Bifunctional influence of 3-chloro substitution on structural and energetic characteristics of N-methyl-salicylidene imines

Journal of Molecular Structure 976 (2010) 19–29

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Bifunctional influence of 3-chloro substitution on structural and energetic characteristics of N-methyl-salicylidene imines A. Koll a,*, J. Janski a, A. Karpfen b, P. Wolschann b a b

Institute of Chemistry, University of Wroclaw, ul. Joliot-Curie 14, PL-50383 Wroclaw, Poland Institute of Theoretical Chemistry, University of Vienna, Währinger Straße 17, A-1090 Vienna, Austria

a r t i c l e

i n f o

Article history: Received 17 July 2009 Received in revised form 8 December 2009 Accepted 14 December 2009 Available online 21 December 2009 Keywords: Intramolecular hydrogen bonds Energy of hydrogen bond Steric effects Structure of hydrogen bond DFT calculations

a b s t r a c t Energetic and structural effects of formation of intramolecular hydrogen bonds in the group of eight differently chloro-substituted ortho-hydroxy aromatic Schiff bases (N-methyl-salicylidene imines) containing the 3-chloro substituent in all compounds were studied. The aim was to explain the specific function of this substituent, giving rise to an especially large increase of the strength of intramolecular hydrogen bonds. DFT B3LYP/6–31 + G(d,p) calculations of the structures of particular conformers as well as the potential energy surfaces were carried out. The method of thermodynamic cycle-like scheme was adapted, where the three open conformers are used to estimate the steric corrections connected with formation of chelate rings in the fourth conformer. Problems arising for the estimation of the energy of intramolecular hydrogen bonds connected with the selection of reference states and various structural aspects are discussed in detail. 3-Chloro-substituted Schiff bases can be accounted as sterically enhanced intramolecular hydrogen bonds. Participation of steric and electronic functions in increasing of the energy of intramolecular hydrogen bonds was analyzed. Low energetic proton transfer reaction was found in these molecules, which can be potentially interesting from the point of view of possible thermochromic applications. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction Salicylidene imines form intramolecular hydrogen bonds with many interesting properties, also from the practical point of view. These are for example thermo- and photochromic properties, related to the shape of the potential energy profile for intramolecular proton transfer. This allows to use salicylidene imines as molecular switches, as elements of molecular memory [1] or as fluorescent probes in biological systems [2]. The title compounds (ortho-hydroxy Schiff bases) form socalled resonance-assisted hydrogen bonds [3,4], with the specific property of direct electronic coupling between acid base centers. It was interesting to study this specificity by comparing with ortho-hydroxy-N,N-dialkylbenzylamines (in the following we will use not exact, but traditionally applied name – Mannich bases), where a methylene bridge distinctly reduces the electronic coupling. In the study of the series of the latter type of intramolecular hydrogen bonds [5] clear structural evidence was found, that substitution by a chlorine atom at the position 3 (cf. Scheme 1) gives a specific strengthening of the intramolecular hydrogen bond, higher than predicted on the basis of pKa modification. The energy of an intramolecular hydrogen bond cannot be defined unambiguously. Using the calculated values of the energy * Corresponding author. Tel.: +48 713757200; fax: +48 71 375 7420. E-mail address: [email protected] (A. Koll). 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.12.028

of the hydrogen-bonded conformer and of three ‘‘open‘‘ forms (like 1,3,4-in Scheme 2 of this publication) we were able to construct a thermodynamic cycle – like Scheme 2 – and to estimate the intramolecular hydrogen bond energy in Mannich bases, together with corrections of steric effects resulting from non-bonding interactions of acid and base centers [6]. Despite strong structural and spectroscopic evidence of the strengthening of the intramolecular hydrogen bond upon 3-Cl substitution, the energy calculated by the method of thermodynamic cycle of the hydrogen bond formation appeared lower than in Mannich bases without such a substituent, which was ascribed to differences in the structure of reference states resulting from steric repulsion [7]. Similar strong interactions were found in Schiff bases with hydrogen at the methylimine centre C–C(H)@N–R substituted by alkyl, in 2-(methylimino-alkyl-methyl)-phenols, as well as aryl groups in 2-(methylimino-phenyl-methyl)-phenols [8]. These compounds form distinctly shorter and stronger (as it is clearly seen from IR spectroscopy) intramolecular hydrogen bonds than non-substituted Schiff bases with the same values of DpKa (= pKa (BH+)  pKa (AH)) in A–H. . .B hydrogen bond) [7]. In this work, we aim to analyze the influence of chloro-substitution at the 3-position in the series of eight differently chlorosubstituted Schiff bases on the properties of the intramolecular hydrogen bond, in comparison to previously studied compounds without such a substituent [9].

