Complexes of cis-bis(triphenylphosphine)ruthenium(II) with N,S-donor thiobenzhydrazones of polycyclic aromatic aldehydes: Synthesis, properties and structures

Article abstract of DOI:10.1016/j.ica.2014.01.002

Reactions of [Ru(PPh₃)₃Cl₂], H ₂Lⁿ (polycyclic aromatic aldehyde thiobenzhydrazones ArCHNNHC(S)Ph, n = 1-4 for Ar = 1-naphthalenyl, 9-anthracenyl, 9-phenanthryl and 1-pyrenyl, respectively) and NaOAc in 1:2:2 mol ratio in methanol produces ruthenium(II) complexes of formula cis-[Ru(PPh₃)₂(HL ⁿ)₂] (1-4) in ～80% yields. Microanalysis (CHN), magnetic susceptibility, solution conductivity, spectroscopic (IR, UV-Vis, NMR and fluorescence) and cyclic voltammetric measurements have been used for the characterization of 1-4. The complexes are non-electrolytic and diamagnetic. In the electronic spectra, the complexes display multiple absorptions within 505-245 nm due to metal-to-ligand charge transfer and ligand centred transitions. The emission spectral features of 1-4 indicate ligand centred emissive states. Molecular structures of 3 and 4 have been determined by X-ray crystallography. In each of 3 and 4, two thioamidate-N,S coordinating (HL ⁿ)^- and two mutually cis oriented PPh₃ molecules assemble a distorted octahedral N₂S₂P ₂ coordination sphere around the metal centre. Cyclic voltammograms of 1-4 show an oxidation response within E_1/2 = 0.76-0.86 V (ΔE_p = 90-100 mV) versus Ag/AgCl.

Full text of DOI:10.1016/j.ica.2014.01.002

Inorganica Chimica Acta 413 (2014) 102–108
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Complexes of cis-bis(triphenylphosphine)ruthenium(II)
with N,S-donor thiobenzhydrazones of polycyclic aromatic aldehydes:
Synthesis, properties and structures
⇑
Koppanathi Nagaraju, Samudranil Pal
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
a r t i c l e i n f o
a b s t r a c t
Reactions of [Ru(PPh3)3Cl2], H2Ln (polycyclic aromatic aldehyde thiobenzhydrazones ArCH@NNHC(@S)Ph,
n = 1–4 for Ar = 1-naphthalenyl, 9-anthracenyl, 9-phenanthryl and 1-pyrenyl, respectively) and NaOAc in
1:2:2 mol ratio in methanol produces ruthenium(II) complexes of formula cis-[Ru(PPh3)2(HLn)2] (1–4) in
ꢀ80% yields. Microanalysis (CHN), magnetic susceptibility, solution conductivity, spectroscopic (IR, UV–
Vis, NMR and fluorescence) and cyclic voltammetric measurements have been used for the characteriza-
tion of 1–4. The complexes are non-electrolytic and diamagnetic. In the electronic spectra, the complexes
display multiple absorptions within 505–245 nm due to metal-to-ligand charge transfer and ligand
centred transitions. The emission spectral features of 1–4 indicate ligand centred emissive states. Molec-
ular structures of 3 and 4 have been determined by X-ray crystallography. In each of 3 and 4, two thioam-
idate-N,S coordinating (HLn)ꢁ and two mutually cis oriented PPh3 molecules assemble a distorted
octahedral N2S2P2 coordination sphere around the metal centre. Cyclic voltammograms of 1–4 show
Article history:
Received 24 September 2013
Received in revised form 1 January 2014
Accepted 6 January 2014
Available online 13 January 2014
Keywords:
Ruthenium(II)
Thiobenzhydrazone
Thioamidate-N,S coordination
Crystal structure
Redox property
an oxidation response within E1/2 = 0.76–0.86 V (
DEp = 90–100 mV) versus Ag/AgCl.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
aldehyde acid hydrazone, there is a choice between the ortho and
the peri position of the aryl group for metallation depending upon
the position of the –C(R)=N– on it (Scheme 1) [26,28–30]. In the
resulting complex, the acid hydrazone acts as a CNO pincer like
ligand and forms 5,5- or 6,5-membered fused chelate rings for
ortho- or peri-metallation, respectively. In general, with the acid
hydrazones of polycyclic aromatic aldehydes, we have found that
palladation occurs at the peri position [26,28], while, ruthenation
occurs at the ortho-position [29,30] of the pendant aryl moiety.
