Ruthenium complexes with Schiff base ligands containing benz(othiazole/imidazole) moieties: Structural, electron spin resonance and electrochemistry studies

Article abstract of DOI:10.1016/j.poly.2014.02.009

Novel ruthenium(II/III) complexes of Schiff bases containing benzimidazole (bz) or benzothiazole (bs) moieties were isolated: the diamagnetic ruthenium complex, cis-[Ru^IICl₂(bzpy)(PPh₃)₂] (1) was formed from the

Full text of DOI:10.1016/j.poly.2014.02.009

Polyhedron 73 (2014) 1–11
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Ruthenium complexes with Schiff base ligands containing
benz(othiazole/imidazole) moieties: Structural, electron spin resonance
and electrochemistry studies
⇑
Irvin N. Booysen , Abimbola Adebisi, Orde Q. Munro, 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:
Novel ruthenium(II/III) complexes of Schiff bases containing benzimidazole (bz) or benzothiazole (bs)
moieties were isolated: the diamagnetic ruthenium complex, cis-[RuIICl2(bzpy)(PPh3)2] (1) was formed
from the 1:1 M reaction between N-((pyridine-2-yl)methylene)-1H-benzimidazole (bzpy) and metal pre-
cursor, trans-[RuCl2(PPh3)3]. The same metal precursor, when reacted with the benzimidazole-derived
Schiff bases [N-(2-hydroxybenzylidene)-benzothiazole (Hbsp) and N-(2-hydroxybenzylidene)-benzimid-
azole (Hbzp)], afforded the paramagnetic ruthenium(III) complexes [RuCl(bsp)2(PPh3)] (2) and
trans-[RuCl(bzp)(PPh3)2] (3), respectively. These metal complexes were characterized via IR, mass and
UV–Vis spectroscopy, elemental analysis, single crystal XRD analysis as well as conductivity measure-
ments. Their redox properties were probed by voltammetry and accompanying UV–Vis spectroelectro-
chemistry experiments. Structural features of complex 1 were further investigated by multinuclear (1H
and 31P) NMR spectroscopy. The presence of the paramagnetic metal centres of 2 and 3 were confirmed
by X-band ESR spectroscopy.
Received 2 November 2013
Accepted 5 February 2014
Available online 15 February 2014
Keywords:
Ruthenium(II/III)
Schiff bases
Benz(imidazole/othiazole)
Electron spin resonance
Structural
Electrochemistry
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
complexes: cis-[RuIICl2(bzpy)(PPh3)2] (1), [RuCl(bsp)2(PPh3)] (2)
and trans-[RuCl(bzp)(PPh3)2] (3), respectively. Despite the similar
The discovery of NAMI A, trans-[RuCl4(DMSO)(Im)](ImH)
{ImH = imidazole} as a potential metallopharmaceutical for meta-
static cancer, has led to a renewed interest into the medicinal inor-
ganic chemistry of ruthenium [1–7]. In particular, ruthenium
complexes with N-donor heterocyclic ligands have been widely
investigated due to their diverse biological activities [8–12]. From
a coordination chemistry perspective, this class of ligand systems
afford ruthenuim complexes with unique coordination environ-
ments, owing to the diverse donor atom combinations and result-
ing metal chelation [13–16]. In addition, they can have variable
stereo-electronic properties arising from their ability to form neu-
tral or multivalent anionic N-donor chelators which inevitably al-
lows for stabilization of the metal centre both in low and high
oxidation states [8–20].
skeletal structures of the ligands, ruthenium complexes with di-
verse structural features were isolated. This diversity is also man-
ifest in their redox and electronic properties.
OH
N
H
X
N
N
N
N
N
bzpy
X = S (Hbsp)
N (Hbzp)
In this account, we report the reactions of trans-[RuCl2(PPh3)3]
with the Schiff bases derived from heterocyclic moieties
[N-((pyridine-2-yl)methylene)-1H-benzimidazole (bzpy), N-(2-
hydroxybenzylidene)-benzothiazole (Hbsp) and N-(2-hydroxyben-
zylidene)-benzimidazole (Hbzp] to afford the ruthenium
2. Experimental
2.1. Materials and methods
Trans-[RuCl2(PPh3)3], 2-aminobenzothiazole (2-NH2-bs), 2-ami-
nobenzimidazole (2-NH2-bz), salicylaldehyde, 2-pyridylaldehyde
and electrochemical analysis grade tetrabutylammonium hexa-
⇑
Corresponding author. Tel.: +27 797378626; fax: +27 332605009.
