Synthesis, spectroscopic and redox properties of some ruthenium(II) thiosemicarbazone complexes: Structural description of four of these complexes

Article abstract of DOI:10.1016/j.ica.2005.11.001

Sixteen neutral mixed ligand thiosemicarbazone complexes of ruthenium having general formula [Ru(PPh₃)₂L₂], where LH = 1-(arylidine)4-aryl thiosemicarbazones, have been synthesized and characterized. All complexes are diamagnetic and hence ruthenium is in the +2 oxidation state (low-spin d⁶, S = 0). The complexes show several intense peaks in the visible region due to allowed metal to ligand charge transfer transitions. The structures of four of the complexes have been determined by single-crystal X-ray diffraction and they show that thiosemicarbazone ligands coordinate to the ruthenium center through the hydrazinic nitrogen and sulfur forming four-membered chelate rings with ruthenium in N₂S₂P₂ coordination environment. In dichloromethane solution, the complexes show two quasi-reversible oxidative responses corresponding to loss of electron from HOMO and HOMO - 1. The E ⁰ values of the above two oxidations shows good linear relationship with Hammett substituents constant (σ) as well as with the HOMO energy of the molecules calculated by the EHMO method. A DFT calculation on one representative complex suggests that there is appreciable contribution of the sulfur p-orbitals to the HOMO and HOMO - 1. Thus, assignment of the oxidation state of the metal in such complexes must be made with caution.

Full text of DOI:10.1016/j.ica.2005.11.001

Inorganica Chimica Acta 359 (2006) 585–592
www.elsevier.com/locate/ica
Synthesis, spectroscopic and redox properties of some
ruthenium(II) thiosemicarbazone complexes: Structural description
of four of these complexes
a
a
b
Dipankar Mishra , Subhendu Naskar , Michael G.B. Drew ,
a,
*
Shyamal Kumar Chattopadhyay
a
Department of Chemistry, Bengal Engineering and Science University, Shibpur College Road, P.O. Botanic Garden, Howrah 711 103, West Bengal, India
b
School of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK
Received 7 July 2005; received in revised form 24 October 2005; accepted 3 November 2005
Available online 13 December 2005
Abstract
Sixteen neutral mixed ligand thiosemicarbazone complexes of ruthenium having general formula [Ru(PPh3)2L2], where LH = 1-(aryli-
dine)4-aryl thiosemicarbazones, have been synthesized and characterized. All complexes are diamagnetic and hence ruthenium is in the
+2 oxidation state (low-spin d6, S = 0). The complexes show several intense peaks in the visible region due to allowed metal to ligand
charge transfer transitions. The structures of four of the complexes have been determined by single-crystal X-ray diffraction and they
show that thiosemicarbazone ligands coordinate to the ruthenium center through the hydrazinic nitrogen and sulfur forming four-
membered chelate rings with ruthenium in N2S2P2 coordination environment. In dichloromethane solution, the complexes show two
quasi-reversible oxidative responses corresponding to loss of electron from HOMO and HOMO ꢀ 1. The E0 values of the above two
oxidations shows good linear relationship with Hammett substituents constant (r) as well as with the HOMO energy of the molecules
calculated by the EHMO method. A DFT calculation on one representative complex suggests that there is appreciable contribution of
the sulfur p-orbitals to the HOMO and HOMO ꢀ 1. Thus, assignment of the oxidation state of the metal in such complexes must be
made with caution.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Ruthenium(II) thiosemicarbazone complexes; Synthesis; X-ray structures; Cyclic voltammetry; Oxidation; EHMO calculations; DFT calcu-
lations; Hammett correlation
1. Introduction
The chemistry of complexes of ruthenium with thiosemic-
arbazones, which can coordinate to the metal either in neu-
tral thione form or in the anionic thiolate form, has
received attention in recent years primarily due to their var-
ied coordination mode, novel electrochemical and elec-
tronic properties [13–19], as well as their biological
importance [13,14,20–23].
