Polyhedron 20 (2001) 327 – 335
Chemistry of ruthenium with some dioxime ligands.
Syntheses, structures and reactivities
Anjan Kumar Das
a, Shie-Ming Peng
b, Samaresh Bhattacharya
a,*
aDepartment of Chemistry, Inorganic Chemistry Section, Jada6pur Uni6ersity, Calcutta
700 032, India
bDepartment of Chemistry, National Taiwan Uni6ersity, Taipei, Taiwan, ROC
Received 24 August 2000; accepted 8 November 2000
Abstract
Reaction of two dioxime ligands, viz. dimethylglyoxime (H2dmg) and diphenylglyoxime (H2dpg), (abbreviated in general as
H2L, where H stands for the oxime protons) with [Ru(PPh3)3Cl2] in 1:1 mole ratio affords complexes of type [Ru(PPh3)2(H2L)Cl2].
Structure of the [Ru(PPh3)2(H2dpg)Cl2] complex has been solved by X-ray crystallography. The coordination sphere around
ruthenium is N2P2Cl2with the two PPh3ligands in trans and the two chlorides in cis positions. Reaction of the dioxime ligands
with [Ru(PPh3)3Cl2] in 2:1 mole ratio in the presence of a base affords complexes of type [Ru(PPh3)2(HL)2]. Structure of the
[Ru(PPh3)2(Hdmg)2] complex has been solved by X-ray crystallography. The coordination sphere around ruthenium is N4P2with
the two PPh3ligands in trans positions. Reaction of the [Ru(PPh3)2(H2dpg)Cl2] complex with a group of bidentate acidic ligands,
viz. picolinic acid (Hpic), quinolin-8-ol (Hq) and 1-nitroso-2-naphthol (Hnn), (abbreviated in general as HL%, where H stands for the acidic proton) in the presence of a base affords complexes of type [Ru(PPh3)2(H2dpg)(L%)]+isolated as perchlorate salts. All
the complexes are diamagnetic (low-spin d6, S = 0) and in dichloromethane solution show several intense MLCT transitions in the
visible region. Cyclic voltammetry on all the complexes shows a reversible ruthenium(II) – ruthenium(III) oxidation within 0.36 – 0.98 V versus SCE followed by a quasi-reversible ruthenium(III) – ruthenium(IV) oxidation within 0.94 – 1.60 V versus SCE. © 2001 Elsevier Science B.V. All rights reserved.
Keywords:Ruthenium; Dioxime ligands; Syntheses; Structures; Reactivities
1. Introduction
The ruthenium chemistry of diimine ligands (1) is an area of significant current interest [1 – 7], particularly with regard to the photophysical and photo-chemical properties exhibited by such complexes. Diimine lig-ands are strongp-acceptors and are recognized stabiliz-ers of the + 2 state of ruthenium (low-spin d6, S = 0).
As a consequence, an interesting aspect of the ruthe-nium – diimine chemistry has been to study the remark-able p-interaction between the filled t2 orbitals of ruthenium(II) and the low-lying vacant p*-orbital of the diimine chromophore. The extent ofp-interaction in these complexes depends primarily on the nature of the diimine ligands, which again depends on the nature of
the groups linked to the two carbons and the two imine-nitrogens. The presence of other p-acceptor lig-ands within the coordination sphere may also have significant influence on the p-interaction between the diimine ligands and ruthenium(II).
In the present study, we have chosen dioximes (2; abbreviated in general as H2L, where H stands for the oxime protons) as the principal ligand, not only be-cause they carry the diimine chromophore but also for their different coordination modes. In our recent stud-ies on the chemistry of ruthenium with some monooxime ligands we have witnessed interesting oxo-transfer and polynucleation reactions [8,9], and these have also encouraged us to study the ruthenium chem-istry of the dioxime ligands. The dioxime ligands are known to coordinate metal ions as neutral dioximes
* Corresponding author. Tel.: + 91-33-483-6223; fax: + 91-33-473-4266.
E-mail address:samaresh –[email protected] (S. Bhattacharya).
0277-5387/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 7 7 - 5 3 8 7 ( 0 0 ) 0 0 6 1 3 - 6
and also as monoanionic dioximates via dissociation of one oxime proton [10,11]. They are also known to act as bridging ligands via coordination through the oxy-gens [12 – 15]. While coordination chemistry of the dioxime ligands has been extensively studied with the 3d metal ions [16 – 20], the dioxime chemistry of ruthe-nium has not been much explored [21 – 26]. Herein we report the chemistry of some mono and bis-dioxime complexes of ruthenium(II), where triphenylphosphine (PPh3) has been used as the coligand.
