J Solution Chem (2014) 43:870–884 DOI 10.1007/s10953-014-0178-z
The Reactivity of vic-dioximes Towards the [(H2O)(tap)2RuORu(tap)2(H2O)]2+ Ion {tap 5 2-(m-tolylazo)pyridine} at Physiological pH Arup Mandal • Sumon Ray • Animesh Chattopadhyay • Parnajyoti Karmakar • Debabrata Nandi • Alak K. Ghosh
Received: 31 May 2013 / Accepted: 29 January 2014 / Published online: 20 May 2014 Ó Springer Science+Business Media New York 2014
Abstract Kinetics of aqua ligand substitution from [(H2O)(tap)2RuORu(tap)2(H2O)]2? {tap = 2-(m-tolylazo)pyridine}, by three vicinal dioximes, namely dimethylglyoxime (L1H), 1,2-cyclohexanedione dioxime (L2H) and a-furil dioxime (L3H), have been studied spectrophotometrically in the 35–50 °C temperature range. The reaction was monitored at 560 nm where the absorbance between the reactant and product is at a maximum. At pH 7.4, the reaction has been found to proceed via two distinct consecutive steps, i.e., it shows a non-linear dependence on the concentration of ligands: the first process is [ligand] dependent but the second step is [ligand] independent. The rate constants for the processes are: k1 * 10-3 s-1 and k2 * 10-4 s-1. The activation parameters, calculated from Eyring plots, suggest an associative mechanism for the interaction process. From the temperature dependence of the outer sphere association equilibrium constants, the thermodynamic parameters were also calculated, which give negative DG° values at all temperatures studied, supporting the spontaneous formation of an outer sphere association complex. The product of the reaction has been characterized with the help of IR and ESI-mass spectroscopic analysis. Keywords Kinetics
Ligand substitution vic-dioximes [(H2O)(tap)2RuORu(tap)2(H2O)]2?
1 Introduction The chemistry of transition metal complexes with vic-dioxime ligands has been well studied and is the subject of several reviews [1, 2]. The coordination compounds of vicinal dioximes have been widely investigated as analytical reagents [3], models for biological
A. Mandal S. Ray A. Chattopadhyay P. Karmakar D. Nandi A. K. Ghosh (&) Department of Chemistry, The University of Burdwan, Burdwan 713104, West Bengal, India e-mail:
[email protected]
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systems [4] such as vitamin B12, compounds with columnar stacking thought to be behind their semiconducting properties [5], and recently in vic-dioxime reactions [6]. On the other hand, ruthenium polypyridyl complexes have been extensively studied as their luminescence and photochemical reactivity make them exceptionally versatile as probes of DNA structures [7–13]. These complexes bind to DNA by non-covalent interactions such as electrostatic binding, groove binding, intercalative binding and partial intercalative binding [14]. Barton et al. have pioneered the application of chiral transitionmetal polypyridyl complexes to probe local variations in double-helical DNA structures and their role in gene expression [15–18]. The interaction between vic-dioximes and [Ru(bpy)2(H2O)2]2? had already been investigated [19], the reaction was studied at low pH and was single step reaction but, in our present work, it is somewhat different. For this reason we were very interested in investigating the anation of [(H2O)(tap)2RuORu(tap)2(H2O)]2? (1) by three vic-dioximes, viz., dimethylglyoxime (L1H), 1,2-cyclohexanedione dioxime (L2H) and a-furil dioxime (L3H), in order to get a better understanding of the kinetic behavior of ruthenium(II). This work has been done in aqueous medium and at physiological pH (7.4) under which conditions most ruthenium(II) complexes are oxidized to ruthenium(III), but the ?2 state of the metal ion in (1) is quite stable due to the presence of an excellent p-acceptor [20] ligand, tap.
