been determined in all the cases, and increase considerably with the increase in concentration of MeOH or 1,4- dioxane in the binary solvent mixtures under ...
Transition Metal Chemistry 27: 52–57, 2002. 2002 Kluwer Academic Publishers. Printed in the Netherlands.
52
Solvent and chelation effects on the photoreduction of cobalt(III)–amine complexes in aqueous–organic solvent media Gopalakrishnan Karthikeyan*, Krishnamoorthy Anbalagan and Kuppanagounder P. Elango Department of Chemistry, Gandhigram Rural Institute (Deemed University), Gandhigram 624 302, India Received 16 January 2001; accepted 07 February 2001
Abstract The photoreduction of trans-[Co(NH3)4Cl2]+, trans-[Co(en)2Cl2]+, [Co(dien)Cl3], [Co(trien)Cl2]+, and [Co(tetren)Cl]2+, ions has been studied using a low pressure Hg vapour lamp as light source (254 nm) in aqueous–organic solvents [0–30% (v/v) MeOH or 1,4-dioxane]. Quantum yields for CoII production by redox decomposition have been determined in all the cases, and increase considerably with the increase in concentration of MeOH or 1,4dioxane in the binary solvent mixtures under investigation. A plot of log(quantum yield) versus the Grunwald– Winstein Parameter, Y, which is a measure of solvent ionizing power, shows that a different blend of general and specific solvent interacts with the solute. This kind of specific solvent interaction on the reactant/excited state has been analysed using multiple regression: viz. Krygowski–Fawcett and Kamlet–Taft equations. Reasons for the difference in reactivity with chelation are also discussed.
Introduction Cobalt(III)–amine complexes are known to undergo photoredox reactions upon excitation of charge-transfer bands, leading to the formation of cobalt(II) species in aqueous solutions [1]. Charge-transfer (c.t.) excited states of transition metal complexes arise from the radial movement of electron density between metal and ligands (or solvent), and this charge redistribution should be inherently sensitive to changes in the solvent. The commonly observed solvatochromism of c.t. absorption and luminescence bands, for example, results from dielectric and/or hydrogen-bonding interactions between the complex and the solvent. That is, the environmental influence on the photoredox chemistry of these complexes is appreciable [2]. There is also sustained interest in the study of redox reactions of cobalt(III) complexes in mixed-solvent media because correlation of reaction rates with various solvent parameters can provide important mechanistic information. The importance of non-specific and specific solvent–solvent–solute interactions on the kinetics and energetics of the redox reactions of cobalt(III) complexes has been emphasized in our recent studies [3–7]. Recently, we studied the photoreduction of some [Co(en)2Cl(R-aniline)]2+ ions in mixed solvent media using methanol and 1,4-dioxane as co-solvents [8]. It was evident that solvation of the initial state and the excited state was strongly influenced by the added cosolvent. The present study is an extension of our
* Author for correspondence
previous investigations. Here we have investigated the photoreduction of a series of cobalt(III)–amine chelates in aquo–organic solvent media in order to establish the possible mechanistic pathways involved.
Experimental The complexes were prepared according to the standard literature procedures: trans-[Co(NH3)4Cl2]Cl [9], trans[Co(en)2Cl2]Cl [10], [Co(dien)Cl3] [10, 11], [Co(trien)Cl2]Cl [12] and [Co(tetren)Cl]Cl2 [13], where en ¼ 1,2-diaminoethane, dien ¼ diethylenetriamine, trien ¼ triethylenetetramine and tetren ¼ tetraethylenepentamine. Analytical quality reagents were generally used. Solutions for photolysis contained the cobalt(III) complex (4 · 10)3 M ), and NaNO3 (0.1 M ). All solutions prepared contained binary solvents of varying compositions: MeOH or 1,4-dioxane in H2O [0–30% (v/v) of co-solvent]. Steady photolysis experiments were carried out using a low pressure mercury vapour pen-ray quartz lamp (254 nm). Air-equilibrated solutions were used for photolysis and the temperature control was maintained at 25 ± 1 C. For quantum yield determinations photolyses were carried out to less than ca. 15% of the total reaction. The incident light intensities were measured by potassium ferrioxalate actinometry [14]. Quantum yields were calculated estimating cobalt(II) formed by Kitson’s method [15]. All absorption measurements were carried out using a Shimadzu u.v.– vis. double beam spectrophotometer. Correlation analysis were made using Microcal origin (version 3.5) computer software. The goodness of fit was
