Synthesis, characterisation and reactivity of trans-[Ru(L)(PPh3)Br2]; L=2-pyridyl-N-(2′-methylthiophenyl)methyleneimine: Crystal structure of trans-[Ru(L)(PPh3)Br2]

Article abstract of DOI:10.1016/S0277-5387(99)00313-7

Reaction of Ru(PPh₃)₂Br₂ with the NNS chelating tridentate ligand 2-pyridyl-N-(2′-methylthiophenyl)methyleneimine (L) led to the isolation of the ruthenium(II) complex [Ru(L)(PPh₃)Br₂]. Reactivity of this complex with different bidentate chelating ligands revealed that the products are quite different from those obtained by reacting Ru(L)(PPh₃)Cl₂ (the corresponding cis dichloro complex) with the same ligands under comparable conditions. The mixed chelates were isolated and characterised by elemental analysis, magnetic moment measurement and by different spectroscopic methods along with their precursor. Electrochemistry of the complexes was examined by cyclic voltammetry using a platinum working electrode and a Ag/AgCl electrode as reference. The crystal structure of [Ru(L)(PPh₃)Br₂] disclosed that, unlike Ru(L)(PPh₃)Cl₂, the two bromo ligands are in trans position and this explained the difference in its reactivity pattern from the corresponding chloro complex. Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(99)00313-7

Polyhedron 18 (1999) 3735–3739
www.elsevier.nl/locate/poly
Synthesis, characterisation and reactivity of trans-[Ru(L)(PPh3)Br2]; L52-
pyridyl-N-(29-methylthiophenyl)methyleneimine
Crystal structure of trans-[Ru(L)(PPh3)Br2]
Milan Majia, Mossaraf Hossaina, Madhumita Chatterjeea, Shyamal Kumar Chattopadhyayb, Vedavati
G. Puranikc, Pinak Chakrabartid, Saktiprosad Ghosha ,
*
aDepartment of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700-032, India
bDepartment of Chemistry, Bengal Engineering College, Deemed University, Howrah 711-103, India
cPhysical Chemistry Division, National Chemical Laboratory, Pune 411-008, India
dDepartment of Biochemistry, Bose Institute, Calcutta 700-054, India
Received 29 July 1999; accepted 4 October 1999
Abstract
Reaction of Ru(PPh3)2Br2 with the NNS chelating tridentate ligand 2-pyridyl-N-(29-methylthiophenyl)methyleneimine (L) led to the
isolation of the ruthenium(II) complex [Ru(L)(PPh3)Br2]. Reactivity of this complex with different bidentate chelating ligands revealed
that the products are quite different from those obtained by reacting Ru(L)(PPh3)Cl2 (the corresponding cis dichloro complex) with the
same ligands under comparable conditions. The mixed chelates were isolated and characterised by elemental analysis, magnetic moment
measurement and by different spectroscopic methods along with their precursor. Electrochemistry of the complexes was examined by
cyclic voltammetry using a platinum working electrode and a Ag/AgCl electrode as reference. The crystal structure of [Ru(L)(PPh3)Br2]
disclosed that, unlike Ru(L)(PPh3)Cl2, the two bromo ligands are in trans position and this explained the difference in its reactivity
pattern from the corresponding chloro complex.
1999 Elsevier Science Ltd. All rights reserved.
Keywords: Ruthenium(II) complexes; Trans dibromo complex; NNS ligand; Chemical and electrochemical studies; X-ray crystal structure
1. Introduction
from those of the corresponding complexes obtained from
(A) and none of the resultant species contain any tri-
phenylphosphine though its precursor is Ru(L)(PPh3)Br2.
Each of the complexes prepared from (A), however,
contains one triphenylphosphine moiety. It is difficult to
explain such observations because the Ru–Br bond is
weaker than the Ru–Cl bond. Also, if complex (1) is
structurally similar to (A), it is expected to yield similar
types of mixed chelates. In order to find an unambiguous
solution to this problem we prepared good single crystals
of (1) and determined their structure by X-ray diffraction.
