DFT studies on tautomerism of C5-substituted 1,2,3-triazoles

Journal of Molecular Structure 651–653 (2003) 697–704 www.elsevier.com/locate/molstruc

DFT studies on tautomerism of C5-substituted 1,2,3-triazoles Wojciech P. Ozimin´skia,*, Jan Cz. Dobrowolskia,b, Aleksander P. Mazureka a b

National Institute of Public Health, 30/34 Chel⁄mska Street, 00-725 Warszawa, Poland Industrial Chemistry Research Institute, 8 Rydygiera Street, 01-793 Warszawa, Poland Received 2 September 2002; revised 16 January 2003; accepted 16 January 2003

Abstract DFT (B3PW91/6-311þþG**), ab initio (HF/6-311þþG**), and single point CCSD(T)/6-311þþ G**//B3PW91/6311þ þG** calculations were performed to investigate the stability and tautomerism of the C5-substituted 1,2,3-triazoles. Three different tautomers are possible for the substituted 1,2,3-triazoles: N1– H, N2– H, and N3– H. For all the substituents applied, the most stable is the N2 –H tautomer. Out of the two less stable tautomers, N1– H and N3 – H, the – F, – CFO, –CH3, – CHO, – Cl, – CN, – CONH2, – NH2, –NO2, and – OH substituents stabilize the N3 –H tautomer, whereas only the – BH2, – BF2, and – COOH substituents stabilize the N1– H form. The relative stability of the C5-substituted 1,2,3-triazoles tautomerism is strongly influenced by the possibility for intramolecular interactions (both attractive and repulsive) between substituent and protons located either at N1 or N3 atom. For all the molecules studied, the Gibbs free energy at 0 and 298 K was estimated, too. q 2003 Elsevier Science B.V. All rights reserved. Keywords: 1,2,3-triazole; DFT; Tautomerism; Gibbs free energy; Substitution effect; CCSD(T)

1. Introduction The 1,2,3-triazole ring is a moiety present in antiallergic [1], antibacterial [2], antifungal [3], antivirial [4], and analgestic [5] drugs, however, its 1,2,4-isomer is used as a drug structures component much more frequently (e.g. anastrozole, estazolam, ribavirin, triazolam, etc.). Two constitutional triazole isomers, which occur in Nature, 1,2,3- and 1,2,4-triazole, may exhibit tautomerism: the (N)H hydrogen atom can be attached to * Corresponding author. Address: National Institute of Public Health, 30/34 Chel⁄mska Street, 00-725 Warszawa, Poland. Tel.: þ 48-22-851-5230; fax: þ48-22-841-0652. E-mail address: [email protected] (W. P. Ozimin´ski).

the 1st, 2nd, or 3rd (or 4th) nitrogen atom. Generally, if two carbon atoms are substituted by two different substituents, for example R – H than three different tautomers can exist (Scheme 1). The triazole tautomerism was a subject of previous studies [6 – 15]. Only a few articles that refer to geometry optimization and tautomerism of 1,2,3triazole can be found in literature during past 30 years. Begtrup et al. [9] studied 1,2,3-triazole using microwave spectroscopy, gas phase electron diffraction, and theoretical ab initio methods. Based on the experimental data, they estimated 1H:2H tautomer ratio in gas phase as 1:1000 and they calculated the SCF energy difference between the 1H and 2H forms to be ca. 2 15 kJ/mol which roughly corresponds to

0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0022-2860(03)00120-0

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Scheme 1.

ratio 1:200 at room temperature. Tornkvist et al. [8] calculated equilibrium geometry, harmonic force field, and vibrational frequencies for the 1H-1,2,3triazole and 2H-1,2,3-triazole molecules at the MP2/ 6-31G* level. They found that the N2 tautomer is ca. 21 kJ/mol more stable than the N1 form. Recently, Billes et al. studied vibrational spectroscopy of selected triazoles and tetrazoles [15]. They calculated vibrational frequencies using the DFT B3P86/ 6-311G** method. Tautomerism of five-membered heterocycles of importance for pharmacy (substituted diazoles and tetrazoles) was a subject of several theoretical and experimental papers prepared in our lab [16 – 23] and such systems are in the field of current interest of many research groups [24 –30]. The aim of this study is systematic investigation of substituent effect and its influence on tautomerism of the C5substituted 1,2,3-triazoles.