53 established using the correlation coefficient (r), coefficient of multiple determination (R2), standard deviation (sd) and Exner’s statistical parameter (w). The relative importance (on a percentage scale) of different solvation effects were analysed using various empirical solvent parameters. The percentage contribution of a parameter to the total effect of reactivities was determined using Equations (1) and (2) [16]. To calculate this value, the regression coefficient of each parameter is statistically quantified as follows. Y ¼ a0 þ a 1 X1 þ a2 X2 þ þ a n Xn
ð1Þ
jai j100 Pxi ¼ Pn i¼1 jai j
ð2Þ
Results and discussion The quantum yields, measured at 254 nm, for the photoreduction of all the cobalt(III)–amine complexes in various water–methanol and water-1,4-dioxane mixtures [0, 5, 10, 15, 20, 25 and 30% (v/v) organic cosolvent], are presented in Table 1. In both solvent mixtures investigated, UCoII increased as the mole fraction of the organic co-solvent increased. This may be due to reduction of the metal centre by the ligand and to solvent reduction (solvent-to-metal charge-transfer). This argument is in line with that suggested by Kutal et al., for the photoreduction of cobalt(III)–am(m)ine complexes [2]. Therefore, an attempt was made to study quantitatively the solvent effect on the photoreduction quantum yields of the cobalt(III)–amine complexes using a linear and multiple regression analysis. Variation of quantum yield with solvent composition Solvent parameters in linear free-energy relations, such as the Grunwald–Winstein equation [17], depend upon macroscopic solvation of the reaction centre. The
correlation of log UCoII with the Grunwald–Winstein solvent ionizing power, Y, parameters (Equation 3) in both the solvent mixtures studied showed satisfactory results. log k ¼ log k0 þ mY
ð3Þ
The negative m value (Table 2) indicates a transition state which is less polar than the reactant. Such a transition state will more easily be attained in a medium of lower ionizing power and, hence, the increase in quantum yield with increase in the amount of organic co-solvent added. When the log of quantum yields of a cobalt(III) complex in the two binary mixtures of solvents studied are correlated, using Equation (3), the phenomenon of dispersion is observed, i.e., lines of different slopes (Figure 1) are observed for water– methanol and water–dioxane systems. The occurrence of dispersion phenomenon is due to the fact that a different blend of non-specific and specific solvent influences interact with the solute for each solvent pair
Table 2. Photoreduction of CoIII–amine complexes in aquo–organic solvent mixtures. Statistical results of Grunwald–Winstein plot Complex*
r
sd
w
m
H2O–MeOH (1) (2) (3) (4) (5)
mixtures 0.990 0.978 0.979 0.961 0.998
0.007 0.038 0.053 0.045 0.011
0.17 0.25 0.25 0.32 0.07
)0.164 )0.574 )0.577 )0.545 )0.563
0.023 0.036 0.042 0.082 0.063
0.29 0.39 0.37 0.25 0.31
)0.215 )0.253 )0.339 )0.913 )0.537
H2O–1,4-dioxane mixtures (1) 0.969 (2) 0.945 (3) 0.951 (4) 0.977 (5) 0.962
* As in Table 1. r – correlation coefficient, sd – standard deviation, w – Exner’s statistical parameter and m – slope of Grunwald–Winstein plot.
Table 1. Quantum yields (102 F) for the photoreduction of CoIII–amine complexes in airequilibrated water–organic solvent mixtures at 25 ± 1 C, concentration of CoIII complex 4 · 10)3 M and ionic strength 0.1 M NaNO3. Irradiation wavelength 254 nm Complex
Co-solvent % (v/v) 0 5 10
15
20
25
30
H2O–MeOH mixtures (1) [Co(dien)Cl3] (2) [Co(NH3)4Cl2]Cl (3) [Co(trien)Cl2]Cl (4) [Co(en)2Cl2]Cl (5) [Co(tetren)Cl]Cl2
3.42 1.27 0.44 0.27 0.23
4.29 1.68 0.72 0.76 0.49
4.43 2.04 0.86 1.00 0.59
4.84 2.50 1.13 1.30 0.70
4.98 2.94 1.39 1.52 0.82
5.20 3.34 1.84 1.61 1.02
5.56 3.70 2.06 1.86 1.18
H2O–1,4-dioxane mixtures (1) [Co(dien)Cl3] (2) [Co(NH3)4Cl2]Cl (3) [Co(trien)Cl2]Cl (4) [Co(en)2Cl2]Cl (5) [Co(tetren)Cl]Cl2
3.42 1.27 0.44 0.27 0.23
3.81 1.55 0.86 0.29 0.29
4.07 1.81 1.14 0.51 0.45
4.53 2.06 1.39 0.89 0.54
5.31 2.13 1.53 1.24 0.64
5.40 2.24 1.63 1.62 0.74
5.61 2.48 1.87 2.02 0.85
54 Krygowski and Fawcett [21], proposed two important solvent scales that can affect the reactivity of a solute. The interaction parallels the Lewis acidity and Lewis basicity character of solvents symbolised by ET and DN respectively. In terms of these dual independent but complementary vectors, the linear free-energy relationship can be represented as in Equation (4). Q ¼ Q0 þ a ET þ b DN
Fig. 1. Plot of log UCoII versus Y for the photoreduction of [Co(tetren)Cl]2+ ion in H2O–MeOH (s) and H2O–dioxane (d) mixtures showing dispersion phenomenon.