Triphenylphos-phine is also a familiarp-acceptor ligand and hence its coordination is expected to result in some interesting effect on the p interaction with the dioxime ligand as well as on the stereochemistry of the complexes. In the present work, the chemistry of dioxime complexes of type [Ru(PPh3)2(H2L)Cl2] and [Ru(PPh3)2(HL)2] has
been studied. The chemistry of the bis-dioximate com-plexes of transition metal ions has been attracting con-tinuous attention because of their importance with reference to models for vitamin B12 [27 – 29], dioxygen carriers [30], catalysis in chemical transformations [31 – 33], intramolecular hydrogen bonding and metal – metal interaction [34 – 36]. The synthesis, structure, spectro-scopic and electron-transfer properties of these com-plexes have been described in this paper with special emphasis on thep-interaction between dioxime ligands and bivalent ruthenium, and reactivities of the mono-dioxime complexes involving Ru – Cl bond cleavage.
2. Experimental
2.1. Materials
Commercial ruthenium trichloride (Arora Matthey, Calcutta, India) was converted to RuCl3·3H2O by
re-peated evaporation with concentrated hydrochloric acid. Triphenylphosphine (PPh3) was purchased from
Loba Chemie, Mumbai, India. Dimethylglyoxime (H2dmg) and diphenylglyoxime (H2dpg) were obtained
from s.d. fine-chem, Mumbai, India. [Ru(PPh3)3Cl2]
was prepared by following a published procedure [37]. Purification of dichloromethane and preparation of te-trabutylammonium perchlorate (TBAP) for electro-chemical work were performed as reported in the literature [38,39]. All other chemicals and solvents were reagent grade commercial materials and were used as received.
2.2. Preparations
2.2.1. [Ru(PPh3)2(H2dmg)Cl2]
Dichloromethane (50 cm3) was added to a mixture of
[Ru(PPh3)3Cl2] (100 mg, 0.10 mmol) and H2dmg (18
mg, 0.15 mmol). The resulting red solution was stirred for 1 h. Upon evaporating the solvent, a solid residue
was obtained which was washed thoroughly with etha-nol and dried in air. Recrystallization from dichloro methane – hexane solution gave [Ru(PPh3)2(H2dmg)Cl2] as a crystalline brown solid. The yield was 83 mg (70%).
Anal. Calc. for C40H38Cl2N2O2P2Ru: C, 59.11; H, 4.68; N, 3.45. Found: C, 59.14; H, 4.71; N, 3.43%.
2.2.2. [Ru(PPh3)2(H2dpg)Cl2]
This was synthesized by following the same above procedure and in the same above scale using diphenyl-glyoxime instead of dimethyldiphenyl-glyoxime. [Ru(PPh3)2
-(H2dpg)Cl2] was obtained as a microcrystalline brown
solid. The yield was 71 mg (73%). Anal. Calc. for C50H42Cl2N2O2P2Ru: C, 64.17; H, 4.49; N, 2.99.
Found: C, 64.14; H, 4.46; N, 2.97%.
2.2.3. [Ru(PPh3)2(Hdmg)2]
Method A: [Ru(PPh3)3Cl2] (100 mg 0.10 mmol) was taken in ethanol (50 cm3) and dimethylglyoxime (30
mg, 0.26 mmol) was added to it followed by triethy-lamine (16 mg, 0.16 mmol). The mixture was refluxed for 1 h. Upon partial evaporation of the solution, [Ru(PPh3)2(Hdmg)2] started to precipitate as a bright
orange crystalline solid. It was collected by filtration, washed thoroughly with ethanol and dried in air. Re-crystallization from dichloromethane – hexane solution gave [Ru(PPh3)2(Hdmg)2] as a bright orange crystalline
solid. The yield was 56 mg (63%). Anal. Calc. for C44H44N4O4P2Ru: C, 61.75; H, 5.15; N, 6.55. Found: C,
61.78; H, 5.13; N, 6.57%.
Method B: to a solution of [Ru(PPh3)2(H2dmg)Cl2] (100 mg, 0.10 mmol) in dichloromethane (30 cm3) was
added dimethylglyoxime (18 mg, 0.15 mmol) followed by triethylamine (16 mg, 0.16 mmol). The resulting solution was stirred for 1 h. The color of the solution gradually changed from brown to orange. Upon evap-orating the solvent a bright orange crystalline solid was obtained, which was thoroughly washed with wa-ter and dried in vacuo over P4O10. Recrystallization
from dichloromethane – hexane solution gave [Ru(PPh3)2(Hdmg)2] as a bright orange crystalline red
solid. The yield was 53 mg (60%).