E-mail address: Booyseni@ukzn.ac.za (I.N. Booysen).
http://dx.doi.org/10.1016/j.poly.2014.02.009
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

2
I.N. Booysen et al. / Polyhedron 73 (2014) 1–11
fluorophosphate were obtained from Sigma Aldrich. All solvents
were obtained from Merck SA. Reagent grade toluene was dried
over sodium wire while the other solvents and chemicals were
used without any further purification. Ultrapure water was pro-
duced from an Elga Purelab Ultra system. The Schiff base ligands
derived from salicylaldehyde were synthesized from modified
experimental procedures as previously reported [21,22]. For Hbsp
and Hbzp, the respective (1:1 M ratio) condensation reactions be-
tween 2-NH2-bs and 2-NH2-bz with salicylaldehyde were done in
dry refluxing toluene, under a nitrogen atmosphere and in the
presence of catalytic amounts of piperidine.
to reflux under a nitrogen atmosphere for 6 h. The volume of the
resultant dark brown solution was reduced to half and then n-hex-
ane was added dropwise to induce precipitation. In turn, the dark
brown precipitate was recrystallized via slow diffusion in a dichlo-
romethane and n-hexane [1:1 (
formation of dark brown XRD quality parallelograms. Yield = 63%,
m.p. >350 °C. IR ( (N–H) 3067 (w), (C@N)Heterocyclic
max/cmꢀ1):
1635 (s), (C@N)Schiff base 1606 (s),
(Ru-[PPh3]2) 696 (s); 1H NMR
v:v)] solution which resulted in the
m
m
m
m
m
(295 K/ppm/d6-DMSO): 8.97 (s, 1H, N3H), 8.47 (s, 1H, H6), 7.66–
7.60 (m, 4H, H1, H2, H3, H4), 7.58–7.54 (m, 4H, H9, H10, H11,
H12), 7.43–7.38 (m, 15H, PPh3), 7.27–7.22 (m, 15H, PPh3);
31P NMR (295 K/ppm/d6-DMSO): 25.57. UV–Vis (DCM, (kmax
The infrared spectra were recorded on a Perkin-Elmer Spectrum
100 in the 4000–650 cmꢀ1 range. The 1H NMR spectra were ob-
tained using Bruker Avance 400 MHz and 500 MHz spectrometers,
respectively. The X-band EPR spectrum was obtained from a Bruker
EMX Ultra X spectrometer. All NMR spectra were recorded in
DMSO-d6. UV–Vis spectra were recorded using a Perkin Elmer
(e
, Mꢀ1 cmꢀ1))): 301 (12800); 358 (sh, 10100); 410 (sh, 7620);
435 (sh, 5480); 576 (3220). Conductivity (DCM, 10ꢀ3 M): 15.51
ohmꢀ1 cmꢀ2 molꢀ1. Anal. Calc. for C49H40Cl2N4P2Ru: C, 64.05; H,
4.39; N, 6.10. Found: C, 63.97; H, 4.01; N, 6.32%. TOF-MS (m/z):
Calcd: 918.11 [M]; Found: 848.164 [Mꢀ2Cl].
Lambda 25. The extinction coefficients (e .
) are given in dm3 molꢀ1 cmꢀ1
Mass spectral analysis of the complexes was done both in positive
and negative modes via the direct injection of the respective sam-
ples into the Water Micromass LCT Premier instrument equipped
with a Time-of-Flight (TOF) Mass spectrometer analyzer and an
Electronspray Ionization (ESI) source. Melting points were deter-
mined using a Stuart SMP3 melting point apparatus. The conduc-
tivity measurements were determined at 295 K on a Radiometer
R21M127 CDM 230 conductivity and pH meter. Elemental compo-
sition of the complexes was determined using ThermoScientific
Flash 2000 CHNS/O Analyser.
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 Autolab Nova 1.7
software was utilized for the operation of the potentiostat and data
analysis. The ruthenium metal complexes were made up in 2 mM
solutions in DCM along with tetrabutylammonium hexafluoro-
phosphate (0.1 M) as a supporting electrolyte. Between each mea-
surement, the GCWE electrode 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 abso-
lute ethanol. Spectroelectrochemical data were attained using a
room temperature Specac optically transparent thin-layer electro-
chemical (OTTLE) cell purchased from the University of Reading
which was connected to the Autolab potentiostat.
2.4. [RuCl(bsp)2(PPh3)] (2)
A two molar ratio of Hbsp (0.0530 g; 0.208 mmol) with respect
to the metal precursor, trans-[RuCl2(PPh3)3] (0.100 g; 0.104 mmol)
were reacted together in refluxing toluene (20 cm3) for 6 h. After
the addition of 10 cm3 acetonitrile to the mother liquor and from
the slow evaporation of the resultant mixture, dark brown crystals
were attained for X-ray analysis. Yield = 66%, m.p. = 236–238 °C. IR
(
m
(e
m
max/cmꢀ1):
(C@C) 1435 (s),
, Mꢀ1 cmꢀ1))): 279 (sh, 16200); 328 (sh, 10900); 524 (1800);
705 (800). Conductivity (DCM, 10ꢀ3 M): 19.38 ohmꢀ1 cmꢀ2 molꢀ1
m
(C@N)Schiff base 1588 (m),
m(C@N)Heterocyclic 1531 (m),
m(Ru-PPh3) 693 (s). UV–Vis (DCM, (kmax
.
Anal. Calc. for C46H33ClN4O2PRuS2: C, 61.02; H, 3.67; N, 6.19.
Found: C, 60.56; H, 4.01; N, 6.39%. TOF-MS (m/z): Calcd: 905.05
[M]; Found: 905.049 [M].
2.5. Trans-[RuCl(bzp)(PPh3)2] (3)
The title compound was formed from the 1:1 M ratio reaction of
Hbzp (0.0247 g; 0.104 mmol) and trans-[RuCl2(PPh3)3] (0.100 g;
0.104 mmol) in (20 cm3) toluene (after 6 h of refluxing). From
the slow evaporation of the mother liquor, brown needle-like crys-
tals suitable for X-ray analysis were obtained after 3 days.