In the present work, we have studied the ruthenium chemistry of
thiobenzhydrazones of some polycyclic aromatic aldehydes
(H2L1–4, the two H of the formula represent the aryl C–H and the
thioamide N–H protons; Scheme 2). Among the four Schiff bases
chosen, except for H2L2 the remaining three provide the scope for
either the ortho- or the peri-ruthenation. The purpose was to pre-
pare new cycloruthenates with CNS pincer like ligands and explore
whether regioselective ortho-ruthenation and formation of fused
5,5-membered fused chelates (with H2L1, H2L3 and H2L4) as
observed for analogous polycyclic aromatic aldehyde benzhydraz-
ones occur or not. Interestingly, no cycloruthenation is taking place
at the polycyclic aryl moiety of any of the thiobenzhydrazones. In-
stead, a series of complexes with the formula cis-[Ru(PPh3)2(HLn)2],
where (HLn)ꢁ acts as four-membered chelate ring forming thioam-
idate-N,S donor, has been isolated (Scheme 2). Herein, we describe
Cyclometallation reactions and cyclometallates are of consider-
able contemporary interest due to their applications in a wide vari-
ety of research areas such as organic synthesis, catalysis,
photochemistry, biological and pharmaceutical chemistry and
materials science [1–21]. Among various types of cyclometallates,
pincer complexes are of particular interest because of their reactiv-
ity and catalytic efficiency [16–19]. The rigid sterically and elec-
tronically easily tunable meridional pincer ligands provide
proximate coordination sites for the reactant molecules and hence
better reactivity in catalytic reactions. In recent times, redox active
and emissive cycloruthenates have also attracted significant atten-
tion for their potentials as light harvesting materials [20,21]. We
have been examining the cyclometallation of the pendant aryl moi-
ety (Ar) in the acid hydrazones (ArCR@NNHC(@O)R0) of various
mono- and polycyclic aromatic aldehydes using platinum metals
for the past several years [22–30]. The aryl group in the
Ar–C(R)@N– (R = H or Me) fragment of a monocyclic aromatic
aldehyde acid hydrazone provides only the ortho-C for metallation
[22–25]. On the other hand, in case of a polycyclic aromatic
⇑
Corresponding author. Tel.: +91 40 2313 4829; fax: +91 40 2301 2460.
E-mail address: spal@uohyd.ac.in (S. Pal).
http://dx.doi.org/10.1016/j.ica.2014.01.002
0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

K. Nagaraju, S. Pal / Inorganica Chimica Acta 413 (2014) 102–108
103
model 620A electrochemical analyser. The measurements were
carried out with a platinum disk working electrode, a platinum
wire auxiliary electrode and an Ag/AgCl reference electrode at
298 K under nitrogen atmosphere. The ferrocenium/ferrocene
couple was observed at E1/2 = 0.65 V under identical conditions.
2.3. Synthesis of cis-[Ru(PPh3)2(HL1)2] (1)
Solid [Ru(PPh3)3Cl2] (200 mg, 0.21 mmol) was added to a meth-
anol solution (30 ml) of H2L1 (153 mg, 0.42 mmol) and NaOAc
(35 mg, 0.42 mmol). The mixture was boiled under reflux for 1 h
and then cooled to room temperature. The red solid deposited
was collected by filtration and dried in air. This material was dis-
solved in minimum amount of dichloromethane and transferred
to a silica gel column packed with dichloromethane. The first yel-
low band moved with the eluent 1:4 mixture of dichloromethane/
n-hexane was discarded. The following red band containing the
complex 1 was eluted with a 2:3 mixture of dichloromethane/n-
hexane. The red solution thus obtained was evaporated to dryness
and the complex was collected as a dark red solid. The yield was
220 mg (78%).
Scheme 1. Possible cyclometallation sites.
the synthesis, characterization and physical properties of these
complexes with the molecular structures of two representative
complexes determined by X-ray crystallography.
2. Experimental
2.1. Materials
The remaining three complexes 2–4 with the general formula cis-
[Ru(PPh3)2(HLn)2] (n = 2–4) were synthesized in 77–81% yields using
[Ru(PPh3)3Cl2], NaOAc and the corresponding Schiff bases in
1:2:2 mol ratiobyfollowing proceduresverysimilartothatused for1.