2. Calculation methods The quantum chemical calculations were carried out with suite of GAUSSIAN 98 programs [31]. Full geometry optimizations were carried out using both the Hartree – Fock (HF), the DFT calculations with hybrid B3PW91 functional [32 –34], and the single point CCSD(T) calculations [35] for frozen B3PW91/ 6-311þ þ G** geometries. The Pople’s type 6-311þ þ G** basis set was used for all calculations [36 – 38]. Geometry optimization was carried out using redundant coordinate algorithm [39]. For optimized structures, the harmonic vibrational frequencies were calculated at the DFT level. This routine allowed introducing the thermo-chemical corrections to DG values at 0 and 298.15 K.

3. Results and discussion The presence of a pyrrole-like and two pyridine-like heteroatoms in triazoles leads to highly perturbed p-electron distribution. According to the total p-charge on their carbon atoms, the N1 –H-1,2,3triazole can be classified as weakly p-excessive system and the N2 –H-1,2,3-triazole is p-deficient system [40]. The B3PW91/6-311þ þ G** total energies, and free Gibbs energies at normal conditions, calculated for 1,2,3-triazole (Table 1), show that the N2 – H form is more stable by 20.520 kJ/mol (DE) or 16.518 kJ/mol (DG298) than the N1 –H/N3 – H form of the molecule. The single point CCSD(T) calculations (CCSD(T)/6311þ þ G**//B3PW91/6-311þ þ G**) yielded the energy difference of 18.707 kJ/mol, so, the value is close to that found previously by Tornkvist et al. [8] at the MP2/6-31G* theory level. For all the substituent studied, the B3PW91/6311þ þ G** and CCSD(T)/6-311þ þ G**//B3PW91 /6-311þ þ G** calculations show the N2 – H form to be more stable than the corresponding N1 –H and N3 –H forms (Tables 1 and 2). This means that despite the character of the substituent (electron donating or electron withdrawing), the two pyridine-like nitrogen atoms N1 and N3, form a kind of electron buffer which isolates the N2 – H moiety from the electron structure changes produced by the substituent in the five-membered cycle. Except for three substituents, the – BH2, – BF2, and – COOH, the second stable form is the N3 – H form. The – BH2 and – BF2 groups are known for their withdrawing activity, however, the latter substituents withdraw the electrons from the triazole system slightly weaker (Table 1), and in our previous study the – COOH group was found to act at the imidazole ring with strength even bigger than the – BH2 group [16]. The other case constitutes the –CH3 substituent,

Table 1 Single point CCSD(T)/B3PW91, B3PW91/6-311þ þG** total energies (E, hartree), and B3PW91 based DG (298.15 K, 1 atm), and isomer relative energies (DE, kJ/mol) for series of the C5 substituted 1,2,3-triazoles. 1 Hartree ¼ 2626.7861 kJ/mol Tautomer form

E (hartree) single point CCSD(T)

DE (kJ/mol) single point CCSD(T)