In this study, we report the synthesis, characterization,
spectroscopic and redox behaviors of 16 ruthenium (II)
complexes of 1-(arylidine) 4-(aryl) thiosemicarbazones
obtained by condensation of 4-(aryl) thiosemicarbazides
with para substituted benzaldehyde. X-ray crystal struc-
tures of four such complexes are also reported here.
Thiosemicarbazones are an important class of N, S
donor ligand which have considerable pharmacological
interest due to their significant antibacterial, antiviral, anti-
malarial, antileprotic and anticancer activities [1–6]. Sev-
eral metal complexes of thiosemicarbazones particularly
with copper, platinum, palladium, rhenium and ruthenium
also show marked and diverse biological activity [7–12].
*
Corresponding author. Fax: +91 33 2668 2916.
E-mail address: shch20@hotmail.com (S.K. Chattopadhyay).
0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.11.001

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D. Mishra et al. / Inorganica Chimica Acta 359 (2006) 585–592
2. Experimental
hyde (L14H) (14), p-chlorobenzaldehyde (L15H) (15),
p-N,N-dimethyl aminobenzaldehyde (L16H) (16) were
prepared using the corresponding amines by the published
procedure [13,14].
Ru(PPh3)3Cl2 was prepared by the published procedure
[24]. Analytical, magnetic, spectroscopic and electrochemi-
cal data were acquired as described in our earlier papers
[25,26].
2.2. Synthesis of the complexes
2.1. Synthesis of the ligands
All the complexes (1–16) of general formulae
[Ru(PPh3)2(L)2] (where L represents the deprotonated
ligand) were synthesized following a common procedure
with almost similar yields. Therefore, synthetic details of
just one representative example is given below:
The ligands (Scheme 1), 4-(phenyl)thiosemicarbazones
of benzaldehyde (L1H) (1), anisic aldehyde (L2H) (2), p-
chlorobenzaldehyde (L3H) (3), p-N,N-dimethyl aminobenz-
aldehyde (L4H) (4), 4-(p-methylphenyl)thiosemicarbazones
of (5) benzaldehyde (L5H) (5), anisic aldehyde (L6H) (6),
p-chlorobenzaldehyde (L7H) (7), p-N,N-dimethyl amino-
benzaldehyde (L8H) (8), 4-(p-chlorophenyl)thiosemicarbaz-
ones of (9) benzaldehyde (L9H) (9), anisic aldehyde (L10H)
(10), p-chlorobenzaldehyde (L11H) (11), p-N,N-dimethyl
aminobenzaldehyde (L12H) (12), 4-(p-fluorophenyl)thiose-
micarbazones of (13) benzaldehyde (L13H) (13), anisic alde-
To a 30 ml methanolic solution of the L2H (142.5 mg,
0.5 mmol), Et3N (50 mg, 0.5 mol) was added, followed by
the addition of solid [Ru(PPh3)3Cl2] (240 mg, 0.25 mmol).
After 3 h of reflux the shiny yellow precipitate that sepa-
rated out during reflux was collected by filtration and
washed thoroughly with cold methanol. Recrystallisation
of the product from 1:1 dichloromethane–methanol solu-
tion leads to golden yellow crystals. Yield: 75–80%.
S
S
R1
R1
C3
N4
C3
H
N4
-H+
H+
N2
N1
N2
H
H
N1
H
H
C
C
R2
R2
(LH)
(L)
Scheme 1.

D. Mishra et al. / Inorganica Chimica Acta 359 (2006) 585–592
587
Anal. Calc. for C64H54N6P2S2Ru (1): C, 67.78; H, 4.76; N,
7.41. Found: C, 67.81; H, 4.80; N, 7.42%. Anal. Calc. for
C67H61ClN6O2.5P2S2 Ru (2): C, 64.24; H, 4.87; N, 6.71.
Found: C, 64.29; H, 4.90; N, 6.73%. Anal. Calc. for C60-
H52ClN6P2S2Ru (3): C, 59.90; H, 4.32; N, 6.98. Found: C,
59.98; H, 4.36; N, 6.94%. Anal. Calc. for C68H64N8P2S2Ru
(4): C, 66.94; H, 5.25; N, 9.18. Found: C, 67.02; H, 5.29; N,
9.13%. Anal. Calc. for C66H59N6O0.5P2S2Ru (5): C, 67.69;
H, 5.04; N, 7.17. Found: C, 67.72; H, 5.08; N, 7.20%. Anal.
