Short communication
A Combined Experimental and DFT Study on the Complexation of the Hydrazinium Cation with a Hexaarylbenzene - Based Receptor
Emanuel Makrlik,1* Petr Toman,2 Petr Vanura3 and Rajendra Rathore4
1 Faculty of Applied Sciences, University of West Bohemia, Husova 11, 306 14 Pilsen, Czech Republic
2 Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovskeho sq. 2,
162 06 Prague 6, Czech Republic
3 Department of Analytical Chemistry, Institute of Chemical Technology, Prague, Technicka 5, 166 28 Prague 6,
Czech Republic
4 Department of Chemistry, Marquette University, P.O. Box 1881, Milwaukee, Wisconsin 53201-1881, U.S.A. * Corresponding author: E-mail: makrlik@centrum.cz Received: 18-01-2011
Abstract
From extraction experiments and /-activity measurements, the exchange extraction constant corresponding to the equilibrium N2H+ (aq) + 1-Cs+(nb) ^ 1.N2H+ (nb) + Cs+(aq) taking place in the two-phase water-nitrobenzene system (1 = hexaarylbenzene - based receptor; aq = aqueous phase, nb = nitrobenzene phase) was evaluated as log Kex(N2H+, 1-Cs+) = -1.2 ± 0.1. Further, the stability constant of the hexaarylbenzene - based receptor 'N2H+ complex (abbrev. 1-N2H+) in nitrobenzene saturated with water was calculated for a temperature of 25 °C: log Pnb (1'N2H+) = 5.6 ± 0.2. By using quantum mechanical DFT calculations, the most probable structure of the 1'N2H+ complex species was solved. In this complex, the cation N2H+5 synergistically interacts with the polar ethereal oxygen fence and with the central hydropho-bic benzene bottom via cation - n interaction. Finally, the calculated binding energy of the resulting complex 1'N2H+ is -270.5 kJ/mol, confirming the relatively high stability of the considered cationic complex species.
Keywords: Hexaarylbenzene - based receptor, N2H+ cation, complexation, extraction and stability constants, water-nitrobenzene system, DFT, complex structure
1. Introduction
Hexaarylbenzene (HAB) derivatives attract a great attention because of unique propeller-shaped structure and potential application in molecular electronics and na-notechnology. 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 because it interacts both with the polar ethereal fence and with the central benzene ring via cation - n interaction.1 Cation - n interaction is a well-established phenomenon in gas phase, as well as in solid state,2-4 and is known to play an important role in the stabilization of tertiary structures of various proteins.5
The dicarbollylcobaltate anion6 and some of its halogen derivatives are very useful reagents for the extrac-
Scheme 1. Structural formula of a hexaarylbenzene (HAB)-based receptor (abbrev. 1).
tion 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,7-44 and on the technological scale for the separation of some high-activity isotopes in the reprocessing of spent nuclear fuel and acidic radioactive waste.45,46
Recently, the interaction of the K+ cation with the HAB - based receptor 1 has been investigated by the extraction method in the water-nitrobenzene system and by using DFT calculations.19 On the other hand, in the current work, the stability constant of the 1.N2H+ cationic complex species in nitrobenzene saturated with water was determined. Moreover, applying quantum mechanical DFT calculations, the most probable structure of the mentioned cationic complex was derived.
2. Experimental
Preparation of the electroneutral HAB - based receptor 1 (see Scheme 1) is described elsewhere.1 Cesium dicarbollylcobaltate (abbrev. CsDCC) was synthesized by means of the method published by Hawthorne et al.47 Hydrazinium chloride (N2H+Cl-) was supplied by Al-drich. The other chemicals used (Lachema, Brno, Czech Republic) were of reagent grade purity. The radionuclide 137Cs+ (Techsnaveksport, Russia) was of standard radioc-hemical purity.