Yield = 71%, m.p. = 256–258 °C. IR (
(C@N)Schiff 1679 (m), (C@N)Heterocyclic 1589 (m),
[Ru-(PPh3)2] 691. UV–Vis (DCM, (kmax
m m(N–H) 3062 (w),
max/cmꢀ1):
m
m
m
(C@C)
base
1433 (m),
m
(e
(103),
2.2. N-((pyridine-2-yl)methylene)-1H-benzimidazole (Bzpy)
Mꢀ1 cmꢀ1))): 270 (sh, 17900); 320 (sh, 10900); 385 (sh, 7030);
544 (sh, 1790); 692 (860). Conductivity (DCM, 10ꢀ3 M): 28.74
ohmꢀ1 cmꢀ2 molꢀ1. Anal. Calc. for C57H47P2ClRuON3: C, 69.26; H,
4.79; N, 4.25. Found: C, 68.87; H, 4.54; N, 4.75%. TOF-MS (m/z):
Calcd: 896.13 [M for 3]; Found: 895.142 [MꢀH].
A mixture of 2-aminobenzimidazole (0.500 g; 3.76 mmol) and
2-pyridinecarboxaldehyde (0.400 g; 3.76 mmol) was heated until
reflux for 3 h in methanol (20 cm3), along with 1 cm3 of piperidine.
The resulting yellow solution was allowed to cool to room temper-
ature and concentrated under reduced pressure. Afterwards dry tol-
uene (40 cm3) was added to the solution and heated to reflux for 6 h
with a Dean and Stark apparatus. A yellow precipitate was filtered
and washed with cold anhydrous toluene. Yield = 75%, m.p. 236–
2.6. X-ray diffraction
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. Crystal and structure refinement data are given in
Table 1. Selected bond lengths and angles are given in Tables
238 °C. IR (
m
m m(N–H) 3051 (w), m(C@N)Schiff base 1612 (s),
max/cmꢀ1):
(C@N)Heterocyclic 1587 (s); 1H NMR (295 K/ppm/d6-DMSO): 12.82
(br, s, 1H, N3H), 9.36 (s, 1H, H6), 8.80 (d, 1H, J = 7.61 Hz, H1), 8.03
(t, 1H, J = 8.03 Hz, H3), 7.67–7.46 (m, 3H, H2, H10, H11), 7.24–7.17
(m, 2H, H9, H12); 13C NMR (295 K/ppm/d6-DMSO): 165.50,
153.67, 150.71, 137.82, 127.01, 122.81, 122.16. UV–Vis (DCM,
2–4. In all three cases the data were collected with Mo K
(k = 0.71073 Å) radiation at crystal-to-detector distance of
a
a
50 mm. The following conditions were used for the Bruker data
collection: 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 the programme SAINT [49] using outlier rejec-
tion, scan speed scaling, as well as standard Lorentz and polariza-
tion correction factors. A SADABS [24] semi-empirical multi-scan
absorption correction was applied to the data [25]. Direct meth-
(kmax (e
, Mꢀ1 cmꢀ1))): 255 (sh, 2300); 284 (1980); 360 (2480).
2.3. Cis-Cl, trans-P [RuIICl2(bzpy)(PPh3)2] (1)
A mixture of bzpy (0.0231 g; 0.104 mmol) and trans-[RuCl2
(PPh3)3] (0.100 g; 0.104 mmol) in dry toluene (20 cm3) was heated

I.N. Booysen et al. / Polyhedron 73 (2014) 1–11
3
Table 1
Crystal data and structure refinement data.
1
2
3ꢁC7H8
Chemical formula
Formula weight
T (K)
Crystal system
Space group
C49H40Cl2N4P2Ru
C46H33ClN4O2PRuS2
905.4
100(2)
monoclinic
P121/n
C57H47P2ClRuON3
988.44
100(2)
918.76
100(2)
triclinic
triclinic
ꢀ
ꢀ
P1
P1
Unit cell dimensions
0
12.2629(6)
13.4789(6)
14.8725(7)
12.6611(5)
23.4989(8)
13.1178(5)
11.9756(10)
12.7076(11)
15.1112(13)
a (AÅ)
0
b (AÅ)
0
c (AÅ)
a
(°)
b (°)
(°)
104.012(2)
101.332(2)
114.289(2)
0.35 ꢂ 0.10 ꢂ 0.08
2048.54(17)
2
1.490
0.63
940.00
90.00
91.807(2)
90.00
0.17 ꢂ 0.12 ꢂ 0.01
3900.89(4)
4
1.54
0.665
1843.7
97.670(4)
91.750(4)
90.061(4)
c
Crystal size (mm)
0.20 ꢂ 0.15 ꢂ 0.08
V (Å3)
2278.0(3)
Z
2
Dcalc (Mg/m3)
Absorption coefficient (mmꢀ1
F(000)
1.441
0.519
1018.0
)
h (°)
1.5; 30.53
1.7; 29.3
1.6; 27.08
Index ranges
Reflections measured
ꢀ17 6 h 6 17, ꢀ19 6 k < 19, ꢀ21 6 l 6 21
ꢀ17 6 h 6 17, ꢀ27 6 k < 32, ꢀ18 6 l 6 18
ꢀ15 6 h 6 15, ꢀ15 6 k < 16, ꢀ18 6 l 6 17
46486
42006
21774
Observed reflections [I > 2
r
(I)]
12336
8949
8911
Independent reflections
11489
10638
7569
Data/restraints/parameters
Goodness of fit (GOF) on F2
Observed R, wR2
11489/0/582
1.044
0.0262, 0.066
0.020
10638/0/509
1.036
0.031, 0.067
0.019
7569/0/587
1.117
0.0564, 0.1404
0.0362
Rint
Table 2
Selected bond lengths (Å) and bond angles (°) for 1.
