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Study of π interaction in metal-schiff base complexes
£ inorg, nucl. Chem. VoL 43, pp. 331-333 © Pergamon Press Ltd.. 1981. Printed in Great Britain
0022-1902/8110201-0331150200t0
STUDY OF :r INTERACTION IN METAL-SCHIFF BASE COMPLEXES RAKESH K. KOHLI, K. GOPALAKRISHNANand P, K. BHATTACHARYA* Chemistry Department, Faculty of Science, M. S. University of Baroda, Baroda 390 002, India
( F'irst received 1 September 1978: received fl)r publication 10 March 1980) Abstract--The formation constants of mixed-ligandcomplexes MAL: where M = Cu(II), A = dipyridyl and L : salicylaldehyde,2-hydroxy-l-naphthaldehyde,2-hydroxyacetophenoneor 2-hydroxy-propiophenoneor Schiff bases N'-phenyl-salicylaldimine, N'-phenyl-2-hydroxy-l-naphthaldimine, N-methyl-2-hydroxy-l-naphthaldimine or 2hydroxy-l-naphthaldimine; have been investigated. It is observed that the value of log KMA MAL iS either, log KMAAL>log KMML:log KMAL~Iog MA KML or log KMAc
RESULTS AND DISCUSSIONS
Study of ternary systems M(dipy)L (L= ~ bonding ligand) has shown that I').VMMt.-,x~rM(dipY)M(di(AK) py)L is very small. This has been attributed to M-~N ~ interaction in [M(dipy)] 2+ complexes[l-6]. In cases where the secondary ligand is also 7r bonding AK is negative or zero as observed in Cu(dipy) catechol[3,7] and Cu(dipy) acac[8] systems, respectively. Similar M ~ L 7r interaction can be envisaged in transition metal complexes of ortho hydroxy aromatic aldehydes, ketones or their Schiff bases. With this point in view the present study of ternary complexes M(dipy)L has been carried out where M = Cu(II), L = Salicylaldehyde, 2-hydroxy- l-naphthaldehyde, 2-hydroxy-acetophenone or 2-hydroxy-propiophenone or N-phenyl-salicylaldimine, 2-hydroxy- 1-naphthaldimine, N-methyl-2-hydroxy-l-naphthaldimine or N-phenyl-2hydroxy-l-naphthaldimine. Studies were not possible for salicylaldiminato, and N-methyl salicylaldiminato ligands as they could not be obtained in pure form. An extension of Irving-Rossotti titration technique has been used [9,10].
It is known that the [Cu(dipy)] 2+ complex formation is complete at lower pH and the complex is stable at higher pH (Fig. 1). The M-dipy curve (3) diverges from the acid curve (1) at pH 6.0, indicating that the formation of hydroxo complex [M(dipy) (OH)2] starts only at higher pH. The curve 5(M + A + L) remains almost merged in the beginning with curve 4(L) indicating that the complexation with secondary ligand does not take place at low pH. The curve 5 diverges from 4 at higher pH (3) showing that M + d i p y + L combination takes place, where, M + dipy 1 : 1 complex formation is complete. In this range hydroxo complex formation also does not take place. The horizontal distances between curves 4 and 5 (Fig. 1) correspond to extra hydrogen ions liberated '.as a result of combination of secondary ligand with [M(dipy)] 2+. t] and pL were calculated using the equations suggested by Irving and Rossotti [ l l]. The aldehyde and ketone ligands have only one pK. value. This was used for the calculation of formation constants of ternary complexes. The calculations were carried out below pH 6.0, Average value of log kc,tJ~ync"ta~oY' were found out using the method of linear plot. These are found to be higher than the formation constants of the corresponding binary complexes Kcut.. cu Schiff base ligands show two pK. values. At lower pH the ligand curve is above the acid curve because the Schiff base solution contains less number of titratable H + ions: due to absorption of protons by nitrogen of C=N. At higher pH it exhibits lower values of pH than the acid titration curve, showing more number of titratable ions, due to dissociation of H+of O-H. Log KcuAtC'~'__ log KCuL, Cu where L = Schiff base of 2-hydroxy-l-napbthaldebyde with ammonia or methylamine: whereas, log K~A~ < log K~:~,~, where L = Schiff base of salicylaldehyde or 2-bydroxy-l-naphthaldehyde with aniline. The values of formation constants of binary and ternary complexes indicate the possibility of M ~ L :~" interaction in these complexes. In case of Cu-sa]icylaldehyde complex, the log Kc,,t_ c, value is significantly larger than expected from the basicity of this ligand. The pK. value for salicylaldehyde is lower than that of 2-hydroxyacetophenone, yet the difference between the log Kc.~. c,, values of salicylaldehyde and 2-hydroxy acetophenone and 2-hydroxy-propiophenone, is not