Calc. for C68.5H62N6O3P2S2Ru (6): C, 65.97; H, 5.21; N,
6.74. Found: C, 66.01; H, 5.24; N, 6.76%. Anal. Calc. for
C66H56Cl2N6P2S2Ru (7): C, 64.39; H, 4.55; N, 6.82. Found:
C, 64.45; H, 4.60; N, 6.77%. Anal. Calc. for C70H68N8P2-
S2Ru (8): C, 67.39; H, 5.45; N, 8.98. Found: C, 67.42; H,
5.47; N, 8.92%. Anal. Calc. for C64H52Cl2N6P2S2Ru (9): C,
63.89; H, 4.32; N, 6.98. Found: C, 63.97; H, 4.35; N,
6.92%. Anal. Calc. for C67H57Cl3N6O2P2S2Ru (10): C,
61.36; H, 4.36; N, 6.43. Found: C, 61.69; H, 4.37; N,
6.45%. Anal. Calc. for C64H50Cl4N6P2S2Ru (11): C, 60.42;
H, 3.93; N, 6.60. Found: C, 60.49; H, 3.97; N, 6.54%. Anal.
Calc. for C68H62Cl2N8P2S2Ru (12): C, 63.35; H, 4.81; N,
8.69. Found: C, 63.42; H, 4.85; N, 8.58%. Anal. Calc. for
C64H52F2N6P2S2 Ru (13): C, 65.69; H, 4.44; N, 7.18. Found:
C, 65.76; H, 4.47; N, 7.09%. Anal. Calc. for C66H56F2N6O2-
P2S2Ru (14): C, 64.44; H, 4.55; N, 6.83. Found: C, 64.52; H,
4.59; N, 6.78%. Anal. Calc. for C64H50Cl2F2N6P2S2Ru (15):
C, 62.03; H, 4.03; N, 6.78. Found: C, 62.11; H, 4.09; N,
6.69%. Anal. Calc. for C68H62F2N8P2S2Ru (16): C, 65.01;
H, 4.94; N, 8.92. Found: C, 65.12; H, 4.97; N, 8.87%.
were solved using direct methods with the SHELX-86 pro-
gram [33]. Non-hydrogen atoms were refined with aniso-
tropic thermal parameters. The hydrogen atoms bonded
to carbon were included in geometric positions and given
thermal parameters equivalent to 1.2 times those of the
atom to which they were attached. Empirical absorption
corrections were applied using DIFABS [34]. The struc-
tures were refined on F2 using SHELXL [35]. CCDC numbers
of the structures are 265804, 265805, 265807 and 265806
for complexes 2, 5, 6 and 10, respectively.
2.5. Description of the crystal structures
Structures 2, 6 and 10 are isomorphous, while 5 is not
isomorphous. The molecular structures (Figs. 1–4) are very
similar in that the metal atom occupies a six-coordinate
octahedral environment with the metal bonded to two tri-
phenylphosphine ligands and two bidentate deprotonated
thiosemicarbazone ligands (L) forming four membered
chelate rings through thiolato sulfur and nitrogen (N2).
The two triphenylphosphine ligands are mutually cis with
angles significantly greater than 90°, ranging from
96.6(10) to 98.1(1)°, no doubt because of the bulky nature
of the ligand. The Ru–P bond lengths range from 2.292(6)–
˚
2.323(4) A. The bidentate ligands show small bite angles,
thus the N–Ru–S angles range from 65.1(2) to 66.0(2)°.
In the structures, the sulfur atoms are mutually trans, but
the nitrogen atoms are cis to each other. The Ru–S dis-
˚
tances range from 2.428(3) to 2.452(3) A, and the Ru–N
˚
distances from 2.132(11)–2.183(10) A. All the observations
2.3. Computational chemistry
are comparable to the values reported earlier [15–19,26]. It
may be noted that among the four structures reported here,
EHMO calculations were carried out on the MM+ opti-
mized structures of the complexes using the Window based
Hyperchem program [27]. For both MM+ and EHMO cal-
culations, the default parameters of the program were used.