It was then confirmed that (1) is trans-Ru(L)(PPh3)Br2.
This finding explained the difference in the reactivity
pattern of (A) and (1). The difference between the stereo-
chemistry of (A) and (1) may be due to steric factors
and/or the electronic character of the monodentate ligands
attached to the Ru(II) centre since the size of the ligands
increase in the order of Cl,Br,PPh3. The steric factor
alone could not preclude the formation of the cis-dibromo
derivative. However, in practice we find that instead of two
bromo ligands, one PPh3 and one bromo ligands occupy
In our exploration of the chemistry of ruthenium coordi-
nated to different types [1–11] of mixed hard-soft N–S
chelating ligands, we reported the synthesis, crystal struc-
ture and reactivity of cis-Ru(L)(PPh3)Cl2 (A) [L52-
pyridyl-N-(29-methylthiophenyl)methyleneimine] in a re-
cent publication [11]. In this work (A) was prepared by
reacting Ru(PPh3)3Cl2 with the ligand (L) in refluxing
benzene. When we reacted Ru(PPh3)3Br2 with the ligand
under identical conditions, a product of similar composi-
tion, Ru(L)(PPh3)Br2 (1), was obtained. However, the
reactivity pattern of (1) was found to be quite different
from that of (A) with respect to similar reagents under
identical experimental conditions. Metathetical reactions of
(1) with bidentate ligands lead to the formation of a
number of compounds having compositions quite different
*Corresponding author. Tel.: 191-33-473-4971; fax: 191-33-473-
2805.
E-mail address: icspg@mahendra.iacs.res.in (S. Ghosh)
0277-5387/99/$ – see front matter
PII: S0277-5387(99)00313-7
1999 Elsevier Science Ltd. All rights reserved.

3736
M. Maji et al. / Polyhedron 18 (1999) 3735–3739
cis positions in (1). This observation points to the fact that
the electronic factor is also playing an important role in
deciding the actual stereochemistry of the compounds (A)
and (1). The importance of the electronic factor is also
evidenced from the fact that the presence of the tri-
phenylphosphine moiety raise the Ru(II)/Ru(III) oxidation
potential compared to those having no triphenylphosphine
moiety.
methanol and the mixture was refluxed for about 12 h.
After cooling, the volume was reduced to one-third of its
original volume in a rotary evaporator and the compound
precipitated with aqueous ammonium hexafluorophosphate
solution. The precipitate was filtered and washed throughly
with water. The compound was dried over fused CaCl2 and
recrystallised from MeOH/EtOH. Yield: 230 mg, 75%.
Anal. found: C, 29.2; H, 3.3; N, 9.0. Calcd. for
RuC31H27N2SPBr2: C, 29.3; H, 3.2, N, 9.1%. Conduct-
ance in CH3CN (LM) 132 Ohm21 cm2 mol21
.
2. Experimental
2.2.3. [Ru(L)(pic)Br] (3)
RuCl3 was purchased from Arora-Matthey Limited.
Ru(PPh3)3Cl2 [12] was prepared using a previously pub-
lished procedure. The ligand 2-pyridyl-N-(29-methyl-
thiophenyl)methyleneimine (L) was prepared according to
the literature [11]. All other common chemicals and
solvents used were of reagent grade and were used as
received. Acetonitrile obtained from E. Merck (India), was
freshly distilled over CaH2 prior to electrochemical experi-
ments.
Again, 375 mg (0.5 mmol) of compound 1 was dis-
solved in 50 ml of methanol. Then, 70 mg (0.5 mmol) of
picolinic acid and 0.60 mg (0.5 mmol) of sodium carbon-
ate was added and the mixture was refluxed for 12 h. After
cooling, the volume of the solution was reduced in a rotary
evaporator. The compound was then precipitated with
ether, filtered and washed with ether. The compound was
dried over fused CaCl2 and recrystalised from MeOH/
Et2O. Yield: 186 mg, 70%. Anal. found: C,.43.0; H, 3.0;
N, 7.8. Calcd. for RuC31H27N2SPBr2: C, 42.9; H, 3.0, N,
7.9%. Conductance in CH3CN (LM) 30 Ohm21 cm2
2.1. Physical measurements
mol21
.
