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A theoretical investigation of furan-Al[X.sub.3], pyrrole-Al[X.sub.3] and thiophene-Al[X.sub.3] (X = H, F, Cl, Br) interactions.

The binding interactions between an electron pair donor (Lewis base) and an electron pair acceptor (Lewis acid) play an important role in many chemical processes. The reaction course of numerous reactions takes place with formation of donor-acceptor adducts as intermediates. The geometrical parameters are sensitive to intra- and intermolecular interactions; hence they can indicate and characterize these processes. The strength of the adduct bond is generally lower than that of a typical covalent bond.

Complexes formed by Lewis acids and bases are widely known and of great importance in modern chemistry. The types of reactions in which trivalent aluminum plays a catalytic role are many and varied. The Friedel-Crafts alkylation and acylation of aromatic rings, removal of tert-butyl groups from phenols, and the well-known ZieglerNatta polymerization reactions are some examples where the aluminum trichloride acts as catalyst. Many of these important compounds have been experimentally and theoretically studied [1-6]. The complexing behavior of aluminum trihalides Al[X.sub.3] has also been the subject of experimental and theoretical works [7-26]. The points that have been more developed are conformational structure, complexation energy and charge transfer. One of the major characteristics of these adducts is the donor-acceptor complexation energy. Mulliken proposed that donor-acceptor complex formation depends on the degree of charge transfer between the HOMO of the donor and the LUMO of the acceptor [27]. According to this point of view, the total charge-transfer QT from donor (D) to acceptor (A) should determine the energy of the donor-acceptor bond and, as a result, the complexation energy. However, in recent computational studies it has been shown that in some systems such a correlation is not valid [28]. This can be attributed to the importance of the terminal atoms in the complex formation.

In this work, we report our investigation on the alane-trihalide (Al[X.sub.3], X = F, Cl, and Br) donor-acceptor complexes [X.sub.3]Al-Y[C.sub.4][H.sub.4] (Y = O in furan, Y = NH in pyrrole, and Y = S in thiophen) compared to the alane [H.sub.3]Al-Y[C.sub.4][H.sub.4] ones. Despite many theoretical works, no comparative ab initio studies of these complexes have been carried out. The electronic structure of these complexes has been analyzed and the relative stabilities are examined.

Material and Methods

Computational details

The geometry optimizations have been carried out at the B3LYP/6-311G(d,p) level. The nature of all stationary point structures were determined by analytical frequency analysis, which also provided zero-point vibrational energies (ZPEs). ZPEs were scaled by the factor 0.9153 [29]. All structures reported here are minima on the potential energy surface (only positive eigenvalues of the Hessian matrix). Final energies were calculated at the B3LYP/6-311G (d,p) + ZPEs level. The basis set superposition error (BSSE) correction was evaluated using the counterpoise method [30]. The electronic structure has been done using the natural bond orbital (NBO) partitioning analysis [31] .The calculations were performed using the GAUSSIAN03 suite of programs [32].

Results and Discussion

Association of Al[X.sub.3] ([D.sub.3h] symmetry; X = H, F, Cl and Br), which act as electron pair acceptors, with Y[C.sub.4][H.sub.4] (Y = O in furan, Y = NH in pyrrole, and Y'= S in thiophen), wish act as electron pair donors, leads to [X.sub.3]Al-Y[C.sub.4][H.sub.4]. For all complexes [C.sub.s] symmetry is found to be favored. The geometry and electronic structures of these complexes have been analyzed and the relative stability is examined. Table 1 lists relevant optimized bond lengths and bond angles for all the complexes studied in this work. The depicted geometrical parameters are reported in figures 1, 2 and 3.

One can see from Table 1 that upon coordination, there are a number of intramolecular distortions that accompany the formation of the complex. In these [C.sub.s] complexes there are two different Al -X bond length (there are two chemically different X atoms, Xa and Xb--see figures 1, 2 and 3), two different [angle]XAlX angles (namely [angle]XaAlXb and [angle]XbAlXb), and two different [angle]XAlY angles (namely [angle]XaAlY and [angle]XbAlY). The calculated Al-X bond lengths in [X.sub.3]Al-Y[C.sub.4][H.sub.4] complexes are little longer than that in isolated moieties AlX3 (X = H, F, Cl and Br). This increase does not exceed 1.5%.




