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Unexpected reactivity of “GaI” towards N,N′-diaryl-β-diketiminate tin(II) chloride: Synthesis, X-ray diffraction analysis and DFT studies
⁎Corresponding author. Tel.: +52 181 83294000. victor.jimenezpr@uanl.edu.mx (Víctor M. Jiménez-Pérez)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
Abstract
Reaction of the [HC(CMeNAr)2]SnCl (Ar = 2,4,6-Me3C6H2) with “GaI” in THF provided an unexpected transhalogenation reaction product [HC(CMeNAr)2]SnI. The crystal structure shows a tin(II) ion with a distorted tetrahedral pyramidal geometry. Theoretical calculations, using density functional theory (B3LYP), suggest the probable reaction mechanism for transhalogenation. The activation energy for moving from reactant ([HC(CMeNAr)2]SnCl to product [HC(CMeNAr)2]SnI) is only 10.2 kcal/mol. The chloride ion is displaced by the iodine ion, probably by the help of the gallium atom when it is coordinated to the chlorine atom.
Keywords
Transhalogenation reaction
“GaI”
β-Diketiminate
X-ray
DFT
1 Introduction
We have been interested in the synthesis of tin(II) complexes stabilized by bulky ligand such as 1,8-diaminonaphthalenes (Jiménez et al., 2008), oxamides (Ibarra et al., 2012), and β-diketiminates (Muñoz et al., 2012; Rosas and Jiménez, 2012) (Fig. 1, I–III). The metal halides complexes (formula RSnX, where R = bulky ligand and X = halide) are considered valuable synthons in chemistry; it has been reported the β-diketiminate Sn(II) chloride is easily replaced by alkoxide, amide, azide, triflate, and hydride (Akkari et al., 2001; Ayers and Dias, 2002; Pineda et al., 2006). However, the transhalogenation reactions on β-diketiminate Sn(II) halide has not been reported, while its germanium(II) analogue used Me3SiF as halide source to convert the chloride to the corresponding fluoride reported by Roesky et al. (2001).
The most recent synthesis of “GaI” (Green et al., 1990) has been reported by ultrasound using gallium metal and half an equivalent of diiodine in toluene to give a pale green, where this versatile reagent has been taken an enormous importance from organic synthesis to materials chemistry (Baker and Jones, 2005). Also, the reactivity of “GaI” towards a variety of Lewis bases has been investigated. Specifically, it has been reported that “GaI” react with Sn(II) derivatives getting the transmetallation (Dias and Jin, 1996). Also, a probable reduction reaction might considered in front of M–X bonds, because it is a soft reducing agent (Lesikar and Richards, 2006). However, in this work we decide to investigate the reactivity “GaI” with [HC(CMeNAr)2]SnCl (Ar = 2,4,6-Me3C6H2) and we obtained the unexpected transhalogenation reaction. The characterization was carried out by X-ray diffraction and the DFT calculations show the probable pathway.
2 Experimental
2.1 Material and equipment
All starting materials were purchased from Aldrich Chemical Company. Solvents were used without further purification. Melting points were determined on an Electrothermal Mel-Temp apparatus and are uncorrected. Elemental analysis was performed with the Perkin–Elmer Series II CHNS/O analyzer. The β-diketiminate Sn(II) chloride (Ayers et al., 2001) and the “GaI” (Green et al., 1990) were prepared according to published procedures.
2.2 Synthesis of compound 1
A solution of [HC(CMeNAr)2]SnCl (Ar = 2,4,6-Me3C6H2) (0.91 g) in toluene (30 mL) was added slowly “GaI” in toluene (10 mL) at −78 °C. The reaction mixture was warmed to room temperature, and stirring was continued for 16 h. The precipitate was filtered, and the solvent was partially reduced (ca. 20 mL). Storage of the remaining solution in a −30 °C freezer for three days afforded yellow crystals of 1 suitable for X-ray diffraction analyses. Yield (0.82 g, 71%); mp 155 °C. Anal. Calcd (%) for C23H29IN2Sn (579.10 g mol−1): C, 47.70; H 5.05; N 4.84. Found: 47.48; H 5.11; N 4.33.
