
versus Ag|AgCl] and 2 [E = 0.56 V versus Ag|AgCl].
½
All the redox processes as found in the respective CVs are
ascribed to the Ru(II)/Ru(III) redox couples since they have similar
half-wave potentials (E ) as the ruthenium(II/III) compounds with
½
Schiff base chelates found in the literature. For example, the
halfwave potential of compound 1 [E = 0.68 V versus Ag|AgCl]
½
was equal to that found for the diamagnetic ruthenium(II) com-
plex, trans-[RuCl(PPh3)2(Htdp)] under the same experimental
conditions [14] while nearly equivalent halfwave potentials
were observed between the paramagnetic ruthenium(III) com-
plexes,
2
[E = 0.56 V versus Ag|AgCl] and trans-[RuIII(nbh)
½
(PPh3)2Cl] (H2nbh = N-benzylidene-4-nitrobenzohydrazide) [E
=
½
0.58 V versus Ag|AgCl] [31].
3.3. Crystal structure of 2
Compound 2 crystallizes in the monoclinic space group P21/c as
the chloroform disolvate, see Fig. 2. The ruthenium metal atom is
at the center of a distorted octahedron which is largely induced
by the constrained RuN2C6C7C15 five-membered chelate ring.
Consequently the equatorial bite angle, N2–Ru–C15 [78.6(2)°] is
considerably smaller than the bond angle formed between
cis-chloro [Cl1–Ru–Cl2 = 88.55(4)°] co-ligands, which are close to
octahedral ideality [i.e. 90°]. Furthermore, the nearly linear
P1–Ru–P2 bond angle of 176.31(4)° is influenced by intramolecular
interactions between the uncoordinated pyridyl moiety of the pch
chelator and selected phenyl groups of the triphenylphosphine
co-ligands, see Fig. S20 {I = 3.729(1) Å and II = 3.663(1) Å}.
Fig. 2. An ORTEP view of complex
ellipsoids and the atom labeling scheme.
2
showing 50% probability displacement
(nabhMe)(PPh3)2] (H2nabhMe 1-naphthaldehyde-4-methyl-ben-
zoylhydrazones) [2.040(6) Å] [7,32]. This is ascribed to the variable
stereoelectronic properties of the naphthalene and pyrenyl rings in
comparison to the chromone moiety. However, the ruthenium–
carbene bond distance of 2 is comparable to the organometallic
complex, trans-[RuCl(bzp)(PPh3)2] with a Ru–CSchiffbase coordination
bond [28].
The difference in the bond lengths of the cis-chloro co-ligands
[Ru–Cl1 = 2.452(1) Å and RuCl2 = 2.575(1) Å] is due to the variable
trans-influence of the N2 and C15 atoms, respectively. The bond
Evaluating the intraligand bond distances of the pch chelator; the
bond orders of the ketonic C8–O2 [1.235(7) Å] and ether C–O
[C15–O1 = 1.370(5) Å and C14–O1 = 1.383(6) Å] bonds are readily
distinguishable. As a result of the formation of the ruthenium–car-
bene bond, the C15–C7 bond distance of 1.433(7) Å is not similar
to the delocalized C–C double bonds within the phenyl ring [e.g.
C16–C17 = 1.377(4) Å] but the C15–C7 bond is still shorter than
the C7–C8 [1.455(7) Å] and C8–C9 [1.481(8) Å] single bonds. In con-
trast to the observation of the C15–C17 double bond (for 2), the
length of the Ru(III)–NSchiff
bond [2.151(4) Å] is similar to
base
the analogous coordination bonds found in other paramagnetic
ruthenium(III) compounds, [RuCl(bsp)2(PPh3)] [2.119(2) Å and
2.096(2) Å] (Hbsp = N-(2-hydroxybenzylidene)-benzothiazole) and
trans-[RuCl (bzp)(PPh3)2] [2.069(4) Å] (Hbzp = N-(2-hydroxybenzy-
lidene)-benzimidazole) [28]. In compound 2, the ruthenium–carbene
bond distance [1.933(5) Å] is shorter than the Ru(III)–Caromatic bond
distances of trans-[Ru(pnbhMe)(PPh3)2Cl] [2.048(3) Å] (H2pnbhMe
= 1-pyrenaldehyde-4-methyl-benzoylhydrazone) and trans-[RuCl

256
I.N. Booysen et al. / Polyhedron 79 (2014) 250–257
uncoordinated analogous bond [1.362(4) Å] of fac-[Re(CO)3
(bsch)Cl] (1) (bsch = 2-benzothiazole-4H-chromen-4-one) were
comparable to delocalized C–C double bonds within its correspond-
ing chromone phenyl ring [33]. Ruthenium complexes containing
chromone moieties are rare in the literature. Among the few exam-
ples are the ruthenium(II) complex salts, [Ru(bpy)2(MCMIP)]2+
(bpy = bipyridine) and [Ru(phen)2(MCMIP)]2+ (phen = phenanthro-
line) containing the bidentate neutral chromone chelator, 2-(6-
N4–C7 double bond. This observation is further supported by
shorterbond lengthof N6–C14 [1.3676 Å for the optimizedstructure
and 1.388(5) Å for the crystal structure] compared to N4–C5
[1.4115 Å for the optimized structure and 1.420(5) Å for the crystal
structure] both observed within the crystal and optimized struc-
tures which implies the latter exhibits single bond character. Conse-
quently, the variable Natural Population Analysis (NPA) of the imino
nitrogens [N4 = ꢀ0.392 and N6 = ꢀ0.282] are computed which
results in the differences in the optimized [Ru–N4 = 2.3055 Å and
Ru–N6 = 2.0196 Å] and experimental [Ru–N4 = 2.189(3) Å and
Ru–N6 = 2.001(3) Å] Ru–Nimino bond distances.
