Chemistry of ruthenium(II) complexes of the tridentate NNS donor methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone. Isolation and structural characterisation of a novel rutheniuni(II) complex containing a co-ordinated imine of an α-N heterocyclic keton

Article abstract of DOI:10.1039/a806341i

A series of ruthenium(II) complexes of the NNS donor ligand methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone (HL) has been synthesized using RuCl₃·xH₂O and [Ru(PPh₃)₃Cl₂]: [Ru(HL)₂][ClO

Full text of DOI:10.1039/a806341i

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Chemistry of ruthenium(II) complexes of the tridentate NNS donor
methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone. Isolation and
structural characterisation of a novel ruthenium(II) complex
containing a co-ordinated imine of an á-N heterocyclic ketone
Milan Maji,a Madhumita Chatterjee,a Saktiprosad Ghosh,*a Shyamal Kumar Chattopadhyay,b
Bo-Mu Wuc and Thomas C. W. Makc
a Department of Inorganic Chemistry, Indian Association for the Cultivation of Science,
Calcutta 700 032, India
b Department of Chemistry, Bengal Engineering College (Deemed University), Howrah 711103,
India
c Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories,
Hong Kong
Received 11th August 1998, Accepted 30th October 1998
A series of ruthenium() complexes of the NNS donor ligand methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone
(HL) has been synthesized using RuCl3ؒxH2O and [Ru(PPh3)3Cl2]: [Ru(HL)2][ClO4]2 1, [Ru(L)(PPh3)2Cl] 2, [Ru(HL)-
(PPh3)2Cl]Cl 3, [Ru(HL)(PPh3)2Cl]PF6 4, [Ru(L)(PPh3)(bpy)]PF6 5, [Ru(L)(PPh3)(dppe)]PF6 6, [Ru(HL)(PPh3)-
(pic)]PF6 7 and [Ru(HL)(PPh3)(mpi)]Cl2 8 [where bpy = 2,2Ј-bipyridine, dppe = 1,2-bis(diphenylphosphino)ethane,
Hpic = pyridine-2-carboxylic acid, mpi = methyl(2-pyridyl)methyleneimine]. Chemical and electrochemical studies
have been carried out. Structures of the compounds 3ؒCH2Cl2 3 and 8ؒCH2Cl2ؒ3H2O have been determined by single
crystal X-ray diffraction. The thione form of the ligand (HL) is chelated to the ruthenium centre through the pyridine
nitrogen, imine nitrogen and the thione sulfur atom. The existence of a new unstable ligand methyl(2-pyridyl)-
methyleneimine (mpi) co-ordinated to RuII through the pyridine and imine nitrogen atoms was confirmed from the
crystal structure of compound 8.
The chemistry of ruthenium bound to nitrogen–sulfur donor
ligands has evinced considerable interest in recent years
primarily due to their ability to form complexes with unusual
stereochemistry,1 uncommon co-ordination number,2,3 interest-
ing electronic structure and bonding situations and with intri-
cate electron-transfer characteristics.4,5 Thiosemicarbazides and
thiosemicarbazones constitute an important class of nitrogen–
sulfur donor ligands, because of their highly interesting
chemical5–9 and biological properties.10 As a part of our pro-
gramme to investigate ruthenium complexes of thiosemicarb-
azides and thiosemicarbazones in general, we undertook the
study of ruthenium complexes of methyl 2-pyridyl ketone
4-(4-tolyl)thiosemicarbazone (HL). During the course of our
investigations we came across two very interesting phenomena.
(1) Under certain reaction conditions RuII-catalysed reductive
cleavage of the hydrazinic N–N bond of the thiosemicarbazone
moiety occurred. Such reductive cleavage is rather common in
the molybdenum complexes of hydrazine,11 but never observed
previously in the metal complexes of thiosemicarbazides and
thiosemicarbazones. (2) We isolated and structurally character-
ised the first metal complex of the imine of a 2(N)-heterocyclic
ketone, i.e. a ruthenium() complex of methyl(2-pyridyl)-
methyleneimine formed during the process mentioned earlier.
and [Ru(HL)(PPh3)(mpi)]Cl2ؒCH2Cl2ؒ3H2O, are described and
discussed.
Experimental
Materials and instrumentation
Elemental analysis were performed with a Perkin-Elmer 240
CHN analyser. Those of complexes 3 and 8 were before crystal-
lisation. The IR and electronic spectra were recorded on a
Perkin-Elmer 783 spectrophotometer (as KBr disks) and on a
Shimadzu UV-VIS recording spectrophotometer respectively.
Solution conductances were measured on a Systronics direct
reading conductivity meter (model 304) and magnetic suscepti-
bility (at room temperature) was determined with a PAR vibrat-
ing sample magnetometer using Hg[Co(SCN)4] as the calibrant.
The NMR spectra were recorded on a Bruker 300 MHz spec-
trometer using SiMe4 as an internal standard. Electrochemical
data were collected with a BAS CV-27 and a BAS model
X-Y recorded at 298 K. Cyclic voltammetry experiments were
carried out with platinum working and auxiliary electrodes and
a Ag–AgCl reference electrode.
The compound RuCl3ؒxH2O was obtained from Arora
Matthey (Calcutta, India) and 2-acetylpyridine from Aldrich.