2.3 X-ray data collection and structure determination
The data for 1 and 2 were covered with a layer of hydrocarbon oil that was selected and mounted with paratone-N oil on a cryo-loop, and immediately placed in the low temperature nitrogen stream at 100(2) K. The data for 1 and 2 were recorded on a Bruker SMART APEX CCD area detector system equipped with an Oxford Cryosystems 700 Series Cryostream cooler, a graphite monochromator, and a Mo Kα fine-focus sealed tube (λ = 0.71073 Å). The structures were solved by direct methods using SHELXS-97 (Sheldrick, 1990) and refined against F2 on all data by full-matrix least-squares with SHELXL-97 (Sheldrick, 1997). Absorption corrections were applied by using SADABS. The CCDC files 1025271–1025272 contain the supplementary crystallographic data. These data can be obtained free of charge via or from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge, CB2 1EZ, UK (http://www.ccdc.cam.ac.uk/conts/retrieving.html).
2.4 Theoretical calculations
The geometry of all structures was fully optimized by using the Gaussian 09 software package (Frisch et al., 2009). The B3LYP hybrid density functional (Becke, 1993), in combination with the def2-TZVP basis set (Metz et al., 2000; Peterson et al., 2003; Weigend and Ahlrichs, 2005), was adopted to calculate exchange–correlation energy since there is some rationale behind the choice of this functional. B3LYP has been used to investigate some reaction mechanisms with gallium (Ga) and tin (Sn) (Joshi and Thomson, 2005; Li et al., 2011; Liao et al., 2013; Pereira and Nascimento, 2005). Their vibrational frequencies were calculated with same level of theory in order to characterize all structures as minima or transition states. The transition state was characterized by having only one imaginary frequency with value of −65.03 and following the vibration mode of that imaginary frequency which generated the reactant (A) and product (C). The Polarizable Continuum Model (PCM) (Tomasi et al., 2005) was used to compute energies in toluene with an electrostatic dielectric constant, ɛ = 2.3741. This calculation is performed in the presence of a solvent by placing the solute in a cavity within the solvent reaction field via a set of overlapping spheres. All output files were visualized with the Chemcraft program v1.6.
3 Results and discussion
The reaction of [HC(CMeNAr)2]SnCl (Ar = 2,4,6-Me3C6H2) with “GaI” in toluene provided the halide exchange product [HC(CMeNAr)2]SnI 1 (Scheme 1) as pale yellow powders that were stable under inert atmosphere and could be stored for regular periods at low temperature (−30 °C). It shows high solubility in organic solvents such as pentane, hexanes, diethyl ether, tetrahydrofuran and toluene. A suitable crystal was isolated by cooling a concentrated solution in n-hexane at −30 °C and its structure was determined by X-ray crystallographic analysis.![Transhalogenation reaction of [HC(CMeNAr)2]SnCl with “GaI”.](/content/184/2019/12/8/img/10.1016_j.arabjc.2015.08.030-fig3.png)
Compound 1 crystallized in the monoclinic space group P21/n as a monomer and unit cell showed two molecules which are not symmetry equivalent. Details of the X-ray crystallographic are shown in Table 1. The crystal structure shows that the metal centre (Sn) is three-coordinated and resides in a distorted tetrahedral environment with a vertex occupied by one lone electron pair (see Fig. 2). The sum of angles around the tin atom is 273.79°. The Sn–I bond length is 2.8866 (4) Å; this is in agreement with the observations to the (Sn–I 2.873 (4) Å) analogue to [HC(CMeNAr)2]SnI (Ar = 2,6-iPr2C6H3) previously reported (Jana et al., 2009). The Sn–N1 [2.177 (3) Å] and Sn–N2 [2.181 (3) Å] distances in the heterocycle corresponded with a delocalized system, while the most acute angle was associated with the bite of the chelating ligand N–Sn–N 85.51 (10)°.