2 Experimental 2.1 Materials and Methods 2-Aminopyridine (99.0 %) was purchased from Sigma–Aldrich and used without further purification. RuCl33H2O was purchased from Sigma–Aldrich and purified according to the literature [21]. The compound cis-diaqua-bis-{2-(m-tolylazo)pyridine} ruthenium(II) diperchlorate monohydrate, cis-[Ru(tap)2(H2O)2](ClO4)2H2O, was prepared following the literature method [21, 22] and the compound was characterized by elemental analyses and spectral data (kmax = 536 nm). The reacting complex ion [(H2O)(tap)2RuORu(tap)2 (H2O)]2? (1) was prepared in situ at pH 7.4. Milli-Q water was used to prepare all the kinetic solutions. All the other chemicals used were of AR quality. 2.2 Product Analysis The product [(tap)2Ru(l-O)(l-LH)Ru(tap)2]? (2) of the reaction between complex (1) and vic-dioximes (LH) was prepared by mixing the reactants in 1:1, 1:2, 1:3, 1:5 and 1:10 molar ratios and then thermostated at 50 °C for 72 h. The spectrum of (2) (Fig. 1) shows good complexation between (LH) and (1). In all cases the products of reaction between the substrate complex and the ligands could not be isolated in their anhydrous forms due to their high solubility and hygroscopic nature. The compositions of the products were confirmed by Job’s method of continuous variation and characterized by spectroscopic analysis. The chemical structures of the above species may be represented as follows:
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Fig. 1 i [(H2O)(tap)2RuORu(tap)2(H2O)2?] = 1.0 9 10-4 moldm-3; ii [(H2O)(tap)2RuORu(tap)2 (H2O)2?] = 1.0 9 10-4 moldm-3, [a-furil dioxime] = 3.0 9 10-3 moldm-3; iii [(H2O)(tap)2RuORu(tap)2(H2O)2?] = 1.0 9 10-4 moldm-3, [1,2-cyclohexane dione dioxime] = 3.0 9 10-3 moldm-3; iv [(H2O)(tap)2RuORu(tap)2(H2O)2?] = 1.0 9 10-4 moldm-3, [dimethylglyoxime] = 3.0 9 10-3 mol dm-3; all at pH 7.4
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The composition of complexes in the solution was determined by Job’s method of continuous variation. The metal:ligand ratio was found to be 2:1 (Fig. 2). This is possible only when a bridged product is formed (vide mechanism and conclusion section). Complex (1) and L1H were mixed in 2:1 molar ratio at pH 7.4 and a violet product was obtained. Then the product was characterized by IR and ESI-MS measurements (Fig. 3). 0.45 0.40 0.35
Δabs
0.30 0.25 0.20 0.15 0.10 0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
[L]/[L]+[M] Fig. 2 Job’s plot for reaction of complex (1) with L1H
Fig. 3 ESI-mass spectrum of the product with L1H
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Fig. 4 Plausible structures of the species giving molecular ion peaks with L1H from the ESI-mass spectra
The IR spectrum [Complex (1) ? L1H] of the violet product, in a KBr disc, shows a strong band at 3443 cm-1 together with medium bands at 1121 and 626 cm-1. The strong bands at 3435 cm-1 indicate that the product is hydrated. The sharp peak at 1121 cm-1 is -1 peak is due to the formation of a Ru–N due to the counter anion (ClO 4 ). The 626 cm bond in the product [23]. It is clear from this spectrum that the ion at m/z * 627 is the molecular ion species in the mixture solution and this is attributed to [(L1H–H?) ? (2Ru) ? (4 tap) ? (1 O) ? (Na?) ? (6H2O)]2?. The precursor ion is shown in Fig. 4. The peaks at m/z * 609 and * 598 are due to [(L1H–H?) ? (2Ru) ? (4 tap) ? (1 O) ? (Na?) ? (4H2O)]2? and [(L1H–H?) ? (2Ru) ? (4 tap) ? (1 O) ? (H?) ? (4H2O)]2?, respectively. 2.3 Measurements All the spectral measurements were done in a Shimadzu UV–Vis spectrophotometer (UV2450 PC), attached to a thermoelectric cell temperature controller (model TCC-240A with an accuracy of ±0.1 °C). IR Spectra (KBr disc, 4000–400 cm-1) were measured in a Perkin–Elmer FTIR model RX1 Infrared Spectrophotometer. ESI-mass spectra were recorded using a micromass Q-Tof microTM mass spectrometer in the positive ion mode. The pHs of the solutions were adjusted with HClO4/NaOH and measured with a Sartorius pH meter (model PB11) with an accuracy of ±0.01. The conventional mixing technique was followed and pseudo-first order conditions were employed throughout. The progress of the reaction was followed by measuring the decrease in absorbance at 560 nm, where the spectral difference between the substrate and the product complex is at a maximum. The k1(obs) and k2(obs) values were calculated graphically (Figs. 5, 6) using the method of Weyh and Hamm [24]. We did not use software like ORIGIN to calculate k1 and k2 as we have observed that when the first step is a curved one, the Weyh and Hamm method give good results. However, we used ORIGIN software for other calculations. The rate data represent an average of duplicate runs that were reproducible within ±4 %.