2.2.4. [Ru(PPh3)2(Hdpg)2]
This was synthesized by following the same above procedures and in the same above scales. In method A diphenylglyoxime was used instead of dimethylgly-oxime and in method B [Ru(PPh3)2(H2dpg)Cl2] and
H2dpg were used instead of [Ru(PPh3)2(H2dmg)Cl2]
and H2dmg, respectively. [Ru(PPh3)2(Hdpg)2] was
ob-tained as a bright-orange crystalline solid. The yield was 69 mg (60%). Anal. Calc. for C64H52N4O4P2Ru: C,
69.62; H, 4.89; N, 5.08. Found: C, 69.62; H, 4.91; N, 5.06%.
2.2.5. [Ru(PPh3)2(Hdpg)(Hdmg)]
Method A: to a solution of [Ru(PPh3)2(H2dpg)Cl2]
(100 mg, 0.10 mmol) in dichloromethane (30 cm3) was
added dimethylglyoxime (18 mg, 0.15 mmol) followed by triethylamine (16 mg, 0.16 mmol). The resulting solution was stirred for 1 h. The color of the solution gradually changed from yellowish – orange to red. Upon evaporating the solvent a solid residue was obtained, which was thoroughly washed with water and dried in vacuo over P4O10. Recrystallization from
dichloro-methane – hexane solution gave [Ru(PPh3)2
(Hdpg)-(Hdmg)] as a crystalline red solid. The yield was 80 mg (82%). Anal. Calc. for C54H50N4O4P2Ru: C, 66.05; H,
5.09; N, 5.70. Found: C, 66.07; H, 5.05; N, 5.68%. Method B: the same above procedure and scale were followed using [Ru(PPh3)2(H2dmg)Cl2] and
diphenyl-glyoxime instead of [Ru(PPh3)2(H2dpg)Cl2] and dimethylglyoxime respectively. [Ru(PPh3)2 (Hdpg)-(Hdmg)] was obtained as a crystalline red solid. The yield was 79 mg (78%).
2.2.6. [Ru(PPh3)2(H2dpg)(pic)]ClO4
An ethanolic solution (30 cm3) of picolinic acid (13
mg, 0.10 mmol) was added to a solution of [Ru(PPh3)2(H2dpg)Cl2] (100 mg, 0.10 mmol) in
dichloromethane (50 cm3). To it triethylamine (16 mg,
0.16 mmol) was added and the resulting solution was initially heated gently to expel dichloromethane as much as possible. The solution was then refluxed for 6 h. The color of the solution changed from yellowish – orange to red. The solution was allowed to cool to room temperature (25°C) and a saturated aqueous solu-tion of sodium perchlorate (0.5 cm3) was added to it.
Upon partial evaporation of the solution, a solid
residue was obtained, which was washed thoroughly with cold water and dried in vacuo over P4O10.
Recrys-tallization from dichloromethane – hexane gave [Ru(PPh3)2(H2dpg)(pic)]ClO4 as a crystalline red solid. The yield was 87 mg (75%). Anal. Calc. for C56H46ClN3O8P2Ru: C, 61.90; H, 4.23; N, 3.87. Found: C, 61.93; H, 4.22; N, 3.90%.
2.2.7. [Ru(PPh3)2(H2dpg)(q)]ClO4
This was synthesized by following the above proce-dure and scale using 8-hydroxyquinoline (Hq) instead of picolinic acid. [Ru(PPh3)2(H2dpg)(q)]ClO4 was
ob-tained as a microcrystalline red solid. The yield was 86 mg (73%). Anal. Calc. for C59H48ClN3O7P2Ru: C,
63.92; H, 4.33; N, 3.79. Found: C, 63.90; H, 4.35; N, 3.78%.
2.2.8. [Ru(PPh3)2(H2dpg)(nn)]ClO4
This was synthesized by following the same proce-dure and scale used for the synthesis of [Ru(PPh3)2 -(H2dpg)(pic)]ClO4using 1-nitroso-2-naphthol (Hnn)
in-stead of picolinic acid. [Ru(PPh3)2(H2dpg)(nn)]ClO4
was obtained as a crystalline red solid. The yield was 92 mg (76%). Anal. Calc. for C60H48ClN3O8P2Ru: C,
63.40; H, 4.22; N, 3.70. Found: C, 63.41; H, 4.25; N, 3.68%.
2.3. Physical measurements
Microanalyses (C, H, N) were performed using a Perkin – Elmer 240C elemental analyzer. IR spectra were obtained on a Perkin – Elmer 783 spectrometer with samples prepared as KBr pellets. Electronic spec-tra were recorded on a Shimadzu UV 240 spectropho-tometer. Magnetic susceptibilities were measured using a PAR 155 Vibrating sample magnetometer. 1H NMR
spectra were recorded on a Bruker AC-200 NMR spec-trometer using TMS as the internal standard. Solution electrical conductivities were measured using a Phillips PR 9500 bridge with a solute concentration of 10− 3M.