EXPERIMENTAL
Materials and methods. The reagents used were of pure grade. Solutions of metal perchlorates in 5(~0 (v]v) dioxan were prepared and standardized. The instruments used, purification of solvent and titrations carried out were similar to those described earlier[8]. The solution in 50%(v/v) dioxan (M = 0.2 M, NaCIO4) were titrated against 0.2M sodium hydroxide solution. Since Schiff bases are prone to undergo hydrolysis, titrations for such systems have been carried out taking excess of ligand solutions (M:L= 1:5). Solutions containing M:L= 1:5 and M:L= 1:2 yield similarresults indicatingthat no hydrolysistakes place at this concentration. One represent:~tivetitration curve has been shown in Fig. I. The values of the formation constants of the binary metalaldehyde or ketone complexes and also metaI-Schiff base complexes are necessary for comparison under identical conditions. Though, such values are available in literature, proton-ligand, and metal-ligand formation constants of metal-aldehyde or ketone complexes and also metaI-Schiff base complexes were determined using lrving-Rossotti titration technique. The values of the formation constants have been recorded in the table.
*Author to whom correspondence should be addressed.
331
332
RAKESH K. KOHLI et aL
5
8 3
6
7 pH
6
5
4
3
2
4D
I
I
50
60
Volume of olkoli in ml.
Fig. 1. Cu(ll) dipy. Salicylaldehyde system-30°C. 1. Acid; 2. Dipy; 3. Cu(II)+dipy(l:l); 4. Salicyaldehyde; 5. Cu(II) + dipy + salicyaldehyde ( 1:1:1) and 6. Cu(II) + Salicylaldehyde ( 1:1). Table 1. Proton and metal-ligand stability constants of secondary ligands at 30°C St. No.
Ligand (LH)
pKIH
pK~H
log K 0u CuL
vCu'dipy log ~Cu.dlpy.L
i. Sallcylaldehyde
9.30
-
7.18
7.93
2. 2-hydroxy-l-naphthaldehTde
8.38
-
7.19
7)94
3 • 2-hTdroxyacetophenone
I0.80
-
7.61
7.83
4. 2-hydroxy-propiophenone
ii.00
-
7.~
7.80
9.67
2.52
9.51
9.51
i0.00
2.79
8.56
8.56
8.19
2.hA
7.84
7.56
9.2~
~.00
8.99
8.~i
~. 2-hyd~oxy-l-naphthaldlmlne 6. N-Methyl-2-hydroxy-lnaphthaldimlne 7. N '- ph enyl-2 -h~nlroxynaphthaldlmlne
8. N)-phenyl salleylaldlmine
much. Another striking feature is that log K c" C,L of Cu-2hydroxy-l-naphthaldehyde complex is almost same as that of Cu-salicylaldehyde, complex though 2-hydroxy-lnaphthaldehyde is more acidic than salicylaldehyde. The formation constant values of these metal complexes indicate a possibility of ~r interaction between the metal d~r orbitals and the p~" of - C = O and phenolate O-. This results in the formation of delocalized ~" electron ring over the metal and the ligand. This imparts a double bond character to the M-L bond and makes the complex more stable. The additional phenyl ring in the case of naphthaldehyde helps in the back donation
because of the ~r electrons being delocalized over two rings. Since the extent of back donation is more, log Cu KC,L value is higher than expected from its basicity alone. In case of ketones basicity is high owing to the electron releasing influence of methyl and ethyl groups. This increases the electron density over C=O and makes it less susceptible to form a ~r bond. This explains, why 2-hydroxy-acetophenone and 2-hydroxy propiophenone, though more basic, do not show much higher values of log KC,L c, than salicylaldehyde or 2-hydroxy-l-naphthaldehyde complexes.
