Catalytic hydrogen transfer in donor-acceptor complexes

ISSN 0012�5008, Doklady Chemistry, 2011, Vol. 437, Part 2, pp. 82–86. © Pleiades Publishing, Ltd., 2011.Original Russian Text © K.K. Kalninsh, E.F. Panarin, 2011, published in Doklady Akademii Nauk, 2011, Vol. 437, No. 4, pp. 491–495.

82

Hydrogen transfer in organic reactions involvessuccessive electron and proton transfer steps, whichcan be experimentally detected in studying excitedcomplexes, exciplexes, by means of laser time�resolved spectroscopy [1]. In real�time thermochemi�cal experiments, the stages of electron and protontransfer are usually inseparable and coalesce in oneprocess of hydrogen transfer. Exceptions are systemsin which stable radical ions form at the stage of elec�tron transfer, for example in the reaction of chloranilwith p�phenylenediamine [2]. Immediately after mix�ing the reagents in polar methanol at low temperature,the spectrum shows absorption bands of chloranil rad�ical anions and p�phenylenediamine radical cations atequilibrium with the starting reagents. An increase intemperature initiates proton transfer followed bythe polycondensation reaction. However, in mostchemical systems, no radical species are detected inthe course of reactions, which encourages research�ers to treat chemical transformations withoutimplying the occurrence of electron and hydrogentransfer (excitation) in the elementary thermo�chemical reaction [3].

In [4, 5], we developed the notions about themechanism of hydrogen transfer in specific electroni�cally excited diradical states, which can be experimen�tally and theoretically studied. This brings about thequestion about the catalytic (from the classical stand�point) properties of the hydrogen transfer reaction,which can shed light onto the factors responsible forextremely low, almost zero, hydrogen transfer activa�tion energies Ea [6]. The major factor is suggested to beproton donor–acceptor (PDA) interactions, which, incombination with electron donor–acceptor (EDA)interactions in the excited state, account for thedecrease in Ea to the range of thermal values (less than1 eV).

Quantum�chemical calculations were performedwith the GAMESS program package [7] by ab initioRHF, ROHF, and GVB methods with the use of theDH, SBK(d,p), and 6�31G(d,p) basis sets (see [8] formethodical details of calculations of molecules in theground (S0) and excited (S1 and T1) states).

Figure 1 shows the effect of the hydrogen bondcharacter on the position of the charge transfer (CT)band in varying the structure of crystalline complexesof p�benzoquinone (p�BQ) with hydroquinone (HQ).The planar complex in solution (without hydrogenbond, curve 1) and crystal (with a hydrogen bond,curve 2) gives rise to the absorption band at, respec�tively, 24 000 and 17 000 cm–1 (the shift is 7000 cm–1).A further decrease in the CT energy is observed ininclusion complexes (curves 3–5) in which the inser�tion of solvent molecules into the crystal lattice of thecomplex changes the state of the hydrogen bond. Themaximal low�frequency shift in this case is 11 000 cm–1,and a fine crystalline suspension thereby becomes bluecolor. A clear correlation between the hydrogen bond

CHEMISTRY

Catalytic Hydrogen Transfer in Donor–Acceptor ComplexesK. K. Kalninsh and Corresponding Member of the RAS E. F. Panarin

Received October 18, 2010

DOI: 10.1134/S0012500811040021

Institute of Macromolecular Compounds, Russian Academy of Sciences, Bol’shoi pr. 31, St. Petersburg, 199004 Russia

D

26 22 18 14ν × 10–3, cm−1

1

5

23

4

Fig. 1. Absorption spectra of the p�benzoquinone–hydro�quinone complexes: (1) a solution in acetonitrile at 25°С;(2) crystalline complex in KBr; (3–5) a fine crystallinesuspension in an acetonitrile–methylene chloride (3, 1 :1.5; 4, 1 : 2) and acetonitrile–chloroform (5, 1.5 : 1) sol�vent at ⎯50°С.