Electrochemical measurements were made using a PAR model 273 potentiostat. A platinum disc working elec-trode, a platinum wire auxiliary electrode and an aqueous saturated calomel reference electrode (SCE) were used in a three electrode configuration. All electro-chemical data were collected at 298 K and are uncor-rected for junction potentials. A RE 0089 X – Y recorder was used to trace the voltammograms.
2.4. Crystallography
[Ru(PPh3)2(H2dpg)Cl2]. Single crystals of [Ru(PPh3)2
-(H2dpg)Cl2] were grown by slow diffusion of hexane
into a dichloromethane solution of the complex. Se-lected crystal data and data collection parameters are given in Table 1. Data were collected on an Enraf –
Table 1
Crystallographic data
[Ru(PPh3)2(H2dpg)Cl2] [Ru(PPh3)2(Hdmg)2]
Empirical formula C50H44N2O3P2Cl2Ru C44H44N4O4P2Ru
Formula weight 954.82 855.87
Space group orthorhombic, P212121 monoclinic, P21/c
12.026(3) a (A, ) 8.6568(21) 17.5853(22) b (A, ) 16.024(3) 21.194(3) c (A, ) 13.8305(22) V (A,3) 4482.0(13) 1884.2(6) 4 Z 2 u (A,) 0.71073 0.7107 0.10×0.15×0.38 0.50×0.13×0.05 Crystal size (mm) 25 T (°C) 25 5.727 v (cm−1) 5.394 0.039a Rf 0.036a 0.036b Rw 0.037b 1.21c 1.36c Goodness-of-fit aR f=S Fo − Fc /S Fo . bR w= [S w( Fo − Fc )2/S w(Fo)2]1/2. cGoodness-of-fit = [S w( F o − Fc )2/(M−N)]1/2, where M is the
Nonius CAD-4 diffractometer using graphite-monochromated Mo Ka radiation (u=0.71073 A,). Three standard reflections measured every 3600 s of X-ray exposure showed no significant intensity varia-tion over the course of data collecvaria-tion. X-ray data reduction, structure solution and refinement were done using the NRCVAX package [40].
[Ru(PPh3)2(Hdmg)2]. Single crystals of [Ru(PPh3)2
-(Hdmg)2] were grown by slow diffusion of hexane into
a dichloromethane solution of the complex. Selected crystal data and data collection parameters are given in Table 1. Data collection and structure solution and refinement were done as described above.
3. Results and discussion
3.1. Synthesis and characterization
3.1.1. [Ru(PPh3)2(H2L)Cl2] complexes
Reaction of the dioxime ligands (H2L) with
[Ru(PPh3)3Cl2] affords, under different experimental
conditions, different products. Simple reaction of the dioxime ligands with [Ru(PPh3)3Cl2] in 1:1 mole ratio in
dichloromethane solution at ambient temperature yields complexes of type [Ru(PPh3)2(H2L)Cl2]. It is interesting
to note that the dioxime ligands do not undergo any proton loss during complexation. Elemental (C, H, N) analytical data agree well with the proposed composi-tion of the complexes. These complexes are diamagnetic which corresponds to the bivalent state of ruthenium (low-spin d6, S = 0) in these complexes. As the dioxime
ligands are symmetric in nature, the [Ru(PPh3)2 -(H2L)Cl2] complexes may exist in three geometrical isomeric forms (3 – 5). To find out the stereochemistry, structure of [Ru(PPh3)2(H2dpg)Cl2] has been
deter-mined by X-ray crystallography.
The structure is shown in Fig. 1 and selected bond parameters are listed in Table 2. The diphenylglyoxime ligand is coordinated to ruthenium as a neutral biden-tate N,N-donor ligand forming five-membered chelate ring with a bite angle of 76.7(2)°. The two PPh3ligands
are mutually trans and the two chlorides are mutually
cis and therefore Ru(PPh3)2(H2dpg)Cl2] has structure 3.
Ruthenium is sitting in a N2P2Cl2 coordination sphere which is distorted significantly from ideal octahedral geometry as reflected in the bond parameters around ruthenium. While the Ru – P and Ru – Cl bond distances are quite normal [41 – 44], the Ru – N lengths are
notice-Fig. 1. View of the [Ru(PPh3)2(H2dpg)Cl2] molecule.
ably shorter than the known Ru(II) – N lengths where the N-donor ligand is not involved inp-interaction with the metal [45 – 49]. The C – N lengths within the coordi-nated dioxime ligands are also significantly longer than localized CN bond [50]. The decrease in Ru–N dis-tance and increase in C – N disdis-tance within the ruthe-nium – dioxime chelate clearly indicate strong p-interaction between ruthenium and the diimine frag-ment of the dioxime ligands. In complexes of rutheniu-m(II) containing the Ru(PPh3)2 moiety, the PPh3 ligands usually take up mutually cis positions because of favorable p-interaction [51–53]. In [Ru(PPh3)2 -(H2dpg)Cl2], the H2dpg ligand appears to function as a
better p-acid by virtue of having the diimine fragment (as also observed in the structural characterization of [Ru(PPh3)2(H2dpg)Cl2]) and hence forces the bulky
PPh3 ligands to mutually trans positions for less steric
hindrance. As properties of the two [Ru(PPh3)2
-(H2L)Cl2] complexes are similar (vide infra), the
[Ru(PPh3)2(H2dmg)Cl2] complex is assumed to have a
similar structure (3) like [Ru(PPh3)2(H2dpg)Cl2].