Thiobenzhydrazones of polycyclic aromatic aldehydes (H2L1–4
(Scheme 2)) were synthesised by following a procedure similar to
that reported for analogous compounds [31,32]. [Ru(PPh3)3Cl2]
was prepared according to the literature method [33]. All other
chemicals were of analytical grade available commercially and were
used as supplied without further purification. Purification of the sol-
vents used was performed by following standard methods [34].
2.4. X-ray crystallography
X-ray quality single crystals of 3 and 4 were obtained by slow
evaporation of the respective dichloromethane-acetonitrile (1:1)
ꢀ
2.2. Physical measurements
solutions. 3 crystallizes as it is in the space group P1, while 4 crys-
tallizes with a dichloromethane molecule in the space group P21/c.
Determination of the unit cell parameters and intensity data col-
lection at 298 K for 3 were performed on a Bruker-Nonius SMART
APEX CCD single crystal diffractometer using graphite monochro-
Microanalysis (CHN) data were obtained with a Thermo Finni-
gan Flash EA1112 series elemental analyzer. Magnetic measure-
ments were performed with the help of a Sherwood scientific
balance. A Shimadzu LCMS 2010 liquid chromatograph mass spec-
trometer was used for the purity verification of the Schiff bases. A
Bruker Maxis HRMS (ESI-TOF analyzer) spectrometer was used to
record the mass spectra of the complexes. The infrared spectra
were collected by using KBr pellets on a Jasco-5300 FT-IR spectro-
photometer. The electronic spectra were recorded on a Cary 100
Conc UV/Vis spectrophotometer. The fluorescence spectra were re-
corded on a Spex FluoroMax-3 spectrofluorimeter. The NMR spec-
tra of the Schiff bases (in (CD3)2SO) and the complexes (in CDCl3)
were collected with the help of a Bruker 400 MHz NMR spectrom-
eter. Cyclic voltammograms of the complexes in dimethylformam-
ide containing tetra-n-butylammonium perchlorate (TBAP) as the
mated Mo K
a radiation (k = 0.71073 Å). The SMART and the SAINT-
Plus programs [35] were used for data acquisition and data
extraction, respectively. The SADABS program [36] was used for
absorption correction. Unit cell parameters and the intensity data
at 298 K for 4ꢂCH2Cl2 were obtained using graphite monochromat-
ed Mo Ka radiation (k = 0.71073 Å) with the help of an Oxford Dif-
fraction Xcalibur Gemini single crystal X-ray diffractometer. The
CrysAlisPro software [37] was used for data collection, reduction
and absorption correction. The structures of both complexes were
solved by direct method and refined on F2 by full-matrix
least-squares procedures. In the case of 3, the 9-phenanthryl moi-
ety of one of the two (HL3)ꢁ is refined with geometrical and
thermal restraints due to severe disorder. Due to the restrained
supporting electrolyte were recorded with
a CH-Instruments
Scheme 2. The thiobenzhydrazones (H2L1–4) and synthesis of cis-[Ru(HL1–4)2(PPh3)2] (1–4).

104
K. Nagaraju, S. Pal / Inorganica Chimica Acta 413 (2014) 102–108
refinement, relatively high residual electron densities are found
around this particular disordered 9-phenanthryl fragment. On the
other hand, in case of 4, ortho- and meta-C atoms of the phenyl ring
in one of the two (HL4)ꢁ were located at eight positions about an
approximate two-fold axis passing through the remaining two C-
atoms. These disordered C atoms were refined isotropically with
half occupancy and geometric restraints. The location of the CH2Cl2
molecule in 4ꢂCH2Cl2 indicates its possible involvement in a very
weak C–Hꢂ ꢂ ꢂS interaction (Cꢂ ꢂ ꢂS, 3.71(2) Å; <C–Hꢂ ꢂ ꢂS, 170°) with
the complex molecule [38]. The crystals of 4ꢂCH2Cl2 were very
weakly diffracting. Hence, the observed to unique data ratio is very
low and R-factors are rather high for 4ꢂCH2Cl2. In both structures,
all non-hydrogen atoms with full occupancy were refined aniso-
tropically. The hydrogen atoms were included at ideal positions
for structure factor calculation by using a riding model. Structure
solution and refinement were done using the SHELX-97 programs
[39] available in the WinGX package [40]. The Platon [41] and Mer-
cury [42] software packages were used for molecular graphics. Se-
lected crystal and refinement data are listed in Table 1.
and the corresponding polycyclic aromatic aldehydes in ethanol.