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 imidazolium N–H bonds of 1 and 2 as well
as the toluene solvate C–H bonds of 3 were located in the differ-
ence density map, and refined isotropically. All hydrogen atoms of
1 and 2 were included as idealized contributors in the least
squares process but for 3, OLEX 2 was utilized where the hydro-
gen atoms were treated by a mixture of independent and con-
strained refinement [27]. One of the phenyl rings of 1,
specifically that attached to P2 (i.e. ring carbons C32–C37), was
disordered over two sites with near-orthogonal orientations.
The two orientations of the phenyl group were assigned to inde-
pendent parts during structure refinement and the site occupancy
factor allowed to freely vary. The final site occupancy factor for
the major orientation of the phenyl ring was 0.53617.
Ru–P1
Ru–Cl1
Ru–N1
C6–N1
C7–N3
Cl1–Ru–Cl2
Cl1–Ru–N1
2.3785(5)
2.4369(3)
2.076(1)
1.311(2)
1.357(2)
91.44(1)
99.24(4)
Ru–P2
Ru–Cl2
Ru–N2
C7–N4
N1–Ru–N2
Cl2–Ru–N2
P1–Ru–P2
2.3964(4)
2.4225(4)
2.045(1)
1.325(2)
78.45(5)
90.87(4)
174.37(1)
Table 3
Selected bond lengths (Å) and bond angles (°) for 2.
Ru1–Cl1
Ru1–N1
Ru1–O1
N3–C21
2.3634(5)
2.119(2)
1.975(1)
1.310(2)
1.297(2)
172.76(4)
175.42(5)
90.51(6)
Ru1–P1
Ru1–N3
Ru1–O2
N1–C7
N4–C22
O1–Ru1–O2
N3–Ru1–O2
–
2.3536(5)
2.096(2)
2.004(1)
1.296(2)
1.287(2)
177.69(5)
87.29(5)
–
N2–C8
Cl1–Ru1–N3
P1–Ru1–N1
N1–Ru1–O1
3. Results and discussion
3.1. Synthesis and spectral characterization
The equimolar ratio reactions of bzpy and Hbzp with
trans-[RuCl3(PPh3)2] afforded
a diamagnetic complex cis-Cl,
trans-P-[RuIICl2(bzpy)(PPh3)2] (1) and a paramagnetic complex
trans-[RuIII(bsp)Cl(PPh3)2] (3), respectively. In addition, a ‘2 + 2’
paramagnetic ruthenium(III) complex, [RuCl(bsp)2(PPh3)] (2) was
attained from the 1:2 M ratio reaction between trans-[RuCl2
(PPh3)3] and Hbsp. The isolated ruthenium(II/III) complexes exhib-
its diverse structural features, despite the fact that similarly struc-
tured ligand systems were reacted with the same metal precursor.
Both 1 and 3 are stabilized by the trans axial-[RuX-(PPh3)2] core
{X = II (for 1) and X = III (for 3)}. In 1, the bzpy chelator acts as a
neutral bidentate chelator through the imino (N1) and pyridyl
(N2) nitrogens, whereas in 3 the coordinated bsp chelator acts as
a dianionic tridentate chelator through the deprotonated imino
carbon (C8) and phenolic oxygen (O1) as well as the neutral
imidazolium nitrogen (N1).
Table 4
Selected bond lengths [Å] and bond angles [°] for 3.
Ru1–P1
Ru1–C8
Ru1–N1
N3–C8
N2–C7
2.386(1)
1.981(5)
2.069(4)
1.326(6)
1.360(6)
80.8(1)
Ru1–P2
Ru1–Cl1
Ru1–O1
N1–C7
N1–Ru1–C8
N1–Ru1–O1
–
2.407(1)
2.436(1)
2.060(4)
1.333(6)
76.0(2)
155.8(1)
–
C8–Ru1–O1
P1–Ru1–P2
178.55(4)
ods, SHELXS-97 [25] and WinGX [26] were used to solve all three
structures. All non-hydrogen atoms were located in the difference
density map and refined anisotropically with SHELXL-97 [25]. All

4
I.N. Booysen et al. / Polyhedron 73 (2014) 1–11
contrast for 2, the C@N [1588 and 1531 cmꢀ1] stretching modes
PPh3
PPh3
appear at low frequencies in comparison to analogous C@N
stretches within in the Hbsp spectrum [1679 and 1573 cmꢀ1].
Weak intensity bands are found both in the IR spectra of 1
N
N
N
N
Cl
Cl
N
N
Ru
Ru
[3051 cmꢀ1] and 2 [3062 cmꢀ1] which are attributed to
m(N–H).