E (hartree) B3PW91/6-311þ þG**

DE (kJ/mol) B3PW91/6-311þ þG**

DG298 (hartree) B3PW91/6-311þþ G**

D(DG298) (kJ/mol) B3PW91/6-311þ þG**

–NH2

N1– H N2– H N3– H

2296.921284 2296.931830 2296.924282

27.703 0 19.825

2297.550293 2297.561993 2297.553611

30.734 0 22.019

2297.502660 2297.513204 2297.505509

27.695 0 20.214

–F

N1– H N2– H N3– H

2340.749387 2340.762546 2340.755047

34.568 0 19.700

2341.415782 2341.429890 2341.421766

37.059 0 21.340

2341.392151 2341.405273 2341.397842

34.468 0 19.520

–Cl

N1– H N2– H N3– H

2700.745264 2700.754810 2700.747562

25.074 0 19.038

2701.759831 2701.770321 2701.762411

27.557 0 20.780

2701.738645 2701.748217 2701.741055

25.141 0 18.812

–H

N1– H N2– H N3– H

2241.684323 2241.691445 2241.684323

18.707 0 18.707

2242.197877 2242.205689 2242.197877

20.520 0 20.520

2242.164858 2242.171146 2242.164858

16.518 0 16.518

–CH3

N1– H N2– H N3– H

2280.903541 2280.910701 2280.903793

18.808 0 18.147

2281.514775 2281.522604 2281.514959

20.566 0 20.080

2281.456742 2281.464268 2281.457101

19.771 0 18.825

–OH

N1– H N2– H N3– H

2316.763790 2316.776582 2316.770108

33.601 0 17.003

2317.410494 2317.424179 2317.417107

35.946 0 18.578

2317.375019 2317.387501 2317.380923

32.785 0 17.280

–CN

N1– H N2– H N3– H

2333.723598 2333.732244 2333.726128

22.714 0 16.066

2334.415614 2334.424171 2334.417317

22.475 0 18.004

2334.386841 2334.394486 2334.388449

20.080 0 15.857

–NO2

N1– H N2– H N3– H

2445.787176 2445.795681 2445.789801

22.341 0 15.447

2446.669426 2446.677875 2446.671427

22.194 0 16.941

2446.638160 2446.645913 2446.640187

20.365 0 15.040

–CFO

N1– H N2– H N3– H

2453.880690 2453.887023 2453.881173

16.635 0 15.367

2454.762032 2454.768542 2454.762174

17.104 0 16.727

2454.731455 2454.737226 2454.731632

15.158 0 14.693

–CHO

N1– H N2– H N3– H

2354.778542 2354.784957 2354.779516

16.849 0 14.291

2355.507812 2355.514354 2355.508442

17.188 0 15.530

2355.468594 2355.474492 2355.469299

15.493 0 13.642 (continued on next page)

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Substituent

699

4.119 0 13.458 2267.592687 2267.594256 2267.589132 5.547 0 15.815 5.488 0 14.228 2267.049202 2267.051291 2267.045875 N1– H N2– H N3– H –BH2

2267.634975 2267.637086 2267.631065

19.021 0 9.854 2410.839138 2410.846380 2410.842629 23.266 0 10.892 22.274 0 9.716 2410.046810 2410.055289 2410.051591 N1– H N2– H N3– H –CONH2

2410.893201 2410.902058 2410.897912

2430.714976 2430.718880 2430.714129 12.081 0 13.261 11.265 0 13.073 2429.896949 2429.901237 2429.896260 N1– H N2– H N3– H –COOH

2430.758432 2430.763031 2430.757983

10.603 0 14.890 2466.223000 2466.227037 2466.221369 12.336 0 16.966 11.654 0 15.237 2465.366732 2465.371168 2465.365367 N1– H N2– H N3– H –BF2

2466.250944 2466.255639 2466.249180

D(DG298) (kJ/mol) B3PW91/6-311þ þG** DG298 (hartree) B3PW91/6-311þþ G** DE (kJ/mol) B3PW91/6-311þ þG** E (hartree) B3PW91/6-311þ þG** DE (kJ/mol) single point CCSD(T) E (hartree) single point CCSD(T) Tautomer form Substituent

Table 1 (continued)

10.256 0 12.479

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700

which neither donates nor withdraws the electrons from the triazole ring, and therefore the N1 –H and N3 –H tautomers are stabilized with almost equal energy (Table 1). It is noticeable that the substituents acting differently on the benzene ring, for example, the – NH2 and – NO2 groups, stabilize the 1,2,3triazole ring in a similar way. This was observed earlier for the analogous imidazole derivatives [21]. The electron withdrawing or donating effect is not the only intramolecular effect that determines the total energy of the substituted 1,2,3-triazole. The substituent at the position C5(4) interacts intramolecularly with the pyridine-like or pyrrol-like group and attractive or repulsive interactions between these groups occur. Indeed, various structures of carboxyl substituted triazoles, depicted in Fig. 1, illustrate the problem. For example, a repulsive interaction of the lone electron pairs of the two oxygen atoms in the – COOH group and repulsive interaction of the –OH group hydrogen atom with the hydrogen atom of the C – H group destabilize the structure 1 (Fig. 1). Analogously, a repulsive interaction of lone electron pair localized on pyridine-like nitrogen atom destabilizes the structure 5 (Fig. 1). On the contrary, the attractive intramolecular hydrogen bond interactions between the –COOH group and pyridine-like N1 atom and attractive interactions of the hydrogen atom at the position C4 cause that the structure 7 (Fig. 1) is in fact the most stable N3 –H form, despite the fact that the cisarrangement of the –OH group with respect to the CyO group, where the lone electron pairs of the two oxygen atom of the carboxylic group interact repulsively, is not an optimal conformation of carboxylic acids. This is why, in the optimal form of the N2 – H carboxy-1,2,3-triazole the OH moiety is in the trans arrangement with respect to the CyO group: structure 6 (Fig. 1). Similar substituent interactions with triazole ring were considered for amide substituents (Fig. 2). However, in this case, the most stable form appeared to be that with an intramolecular hydrogen bond present in the system: e.g. the N1 – H· · ·OyC H-bond in the N1 –H tautomer 1 (Fig. 2), the N –H· · ·N1 H-bond in the N2 – H tautomer 4 (Fig. 2), and the N – H· · ·N1 Hbond in the N3 – H tautomer 7 (Fig. 2). Again, the aldo1,2,3-triazoles are stabilized by intramolecular hydrogen bond interaction between the C – H or CyO