4-(4-Tolyl)thiosemicarbazide10 and [Ru(PPh3)3Cl2]13 were pre-
pared according to published procedures. Acetonitrile (pure)
obtained from E. Merck (India) was freshly distilled over cal-
cium hydride for electrochemical experiments. Other reagents
were used without further purification.
Most ketone imines (HN᎐CR1R2) are unstable at room tem-
᎐
perature12 and are found to react with some metal ions as a
monodentate ligand through the imine nitrogen or as an exo-
bidentate ligand bridging two metal ion centres through its
deprotonated iminate nitrogen. However, no report on a metal
complex containing
a co-ordinated imine of a 2(N)-
heteroaromatic ketone has appeared previously. This paper
reports the results of our studies on several ruthenium com-
plexes involving HL as well as the complex [Ru(HL)(PPh3)-
(mpi)]Cl2 containing methyl(2-pyridyl)methyleneimine (mpi).
Structures of two complexes, [Ru(HL)(PPh3)2Cl]ClؒCH2Cl2
Preparations
Methyl 2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone (HL).
4-(4-Tolyl)thiosemicarbazide (2.42 g, 0.02 mol) was dissolved
in ethanol (100 cm3) by heating and 2-acetylpyridine (3.62 g,
J. Chem. Soc., Dalton Trans., 1999, 135–140
135

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0.02 mol) was added. The mixture was stirred for 45 min.
Then acetic acid (2 cm3) was added and stirred again for 3 h.
The product was filtered off, washed with water and diethyl
C, 54.49; H, 4.01; N, 8.87%). Conductance in CH3CN 125.21
ΩϪ1 cm2 molϪ1. Electronic spectrum in CH3CN [λmax/nm
(103εmax/MϪ1 cmϪ1)]: 456 (8.01), 376 (15.59), 296 (28.57) and
209 (81.19).
1
ether and dried. The yield was 90%. H NMR (CDCl3, room
temperature): δ 9.32 (s, 1 H, NH), 8.88 (s, 1 H, NH), 8.76 (d),
8.61 (d), 8.01 (d), 7.87 (t), 7.74 (t), 7.56 (t), 7.37 (t), 7.31 (t),
7.19 (t), 2.49 (s, 3 H, CH3) and 2.36 (s, 3 H, CH3).
[Ru(L)(PPh3)(dppe)]PF6 6. The complex [Ru(L)(PPh3)2Cl]
(120 mg, 0.12 mmol) was dissolved in dichloromethane (20
cm3). 1,2-Bis(diphenylphosphino)ethane (49 mg, 0.12 mmol)
was added followed by methanol (25 cm3). The mixture was
refluxed for 8 h then concentrated in a rotary evaporator. The
compound was isolated by adding an aqueous solution of
ammonium hexafluorophosphate. It was filtered off, washed
with water and dried over fused calcium chloride. The dry com-
pound was finally washed with ether and dried (Found: C,
60.13; H, 4.67; N, 4.59. Calc. for C59H54F6N4P4RuS: C, 59.54;
H, 4.54; N, 4.71. Conductance in CH3CN 127.10 ΩϪ1 cm2
molϪ1. Electronic spectrum in CH3CN [λmax/nm(103εmax/MϪ1
cmϪ1)]: 481 (0.820), 424 (5.94), 373 (11.91), 261 (21.75) and 216
(45.96).
[Ru(HL)2][ClO4]2 1. CAUTION! perchlorate salts of metal
complexes with organic ligands are potentially explosive. Only a
small amount of compound should be prepared, and handled
with caution.
The ligand (HL) (568 g, 0.2 mmol) was suspended in meth-
anol (30 cm3) and RuCl3ؒxH2O (261 mg, 0.2 mmol) dissolved in
methanol (25 cm3) was added drop by drop. The mixture was
stirred for 3 h. It was filtered and the filtrate concentrated to
about 10 cm3 using a rotary evaporator. An aqueous solution of
lithium perchlorate was added to the concentrated solution and
the desired compound precipitated. It was washed with water
followed by diethyl ether and dried over fused calcium chloride
(Found: C, 41.03; H, 3.60; N, 12.90. Calc. for C30H32Cl2N8-
O8RuS2: C, 41.47; H, 3.69; N, 12.90%). Conductance in CH3CN
[Ru(HL)(PPh3)(pic)]PF6 7. The complex [Ru(L)(PPh3)2Cl]
(120 mg, 0.12 mmol) was dissolved in dichloromethane (20
cm3). Pyridine-2-carboxylic acid (16 mg, 10.12 mmol) was
added, followed by methanol (25 cm3). The mixture was
refluxed for 8 h (Hpic) then concentrated in a rotary evaporator.
Compound 7 was isolated by adding an aqueous solution of
ammonium hexafluorophosphate. It was filtered off, washed
thoroughly with water and dried over calcium chloride. The dry
compound was washed again with ether and dried (Found: C,
50.8; H, 4.03; N, 4.75. Calc. for C39H34F6N5O2P2RuS: C, 51.26;
H, 3.72; N, 7.67%). Conductance in CH3CN 139.30 ΩϪ1 cm2
molϪ1. Electronic spectrum in CH3CN [λmax/nm(103εmax/MϪ1
cmϪ1)]: 577 (0.811), 481 (4.82), 376 (21.93), 256 (23.09) and 220
(54.13).
(ΛM): 230 ΩϪ1 cm2 molϪ1. Electronic spectrum in CH3CN [λmax
/
nm(103εmax/MϪ1 cmϪ1)]: 650 (2.536), 382 (86.032), 255 (100.9)
and 210 (151.58).