All the Schiff bases were characterized by microanalysis (CHN)
and various spectroscopic (infrared, electronic, emission, mass
and 1H NMR) measurements. These characterization data are listed
in Tables S1 and S2 (Supplementary material). In our attempts to
prepare ruthenium(III) cyclometallates with CNS pincer like li-
gands, [Ru(PPh3)3Cl2], H2Ln and NaOAc (1:1:2 mol ratio) were re-
acted in methanol. Under similar reaction conditions, the
analogous benzhydrazones are known to provide cycloruthenates
[29,30]. However, instead of the cycloruthenates, complexes with
the general formula [Ru(PPh3)2(HLn)2] (1–4), where (HLn)ꢁ acts as
thioamidate-N,S donor four-membered chelate ring forming li-
gand, have been formed. Based on the metal to ligand ratio in these
complexes, the reactions were performed again using [Ru(PPh3)3-
Cl2], H2Ln and NaOAc in 1:2:2 mol ratio (Scheme 2) and the com-
plexes have been isolated in much improved yields (77–81%)
compared to the previous reactions. The microanalysis (CHN) and
the ESI mass spectral data for 1–4 are in very good agreement with
their molecular formulae (Table 2). The bivalent low-spin state of
ruthenium in 1–4 is indicated by the diamagnetic nature of them.
All the complexes dissolve easily in dichloromethane, chloroform,
dimethylformamide and dimethylsulfoxide and provide red solu-
tions. In solution, they are electrically non-conducting.
3. Results and discussion
3.1. Synthesis and some properties
The Schiff bases H2L1–4 were prepared in 92–95% yields by con-
densation reactions of equimolar amounts of thiobenzhydrazide
3.2. Spectroscopic characteristics
Infrared spectra of H2L1–4 display two sharp and moderately
strong bands in the range 3184–3293 and 953–975 cmꢁ1 due to
the thioamide N–H and C@S, respectively [43]. The absence of both
of these bands in the spectra of 1–4 indicates deprotonated state of
the thioamide fragment of the complexed (HLn)ꢁ. A medium inten-
sity band within 1575–1595 cmꢁ1 observed for H2Ln is assigned to
its azomethine fragment. The spectra of 1–4 display a similar band
at a slightly lower frequency range of 1567–1589 cmꢁ1. In the
complexes, the azomethine-N does not coordinate the metal cen-
tre, while the adjacent thioamidate acts as four-membered chelate
ring forming an N,S-donor (vide infra). Perhaps the band observed
for 1–4 is due to the conjugated –HC@N–N@C(Sꢁ-)– moiety of
(HLn)ꢁ.
Proton NMR spectroscopic data of the Schiff bases and the
corresponding complexes have been listed in Tables S2 and S3
(Supplementary material), respectively. The spectra of H2Ln
indicate the thioamide-iminothiol tautomerism. Both NH and SH
protons appear as singlets in the ranges d 13.33–13.45 and d
7.26–7.53, respectively. Neither of these two signals is observed
in the spectra of 1–4. The nonappearance of these signals corrobo-
rates the infrared spectral observation regarding the deprotonated
state of the thioamide fragment of (HLn)ꢁ in each complex. The
spectra of the complexes are quite complex and hence difficult to
Table 1
Selected crystal data and structure refinement summary.
Complex
3
4ꢂCH2Cl2
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
C80H60N4P2S2Ru
1304.45
triclinic
C85H62N4P2S2Cl2Ru
1437.42
monoclinic
P21/c
14.0029(12)
17.7848(14)
27.8436(19)
90
96.712(6)
90
6886.6(9)
4
1.386
0.463
27143
12106
3192
861
0.0946, 0.1202
0.3174, 0.2001
0.824
0.510/ꢁ0.373
ꢀ
P1
13.6102(18)
13.8872(18)
17.727(2)
90.939(2)
106.070(2)
94.427(2)
3207.5(7)
2
1.351
0.409
26861
11238
8397
802
0.0695, 0.1796
0.0962, 0.1949
1.036
1.868/ꢁ0.810
b (Å)
c (Å)
a
(°)
b (°)
c
(°)
V (Å3)
Z
q
l
(g cmꢁ3
(mmꢁ1
)
)
Reflections collected
Reflections unique
Reflections [I P 2
Parameters
R1, wR2 [I P 2
r
(I)]
r
(I)]
R1, wR2 [all data]
Goodness-of-fit (GOF) on F2
Maximum/minimum peaks (e Åꢁ3
)
Table 2
Elemental analysis, ESI-Ms,a electronic absorption and emission spectroscopica and cyclic voltammetric datab,c
.
