Experimental and theoretical study on the cooperative interaction of the ethanolammonium cation with a hexaarylbenzene-based receptor

Chemical Physics 406 (2012) 86–90

Contents lists available at SciVerse ScienceDirect

Chemical Physics journal homepage: www.elsevier.com/locate/chemphys

Experimental and theoretical study on the cooperative interaction of the ethanolammonium cation with a hexaarylbenzene-based receptor Emanuel Makrlík a,⇑, Petr Toman b, Petr Vanˇura c, Rajendra Rathore d a

´cká 129, 165 21 Prague 6, Czech Republic Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Kamy Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovského sq. 2, 162 06 Prague 6, Czech Republic c Department of Analytical Chemistry, Institute of Chemical Technology, Prague, Technická 5, 166 28 Prague 6, Czech Republic d Department of Chemistry, Marquette University, P.O. Box 1881, Milwaukee, WI 53201 – 1881, USA b

a r t i c l e

i n f o

Article history: Available online 24 August 2012 Keywords: Ethanolammonium cation Hexaarylbenzene-based receptor Complexation DFT calculations Complex structures

a b s t r a c t From extraction experiments and c-activity measurements, the extraction constant corresponding to the equilibrium HL+(aq) + 1  Cs+(nb) , 1  HL+(nb) + Cs+(aq) taking place in the two–phase water–nitrobenzene system (HL+ = ethanolammonium, 1 = hexaarylbenzene-based receptor; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex (HL+,1  Cs+) = 1.1 ± 0.1. Further, the stability constant of the 1  HL+ complex in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log bnb (1  HL+) = 5.4 ± 0.2. Finally, by using quantum mechanical DFT calculations, the most probable structures A and B of the 1  HL+ complex species, which are obviously in a dynamic equilibrium, were indicated. In both of these structures of the resulting complex 1  HL+, the cation HL+ synergistically interacts with the polar ethereal oxygen fence and with the central hydrophobic benzene bottom of the parent receptor 1 via cation–p interaction. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In recent years, the hexaarylbenzene (HAB) core has received great attention because of its usage for the preparation of modern graphitic materials [1,2] that could potentially be employed in the emerging areas of molecular electronics and nanotechnology [3,4]. It has been previously described by employing NMR spectroscopy and X-ray crystallography that a HAB-based receptor (abbrev. 1; see Scheme 1) binds a single potassium cation since it interacts with the polar ethereal fence and with the central benzene ring via cation–p interaction [5,6]. Cation–p interaction is a well-established phenomenon in gas phase, as well as in solid state [7–14], and is known to play an important role in the stabilization of tertiary structures of various proteins [15]. The dicarbollylcobaltate anion (DCC) [16] and some of its halogen derivatives are very useful reagents for the extraction of various metal cations (especially Cs+, Sr2+, Ba2+, Eu3+, and Am3+) from aqueous solutions into a polar organic phase, both under laboratory conditions for purely theoretical or analytical purposes [17– 21], and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste [22,23].

In the current work, the stability constant of the HAB-based receptor. ethanolammonium complex species (abbrev. 1  HL+; see Scheme 1) in nitrobenzene saturated with water was determined. Moreover, applying quantum mechanical density functional theory (DFT) calculations, the most probable structures of this cationic complex were derived. 2. Experimental Preparation of the electroneutral HAB-based receptor 1 (see Scheme 1) is described elsewhere [5]. Cesium dicarbollylcobaltate (CsDCC) was synthesized by means of the method published by Hawthorne et al. [24]. Ethanolammonium chloride (abbrev. HL+Cl; see Scheme 1) was purchased from Fluka and it was employed as received. The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. The radionuclide 137 Cs+ was supplied by Techsnaveksport, Russia. The extraction experiments were carried out in 10 mL glass test-tubes with polyethylene stoppers: 2 mL of an aqueous solution of HL+Cl (1  103–3  103 mol/L) and micro amounts of 137 Cs+ were added to 2 mL of a nitrobenzene solution of 1 and CsDCC, whose initial concentrations varied also from 1  103 to 3  103 mol/L (in all experiments, the initial concentration of 1 , was equal to the initial concentration of in nitrobenzene, C in;nb 1

⇑ Corresponding author. E-mail address: [email protected] (E. Makrlík). 0301-0104/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chemphys.2012.08.012

CsDCC in this medium, C in;nb CsDCC ). The test-tubes filled with the solutions were shaken for 2 h at 25 ± 1 °C, using a laboratory shaker.