Although a soft-ionization technique was employed, the posi-
tive mode mass spectrum of 1 showed a peak corresponding to a
fragment of the complex minus the chloro co-ligands (see
Fig. S6). The peaks of 2 and 3 are detected at 905.049 m/z [M]
and 895.142 m/z [M–H], respectively. A series of intra-ligand
Cl
2
PPh3
1
PPh3
p–
p⁄ transitions [301, 358, 401 and 435 nm] are observed in the
Cl
O
N
C
UV–Vis spectrum of 1 which were similar to that of the free li-
gand’s electronic transitions (see Fig. S7). A metal-to-ligand-charge
transfer transition (MLCT) is observed at 576 nm while no d–d
transition was found at longer wavelengths which are as expected
for a low spin d6 octahedral complex [31]. Characteristic of 1, the
Ru
PPh3
3
highly delocalized chelators afford multiple p–
p⁄ transitions below
400 nm in the UV–Vis spectra of both complexes 2 and 3 (see
Fig. S8). Above 400 nm, MLCT [524 (for 2) and 544 (for 3) nm]
and d–d [705 (for 2) and 692 (for 3) nm] transitions are found with
significantly lower extinction coefficients.
It is commonly found that multidentate Schiff base ligands sta-
bilize the trans-[RuII(PPh3)2]2+ core through their chelation and di-
The paramagnetic centres of complexes 2 and 3 were confirmed
by room temperature X-band ESR spectroscopy. The anisotropic
solid state ESR spectrum of 2 is nearly identical to the classical
rhombic ESR spectra attained for low-spin ruthenium(III) Schiff
base complexes, see Fig. 1 [32,33]. The deviation (between 3130
and 3370 G) from the typical rhombic ESR spectrum reflects distor-
tion of the octahedral geometry for 2 [34,35]. The same phenome-
non was observed within the poorly resolved solid state ESR
spectrum of 3, where only gx- and gy- values were observed, refer
to Table 5 and Fig. 2. In the case of the isotropic solution ESR spec-
tra of 2 and 3 in dichloromethane: toluene (1:1), both compounds
exhibit broad first derivative features devoid of additional fine
structure with centralized giso-values of 2.113 and 2.110,
respectively.
verse donor capabilities. For example, the metal carbonyl complex
trans-[RuH(cops)CO(PPh3)2]
{Hcops = 2-chlorophenylsalicylaldi-
mine}, where the cops Schiff base moiety acts as a monoanionic
bidentate chelator, affords a stable 6-membered chelate ring
through the deprotonated phenolic oxygen and imino nitrogen
[28]. This is further exemplified, within the bicyclometalled com-
plex, trans-[Ru(mbo)CO(PPh3)2] {H2mbo = 2-mercaptophenylimi-
no-4-bromophenol}, in which the mbo Schiff base chelates the
metal centre via the SNO donor set thereby forming 5- as well as
6-membered chelate rings [29]. In another study, efforts to isolated
a ruthenium complex with the potentially tetradentate bis-Schiff
base ligand H2pmb, {1,2-bis(20-pyridylmethyleneimino)benzene},
the ligand transformed when reacted with trans-[RuCl2(PPh3)3] to
afford the dicationic complex cation trans-[Ru(PPh3)2(pbz)2](ClO)2
{pbz = 2-pyridylbenzimidazole} [30].
3.2. Electrochemistry
All the metal complexes (1, 2 and 3) exhibit good solubility in
most polar aprotic solvents but partial solubility in alcoholic med-
ia. The low molar conductivities of the respective complexes are
typical of charge neutrality for ruthenium(II/III) complexes [31].
The 1H NMR spectrum of 1 is dominated by the intense signals of
the triphenylphosphine co-ligands which appear as two separate
multiplets (at 7.43–7.38 ppm and 7.27–7.22 ppm) (see Fig. S1).
More up-field, the aromatic signals of the pyridyl and benzimid-
azole moieties resonate as less intense multiplets [7.66–7.60 ppm
and 7.58–7.54 ppm] respectively. Shifts in both the imino [H3 at
8.97 ppm] and imidazolium [H10 at 8.47 ppm] protons of 1 relative
to the corresponding signals found within the free-ligand’s 1H NMR
spectrum (Schiff base and imidazolium protons found at 12.42 and
8.40 ppm, respectively) affirms coordination of the bzpy chelator.
Only one signal at 25.57 ppm is found for the two triphenylphos-
phine co-ligands which indicate that these co-ligands are in similar
chemical environments (see Fig. S2).
Selected cyclic voltammogram (CV) parameters of the com-
plexes are summarized in Table 6 and their respective CVs are
shown in Figs. 3 and 4 and S9. The CVs of 1 and 3 showed one redox
couple each whereas the CV of 2 showed two redox couples labeled
A and B, respectively. For 2, the peak potentials on the squarewave
voltammogram (SWV, see Fig. 5) are as expected equal to the
calculated halfwave potentials. All the redox couples are quasi-
reversible since their peak to peak separations (
DE) are different
than ferrocene ( E = 90 mV at 100 mV/s). In addition, all the redox
D
couples showed diffusion controlled behavior with increasing scan
rates. For example, see Fig. S10 for the overlay CVs of complex 3 for
scan rates ranging from 50 to 200 mV/s, at increments of 25 mV/s.