A DFT calculation on one of the complexes (1) was per-
formed using the ADF program [28]. Geometry optimiza-
tion for the DFT calculation was carried out using the
default criteria for convergence in the ADF program. For
all elements the ZORA approximation was used together
with the default TZP basis sets using a small core. In addi-
tion Vosko, Wilk and NusairÕs local exchange correlation
potential was used [29] together with BeckeÕs non-local
exchange [30] and PerdewÕs correlation corrections [31].
2.4. X-ray crystallography
Single crystals for 2 and 6 were grown from slow evap-
oration of dichloromethane–methanol solution and those
of 5 and 10 were obtained from dichloromethane–acetoni-
trile solution of the complexes. Data were measured with
Mo Ka radiation using the MAR research Image Plate Sys-
tem at 293 K. The crystals were positioned at 70 mm from
the Image Plate. One hundred frames were measured at 2°
intervals with a counting time of 2 min. Data analysis was
carried out with the XDS program [32]. The structures
Fig. 1. X-ray crystal structure of 2.

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D. Mishra et al. / Inorganica Chimica Acta 359 (2006) 585–592
Fig. 2. X-ray crystal structure of 5.
Fig. 3. X-ray crystal structure of 6.
Fig. 4. X-ray crystal structure of 10.
while the Ru–P and Ru–S bond distances are not so sensi-
tive to the variation at R1 (H, Me, Me and Cl for 2, 5, 6,
10, respectively) and R2 (OMe for 2, 6 and 10, H for 5),
the Ru–N bond is found to be appreciably shorter in 5
respectively). It may be noted that due to the formation
of the four-membered chelate rings the carbon atom of
the thiosemicarbazone moiety (C72 and C82) is at appre-
ciably short distance from the ruthenium (Ru–C72/C82
distances lie within a range of 2.71–2.82 A, which may be
compared with sum of the covalent radii of Ru and C
˚
˚
(average = 2.137 A, R2 = H) compared to the other three
˚
structures (average values are 2.179, 2.176, 2.164 A for 2,
˚
˚
6 and 10, respectively; R2 = OMe). Between 2, 6 and 10
the average Ru–N, Ru–P distances are all shorter in 10
(R1 = Cl, average Ru–N, Ru–P distances are 2.164,
(2.33 A) and sum of their van der WaalÕs radii (4.00 A)).
˚
In fact the Ru–C72 distance (2.71(2) A) in compound 5 is
so short that it may be considered as pseudo-seven coordi-
nated species. Moreover, there are also several Ru ꢁ ꢁ ꢁ H
short contacts (see supplementary table ST1), particularly
involving the H atoms attached to aldimine moieties
˚
2.305 A, respectively) compared to 2 (R1 = H, Ru–N,
˚
Ru–P distances are 2.179, 2.316 A, respectively) and 6
˚
(R1 = Me, Ru–N, Ru–P distances are 2.176, 2.310 A,

D. Mishra et al. / Inorganica Chimica Acta 359 (2006) 585–592
589
(H75 and H85). These Ru ꢁ ꢁ ꢁ H–C hydrogen bonds along
Table 2
Bond distances (A) and bond angles (°) for complexes 2, 5, 6 and 10
˚
with some additional Ru ꢁ ꢁ ꢁ H interactions involving phe-
nyl protons of the PPh3 ligand and the N76–H76 ꢁ ꢁ ꢁ N74
and N86–H86 ꢁ ꢁ ꢁ N84 hydrogen bonds probably contrib-
ute towards stabilizing the four membered chelate rings
in these complexes. The details list of hydrogen bonds for
the compounds 2, 5, 6 and 10 are given in supplementary
tables ST2, ST3, ST4 and ST5, respectively.
