Journal of Chemistry

Volume 2014 (2014), Article ID 619196, 6 pages

http://dx.doi.org/10.1155/2014/619196

Research Article

Aluminum Dimer Containing Bulky 1,2,3-Triazolate Ligand

Issam Kobrsi

The Petroleum Institute, P.O. Box 2533, Abu Dhabi, United Arab Emirates

Received 27 April 2014; Revised 15 June 2014; Accepted 16 June 2014; Published 17 August 2014

Academic Editor: Theocharis C. Stamatatos

Copyright © 2014 Issam Kobrsi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The first molecular aluminum 1,2,3-triazolato complex was synthesized bearing a bulky 1,2,3-triazolate ligand. Oligomers and polymers were avoided due to the bulkiness and noncoordinating nature of the substituents. The novel Al₂N₄ ring formed contains symmetrical Al-N bond distances unexpectedly having asymmetric Al-N-N angles of 144.55(15)° and 115.83(14)°. This asymmetry demonstrates the effect of the steric hindrance of the ligand.

1. Introduction

There is considerable interest in the development of new molecular azolato metal complexes. Motivations in this area are diverse and include the preparation of new single-site olefin polymerization catalysts, attempts to understand and mimic hydrodenitrogenation catalysts, and the preparation of improved precursors to metal nitride phases, to name a few [1].

The chemistry of 3,5-disubstituted-1H-1,2,4-triazoles is dominated by polymeric complexes due to the presence of three donor sites on the triazole ring [2–29]. However, molecular complexes can also be synthesized [1, 30–43]. On the other hand, the chemistry of 4,5-disubstituted-1H-1,2,3-triazoles is virtually nonexistent, with only very few molecular complexes isolated thus far [44–46].

This scarcity can be attributed to several factors, mainly the difficulty in synthesizing 4,5-disubstituted-1H-1,2,3-triazole ligands with sufficient steric bulk to favor molecular complexes. Most of the attention in synthesizing 1,2,3-triazole ligands is given to the “click” chemistry approach due to its simplicity, versatility, and reputation for being a “green” process [47–50]. Most click chemistry 1,2,3-triazole ligands contain an alkyl group bonded to one of the nitrogen atoms as a result of the use of organic azide as one of the reactants. The other reactant is usually a terminal alkyne that leaves one of the carbon atoms unsubstituted. The resulting triazole requires the cleavage of the N-C bond prior to use as an anionic ligand, or it can be used as a C-donor ligand. However, no click chemistry reaction has been able to synthesize a 4,5-disubstituted-1H-1,2,3-triazole ligand with noncoordinating side groups that is bulky enough to produce molecular complexes.

One reaction that has gone unnoticed is the deprotonation of trimethylsilyldiazomethane. This reaction can be further used in cycloaddition reactions to produce various 1,2,3-triazoles in fairly high yields. For example, trimethylsilyldiazomethanide can react with trimethylacetonitrile and upon acidification produce the very bulky ligand 4-tert-butyl-5-trimethylsilyl-1H-1,2,3-triazole (tzH) [45, 46, 51] (see Figure 1). Two complexes have been reported to contain this ligand formed in situ [45, 46]; however, no reactions have taken advantage of this bulky triazole in its isolated neutral form.

Figure 1: Reaction scheme for the production of tzH.

I hereby report the first aluminum complex bearing a 4,5-disubstituted-1,2,3-triazolate ligand. The complex is molecular, dimeric, and prepared by the reaction of trimethylaluminum (TMA) with free tzH. This reaction proves that sufficiently bulky 4,5-disubstituted-1H-1,2,3-triazole ligands can be used to synthesize new and interesting molecular complexes and should set the foundation to expand the study of the coordination of these poorly researched ligands. The complex synthesized in this report is the only 1,2,3-triazolato aluminum complex where the triazolato ligand is coordinated by virtue of the triazole core itself due to noncoordinating substituents. A previously reported 1,2,3-triazolato aluminum complex contains a triazolato ligand bearing coordinating phosphine chalcogenide substituents that dominate the coordination of the ligand [52, 53].