Infrared spectra of the [Ru(PPh3)2(H2L)Cl2]
com-plexes show strong vibrations due to the Ru(PPh3)2 fragment near 520, 695 and 740 cm− 1[54,55]. A sharp
band observed near 1030 cm− 1 in both the complexes
is assigned to the w(N–O) vibration. The w(O–H) stretches appear near 3400 cm− 1 and the w(Ru–Cl)
vibrations are observed at 310 – 335 cm− 1. The
[Ru(PPh3)2(H2L)Cl2] complexes are moderately soluble
in common organic solvents like dichloromethane, chloroform, acetone, etc. producing red solutions. 1H
NMR spectra of these complexes have been recorded in CDCl3 solution. An isolated signal, observed near 10.5
Table 2
Selected bond lengths (A, ) and bond angles (°) for [Ru(PPh3)2(H2dpg)Cl2] and [Ru(PPh3)2(Hdmg)2]
[Ru(PPh3)2(H2dpg)Cl2] [Ru(PPh3)2(Hdmg)2]
Ru–Cl(1) 2.442(2) O(1)–N(1) 1.376(8) Ru–P 2.417(1) O(1)–Ha 1.68(5)
Ru–Cl(2) 2.461(2) O(2)–N(2) 1.379(8) Ru–N(1) 2.030(3) N(1)–C(2) 1.317(6) Ru–P(1) 2.387(2) N(1)–C(1) 1.321(10) Ru–N(2) 2.002(3) N(2)–C(3) 1.290(6) Ru–P(2) 2.410(2) N(2)–C(2) 1.322(10) O(1)–N(1) 1.329(5) C(2)–C(3) 1.466(7) Ru–N(1) 1.973(5) C(1)–C(2) 1.45(1) O(2)–N(2) 1.387(5) C(1)–C(2) 1.484(7)
Ru–N(2) 1.962(6) C(1)–C(3) 1.47(1) O(2)–H 1.15(5) C(3)–C(4) 1.488(7)
C(2)–C(9) 1.48(1)
P(1)–Ru–P(2) 173.1(8) N(1)–Ru–N(2) 76.7(2) P–Ru–Pa 179.9 N(2)–Ru–N(2a) 180.0 Cl(1)–Ru–N(1) 167.3(2) Cl(1)–Ru–Cl(2) 102.79(8) N(1)–Ru–N(1a) 180.0 N(1)–Ru–N(2) 77.1(1) Cl(2)–Ru–N(2) 166.6(2)
O – H proton. The aromatic protons are observed within 6.1 – 7.6 ppm. The methyl signal of the coordi-nated dimethylglyoxime ligand in [Ru(PPh3)2 -(H2dmg)Cl2] is observed as a sharp resonance at 1.92 ppm. Electronic spectra of the [Ru(PPh3)2(H2L)Cl2]
complexes have been recorded in dichloromethane solu-tion. Spectral data are presented in Table 3. Each complex shows several intense absorptions in the visible and ultraviolet region. The absorptions in the ultravio-let region are assignable to transitions involving ligand orbitals. Three absorptions are displayed by both the [Ru(PPh3)2(H2L)Cl2] complexes in the visible region, of
which the lowest energy one is much weaker in intensity1and is assigned to the d – d (1A
11T1)
transi-tion. The other probable d – d (1A
11T2) transition
could not be identified due to intense absorptions at higher energies. The other two intense absorptions in the visible region are probably due to allowed metal-to-ligand transfer transitions. Multiple charge-transfer transitions in such mixed-ligand complexes may result from lower symmetry splitting of the metal level, the presence of different acceptor orbitals and from the mixing of singlet and triplet configurations in the excited state through spin – orbit coupling [56 – 59]. To have an insight into the nature of the observed charge-transfer transitions, qualitative EHMO calcula-tions have been performed [60,61] on a model of the [Ru(PPh3)2(H2L)Cl2] complexes replacing the methyl/
phenyl groups of the dioxime ligands and the phenyl groups of triphenylphosphines by hydrogen. Partial MO diagram is shown in Fig. 2. The highest occupied molecular orbital (HOMO) and the next two occupied orbitals (HOMO-1 and HOMO-2) have major contri-butions from the metal t2 orbitals. Hence these three
filled orbitals may be attributed to the metal t2orbitals.