| 1 | 2 | |
|---|---|---|
| Empirical formula | C23H29IN2Sn | C23H31IN2 |
| Formula weight | 579.07 | 462.40 |
| Temperature, K | 100(2) | 100(2) |
| Wavelength | 0.71073 | 0.71073 |
| Crystal system | Monoclinic | Monoclinic |
| Space group | P2(1)/c | P2(1)/c |
| a, Å | 10.6362(8) | 11.5856(5) |
| b, Å | 11.4235(8) | 13.3516(6) |
| c, Å | 38.542(3) | 14.3348(6) |
| α | 90.00° | 90.00° |
| β | 97.875(1)° | 92.7520(10)° |
| γ | 90.00° | 90.00° |
| V, Å3 | 4638.7(6) | 2214.84(17) |
| Z | 8 | 4 |
| 1.658 | 1.306 | |
| μ, mm−1 | 2.442 | 1.454 |
| 2θ range for data collection | 2.40–27.97° | 2.77–28.30° |
| Index ranges | −13 ⩽ h ⩽ 13 | −15 ⩽ h ⩽ 15 |
| −14 ⩽ k ⩽ 14 | −17 ⩽ k ⩽ 17 | |
| −48 ⩽ l ⩽ 48 | −18 ⩽ l ⩽ 19 | |
| No. of reflns collected | 9627 | 5500 |
| No. of indep reflns | 8336 | 5203 |
| [Rint] | 0.0332 | 0.0212 |
| Fit goodness | 1.053 | 1.059 |
| R1, wR2 (I > 2σ(I)) | 0.0400/0.0805 | 0.0269/0.0656 |
| R1, wR2 (all data) | 0.0329/0.0768 | 0.0253/0.0637 |
The presence of water in the former reaction gives the β-diketiminate salt hydroiodide in a 3% yield (Scheme 1), where salt showed a W-shaped conformation (Jiménez et al., 2009). The structure of 2 was confirmed by X-ray diffraction analysis (Fig. 3). The hydrogen iodide comes from the hydrolysis reaction of the “GaI” (Eq. (1)).
Regarding the crystal structure, both amine and imine NH functionalities, N(1) and N(2), are involved in strong N(1)–H⋯I 2.783(3) Å, N(2)–H⋯I 2.819(3) Å hydrogen bonds with symmetry-related I− ions. The network of hydrogen bonds forms a 1D supramolecular structure along the short cell axis c. The π⋯π interaction observed in the asymmetric unit, characterized by a centroid-to-centroid separation of 4.436(4) Å.
In order to explain the substitution of the chlorine atom by one iodine atom, was performed a theoretical study using density functional theory (B3LYP) in combination with the def2-TZVP basis set. Results are shown in Fig. 4. As we can observe in structure A, the gallium atom, from gallium iodide, coordinates the chlorine atom. This interaction has a value of 2.84 Å. According to experimental results, the iodine must be coordinated to the tin atom to displace the chlorine atom. We have explored the potential energy surface for this reaction and only one transition state (B) was found, which is shown in Fig. 4. The B structure is higher in energy than A by 10.16 kcal/mol and a four-membered heterocycle is formed. Sn–Cl and Ga–I bond lengths are significantly longer (0.46 and 0.16 Å, respectively) than those for structure A, and Ga–Cl bond length is much shorter by 0.36 Å. The Sn–I interaction has a value of 3.87 Å. The transition state was characterized by having only one imaginary frequency with value of −65.03 and we followed the vibration mode of that imaginary frequency which generated the reactant (A) and product (C). Structure C is the final product, in which the iodine atom has replaced the chlorine atom. Gallium atom is still coordinated to the iodine atom (3.26 Å). The product (C) has an energy difference, with respect to the first structure, of only 0.76 kcal/mol. The last energy value is almost negligible in theoretical terms. We can say that both, structure A and structure B, have almost the same stabilization energy.
In order to know the influence of the solvent on the energies of the three structures shown in Fig. 4, we carried out a study of SCRF in toluene using the polarizable continuum model (PCM). The computed energies do not vary much from the electronic energies. The AE has a slightly smaller value (9.34 kcal/mol), while the energy difference between structures A and C is 0.82 kcal/mol, A is still more stable than C. Finally, we have also analysed the zero-point energies (ZPE) and are also consistent with all previous energy, and the AE has a value of 9.16 kcal/mol and A is more stable than C by 0.69 kcal/mol.
4 Conclusions
In summary a new Sn(II) compound has been prepared by simple transhalogenation with “GaI”. The reactivity of “GaI” towards β-diketiminate Sn(II)Cl was unexpected being the first time reported so far. DFT calculations suggest that transhalogenation reaction is viable and the iodine atom is capable of displacing the chlorine atom. This is possible, probably by the help of the gallium atom when it is coordinated to the chlorine atom.
Acknowledgements
This research was financially supported by Consejo Nacional de Ciencia y Tecnología (Grant: 156450). The authors thank Prof. H.V. Rasika Dias for performing the X-ray diffraction studies.
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Appendix A
Supplementary material
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.arabjc.2015.08.030.
Appendix A
Supplementary material
Supplementary data 1
Supplementary data 1