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A. Koll et al. / Journal of Molecular Structure 976 (2010) 19–29

H O7 H 6 5

H

1 4

H11

2 3

C10 N9 C8

3-positions are occupied by chlorine atoms in all compounds. The pKa values, given in parentheses, are also different, because of the additional 3-chloro substitution [10,11]. Our basic approach is to make correlations of various parameters with pKa values. This will describe the electronic interaction of particular sets of substituents, and allow for an analysis of the deviations as resulting from steric interactions. The data obtained show that the substituents in Schiff bases clearly play a bifunctional role on the properties of the intramolecular hydrogen bond. They significantly depend on both, electronic and steric effects. In the discussion of the structures of related compounds with intramolecular resonance-assisted hydrogen bonds, Gilli et al. [12– 14] suggested that in such compounds the pKa values should not be used to characterize the strength of the hydrogen bond, because of the simultaneous influence of substituents on acidic and basic functions of interacting moieties existing in a molecule. This can lead to a compensation of the effects on DpKa. Even more, in molecules with intramolecular p-electron coupling there is mutual – through the molecule – compensation of charge modifications resulting upon hydrogen bond formation. A weak dependence of some structural parameters on pKa was also found in Schiff bases [9]. Nevertheless, the results obtained by us previously [9] show that such a compensation is not complete and that properties like the extension of the O–H distance, steric corrections or corrected intramolecular hydrogen bond energies show clear dependences on the pKa of phenols. The pKa turns out to be the main factor expressing variations of these parameters. One could, at this point, make some more general considerations; the function of relative stability of the enol form before the proton transfer and of the zwitterionic form after the proton transfer, represented by DpKa in a classical theory of Huyskens and Zeegers-Huyskens [15] in our case, may be identified with the difference in energy between conformers 2 and 5 in Scheme 2, despite the fact, that in Schiff bases the structure 5 contains only about 60% of zwitterionic form, and 40% of keto form [9]. A correlation between the difference in the stability of enol and proton transfer forms and the pKa of phenols was demonstrated earlier [9]. Therefore, the pKa can be accepted as a measure of the electronic influence of the substituents on the properties of the hydrogen bond also in Schiff bases. Because the energy differences between structures 5 and 2 depend on the strength of

H H

H16

H

H Position of 0 a* b c d e f g substitution (9.11)** (8.61) (8.18) (7.80) (7.84) (7.18) (6.98) (6.25) 3

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

4

H

Cl

H

H

Cl

Cl

H

Cl

5

H

H

Cl

H

Cl

H

Cl

Cl

6

H

H

H

Cl

H

Cl

Cl

Cl

*

Short labels for particular compounds

** pKa values of the phenolic part Scheme 1.

The first problem to be discussed will be the influence of 3-Clsubstitution on the geometric characteristics of the intramolecular hydrogen bonds. The second problem is the question about the difference in energetics of both types of intramolecular hydrogen bonds, especially, how it is related to the conformational modifications of particular structures. Because of mesomeric interactions in Schiff bases, we expect a stronger tendency to planarity in these molecules, which could give a more simple structural situation than in Mannich bases. The studied compounds along with appropriate atom labeling are presented in Scheme 1. The scheme is directly related to that given for non-3-chlorosubstituted Schiff bases. Contrary to compounds in [9], here the

H

O

CH3

N

H

H

H

Cl H

O ΔET+S2

H

ΔH(OH..Cl)

H

H

N

Cl

H

Cl H

ΔET+S1

+S1-ΔH(OH..Cl) O

H

H

H

5

2 ΔET+S2

N

H

H

H

S1

O

PT

H

1

H

+ CH3 -O H N

CH3

CH3

-O

H

H

N

S2-ΔH(OH..Cl) H

Cl

H

Cl

4

H

+ CH 3

H

N Cl

H

H 3

Scheme 2.

CH3

H

H

H

opening PT

6

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A. Koll et al. / Journal of Molecular Structure 976 (2010) 19–29

nols containing the 6-Cl substituents in hydrogen-bonded and ‘‘open” forms. The description of the structures of particular tautomers was based on the concept of resonance between assumed structures, as it is described in detail in Ref. [9]. The particular ring bond lengths were fitted as a combination of appropriate bond lengths in two resonance structures. The six equations for the ring bond lengths were solved simultaneously. To represent the ring bond lengths in particular resonance structures, appropriate reference molecules without hydrogen bond were selected: ortho-cresol, ortho-cresolate and methidine [5], (cf. also Scheme S5 in Supplementary materials). The last one describes the bond lengths pattern in the ortho-quinoid keto structure. The amount of ketocontent of particular tautomers appears to be a measure of electron charge redistribution upon formation of the hydrogen bond and upon proton transfer [5,9].