Upon complexation, the lengthening of the Al-X bond increases when going from Al[H.sub.3] to Al[Br.sub.3]. This is because in the isolated Al[X.sub.3] strong [pi]-donation from the halogen lone pairs into the formally empty p ([pi]) orbital at aluminum stabilizes the molecule, yielding shorter Al-X bonds. Of particular interest is the Y-C (Y = O in furan, Y = NH in pyrrole, and Y = S in thiophen) bond distance. Upon complexation the calculated geometrical parameters show a lengthening of the Y-C (Y = O in furan, Y = NH in pyrrole, and Y = S in thiophen) bonds. Indeed, the NBO calculations show that in isolated furan donor fragment, the pairs on Y (Y = O in furan) atom have lower "s" character than that in [X.sub.3]Al-furan complexes. On the other hand one notices the reverse in the case of the other complexes [X.sub.3]Al-pyrrole and [X.sub.3]Al-thiopnene.

Taking into account that greater "s" character in the complex favors a shorter and stronger bond, we can deduce that this change alone would imply a shortening of the Y-C bond lengths because it increases upon coordination. Moreover, Table 2 shows that the 2s atomic orbital (AO) contribution of Y = O, in the O-C bond is more important in [X.sub.3]Al-furan (X = H, F, Cl and Br) complexes than that in isolated furan moiety. In the even feel, the calculated Wiberg bond index, from the NBO analysis, of the Y-C bond decreases upon coordination for Y = O, NH and S atoms (Table 2).

On the other hand, the bond angle [angle]X-Al-Y (Y = O, NH, and S) varies slightly in going from Al[X.sub.3] free moiety (90[degrees]) to [X.sub.3]AlY[C.sub.4][H.sub.4] complex adducts. It increases on average only by about 10[degrees]. This has a consequence for the Al geometrical environment, which passes from [D.sub.3h] (flat) in free Al[X.sub.3] to pseudo-pyramidal in the complex. For the bond angles [angle]X-Al-X and [angle]C-Y-C we note that no notable deviation in going from isolated Al[X.sub.3] to [X.sub.3]AlY[C.sub.4][H.sub.4] complex. One can see that [angle]X-Al-X bond angle decreases by about 4[degrees] in going from the isolated Al[X.sub.3] (X = H, F, Cl and Br) ligand to the complex adduct. The [angle]C-Y-C bond angle increases by about ~1[degrees] in going from the isolated Y[C.sub.4][H.sub.4] ligand to the complex adduct. This trend is consistent with the observed Y-C bond lengths which are affected very little by coordination.

Table 3 lists the computed complexation energies for the [X.sub.3]AlY[C.sub.4][H.sub.4] (X = H, F, Cl and Br; Y = O in furan, Y = NH in pyrrole, and Y = S in thiophen), donor-acceptor complexes and the charge transfer from Y[C.sub.4][H.sub.4] Lewis bases to Al[X.sub.3] Lewis acids ([Q.sub.t]). The complexation energies are calculated as the difference between the energies of the complexes and the respective donor-acceptor moieties. The estimation of the basis set superposition error (BSSE) for all the structures presented here was performed by the full counterpoise method at the B3LYP/6-311G (d,p) level. These results are also presented in Table 3. The BSSE goes from 0.89 kcal/mol for the [H.sub.3]Al-thiophen complex to 5.52 kcal/mol for the [F.sub.3]Al-pyrrole complex. Table 3 shows that BSSE has slightly significant values and must be taken into account. The calculated complexation energies [E.sub.comp] of the halogen alane Lewis acids with Y[C.sub.4][H.sub.4] (Y = O in furan, Y = NH in pyrrole, and Y = S in thiophen) Lewis bases show the trend furan > pyrrole > thiophen at the B3LYP/6-311G (d,p) + BSSE corrections level of theory.