Study of rr interaction in metaI-Schiff base complexes The formation constants of the ternary complexes are higher than the corresponding binary complexes. This is in keeping with the behaviour of oxygen binding secondary ligand complexes [3]. The higher value of log KCc~,l. can be attributed to three factors. First is special behaviour of dipyridyl, i.e. besides N-~ Cu cr bonding, there exists M--, N 7r interaction in dipyridyl complexes. This retains the electronegativity of the metal ion in [Cu(dipy)]2~ same as in [Cu(H20)~] 2÷. Secondly, the distorted octahedron [Cu(H20)6] 2+ will be somewhat more strongly distorted towards the square planar coordination by the coordination of a strongly chelating ligand c~~r-dipyridyl, thus creating the right geometry for the coordination of the secondary ligand resulting in the increased value of log gcuAl~. c~A Besides the above two factors, another plausible operating factor could be the existence of r bonding in the secondary ligands, i.e. between Cu and aromatic aldehyde or ketone. The 7r interactions between M-A and M-L, mutually stabilize each other and this contributes to higher value of log KCuAI,. C~A This effect is expected to be higher for the complexes of Cu-dipy-L, where L--salicylaldehyde or 2-hydroxy-l-naphthaldehyde than where L=2-hydroxyacetophenone or 2hydroxy propiophenone. The binary complex of schiff base 2-hydroxy-l-naphthaldimine shows higher value of log Kcuc cu than the Schiff base complex of N-methyl-2-hydroxy-l-naphthaldimine, though from the basicity consideration the order should have been reverse. This indicates that the extent of 7r bonding outweighs the inherent basicity of the system. The presence of Cu-N ~" bonding in the schiff base complex is also supported by the fact that log Cu(dipy) KCu(dipy)t. is same as log KCuL, cu i.e. AK = 0 when L = 2hydroxy-l-naphthaldimine or N-methyl-2-hydroxy-lnaphthaldimine. As explained above, the mutual stabilization of 7r bonding in the dipyridyl and the Schiff base in the mixed ligand complex makes log KcuAL°'a__ log ell Kc,,L. Such ~- delocalization can take place due to the interaction of metal d r orbital with the p~r orbitals of the
333
ring carbon, azomethine nitrogen and the O- of tl~e phenolate group. The value of AK is positive in the cases where L = N'phenyl-2-hydroxy-l-naphthaldimine or N'-phenyl salicylaldimine. This could probably be due to steric hindrance between dipyridyl and these bulky Schiff base ligands, in the mixed ligand complex. Because of tihe steric interaction, the planarity of the molecule gets distorted, thus reducing the extent of Cu--,N ~r interaction. Hence log KCuALCUA
REFERENCES
I. M. V. Chidambaram and P. K. Bhattacharya, J. Inorg. Nacl. Chem. 32, 3271 (1970). 2. R. L. Datta and De Dhrubendra, J, Indian Chem. Soc. 46, 1 (1%9). 3. R. Greisser and H. Sigel, lnorg. Chem. 9, 1238 (1970). 4. P. R. Huber, R. Greisser, B. Prijs and H. Sigel. Europ. J. Biochem. 10, 238 (1%9). 5. D. H. Busch and J. C. Bailer, J. Am. Chem. Soc. 78, 1137 (1959). 6. B. J. Bathaway, D. E. Billina, R. J. Dudley and P. Nicholls, Z Chem. Soc. 2312 (1%9). 7. G. A. L. Heureux and A. E. Martell, J. lnorg. Nucl. Chem. 28, 481 (1%6).
8. Uma Doraswamy and P. K. Bhattacharya, Indian L Chem. 13, 1069 (1975). 9. H. M. Irving and H. S. Rossotti, J. Chem. Soc. 2904 (1954). 10. F. J. C. Rossotti and H. S. J. Rossotti, Acta. Chem. Scand. 9, 1166 (1955). 11. H. M. Irving and H. S. Rossotti, J. Chem. Soc. 3397 (1953).