intramolecular hydrogen bond, the energy difference between 4 and 6 conformers (Scheme 2) can be better related to DpKa in Huyskens’ theory. Scheme 3 shows that there is a correlation of the energy difference between 4 and 6 conformers (Table 1 [9]) on the pKa of phenols (including the correction of the energy of the intramolecular hydrogen bond in 4, when it contains a 6-Cl-substituent). On this basis, we will characterize the effects of the electronic influence of the substituents on the properties of intramolecular hydrogen bond in a classical way, i.e. in dependence on DpKa [16]. In this work we use pKa values instead DpKa, because they are linearly related, due to the fact, that the ‘‘basic” part of interacting groups is the same in each series of compounds. 2. Calculations

3. Results and discussion

The values of the energy of particular conformers (see Scheme 2) for 8 differently chloro-substituted ortho-Schiff bases, containing the 3-Cl substituent throughout were calculated by DFT methods at the B3LYP/6–31 + G(d,p) level using Gaussian03 [17]. As it is shown in Scheme 2, the combination of the energy values for structures 1–4 allows for the determination of the energy of the intramolecular hydrogen bond (DET) corrected for steric effects of non-bonding interactions of base and acid moieties (S1 and S2), respectively, while approaching each other upon closing the chelate hydrogen bond ring [9]. The effects of intramolecular O–H. . .Cl hydrogen bonds (DH(O–H. . .Cl)) in tautomers 1 and 4 were estimated on the basis of the calculations for chloro-substituted phe-

(a)

O H

N

3.1. Structural effects of 3-Cl atom substitution on hydrogen bond parameters Introducing the 3-chloro substituent leads to a change of the structural parameters mainly in the hydrogen bond, as well as modifications of the structural parameters in the phenyl and chelate rings. We shall use the correlations obtained for non-3chloro-substituted Schiff bases [12] and compare with the values obtained for 3-Cl-substituted compounds at appropriate values of pKa.

O-

R

C 6 H n Cl (4-n) C H

(b)

Δ E (kcal/mol)

(c)

H l

O

H

+ N

R

C 6 H n Cl (4-n) C H

C 6 H n Cl (4 - n) C(H)

NR

O C 6 H n Cl (4 - n) C(H)

N(R)H +

5.5 5 4.5 4 3.5 3 2.5 2 6.5

7.5

8.5

9.5

pKa Scheme 3.

Table 1 The values of bond lengths variance – Aa- in 3-Cl-substituted Schiff bases in planar conformations. In parentheses, there are increase factors in relation to analogously substituted Schiff base, but without 3-Cl substituent [12].

a

A-pl

1

3-Cl 3,4-Cl2 3,5-Cl2 3,6-Cl2 3,4,5-Cl3 3,4,6-Cl3 3,5,6-Cl3 3,4,5,6-Cl4

134.1 139.0 142.7 156.4 105.8 151.2 125.1 46.1

For definition see Eq. (1).

2 [2.0] [1.7] [1.8] [1.8] [1.8] [1.5] [1.5] [0.8]

201.1 185.6 206.4 210.6 154.6 185.7 182.6 91.5

3 [1.7] [1.4] [1.6] [1.6] [1.4] [1.3] [1.5] [0.9]

189.2 202.2 194.8 191.6 167.0 188.8 147.5 65.4

4 [2.2] [2.1] [2.2] [2.2] [2.1] [1.9] [1.8] [1.9]

118.0 135.4 125.9 129.0 113.4 137.4 93.2 42.8

5 [3.2] [3.0] [3.2] [2.7] [3.4] [2.6] [2.2] [1.1]

1364.9 1294.6 1381.1 1404.0 1163.2 1340.1 1368.2 1113.5

6 [1.2] [1.1] [1.1] [1.1] [1.0] [1.0] [1.1] [1.1]

2409.3 2285.8 2396.4 2454.8 2080.3 2313.4 2344.7 1971.8

[1.1] [1.0] [1.1] [1.1] [1.0] [1.0] [1.1] [1.0]

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A. Koll et al. / Journal of Molecular Structure 976 (2010) 19–29

Fig. 1 presents the dependence of C–O bond lengths (d(C–O)) in phenols on pKa. As one can see, substitution at position 3 by chlorine has only very weak influence on d(C–O). All four correlation lines show a small decrease of d(C–O) with increasing acidity of the phenols. A somewhat stronger effect is observed for 6-Cl substitution. It can be understood as the effect of a direct electronic interaction of the 6-Cl substituent on the C–O bond in proximity, or as an increase of the intramolecular hydrogen bond strength due to steric interaction of this substituent. These possibilities will be discussed further, on the basis of other properties of the systems. Fig. 2 presents the increase of O–H bond lengths upon formation of the intramolecular hydrogen bond in 2 in relation to conformer 3, in which the O-H group is neither involved in O–H. . .N nor in O–H. . .Cl interactions (Scheme 2). There are two separate correlations between increments of O–H bond lengths. The values for 3-Cl substituted compounds are distinctly higher; the hydrogen bonds appear to be strengthened beyond the effects of pKa, a very clear evidence of the importance of steric effects. In the Figure, the increase of the N–H+ distance in conformers 5 in relation to 6, on pKa is shown, too. The increments of N–H+ distances (5–6) for 3Cl substituted and non-substituted Schiff bases form one set. Formation of the hydrogen bond leads to a smaller increase of the N–H+ bond length than for the O–H bond in the molecular form of the hydrogen bond. Fig. 2 demonstrates an increase of the O– H. . .N hydrogen bond strength and a decrease of the O. . .H–N+ bond strength upon increasing the phenol acidity. Fig. 3 displays the dependence of the O(H). . .N distance (hydrogen bond length) on pKa. Here one observes separate lines for each correlation. The hydrogen bond length is distinctly shorter in compounds containing the 3-Cl substituent, despite the fact, that the