[18]. Furthermore, close similarity in m values suggests that a similar mechanism is operating in the series of cobalt(III) complexes studied. The above results parallel the correlation of log UCoII versus the inverse of relative permittivity, er (Laidler and Erying equation [19]), which is also satisfactory (0.979 ‡ r ‡ 0.933, 0.093 ‡ sd ‡ 0.028, straight line with positive slope) in both solvent systems. The satisfactory correlation of quantum yield with Y and e1 may be r related to a solvation phenomenon. The solvent-effect equations above involve only one solvent vector, the solvent ionizing power, Y, or relative permittivity, er, which appears to be insufficient for the linear free-energy correlation in the present context. Thus, the approach by idealized theories is often inadequate since these theories regard solvents as a nonstructured continuum, not composed of individual solvent molecules with their own solvent–solvent interactions. Furthermore, they do not take into account specific solute–solvent interactions, such as hydrogenbonding and electron pair donor–electron pair acceptor interactions, which often play a dominant role in solute– solvent interactions. No single macroscopic physical parameter can possibly account for the multitude of solute–solvent interactions on the molecular microscopic level. Hence satisfactory quantitative descriptions of solvent effects have to take into account all nonspecific and specific solvent–solvent–solute interactions. The separation of solvent polarity into non-specific and specific solvent–solvent–solute interaction mechanisms is purely formal, but, if this separation can be reasonably done, the resulting parameters may be used to interpret solvent effects through such multiple correlations, thus providing information about the type and magnitude of interactions with the solvent [20].
ð4Þ
where ET is Reichardt’s [20] solvatochromic parameter which provides an excellent and very sensitive characterization of the micropolarity of the solvation shell on the molecular microscopic level of solvents, whereas DN is the donor number (or donicity) proposed by Gutmann [22] which is an empirical semiquantitative measure of the nucleophilic properties of the electron pair donor (EPD) solvent, Q0 is the value of the solventdependent physicochemical property of the solute under investigation in the gas phase (or in an inert solvent) and a and b are the regression coefficients describing the relative sensitivity of the solute property Q to the Lewis acidic and Lewis basic solvent properties, respectively. For use in multiparameter correlation equations, the dimensionless normalized ETN and DNN values seem to be more suitable [23] and these values for the present study were calculated as described earlier [8]. All the cobalt(III) complexes studied show an excellent correlation (0.999 ‡ R2 ‡ 0.952, 0.075 ‡ sd ‡ 0.007, 0.25 ‡ w ‡ 0.04) according to Equation (4), in which, the solute property is given in log UCoII values in varying solvent compositions, while Q0 represents the log UCoII value in the reference solvent. Such an excellent correlation indicates the existence of specific local solute–solvent interactions. From the magnitude of the coefficients of the terms ETN and DNN the percentage contributions of the parameters on the reactivity were calculated [24] and are given in Table 3. In both solvent systems studied, the sign of the coefficient of the DNN term is positive, indicating that specific interactions between the excited state and the solvent, are more than the reactant–solvent interactions [24]. The results (Table 3) also indicate that the contribution of the donor number, DNN (Lewis basicity), to reactivity is greater in aqueous methanol than in aqueous dioxane mixtures. Thus, in water–methanol mixtures, reduction of the cobalt(III) complexes by the solvent molecules is more favourable than in water–dioxane mixtures, resulting in higher quantum yields. This is due to the predominance of solvent-to-metal charge-transfer bands [2]. An attempt was also made to correlate the quantum yield data with Richardt’s solvatochromic Lewis acidity parameter, ETN , and Kamlet–Taft’s [25] Lewis basicity parameter, b, in the form of Equation (5). Q ¼ Q0 þ a ETN þ b b
ð5Þ
For all the cobalt(III) complexes studied, in both the aquo–organic solvent mixtures, a good correlation exists
55 Table 3. Photoreduction of CoIII–amine complexes in water–organic solvent mixtures. Multiple correlation analysis of log UCoII against N Krygowski–Fawcett parameters EN and EN T DN T b and their weighted percentage contributions Percentage contributions (Pxi)b
Complexesa (1) (2)
(3)
(4)
(5)
H2O–MeOH mixtures Results of Equation (4) PETN PDNN
17 83
22 78
25 75
14 86
17 83
Results of Equation (5) PETN Pb
53 47
61 39
65 35
48 52
53 47
H2O–1,4-dioxane mixtures Results of Equation (4) PETN 79 PDNN 21
71 29
72 28
60 40
79 21
Results of Equation (5) PETN Pb
12 88
10 90
20 80
13 87
a
15 85
As in Table 1; b as calcd. using Equations (1) and (2).