[Ru(L)(PPh3)2Cl] 2, [Ru(HL)(PPh3)2Cl]Cl 3 and [Ru(HL)-
(PPh3)2Cl]PF6 4. The ligand (HL) (71 mg, 0.25 mmol) was dis-
solved in ethanol (20 cm3) and [Ru(PPh3)3Cl2] (240 mg, 0.25
mmol) added. The mixture was refluxed for 4 h under dry nitro-
gen, then cooled. The solid product (2) was filtered off, washed
with ether and dried in a calcium chloride desiccator. The
filtrate was concentrated in a rotary evaporator to about 10
mL. Compound 4 was isolated by adding saturated aqueous
ammonium hexafluorophosphate to the concentrated solution.
It was filtered off, washed thoroughly with water and then with
ether and finally dried over fused calcium chloride. Alter-
natively, the chloride compound 3 was obtained by concentrat-
ing the filtrate to about 10 mL and adding ether. The solid
separated was filtered off, washed thoroughly with ether and
recrystallised from dichloromethane (Found: C, 64.63; H, 4.78;
N, 6.01. Calc. for C51H45ClN4P2RuS 2: C, 64.86; H, 4.77; N,
[Ru(HL)(PPh3)(mpi)]Cl2 8. The complex [Ru(L)(PPh3)2Cl]
(120 mg, 0.12 mmol) was dissolved in ethanol (40 cm3) and
a sixfold excess of ligand (HL) (0.72 mmol) added. The mix-
ture was refluxed for 24 h then cooled and filtered. The filtrate
was evaporated to dryness. The solid residue was stirred with
n-hexane to wash out excess of ligand and triphenyl-
phosphine, filtered off, dried and recrystallised from a mixture
of dichloromethane and n-hexane. The components in the fil-
trate were separated by column chromatography using neutral
silica gel. The first component was triphenylphosphine eluted
with light petroleum (bp 60–80 ЊC), the middle fraction was
N-(p-tolyl)thiourea and the last fraction with the ligand (HL)
eluted with 10% ethyl acetate in light petroleum (Found: C,
57.21; H, 4.62; N, 10.38. Calc. for C40H39Cl2N6PRuS 8: C,
57.28; H, 4.65; N, 10.02%). Conductance in CH3CN 210.00 ΩϪ1
5.93%). Conductance in CH3CN (ΛM): 15.4 ΩϪ1 cm2 molϪ1
.
Electronic spectrum in CH3CN [λmax/nm(103εmax/MϪ1 cmϪ1)]:
580 (0.9007), 481 (2.256), 398 (9.760), 376 (11.192), 274 (11.870)
and 215 (40.85) (Found: C, 62.67; H, 4.83; N, 5.59. Calc. for
C51H46Cl2N4P2RuS 3: C, 62.45; H, 4.69; N, 5.71%). Conduct-
ance in CH3CN 123.09 ΩϪ1 cm2 molϪ1. Electronic spectrum
in CH3CN [λmax/nm(103εmax/MϪ1 cmϪ1)]: 478 (0.3218), 445
(9.381), 402 (10.67), 378 (12.58) and 210 (73.15). 1H NMR
(CDCl3, room temperature): δ 12.16 (s, 1 H, NH), 11.12 (s, 1 H,
NH), 8.76 (d), 7.41 (m), 7.36 (t), 7.24 (m), 7.06 (t), 6.79 (t), 2.31
(s, 3 H, CH3) and 2.16 (s, 3 H, CH3) (Found: C, 56.27; H, 4.34;
N, 5.19. Calc. for C51H46ClF6N4P3RuS 4: C, 56.17; H, 4.22; N,
5.14%). Conductance in CH3CN 153.24 ΩϪ1 cm2 molϪ1. Elec-
tronic spectrum in CH3CN [λmax/nm(103εmax/MϪ1 cmϪ1)]: 478
(0.3712), 442 (11.37), 403 (16.15), 362 (12.37), 265 (35.41) and
212 (52.27).
cm2 molϪ1. Electronic spectrum in CH3CN [λmax/nm(103εmax
/
MϪ1 cmϪ1)]: 464 (3.469), 378 (10.94), 317 (15.65), 260 (20.94)
and 216 (45.96). 1H NMR (CDCl3, room temperature): δ 12.25
(s, 1 H, NH), 8.14 (d), 8.04 (t), 7.87 (q), 7.6 (m), 7.3 (s), 7.05 (d),
6.66 (s), 2.31 (s, 3 H, CH3) and 2.35 (s, 3 H, CH3) (Found: C,
57.68; H, 5.93; N, 17.03. Calc. for N-(p-tolyl)thiourea (C8H10-
N2S): C, 57.83; H, 6.02; N, 16.87%). IR in CHCl3: ν(NH) 3400,
1
3380, ν(SH) 2420, ν(C–S) 840 cmϪ1. H NMR (CDCl3, room
temperature): δ 8.62 (d, 1 H, Ph), 8.24 (d, 1 H, Ph), 7.77 (t, 1 H,
Ph), 7.31 (t, 1 H, Ph) and 3.37 (s, 3 H, CH3).
[Ru(L)(PPh3)(bpy)]PF6 5. The complex [Ru(L)(PPh3)2Cl]
(119 mg, 0.12 mmol) was dissolved in dichloromethane (20
cm3). 2,2Ј-Bipyridine (219.5 mg, 0.12 mmol) followed by
methanol (25 cm3) was added. The mixture was refluxed for 8 h.