DOKLADY CHEMISTRY Vol. 437 Part 2 2011

CATALYTIC HYDROGEN TRANSFER IN DONOR–ACCEPTOR COMPLEXES 83

Calculated hydrogen transfer energies (Ea) in the excited T1 state, the singlet–triplet splitting E(S1) – E(T1), and the energyEH and length RO⋅⋅⋅H of the hydrogen bond in the ground state S0

Complex Ea in the T1 state (kcal/mol); computation method E(S1) – E(T1) EH (kcal/mol); RO⋅⋅⋅H (Å);

computation method

p�BQ ⋅ HQ –0.03; ROHF/DH21.83; ROHF/SBK(d,p)

0.0000.004

4.74; 2.00; RHF/SBK(d,p)5.39; 2.03; RHF/6�31G(d,p)

CBQ ⋅ HQ –1.56; ROHF/DH20.79; ROHF/SBK(d,p)

0.013– 4.40; 2.04; RHF/SBK(d,p)

CBQ ⋅ CHQ –0.78; ROHF/DH21.28; ROHF/SBK(d,p)

–– 5.23; 2.00; RHF/SBK(d,p)

p�BQ ⋅ MHQ –0.76; ROHF/DH–

0.004– 4.59; 2.02; RHF/SBK(d,p)

p�BQ ⋅ P 2.19; ROHF/DH26.51; ROHF/SBK(d,p)

–0.002 4.81; 1.99; RHF/SBK(d,p)

CBQ ⋅ P –0.71; ROHF/DH25.74; ROHF/SBK(d,p)

–0.004 4.49; 2.03; RHF/SBK(d,p)

MBQ ⋅ P 4.71; ROHF/DH30.91; ROHF/SBK(d,p)

–0.004 4.69; 2.00; RHF/SBK(d,p)

p�BQ ⋅ p�CP 3.33; ROHF/DH27.21; ROHF/SBK(d,p)

–0.010 4.48; 2.03; RHF/SBK(d,p)

p�BQ ⋅ Anl 9.54; ROHF/DH27.21; ROHF/SBK(d,p)

0.006– 2.97; 2.47; RHF/SBK(d,p)

p�BQ ⋅ p�PD4.16; ROHF/DH

22.19; ROHF/SBK(d,p)22.15; ROHF/6�31G(d,p)

0.003– 2.96; 2.48; RHF/SBK(d,p)

Note: p�BQ is p�benzoquinone, HQ is hydroquinone, CBQ is 2�chloro�1,4�benzoquinone, CHQ is chlorohydroquinone, MHQ is 2�methylhy�droquinone, P is phenol, MBQ is 2�methyl�1,4�benzoquinone, p�CP is p�chlorophenol, Anl is aniline, and p�PD is p�phenylenediamine.

84


KALNINSH, PANARIN

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F = 0.05F = 0.30

F = 0.03

F = 0.14

F = 0.10 F = 0.15

F = 0.20

F = 0.03

F = 0.03

F = 0.06

F = 0.04

F = 0.80F = 0.06

1.79 Å

F = 0.03

F = 0.14

F = 0.15

F = 0.12

F = 0.19F = 0.03

F = 0.30

F = 0.05

1.79 Å

1.76 Å

F = 0.01

F = 0.03

F = 0.05

F = 0.03F = 0.05

F = 0.81

S0

E(T1) ~ E(S1) = –1.56 kcal/mol (ROHF/DH)

+20.79 kcal/mol (ROHF/SBK(d,p)

1

2

E(T1) ~ E(S1) = –16.03 kcal/mol (ROHF/DH)

+9.85 kcal/mol (ROHF/SBK(d,p)

Cl21

Cl26

Cl31

Cl32

Cl21

Fig. 2. Calculated (ROHF/DH) molecular structures of reaction complexes in the excited triplet state. Complexes: 1 is chloro�p�ben�zoquinone–hydroquinone, and 2 is chloro�p�benzoquinone–chloroacetic acid–hydroquinone. Inset: complex 2 in the ground elec�tronic state S0. The free valence indices (F) and excitation energies (E(T1) and E(S1)) with respect to the free reagent level are given.


CATALYTIC HYDROGEN TRANSFER IN DONOR–ACCEPTOR COMPLEXES 85

state and the CT band position in near�IR region(5000–7000 cm–1) has been demonstrated by X�raycrystallographic and optical studies of EDA inclusioncomplexes of tetracyanoquinodimethane with benzi�dine [9, 10].