0022-1902/8110201-0331150200t0
STUDY OF :r INTERACTION IN METAL-SCHIFF BASE COMPLEXES RAKESH K. KOHLI, K. GOPALAKRISHNANand P, K. BHATTACHARYA* Chemistry Department, Faculty of Science, M. S. University of Baroda, Baroda 390 002, India
( F'irst received 1 September 1978: received fl)r publication 10 March 1980) Abstract--The formation constants of mixed-ligandcomplexes MAL: where M = Cu(II), A = dipyridyl and L : salicylaldehyde,2-hydroxy-l-naphthaldehyde,2-hydroxyacetophenoneor 2-hydroxy-propiophenoneor Schiff bases N'-phenyl-salicylaldimine, N'-phenyl-2-hydroxy-l-naphthaldimine, N-methyl-2-hydroxy-l-naphthaldimine or 2hydroxy-l-naphthaldimine; have been investigated. It is observed that the value of log KMA MAL iS either, log KMAAL>log KMML:log KMAL~Iog MA KML or log KMAc
RESULTS AND DISCUSSIONS
Study of ternary systems M(dipy)L (L= ~ bonding ligand) has shown that I').VMMt.-,x~rM(dipY)M(di(AK) py)L is very small. This has been attributed to M-~N ~ interaction in [M(dipy)] 2+ complexes[l-6]. In cases where the secondary ligand is also 7r bonding AK is negative or zero as observed in Cu(dipy) catechol[3,7] and Cu(dipy) acac[8] systems, respectively. Similar M ~ L 7r interaction can be envisaged in transition metal complexes of ortho hydroxy aromatic aldehydes, ketones or their Schiff bases. With this point in view the present study of ternary complexes M(dipy)L has been carried out where M = Cu(II), L = Salicylaldehyde, 2-hydroxy- l-naphthaldehyde, 2-hydroxy-acetophenone or 2-hydroxy-propiophenone or N-phenyl-salicylaldimine, 2-hydroxy- 1-naphthaldimine, N-methyl-2-hydroxy-l-naphthaldimine or N-phenyl-2hydroxy-l-naphthaldimine. Studies were not possible for salicylaldiminato, and N-methyl salicylaldiminato ligands as they could not be obtained in pure form. An extension of Irving-Rossotti titration technique has been used [9,10].
It is known that the [Cu(dipy)] 2+ complex formation is complete at lower pH and the complex is stable at higher pH (Fig. 1). The M-dipy curve (3) diverges from the acid curve (1) at pH 6.0, indicating that the formation of hydroxo complex [M(dipy) (OH)2] starts only at higher pH. The curve 5(M + A + L) remains almost merged in the beginning with curve 4(L) indicating that the complexation with secondary ligand does not take place at low pH. The curve 5 diverges from 4 at higher pH (3) showing that M + d i p y + L combination takes place, where, M + dipy 1 : 1 complex formation is complete. In this range hydroxo complex formation also does not take place. The horizontal distances between curves 4 and 5 (Fig. 1) correspond to extra hydrogen ions liberated '.as a result of combination of secondary ligand with [M(dipy)] 2+. t] and pL were calculated using the equations suggested by Irving and Rossotti [ l l]. The aldehyde and ketone ligands have only one pK. value. This was used for the calculation of formation constants of ternary complexes. The calculations were carried out below pH 6.0, Average value of log kc,tJ~ync"ta~oY' were found out using the method of linear plot. These are found to be higher than the formation constants of the corresponding binary complexes Kcut.. cu Schiff base ligands show two pK. values. At lower pH the ligand curve is above the acid curve because the Schiff base solution contains less number of titratable H + ions: due to absorption of protons by nitrogen of C=N. At higher pH it exhibits lower values of pH than the acid titration curve, showing more number of titratable ions, due to dissociation of H+of O-H. Log KcuAtC'~'__ log KCuL, Cu where L = Schiff base of 2-hydroxy-l-napbthaldebyde with ammonia or methylamine: whereas, log K~A~ < log K~:~,~, where L = Schiff base of salicylaldehyde or 2-bydroxy-l-naphthaldehyde with aniline. The values of formation constants of binary and ternary complexes indicate the possibility of M ~ L :~" interaction in these complexes. In case of Cu-sa]icylaldehyde complex, the log Kc,,t_ c, value is significantly larger than expected from the basicity of this ligand. The pK. value for salicylaldehyde is lower than that of 2-hydroxyacetophenone, yet the difference between the log Kc.~. c,, values of salicylaldehyde and 2-hydroxy acetophenone and 2-hydroxy-propiophenone, is not