Complex Found (calc) (%)
Found (calc) m/z
Absorption kmax (nm) (10ꢁ4
(Mꢁ1 cmꢁ1))
ꢃ
e
Emission kmax
(nm)
E1/2 (V) (DEp
(mV))
(M + 1)+
C
H
N
1
2
3
4
71.56
(71.80)
73.48
(73.66)
73.39
(73.66)
74.43
4.81
(4.69)
4.72
(4.64)
4.85
(4.64)
4.54
4.71
(4.65)
4.43
(4.29)
4.38
(4.29)
4.27
1205.28 (1205.25)
1305.28 (1305.28)
1304.98 (1305.28)
1352.77 (1352.28)
465 (0.65), 348 (1.83), 301 (2.47), 245
(2.81)
372
0.83 (100)
0.84 (90)
0.86 (90)
0.76 (100)
497d (0.47), 415 (0.89), 259 (3.69)
416, 442, 466,
501d
466 (0.69), 347 (2.09), 254 (4.33)
367,d 381, 401d
505 (1.02), 391 (3.08), 295 (2.15), 245
(2.85)
395,d 424, 349d
(74.59)
(4.47)
(4.14)
a
b
c
In dichloromethane.
In dimethylformamide.
E1/2 = (Epa + Epc)/2 and
Shoulder.
D
Ep = Epa ꢁ Epc, where Epa and Epc are cathodic and anodic peak potentials, respectively.
d

K. Nagaraju, S. Pal / Inorganica Chimica Acta 413 (2014) 102–108
105
assign due to the overlapping of the (HLn)ꢁ protons with the PPh3
protons. Nevertheless, the spectra clearly indicate that each pair
of (HLn)ꢁ and PPh3 in all four complexes is chemically equivalent.
The coordination sphere around the metal centre as found in each
of the two X-ray structures of 3 and 4 (vide infra) suggests that
the two (HLn)ꢁ as well as the two cis-oriented PPh3 molecules are
very likely to be related to each other by a two-fold axis passing
through the metal centre in solution-state. The azomethine proton
of H2Ln appears as a singlet in the range d 8.71–9.96. A similar
singlet observed within d 9.70–10.10 for 1–4 is assigned to the
azomethine proton of (HLn)ꢁ. The downfield shift of the azomethine
proton in 1–4 is possibly the combined influence of electron density
lowering caused by the metal coordination at the adjacent biden-
tate thioamidate and the neighbour-anisotropy effect. The remain-
ing few protons of the complexes that could be assigned do not
show any particular trend in the chemical shifts when compared
with the chemical shifts of the corresponding protons in the free
Schiff bases (Tables S2 and S3). The 31P NMR spectra of the com-
plexes display a singlet within the narrow range of d 49.19–49.51.
As indicated by the proton NMR spectra, this 31P resonance also
corroborates the chemical equivalence of the two cis oriented
PPh3 ligands in 1–4.
Fig. 2. Emission spectra of H2L2 (0.59 ꢃ 10ꢁ4 M) (———) and cis-[Ru(PPh3)2(HL2)2]
(2) (0.14 ꢃ 10ꢁ4 M) (—) in CH2Cl2.
emission response is typical of the polycyclic aromatic fragment
of (HLn)ꢁ and indicates the involvement of its p⁄ state [44].
p–
Electronic spectra of H2L1–4 and the corresponding complexes
(1–4) were recorded in dichloromethane. The spectra of H2L1 and
1 are illustrated in Fig. 1 and the spectra of the remaining thi-
obenzhydrazones (H2L2–4) and the corresponding complexes (2–
4) are provided in the supplementary material (Fig. S1). Other than
two shoulders at 435 and 405 nm for H2L2 and H2L4, respectively,
the remaining absorptions for all the Schiff bases are below
390 nm (Table S1). On the other hand, the spectra of 1–4 display
three to four strong absorptions in the range 505–245 nm (Table 2).