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-0.6 -0.8
ln(A t-A inf.)
-1.0
X
-1.2
Δ
-1.4
Y
-1.6 -1.8 -2.0 -2.2 0
20
40
60
80
100
Time(min.) Fig. 5 A typical plot of ln (At - A?) versus time [L1H] = 4.0 9 10-3 moldm-3, pH 7.4, temperature = 40 °C
t.
[complex] = 1.0 9 10-4 moldm-3,
-2.4
-2.5
lnΔ
-2.6
-2.7
-2.8
-2.9 0
1
2
3
4
5
Time(min.) Fig. 6 A typical plot of ln D versus time (t). [L1H] = 3.0 9 10-3 moldm-3, pH 7.4, temperature = 40 °C
[complex] = 1.0 9 10-4 moldm-3,
3 Results and Discussion The pKa values of the ligands L1H, L2H and L3H are given in Table 1. From the pKa values of all the ligands we can say that at pH 7.4, all of these three ligands remain in the neutral form.
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Table 1 pKa values of the ligands Ligand
Structures
Reference
Dimethylglyoxime (L1H) pK1 = 10.66
[25]
1,2-Cyclohexanedione dioxime (L2H) pK1 = 12
[25]
A-Furil dioxime (L3H) pK1 = 11.5
[25]
On the other hand, the first acid dissociation equilibrium constant pK1 of the complex [Ru(tap)2(H2O)2]2? is 6.6 [26] at 25 °C. At pH 7.4, the complex ion exists in a dimeric oxo-bridged form, [(H2O)(tap)2RuORu(tap)2(H2O)]2? [27–30]. At pH 7.4, the mononuclear species exists in the hydroxoaqua form. Two such species assemble to form the dinuclear oxo-bridged diaqua complex due to a thermodynamic force mainly arising from p-bonding [31] (O2- donor, RuII acceptor), which is favorable for the 4d ion RuII. Now, such strong covalency reduces the acidity of the coordinated water. The oxobridge formation is solely dependent on pH. Electrochemical studies show that there is pH potential domain where the l-oxo structures stay intact. Our variable temperature study does not show any effect, which is in line with the fact that oxobridge formation is solely pH-dependent [32, 33]. The plot of ln (At - A?) versus time indicates that the reaction is not a single step process, therefore a two-step consecutive process may be assumed, the first step being dependent and the final step being independent of the concentration of ligand. The rate constant for such a process can be evaluated by assuming Scheme 1. A is the oxo-bridged diaqua complex, B is the intermediate with ligand and C is the final chelated product complex [(tap)2Ru(l-O)(l-LH)Ru(tap)2]?.