After cooling the solution was concentrated in a rotary evapor-
ator to about 10 cm3. Compound 5 was isolated by adding
saturated aqueous ammonium hexafluorophosphate to the
concentrated solution. The precipitated compound was filtered
off, washed thoroughly with distilled water and dried over fused
calcium chloride. It was then washed with ether and dried
(Found: C, 54.93; H, 4.29; N, 9.1. Calc. for C43H38F6N6P2RuS:
X-Ray crystallography
Brown prismatic crystals were grown by the slow diffusion of
n-hexane into dichloromethane solution of complexes 3 and 8
at room temperature. Single crystals 0.15 × 0.19 × 0.20 and
0.10 × 0.20 × 0.40 mm were chosen for diffraction study res-
pectively. Crystal data are in Table 1. Intensity data were col-
lected at 294 K on a MSC/Rigaku-IIC imaging plate diffract-
ometer using graphite-monochromatized Mo-Kα (λ = 0.71073
136
J. Chem. Soc., Dalton Trans., 1999, 135–140

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Å) radiation from a rotating anode generator. For 3 a total of
15190 reflections were collected, with 5221 independent reflec-
tions (Rint = 6.44%).14 For complex 8, 8178 (Rint = 0.00) unique
data were collected. The intensities were corrected for Lorentz-
polarisation effects and absorption using the ABSCOR pro-
gram.15 The structure of 3 and 8 were solved by Patterson
methods and direct methods respectively. All non-hydrogen
atoms were refined anisotropically by full matrix least squares,
with a riding model for hydrogen atoms, using the SHELXTL
PLUS (PC Version) package.16 For compound 3 with 2966
[F > 6.0σ(F)] observed reflections, refinement converged with
Rf = 0.042 and RЈ = 0.049. Largest difference peak and hole are
0.83 and Ϫ0.93 e ÅϪ3 respectively. For compound 8 with 6127
observed reflection (|Fo| ≥ 6σ|Fo|), refinement converged with
Rf = 0.072 and RЈ = 0.078. Largest difference peak and hole are
0.95 and Ϫ0.93 e ÅϪ3 respectively. Selected bond lengths and
bond angles are given in Table 2.
CCDC reference number 186/1231.
Results and discussion
Reaction of ruthenium chloride with HL affords the bis chelate
complex [Ru(HL)2][ClO4]2 1. The compound is diamagnetic
and behaves as a 1:2 electrolyte in acetonitrile solution. Previ-
ous works17–19 with thiosemicarbazones of 2-acetylpyridine
have established that the ligand behaves as a planar NNS donor,
co-ordinating through the pyridine nitrogen, the imine nitrogen
and the thione sulfur atom. The IR spectrum of compound 1
indicates a similar co-ordination behaviour of the ligand. Thus
1 may be considered as a ruthenium() bis chelate, where each
tridentate NNS ligand occupies a meridional plane. Reaction of
(HL) with [Ru(PPh3)3Cl2] in refluxing ethanol leads to the iso-
lation of the monochelates [Ru(L)(PPh3)2Cl] 2 and [Ru(HL)-
(PPh3)2Cl]X [X = Cl 3 or PF6 4]. The neutral complex 2 separ-
ated out from the reaction mixture, whereas the cationic com-
plexes [3 and 4] were isolated from the mother-liquor by add-
ition of the appropriate anion. Compounds 2 and 3/4 can easily
be converted into each other by the addition of acid and base
respectively. Crystal structure analysis of 3 established that in
the distorted octahedral complex the two triphenylphosphine
moieties are trans to each other, while the three NNS donor
points of the ligand and the co-ordinated chloride constitute
the equatorial square plane. The electronic spectrum of the
bulk compound 3 in acetonitrile is identical to that of the crys-
tals dissolved in the same solvent, indicating that the bulk com-
pound is the trans isomer. The ready interconversion of 2 and 3
and very similar IR and electronic spectra suggest that the trans
structure also prevails in 2. The steric repulsion between the two
bulky triphenylphosphine moieties, as well as the p-tolyl moiety
of the ligand leads to the formation of only the trans com-
pounds. When compound 2 is dissolved in acetonitrile the co-
ordinated chloride is solvolysed and the resulting solution
behaves as a 1:1 electrolyte. However, compounds 3 and 4 did
not suffer such a change. It is well known that RuII, a low spin
d6 system, undergoes substitution by a dissociative mechan-
ism.20 Complex 2 being neutral, can dissociate the chloride ion
much more easily than 3 and 4 which are monocationic. Such
an effect of the overall charge of the complex unit on the dis-
sociation of chloride ion is well documented.21 Compound 2
reacts with bidentate donors like bipyridine (bpy) and dppe to
Scheme 1
ligand HL is in its protonated form while the imine (mpi)
retains its neutral (non-deprotonated) form. Though a number
of diphenylmethyleneimine complexes are reported in the
literature involving a variety of co-ordination modes, no [α(N)-
heterocyclic]methyleneimine complexes have been reported to
date. To our knowledge this is the first report of an [α(N)-
heterocyclic]iminato complex which has been fully character-
ised by X-ray crystallography. The formation of the imine com-
plex from the thiosemicarbazone may be visualised to proceed
via a two-electron reductive cleavage of the hydrazinic N–N
bond of the thiosemicarbazone by the ruthenium() acceptor
centre in 2. The resulting ruthenium() complex could be
reduced subsequently by the triphenylphosphine or by the
excess of ligand present in the system. If the ligand plays the
role of reductant, it should be converted into the N-(4-
tolyl)thiourea. The latter is actually isolated from the reaction
medium and identified by its characteristic NMR and IR spec-
tra and elemental analysis. The two-electron reductive cleavage
of the N–N bond is one of the elementary reaction steps in the
reduction of nitrogen to ammonia. Examples of such reductive
cleavage are abundant in molybdenum complexes of hydra-
zine.10 It is also known that diphenylmethyleneimine complexes
may be generated by the reaction of an appropriate precursor
metal complex and azines like Ph C᎐N–N᎐CPh . However,
᎐
᎐
2
2
this is the first report of the generation of an imine complex by
such cleavage of the N–N bond of a thiosemicarbazone co-
ordinated to a metal centre.