Elemental analyses were performed on a Perkin Elmer
240 CHNS/O analyser. Solution conductance was mea-
sured on a Systronics direct reading conductivity meter
(model 304), and room temperature magnetic moments
were measured with a PAR vibrating sample magnetometer
using Hg[Co(SCN)4] as calibrant. IR and electronic spec-
tra were recorded on a Perkin Elmer 783 IR spec-
trophotometer and Shimadzu UV–Vis recording spec-
trophotometer, respectively. NMR spectra were recorded
on a Bruker 300-MHz NMR spectrometer using TMS as
the internal standard. Cyclic voltammetry experiments
were carried out on a BioAnalytical System (BAS) 27
electrochemical analyser and a BAS Model X–Y recorder
using a platinum disc electrode.
2.2.4. [Ru(L)(ox)Br] (4)
First, 375 mg (0.5 mmol) of compound 1 was dissolved
in 50 ml of methanol, then 80 mg (0.5 mmol) of oxine and
60 mg (0.5 mmol) of sodium carbonate was added and the
mixture refluxed for 12 h. After cooling, the volume of the
solution was reduced in a rotary evaporator. The com-
pound was precipitated with ether. It was then filtered,
washed with ether, dried over fused CaCl2 and recrystal-
lised from MeOH/Et2O. Yield: 199 mg, 72%. Anal. found:
C, 47.5; H, 3.3; N, 7.5. Calcd. for RuC31H27N2SPBr2: C,
47.6; H, 3.2, N, 7.6%. Conductance in CH3CN (LM) 37
Ohm21 cm2 mol21
.
2.3. Crystallographic structure determinations
2.2. Preparation of the complexes
A single crystal of the complex Ru(L)(PPh3)Br2 suitable
for X-ray structure determination was grown by slow
diffusion of petroleum ether into a dichloromethane solu-
tion of the compound. A deep brown crystal of dimension
0.2530.1331.2 mm was used for data collection. Accurate
unit cell parameters were determined by a least-square fit
of 25 machine-centered reflections in the range 24,2u,
368. Data were collected at 293 K on a PC-controlled
Nonius CAD-4 single crystal X-ray diffractometer using
2.2.1. Trans-Ru(L)(PPh3 )Br2 (1)
First, 1049 mg (1 mmol) of Ru(PPh3)3Br2 was dis-
solved in 40 ml benzene and the solid ligand (470 mg, 2
mmol) was added to it followed by 30 ml of benzene. The
mixture was refluxed for 5 h. The compound separated out
within 30 min. The reaction mixture was cooled and
filtered. The dark violet residue was washed thoroughly
with benzene and dried in vacuo over fused CaCl2. Yield:
600 mg, 80%. Anal. found: C, 49.7; H, 3.4; N, 3.9. Calcd.
for RuC31H27N2SPBr2: C, 49.5; H, 3.6, N, 3.7%. Conduct-
˚
Mo Ka radiation (l50.7107 A). Three standard reflections
measured every hour showed ,4% variation in average
intensity. The structure was solved using MULTAN-80
(NRCVAX programs) [13]. Full matrix least-squares refine-
ment on F2 of scale factor and positional anisotropic
thermal parameters for non-hydrogen atoms using SHELXL-
93 [14] converged to a final R50.073 and and Rw50.219
ance in CH3CN (LM) 40 Ohm21 cm2 mol21
.