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k1
A
k2
B
C
Scheme 1 Possible reaction pathways for product formation
Table 2 103 k1(obs) (s-1) values for different ligand concentrations at different temperatures: [complex 1] = 1.0 9 10-4 moldm-3, pH = 7.4, ionic strength = 0.1 moldm-3 NaClO4 Ligandsa
L1H
L2H
L3H
Temperature (°C)
103 [ligand] (moldm-3) 1.00
2.00
3.00
4.00
5.00
35
0.93 (0.03)
1.63 (0.01)
2.28 (0.03)
2.71 (0.03)
3.04 (0.04)
40
1.10 (0.02)
1.95 (0.02)
2.63 (0.02)
3.20 (0.02)
3.59 (0.05)
45
1.28 (0.01)
2.24 (0.03)
3.05 (0.04)
3.68 (0.02)
4.17 (0.06)
50
1.52 (0.08)
2.67 (0.01)
3.53 (0.03)
4.27 (0.01)
4.84 (0.05)
35
0.63 (0.05)
1.10 (0.06)
1.58 (0.03)
1.99 (0.02)
2.17 (0.03)
40
0.81 (0.02)
1.48 (0.05)
2.02 (0.04)
2.42 (0.01)
2.72 (0.06)
45
0.99 (0.06)
1.77 (0.02)
2.43 (0.03)
2.92 (0.02)
3.25 (0.05)
50
1.25 (0.01)
2.18 (0.03)
2.98 (0.01)
3.64 (0.03)
4.05 (0.03)
35
0.54 (0.03)
0.92 (0.02)
1.22 (0.02)
1.48 (0.05)
1.66 (0.01)
40
0.71 (0.03)
1.22 (0.07)
1.60 (0.03)
1.91 (0.02)
2.03 (0.02)
45
0.93 (0.04)
1.52 (0.01)
2.02 (0.01)
2.42 (0.03)
2.58 (0.02)
50
1.14 (0.06)
1.92 (0.03)
2.46 (0.02)
2.86 (0.01)
3.14 (0.04)
Standard deviations are given in parentheses a
L1H ? dimethylglyoxime; L2H ? 1,2-cyclohexanedione dioxime; L3H ? a-furil dioxime
3.1 Calculation of the k1 Value The rate constant for this path was calculated from the absorbance data using the Weyh and Hamm equation as discussed earlier [34]. A similar procedure was applied for each L1H concentration in the 1.0 9 10-3 to 5.0 9 10-3 moldm-3 range using the experimental conditions specified in Table 2. The k1(obs) values are collected in Table 2. The rate increases with increase in [LH] and reaches a limiting value (Fig. 7). The limiting rate is probably due to the completion of outer sphere association complex formation. Since the metal ion reacts with its immediate environment, further changes in [L1H] beyond the saturation point will not affect the reaction rate. The outer sphere association complex may be stabilized through H-bonding. Based on the experimental findings, the following Scheme 2 may be proposed for the path (A) ? (B) (k1 path): The k1 and KE values were calculated (as discussed in Ref. [34]) from the intercept and slope (Fig. 8) and are collected in Table 3. 3.2 Calculation of k2 for B ? C The B ? C step is an intramolecular ring closure and is independent of ligand concentration.
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D
5.0 4.5
C
4.0
B
3
-1
10 k1(obs) (s )
3.5
A
3.0 2.5 2.0 1.5 1.0 0.5 0.0 0
1
2 3
3 1
4
5
-3
10 [L H] (mol. dm ) Fig. 7 Plots of k1(obs) versus [L1H] at different temperature: A = 35, B = 40, C = 45 and D = 50 °C
KE
(A) + (LH) (A) · (LH)
k1
(A) · (LH)
Outer sphere association complex
(B)
Scheme 2 Possible reaction sequences for the step A ? B
At any particular temperature the slopes of ln(At - A?) versus time plots at different ligand concentrations were found to be constant in the region where the plot is linear (Fig. 5). For different temperatures, the k2 values are obtained directly from the limiting slopes and are collected in Table 3. Based on the experimental findings, a two-step interchange associative mechanism is proposed for the substitution process. In the first path, an outer sphere association complex is formed between the ligand and the two ruthenium(II) centers, which is stabilized by H-bonding between the incoming ligand and the coordinated aqua molecules. Next the interchange of the ligand from the outer sphere to the inner sphere occurs. 3.3 Effect of Temperature Four different temperatures were chosen for study and the activation parameters for the steps A ? B and B ? C were evaluated from the linear Eyring plots (Figs. 9, 10) and are collected in Table 4. The low DH= values are in support of the ligand participation in the transition state for both of the steps. The energy required to break the departing metal–ligand bond is partly compensated for by the formation of a metal–incoming ligand bond and a lower value of DH= is observed. The high negative DS= values suggest a more compact transition state, where both the incoming and departing ligands are attached in the transition state, and this