Thus for the complexes the lowest energy band is likely to be due
to the metal to ligand charge transfer, while the following absorp-
tions are attributed primarily to ligand centred transitions.
All the Schiff bases and their corresponding complexes are
emissive in nature. Emission properties have been studied by using
their dichloromethane solutions. Representative emission spectra
of a thiobenzhydrazone and its complex are illustrated in Fig. 2
and the spectra of the other three thiobenzhydrazones and their
corresponding complexes are given in the supplementary material
(Fig. S2). On excitation at the wavelength of either the lowest en-
ergy absorption band or the most intense absorption band the
Schiff bases display a broad emission in the wavelength range
410–515 nm (Table S1, Figs. 2 and S2). The emission spectra of
1–4 have been collected with excitation at ꢀ300 nm. The data have
been summarized in Table 2. A group of three or four overlapping
emission bands are observed for 2–4, while 1 displays a signifi-
cantly broader emission band (Figs. 2 and S2). This type of
3.3. X-ray molecular structures of cis-[Ru(PPh3)2(HL3)2] (3) and
cis-[Ru(PPh3)2(HL4)2] (4)
The molecular structures of 3 and 4 are illustrated in Fig. 3 and
the bond lengths and angles in the coordination spheres of the me-
tal centres are listed in Table 3. In both complex molecules, the me-
tal centre is in
a distorted octahedral N2S2P2 coordination
environment. Both thiobenzhydrazonates act as thioamidate-N,S
donors and form four-membered chelate rings. The deprotonated
state of the thioamide fragments of (HLn)ꢁ in 3 and 4 is indicated
by the shorter C–N (1.289(12)–1.320(11) Å) and longer C–S
(1.713(11)–1.723(6) Å) bond lengths than the corresponding bond
lengths in free substituted thiosemicarbazide/ thiobenzhydrazide
[45,46] or thiosemicarbazones [47–50] and in neutral thiosemicar-
bazone coordinated to metal ion via azomethine-N and thioamide-
S [51]. The mutually cis PPh3 ligands are trans to the two thioami-
date-N atoms in both complexes. The bond parameters associated
with the metal centres in both complexes are also very similar
(Table 3). The bite angles of the four-membered chelate rings
formed by (HLn)ꢁ are within 65.12(13)–66.4(3)°, while the remain-
ing cis bond angles are in the range 79.9(3)–104.42(11)°. The trans
bond angles span a smaller range of 158.41(6)–166.7(3)° compared
to the cis bond angles. Overall, the bond angles and the bond
lengths involving the metal centre in each of 3 and 4 are compara-
ble with those observed for similar complexes of cis-{Ru(PPh3)2}2+
with four membered chelate ring forming thioamidate-N,S donor
thiosemicarbazonates [52–59]. The very similar physical proper-
ties of all four complexes indicate that 1 and 2 have the same gross
molecular structure as that of 3 and 4.
3.4. Coordination mode and cyclometallation
Thiosemicarbazonates are analogous to the thiobenzhydrazo-
nates regarding the identical >C@N–N@C(Sꢁ)– fragment in both
which can have two coordination modes. For ruthenium and os-
mium complexes of thiosemicarbazonates both four-membered
chelate ring forming thioamidate-N,S and five-membered chelate
ring forming azomethine-N and thioamidate-S coordination modes
have been observed [50,59,60]. In general, between the two coor-
dination modes the thioamidate-N,S coordination is more common
for platinum metal ions such as ruthenium that prefer hexacoordi-
nation [50,52–59]. It has been reported that if the group R trans to
Fig. 1. Electronic spectra of H2L1 (———) and cis-[Ru(HL1)2(PPh3)2] (1) (—) in CH2Cl2.

106
K. Nagaraju, S. Pal / Inorganica Chimica Acta 413 (2014) 102–108
Table 3
Selected bond lengths (Å) and bond angles (°) for 3 and 4ꢂCH2Cl2.