acidity difference was already counted by increasing the pKa values. The hydrogen bonds in the proton transferred forms are generally shorter than those in the molecular ones. The 3-Cl substitution decreases also the length of the zwitterionic form of the hydrogen bond. Different slopes lead even to crossings of the correlations of O. . .N distance on pKa. Molecular hydrogen bonds become even shorter than their proton transferred counterparts. Fig. 4 shows the dependence of OH. . .N and O. . .NH+ distances on pKa. As previously, all distances for 3-Cl-substituted molecules are shorter than for non-3-Cl-substituted. For molecular hydrogen bonds, these distances decrease when pKa decreases, while for proton-transferred tautomers – they increase. Figs. 2–4 consistently show, that the hydrogen bond strength increases with pKa in the case of the molecular hydrogen bond and decrease for the ionic forms. The last three Figures demonstrate the bifunctional role of the 3-Cl substitution. There are effects of the increase of the hydrogen bond strength related to the decreased acidity of these compounds, but there is also an evident increase of the strength due to steric effects. The 3-chloro-substituted Schiff bases one should account as sterically enhanced intramolecular hydrogen bonds [18]. 3.2. Influence of 3-Cl substitution on valence angles Direct evidence of steric effects of 3-Cl substitution stems also from the study of the geometry change upon 3-Cl substitution. There are characteristic changes of the valence angles, in proximity to this substituent. The effects are illustrated in Scheme 4. When comparing the structures a and b in Scheme 4 with intramolecular O–H. . .N hydrogen bonds (conformer 2, in Scheme 2), one observes an increase of the C3–C2–C8 (for labeling see Scheme

Fig. 1. Dependence of the C–O bond length on pKa values of particular chloro-substituted phenols. Black circles represent points for the enol forms (conformer 2) of hydrogen bonds for non 3-Cl substituted derivatives, black triangles indicate points for 3-chloro substituted compounds. Open circles give values for proton transfer forms (5 in Scheme 2) in non 3-Cl substituted, open triangles for 3-Cl substituted compounds.

A. Koll et al. / Journal of Molecular Structure 976 (2010) 19–29

23

Fig. 2. Dd(O–H) – increase of the O–H bond lengths upon formation of the intramolecular hydrogen bond in 2 in relation to conformer 3, (no hydrogen bond, Scheme 2), and the corresponding increase of the N–H+ distances in 5 in relation to conformer 6, respectively. The symbols are the same as in Fig. 1. Dependence on pKa of phenols.

Fig. 3. Dependence of the O(H). . .N distance (the length of hydrogen bond) on pKa. Labeling is the same as in Fig. 1.

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A. Koll et al. / Journal of Molecular Structure 976 (2010) 19–29

Fig. 4. Dependence of OH. . .N and O. . .NH+ distances on pKa. Labeling is the same as in Fig. 1.

Scheme 4.

1) angle (2,5°), external to the hydrogen bond and a simultaneous decrease (in average, 1°) of internal C2–C8–N9, C1–C2–C8 and C1– C2–C3 angles. It makes the hydrogen bond shorter, but with simultaneous extension of the O–H bond length – the effect is opposite

to that resulting from steric strain but is in accordance with the strengthening of hydrogen bonding. It increases also the ortho-quinoic character of the phenyl ring (for quantitative description see further).

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A. Koll et al. / Journal of Molecular Structure 976 (2010) 19–29