The lowest unoccupied molecular orbital (LUMO) and the next unoccupied orbital (LUMO + 1) are relatively closely spaced and these are localized almost completely
on the diimine part of the dioxime ligand. Hence these two vacant orbitals may be assumed to be imine p*-or-bital of the dioxime ligands. The two charge-transfer transitions observed in the [Ru(PPh3)2(H2L)Cl2] com-plexes may therefore be assigned to transitions occur-ring from the filled metal t2 levels to the
p*-(imine)orbitals of the dioxime ligands.
Table 3
Electronic spectral and cyclic voltammetric data
Compounds Electronic spectral dataa Cyclic umax(nm) (m, M−1cm−1) valtammetric dataa,b E1/2(V) (DEp, mV) 222 (44 900), 247 (44 200), 0.84(80), [Ru(PPh3)2(H2dmg)Cl2] 300c(9100), 360(9200), 1.31(140) 440c(1500) [Ru(PPh3)2(H2dpg)Cl2] 232 (40 100), 278 (39 600), 0.98(80), 1.38(110) 312c(6900), 406(10 200), 467c(1100) [Ru(PPh3)2(Hdmg)2] 235(34 800), 260(37 400), 0.36(80), 300c(5700), 375(7400), 0.94(120) 440c(1000) 0.52(70), [Ru(PPh3)2(Hdpg)2] 240(44 400), 270c(35 400), 1.03(120) 315c(11 800), 406(7300), 438c(5000) [Ru(PPh3)2(Hdmg)- 240(40 200), 254(37 600), 0.46(70), 1.00(100) (Hdpg)] 324c(9400), 390(9400), 440c(2600) 0.66(70), [Ru(PPh3)2(H2dpg)- 224(82 800), 256(81 400), (pic)]ClO4 328c(18 100), 392(18 100) 1.60(250) 0.60(70), [Ru(PPh3)2(H2dpg)- 224(56 600), 252c(44 900), (q)]ClO4 292(34 200), 364(11 500) 1.50(240) [Ru(PPh3)2(H2dpg)- 224(84 100), 252(71 000), 0.68(80), 308c(22 700), 400(5500), (nn)]ClO4 1.42(320) 490(22 200) aIn dichloromethane solution.
bSupporting electrolyte, TBAP; reference electrode, SCE; E 1/2=
0.5(Epa+Epc), where Epa and Epc are anodic and cathodic peak
potentials respectively;DEp= Epa−Epc; scan rate, 50 mV s−1. cShoulder.
Fig. 2. Partial MO diagram of [Ru(PPh3)2(H2L)Cl2].
3.1.2. [Ru(PPh3)2(HL)2] complexes
Reaction of two moles of the dioxime ligands with one mole of [Ru(PPh3)3Cl2] proceeds smoothly in
refluxing ethanol in the presence of a base to afford bis-dioximato complexes of type [Ru(PPh3)2(HL)2] in decent yields. The bis-dioximato complexes can also be synthesized from [Ru(PPh3)2(H2L)Cl2] by reacting them with the respective dioxime ligand in presence of a base. A mixed-bis-dioximato complex, viz. [Ru(PPh3)2 -(Hdpg)(Hdmg)], has been prepared by reacting either [Ru(PPh3)2(H2dpg)Cl2] or [Ru(PPh3)2(H2dmg)Cl2] with H2dmg or H2dpg respectively in the presence of a base.
Composition of the bis-dioximato complexes has been verified by their microanalytical data. All the three bis dioximato complexes are diamagnetic, which corre-sponds to the + 2 state of ruthenium in these com-plexes. The structure of [Ru(PPh3)2(Hdmg)2] has been
determined by X-ray crystallography. The structure is shown in Fig. 3 and relevant bond distances and angles are presented in Table 2. The dioxime ligands are coordinated to ruthenium, via loss of one oxime pro-ton, as monoanionic bidentate N,N-donor ligands forming five-membered chelate ring with a bite angle of 77.1(1)°. The two Hdmg ligands which share the equa-torial plane, are hydrogen bonded in the usual in-tramolecular fashion. The two PPh3 ligands occupy mutually trans positions, the two oxime-nitrogens and the two oximato-nitrogens are also mutually trans. From the bond parameters around ruthenium, the RuN4P2 core appears to be much less distorted from ideal octahedral geometry relative to the [Ru(PPh3)2(H2L)Cl2] complexes. While the Ru – P
dis-tance is normal, bond disdis-tances within the Ru(Hdmg)
fragment are quite interesting. The two sets of Ru – N, C – N and N – O distances within the Ru(Hdmg) chelate are all very different. All these features indicate that deprotonation of one oxime function has made the Hdmg ligand unsymmetrical and the diimine character of it is also reduced to a great extent. The other two bis-dioximato complexes are assumed to have similar structure as [Ru(PPh3)2(Hdmg)2] as all three
bis-dioxi-mato complexes display similar spectroscopic and elec-tron-transfer properties (vide infra).