Peak current ratios approaching one, were observed for all com-
plexes which imply the redox couples are for one electron redox
processes.
The quasi-reversible redox processes of 1 and 3 were ascribed
to the Ru(II/III) redox couples since they have similar halfwave
Within the IR spectra, the single Ru–P bond of 2 [693 cmꢀ1
]
shows a strong vibrational mode at nearly the same frequency as
the trans-[Ru(PPh3)2] unit in 1 [693 cmꢀ1] and 2 [692 cmꢀ1] (see
Figs. S3–S5). IR spectral analysis also confirmed coordination
through the shifting of the C@N bands of the free–ligands in com-
parison to the corresponding bands found within the spectra of
their complexes. For 1, the C@N bands of heterocyclic
[1635 cmꢀ1] and Schiff base [1606 cmꢀ1] moieties appear at higher
frequency compared to the same bands found in the free ligand’s
[bzpy] IR spectrum [1613 and 1587 cmꢀ1]. Similarly for 3, the
C@N [1679 and 1589 cmꢀ1] bands are more red-shifted with
respect to the bands of the free-ligand [1605 and 1589 cmꢀ1]. In
potentials (E ) found for other ruthenium(II/III) complexes within
½
literature. Like in the case of the ruthenium(II) complexes, trans-
[Ru(HRbmp)(PPh3)2(CO)(Cl)], 2-benzylimino-methyl)-4-R-phenol
(HRbmp) in DCM (vs Ag|AgCl) with halfwave potentials ranging
from 0.62 V to 1.16 V [36]. Similarly, the paramagnetic trans-[RuIII-
Cl(L)(PPh3)2] {Schiff base (H2L) ligands derived from benzaldehyde
and various functionalized acetic hydrazides} complexes exhibited
comparable Ru(II/III) redox couples, under similar experimental
conditions [37].
The two redox processes (A and B) of 2 were both assigned to be
metal based redox processes as the CV of the free-ligand (i.e. Hbsp)

I.N. Booysen et al. / Polyhedron 73 (2014) 1–11
5
Fig. 1. Solid state and Solution X-band ESR spectra of 2 at 298 K. Instrument settings: microwave bridge frequency, 9.8 GHz; microwave bridge attenuator, 20 dB; modulation
frequency, 100 kHz; modulation amplitude, 5 G; centre field, 3500 G.
Table 5
mononuclear ruthenium complexes is 1.2–1.7 V. Within this study,
G-values of the respective complexes in the solid state (A) and in solution (B).
a
DE of 1.358 V for complex 2 was attained which supports the
½
assignments of the respective couples.
Spectrum
gx
gy
gz
giso
The redox couples of the respective complexes were further
investigated with spectroelectrochemistry to corroborate the vol-
tammetric assignments. The overlay UV–Vis spectra of 1 showed
a distinctive isosbestic point at 603 nm which is due to the appear-
ance of a shoulder (at 681 nm) ascribed to a d–d electronic transi-
tion (see Fig. 6). Occurrence of this metal-based electronic
transition implies that the d6–d5 system conversion transpired,
confirming the Ru(II/III) redox couple in the CV assignment [39].
A characteristic feature is the accompanying decrease in the MLCT
(at 576 nm) and becomes progressively red-shifted until 585 nm
[40,41].
2-A
2-B
3-A
3-B
2.198
–
2.309
–
2.090
–
2.018
–
1.910
–
–
–
–
2.113
–
2.110
Not observed.
indicated that the compound was not redox active within the same
potential window as A and B, respectively. These redox couples
were assigned to the Ru(II/III) [for A] and Ru(III/IV) [for B] couples,
consistent with analogous electrochemical behavior as the ruthe-
nium(III) bipyridine (bpy) complexes, [Ru(bpy)Cl3(X)] {X = MeOH,
PPh3, 4,40-bipyridine or CH3CN} [38]. It has been noted, that the lit-
Applying a controlled negative overpotential (at ꢀ0.55 V) for 2
while investigating the redox couple A, the reduction of the para-
magnetic ruthenium(III) centre is confirmed by the disappearance
of the d–d transition (at 705 nm) and the formation of a new MLCT
erature range of the halfwave potential differences (DE ) for
½
Fig. 2. Solid state and Solution X-band ESR spectra of 3 at 298 K. Instrument settings: microwave bridge frequency, 9.8 GHz; microwave bridge attenuator, 20 dB; modulation
frequency, 100 kHz; modulation amplitude, 5 G; centre field, 3500 G.

6
I.N. Booysen et al. / Polyhedron 73 (2014) 1–11
Table 6
noted, which could imply that ligand-induced oxidation of the me-
tal centre took place (see Fig. 8) [43,44]. These spectroelectrochem-
ical observations of 2 confirm the CV and SWV assignments for
redox couples A and B. Unfortunately for 3, upon applying poten-
tials between 0.865 and 0.90 V, only diffuse isosbestic points were
observed which is typical of the presence of two species within the
solution.
Selected CV parameters (at 100 mV/s) for the complexes 1, 2 and 3.