For comparison also the influence of 3-Cl substitution for the tautomer 1 can be discussed (structures b and c, in Scheme 4). The external angle C3–C2–C8 increases as in the case of hydrogen-bonded forms. Because of the missing attraction, the N. . .O distances are longer than in tautomers with intramolecular hydrogen bond. One observes also in this case the decrease of the N. . .O distance upon 3-Cl substitution, but the O–H distance is not expanded, because it does not take part in the hydrogen bond. One can also compare the effects of formation of the intramolecular hydrogen bond for molecules with (b and d) and without 3-Cl (structures a and c). For compounds without the 03-Cl substituent, the O. . .N distance decreases from 2.811 to 2.627 Å A. Due to this, the C3–C2–C8 angle increases by 3.1°, the C1–C2–C8 angle decreases by 4.4° and the C2–C8–N9 angle decreases by 3.6°. The O–H dis0 tance increases from 0.967 to 0.999 Å A. In the compound with 3Cl substituent, the O. . .N distance decreases from 2.721 to 0 2.576 Å A, the C1–C2–C8 angle decreases from 123.7 to 120.1° and the C2–C8–N9 angle decreases from 125.2 to 121.9°. The O–H dis0 tance increases from 0.967 to 1.006 Å A. The last effect combines both steric and electronic interactions. Both effects, steric and formation of intramolecular hydrogen bonds, act in the same direction. The structural changes discussed above are in full agreement with the hypothesis that the steric effect of the 3-Cl substituent increases the strength of intramolecular hydrogen bond. Very similar effects result from MP2/6–31 + G(d,p) calculations (see Supplementary materials). 3.3. The changes of collective characteristics of phenol ring Two collective characteristics of distance patterns in the phenol ring can be applied in the discussion about the character of the intramolecular hydrogen bond – the content of keto-resonance form in the description of the structure of particular tautomers ([19], see also above) and the variance of bond lengths A;

P A¼

 2  dÞ  106 n

i ðdi

ð1Þ

where di is a particular bond length in the n member aromatic ring, 0  – an average bond length in the ring, in A and d Å. It was stated that formation of the hydrogen bond leads to an increase of the value of the last characteristics. The stronger the hydrogen bond is, the higher is the A factor [5,9]. It was particularly large for zwitterionic forms, where the content of o-quinoid resonance form increases and consequently the difference in ring bond lengths is growing up. The highest values were found for the open forms in zwitterionic molecules without hydrogen bond [9], where the content of keto structure was the highest, about 60–70%. It demonstrates that in conformer 2 the intramolecular hydrogen bond is stabilized by a resonance between zwitterionic and keto forms, which is the most effective when both forms are in nearly equal content. In Table 1, there are values of A calculated for planar structures of 3-Cl-substituted Schiff bases for more direct comparison of steric effects (in parentheses are factors of increase relative to non-3Cl substituted analogues). A significant increase of A is observed in the conformer 2, which is an evidence for the strengthening of intramolecular hydrogen bonds upon 3-Cl substitution. Nevertheless, also for non-hydrogen-bonded conformers, A increases upon this substitution, which results from the steric distortion of the phenyl ring with increasing number of bulky substituents, especially for conformers 3 and 4, where the 3-Cl substituent interacts directly with the nitrogen atom. Nevertheless, inspecting the results in Table 1, one finds that the largest A parameters generally belong to conformer 2, not counting 5 and 6 with large content

of keto-resonance form. The very strong reduction of the A parameter is seen in the last rows of Tables 1 and 1S. This is a similar effect like in Mannich bases [6], where complete substitution of H atoms in phenyl ring introduced regularity in phenyl bonds lengths, and a serious reduction of bond length variances. Table 1S (Supplementary materials) contains the values for non-planar molecules 1, 3, and 4. Steric repulsions are released for non-planar molecules and bond lengths variance decreases. In structures 3, even lower values of variance were found than in molecules without 3-Cl (given in square brackets). This is not true for conformers 4. The results seem to depend not only on the conformation, but also on the particular substitution pattern. For these reasons it is not obvious, if particular forms of non-planar molecules can be used in a uniform way as reference states when discussing the role of the hydrogen bond. Similar conclusions are seen in Tables 2 and 2S, which display the amount of keto forms presented in an analogous way as for A parameters. The amount of keto forms in particular structures increases less upon 3-Cl substitution for 1 and 2, than for 3 and 4. This amount is about 14.5 for conformer 2 of all substances and is higher than for the other conformers (1, 3, and 4), but less than for the tautomers 5 and 6. This can be explained by the strengthening of the hydrogen bonds by 3-Cl substitution. In the non-planar conformers 1, 3, and 4 one finds a considerable decrease of the content of keto form, which results from released steric strain and decreased resonance coupling after turning the C(Cl)–N@R group out of the molecular plane. Steric effects of 3-Cl-introduction are too weak to break the intramolecular hydrogen bonds in 2 conformer thus preserving its planar structure. 3.4. Energetic effects of 3-chloro substitution In order to discuss the energetic relations between different conformers and to establish the structure of non-planar conformers, 2D potential energy surfaces were calculated as a function of the rotation of the O–H bond around the C–O bond and of the imino group around the C2–C8 bond for all eight differently chlorosubstituted Schiff bases, containing the 3-Cl substituent. These are compared to an unsubstituted Schiff base (S0 in our notation). Fig. 5 presents three examples of calculated potential energy surfaces (in kcal/mol). All stable conformers are planar, and conformer 2, with an intramolecular hydrogen bond, is located much lower (at least 10 kcal/mol) than the others, so in practice only this conformer can be observed. Because the reference states are planar, one can safely compare the energy of particular states, estimate the steric corrections and establish the corrected energies of intramolecular hydrogen bonds, as it was made in [9]. SCG is the 3-chloro substituted derivative, and one can follow the changes in conformational equilibria, resulting from this substitution. The only planar conformer is 2, with an intramolecular hydrogen bond.