The extraction experiments were carried out in 10 mL glass test-tubes with polyethylene stoppers: 2 mL of an aqueous solution of N2H+Cl- of the concentration in the range from 1 x 10-3 to 3 x 10-3 mol/L and microa-mounts of 137Cs+ were added to 2 mL of a nitrobenzene solution of 1 and CsDCC, the concentrations of which varied also from 1 x 10-3 to 3 x 10-3 mol/L (in all experiments, the initial concentration of 1 in nitrobenzene, C™,nb, was equal to the initial concentration of CsDCC in this medium, CJ?sDbCC). The test-tubes filled with the solutions were shaken for 2 hours at 25 ± 1 °C, using a laboratory shaker. Then the phases were separated by centrifugation. Afterwards, 1 mL samples were taken from each phase and their /-activities were measured using a well-type NaI(Tl) scintillation detector connected to a /-analyzer NK 350 (Gamma, Budapest, Hungary).
The equilibrium distribution ratios of cesium, DCs, were determined as the ratios of the measured radioactivities of 137Cs+ in the nitrobenzene and aqueous samples.
3. Results and Discussion
Regarding the results of previous papers,6,48,49 the two-phase water-N2H+Cl-nitrobenzene- cesium dicarbollylcobaltate (CsDCC) extraction system can be described by the following equilibrium
N2H+ (aq) + Cs+(nb) ^
N2H+ (nb) + Cs+(aq); Kex (N2H+, Cs+)
(1)
with the corresponding exchange extraction constant Kex (N2H+, Cs+); aq and nb denote the presence of the species in the aqueous and nitrobenzene phases, respectively. For the constant Kex (N2H+, Cs+) one can write48
iogK JN2h:,CV) = logK'N ][. -logK;v (2)
where K'V||+ and K'(v+ are the individual extraction constants for N52H+5 and Cs+, respectively, in the water-nitrobenzene system.48,49 Knowing the values log K'N2H+ = -4.849 and log K'Cs+= -2.7,48 the exchange extraction constant Kex (N2H+, Cs+) was simply calculated from Eq. (2) as log Kex (N2H+, Cs+) = -2.1.
Previous results50-54 indicated that the two-phase water-N2H+5Cl--nitrobenzene-1 (1 = HAB - based receptor)- CsDCC extraction system (see Experimental), chosen for determination of the stability constant of the complex 1.N2H+5 in water-saturated nitrobenzene, can be characterized by the main chemical equilibrium
N2H+(aq) + 1Cs+(nb) ? 1N2H5 (nb) + Cs+(aq)
(3)
with the respective equilibrium extraction constant Ke (N2H+5, 1.Cs+):
k„<n,h;,i-cs+) =
WUÇga [N:HajI-Cs+L
(4)
It is necessary to emphasize that 1 is a considerably hydrophobic receptor, practically present in the nitrobenzene phase only, where it forms - with N2H+5 and Cs+ - the relatively stable complexes 1.N2H+5 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 obtain the final expression for K (N2H+, 1.Cs+) in the form
Keil(N2HÎ, l-Cs+) =
1
pin.nb CsDCC
(5)
dCs (i + DCs)c;^:
CI- ^
in,nb CsDCC
where C^H+cr is the initial concentration of N2H+Cl- in the aqueou2 s5 phase and CCins,DnbCC denotes the initial concentration of CsDCC in the organic phase of the system under consideration.
In this study, from the extraction experiments and /-activity measurements (see Experimental) by means of
Eq. (5), the following value of the constant Kex (N2H+, 1-Cs+) was determined as log Kex (N2H+, 1-Cs+) = -1.2 ±0.1.
Furthermore, with respect to previous results,50-54 for the extraction constants Kex (N2H+5, Cs+) and Kex (N2H+, 1-Cs+) defined above, as well as for the stability constants of the complexes 1*N2H+ and 1*Cs+ in nitrobenzene saturated with water, denoted by Pnb (1'N2H+) and Pnb (1*Cs+), respectively, one gets
log Pnb(1-N2H+5) = log pnb(1'Cs+) + log Kex(N2H+5,1-Cs+) - log KJN2H+, Cs+)
(6)
Using the constants log Kex (N2H+, Cs+) and log Kex (N2H+, 1-Cs+) given above, the value log Pnb (1-Cs+) = 4.7 ± 0.1,62 and applying Eq. (6), we obtain the stability constant of the 1*N2H+ complex in water-saturated nitrobenzene at 25 °C as log Pnb (1N2H+) = 5.6 ± 0.2. This means that in the mentioned nitrobenzene medium, the stability of the 1'N2H+ complex under study is somewhat higher than that of the cationic complex species 1*Cs+.