EXPERIMENTAL
Materials and methods. The reagents used were of pure grade. Solutions of metal perchlorates in 5(~0 (v]v) dioxan were prepared and standardized. The instruments used, purification of solvent and titrations carried out were similar to those described earlier[8]. The solution in 50%(v/v) dioxan (M = 0.2 M, NaCIO4) were titrated against 0.2M sodium hydroxide solution. Since Schiff bases are prone to undergo hydrolysis, titrations for such systems have been carried out taking excess of ligand solutions (M:L= 1:5). Solutions containing M:L= 1:5 and M:L= 1:2 yield similarresults indicatingthat no hydrolysistakes place at this concentration. One represent:~tivetitration curve has been shown in Fig. I. The values of the formation constants of the binary metalaldehyde or ketone complexes and also metaI-Schiff base complexes are necessary for comparison under identical conditions. Though, such values are available in literature, proton-ligand, and metal-ligand formation constants of metal-aldehyde or ketone complexes and also metaI-Schiff base complexes were determined using lrving-Rossotti titration technique. The values of the formation constants have been recorded in the table.
*Author to whom correspondence should be addressed.
331
332
RAKESH K. KOHLI et aL
5
8 3
6
7 pH
6
5
4
3
2
4D
I
I
50
60
Volume of olkoli in ml.
Fig. 1. Cu(ll) dipy. Salicylaldehyde system-30°C. 1. Acid; 2. Dipy; 3. Cu(II)+dipy(l:l); 4. Salicyaldehyde; 5. Cu(II) + dipy + salicyaldehyde ( 1:1:1) and 6. Cu(II) + Salicylaldehyde ( 1:1). Table 1. Proton and metal-ligand stability constants of secondary ligands at 30°C St. No.
Ligand (LH)
pKIH
pK~H
log K 0u CuL
vCu'dipy log ~Cu.dlpy.L
i. Sallcylaldehyde
9.30
-
7.18
7.93
2. 2-hydroxy-l-naphthaldehTde
8.38
-
7.19
7)94
3 • 2-hTdroxyacetophenone
I0.80
-
7.61
7.83
4. 2-hydroxy-propiophenone
ii.00
-
7.~
7.80
9.67
2.52
9.51
9.51
i0.00
2.79
8.56
8.56
8.19
2.hA
7.84
7.56
9.2~
~.00
8.99
8.~i
~. 2-hyd~oxy-l-naphthaldlmlne 6. N-Methyl-2-hydroxy-lnaphthaldimlne 7. N '- ph enyl-2 -h~nlroxynaphthaldlmlne
8. N)-phenyl salleylaldlmine
much. Another striking feature is that log K c" C,L of Cu-2hydroxy-l-naphthaldehyde complex is almost same as that of Cu-salicylaldehyde, complex though 2-hydroxy-lnaphthaldehyde is more acidic than salicylaldehyde. The formation constant values of these metal complexes indicate a possibility of ~r interaction between the metal d~r orbitals and the p~" of - C = O and phenolate O-. This results in the formation of delocalized ~" electron ring over the metal and the ligand. This imparts a double bond character to the M-L bond and makes the complex more stable. The additional phenyl ring in the case of naphthaldehyde helps in the back donation
because of the ~r electrons being delocalized over two rings. Since the extent of back donation is more, log Cu KC,L value is higher than expected from its basicity alone. In case of ketones basicity is high owing to the electron releasing influence of methyl and ethyl groups. This increases the electron density over C=O and makes it less susceptible to form a ~r bond. This explains, why 2-hydroxy-acetophenone and 2-hydroxy propiophenone, though more basic, do not show much higher values of log KC,L c, than salicylaldehyde or 2-hydroxy-l-naphthaldehyde complexes.