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701

Table 2 The B3PW91/6-311þþ G** calculated AIM charges in the most stable N2 tautomer of C5-substituted 123-triazoles Substituent

NH2 F Cl H CH3 OH BF2 CN NO2 CFO CHO COOH CONH2 BH2

Atomic charge (e) N1

N2

N3

C4

C5

H2

H4

R

R(sum)

20.662 20.621 20.594 20.620 20.637 20.671 20.567 20.549 20.559 20.559 20.590 20.573 na 20.555

20.293 20.281 20.280 20.284 20.293 20.288 20.272 20.269 20.264 20.268 20.275 20.273 na 20.277

20.612 20.599 20.609 20.620 20.627 20.609 20.627 20.605 20.602 20.614 20.618 20.621 na 20.639

0.458 0.491 0.504 0.461 0.459 0.480 0.480 0.510 0.528 0.512 0.497 0.503 na 0.473

0.902 1.077 0.579 0.461 0.480 1.069 20.199 0.531 0.776 0.474 0.428 0.464 Na 20.203

0.442 0.452 0.452 0.445 0.443 0.446 0.452 0.460 0.463 0.458 0.454 0.453 na 0.448

0.065 0.094 0.092 0.074 0.066 0.085 0.091 0.105 0.125 0.111 0.107 0.107 na 0.082

21.078 20.624 20.155 0.074 0.006 21.114 2.217 0.889 0.393 1.578 1.018 1.566 na 1.876

20.314 20.624 20.155 0.074 0.1 20.522 0.6 20.194 20.485 20.129 20.015 20.076 na 0.658

na stands for not available because of convergence problems.

Fig. 1. Dependence of the B3PW91/6-311þ þG** relative energy (kJ/mol) of 5-carboxy-1,2,3-triazoles on rotation of the –COOH substituent.

Fig. 2. Dependence of the B3PW91/6-311þþ G** relative energy (kJ/mol) of 5-amido-1,2,3-triazoles on rotation of the –CONH2 substituent.

702

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moieties of aldehyde group with the N1 or N1 –H moiety of triazole ring (structure 3, Fig. 3). The – CFO group constitutes the last example showing importance of an arrangement of the substituent (Fig. 4). From Table 2, it is clear that substituents have some influence on charge in the positions C5, only. Indeed, the charge at the N1, N2, N3, C4, H2, and H4 atoms differs a little from that in unsubstituted 1,2,3triazole solely for the most strongly acting electron donating or electron withdrawing groups, i.e. – NH2, – F, and – OH or – BH2 and BF2 substituents, respectively. Unexpectedly, the substituents, which have lone electron pair(s), generate increase of charge at the C5 atom. This phenomenon can be related to electron flow from the substituent to the p-electron

Fig. 4. Dependence of the B3PW91/6-311þþ G** relative energy (kJ/mol) of 5-CFO-1,2,3-triazoles on rotation of the – CFO substituent.

Fig. 3. Dependence of the B3PW91/6-311þ þG** relative energy (kJ/mol) of 5-aldo-1,2,3-triazoles on rotation of the – CHO substituent.

systems of the five-membered triazole system. As for the other atoms, only the most strongly acting electron withdrawing groups, – BH2 and – BF2, produce decrease of charge at the C5 atom. This can be connected with simultaneous, however, small, increase of charge at the N1 atom. For the other electron withdrawing substituents the effect is negligible, except for the NO2 group, which causes increase of charge at the C5 atom. Perhaps, the most striking tendency can be observed at the first atom of the substituent group attached directly to the C5 atom. The charge at this atom of the substituent is quite negative for electron donating groups and definitely positive for the electron withdrawing groups. A similar trend can be

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detected for the charge summed over the whole substituent, however, it is quite clear only for the most strongly acting groups.

4. Conclusions Theoretical DFT, HF, and single point CCSD(T) calculations were performed to investigate the stability and tautomerism of the C5 substituted 1,2,3-triazoles. Regardless the substituent used, the N2 – H 1,2,3-triazole tautomer appeared to be the most stable. Out of the two less stable tautomers, N1 –H and N3 – H, only the – BH2, –BF2, and – COOH substituents stabilize the N1 – H form. Thus usually, the N3 – H tautomer of the 1,2,3-triazole system is the second stable form of the molecule. Some, substituents that act quite differently on the benzene ring systems, e.g. – NH2 and – NO2 groups, influence the 1,2,3-triazole ring in a similar way. The relative stability of the C5 substituted 1,2,3-triazoles tautomerism is also influenced by intramolecular interactions (both attractive and repulsive) between substituent and protons located either at the N1 or N3 atom.

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