87

E. Makrlík et al. / Chemical Physics 406 (2012) 86–90

K ex ðHLþ ; 1  Csþ Þ ¼

½1  HLþ nb ½Csþ aq

ð4Þ

½HLþ aq ½1  Csþ nb

It is necessary to emphasize that 1 is a considerably hydrophobic receptor, practically present in the nitrobenzene phase only, where it forms – with HL+ and Cs+ – the relatively stable complexes 1  HL+ and 1  Cs+. Taking into account the conditions of electroneutrality in the organic and aqueous phases of the system under study, the mass balances of the univalent cations studied at equal volumes of the nitrobenzene and aqueous phases, as well as the measured distribution ratio of cesium, DCs = [1  Cs+]nb/[Cs+]aq, combined with Eq. (4), we gain the final expression for Kex (HL+, 1  Cs+) in the form:

K ex ðHLþ ; 1  Csþ Þ ¼

Scheme 1. Structural formulas of a hexaarylbenzene (HAB)-based receptor (abbrev. 1) and ethanolammonium (abbrev. HL+).

Then the phases were separated by centrifugation. Afterwards, 1 mL samples were taken from each phase and their c-activities were measured by means of a well-type NaI(Tl) scintillation detector connected to a c-analyzer NK 350 (Gamma, Budapest, Hungary). The equilibrium distribution ratios of cesium, DCs, were determined as the ratios of the corresponding measured radioactivities of 137Cs+ in the nitrobenzene and aqueous samples.

1 C in;nb CsDCC DCs ð1 þ DCs ÞC in;aq  C in;nb CsDCC HLþ Cl

ð5Þ

where C in;aq is the initial concentration of HL+Cl in the aqueous HLþ Cl in;nb phase and C CsDCC denotes the initial concentration of CsDCC in the organic phase of the system under consideration. In this study, from the extraction experiments and c-activity measurements (see Experimental) by means of Eq. (5), the following value of the constant Kex (HL+, 1  Cs+) was determined as log Kex (HL+, 1  Cs+) = 1.1 ± 0.1 (see Table 1). Furthermore, with respect to previous results [28–31], for the extraction constants Kex (HL+, Cs+) and Kex (HL+, 1  Cs+) defined above, as well as for the stability constants of the complexes 1  HL+ and 1  Cs+ in nitrobenzene saturated with water, denoted by bnb (1  HL+) and bnb (1  Cs+), respectively, one gets

log bnb ð1  HLþ Þ ¼ log bnb ð1  Csþ Þ þ log K ex ðHLþ ; 1  Csþ Þ  log K ex ðHLþ ; Csþ Þ

ð6Þ

Regarding the results of previous papers [16,25–27], the two– phase water–HL+Cl–nitrobenzene–cesium dicarbollylcobaltate (CsDCC) extraction system can be described by the following equilibrium

Using the constants log Kex (HL+,Cs+) and log Kex(HL+,1  Cs+) given above, the value log bnb(1  Cs+) = 4.7 ± 0.1 [32], and applying Eq. (6), we obtain the stability constant of the 1  HL+ complex in water-saturated nitrobenzene at 25 °C as log bnb(1  HL+) = 5.4 ± 0.2. This means that in the mentioned nitrobenzene medium, the stability of the 1  HL+ complex under study is somewhat higher than that of the cationic complex species 1  Cs+ (1 = HAB-based receptor).

HLþ ðaqÞ þ Csþ ðnbÞ () HLþ ðnbÞ þ Csþ ðaqÞ;