Table 2 The amount of keto-resonance forms in particular tautomers for eight 3-Clsubstituted Schiff bases. X-pl

1

2

3-Cl 3,4-Cl2 3,5-Cl2 3,6-Cl2 3,4,5-Cl3 3,4,6-Cl3 3,5,6-Cl3 3,4,5,6-Cl4

8.1 [1.2] 8 [1.2] 7.8 [1.2] 8.7 [1.2] 7.8 [1.2] 8.7 [1.2] 8.6 [1.2] 8.6 [1.2]

14.4 14.6 14.4 14.2 14.7 14.5 14.5 14.8

3 [1.2] [1.2] [1.2] [1.2] [1.2] [1.2] [1.2] [1.3]

10.3 10.4 10.1 10.1 10.1 10.3 10.1 10.2

4 [1.3] [1.3] [1.3] [1.3] [1.2] [1.3] [1.3] [1.3]

4.7 4.9 4.5 5.2 4.2 5.0 5.0 4.8

[1.3] [1.4] [1.4] [1.4] [1.2] [1.3] [1.4] [1.2]

5*

6a

45.1 45.9 46.7 44.1 46.8 45.7 46.4 47.5

67.4 67.4 66.7 66.5 66.4 66.9 66.5 66.6

a The content of keto form is regularly increased by 4% for 5, and decreased by 2% for 6 (cf. Table 2s).

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A. Koll et al. / Journal of Molecular Structure 976 (2010) 19–29

Fig. 5. Potential energy surface for selected Schiff bases calculated at the level B3LYP/6–31 + G(d,p) method. The isoenergetic lines describe the calculated energies in kcal/ mol. Sequences of atoms along particular axis define the torsion angles describing these axis. Numbers at corners define the conformers, as it is in Scheme 2.

The minima for non-planar conformers 1 and 4 are located only 0.2 kcal/mol lower than the related planar ones. Only in conformer 3 the energetic distance between planar and non-planar states is more substantial – 1.2 kcal/mol. The minimum is located at 50° of rotation of the imine group and the O–H group is rotated out of plane on 15°. In the conformers 1 and 4 the imino group is rotated by about 30°. The O–H bond is in plane. The last example (SCG) describes the energetic behavior of the completely (3,4,5,6-) chloro-substituted Schiff base. Because the H ring atoms are replaced by chlorine atoms, the relative energy of conformer 3 is further increased, but the energy of 1 and 4, due to intramolecular O-H. . .Cl hydrogen bond, decreases. Minima for 1 and 4 non-planar states are more distinct. The rotation angles of the imino group are 40° and 35°, respectively. There is no minimum for conformation 3. Thus, the energy is only estimated for the point on the potential energy profile with the lowest slope (at about 120°) Numerical values of the energy of all planar and non-planar conformers are given in Tables 3 and 3S, respectively. 3.5. Estimation of the intramolecular hydrogen bond energy In this work, the same procedure for the estimation the energy of intramolecular hydrogen bond is applied as in the case of the compound without 3-Cl substituent [9], cf. Scheme 2.

Table 3 Calculated relative energies [B3LYP/6–31 + G(d,p)] (in kcal/mol) of particular planar conformers (1–6) of 3-chloro-substituted Schiff bases. Substituents

1

2

3

4

5

6

3-Cl 3,4-Cl2 3,5-Cl2 3,6-Cl2 3,4,5-Cl3 3,4,6-Cl3 3,5,6-Cl3 3,4,5,6-Cl4

15.60 15.92 15.83 12.80 15.99 13.18 12.97 13.23

0 0 0 0 0 0 0 0

17.29 17.68 17.43 17.83 17.80 18.19 17.93 18.48

15.30 15.70 15.49 12.38 15.79 12.83 12.46 12.83

2.66 2.17 1.77 1.42 1.81 0.97 0.51 0.62

12.61 12.00 11.63 11.19 11.71 10.58 10.11 10.31

In the case of 3-Cl substituted compounds some difficulties arise, as it was shown in the case of Mannich bases [8]. Calculations of the structures with 3-Cl substituent all gave geometry changes characteristic for hydrogen bonds with increased strength, but the estimated energy was less than for the series of non 3-Cl substituted compounds. In Schiff bases, we already reported the results of calculations for series without 3-Cl substitution. In this work, we performed calculations of the potential energy surfaces of Schiff bases with this substituent, in order to estimate the structures of all reference states (conformers in our case) that occur in the thermodynamic cycle-like scheme (Scheme 2) to estimate the energy of the intramolecular hydrogen bond and to establish

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A. Koll et al. / Journal of Molecular Structure 976 (2010) 19–29

Table 4 Corrections for steric interactions (S1, S2) and corrected values of the energy of intramolecular hydrogen bond formation in enol (HB) and zwitterionic forms (PT). For planar structures of all conformers. Substituent

S1(1–4)

S20 (3–4)

O–H. . .Cl corr.