These and other (see, e.g., [11]) studies deal withvertical optical transitions. As for equilibrium excitedstates active in thermochemical reactions, emissionspectra of hydrogen�bonded EDA complexes, as arule, cannot be detected because of almost completequenching. Therefore, quantum�chemical calcula�tions are a major source of information on the natureand energy of these states.

The table presents the spatial structures of somedonor–acceptor complexes in the excited triplet stateand calculated hydrogen transfer energies, which havethe meaning of the activation energy. Of the two usedab initio methods, ROHF/DH and ROHF/SBK(d,p),the former satisfactorily describes the electronicallyexcited states of molecules and complexes (see [12]),whereas the latter provides hydrogen bond energiesconsistent with the experimental values. It is worthnoting that calculations predict low, close to zero,Ea values, which agrees with the experimentallyobserved high rate of hydrogen atom transfer betweenhydroquinone and chloro�p�benzoquinone in solu�tion [13].

For the p�BQ ⋅ HQ complex, the experimental ver�tical transition energy in solution is 24 000 cm–1 (3 eV)(Fig. 1, curve 1). The equilibrium level, with inclusionof solvation of the polar excited state, is considerablylower on the energy scale, but no more than by 1.5 eV.Then, the energy of the equilibrium relaxed stateactive in the hydrogen transfer reaction is ECT > 1.5 eV,which noticeably exceeds the hydrogen transfer energycalculated by known quantum�chemical methods(table). These simple estimates support the conclusion[11] that the hydrogen transfer reaction is catalyzedboth by the external catalyst agent and by means ofconjugation of EDA and PDA interactions (electron–proton effect [14]) in the reaction complex. PDAreagents function as the catalyst owing to their inher�ent properties, such as the ability to form inter� orintramolecular hydrogen bond. From the standpointof the nature of elementary stages, the suggested cata�lytic mechanism occurs in any reactions of reagentscontaining mobile hydrogen capable of forminghydrogen bond between π�electron systems andinvolved in the chemical process.

The latter conclusion is favored by the similarityof the structures and free valence index (F) distribu�tions in the catalytic reaction complexes (Fig. 2). Incomplex 1, the reagent molecules are directly boundto each other; in complex 2, modeling bifunctional

catalysis, the reagent molecules interact through theacid molecule that acts as the external catalystagent. During the quantum�chemical optimizationof the excited triplet electronic state (T1), thehydrogen atom is transferred from hydroquinone tothe chloro�p�benzoquinone molecule to form twochemically active radical species. The energy of sucha state calculated at the ROHF/DH level has a smallnegative value of ⎯1.56 kcal/mol, whereas theROHF/SBK(d,p) method gives +20.79 kcal/mol forthis energy. The excited singlet state S1 is character�ized by nearly the same energies and spatial and elec�tronic structure parameters, which allows us to callboth lowest�lying excited intermolecular states S1

and T1 “degenerate.” Analogous hydrogen transferoccurs in the presence of the molecule of chloroace�tic acid (Fig. 2), which acts as a bridge and bifunc�tional [6] catalyst and noticeably decreases thehydrogen transfer activation energy. In the groundelectronic state S0 of complex 2 (see the inset in Fig.2), the reagents are chemically inert and a involvedonly in the formation of a moderate hydrogen bond(E = 11.62 kcal/mol for two such bonds). The calcu�lations with the SBK(d,p) or 6�31G(d,p) basis setsgive higher (as compared with the DH basis set) abso�lute energies of the excited states, but they ade�quately reproduce the above catalytic effects.

Thus, the conjugation of two π�electron systems ofthe donor–acceptor complex through the commonhydrogen atom sharply decreases the energy of theexcited electronic state (Ea) down to zero or even neg�ative values and thus catalyzes the hydrogen transferreaction. In the case of bifunctional catalysis, hydro�gen transfer occurs through the bridging acid moleculeacting as the catalyst. In this case, electronic excita�tion involves both molecules of the hydrogen donorand acceptor, which are not directly bound to eachother. This conclusion follows from the distribution offree valence indices F. The ground state of the reactioncomplex is not involved in the hydrogen transfer reac�tion and forms only weak intermolecular donor–acceptor and hydrogen bonds.

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Catalytic hydrogen transfer in donor-acceptor complexes