Infrared spectra of the bis-dioximato complexes show strong absorptions due to the Ru(PPh3)2 fragment near
520, 700 and 750 cm− 1 as before. A strong band
displayed near 1225 cm− 1 by all these complexes is
assigned to the w(N–O) stretch. The bis dioximato complexes are soluble in common organic solvents like dichloromethane, chloroform, etc., producing orange solutions. 1H NMR spectra, recorded in CDCl
3
solu-tion, show the aromatic proton signals within 7.0 – 8.0 ppm. An isolated resonance observed near 10.0 ppm is assigned to the oxime O – H signal. The methyl signal of the dimethylglyoximate ligand in [Ru(PPh3)2(Hdmg)2] is
observed at 2.19 ppm. Electronic spectra of these com-plexes have been recorded in dichloromethane solution. Spectral data are listed in Table 3. Intense absorptions are observed in both visible and ultraviolet regions. The absorptions in the ultraviolet region are believed to be occurring within the ligand orbitals. Qualitative EHMO calculations on the bis-dioximato complexes show simi-lar results as obtained in the [Ru(PPh3)2(H2L)Cl2] com-plexes. The top three filled orbitals having predomi-nantly ruthenium t2 character and the first two vacant
orbitals having ligand (HL) p*-character. Hence the absorptions in the visible region are assigned to metal (t2)-to-ligand (p* of HL) charge-transfer transitions. 3.1.3. The [Ru(PPh3)2(H2L)(L%)]ClO4 complexes
Complexes of ruthenium(II) having a cis-RuCl2
frag-ment have always been of particular interest with refer-ence to their possible reactivities arising from the dissociation of the Ru – Cl bonds. The coordinated chlorides are found to be displaceable by chelating bidentate ligands under relatively mild condition. Such reactivities have already been utilized in the preparation of the bis-dioximate complexes (vide supra). Though both of the [Ru(PPh3)2(H2L)Cl2] complexes display
sim-ilar reactivities with regard to displacement of the chlo-rides by bidentate chelating ligands, only the reactions of [Ru(PPh3)2(H2dpg)Cl2] are reported here. A group of
three acidic ligands, viz., picolinic acid(Hpic), quinolin-8-ol(Hq) and 1-nitroso-2-napthol(Hnn), (abbreviated in general as HL%, where H stands for the acidic hydrogen) have been used for these reactions. All these ligands are known to coordinate ruthenium, via loss of the acidic proton, as bidentate N,O-donors forming five-mem-bered chelate rings [9,62,63]. Reaction of these ligands with [Ru(PPh3)3Cl2] proceeds smoothly in stirring dichloromethane in the presence of a base to afford complexes of type [Ru(PPh3)2(H2dpg)(L%)]+ which have
been isolated as perchlorate salts in the solid state. Elemental (C, H, N) analytical data of these complexes agree well with their compositions. The [Ru(PPh3)2
-(H2dpg)(L%)]ClO4complexes are diamagnetic, which
in-dicates that ruthenium is in its + 2 oxidation state in these complexes. The [Ru(PPh3)2(H2dpg)(L%)]+
com-plexes are assumed to have a geometry similar to that
of the [Ru(PPh3)2(H2dpg)Cl2] complex with the two
PPh3 ligands in mutually trans positions and the two
chlorides replaced by L%.
Infrared spectra of the [Ru(PPh3)2(H2dpg)(L%)]ClO4 complexes are mostly similar to the spectrum of [Ru(PPh3)2(H2dpg)Cl2], which is obviously due to pres-ence of the common Ru(PPh3)2(H2dpg) fragment. Some
new vibrations are of course observed in the [Ru(PPh3)2(H2dpg)(L%)]ClO4 complexes, of which the
two vibrations uniformly displayed by all the [Ru(PPh3)2(H2dpg)(L%)]ClO4 complexes near 1100 and
620 cm− 1, are due to the presence of perchlorate ion.
The [Ru(PPh3)2(H2dpg)(L%)]ClO4 complexes are soluble
in polar organic solvents like acetonitrile, dichloromethane, etc. Conductivity studies in acetoni-trile solution show that these complexes behave as 1:1 electrolytes (\M= 1140 – 155V− 1cm2M− 1).1H NMR
spectra of the [Ru(PPh3)2(H2dpg)(L%)]+ complexes
recorded in CDCl3solution, show the oxime OH reso-nance near 10.4 ppm. The aromatic protons are ob-served within 6.0 – 8.0 ppm as overlapping signals. However, intensity measurements correspond nicely to the total number of aromatic protons in the respective complexes. Electronic spectra of the [Ru(PPh3)2
-(H2dpg)(L%)]+ complexes has been recorded in
dichloromethane solution (Table 3). Each complex shows intense absorptions in the visible region probably due to the allowed metal-to-ligand charge-transfer tran-sitions. Intense absorptions are also observed in the ultraviolet region as before.