S2 corr.

HB corr.

PT(6–5)

3-Cl 3,4-Cl2 3,5-Cl2 3,6-Cl2 3,4,5-Cl3 3,4,6-Cl3 3,5,6-Cl3 3,4,5,6-Cl4

0.295 0.227 0.336 0.41 0.197 0.355 0.517 0.397

1.992 1.984 1.939 5.449 2.005 5.364 5.466 5.642

0 0 0 3.06 0 2.94 3.16 3.15

1.992 1.984 1.939 2.389 2.005 2.424 2.306 2.492

17.59 17.91 17.77 18.24 17.99 18.54 18.48 18.87

9.94 9.82 9.86 9.78 9.90 9.60 9.60 9.69

which factors determine the specificity of such a substitution. The energy of the particular planar and non-planar structures was taken from Tables 3 and 3a, respectively. The results are presented in Tables 4 and 4S. The steric effects of approaching the nitrogen atom to all heavy atoms in the chelate ring, are defined as a difference between 1 and 4 conformers [9]. For non 3-chloro substituted compounds this effect is about 4 kcal/mol. Introducing the 3-Cl substituent, the relative energy of 4 conformers increases and the 1–4 difference both for planar and non-planar structures becomes very small. It appears that repulsion of N and O atoms and of N and Cl atoms is almost equivalent. Here, we encounter the problem of the definition of the reference state. As reference states we are forced to use the 3-Cl substituted compounds. Nevertheless, it seems that established energy N. . .O interactions, S1 in the non-substituted Schiff bases (about 4 kcal/mol) is more istic than the close to 0 kcal/mol obtained in this work. This leads to an underestimation of the energy of the intramolecular hydrogen bond in 3-Cl-substituted Schiff bases. Because the reference states in 3-Cl-substituted Schiff bases differ from molecules unsubstituted in this position, the numerical values of the energy of intramolecular bond, strictly speaking, should not be compared directly. The energy of non-bonding interaction of the O–H group with the N atom was previously established as 2 kcal/mol. In this work, it is 2 kcal/mol for the planar reference structures and about 1 kcal/ mol for the non-planar structures, because the H. . .H interactions are weaker in bent structures than in planar ones. In this case, these differences are almost negligible, this effect does not change seriously the established energy of intramolecular hydrogen bonds. When using strictly the same formalism as for molecules without 3-Cl substitution one obtains the energy of the intramolecular hydrogen bond as 18–19 kcal/mol, when using planar reference state and as 16.5–17 kcal/mol when using non-planar minima for 1, 3, and 4 conformers as a reference state. Those results (Tables 4 and 4a) are slightly higher than found in [9] if one selects more istic S2 values for planar structures. One should also have in mind the general underestimation of S1 values just discussed. The energy of the ionic intramolecular hydrogen bond this time is given as the difference between the energies of 6 and 5 structures and should not be directly compared with results obtained in [9]. Values of the relative (to 2) energy of conformer 5 can give information on possible proton-transfer equilibria in 3-chloro-substituted Schiff bases. The obtained values (see Table 3) are very low for the gas phase. In the entire series they are within the range of 2.5–0.5 kcal/mol. For non 3-chloro-substituted Schiff bases this range was 4.4–2.0 kcal/mol. It is further evidence, how strong intramolecular hydrogen bonds are in 3-chloro substituted orthohydroxy Schiff bases. The molecules with enol form of hydrogen bonds are well ‘‘prepared” for the proton transfer. The hydrogen bond lengths in enol conformers are similar to the proton-transferred ones, (see Fig. 3). For the last two compounds, the O. . .(H)N+ distance is even larger than for O(H). . .N. The energetic