Study of rr interaction in metaI-Schiff base complexes The formation constants of the ternary complexes are higher than the corresponding binary complexes. This is in keeping with the behaviour of oxygen binding secondary ligand complexes [3]. The higher value of log KCc~,l. can be attributed to three factors. First is special behaviour of dipyridyl, i.e. besides N-~ Cu cr bonding, there exists M--, N 7r interaction in dipyridyl complexes. This retains the electronegativity of the metal ion in [Cu(dipy)]2~ same as in [Cu(H20)~] 2÷. Secondly, the distorted octahedron [Cu(H20)6] 2+ will be somewhat more strongly distorted towards the square planar coordination by the coordination of a strongly chelating ligand c~~r-dipyridyl, thus creating the right geometry for the coordination of the secondary ligand resulting in the increased value of log gcuAl~. c~A Besides the above two factors, another plausible operating factor could be the existence of r bonding in the secondary ligands, i.e. between Cu and aromatic aldehyde or ketone. The 7r interactions between M-A and M-L, mutually stabilize each other and this contributes to higher value of log KCuAI,. C~A This effect is expected to be higher for the complexes of Cu-dipy-L, where L--salicylaldehyde or 2-hydroxy-l-naphthaldehyde than where L=2-hydroxyacetophenone or 2hydroxy propiophenone. The binary complex of schiff base 2-hydroxy-l-naphthaldimine shows higher value of log Kcuc cu than the Schiff base complex of N-methyl-2-hydroxy-l-naphthaldimine, though from the basicity consideration the order should have been reverse. This indicates that the extent of 7r bonding outweighs the inherent basicity of the system. The presence of Cu-N ~" bonding in the schiff base complex is also supported by the fact that log Cu(dipy) KCu(dipy)t. is same as log KCuL, cu i.e. AK = 0 when L = 2hydroxy-l-naphthaldimine or N-methyl-2-hydroxy-lnaphthaldimine. As explained above, the mutual stabilization of 7r bonding in the dipyridyl and the Schiff base in the mixed ligand complex makes log KcuAL°'a__ log ell Kc,,L. Such ~- delocalization can take place due to the interaction of metal d r orbital with the p~r orbitals of the
333
ring carbon, azomethine nitrogen and the O- of tl~e phenolate group. The value of AK is positive in the cases where L = N'phenyl-2-hydroxy-l-naphthaldimine or N'-phenyl salicylaldimine. This could probably be due to steric hindrance between dipyridyl and these bulky Schiff base ligands, in the mixed ligand complex. Because of tihe steric interaction, the planarity of the molecule gets distorted, thus reducing the extent of Cu--,N ~r interaction. Hence log KCuALCUA
REFERENCES
I. M. V. Chidambaram and P. K. Bhattacharya, J. Inorg. Nacl. Chem. 32, 3271 (1970). 2. R. L. Datta and De Dhrubendra, J, Indian Chem. Soc. 46, 1 (1%9). 3. R. Greisser and H. Sigel, lnorg. Chem. 9, 1238 (1970). 4. P. R. Huber, R. Greisser, B. Prijs and H. Sigel. Europ. J. Biochem. 10, 238 (1%9). 5. D. H. Busch and J. C. Bailer, J. Am. Chem. Soc. 78, 1137 (1959). 6. B. J. Bathaway, D. E. Billina, R. J. Dudley and P. Nicholls, Z Chem. Soc. 2312 (1%9). 7. G. A. L. Heureux and A. E. Martell, J. lnorg. Nucl. Chem. 28, 481 (1%6).
8. Uma Doraswamy and P. K. Bhattacharya, Indian L Chem. 13, 1069 (1975). 9. H. M. Irving and H. S. Rossotti, J. Chem. Soc. 2904 (1954). 10. F. J. C. Rossotti and H. S. J. Rossotti, Acta. Chem. Scand. 9, 1166 (1955). 11. H. M. Irving and H. S. Rossotti, J. Chem. Soc. 3397 (1953).