The furan complexes with Al[X.sub.3] (X = H, F, Cl and Br) Lewis acids are calculated to be more strongly bound than the respective pyrrole and thiophen complexes. In addition, the energetic results show that the stability decreases when going from Y = O in furan to Y = S in thiophen for all complexes. Indeed, the complexation energies of [H.sub.3]Al-furan, [H.sub.3]Al-pyrrole, and [H.sub.3]Al-thiophen complexes are -9.54, -5.86, and -4.93 kcal/mol, respectively, while the complexation energies of [F.sub.3]Al-Furan, [F.sub.3]Al-Pyrrole, and [F.sub.3]Al-Thiophen are -19.17, -15.59, and -10.96 kcal/mol, respectively, the complexation energies of [Cl.sub.3]Al-furan, [Cl.sub.3]Al-pyrrole, and [Cl.sub.3]Al-thiophen are -14.77, -12.23, and -8.52 kcal/mol, respectively, and the complexation energies of [Br.sub.3]Al-furan, [Br.sub.3]Al-pyrrole, and [Br.sub.3]Al-thiophen are -11.85, -9.49, and -6.32 kcal/mol, respectively. On the other hand, one can observe that the complexation energy of the alane complexes show the trend Al[F.sub.3] > Al[Cl.sub.3] > Al[Br.sub.3] > Al[H.sub.3] at the both levels of calculation B3LYP/6-311G (d,p) and B3LYP/6-311G(d,p) + BSSE. This confirms that the values of the BSSE correction have no influence on the established trend. Indeed, Figure 4 shows nicely that furan leads always to the more stable complex among the Lewis bases.


The charge transfer from furan to Al[X.sub.3] (X = H, F, Cl and Br) is lower than that from pyrrole and thiophene, while the complexation energies of [X.sub.3]Al-furan complexes are higher than that for [X.sub.3]Al-pyrrole and [X.sub.3]Al-thiophene complexes (see Table 3). Moreover, the F3Al-furan complex is the most stable and it show only a lower charge transfer (0.081 e), whereas the less stable complex is [H.sub.3]Al-thiophene and is shows a charge transfer of 0.192 e.

Hence, one can see that from the NBO results it follows that there is no correlation between charge transfer and the calculated complexation energy of [X.sub.3]AlY[C.sub.4][H.sub.4] (X = H, F, Cl and Br; Y = O in furan, Y = NH in pyrrole, and Y = S in thiophene) donoracceptor complexes. However and as that is shown on Table 3, for a same base Y[C.sub.4][H.sub.4], the stability of the complxes increases with the electronegativity of halogen X ([X.sub.F] > [X.sub.CI] > [X.sub.Br] > [X.sub.H]). This result can also be confirmed by another descriptor. It is about the gap between the HOMO of the donor and the LUMO of the acceptor. It is known that more this gap is narrow more coordination between the donor and the acceptor is strong and more the complex formed donor-acceptor is stable.

The results of the calculations carried out on B3LYP/6-311G (d,p) level shows the opposite as it is shown by Table 4. Indeed, the weakest gap (346, 39 kcal/mol) and the highest gap (491, 98 kcal/mol) correspond to the Al[H.sub.3]-thiophene complexes the least stable and most stable Al[F.sub.3]-furan, respectively. It is significant to note within this framework which it is known the frontier molecular orbital theory proposed by Fukui takes into account only the interactions between a LUMO and a HOMO. It is obvious that the chemical reactivity is not reduced simply to this type of interaction. One must also consider repulsive terms which will intervene between the occupied OM. Consequently, the donor orbital of the [C.sub.4][H.sub.4]Y species is not really the HOMO. So, one must be very careful during the use of the concept of the HOMO in a DFT calculation.

DFT calculations have been carried out to study the interaction in [X.sub.3]Al-Y[C.sub.4][H.sub.4] (X = H, F, Cl and Br; Y = O in furan, Y = NH in pyrrole, and Y = S in thiophen) donor-acceptor complexes. We have shown that the formation of the donor-acceptor complexes is accompanied by a certain number of geometrical distortions such as the increase length of Al-X bond while passing by Al[H.sub.3] to Al[Br.sub.3]. The same remark is valid for Y-C bond. One can also note significant changes such as the growth of [angle]XAlY angle because of the passage of the aluminum atom of symmetry [D.sub.3h] (in isolated species Al[X.sub.3]) to a pseudo-pyramidal geometry in the studied complexes.

We have also shown that the stability of the complexes [X.sub.3]Al-Y[C.sub.4][H.sub.4] donor-acceptor decreases in the order furane, pyrrole and thiophen. It is significant here not to announce the need for taking into account the values of the BSSE in the evaluation of energies of the donor-acceptor complexes studied in this work. The analysis of the electronic structure based on natural bond orbitals (NBO) partitioning indicates that there is no correlation between the charge transfer and the stability of the complex.

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Mustapha Cherkaoui * and Abderrahim Boutalib

Departement de Chimie, Faculte des Sciences Semlalia, Universite Cadi Ayyad, B.P. 2390, Marrakech, Morocco.