3.2. Cyclic 6oltammetric studies
Electrochemical properties of all the complexes have been studied by cyclic voltammetry in dichloromethane solution (0.1 M TBAP). Voltammetric data are pre-sented in Table 3 and selected voltammograms are shown in Fig. 4. Each complex shows two one-electron oxidative oxidative responses on the positive side of SCE. The first response is assigned to ruthenium(II) – ruthenium(III) oxidation and the second to rutheniu-m(III) – ruthenium(IV) oxidation. One-electron nature of both the responses has been established by compar-ing current height of each response with that of fer-rocene – ferrocenium couple under identical ex-perimental conditions. The ruthenium(II) – rutheniu-m(III) oxidation is reversible, characterized by a peak-to-peak separation of 70 – 80 mV and the anodic-peak-current (ipa) is almost equal to the
cathodic-peak-current (ipc). The ruthenium(III) – ruthenium(IV)
oxida-tion is quasi-reversible. It is interesting to note here that the ruthenium(III) – ruthenium(IV) oxidation potential is rather close to the ruthenium(II) – ruthenium(III) oxi-dation potential than is usually observed. This closeness indicates that this second electron-transfer reaction is probably associated with proton-transfer from the
ox-Fig. 4. Cyclic voltammograms of (a) [Ru(PPh3)2(H2dmg)Cl2]; (b)
[Ru(PPh3)2(Hdmg)2]; and (c) [Ru(PPh3)2(H2dpg)(nn)]ClO4 in
dichloromethane solution (0.1 M TBAP) at a scan rate of 50 mV s− 1.
4. Conclusions
The present study shows that reaction of dioxime ligands with [Ru(PPh3)3Cl2] affords stable complexes of ruthenium(II) where the dioxime ligands bind to ruthe-nium(II) either in the neutral dioxime form or in the monoanionic dioximate form. While the [Ru(PPh3)2
-(H2L)Cl2] complexes have been found to be useful
start-ing material for the synthesis of complexes of type [Ru(PPh3)2(H2L)(L%)]+, the bis-dioximato complexes
appear to be suitable as ‘bridging ligand’ (via disso-ciation of the remaining two oxime protons), in the synthesis of trinuclear complexes of type 6.
Such possibilities are currently under exploration.
5. Supplementary material
Crystallographic data for the structural analysis have been deposited with the Cambridge Crystallographic Data Centre, CCDC no. 148069 and CCDC no. 148070. Copies of this information may be obtained from The Director, CCDC, 12 Union Road, Cam-bridge, CB2 1EZ, UK (fax: + 44-1233-336033; e-mail: [email protected] or www: http://www.ccdc. cam.ac.uk).
Acknowledgements
Financial assistance received from the Department of Science and Technology, New Delhi (grant no. SP/S1/ F33/98) is gratefully acknowledged. Thanks are also due to the Third World Academy of Sciences for finan-cial support enabling the purchase of an electrochemi-cal cell system. The authors thank Professor Ramgopal Bhattacharyya of Jadavpur University and Dr Sree-brata Goswami of the Indian Association for the Culti-vation of Science for their help.
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![Fig. 1. View of the [Ru(PPh 3 ) 2 (H 2 dpg)Cl 2 ] molecule.](https://thumb-ap.123doks.com/thumbv2/9libinfo/8666102.195490/4.918.472.820.62.386/fig-view-ru-pph-h-dpg-cl-molecule.webp)
![Fig. 2. Partial MO diagram of [Ru(PPh 3 ) 2 (H 2 L)Cl 2 ].](https://thumb-ap.123doks.com/thumbv2/9libinfo/8666102.195490/6.918.200.690.71.419/fig-partial-mo-diagram-ru-pph-h-cl.webp)
![Fig. 4. Cyclic voltammograms of (a) [Ru(PPh 3 ) 2 (H 2 dmg)Cl 2 ]; (b) [Ru(PPh 3 ) 2 (Hdmg) 2 ]; and (c) [Ru(PPh 3 ) 2 (H 2 dpg)(nn)]ClO 4 in dichloromethane solution (0.1 M TBAP) at a scan rate of 50 mV s − 1 .](https://thumb-ap.123doks.com/thumbv2/9libinfo/8666102.195490/8.918.118.367.70.490/fig-cyclic-voltammograms-hdmg-dichloromethane-solution-tbap-rate.webp)