3.2. Quantum mechanical calculations

3. Results and discussion 3.1. Extraction experiments

K ex ðHLþ ; Csþ Þ

ð1Þ +

+

with the corresponding exchange extraction constant Kex (HL , Cs ); aq and nb denote the presence of the species in the aqueous and nitrobenzene phases, respectively. For the constant Kex (HL+, Cs+) one can write [25–27]

log K ex ðHLþ ; Csþ Þ ¼ log K iHLþ  log K iCsþ K iHLþ +

ð2Þ

K iCsþ

where and are the individual extraction constants for HL+ and Cs , respectively, in the water–nitrobenzene system [25–27]. Knowing the values log K iHLþ = 4.5 (HL+ = ethanolammonium) i [26] and logK Csþ = 2.7 [25], the exchange extraction constant Kex + + (HL , Cs ) was simply calculated from Eq. (2) as log Kex (HL+, Cs+) = 1.8. Previous results [28–31] indicated that the two–phase water– HL+Cl–nitrobenzene–1 (HAB-based receptor)–CsDCC extraction system (see Experimental), chosen for determination of the stability constant of the complex 1  HL+ in water-saturated nitrobenzene, can be characterized by the main chemical equilibrium þ

HLþ ðaqÞ þ 1  Cs ðnbÞ () 1  HLþ ðnbÞ þ

þ Cs ðaqÞ;

þ

þ

K ex ðHL ; 1  Cs Þ

ð3Þ

with the respective equilibrium extraction constant Kex (HL+, 1  Cs+):

The quantum mechanical calculations were carried out at the density functional level of theory (DFT, B3LYP functional) [33,34] using the Gaussian 03 suite of programs [35]. The 6–31G(d) basis set was used and the optimizations were unconstrained. In order to increase the numerical accuracy and to reduce oscillations during the molecular geometry optimization, two-electron integrals and their derivatives were calculated by using the pruned (99,590) integration grid, having 99 radial shells and 590 angular points per shell, which was requested by means of the Gaussian 03 keyword ‘‘Int = UltraFine’’. Although a possible influence of a polar solvent on the detailed structures of 1 and the 1  HL+ complex species could be imagined,

Table 1 Experimental data concerning determination of log Kex (HL+, 1  Cs+) on the basis of Eq. (5). C in;aq  (M) HLþ Cl

C in;nb CsDCC (M)

DCs

log Kex (HL+, 1  Cs+)

1.0  103 1.5  103 2.0  103 2.5  103 3.0  103

1.0  103 1.5  103 2.0  103 2.5  103 3.0  103

3.67 3.32 3.60 3.30 3.77

1.1 1.0 1.1 1.0 1.2

88

E. Makrlík et al. / Chemical Physics 406 (2012) 86–90

Fig. 1. Two projections of the DFT optimized structure of free receptor 1 [B3LYP/631G(d)] (hydrogen atoms omitted for clarity). The depth of the cavity in 1: 2.15 Å; the diameter of the cavity in 1: 6.2 Å.

our quantum mechanical calculations in similar cases, performed in an analogous way, showed very good agreement of experiment with theory [36–43]. In the model calculations, we optimized the molecular geometries of the parent HAB-based receptor 1 and the cationic complex 1  HL+. The optimized structure of the free receptor 1, involving a bowl-shaped cavity, which is comprised of an aromatic bottom (i. e., central benzene ring) and an ethereal fence formed by six oxygen atoms from the peripheral aryl groups, is illustrated in Fig. 1. The depth of the cavity, i.e., the distance between the mean plane of the aromatic bottom and that of the ethereal fence, is 2.15 Å (1Å = 0.1 nm); the diameter of this cavity in 1 is 6.2 Å (Fig. 1). The structures A and B obtained by the full DFT optimizations of the cationic complex species 1  HL+ are depicted in Figs. 2 and 3, respectively, together with the lengths of the corresponding bond interactions (in Å). In both of these structures of the complex 1  HL+, the cation HL+ synergistically interacts with the hydrophilic polar ethereal oxygen fence and with the central hydrophobic benzene bottom via cation–p interaction, as pictured in Figs. 2 and 3.

Fig. 2. Two projections of the DFT optimized structure A of the 1  HL+ complex [B3LYP/6–31G(d)] (hydrogen atoms of 1 omitted for clarity). The distance between the mean plane of the bottom benzene ring of 1 and nitrogen atom of the cation HL+ in the 1  HL+ complex: 2.93 Å; the distances between three hydrogens of the cation HL+ and the corresponding six oxygens of the ethereal fence of 1: 2.03, 2.57, 2.68, 2.87, 2.82, and 1.96 Å.