Complex
3
4ꢂCH2Cl2
Ru–N(2)
Ru–N(4)
Ru–P(1)
Ru–P(2)
Ru–S(1)
Ru–S(2)
2.141(4)
2.149(5)
2.164(10)
2.154(9)
2.309(3)
2.308(3)
2.400(3)
2.396(3)
79.9(3)
92.7(3)
162.0(3)
65.1(3)
98.8(3)
166.7(3)
90.0(3)
95.8(2)
66.4(3)
2.3216(16)
2.3331(15)
2.4133(14)
2.4190(15)
80.29(17)
90.12(13)
164.12(13)
65.86(12)
96.57(12)
164.21(13)
90.93(13)
98.22(13)
65.12(13)
101.12(6)
89.12(5)
N(2)–Ru–N(4)
N(2)–Ru–P(1)
N(2)–Ru–P(2)
N(2)–Ru–S(1)
N(2)–Ru–S(2)
N(4)–Ru–P(1)
N(4)–Ru–P(2)
N(4)–Ru–S(1)
N(4)–Ru–S(2)
P(1)–Ru–P(2)
P(1)–Ru–S(1)
P(1)–Ru–S(2)
P(2)–Ru–S(1)
P(2)–Ru–S(2)
S(1)–Ru–S(2)
N3
S2
N4
S1
Ru
P1
N1
N2
P2
99.93(12)
90.89(11)
104.42(11)
101.60(11)
90.49(11)
158.65(11)
103.99(5)
102.78(5)
91.60(5)
158.41(6)
(a)
acts as a CNS pincer like ligand in the complex [59]. On the other
hand, the large P–Ru–P angle creating two mutually cis triphenyl-
phosphines and chelating 1,4-bis(diphenylphosphino)butane as
ancillary ligands do not allow the thiosemicarbazonate to have
the five-membered ring forming coordination mode. Instead,
four-membered chelate ring with a smaller N–Ru–S bite angle
forms and the aromatic ring is positioned away from the metal
centre [57–59]. Contrary to the above steric arguments, in the
dicationic bis complexes of cis-{Ru(PPh3)2}2+ with neutral thio-
semicarbazones derived from 4-R-3-thiosemicarbazides, each of
the two ligands forms five-membered chelate ring [51]. Perhaps
the protonated thioamide in the >C@N–NHC(@S)NHR fragment
and the bulky substituent (R = cyclohexyl and substituted phenyl)
impose a different type of steric hindrance compared to that in the
complexes of the thiosemicarbazonates where the thioamide is
deprotonated [50,52–60]. As a result, these neutral thiosemicarba-
zones prefer the five-membered instead of the four-membered
ring forming coordination mode.
N1
S1
N2
S2
Ru
P2
P1
N3
N4
In our earlier works on ruthenium complexes with acid hydra-
zones (H2L0, ArCR@NNHC(@O)R0 (R = H or Me and R0 = Me or Ph)),
the –CR@N–N@C(Oꢁ)– moiety of (HL0)ꢁ has the five-membered
ring forming azomethine-N and amidate-O coordination mode
[24,25,29,30]. As seen for thiosemicarbazonates [59], here also this
type of coordination provides the orientation of the aromatic ring
(Ar) required for its cycloruthenation. Perhaps the hard amidate-
O coordination leads to the subsequent aerobic oxidation of the
metal centre and formation of the ruthenium(III) complex of for-
mula trans-[Ru(L0)(PPh3)2Cl] where (L0)2ꢁ acts as a CNO pincer li-
gand. Like 1–4, the complexes with H2L0 were also prepared from
[Ru(PPh3)3Cl2] in presence of base (NaOAc/NEt3) in methanol
[24,25,29,30]. Thus a similar five-membered chelate ring forming
azomethine-N and thioamidate-S coordination and cyclometalla-
tion of the polycyclic aromatic fragment was expected for the pres-
ent series of thiobenzhydrazonates. However, (HLn)ꢁ in each of 1–4
adopts the four-membered ring forming thioamidate-N,S coordi-
nation mode with an orientation of its polycyclic aromatic ring that
makes the cycloruthenation impossible (Fig. 3). The (HL0)ꢁ and the
(HLn)ꢁ differ only in one coordination site (O or S) and also very
similar procedures have been used for the synthesis of their com-
plexes. Thus the difference in the coordination modes of these two
ligands may not be controlled by only ancillary ligand imposed
steric hindrance as apparent for the complexes of thiosemicarbaz-
onates [50,59,60]. The X-ray structures of free thiosemicarbazones
(b)
Fig. 3. Molecular structures of (a) cis-[Ru(PPh3)2(HL3)2] (3) and (b) cis-[Ru(PPh3)2(-
HL4)2] (4). Thermal ellipsoids of all non-hydrogen atoms are drawn at the 30%
probability level. For clarity only non-carbon atoms are labelled and only one
orientation of the disordered phenyl ring of 4 is shown.