2
5
6
10
Ru(1)–N(83)
Ru(1)–N(73)
Ru(1)–P(1)
Ru(1)–P(2)
Ru(1)–S(81)
Ru(1)–S(71)
N(83)–Ru(1)–N(73)
N(83)–Ru(1)–P(1)
N(83)–Ru(1)–P(2)
N(83)–Ru(1)–S(81)
N(83)–Ru(1)–S(71)
N(73)–Ru(1)–P(1)
N(73)–Ru(1)–P(2)
2.173(9)
2.183(10)
2.309(4)
2.323(4)
2.452(3)
2.443(4)
82.7(5)
164.1(2)
90.4(2)
65.5(2)
99.7(3)
90.4(2)
167.5(2)
2.132(11)
2.143(14)
2.315(5)
2.292(6)
2.432(4)
2.441(5)
83.3(5)
169.8(3)
91.2(4)
65.6(3)
95.8(3)
89.5(3)
171.5(3)
96.5(3)
65.7(3)
96.7(2)
108.3(2)
87.8(2)
87.1(2)
108.5(2)
156.4(2)
2.173(6)
2.178(6)
2.308(3)
2.313(3)
2.431(3)
2.424(3)
83.7(2)
165.1(2)
90.8(2)
65.9(2)
98.8(2)
90.0(2)
168.6(2)
99.4(2)
65.9(2)
97.6(10)
102.1(1)
90.9(1)
87.3(1)
105.3(1)
160.7(1)
2.168(7)
2.165(7)
2.299(3)
2.312(3)
2.433(3)
2.428(3)
83.6(2)
165.5(2)
91.0(2)
65.9(2)
99.0(2)
90.3(2)
168.6(2)
99.8(2)
65.6(2)
97.2(1)
102.5(1)
90.4(1)
87.0(1)
105.6(1)
160.9(1)
3. Results and discussion
3.1. Synthesis and some properties
A series of ruthenium complexes using 4-(aryl) thiose-
micarbazones of aromatic aldehydes (LH) as ligands of
general formulae [Ru(PPh3)2(L)2] (1–16) have been synthe-
sized. Detailed synthetic methods have been described
above. The elemental analyses, molar conductivity data,
room temperature magnetic moment and spectroscopic
data are consistent with their respective formulae as stated
above (see Tables 1 and 2).
N(73)–Ru(1)–S(81) 100.6(3)
N(73)–Ru(1)–S(71)
P(1)–Ru(1)–P(2)
P(1)–Ru(1)–S(81)
P(1)–Ru(1)–S(71)
P(2)–Ru(1)–S(81)
P(2)–Ru(1)–S(71)
S(71)–Ru(1)–S(81)
65.1(3)
98.1(1)
102.1(1)
90.2(1)
86.6(1)
105.5(1)
161.5(1)
3.2. Electronic spectra
Electronic spectra were recorded in dichloromethane
solution and data are displayed in Table 3. Each complex
shows intense absorption in the visible region that is prob-
ably due to the allowed MLCT transitions as is generally
observed in low spin d6 ruthenium complexes [36–38]. Mul-
tiple charge transfer transitions in such mixed ligand com-
plexes may result from splitting of metal orbitals in lower
symmetry and presence of different acceptor levels in the
ligands. All other transitions in the UV region are assigned
to intra ligand transitions.
the presence of 0.1 M tetraethylammonium perchlorate
(TEAP)) solution and results are presented in Table 3.
All the complexes show two oxidative responses on the
positive side of Ag/AgCl reference electrode. One selected
voltammogram is shown in Fig. 5. This first oxidation is
quasi reversible, with a peak-to-peak separation of 76–
123 mV as evident from cyclic voltammetric data, and
the anodic peak current (ipa) is almost equal to the catho-
dic peak current (ipc). The one-electron nature of this oxi-
dation has been tentatively established by comparing its
current height with that of standard ferrocene/ferroce-
nium couple under the same experimental condition.