Structures of complexes 3 and 8
In both complexes 3 and 8 the ligand occupies a meridional
plane co-ordinating through pyridine nitrogen [N(1)], imine
nitrogen [N(2)] and the thiolate sulfur [S(1)] atom. Along with
these three donor atoms a chlorine [Cl(1)] atom in 3 (Fig. 1) and
an imine nitrogen [N(6)] in 8 (Fig. 2) complexes a square plane
around the metal ion. The Ru–Cl(1) distance (2.459 Å) in 3 is
somewhat long {cf. Ru–Cl distance of 2.387 Å in [Ru(PPh3)-
Cl2]13}. Two trans triphenylphosphine groups in 3 and one
triphenylphosphine and one pyridine nitrogen [N(5)] of methyl-
(2-pyridyl)methyleneimine ion 8 complete the octahedron. It is
worthwhile to make a comparison of structures of 3 and 8 with
that of [Ru(LЈ)(PPh3)2]ClO4 9; [LЈ = monoanion of 2,6-diacetyl-
pyridine 4-(4-tolyl)thiosemicarbazone].6 The trans triphenyl-
give [Ru(L)(PPh3)(bpy)]PF6 5 and [Ru(L)(PPh3)(dppe)]PF6
6
(Scheme 1). However, reaction with Hpic produces [Ru(HL)-
(PPh3)(pic)]PF6 7 in which the ligand is present in its protonated
form, picolinic acid displaying its usual behaviour by acting in
the monoanionic bidentate fashion. The proton dissociated
from picolinic acid appears to transform the deprotonated
ligand into its protonated form. A very interesting reaction
took place when compound 2 was refluxed with an excess
of ligand HL. From the reaction medium it was possible to
isolate the complex [Ru(HL)(PPh3)(mpi)]Cl2 8, in which the
J. Chem. Soc., Dalton Trans., 1999, 135–140
137

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Table 2 Selected bond distances (Å) and angles (Њ) for [Ru(HL)-
(PPh3)2Cl]ClؒCH2Cl2 3 and [Ru(HL)(PPh3)(mpi)]Cl2ؒCH2Cl2ؒ3H2O 8
Table 1 Crystal data for [Ru(HL)(PPh3)2Cl]ClؒCH2Cl2 3 and [Ru-
(HL)(PPh3)(mpi)]Cl2ؒCH2Cl2ؒ3H2O 8
Formula
C52H48Cl4N4P2RuS
C41H47Cl4N6O3PRuS
977.7
Ru(1)–N(1)
Ru(1)–N(2)
Ru(1)–S(1)
Ru(1)–P(1)
Ru(1)–P(1A)
Ru(1)–Cl(1)
C(6)–N(2)
N(2)–N(3)
N(3)–C(8)
C(8)–S(1)
2.085(5)
1.984(5)
2.386(2)
2.399(1)
2.399(1)
2.459(2)
1.304(7)
1.383(7)
1.359(7)
1.707(6)
1.334(8)
1.462(9)
1.416(8)
Ru(1)–N(1)
Ru(1)–N(2)
Ru(1)–S(1)
Ru(1)–P(1)
Ru(1)–N(5)
Ru(1)–N(6)
C(6)–N(2)
N(2)–N(3)
N(3)–C(8)
C(8)–S(1)
2.092(6)
1.991(5)
2.358(2)
2.334(2)
2.110(6)
2.075(7)
1.336(8)
1.371(9)
1.347(8)
1.698(7)
1.344(11)
1.478(12)
1.418(9)
M
1065
Pnma
19.279(4)
15.947(3)
15.752(3)
¯
Space group
P1
a/Å
b/Å
c/Å
α/Њ
β/Њ
11.286(1)
13.629(1)
16.109(2)
98.36
97.19
γ/Њ
109.02(1)
2277.99(11)
2
1000
1.424
0.072
0.078
V/Å3
Z
4843(2)
4
2184
1.462
0.0418
0.0493
C(8)–N(4)
C(5)–C(6)
N(4)–C(9)
C(8)–N(4)
C(5)–C(6)
N(4)–C(9)
F(000)
Dc/g cmϪ3
R
N(1)–Ru(1)–N(2)
N(1)–Ru(1)–S(1)
N(1)–Ru(1)–Cl(1)
77.4(2)
160.8(1)
99.0(1)
N(1)–Ru(1)–N(2)
N(1)–Ru(1)–S(1)
N(1)–Ru(1)–N(6)
N(2)–Ru(1)–N(6)
N(2)–Ru(1)–S(1)
N(6)–Ru(1)–S(1)
P(1)–Ru(1)–N(1)
P(1)–Ru(1)–N(2)
P(1)–Ru(1)–S(1)
P(1)–Ru(1)–N(6)
P(1)–Ru(1)–N(5)
N(5)–Ru(1)–N(1)
N(5)–Ru(1)–N(2)
N(5)–Ru(1)–S(1)
N(1)–Ru(1)–N(6)
C(5)–C(6)–N(2)
78.0(2)
161.1(2)
98.7(3)
169.5(2)
83.4(2)
98,9(2)
97.0(2)
92.0(2)
87.3(1)
98.3(2)
171.6(1)
90.4(2)
93.5(2)
87.0(2)
76.5(2)
110.8(6)
RЈ
N(2)–Ru(1)–Cl(1) 176.4(1)
N(2)–Ru(1)–S(1)
Cl(1)–Ru(1)–S(1)
P(1)–Ru(1)–N(1)
P(1)–Ru(1)–N(2)
P(1)–Ru(1)–S(1)
P(1)–Ru(1)–Cl(1)
83.3(1)
100.2(1)
91.9(1)
91.9(1)
88.7(1)
88.2(1)
P(1)–Ru(1)–P(1A) 175.2(1)
P(1A)–Ru(1)–N(1)
P(1A)–Ru(1)–N(2)
P(1A)–Ru(1)–S(1)
P(1A)–Ru–Cl(1)
C(5)–C(6)–N(2)
91.9(1)
91.9(1)
88.7(1)
88.2(1)
112.5(5)
C(38)–C(39)–N(6) 115.3(7)
diacetylpyridine monothiosemicarbazone complex 9 the oppos-
ite trend is observed.6 In 9 the Ru–N (py) distance is appreciably
shorter than the normally observed value and the Ru–N (py)
distances increase in the order 9 < 3 < 8. The Ru–N(2) (imine)