between log UCoII with ETN and b (0.999 ‡ R2 ‡ 0.954, 0.076 ‡ sd ‡ 0.005, 0.25 ‡ w ‡ 0.04). The percentage contributions of these solvent vectors are also given in Table 3. In both solvent mixtures the signs of the coefficients of these two terms are negative, indicating that the reactant is better solvated than the excited state. It is evident, from the percentage contribution values, that in water–methanol mixtures the specific interaction between the excited state and the solvent is more through solvent Lewis basicity, b, interactions than that of Lewis acidity. Since methanol is a hydrogen bond donor (HBD) as well as a hydrogen bond acceptor (HBA) solvent, it stabilizes the excited state through specific solvation, resulting in higher quantum yields. Since dioxane, however, is a HBA solvent, the specific interaction between the excited state and the solvent through Lewis basicity interactions is less, and the excited state is solvated to a lesser extent, resulting in fall of quantum yields. Considering the correlation between Lewis acidity and Lewis basicity with quantum yield data, the influence of specific solvation on reactivity is more pronounced. Therefore, it seems reasonable to use the polarizability of the mixture as an adequate solvent parameter to consider the influence of non-specific solvation on reactivity. Hence, the photoreduction quantum yield values were also fitted to another expression, namely a solvatochromic comparision method developed by Kamlet and Taft to quantify and rationalize multiple interacting solvent effects on reactivity [25]. The quantum yield data were subjected to correlation analysis with the solvatochromic parameters a, b and p* in the form of a linear solvation energy relationship (LSER) as given in Equation (6). log k ¼ A0 þ s p þ a a þ b b
ð6Þ
where p* is an index of solvent polarity/polarizability which measures the ability of the solvent to stabilize a charge, or a dipole by virtue of its dielectric effect, a is the solvent HBD acidity which describes the ability of the solvent to donate a proton in a solvent to solute hydrogen bond, b is the solvent HBA basicity which provides a measure of the solvent’s ability to accept a proton (or donate an electron pair) in a solute to solvent hydrogen bond and A0 is the regression value of the solute property in the reference solvent cyclohexane. The regression coefficients s, a and b measure the relative susceptibilities of the solvent-dependent solute property, log k, to the indicated solvent parameter. These solvatochromic parameters for the two solvent mixtures used in the present study were calculated as described in the literature [3, 5].The log UCoII values for photoreduction of cobalt(III) complexes in different water– methanol and water–dioxane mixtures show an excellent correlation (0.999 ‡ R2 ‡ 0.995, 0.776 ‡ sd ‡ 0.011, 0.09 ‡ w ‡ 0.04). From the values of the regression coefficients, the contributions of each parameter on a percentage basis were calculated [24] and are listed in Table 4. The results of the systematic multiple regression analysis indicate that, in water–methanol mixtures the overall (specific and non-specific) interactions between the excited state and the solvent is more (as indicated by the positive signs of the coefficients of p* and b terms) than in water–dioxane mixtures, resulting in higher quantum values. The specific interactions between the excited state and the solvent is also more dominant in water–methanol mixtures, than in water– dioxane mixtures, as indicated by the positive sign of the b term. Since methanol is an amphiprotic solvent, its specific interactions with the excited state through HBA and HBD properties is more, resulting in higher quantum yields. Whereas, dioxane is a typical HBA solvent, its specific interaction with the excited state is relatively less, resulting in lower quantum yields. Further, in water–dioxane mixtures the non-specific long-range interactions between the excited state and the solvent alone, dominate as indicated by the positive sign Table 4. Photoreduction of CoIII–amine complexes in aquo–organic solvent mixtures. Multiple correlation analysis of log UCoII against Kamlet–Taft’s solvatochromic parameters and their weighted percentage contributions Percentage contributions (Pxi)b
Complexesa (1) (2)
(3)
(4)
(5)
H2O–MeOH mixturesc Pa Pb Pp
40 19 41
39 25 36
39 23 38
40 20 40
39 24 37
H2O–1,4-dioxane mixturesd Pa 22 Pb 46 32 Pp
23 46 31
23 45 32
22 46 32
23 45 32
a As in Table 1; b as calcd. using Equations (1) and (2); c sign of coefficient of a is negative and that of b and p* is positive; d sign of coefficients a and b is negative and that of p* is positive.