The quantum mechanical calculations were carried out at the density functional level of theory (DFT, B3LYP functional) using the Gaussian 03 suite of programs.55 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-N2H+ complex species could be imagined, our quantum mechanical calculations in similar cases, performed in an analogous way, showed very good agreement of experiment with theory.56-61
In the model calculations, we optimized the molecular geometries of the parent HAB - based receptor 1 and its complex with N2H+5. The optimized structure of a free receptor 1 having C3 symmetry, 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 presented in our previous work.19 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 A (1 A = 0.1 nm); the diameter of this cavity in 1 is 6.19 A.19
The structure obtained by the full optimization of the cationic complex species 1*N2H+ is depicted in Figure 1, together with the lengths of the three two-center hydrogen bonds (in A). In this complex, the cation N2H+5 syner-gistically interacts with the hydrophilic polar ethereal oxygen fence and with the central hydrophobic benzene bottom via cation - n interaction (the distance between the mean plane of the bottom benzene ring and the "near"
nitrogen atom of N2H+ in the resulting complex TN2H+is 2.86 A, as pictured in Figure 1). At this point it is necessary to emphasize that the formation of the complex species 1*N2H+ results in the small getting longer of the respective cavity, as follows from comparison of Figure 1 with our previous results.19
Figure 1: Two projections of the DFT optimized structure of the 1-N2H+ 1. complex [B3LYP/31G(d)] (hydrogen atoms omitted for clarity except those of N2H+). The distance between the mean plane of the bottom benzene ring and the "near" nitrogen atom of N2H+ in the 1-N2H+ complex: 2.86 Â; the lengths of the three two-center H-bonds in the 1-N2H+ complex: 1.94, 2.72, 2.20, 2.70, 2.00, and 2.70 Â; the depth of the cavity in 1-N2H+: 2.22 Â.
Finally, the interaction energy, E(int), of the 1'N2H+ complex [calculated as the difference between electronic energies of 1'N2H+and isolated N2H+ and 1 species: E(int) = E(1-N2H+) - E(N2H+) - E(1)] was found to be -270.5 kJ/mol, which confirms quite unambiguously the formation of the considered cationic complex 1*N2H+.
4. Acknowledgements
This work was supported by the Czech Ministry of Education, Youth and Sports (Projects MSM 4977751303 and MSM 6046137307) and by the Czech Science Foundation (Project P205/10/2280). 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.
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Povzetek
S pomočjo ekstrakcije in meritev y-aktivnosti smo za konstanto ekstrakcije za ravnotežje N2H+ (aq) + 1'Cs+(nb) ^ 1-N2H+ (nb) + Cs+(aq) v dvo faznem sistemu voda-nitrobenzen (1 = heksaarilbenzen - receptor; aq = vodna faza, nb = nitrobenzen ) določili vrednost log Kex (N2H+, 1Cs+) = -1.2 ± 0.1. Pri 25 °C smo določili tudi konstanto stabilnosti kompleksa heksaaril N2H+ (LN2H+) v nitrobenzenu, nasičenem z vodo, ki znaša log Pnb (LN2H+) = 5.6 ± 0.2. S pomočjo kvantomehanskih DFT izračunov smo predvideli najbolj verjetno strukturo LN2H+ kompleksa. Izkazalo se je, da kation N2H+ sinergistično interagira s polarno etrsko skupino in centralnim hidrofobnim benzenovim obročem preko kation -n interakcij. Izračunali smo tudi vezno energijo nastalega kompleksa LN2H+, ki znaša -270.5 kJ/mol in potrjuje njegovo relativno visoko stabilnost.