Finally, the interaction energies, E(int), corresponding to the structures A and B of the 1  HL+ complex under study [calculated as the difference between the pure electronic energies of 1  HL+ and isolated 1 and HL+ species: E(int) = E(1  HL+)  E(1) – E(HL+)], are very close: -208.6 and –206.9 kJ/mol, respectively. In conclusion, it should be noted that the structures A and B of the 1  HL+ complex are apparently in a dynamic equilibrium. Besides, from this point of view, the experimentally determined value of the stability constant of 1  HL+ in the nitrobenzene medium given above, corresponding to the equilibrium 1 (nb) + HL+(nb) , 1  HL+(nb), can be obviously considered as a certain ‘‘average’’ stability constant of the two DFT calculated structures.

E. Makrlík et al. / Chemical Physics 406 (2012) 86–90

89

cation in the cavity of the parent HAB-based receptor 1 as well as the significant interatomic distances within the complex species under study, were obtained. Acknowledgements This work was supported by the Grant Agency of Faculty of Environmental Sciences, Czech University of Life Sciences, Prague, Project No.: 42900/1312/3114 ‘‘Environmental Aspects of Sustainable Development of Society’’, by the Czech Ministry of Education, Youth, and Sports (Project MSM 6046137307), and by the Czech Science Foundation (Project P 205/10/2280). The computer time at the MetaCentrum (Project LM 2010005), as well as at the Institute of Physics (computer Luna/Apollo), Academy of Sciences of the Czech Republic, is gratefully acknowledged. Finally, R. R. thanks the National Science Foundation for financial support. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemphys.2012. 08.012. References

Fig. 3. Two projections of the DFT optimized structure B of the 1  HL+ complex [B3LYP/6-31G(d)] (hydrogen atoms of 1 omitted for clarity). The distance between the mean plane of the bottom benzene ring of 1 and nitrogen atom of the cation HL+ in the 1  HL+ complex: 2.92 Å; the distances between three hydrogens of the cation HL+ and the corresponding six oxygens of the ethereal fence of 1: 2.40, 2.17, 2.06, 2.76, 2.99, and 2.57 Å.

4. Conclusions In summary, we have demonstrated that a complementary experimental and theoretical approach can provide important information on the HAB-based receptor (1) complexation with the ethanolammonium cation HL+. From the experimental investigation of the resulting complex 1  HL+ in the two-phase water– nitrobenzene system, the strength of the considered 1  HL+ cationic complex species in nitrobenzene saturated with water was characterized quantitatively by the stability constant, log bnb (1  HL+) = 5.4 ± 0.2 (for a temperature of 25 °C). By using theoretical quantum mechanical DFT calculations, the structural details of the 1  HL+ complex, such as position and orientation of the HL+