distances are slightly shorter than the Ru–N(1) (py) distances,
but they follow the same order, e.g. 9 < 3 < 8. The Ru–S(1) dis-
tances are normal, but they follow an order opposite to that of
the Ru–N(1) distances, e.g. 9 > 3 > 8. The C(8)–S(1) distances
in all the three compounds are similar (1.69–1.71 Å) and close
Fig. 1 Perspective view of the [Ru(HL)(PPh3)2Cl]ϩ cation of [Ru-
(HL)(PPh3)Cl]ClؒCH2Cl2 with atom labelling.
to the C᎐S distances observed in the free thiosemicarbazides
᎐
and thiosemicarbazones.24,25 Again, though the imine C(6)–
N(2) distances are close to their expected values, both the C(8)–
N(3) bond distances in the thiosemicarbazone moiety are
appreciably shorter than the C–N single bond distance. The
C(8)–N(4) distance in 8 is shorter than the C(8)–N(3) distance,
but in complex 9 the opposite is true. Similarly the N(2)–N(3)
distances in all the complexes are appreciably shorter than that
reported for free thiosemicarbazide or for hydrazine.23,24 It has
been suggested that in thiosemicarbazides and thiosemicarb-
azones there is an extensive π delocalisation over the entire
chain, so that none of the bonds can be considered a true single
or double bond. Rheingold and co-workers9 proposed that,
even in deprotonated thiosemicarbazones, the iminothiolate
sulfur S(1) undergoes rehybridisation to sp2 and the lone pair
on the p orbital can participate in conjugation with the
imine moiety. Such extensive π delocalisation within the ligand
moiety coupled with the π backbonding from the metal is
responsible for the apparent anomalies in bond distances
mentioned above.
Fig. 2 Perspective view of the [Ru(HL)(PPh3)(mpi)]2ϩ cation of [Ru-
(HL)(PPh3)(mpi)]Cl2ؒCH2Cl2ؒ3H2O with atom labelling.
phosphine groups in 3 have identical Ru–P bond lengths (2.399
Å). These bonds are longer than reported (2.370, 2.373 Å) for
trans-[Ru(LЈ)(PPh3)2]ClO4 but similar to that observed in
[Ru(CO)(C2HN2S3)2(PPh3)2]22 (2.397 Å, 2.399 Å). However, the
Ru–P bond lengths in both 3 and 9 are longer than that
observed in 8 (2.334 Å), the latter being on the shorter side of
the range normally observed for Ru–P bonds.23 Again, in the
2-acetylpyridine Schiff base complexes 3 and 8, the Ru–N (py)
distances are larger than the Ru–N (imine) distances, but in 2,6-
Electrochemistry
The electrochemical data for the complexes are given in Table
3. The electrochemistry of the complexes is dominated by a
reversible RuII–RuIII oxidation. Peak potential separations
between anodic and cathodic peaks, Epa Ϫ Epc, vary between 60
and 90 mV and are virtually independent of scan rate. These
peak separations, though larger than the ideal Nernstian value
of 59 mV, are commonly observed for this type of com-
138
J. Chem. Soc., Dalton Trans., 1999, 135–140

View Article Online
Table 3 Cyclic voltammetric dataa of the complexes in acetonitrile at
298 K
₁
E /V (∆Ep/mV)
₂
Compound
Oxidation
Reduction Donor sitesb
1 [Ru(HL)2][ClO4]2
0.005(90)
0.36(75)
0.65(60)
0.70(60)
0.65(60)
0.77(80)
0.47(60)
N4S2
2 [Ru(L)(PPh3)2Cl]
N2P2SЈCl
N2P2SCl
N2P2SCl
N4PSЈ
N2P3SЈ
N3PSO
N4PS
3 [Ru(HL)(PPh3)2Cl]Cl
4 [Ru(HL)(PPh3)2Cl]PF6
5 [Ru(L)(PPh3)(bpy)]PF6
6 [Ru(L)(PPh3)(dppe)]PF6
7 [Ru(HL)(PPh3)(pic)]PF6
Ϫ1.42(100)
Ϫ1.56(80)
8 [Ru(HL)(PPh3)(mpi)]Cl2 0.67(60)
a Conditions: supporting electrolyte, NEt4ClO4 (0.1 M); working elec-
trode, platinum; reference electrode, Ag–AgCl; solute concentration,
10Ϫ3 M. E is calculated as the average of anodic (Epa) and cathodic (Epc)
₁
₂
peak potentials; ∆Ep = Epa Ϫ Epc, Ipc/Ipa = 1, and scan rate = 50 mV sϪ1
b S refers to thiocarbonyl sulfur and SЈ to thiolato sulfur.