The complexes show a second oxidative response at the
potential range 0.77–1.25 V. The earlier authors [16,17]
have assigned these two oxidations to RuII/RuIII and
3.3. Electrochemistry
Electron transfer properties of the complexes were
examined by cyclic voltammetry in dichloromethane (in
Table 1
Crystallographic data for 2, 5, 6 and 10
Complex
2
5
6
10
Empirical formula
Formula weight
C
67H61ClN6O2.5P2RuS2
C66H59N6O0.5P2RuS2
1171.31
C68.5H65N6O3P2RuS2
1247.39
C67H57Cl3N6O2P2Ru
1304.68
1251.82
Crystal system, space group
Unit cell dimensions
monoclinic, P21/c
monoclinic, P21/n
monoclinic, P21/c
monoclinic, P21/c
˚
a (A)
12.660(14)
23.800(27)
21.765(24)
105.54(1)
6322.6
4, 1.330
0.457
10180
0/730
11.822(14)
40.108(45)
12.371(14)
92.48(1)
5860.3
4, 1.333
0.441
4094
0/330
12.555(14)
23.362(25)
22.513(25)
101.70(1)
6466.1
4, 1.280
0.406
9720
3/753
12.498(14)
23.299(25)
22.412(24)
101.43(1)
6396.7
4, 1.364
0.535
9031
3/749
˚
b (A)
˚
c (A)
b (°)
3
˚
Volume (A )
Z, calculated density (Mg mꢀ3
)
Absorption coefficient (mmꢀ1
Unique reflections collected
Restraints/parameters
)
Final R indices [1 > 2r(I)] R1, wR2
R indices (all data)
Largest difference in peak and hole (e A
0.1201, 0.2013
0.1961, 0.2746
0.869, ꢀ0.704
0.0978, 0.2781
0.1641, 0.3003
0.749, ꢀ0.820
0.0769, 0.2024
0.1554, 0.2318
1.168, ꢀ1.124
0.0796, 0.1987
0.1841, 0.2313
0.903, ꢀ0.913
ꢀ3
˚
)

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D. Mishra et al. / Inorganica Chimica Acta 359 (2006) 585–592
Table 3
Electronic spectral and cyclic voltammetric data
Complexes
Electronic spectral datac in CH2Cl2, kmax/nm (e/l molꢀ1 cmꢀ1
)
E0/V (DEp/mV) in CH2Cl2
y
y
E10
E20
E30
1
2
431a(6178), 340 (20758), 265a(43247), 227 (78586)
421a(8334), 335 (22782), 243 (33756), 228 (48620)
420a (20397), 344 (33146), 285a (53543), 228 (101987)
379 (13497), 277a(9508),262a(10439), 228 (20280)
410a (15085), 344 (22930) 271a (41033), 225 (70902)
400a (21060), 348 (28258) 262 (53317), 227 (79975)
412a (13972), 352 (18663), 275a (32735), 240 (55091)
418a (15757), 365 (19069), 273a (18667), 235 (40146)
412a (15018), 337 (26314), 287a (49674), 227 (84094)
406a (15339), 341 (26295), 264 (45535), 227 (66725)
418 (16581), 341 (28634), 287a(44641), 227 (84687)
378 (87446), 270a (57212), 263a (61612), 227 (126713)
408a (15339), 337 (23711), 269a (45416), 227 (82932)
406a (19380), 337 (30847), 262 (57333), 227 (87498)
405 (17047), 343 (26295), 272a (46705), 227 (83361)
376 (55411), 264a (43797), 227 (98543)
0.51 (107)
0.47 (110)
0.53(106)
0.34 (89)
0.47 (88)
0.43(110)
0.50 (91)
0.33 (96)
0.52 (95)
0.47(85)
0.55 (123)
0.40 (76)
0.51 (94)
0.46 (95)
0.54 (101)
0.38(76)
1.25 (238)
1.05 (236)
1.20 (76)
3
4
0.77 (165)
1.15 (120)
1.10 (144)
1.15 (120)
0.77 (190)
1.17 (146)
1.07 (106)
1.17 (146)
0.78 (128)
1.22 (135)
1.08 (104)
1.12 (214)
0.77 (156)
1.14 (107)
1.12 (110)
1.17 (82)
1.14 (113)
5
6
7
8
9
10
11
12
13
14
15
16
yE10 and E02 are tentatively RuIII/RuII and RuIV/RuIII couples. However, due to appreciable admixture of sulfur orbitals with HOMO and HOMO ꢀ 1 the
oxidation state of the metal cannot be defined unequivocally.
a
Shoulder.
such molecules must be made with caution as the electron
is lost from a molecular orbital having appreciable ligand
contribution rather than from a pure metal centered
orbital.