the thioamidate-N in the –C(R)@N–N@C(Sꢁ-)– fragment of the li-
gand is large (phenyl) four-membered chelate ring formation oc-
curs, while small R (methyl) leads to five-membered chelate ring
formation [50,60]. This difference has been shown to be due to
the steric hindrance caused by primarily the ancillary ligands. A
relatively more recent report [59] on ruthenium complexes dem-
onstrate the effect of steric hindrance imposed by the ancillary
phosphine ligands on the coordination mode of the thiosemicar-
bazonate and subsequent occurrence or non-occurrence of cyclo-
ruthenation. In the case of chelating bis(diphenylphosphino)
methane as the ancillary ligand, the small P–Ru–P bite angle allows
the five-membered ring forming coordination mode of the Ar–
CH@N–N@C(Sꢁ)– fragment with a larger N–Ru–S bite angle. As a
consequence the aromatic ring (Ar) comes close to the metal centre
and cyclometallation occurs. Eventually the thiosemicarbazonate

K. Nagaraju, S. Pal / Inorganica Chimica Acta 413 (2014) 102–108
107
4. Conclusion
In our attempts to explore the cycloruthenation of the fused
aromatic ring fragments in the thiobenzhydrazones of polycyclic
aromatic aldehydes (H2Ln) and to prepare new CNS pincer com-
plexes, we have obtained a series of cis-{Ru(PPh3)2}2+ bis-thi-
obenzhydrazonate complexes. The synthesis, characterization and
physical properties of these complexes of the general formula cis-
[Ru(PPh3)2(HLn)2] are reported. In these complexes, (HLn)ꢁ acts as a
four-membered chelate ring forming thioamidate-N,S donor. Elec-
tronic spectra of these species display absorptions for both metal-
to-ligand charge transfer and intraligand transitions. However,
emission spectral profiles of the complexes suggest involvement
of only the (HLn)ꢁ polycyclic aromatic fragment centred excited
states. The complexes are redox active and display a one electron
oxidation. At present, we are trying to synthesize the so far elusive
cyclometallated species with suitably substituted thiobenzhydraz-
ones and analogous thiosemicarbazones using a variety of ruthe-
nium starting materials under various different reaction
conditions.
Acknowledgements
Fig. 4. Cyclic voltammogram of cis-[Ru(PPh3)2(HL3)2] (3) in dimethylformamide
(0.1 M TBAP) at 298 K.
K. Nagaraju thanks the Council of Scientific and Industrial Re-
search for a research fellowship. We are grateful to the Department
of Science and Technology (IYC Grant and FIST program) and the
University Grants Commission (CAS program) for the financial
support.
show that the thioamide-S atom is trans to the azomethine-N [47–
50]. It is very likely that the present thiobenzhydrazones have the
same configuration (Scheme 2) which becomes rigid on deprotona-
tion of the thioamide/iminothiol moiety. Perhaps the soft S-atom
coordinates the soft ruthenium(II) [61] first and then the nearby
thioamidate-N completes the four-membered chelate ring. On
the other hand, it is very likely that the acid hydrazonate coordi-
nates the soft metal centre first through the borderline
azomethine-N and then by the hard O-atom and forms the five-
membered ring.
Appendix A. Supplementary material
CCDC 960693 and 960694 contains the supplementary crystal-
lographic data for 3 and 4ꢂCH2Cl2. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data (Tables
S1–S3 and Figs. S1 and S2) associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.ica.2014.01.002.
3.5. Redox properties
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Electron transfer properties of 1–4 in dimethylformamide have
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listed in Table 2 and a representative cyclic voltammogram is illus-
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the potential range 0.76–0.86 V (vs. Ag/AgCl). Comparison of the
current heights of the observed responses with the current heights
of known one electron transfer processes under comparable condi-
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Considering the higher potential required for the oxidation of 1–
4 compared to that needed for the first oxidation of analogous
complexes of thiosemicarbazonates and the very similar coordina-
tion environment around the metal centre in both type of com-
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