[1] M.D. Watson, A. Fechtenkötter, K. Müllen, Chem. Rev. 101 (2001) 1267. [2] R. Rathore, C.L. Burns, J. Org. Chem. 68 (2003) 4071. [3] M.C. Petty, M.R. Bryce, D. Bloor, Introduction to Molecular Electronics, Oxford University Press, New York, 1995. [4] B.G. Maiya, T. Ramasarma, Curr. Sci. 80 (2001) 1523. [5] R. Shukla, S.V. Lindeman, R. Rathore, J. Am. Chem. Soc. 128 (2006) 5328. [6] R. Shukla, S.V. Lindeman, R. Rathore, Org. Lett. 9 (2007) 1291. [7] J.C. Ma, D.A. Dougherty, Chem. Rev. 97 (1997) 1303. [8] P.B. Armentrout, M.T. Rodgers, J. Phys. Chem. A 104 (2000) 2238. [9] A. Gapeev, C.N. Yang, S.J. Klippenstein, R.C. Dunbar, J. Phys. Chem. A 104 (2000) 3246. [10] S. Tsuzuki, M. Yoshida, T. Uchimaru, M. Mikami, J. Phys. Chem. A. 105 (2001) 769. [11] H. Huang, M.T. Rodgers, J. Phys. Chem. A 106 (2002) 4277. [12] Y. Mo, G. Subramanian, J. Gao, D.M. Ferguson, J. Am. Chem. Soc. 124 (2002) 4832. [13] A.S. Reddy, G.N. Sastry, J. Phys. Chem. A 109 (2005) 8893. [14] D. Vijay, G.N. Sastry, Phys. Chem. Chem. Phys. 10 (2008) 582. [15] K. Sakurai, T. Mizuno, H. Hiroaki, K. Gohda, J. Oku, T. Tanaka, Angew. Chem. Int. Ed. 44 (2005) 6180. [16] E. Makrlík, P. Vanˇura, Talanta 32 (1985) 423. [17] E. Makrlík, P. Vanˇura, P. Selucky´, J. Solution Chem. 38 (2009) 1129. [18] E. Makrlík, P. Vanˇura, P. Selucky´, V.A. Babain, I.V. Smirnov, Acta Chim. Slov. 56 (2009) 718. [19] E. Makrlík, P. Vanˇura, Z. Sedláková, J. Radioanal. Nucl. Chem. 280 (2009) 607. ˇ ura, P. Selucky´, J. Radioanal. Nucl. Chem. 283 (2010) 571. [20] E. Makrlík, P. Van [21] E. Makrlík, P. Vanˇura, P. Selucky´, J. Radioanal. Nucl. Chem. 284 (2010) 137. [22] V.N. Romanovskiy, I.V. Smirnov, V.A. Babain, T.A. Todd, R.S. Herbst, J.D. Law, K.N. Brewer, Solvent Extr. Ion Exch. 19 (2001) 1. [23] J.D. Law, R.S. Herbst, T.A. Todd, V.N. Romanovskiy, V.A. Babain, V.M. Esimantovskiy, I.V. Smirnov, B.N. Zaitsev, Solvent Extr. Ion Exch. 19 (2001) 23. [24] M.F. Hawthorne, D.C. Young, T.D. Andrews, D.V. Howe, R.L. Pilling, A.D. Pitts, M. Reintjes, L.F. Warren, P.A. Wegner, J. Am. Chem. Soc. 90 (1968) 879. [25] J. Rais, Collect. Czech. Chem. Commun. 36 (1971) 3253. [26] E. Makrlík, F. Bozˇek, Polish J. Chem. 72 (1998) 949. ˇ ura, J. Budka, J. Radioanal. Nucl. Chem. 286 (2010) [27] E. Makrlík, P. Selucky´, P. Van 155. [28] E. Makrlík, J. Hálová, M. Kyrš, Collect. Czech. Chem. Commun. 49 (1984) 39. [29] E. Makrlík, P. Vanˇura, ACH – Models Chem. 135 (1998) 213. ˇ ura, M. Dan ˇ ková, J. Radioanal. Nucl. Chem. 240 (1999) 579. [30] E. Makrlík, P. Van [31] E. Makrlík, P. Vanˇura, Monatsh. Chem. 137 (2006) 157. ˇ ura, P. Selucky´, R. Rathore, J. Radioanal. Nucl. Chem. [32] E. Makrlík, P. Toman, P. Van 286 (2010) 55. [33] C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. [34] A.D. Becke, J. Chem. Phys. 98 (1993) 5648. [35] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K.

90

E. Makrlík et al. / Chemical Physics 406 (2012) 86–90

Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M. W. Gill, B. Johnson, W. Chen, M.W. Wong, C. Gonzalez, J.A. Pople, Gaussian 03, Revision C. 02, Gaussian, Inc., Wallingford, CT, 2004. [36] J. Krˇízˇ, J. Dybal, E. Makrlík, Biopolymers 82 (2006) 536. [37] J. Krˇízˇ, J. Dybal, E. Makrlík, P. Vanˇura, J. Lang, Supramol. Chem. 19 (2007) 419. ˇ ura, Supramol. Chem. 20 (2008) 387. [38] J. Krˇízˇ, J. Dybal, E. Makrlík, P. Van

ˇ ura, Supramol. Chem. 20 (2008) 487. [39] J. Krˇízˇ, J. Dybal, E. Makrlík, J. Budka, P. Van [40] J. Krˇízˇ, J. Dybal, E. Makrlík, J. Budka, J. Phys. Chem. A 112 (2008) 10236. [41] J. Krˇízˇ, J. Dybal, E. Makrlík, J. Budka, P. Vanˇura, J. Phys. Chem. A 113 (2009) 5896. [42] J. Krˇízˇ, P. Toman, E. Makrlík, J. Budka, R. Shukla, R. Rathore, J. Phys. Chem. A 114 (2010) 5327. [43] J. Krˇízˇ, J. Dybal, E. Makrlík, P. Vanˇura, B.A. Moyer, J. Phys. Chem. B 115 (2011) 7578.