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879
A 1.1 1.0
B
0.9
C
1/103k1(obs) (s)
0.8
D
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0.2
0.4
0.6 3
1
0.8 3
1.0
-1
1/10 [L H] (dm mol ) Fig. 8 Plots of 1/k1(obs) against 1/[L1H]: A = 35, B = 40, C = 45 and D = 50 °C Table 3 The k1, k2, and KE values for the substitution reaction between ligands and complex (1) Ligandsa L1H
L2H
L3H
Temp. (°C)
103 k1 (s-1)
KE (dm3mol-1)
104 k2 (s-1)
35
7.32 (0.007)
145
1.36 (0.008)
40
8.44 (0.002)
150
1.78 (0.002)
45
9.57 (0.002)
154
2.28 (0.005)
50
10.58 (0.002)
168
2.68 (0.003)
35
5.96 (0.020)
118
1.15 (0.002)
40
7.12 (0.009)
129
1.56 (0.002)
45
8.17 (0.006)
138
2.08 (0.001)
50
9.40 (0.003)
153
2.53 (0.003)
35
3.39 (0.010)
189
0.98 (0.004)
40
4.05 (0.010)
215
1.37 (0.005)
45
4.80 (0.010)
240
1.88 (0.003)
50
5.67 (0.002)
253
2.30 (0.001)
Standard deviations are given in parentheses a
L1H ? dimethylglyoxime; L2H ? 1,2-cyclohexanedione dioxime; L3H ? a-furil dioxime
also supports the assumption of a ligand participating in the transition state. The following -1 (for L1H), 88.6, 89.7, 90.8, 91.8 kJmol-1 (for L2H), DG= 1 [88.0, 89.2, 90.3, 91.4 kJmol -1 -1 90.0, 91.0, 92.0, 93.1 kJmol (for L3H)] and DG= 2 [98.1, 99.2, 100.2, 101.2 kJmol 1 -1 2 (for L H), 98.7, 99.6, 100.5, 101.5 kJmol (for L H), 99.1, 100.0, 100.8, 101.7 kJmol-1 (for L3H)] values also were calculated for both of the steps at all temperatures studied (35, 40, 45 and 50 °C respectively).
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ln(k1h/KBT)
-34.20 -34.25 -34.30 -34.35 -34.40 -34.45 3.08
3.10
3.12
3.14
3.16
3.18
3
-1
3.20
3.22
3.24
3.26
3.24
3.26
10 /T (K ) Fig. 9 Eyring plot {ln (k1h/kBT) versus 1/T} for the step A ? B (For L1H)
-37.7 -37.8
ln(k2h/K BT)
-37.9 -38.0 -38.1 -38.2 -38.3 -38.4 3.08
3.10
3.12
3.14
3.16
3.18
3.20
3.22
103/T(K -1) Fig. 10 Eyring plot {ln (k2h/kBT) versus 1/T} for the step B ? C (For L1H)
3.4 Effect of Substituent in the Ligand Frame The substituent effect was studied by successive replacement of methyl groups in L1H by a cyclohexane ring in L2H and two furan rings in L3H. The rate constant values were found
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Table 4 Activation parameters for [complex 1] by ligands in aqueous medium, pH 7.4 Ligandsa
-1 DH= 1 (kJmol )
-1 -1 DS= 1 (JK mol )
-1 DH= 2 (kJmol )
-1 -1 DS= 2 (JK mol )
L1H
17.8 ± 0.2
-228 ± 1
35.3 ± 0.8
-204 ± 3
L2H
22.4 ± 0.9
-215 ± 3
41.4 ± 0.9
-186 ± 3
L3H
25.9 ± 1.4
-208 ± 4
45.5 ± 1.2
-174 ± 4
a
1
2
3
L H ? dimethylglyoxime; L H ? 1,2-cyclohexanedione dioxime; L H ? a-furil dioxime
Table 5 The 103 k1(obs) and 105 k2(obs) values at different pHs: [complex 1] = 1.0 9 10-4 moldm-3, [LH] = 3.0 9 10-3 moldm-3, temperature = 50 °C Ligandsa
pH
103 k1(obs) (s-1)
L1H
5.5
0.50
5.00
6.0
0.56
6.67
6.5
0.63
8.33
7.0
1.06
12.06
7.4
3.53
26.80
5.5
0.45
4.80
6.0
0.51
6.06
6.5
0.57
6.67
7.0
0.75
11.10
7.4
2.98
25.30
5.5
0.42
1.90
6.0
0.49
2.27
6.5
0.56
3.70
7.0
0.70
9.09
7.4
2.46
23.00
L2H
L3H
a
105 k2(obs) (s-1)
L1H ? dimethylglyoxime; L2H ? 1,2-cyclohexanedione dioxime; L3H ? a-furil dioxime