56 of the p* term. Absence of specific solvent-excited state interactions in water–dioxane mixtures may be the cause of the observed lower quantum yield values in the mixture. A dynamic exchange of solvent molecules exists between the solvation shell of the excited state and the bulk [26]. As the organic solvent concentration increases in the mixture, more and more organic molecules are introduced into the solvation shell, thereby increasing the hydrophobic environment of the excited state, and consequently lowering the energy of the ligand-to-metal charge-transfer (LMCT) band [2]. Increase in hydrophobicity stabilizes the excited state (which is less polar than the reactant) through specific solute–solvent interactions and consequently increases the reduction quantum yields as the organic solvent proportion in the mixture increases. Plots of log UCoII versus mole-fraction of the co-solvents, x2, show separate lines for water– methanol and water–dioxane mixtures (Figure 2). The difference in slopes between the two lines is a measure of solvent participation in the reduction process. Therefore, it is presumed that solvent reduction is more predominant in water–methanol mixtures, where there exist both specific and non-specific excited state-solvent interactions, than in water–dioxane mixtures, where such interactions are less pronounced. Variation of quantum yield with chelation The results in Table 1 indicate that the UCoII value decreases from [Co(dien)Cl3] to [Co(tetren)Cl]2+ and that this trend is observed both in water–methanol and water–dioxane mixtures. The observed variation may be due to a chelation effect in addition to other effects, such
Fig. 2. log UCoII versus mole fraction of co-solvent, x2, for the photoreduction of trans-[Co(NH3)4Cl2]+ ion in H2O–MeOH (s) and H2O–dioxane (d) mixtures.
as the number of oxidisable ligands and the number of acid hydrogens present in the coordination sphere. Therefore variations in quantum yields have been analysed in terms of structural effects, in addition to the solvent effects discussed in the foregoing section. Table 1 shows that there is an overall decrease in quantum yield with a decrease in the number of chloride ligands in the coordination sphere, that complex (1) shows the higher quantum yield value, while complex ions (2), (3) and (4) show moderatley high values, and complex (5) shows the relatively lower value. This may be due to the fact that with a decrease in the number of chloride ligands, there will be less reduction of metal center by LMCT transitions. The second factor, which is expected to alter the UCoII of the reaction of these complexes, is that of chelation. It is evident from the data (Table 1) that the quantum yields for a series of complex ions, which contain two chloride ligands [(2) and (3, 4)], decreases from the ammine complex to the chelating complex ion. This may be due to the fact that chelation hinders the stability of the excited state and, hence, leads to a decrease in the quantum yields. Finally, changes in the effective distribution of positive charge are expected to alter the UCoII of the reactions. Based on the arguments of Pearson et al. [13] it is believed that the positive charge on the complex ions under investigation is primarily transferred to the acid hydrogens (NAH) and this transfer is enhanced by an increasing number of such hydrogens. There is a considerable reduction in positive charge density on the metal centre on excitation. This decrease is expected to take place more readily with complexes that can less effectively distribute the additional charge, i.e., a complex ion having less acid hydrogens. Since the decrease in positive charge is most readily accommodated by the complex with the least number of acid hydrogens, it is believed that this would result in greater stabilization of the excited state and, consequently, a more rapid reaction. In agreement with the above three factors complex ion (1), with a lower number of acid hydrogens and greater number of chloride ligand shows the highest quantum yield value among the series investigated. Complex ions (3) and (4) have almost the same number of acid hydrogens and same number of chloride ligands exhibit lower quantum yield values. Similarly, in the case of complex ions (3) and (5), though they have nearly equivalent numbers of acid hydrogens, there is a less probable reaction due to an LMCT transition in the case of complex (5); in addition, more strain exists due to the increase in the size of chelation, which leads to the lower UCoII value of the [Co(tetren)Cl]2+ in the series. To conclude, linear and multiple regression analyses may be used to separate and quantify the specific and non-specific solvational effects on the photoreduction reaction of cobalt(III) complexes. Using the above methods the significance of those solvent–solvent–solute interactions on the reactivity has been established.
57 Furthermore, quantum yield values of the title reaction were found to depend on the number of oxidisable ligands and charge distribution, in addition to chelation effects.
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TMCH 5012