2.2.2. [Ru(L)(en) Br](PF6 ) (2)
First, 375 mg (0.5 mmol) of compound 1 and 30 mg
(0.5 mmol) of ethylenediamine was dissolved in 50 ml of

M. Maji et al. / Polyhedron 18 (1999) 3735–3739
3737
sponding chloro compound but they differ in stereochem-
istry, i.e. disposition of the donor atoms around the
ruthenium centre. In bromo compound 1, two bromo
ligands are trans but in chloro compound (A) the two
chloro ligands are cis to each other. The coordination
geometry around ruthenium is distorted octahedral. The
ligand acts as a tridentate donor coordinating to the
ruthenium(II) center through pyridine nitrogen N(1), imine
nitrogen N(2), and thioether sulfur(S), which occupy three
positions of the equatorial plane; the forth position being
occupied by a triphenylphosphine moiety. The two axial
positions are occupied by two bromo ligands. The Ru–S
˚
bond length of the complex 1 (2.314 A) is nearly equal to
˚
the corresponding bond length (2.309 A) of the chloro
complex (Table 2). The rather short distance of this bond
[15] indicates a strong ruthenium thioether interaction. The
Fig. 1. ORTEP diagram and atom numbering scheme of trans-
Ru(L)(PPh3)Br2 (1).
˚
Ru–P distance in the bromo complex (2.341 A) is larger
˚
than that of the chloro (2.292 A) complex. In the bromo
complex there is a competition of p-back bonding between
PPh3 and imine nitrogen but in the chloro complex such
p-bonding of the chloro ligand trans to PPh3 is not so
effective. This phenomenon is reflected in the slightly
(using reflections with I $ 2s(I) only). One molecule of
dichloromethane was located from the difference Fourier.
It was found to be disordered, with each chlorine atom
occupying two different positions of equal occupancy. The
solvent atoms were refined isotropically (Fig. 1). Coordi-
nates of the hydrogen atoms (except the one for solvent
molecule) were geometrically determined and held fixed
during the refinement. The hydrogen atom scattering
contribution was included in the subsequent calculations.
The crystal data, data collection and refinement parameters
are summarised in Table 1.
˚
longer Ru–N(2) imine (2.034 A) bond distance in the
bromo complex compared to that of the Ru–N(9) (1.988
˚
A) distance in the chloro complex.
All compounds are diamagnetic at room temperature.
Compounds 1, 3, and 4 are non-electrolytes and compound
2 behaves as a 1:1 electrolyte in acetonitrile. The ligand
(L) behaves as a neutral tridentate ligand. The involvement
of the pyridine nitrogen, imine nitrogen and thioether
sulfur of the ligand in coordination to the ruthenium centre
is indicated by analysis of IR spectra (Table 3). The
pyridine ring vibrations at 630 and 430 cm21, a fairly
strong band at 1620 cm21, assigned to n(CN) vibration
and the n(CS) band present at 790 and 760 cm21 in the
free ligand are found to undergo a red shift in the
3. Results and discussion
3.1. Description of the crystal structure of compound 1
Compound 1 has a similar composition to the corre-
Table 2
˚
Table 1
Selected bond lengths (A) and bond angles (8) for trans-