to decrease from L1H to L3H through L2H. The relative kmax positions of the LH substituted products are 537, 538.5, 543 nm for L1H, L2H and L3H, respectively (Fig. 1), in aqueous medium. A sharp, lower energy shift of the kmax position in going from complexed L1H to L3H through L2H reflects the trend in weak donicity. The rate constant values are a reflection of the joint effects of electronic and steric factors. For an associative activation process, with increase in size of the incoming ligand the activated state become more crowded and, as a result, a decrease in rate with increased crowding is observed. This observation also is supportive of a ligand-assisted mechanism. 3.5 Effect of pH Variation The reaction was studied at four different pH values. The kobs values are found to increase with increase in pH in the studied pH range (pH 5.5–7.4). The kobs values are collected in Table 5. With increase in pH the complex changes its form from aqua to hydroxoaqua and then to the oxo-bridged dimer. The hydroxo species is more reactive due to the well-known labilizing effect of the –OH group via its p-bonding ability and strong electromeric effect
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2+
O (tap)2 Ru
Ru (tap)2
H2O
2+
O N
OH
KE
(tap)2 Ru
R
+
N
OH2
OH
H
LH
Complex (1)
O H
H
Ru (tap)2
O
H
O .. N
.. N
H
H O
R Outer sphere complex k1, -H2O
+
O (tap)2 Ru H3O+
O
k2
Ru (tap)2
+
(tap)2 Ru
Chelation
N
N
OH
H2O HO
Ru (tap)2 .. N
N
OH
R
R (2)
R
2+
O
= R
for dimethylglyoxime (L1H)
=
for 1, 2 -cyclohexane dionedioxime (L2H)
O R
=
O
for alfa- furil dioxime (L3H)
Fig. 11 Plausible mechanism for the substitution of aqua ligands from [(H2O)(tap)2RuORu(tap)2(H2O)]2? by vicinal dioximes
and, as the percentage of the dimer is increased, the dimer with two metal centers will welcome the ligand better.
4 Mechanism and Conclusion The interaction of dimethylglyoxime (L1H), 1,2-cyclohexane dionedioxime (L2H), and afuril dioxime (L3H) with the title ruthenium complex proceeds via two distinct consecutive
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steps (k1 * 10-3 s-1 and k2 * 10-4 s-1). The first path proceeds via an associative interchange activation mechanism and the second step is the ring closure. At the outset of the first path an outer sphere association complex results; this is stabilized through Hbonding and is followed by an interchange from the outer sphere to the inner sphere complex. The outer sphere association equilibrium constants, a measure of the extent of Hbonding for each path at different temperatures, were evaluated (Tables 2 and 3). From the temperature dependence of the KE the thermodynamic parameters are calculated to be: DH1 = 7.5 ± 2.0 kJmol-1, DS1 = 66 ± 6 JK-1mol-1 (for L1H), DH1 = 13.9 ± -1 -1 -1 2 (for L H) and DH1 = 16.4 ± 1.2 kJmol-1, 1.4 kJmol , DS1 = 85 ± 4 JK mol -1 -1 3 DS1 = 97 ± 4 JK mol (for L H). The DG1 [-12.8, -13.2, -13.5, -13.8 kJmol-1 1 (for L H), -12.3, -12.7, -13.1, -13.6 kJmol-1 (for L2H), -13.5, -14.0, -14.4, -14.9 kJmol-1 (for L3H)] values, calculated for both steps at all temperatures studied (35, 40, 45 and 50 °C respectively), have a negative magnitude which once again favors the spontaneous formation of an outer sphere association complex. The formation of a seven membered structure of the product occurs by the coordination of two N atoms of LH with the two Ru(II) centers. Now one N atom first attacks one of the ruthenium(II) centers by the removal of one H3O?, i.e., it follow the k1 path and another N atom of the LH finishes the ring closing process. From a comparison of the ligands used, it can be concluded that the variation in size, bulkiness and electronic effect of the entering vic-dioximes reflect their properties as nucleophiles. The differences in nucleophilicity of the ligands are obvious and their reactivity follows the order: L3H \ L2H \ L1H. The sensitivity of the reaction rate towards donor properties of the entering ligands are in the line with that expected for an associative mode of activation. Due to the higher steric effect the reactivity of the ligand L3H decreases, which is reflected in the rate constant values. A plausible mechanism is shown in Fig. 11.
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