2. Experimental

2.1. General Considerations

All reactions were performed under an inert atmosphere of argon or nitrogen using either glovebox or Schlenk line techniques. Hexane was distilled from phosphorus pentoxide. THF was distilled from sodium/benzophenone. Al(CH₃)₃ was purchased from Strem Chemicals Inc. and used as received. Butyl lithium, trimethylacetonitrile, and trimethylsilyldiazomethane were purchased from Aldrich and used as received. ¹H and ¹³C NMR were obtained at 500 MHz and 125 MHz, respectively. Elemental analysis was performed in house.

2.2. Preparation of Bis(μ-4-tert-butyl-5-trimethylsilyl-1,2,3-triazolato) (tetramethyl) Dialuminum (1)

TzH was prepared according to literature [51] and was used for the preparation of complex 1 as described below and illustrated in Figure 2. A 100 mL Schlenk flask was charged with tzH (0.4070 g, 2.062 mmol) and hexane (30 mL). Trimethylaluminum (0.1449 g, 2.010 mmol) was added and the solution was allowed to stir at room temperature for 18 h in the glovebox. The reaction mixture was then filtered through a pad of Celite, the volume was reduced to 5 mL, and the reaction mixture was allowed to stand undisturbed at −20°C. Complex 1 (0.6610 g, 63.25%) was isolated as colorless rods. Mp 139°C dec; ¹H NMR (C₆D₆, 22°C, δ) 1.27 (s, 9 H, C(CH₃)₃), 0.59 (s, 9 H, Si(CH₃)₃); −0.50 (s, 12 H, Al(CH₃)₂; ¹³C NMR (C₆D₆, 22°C, δ) 150.01 (s, triazolate ring C-C), 136.82 (s, triazolate ring C-Si), 38.96 (s, C(CH₃)₃), 31.28 (s, C(CH₃)₃), −0.21 (s, Si (CH₃)₃), −8.31 (s, Al(CH₃)₂). Anal. calcd for C₂₂H₄₈Al₂N₆Si₂: C, 52.14; H, 9.54; N, 16.58. Found: C, 53.18; H, 9.63; N, 16.25.

Figure 2: Scheme for the production of 1.

2.3. X-Ray Crystallographic Structure Determination

Diffraction data for 1 were measured on a Bruker diffractometer equipped with Mo radiation and a graphite monochromator. The samples were mounted in thin-walled glass capillaries under a dry nitrogen atmosphere. The frame data were indexed and integrated with the manufacturer’s SMART software [54]. All structures were refined using Sheldrick’s SHELX-97 software [55]. Hydrogen atom positions were calculated or observed.

3. Results and Discussion

3.1. Synthetic Parameters

Complex 1 was synthesized by reacting Al(CH₃)₃ and tzH in a 1 : 1 ratio. The reaction was clean and only one product was present as indicated by solid state structure and solution ¹H NMR. Varying the ratio of Al(CH₃)₃ and tzH to 1 : 2 or 1 : 3 produced complex reaction mixtures as indicated by solution ¹H NMR. These reactions were not further characterized. These mixtures could be due to incomplete protonation of the methyl groups resulting in tzH adducts or to equilibria between bridging and terminal complexes. A separate study is needed to determine the nature of these complexes and to optimize their synthesis.

3.2. X-Ray Crystal Structures

1 was characterized spectroscopically and structurally by single crystal X-ray diffraction. The composition was also verified by elemental analysis. All the atoms except the methyl groups lie in the same plane, with the two Al atoms being part of an Al₂N₄ 6-membered ring. The space-filling model illustrates the steric hindrance provided by tz (see Figure 3).

Figure 3: Space-filling model of 1 illustrating how the steric bulk provided by the bulky substituents blocks the nitrogen atoms from reacting further.

Experimental crystallographic data are summarized in Table 1 and selected bond lengths and angles are presented in Tables 2 and 3. A representative perspective view of 1 is shown in Figure 4.

Table 1: Experimental crystallographic data for 1.

1

Empirical formula

Fw
506.80

Space group
P2

6.4680(8)

15.477(2)

15.599(2)

(deg)
90.00

(deg)
98.079(3)

(deg)
90.00

1546.0(4)

2

0.71073

(mm⁻¹)
0.191

0.0515

Rw
0.1390

Table 2: Selected bond lengths for 1.