Complex
1
2
3
A
B
Epa (V)
Epc (V)
0.896
0.820
0.858
76
ꢀ0.433
ꢀ0.497
ꢀ0.465
64
0.965
0.899
0.932
66
0.660
0.739
0.700
79
E
½
(mV)
D
E (mV)
3.3. Crystallographic studies
ꢀ
band at 574 nm between the two defined isosbestic points (at 447
and 649 nm), see Fig. 7. Another study of a ruthenium(III) complex,
[Ru(H2bpyp)(acac)] {H2bpyp = 1,2-benzyl-bis-(2-(pyrazol-4-yl)phe-
nol)}, with similar CV traces as 2 showed that the reduction to the
Ru(II) species also resulted in the formation of a new MLCT [42].
Nearly quantitative conversion back to 2 occurred when a zero
potential was applied, leading to the regeneration of the d–d tran-
sition. At incrementing applied positive potentials, the characteris-
Complexes 1 and 3ꢁC7H8 crystallize in the space group P1, with
two molecules of each occupying the respective triclinic unit cells
(i.e. Z = 2) whereas four crystallographically identical molecules of
2
(i.e. Z = 4) are found within its monoclinic unit cell (see
Figs. S10–S12). Similar crystal packing arrangements were attained
for complexes 1 and 3.C7H8: two mutual classical hydrogen bonds
[N2–H2ꢁ ꢁ ꢁN3 = 2.11294 Å] in 1 occur between two respective mol-
ecules resulting in a series of molecules aligned parallel with the
[c]-axis; stabilization of the crystal lattice of 3 is rendered through
non-classical hydrogen-bonding which allows the molecules to
tic feature of the redox couple A, is the intense
259 and 286 nm while a decrease in the rest of the bands were
p–
p⁄ transitions at
Fig. 3. CV of complex 2 at 100 mV/s between the potential range of ꢀ1.0 and +1.2 V.
Fig. 4. CV of complex 3 at 100 mV/s between the potential range of ꢀ0.4 and +1.0 V.

I.N. Booysen et al. / Polyhedron 73 (2014) 1–11
7
Fig. 5. SWV of complex 2 at 100 mV/s between the potential range of ꢀ0.9 and +1.2 V.
Fig. 6. UV–Vis spectral changes observed for reduction of complex 1 at an applied potential of ꢀ0.90 V. The initial spectrum is shown as a dashed blue line. (Colour online.)
stack in columns along the [c]-axis. The molecules of 2 run parallel
with the [a]-axis due to a series of intermolecular interactions
[3.87 Å] between the S1-benzothiazole moieties of adjacent
molecules.
interactions between selected phenyl rings of the PPh3 co-ligands
and the imidazolium/pyridyl moieties of the bzpy chelator, see
Fig. S14 {I = 3.748 Å, II = 3.553 Å and III = 3.575 Å}. Consequently,
the respective non-coordinating heterocyclic moieties are found
out of plane with the pyridyl (of 1) and deprotonated phenolic
(of 2) rings. More specifically, the pyridyl moiety is slightly out
of plane (by 11.07°) with respect to the benzimidazole moiety. Fur-
thermore, the benzothiazole moieties are at different angles
[54.72° with respect to the S1-benzothiazole and 11.53° with re-
spect to the S2-benzothiazole] out of the plane of the deprotonated
phenolic rings which are due to the combined effects of the inter-
and intramolecular steric interactions. Unlike 1 and 2, no co-planar
phenyl rings of the PPh3 co-ligands were found with respect to the
bzp chelator of 3 which lead to a straighter backbone [P1–Ru1–P2
= 178.55(4)°]. The axial linearity of 3 is also clearly evident from
the Ru–P [Ru1–P1 = 2.386(1) Å and Ru1–P2 = 2.407(1) Å] bond dis-
tance being nearly equal.
The effects of cyclometallation are clearly evident from the dis-
tortion of the equatorial bond angles compared to the ideal octahe-
dral values. For 1, the constrained N1–Ru–N2 [78.45(5)°] bite angle
forces the Cl1–Ru–Cl2 [91.44(1)°], Cl2–Ru–N2 [90.87(4)°] and
[Cl1–Ru–N1 = 99.24(4)°] bond angles wider than the idealized 90°
(see Fig. 9). Similarly, the bicyclometalled complex 3 with its two
5-membered chelate rings [bite angles: N1–Ru1–C8 = 76.0(2)° and
C8–Ru1–O1 = 80.8(1)°] force the bond angles Cl–Ru1–C8 = 176.4(1)°
and N1–Ru1–O1 = 155.8(1)° to deviate from linearity (see Fig. 10).
The same phenomenon was observed with complex 2’s equatorial
bond [Cl1–Ru1–N3 = 172.76(4)° and O1–Ru1–O2 = 177.69(5)°] an-
gles which were all less than 180° as ascribed to the 6-membered
chelate rings [N3–Ru1–O2 = 87.29(5)° and N1–Ru1–O1 =
90.51(6)°] (see Fig. 11).
The cis–chloro coordination bonds of 1 [Ru-Cl1 = 2.4369(3) Å
and Ru–Cl2 = 2.4225(4) Å] are nearly equidistant which implies
that the trans-influence on the N1 and N2 nitrogen atoms are
approximately similar. The other coordination bonds of Ru–N1
In fact, the latter bond angles induces a P1–Ru–N1 [175.42(5)°]
axial bond angle which deviates from linearity. For 1, non-linearity
of its axial bond angle [P1–Ru–P2 = 174.37(1)°] is accounted to the

8
I.N. Booysen et al. / Polyhedron 73 (2014) 1–11
Fig. 7. UV–Vis spectral changes observed for reduction of complex 2 at potentials of redox couple B, at an applied potential ꢀ0.55 V. The initial spectrum is shown as a dashed
blue line. (Colour online.)