methyl-3-chromonyl)imidazo[4,5-f]
[1,10]-phenanthroline
(MCMIP) [34].
Several ruthenium compounds with derivatives of uracil and
other nucleotide bases have been isolated, e.g. the cationic arene–
3.4. Crystal structure of 3
ruthenium compound, [RuII(Ur@C@C)(PPh3)2( 5-C5H5)](PF6) which
g
The octahedron of 3 is defined by the N4-donor set of the mono-
anionic H3ucp chelator occupying the equatorial plane while the
axial plane is definedbytheClRuP atoms, seeFig. 3. Indicative tocom-
pound 2, the octahedron of 3 is severely distorted as a result of the
combined effect of the three five-membered chelate rings which
results in the formation of three constrained bite angles [N4–Ru–
N5 = 76.4(1)°, N5–Ru–N6 = 81.2(1)° and N6–Ru–N7 = 79.1(1)°]
which induces non-linear N4–Ru–N6 [157.4(1)°] and N5–Ru–N7
[159.9(1)°] bond angles. Similar to 2, the axial Cl–Ru–P
was formed from the uracil (Ur)-substituted alkyne UrAC@CH,
NH4PF6 and [Ru(PPh3)2(
g
5-C5H5)Cl] [38]. Another example is the
paramagnetic ruthenium(III) compound, trans-[RuCl4(DMSO)
(H2mtpo)]ꢁ4H2O which was isolated from the reaction between
the adenine derivative, 5-methyl-1,2,4-triazolo [1,5-a]pyrimidin-
7(4H)-one (H2mtpo) and [H(DMSO)2][trans-Ru(DMSO)2Cl4] [39].
3.5. Crystal structure of 4
[172.96(3)°] bond angle is influenced by classical
p–p stacking
between the uracil rings and selected phenyl groups of the PPh3
co-ligand given by the interplanar spacings of 3.431(1) Å (I) and
3.430(1) Å (II), see Fig. S21. Furthermore, the intramolecular interac-
tions cause the uracil moiety to lie substantially out of the N4-
equatorial plane (at 42.64°).
The bridging pyridyl nitrogen to ruthenium [1.957(5) Å] and
Ru–Cl [2.4507(15) Å] bonds of trans-[Ru(Cl)(btrpy)(PPh3)2]PF6
(btrpy = 4,40,40-tri-t-butyl-2,20:60,200-terpyridine) are similar to the
Ru–N5 [1.954(4) Å] and the Ru–Cl [2.476(3) Å] bond distances of
3 [35]. The metal–amido bond distance of 3 [Ru N7 = 2.143(4) Å]
is within the range of 2.036(3)–2.292(9) Å found for other amido
ruthenium(II) complexes [14,36,37]. Interestingly, a large differ-
ence in the metal-imino bond distances [Ru–N4 = 2.189(3) Å and
Ru–N6 = 2.001(3) Å] is noted and this phenomenon is also evident
in the bond distances of the Schiff base moieties [N4–
C7 = 1.314(5) Å and N6–C13 = 1.324(5) Å].
The triclinic unit cell of 4 is occupied with two molecules of 4 as
well as four chloroform molecules of recrystallization, see Fig. 4.
The intermolecular interaction (at 3.912(1) Å) between the bridg-
ing pyridyl rings of adjacent molecules within each triclinic unit-
cell allows all the molecules within the crystal lattice to pack in
columns parallel to the [a]-axis. As seen in 2 and 3, the crystal lat-
tice of 4 is stabilized by a series of intramolecular interactions
between the antipyrine moieties and respective phenyl rings of
the triphenylphosphine co-ligand, see Fig. S22 {I = 4.308(1) Å,
II = 3.515(1) Å and III = 3.817(1) Å}. Noticeably, the intermolecular
interactions I and II differ considerably which could potentially be
due to the non-classical interactions between one of the chloro-
form molecules of recrystallization and the ketonic O1 atom
[O1ꢁ ꢁ ꢁCl2S = 3.091(3) Å]. In turn, the nature of the aforementioned
intermolecular interaction results in the different Ru–N3/N5
[2.125(3) Å/2.105(3) Å] bond distances of 4.