.
Fig. 3 Plot of EMLCT versus RuIII–RuII potential (Eobs, on NHE scale).
plexes.6,26,27 In most of the cases no well defined peaks are
observed at the cathodic side of the cyclic voltammograms. This
is probably due to the reduction of the ligand followed by
decomposition of the resultant complex. For complexes 5 and 8
a reductive couple observed around Ϫ1.5 V may be ascribed to
a ligand (bipyridine/mpi) centered reduction.28 It may be noted
that in this study we have extensively varied the co-ordination
environment around the ruthenium() acceptor centre employ-
ing a variety of nitrogen, sulfur, phosphorus, oxygen and
chloride donors. So, it is worthwhile to follow the trend in the
variation of RuIII–RuII potential with the change of donor
environment around the metal ion, particularly because such
studies are rather scanty.29 It is well established that such poten-
tials are affected by both the nature of the donor sets as well as
the overall charge of the complex, the latter being the dominat-
ing factor. So, a meaningful correlation is possible only when
well with the lower basicity of the latter as described by
Rheingold and co-workers.9
Electronic spectra
The electronic spectra of low-spin d6 complexes are generally
dominated by metal to ligand charge transfer in the visible
region.30–32 As most of the complexes discussed in this work are
of Cs or lower symmetry all the d orbitals are non-degenerate.
So, a number of MLCT transitions are expected. However,
due to the small energy separation between some of these d
orbitals, as well as poor overlap between them and the excited
state orbitals, some of the expected MLCT transitions may
not be resolved. In general, all the complexes exhibit two
well resolved MLCT transitions around 420–480 (band I) and
373–378 nm (band II). When the energy of band I is plotted
against EЊRu(III)/Ru(II) a nice linear correlation EMLCT = 1.18
EЊRu(III)/Ru(II) ϩ 2.42 is obtained (Fig. 3). Besides, for some of the
complexes, there is an additional low energy band at 480–580
nm (band III). While bands I and III are substituent dependent,
II is unaffected by substituents. The two highest energy bands at
260–290 and 210–220 nm are probably due to intraligand
transitions.
one compares complexes having identical charges. Thus, we
2ϩ
may compare the ERu(III)/Ru(II) of Ru(bpy)3
(1.38 V, N6
donors) with that of [Ru(HL)2]2ϩ (Ϫ0.005 V, N4S2 donors) and
conclude that replacement of two pyridine nitrogens by two
thiocarbonyl sulfurs has stabilised the RuIII by 1.43 V. This
may be rationalised by referring to the higher polarisability
and poorer π-accepting capability of the thiocarbonyl sulfur
compared to pyridyl nitrogen, and both the factors tend to
stabilise the ruthenium() state. Again, we can compare the
RuIII–RuII potential of the complex [Ru(L)(PPh3)2Cl] (0.36 V,
N2P2SCl donors) with that of [Ru(bpy)2Cl2] (0.34 V, N4Cl2
donors); in this case the replacement of two pyridine nitrogens
and a chloride by two phosphorus and a thiolato donor
set keeps the potential almost unaltered. Though thiolato
ligands are known to be efficient in stabilising higher ( and
) oxidation states of ruthenium, in the present case that effect
is compensated by the introduction of two phosphine donors,
which are even more efficient in stabilising ruthenium()
than the pyridine nitrogens. Similarly one can compare the
series of five monocationionic complexes [Ru(HL)(PPh3)2Cl]-
Cl (0.65 V, N2P2SCl donors), [Ru(HL)(PPh3)2Cl]PF6 (0.70 V,
N2P2SCl donors), [Ru(L)(PPh3)(bpy)]PF6 (0.65 V, N4PSЈ
donors), [Ru(L)(PPh3)(dppe)]PF6 (0.77 V, N2P3SЈ donors)
and [Ru(HL)(PPh3)(pic)]PF6 (0.47 V, N3PSO donors) and
conclude that the presence of bipyridine nitrogen or imine
nitrogen as well as phosphine donors tends to stabilise the
ruthenium() state, whereas thiolato and carboxylato donors
stabilise the ruthenium() state. One may also compare
1H NMR spectra
1
The H NMR spectrum of the ligand (HL) exhibits signals at
δ 2.49 (3 H) and 2.35 (3 H) which are assigned to the CH3 group
of the p-tolyl moiety and that attached to the imine moiety of
the ligand. The signals at δ 9.32 (1 H) and 8.9 (1 H) are due to
NH protons and all aromatic protons exhibit signals in the
region δ 7.19–8.61.33 For complex 3 the signal of the CH3 group
attached to the imine moiety was shifted upfield to δ 2.16,
whereas the CH3 proton signal of the p-tolyl moiety remains
unaffected at δ 2.31. The NH proton signals are at δ 12.16 and
11.12. The aromatic protons are observed between δ 6.8 and
8.76. Compound 8 exhibits three CH3 proton signals at δ 2.31 (3
H), 2.35 (3 H) and 2.46 (3 H). The signal at δ 2.35 is due to the
CH3 group of the p-tolyl residue. One NH signal is observed at
δ 12.25. The broken organic fragment isolated from the reaction
mixture [N-(p-tolyl)thiourea] exhibits a signal at δ 3.37 due to
the CH3 group33 of the tolyl part. The phenyl protons are
observed between δ 7.3 and 8.62. The NH proton signals are
not observed and similar observations were reported34 previ-
ously in the case of N-(p-nitrophenyl)thiourea.