* Corresponding author. E-mail:

Received: 22 February 2012; revised: 23 September 2012; accepted: 02 October 2012.

Available online: 30 December 2012.
Table 1. B3LYP/6-311G (d, p) calculated geometries (Bond
length in [Angstrom] and Bond angle in degree)

Complex                  Al-Xa(Xb)     Al-Y     Y-C    [C.sub.1]-

Al[H.sub.3]                1.584
Al[F.sub.3]                1.651
Al[Cl.sub.3]               2.081
Al[Br.sub.3]               2.245
Furan                                          1.363     1.358
Pyrrole                                        1.374     1.376
Thiophen                                       1.734     1.364
[H.sub.3]Al-Furan       1.592(1.591)   2.119   1.384     1.349
[F.sub.3]Al-Furan       1.668(1.673)   1.978   1.393     1.346
[Cl.sub.3]Al-Furan      2.112(2.116)   2.023   1.399     1.345
[Br.sub.3]Al-Furan      2.278(2.282)   2.045   1.398     1.345
[H.sub.3]Al-Pyrrole     1.587(1.598)   2.238   1.420     1.354
[F.sub.3]Al-Pyrrole     1.669(1.679)   2.074   1.434     1.349
[Cl.sub.3]Al-Pyrrole    2.112(2.129)   2.102   1.441     1.347
[Br.sub.3]Al-Pyrrole    2.276(2.297)   2.120   1.440     1.347
[H.sub.3]Al-Thiophen    1.588(1.592)   2.656   1.749     1.356
[F.sub.3]Al-Thiophen    1.669(1.673)   2.485   1.756     1.353
[Cl.sub.3]Al-Thiophen   2.119(2.117)   2.532   1.757     1.352
[Br.sub.3]Al-Thiophen   2.276(2.284)   2.551   1.756     1.353

Complex                 [C.sub.2]-    [angle] XaAlXb
                        [C.sub.3]    ([angle] XbAlXb)

Al[H.sub.3]                               120.0
Al[F.sub.3]                               120.0
Al[Cl.sub.3]                              120.0
Al[Br.sub.3]                              120.0
Furan                     1.435
Pyrrole                   1.424
Thiophen                  1.427
[H.sub.3]Al-Furan         1.439        118.0(119.7)
[F.sub.3]Al-Furan         1.442        116.3(120.5)
[Cl.sub.3]Al-Furan        1.441        116.3(117.5)
[Br.sub.3]Al-Furan        1.441        116.4(117.0)
[H.sub.3]Al-Pyrrole       1.443        119.4(116.2)
[F.sub.3]Al-Pyrrole       1.448        117.6(116.0)
[Cl.sub.3]Al-Pyrrole      1.449        116.9(114.2)
[Br.sub.3]Al-Pyrrole      1.449        117.0(113.9)
[H.sub.3]Al-Thiophen      1.436        119.2(118.7)
[F.sub.3]Al-Thiophen      1.440        117.2(118.5)
[Cl.sub.3]Al-Thiophen     1.441        116.6(117.4)
[Br.sub.3]Al-Thiophen     1.441        116.9(117.4)

Complex                  [angle] XaAlY    [angle]
                        ([angle] XbAlY)     CYC

Furan                                      106.8
Pyrrole                                    109.8
Thiophen                                   91.4
[H.sub.3]Al-Furan         99.7(95.6)       107.4
[F.sub.3]Al-Furan         103.7(96.5)      107.8
[Cl.sub.3]Al-Furan       101.5(100.2)      107.2
[Br.sub.3]Al-Furan       101.2(100.5)      107.2
[H.sub.3]Al-Pyrrole       101.6(95.3)      106.8
[F.sub.3]Al-Pyrrole       106.2(96.7)      106.3
[Cl.sub.3]Al-Pyrrole      106.0(99.5)      105.8
[Br.sub.3]Al-Pyrrole      106.3(99.3)      105.7
[H.sub.3]Al-Thiophen      98.6(94.1)       91.5
[F.sub.3]Al-Thiophen      103.3(95.3)      91.6
[Cl.sub.3]Al-Thiophen     104.6(98.2)      91.7
[Br.sub.3]Al-Thiophen     104.8(97.7)      91.7

Table 2. The Optimized Y-C bond length of the Y[C.sub.4][H.sub.4]
moiety and their complexes with Al[X.sub.3], Wiberg bond index,
and the [n.sub.s] NBO contribution of Y Atom in the Y-C bond