In addition, complexes 4, 8, 12 and 16 also show a third
quasi-reversible oxidative response at the potential range
1.12–1.17 V, which is tentatively assigned as ligand cen-
tered oxidation.
For all the complexes, the E10 and E02 values are found
to be sensitive to the substituents (R1 and R2) on the aro-
Fig. 5. Cyclic voltammogram of 4 in CH2Cl2.
Table 4
Results of DFT calculation on 1
Energy (a.u.)
% Atomic orbital coefficients
LUMO
HOMO
HOMO ꢀ 1
HOMO ꢀ 2
HOMO ꢀ 3
ꢀ0.095649
ꢀ0.159906
ꢀ0.166758
ꢀ0.170253
ꢀ0.175612
No metal contribution
31.6 Ru dxy, 14.4 S py, 5.1 S pz
50.6 Ru dxz, 12.7 S px, 5.0 S pz
20.1 Ru d2z , 17.7 dxy, 9.8 S py
No metal contribution
RuIII/RuIV, respectively. However, DFT calculation car-
ried out on a representative molecule (1) shows (Table
4), that the HOMO has ꢂ30% metal character and
ꢂ20% contributions from the sulfur orbitals. Thus,
assignment of the oxidation state of the metal ion for
Fig. 6. Plot of E10 ðVÞ vs. Hammett substituents constant (r) of R2 for a
fixed R1.

D. Mishra et al. / Inorganica Chimica Acta 359 (2006) 585–592
591
Fig. 9. Plot E10 ðVÞ versus energy of the HOMO of the metal complexes.
Fig. 7. Plot of E20 ðVÞ vs. Hammett substituents constant (r) of R2 for a
fixed R1.
regression coefficient is much less satisfactory than that
for E10.
4. Conclusion
Study of 16 Ru(II) complexes of 1-(arylidine) 4-(aryl)
thiosemicarbazones reported in this paper reveals that the
structural parameters (e.g., Ru–N and Ru–P bond dis-
tances) and the oxidation potentials of these complexes
can be fine tuned by changing the substitution in the 4-aryl
(R1) as well as arylidine (R2) part. As expected the sensitiv-
ity towards variation at R2 is more than that at R1. A DFT
calculation shows that the HOMO along with HOMO ꢀ 1
and HOMO ꢀ 2 orbitals has substantial contribution from
the sulfur orbitals. Thus, assignment of the oxidation state
of the metal ion in the oxidized form of these complexes
must be made with caution unless supplemented with the-
oretical calculations or detailed spectroscopic characteriza-
tion of the oxidized species.
Fig. 8. Plot of E10 ðVÞ vs. Hammett substituents constant (r) of R1 for a
Acknowledgments
fixed R2.
D.M. and S.N. acknowledge CSIR and UGC, respec-
tively, for their fellowships. S.K.C. acknowledges AICTE
and UGC, New Delhi, for financial support. We thank
EPSRC (UK) and the University of Reading for funds
for the Image Plate System. We also acknowledge the use
of UV–Vis spectrophotometer purchased from a DST (In-
dia)-FIST grant.
matic rings of the ligands. Electron-donating substituents
decreases, the redox potential and electron-withdrawing
substituents increase the potential as expected. This is
clearly revealed in the plots (Figs. 6–8) of E0 values versus
Hammett substituents constant (r) of R1 and R2. Plots of
the E10 versus the energy of the HOMO of the molecules
are also found to be approximately linear (Fig. 9), indicat-
ing that by suitable substitution in the R1 and R2 posi-
tions one can fine tune the HOMO energy over a range
of 0.6 eV and the redox potential over a range of 0.2 V.
The corresponding plot for E20 is also linear, though the
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.ica.2005.11.001.

592
D. Mishra et al. / Inorganica Chimica Acta 359 (2006) 585–592
[21] F. Bregant, S. Pacor, S. Ghosh, S.K. Chattopadhyay, G. Sava,
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