Al1–C8
1.945(3)
Si1–C10
1.828(4)

Al1–C7
1.952(3)
Al1–N2
1.966(2)

Al1–N3
1.972(2)
Si1–C9
1.844(5)

Si1–C11
1.849(4)
Si–C2
1.872(2)

N1–N2
1.327(3)
N2–N3
1.337(3)

N1–C2
1.356(3)
N3–C1
1.366(3)

N3–Al1
1.972(2)
C1–C2
1.405(3)

C1–C3
1.604(3)
C3–C5
1.523(5)

C3–C4
1.567(4)
C3–C6
1.569(5)

Table 3: Selected bond angles (deg) for 1.

C8–Al1–C7
122.73(17)
C8–Al1–N2
104.69(12)

C8–Al1–N3
112.10(14)
N2–Al1–N3
99.58(8)

N2–N1–C2
107.88(19)
N1–N2–N3
110.98(18)

N1–N2–Al1
104.31(14)
N3–N2–Al1
144.55(15)

N2–N3–C1
107.38(18)
N2–N3–Al1
115.83(14)

C1–N3–Al1
136.69(16)
N3–C1–C2
106.7(2)

N3–C1–C3
125.01(19)
C2–C1–C3
128.32(19)

N1–C2–C1
107.1(2)
N1–C2–Si1
113.61(18)

Figure 4: ORTEP view of the structure of 1 showing the asymmetric Al-N-N angles. Thermal ellipsoids drawn at 50% probability level and hydrogens omitted for clarity.

The Al₂N₄ ring exhibits novel structure in that the Al-N distances are equal, yet the Al-N-N angles are asymmetric. The Al1-N2-N3 angle is 144.55(15)° and the Al1-N3-N2 angle is 115.83(14)° (see Figure 4).

This angular asymmetry is believed to be due to the Van der Waals repulsion between the bulky substituents and the alkyl groups on the Al. Figure 5 illustrates the proposed reason behind this asymmetry. Other aluminum azolato complexes with either pyrazoles, 1,2,4-triazoles, or tetrazoles that exhibit a similar Al₂N₄ ring structure do not exhibit this asymmetry [52, 56–60]. Instead, the Al-N-N angles are symmetrical. This report thus demonstrates the novelty that can arise by using bulky 4,5-disubstituted-1H-1,2,3-triazole ligands such as tzH and illustrates the need to explore this accessible ligand further.

Figure 5: The steric hindrance is reduced by the skewing of the Al₂N₄ ring producing a more stable, less sterically hindered structure with asymmetric Al-N-N angles.

3.3. Thermogravimetric Analysis (TGA)

To identify the thermal stability of 1, TGA analysis was carried out on a Delta Series TA-SDTQ600 analyzer in a nitrogen atmosphere from room temperature to 700°C (10°C min⁻¹) using aluminum crucibles (Figure 6). The result shows that the complex is stable up to 130°C, after which it begins to decompose steadily upon further heating. This is consistent with the observation of a decomposition point at around 139°C when measuring the melting point.

Figure 6: The TGA data of 1.

4. Conclusions

An unprecedented molecular aluminum complex containing the ligand tz was synthesized by virtue of the steric bulk and noncoordinating nature of the substituents. This steric bulk blocked the nitrogen atoms of the tz forming polymeric species. The complex exhibits a novel Al₂N₄ ring structure where the Al-N distances are the same but the Al-N-N angles vary greatly. These novel asymmetric angles are due to the Van der Waals repulsion between the bulky substituents and the alkyl groups on the aluminum. This report demonstrates the ease at which tzH can be accessed and the novelty that can arise when used to synthesize metal complexes. It should set the foundation for further research into the coordination of bulky 1,2,3-triazole ligands that are extremely scarce in the literature.

Conflict of Interests

The author declares that there is no conflict of interests regarding the publication of this paper.

Supporting Information

Crystallographic data (excluding structure factors) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no. CCDC-969163. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).

Acknowledgment

The author is grateful for funding from the Petroleum Institute (RIFP Grant no. 12327).

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