                          d (Y-C)        Wiberg     [n.sub.s]
                        (A[degreea])   bond index      (%)

Furan                      1.363         1.0475       31.33

Al[H.sub.3]-Furan          1.384         0.9593       32.21
Al[F.sub.3]-Furan          1.393         0.9266       32.70
Al[Cl.sub.3]-Furan         1.399         0.9189       32.56
Al[Br.sub.3]-Furan         1.398         0.9216       32.49

Pyrrole                    1.374         1.1854       36.02

Al[H.sub.3]-Pyrrole        1.420         1.0465       32.05
Al[F.sub.3]-Pyrrole        1.434         1.0134       31.92
Al[Cl.sub.3]-Pyrrole       1.441         0.9966       30.26
Al[Br.sub.3]-Pyrrole       1.440         0.9985       30.13

Thiophen                   1.734         1.2148       19.75

Al[H.sub.3]-Thiophen       1.749         1.1395       18.97
Al[F.sub.3]-Thiophen       1.756         1.1163       19.12
Al[Cl.sub.3]-Thiophen      1.757         1.1092       18.90
Al[Br.sub.3]-Thiophen      1.756         1.1104       18.88

Table 3. [E.sub.comp] (B3LYP) (Complexation energies), BSSE,
[E.sub.comp+BSSE] (kcal/mol), and charge transfer [Q.sub.t]

Complex                  [E.sub.    BSSE   [E.sub.comp   [Q.sub.t]
                        comp] (a)   (b)      +BSSE]

Al[H.sub.3]-Furan        -11.18     1.64      -9.54        0.099
Al[H.sub.3]-Pyrrole       -7.42     1.56      -5.86        0.120
Al[H.sub.3]-Thiophen      -5.82     0.89      -4.93        0.192
Al[F.sub.3]-Furan        -24.54     5.38     -19.17        0.081
Al[F.sub.3]-Pyrrole      -21.11     5.52     -15.59        0.101
Al[F.sub.3]-Thiophen     -15.36     4.40     -10.96        0.191
Al[Cl.sub.3]-Furan       -17.34     2.57     -14.77        0.110
Al[Cl.sub.3]-Pyrrole     -14.85     2.62     -12.23        0.146
Al[Cl.sub.3]-Thiophen    -10.09     1.57      -8.52        0.261
Al[Br.sub.3]-Furan       -14.42     2.57     -11.85        0.109
Al[Br.sub.3]-Pyrrole     -12.03     2.54      -9.49        0.148
Al[Br.sub.3]-Thiophen     -8.01     1.69      -6.32        0.262

(a) [E.sub.comp] = E([X.sub.3]Al-Y[C.sub.4][H.sub.4])-[E(Al[X.sub.3])
+ E(Y[C.sub.4][H.sub.4])] (X = H, F, Cl and Br; Y = O, S and NH)

(b) Calculated using the counterpoise method.

Table 4. B3LYP/6-311G(d,p) level [Gap.sub.(HOMO-LUMO)]
(kcal/mol), [E.sub.comp+BSSE] (kcal/mol), and charge
transfer (electron)

Complex                  [Gap.sub.      [E.sub.     [Q.sub.t]
                        (HOMO-LUMO)]   comp+BSSE]

Al[H.sub.3]-Furan          368.35        -9.54        0.099
Al[H.sub.3]-Pyrrole        364.59        -5.86        0.120
Al[H.sub.3]-Thiophen       346.39        -4.93        0.192
Al[F.sub.3]-Furan          491.98        -19.16       0.081
Al[F.sub.3]-Pyrrole        488.21        -15.59       0.101
Al[F.sub.3]-Thiophen       470.01        -10.96       0.191
Al[Cl.sub.3]-Furan         392.20        -14.77       0.110
Al[Cl.sub.3]-Pyrrole       389.06        -12.23       0.146
Al[Cl.sub.3]-Thiophen      370.24        -8.52        0.261
Al[Br.sub.3]-Furan         363.96        -11.85       0.109
Al[Br.sub.3]-Pyrrole       360.20        -9.49        0.148
Al[Br.sub.3]-Thiophen      344.51        -6.32        0.262
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Title Annotation:Full Paper
Author:Cherkaoui, Mustapha; Boutalib, Abderrahim
Publication:Orbital: The Electronic Journal of Chemistry
Date:Oct 1, 2012
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