distance between those two conformers is 0.5 kcal/mol only. These compounds seem to be interesting from the point of view of thermochromic properties. Generally, the estimated energy of intramolecular hydrogen bond is strongly related to selected reference states. Reference states with and without 3 chlorine substitution reveal a different geometry because of strong steric effects of this substituent. For this reason, the results obtained are not completely compatible. The choice of the planar reference state appears to be the better solution because of improved geometrical resemblance [20] to planar structure of conformer 2 with hydrogen bond. The energy of the intramolecular hydrogen bond defined in this manner is slightly higher than in analogous molecules without 3-Cl substitution. The analysis of all geometric modifications shows without doubts that 3-Cl substituents increase the strength of the intramolecular hydrogen bond, which results both from steric enhancement and from electronic effects, increasing the acidity of the donor part of intramolecular complexes. When correlating the estimated energy of intramolecular hydrogen bonds of molecules with and without 3 chloro-substituent with an internal measure of the strength of hydrogen bond, as is e.g. the (O)H. . .N distance, one finds a rather common dependence for both classes of compounds in Fig. 6. A similarly common correlation was found for Mannich bases, there between the electron density at the critical point of H. . .N bonds and the (O)H. . .N distance [8]. It was stated that the internal characteristic properties of the hydrogen bond are related, independent of the source of the modification the strength of hydrogen bond. In our case, we demonstrated that there are steric and electronic effects of 3-chloro substitution. Such a correlation in our case (Fig. 6) can be taken as evidence that the energy of the intramolecular hydrogen bonds estimated in this work is rather istic for 3-chloro substituted ortho-hydroxy Schiff bases, in spite of problems connected with the selection of the reference states.

4. Summary and conclusion Applying the B3LYP/6–31 + G(d,p) method, the calculations of the potential energy surfaces and structures of all eight conceivable chlorine-substituted N-methyl-salicylidene imines were carried out, with the restriction that the position 3 is permanently Cl-substituted. The main question in this study was to explain the very specific interactions of chlorine atom at 3-position with other substituents. In our previous investigations, it was established that also the 3-Cl substituted Mannich bases form a separate group with specific properties. The estimated energy of intramolecular hydrogen bonds was less than for molecules without this substitution despite the fact that all other characteristics of hydrogen bond demonstrated strong enhancement of interactions upon this substitution. Detailed analysis of geometry modulation upon 3-chloro substitution in Schiff bases performed in this work demonstrates that

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A. Koll et al. / Journal of Molecular Structure 976 (2010) 19–29

0

Fig. 6. Correlation between estimated energy of intramolecular hydrogen bonds (in kcal/mol) with the (O)H. . .N bond lengths (in A Å) for 3-chloro-substituted Schiff bases (black circles) and the unsubstituted Schiff bases in the 3-position (open circles).

there is also a strong enhancement of the strength of intramolecular hydrogen bonds. We estimated the value of these effects in relation to the pKa of the phenolic part of these intramolecular complexes. The calculated effects are, however, higher than expected on the basis of pKa. Additional effects were attributed to steric influence on the geometry on formed chelate ring. Detailed analysis of the geometry changes demonstrated that steric shortening of the hydrogen bonds leads to parallel effects, resulting from formation of intramolecular hydrogen bonds. As a result of our calculations, we are able to state that 3-chloro substitution acts bifunctionally. It increases the acidity of phenols and strongly shortens the hydrogen bridges due to steric interactions. Hydrogen bonds in 3-chloro-substituted Schiff bases can be accounted as sterically enhanced hydrogen bonds, discovered previously in salicylidene imines where the hydrogen atom was replaced by aryl or alkyl substituents in the C–C(H)–N–R moiety[18]. With the aim to estimate the energy of intramolecular hydrogen bonds, we adapted the thermodynamic cycle like in Scheme 2. In calculations it turned out that conformers not forming intramolecular hydrogen bonds become non-planar. In order to establish their geometries and to estimate the energies of non-planar conformers, we have performed the calculations of the potential energy surface for all eight 3-chloro substituted molecules and for N-methyl-salicylidene imine as the reference system. It was found, that the conformers with intramolecular hydrogen bonds stay planar. Steric effects lead to deviations from planarity of the conformers without intramolecular hydrogen bonds, but the steric strain is not sufficiently large to make the conformations with intramolecular hydrogen bond non-planar. It shortens and strengthens the intramolecular hydrogen bonds. Two versions of reference molecules were applied – planar and non-planar ones. The role of selection of the reference states in estimating the intramolecular hydrogen bond energy was analyzed. It was found, that choosing 3-chlorosubstituted reference molecules leads to an underestimation of steric effects and the energy of interactions. Nevertheless, a good correlation was obtained between hydrogen bond energies estimated in this work and the (O)H. . .N distance, similar to

estimates [9] for non 3-chloro-substituted derivatives. It suggests that the estimation of the energy of the intramolecular hydrogen bond in 3-chloro substituted compounds, gave reasonable values. This probably underestimated energy was found to be about 18–19 kcal/mol, distinctly higher than for non-3-Cl-substituted molecules. The energy of proton transfer can be established unambiguously, it was found that for 3-chloro-substituted compounds it is strongly decreased and drops even to 0.5 kcal/mol, which is of interest for in study of thermochromic properties of 3-chlorosubstituted Schiff bases forming the low-barrier hydrogen bonds. Acknowledgements The authors wish to thank the ZID of the University of Vienna and WCSS Wroclaw for computing time. Technical assistance of Mrs. E. Liedl, Ms. A. Stummer and Ms. M. Ziehengraser is gratefully acknowledged. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.molstruc.2009.12.028. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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