Crystallographic data for trans-[Ru(L)(PPh3)Br2] (1)
[Ru(L)(PPh3)Br2] (1)
Formula
C31H27Br2N2PRuS?CH2Cl2
835.38
Triclinic
¯
P1
Ru–Br(1)
Ru–N(2)
Ru–S
C(13)–S
C(7)–C(12)
C(5)–C(6)
P–C(14)
P–C(26)
2.502(2)
2.034(8)
2.314(3)
1.814(11)
1.41(2)
1.45(2)
1.848(11)
1.841(11)
Ru–Br(2)
Ru–N(1)
Ru–P
C(12)–S
C(6)–N(2)
C(5)–N(1)
P–C(27)
2.531(2)
2.089(8)
2.341(3)
1.789(11)
1.276(14)
1.340(13)
1.823(10)
Formula weight
Crystal system
Space group
˚
a (A)
10.651(2)
12.065(2)
14.918(2)
113.53(2)
89.78(2)
103.23(2)
1702.7(5)
2
0.70930
1.629
3.097
0.0730
0.2189
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
N(1)–Ru–N(2)
N(1)–Ru–S
N(1)–Ru–P
N(2)–Ru–Br(1)
S–Ru–Br(1)
N(2)–Ru–Br(2)
S–Ru–Br(2)
78.6(3)
162.8(2)
100.6(2)
84.3(2)
94.11(8)
87.9(2)
83.81(8)
172.06(6)
98.5(4)
N(2)–Ru–S
N(2)–Ru–P
S–Ru–P
N(1)–Ru–Br(1)
Br(1)–Ru–P
N(1)–Ru–Br(2)
Br(2)–Ru–P
C(12)–S–C(13)
C(13)–S–Ru
84.2(3)
177.2(3)
96.59(10)
84.7(2)
98.29(8)
95.0(2)
89.57(8)
100.9(6)
114.3(4)
3
˚
V (A )
Z
˚
l (A)
rcalc (g cm23
)
m (mm21
R (F)
)
wR (F)
Br(1)–Ru–Br(2)
C(12)–S–Ru
GOF (F2)
1.027

3738
M. Maji et al. / Polyhedron 18 (1999) 3735–3739
Table 3
Important IR bands (in cm21) of the complexes
Compound
n(NH2)
n(C=C)1n(C=N)
n(CS)
Other frequencies
L
1620, 1580, 1570, 1510
1620 (sh)a, 1590, 1515
1645, 1595, 1575,
1640, 1595, 1575, 1555
1605, 1580, 1550
790, 760
775, 750
770, 740
770, 745
775, 745
630, 580, 490, 420
620, 570, 480, 420
850, 550, 520, 460, 420
695, 520, 445, 420
690, 570, 505, 455, 420
[Ru(L)(PPh3)Br2] (1)
[Ru(L)(en)Br]PF6 (2)
[Ru(L)(pic)Br] (3)
[Ru(L)(oxin)Br] (4)
3320, 3220
a sh, shoulder.
bromo ligands. This effect may be due to the higher
electronegativity of the chlorine atom [6,7]. Reactions of
[Ru(L)(PPh3)Br2] with a different bidentate ligand (L9)
gives compounds of the general formula [Ru(L)(L9)Br]0 / 1
formed by the substitution of a PPh3 and a Br ligand. All
these compounds exhibit a lower E8Ru(II) / Ru(III) potential
than that of the mother compound, which indicates that all
the three bidentate ligands are less effective in stabilising
the Ru(II) oxidation state compared to PPh3 and Br jointly.
Among the three complexes, monocation compound 2
shows a higher E8Ru(II) / Ru(III) potential than that of the
neutral compounds [18,19]. Though both picolinic acid and
oxine are uninegative donors, the oxinate compound is
more difficult to oxidise than that of the picolinate
compound due to greater delocalisation of the dp electrons
in the oxinate complex than in the picolinate complex,
because oxine has more low lying p* orbital to receive
back donated electron from the Ru(II) centre and stabilise
the Ru(II) state.
complexes due to M–N and M–S bond formation [16,17].
The strong, broad band at 850 cm21 indicates the presence
of ionic hexafluorophosphate in compound 2. The broad
band centered at 1690 cm21 due to the presence of free
carboxylic acid group of picolinic acid is lowered to
160521 on complexation. The presence of the NH2 proton
of ethylenediamine in complex 2 is revealed by the
symmetric and assymmetric n(N–H) modes at 3320 and
3220 cm21 and the NH2 bending at 1645 cm21
.