To understand these geometrical discrepancies, the geometry of
3 was optimized at the DFT level. The difference in the theoretical
bond distances of the Schiff base moieties [N4–C7 = 1.3010 Å and
N6–C13 = 1.3225 Å] is also reflected in the optimized structure of
3. The longer bond distance of the N6–C13 appears to emanate from
The other coordination sphere bonds of 4 were comparable to 3,
as expected considering the similarity of the RuII–Npyridyl bond
lengths [1.945(3) Å and 1.954(4) Å for 4 and 3, respectively]. In addi-
tion, it was found that the cis-[RuIICl2] core [Ru–Cl1 = 2.4448(9) Å
and Ru–Cl2 = 2.4420(8) Å] of 4 has similar bond distances as the
Ru–Cl bond [2.476(3) Å] of 3. As expected, the Ru–P bonds of 3
[2.294(1) Å] and 4 [2.3155(8) Å] are shorter in comparison to the
nearly equidistant trans-axial [Ru–P1 = 2.393(1) Å and Ru–P2 =
2.391(1) Å] bonds of 2 due to the increase in the Lewis acidity of
the latter’s ruthenium center.
the C13–N6–C14 delocalized
p-system as opposed to localized
Octahedral distortion is induced by the constrained equatorial
N3–Ru–N4 [78.4(1)°] and N4–Ru–N5 [78.2(1)°] bite angles of the
bpap chelator. This results in the N3–Ru–N5 [156.6(1)°] and
N4–Ru–Cl1 [174.07(9)°] bond angles deviating from octahedral ide-
ality. The linear deviation of the axial P–Ru–Cl2 [176.99(3)°] bond
angle is due to the influence of the intramolecular interactions
I–III. The hybridization of the nitrogens (for 3 and 4) is further sup-
ported by the Schiff base bond distances [C7–N4 = 1.314(5) Å and
C13–N6 = 1.324(5) Å for 3; C12–N3 = 1.313(4) Å and C18–N5
= 1.311(5) Å for 4] which were comparable to other chelating Schiff
base moieties coordinated to the ruthenium(II) core [40,41]. How-
ever, the effect of cyclometallation in 3 and 4 causes their
C–NSchiff base–C angles [C5–N4–C7 = 117.1(3)° and C13–N6–C14
= 128.8(3)° for 3; C10–N3–C12 = 115.4(3)° and C18–N5–C19 =
114.8(3)° for 4] to be inconsistent with respect to the expected
120° value.
Fig. 3. An ORTEP view of complex
ellipsoids and the atom labeling scheme. The hydrogen atoms were omitted for
clarity.
3 showing 50% probability displacement

I.N. Booysen et al. / Polyhedron 79 (2014) 250–257
257
Fig. 4. An ORTEP view of complex 4 showing 50% probability displacement ellipsoids and the atom labeling scheme. The hydrogen atoms were omitted for clarity.
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Novel ruthenium(II) and (III) complexes with multidentate
Schiff base chelates bearing uracil, antipyrine or chromone moie-
ties were successfully synthesized and spectroscopically character-
ized. X-ray analysis indicated that the mono-imine chelators (pap
for 1 and pch for 2) coordinated to the trans-[RuII/III(PPh3)2] cores,
respectively while the highly delocalized diimine chelators (H3ucp
for 3 and bpap for 4) replaces two bulky PPh3 co-ligands of the
metal precursor in the formation of 3 and 4. The distinctive differ-
ence in the structural features of the diimines and mono-imine
ruthenium compounds are also reflected in their redox behaviors
whereby the more delocalized nature of the diimine chelators of
3 and 4 promotes faster electron transfer resulting in lower redox
potentials in contrast to the mono-imine chelators of 1 and 2. The
optimized geometrical parameters of 3 were in reasonable agree-
ment with the corresponding geometrical parameters attained
from the crystal structure. In addition, the computational calcula-
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nium-imino bond distances of complex 3.
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Acknowledgements
We are grateful to the University of KwaZulu-Natal and the
National Research Foundation of South Africa for financial support.
Appendix A. Supplementary information
CCDC 986308, 986309 and 986310 contains the supplementary
crystallographic data for 2ꢁ2CHCl3, 3ꢁCH2Cl2 and 4ꢁ2CHCl3. These
data can be obtained free of 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.2014.05.021.
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