Fig. 8. UV–Vis spectral changes observed for reduction of complex 2 at potentials of redox couple B, at incrementing applied potentials. The initial spectrum is shown as a
dashed blue line. (Colour online.)
[2.076(1) Å] and Ru–N2 [2.045(1) Å] are typical of a ruthenium(II)
metal centre bonded to a pyridyl or Schiff base nitrogens, respec-
tively. For example, the organoruthenium(II) complex, trans-Cl,
cis-CO-[Ru(CO)2(spy)Cl2] {spy = N-((pyridine-2-yl)methylene)-thi-
[Ru1–O2 = 2.004(1) Å] and
3
[Ru1–O1 = 1.975(1) and Ru1–
O2 = 2.004(2) Å], respectively. The difference in the imino coordi-
nation bonds are ascribed to the varied trans-influence on the
imino nitrogens.
azole} has similar Ru–NSchiff
and Ru–Npyridyl bond lengths of
The rare metal carbene [Ru1–C8 = 1.981(5) Å] bond distance
were shorter than the analogous bond [Ru–CSchiff base = 2.048(7) Å]
base
2.169(4) and 2.091(5) Å [45]. Comparatively, the stronger Lewis
acidic character of the paramagnetic ruthenium(III) centres
compared to the diamagnetic metal centre of 1, afforded selected
found in the ruthenium(II) complex, trans-[Ru(cmp)(CO)Cl(PPh )2]
3
{cmp = methyl-4-((5-chloropyridin-2-yl-imino)methyl)benzoate}
which as expected due to the lower oxidation state of the latter
[46]. Nitrogen heterocyclic carbene (NHC) ruthenium complexes
have been widely researched due to their optimal catalytic proper-
ties, like in hydroformylation, olefin methasis as well as hydrogen-
transfer reactions [40,41]. More recently, the first ruthenium
chemotherapeutic drug, NAMI-A, trans-[RuCl4(DMSO)(Im)](ImH)
{Im = imidazole} has recently entered Phase II clinical trials due
to its optimal antimetastatic cancer activity which is accompanied
shorter analogous coordination sphere bonds for
= 2.3536(5) Å, Ru1–N1 = 2.119(2) Å and Ru–N3 = 2.096(2) Å] and
[Ru–P1 = 2.3785(5) Å, Ru–P2 = 2.3964(4) Å and Ru1–N1 =
2 [Ru1–P1
3
2.069(4) Å]. However, the metal to chloride bond distances for 2
[2.3634(5) Å] and 3 [2.436(1) Å] were different, which is largely
accounted to the difference in trans-influence experience by the
various chloride ions. In addition, similar deprotonated phenolic
oxygens to ruthenium(III) bond lengths were found for
2

I.N. Booysen et al. / Polyhedron 73 (2014) 1–11
9
Fig. 9. An ORTEP view of complex 1 showing 50% probability displacement ellipsoids and the atom labeling.
Fig. 10. An ORTEP view of complex 2 showing 50% probability displacement ellipsoids and the atom labeling. Hydrogen atoms have been omitted due to clarity.
with fewer significant side effects than platinum-based metallo-
pharmaceuticals [47]. This has led to an interest of exploring the
biological activities of NHC ruthenium complexes, e.g., the NHC
nitrogen atoms of the bz moiety} have shown to exhibit various
biological activities ranging from DNA intercalation to protease
inhibitor capabilities [48].
Similar bond lengths were attained for the individual {C@N}Schiff base
[C6–N1 = 1.311(2) Å (for 1) and C8–N3 = 1.327(6) Å] (for 3)] and
ruthenium(II) complexes, cis-[Ru(
g
6-cymene)Cl2(2R-bz)] {where R
can be methyl, ethyl, isopropanol or benzyl subsitituents on the

10
I.N. Booysen et al. / Polyhedron 73 (2014) 1–11
Fig. 11. An ORTEP view of complex 3ꢁC7H8 showing 50 % probability displacement ellipsoids and the atom labeling.
{C@N}Heterocyclic [C7–N4 = 1.325(2) Å (for 1) and C7–N1 = 1.333(6) Å
(for 3)] bond distances as the nitrogen atoms are sp2 hydridized
nitrogens; but the aforementioned C@N bonds were still shorter
than the carbon to sp3 hybridized nitrogen bonds within their indi-
vidual heterocyclic moieties [C7–N3 = 1.357(2) Å (for 1) and N2–
C7 = 1.360(6) Å (for 3)]. Within the bsp chelators (of 2), the imino
bond [N3–C21 = 1.310(2) Å and N1–C7 = 1.296(2) Å] distances
were typical for ruthenium(III) complexes with Schiff base che-
lates; [49] while the C@N bond [C8–N2 = 1.297(2) Å and C22–
N4 = 1.287(2) Å] (within the benzothiazole rings) lengths were
shorter despite having the same bond order but were comparable
for transition metal complexes with non coordinating benzothia-
zole moieties [50,51].
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.2014.02.009.
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