the ERu(III)/Ru(II) values of [Ru(bpy)2(SPh)2] (Ϫ0.28 V, N4SϪ
2
donors)29 and [Ru(bpy)2(pybt)]ϩ [0.32 V, N5SϪ donors;
pybt = 2-(2-pyridyl)benzenethiolate]28 with those of thiolato
complexes reported in this paper and conclude that the ben-
zenethiolato group is more efficient in stabilising RuIII than the
iminethiolates described in this paper, a fact which correlates
Conclusion
This paper describes the ruthenium() complexes of methyl
2-pyridyl ketone 4-(4-tolyl)thiosemicarbazone, in which the
J. Chem. Soc., Dalton Trans., 1999, 135–140
139

View Article Online
1988, 1119; S. N. Anderson, M. E. Fakley, R. L. Richards and
J. Chatt, J. Chem. Soc., Dalton Trans., 1981, 1973.
12 P. L. Andreu, J. A. Cabeja, I. Rio, V. Riera and C. Boiss,
Organometallics, 1996, 15, 3004.
13 P. S. Hallman, T. A. Stephenson and G. Wilkinson, Inorg. Synth.,
1970, 12, 237.
14 M. Sato, M. Yamamoto, K. Imada, N. Tanaka and T. Higashi,
J. Appl. Crystallogr., 1992, 25, 146; K. L. Kraus and G. N. Philips,
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15 ABSCOR, An empirical absorption correction based on Fourier
coefficient fitting, T. Higashi Rigaku Corporation, Tokyo, 1995.
16 G. M. Sheldrick, Computational Crystallography, ed. D. Sayre,
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17 D. X. West and D. S. Galloway, Transition Met. Chem., 1988, 13,
410.
ligand behaves either as a monoanionic tridentate NNS donor
(thioenol form) or as a neutral tridentate NNS donor (thione
form). The pH-dependent interconversion of the compounds
[Ru(L)(PPh3)2Cl] and [Ru(HL)(PPh3)2Cl]Cl is a manifestation
of thione–thioenol tautomerisation of the co-ordinated ligand.
Formation of the compound [Ru(HL)(PPh3)(mpi)]Cl2 from the
complex [Ru(L)(PPh3)2Cl] is an extremely interesting mani-
festation of the unusual reactivity of the co-ordinated thio-
semicarbazone moiety. The complex [Ru(HL)(PPh3)(mpi)]Cl2,
produced through reductive cleavage of the hydrazinic N–N
bond of the thiosemicarbazone ligand, is the first structurally
characterised metal complex of an imine of a heterocyclic
ketone. The crystal structure of the compound [Ru(HL)-
(PPh3)2Cl]ClؒCH2Cl2 has been of great help in understanding
the rather unusual chemical reaction leading to the formation
of [Ru(HL)(PPh3)(mpi)]Cl2.
18 D. X. West and D. S. Galloway, Transition Met. Chem., 1988, 13,
415.
19 D. W. West, R. D. Profilit and J. L. Hines, Transition Met. Chem.,
1988, 13, 467.
20 F. Basolo and R. G. Pearson, Mechanism of inorganic reaction.
A study of metal complexes in solution, Wiley, New York, 1958,
pp. 108, 163.
Acknowledgements
21 J. E. Huheey, E. A. Keiter and R. L. Keiter, Inorganic Chemistry.
Principles of Structure and Reactivity, 4th edn., Harper Collins
College Publishers, 1993, p. 551.
M. M. gratefully acknowledges the Council of Scientific and
Industrial Research (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. T. C. W. M. acknow-
ledges support from the Hong Kong Research Grants Council.
22 P. Mura, B. G. Olby and S. D. Robinson, Inorg. Chem., 1985, 108,
45.
23 S. R. Fletcher and A. C. Skapski, J. Chem. Soc., Dalton Trans., 1972,
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24 A. K. Nandi, S. Chaudhuri, S. K. Chaudhuri and S. Ghosh, Acta
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25 A. K. Nandi, S. Chaudhuri, S. K. Chaudhuri and S. Ghosh,
J. Chem. Soc., Perkin Trans. 2, 1984, 1729.
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