3.2. Electrochemistry
The redox behaviour of the complexes were studied by
cyclic voltammetry in acetonitrile (Table 4). All the
compounds show one Ru(II)/Ru(III) oxidation on the
positive side. The separation between the anodic and the
cathodic peak (DEp) value is close to the ideal Nernstian
value of 59 mV and ipc /ipa 5 1. So the Ru(II)/Ru(III)
couple is reversible. The E8Ru(II) / Ru(III) value of the bromo
complex is lower than that of the chloro complex indicates
that chloro ligands stabilize the Ru(II) state more than the
3.3. Electronic spectra
The electronic spectra of Ru(II) complexes is dominated
by charge transfer transitions (Table 5). In the free ligands
there are three bands at 380, 251 and 210 nm. The bands at
251 and 210 nm can be interpreted as p→p* transitions
of the pyridine and phenyl ring respectively, the band at
380 nm is probably a charge transfer transition from the
filled sulfur orbital to one of the p* orbital of the diimine
moiety. In the asymmetric crystal field prevailing in the
complexes, the dp orbitals of Ru(II) are expected to be
non degenerate. Considering the number of vacant p*
orbitals of suitable symmetry in the thioether ligand as
well as in the coligands, a large number of metal-to-ligand
charge transfer transitions are expected, in addition to the
intraligand charge transfer transitions. This makes the
Table 4
Cyclic voltammetric resultsa,b of the complexes in CH3CN
Complex
E1 / 2 /V (DEp, mV)
Oxidation
[Ru(L)(PPh3)Br2] (1)
[Ru(L)(en)Br]PF6 (2)
[Ru(L)(pic)Br] (3)
[Ru(L)(oxin)Br] (4)
0.39(60)
0.59(60)
0.22(60)
0.42(60)
a Working electrode, glassy carbon; reference electrode, Ag/AgCl;
E
1 / 2 5 0.5 (Epc 1 Epa), where Epc and Epa are cathodic and anodic peak
potentials, respectively. Supporting electrolyte 0.1 M TEAP; solute
concentration 1023 M; scan rate 50 mV s21
.
b Peak height of the couple compared with one-electron oxidation of
Ru(II)/Ru(III) of cis-[Ru(bpy)2Cl2] under identical molar concentration
and experimental conditions.
Table 5
Electronic spectral data of the complexes in CH3CN
Complex
lmax /nm (e31023, M21 cm21
)
[Ru(L)(PPh3)Br2] (1)
[Ru(L)(en)Br]PF6 (2)
[Ru(L)(pic)Br] (3)
[Ru(L)(oxin)Br] (4)
282(4.3), 314(0.664), 338(0.463), 516(0.142), 596(0.0157)
226(1.906), 307(0.6515), 341 (sh)a (0.4228), 365 (sh)a (0.2876), 490(0.1525)
227(2.3112), 307(0.8985), 341 (sh)a (0.6847), 359 (sh)a (0.4599), 518(0.1907)
227(1.9728), 250(1.3929), 315(0.3717), 357 (sh)a (0.2710), 406(0.3008), 468 (sh)a (0.2103)
a sh, shoulder.

M. Maji et al. / Polyhedron 18 (1999) 3735–3739
3739
interpretation of the electronic spectra extremely difficult.
However, it may be concluded that bands in the 400–600
nm region are due to MLCT involving Ru(dp)→S(dp)
and Ru(dp)→p* (imine) transitions. Besides, in some of
the complexes there is an additional MLCT band around
360 nm. The two bands around 340 and 310 nm are
probably intraligand charge transfer transitions, while the
band below 300 nm are due to p→p* transitions of
pyridine and phenyl rings.
CB2 1EZ, UK on request, quoting the deposition number
132815.
Acknowledgements
M.M. thanks the CSIR, New Delhi, for the award of a
fellowship. Financial assistance from the Department of
Science and Technology (DST), Government of India,
New Delhi, is also gratefully acknowledged.
3.4. NMR spectra
The methyl signal of the free ligand is observed at 2.37
ppm [20]. On complexation, this signal appears at different
d values for different complexes, ranging from d 1.25 to
2.1 ppm. The two CH2 signals of the ethylenediamine
complex appear at 2.95 and 2.88 ppm. The phenyl protons
are observed in the d 6.0–7.3 range.
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This study points to the fact that a particular procedure
involving similar reactants can lead to chemically and
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N-(29-methylthiophenyl)methyleneimine reacts with the
very similar Ru(II) complexes Ru(PPh3)3Cl2 and
Ru(PPh3)3Br2, two different complexes of the same
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Supplementary data
Supplementary data are available from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge