Dichlorobis[1,3-dimethyl-2(3H)-imidazoleselone]zinc(II): a potential zinc selenide synthon

Inorganic Chemistry Communications 5 (2002) 124–126 www.elsevier.com/locate/inoche

Dichlorobis[1,3-dimethyl-2(3H)-imidazoleselone]zinc(II): a potential zinc selenide synthon Daniel J. Williams a

a,*

, Kathleen M. White a, Donald VanDerveer b, Angus P. Wilkinson

b

Department of Chemistry and Biochemistry, Kennesaw State University, 1000 Chastain Road, Mail Stop 1203, Kennesaw, GA 30144-5591, USA b School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400, USA Received 8 October 2001; accepted 1 December 2001

Abstract The synthesis and characterization of a potential ZnSe synthon is reported. Dichlorobis[1,3-dimethyl-2(3H)-imidazoleselone] is prepared with 57% yield by direct combination of ZnCl2 and the ligand in boiling acetonitrile. X-ray crystallography shows a discrete molecular structure with a tetrahedral coordination sphere around the zinc. Thermogravimetric analysis from 50 to 600 °C indicates a 67.22% weight loss which results in a 10% higher residual mass than that expected for pure ZnSe. X-ray powder diffraction on the residue from a larger scale thermolytic decomposition experiment confirmed the presence of both hexagonal and cubic ZnSe. Ó 2002 Published by Elsevier Science B.V. Keywords: Crystal structure; Selone; Zinc chloride; Zinc selenide

1. Introduction The application of zinc selenide-based materials in optical electronics, laser technology, and semiconductor research with specific interest in developing material suitable for making blue-green light emitting diodes (LEDs) has been the focus of many studies [1–3]. Those interested in the synthesis of nanoparticle semiconductors have investigated various precursors and methods for the synthesis of ZnSe [4,5]. We have recently synthesized a selenourea addition product which could be of potential interest to material scientists working in the field described above. The compound, dichlorobis[1,3-dimethyl-2(3H)-imidazoleselone]zinc(II) or ZnCl2 ðdmiseÞ2 , is prepared by direct combination of ZnCl2 and the ligand in boiling acetonitrile. This selenourea ligand (dmise, Fig. 1) has been incorporated in some of our previous studies as well as those of other investigators [6,7]. We have been primarily interested in both main group and transition metal halide dmise adducts, one example being CoCl2 ðdmiseÞ2 [7]. *

Corresponding author. Tel.: +1-770-423-6174; fax: +1-770-4236744. E-mail address: [email protected] (D.J. Williams).

Our purpose in this communication is to alert the materials science community of a new compound which is a potential ZnSe precursor. We are reporting the experimental details for the synthesis, characterization, and results of a single crystal X-ray diffraction for this new zinc(II) chloride adduct. To confirm the adduct’s potential as a ZnSe synthon, the results of a thermogravimetric analytical (TGA) study and a larger scale thermolytic decomposition and subsequent X-ray powder diffraction experiment are reported below as well.

2. Experimental 2.1. General All chemicals were reagent grade and used as obtained without further purification. Dmise was synthesized via methods outlined in the literature [6,8]. The melting point (uncorrected) was determined in an open capillary tube on a Mel-Temp melting point apparatus. Fourier transform infrared data (FTIR) were collected on a powdered sample using a Perkin–Elmer Model Spectrum One FTIR spectrophotometer fitted with a diamond attenuated reflectance stage. Values are

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Analyzer. A 16.663 mg sample of ZnCl2 ðdmiseÞ2 was heated under a nitrogen sweep from 50 to 600 °C at a rate of 20 °C/min. Onset of decomposition was recorded at 301.426 °C. A total weight loss of 67.22% of the original sample mass was recorded. 2.4. X-ray crystallographic study

Fig. 1. The molecular structure of ZnCl2 ðdmiseÞ2 . Estimated standard ): Zn (1)–Cl (1) 2.2619 (8), deviations in parentheses. Bond lengths (A Zn (1)–Cl (2) 2.2905 (8), Zn (1)–Se (1) 2.4691 (4), Zn (1)–Se (2) 2.4873 (4). Bond angles (°): Cl (1)–Zn (1)–Cl (2) 108.75 (3), Cl (1)–Zn (1)–Se (1) 118.34 (2), Cl (2)–Zn (1)–Se (1) 107.07 (2), Cl (1)–Zn (1)–Se (2) 108.43 (2), Cl (2)–Zn (1)–Se (2) 110.49 (2), Se (1)–Zn (1)–Se (2) 103.58 (2).

reported in cm1 (2 cm1 ) and collected in the 4000– 600 cm1 range. Key: v – very; w – weak, m – medium; s – strong. Elemental analysis was performed by Atlantic Microlabs. Thermogravimetric analysis (TGA) and crystallographic experimental details are reported below. 2.2. Dichlorobis[1,3-dimethyl-2(3H)-imidazoleselone] zinc(II) Into a 25 ml Erlenmeyer flask with magnetic stir bar was placed 0.40 g (2.3 mmol) of dmise dissolved in 25 ml of boiling acetonitrile. ZnCl2 (0.20 g; 1.5 mmol) was added with stirring, and the mixture was permitted to continue a slow boil. The solution volume was reduced to approximately 10 ml, and the flask was removed from the heat. Small pale-yellow crystals were noted to form upon cooling of the solution to room temperature. The flask was stoppered and refrigerated. Filtration and washing with small portions of solvent yielded 0.40 g of ZnCl2 ðdmiseÞ2 (57% yield). Melting point 197–199 °C. Analytical: Calc. for C10 H16 Cl2 Se2 Zn: C – 25.01%, H – 3.28%, N – 11.60%. Found: C – 24.94%; H – 3.36%; N – 11.53%. FTIR: 3148m, 3121m, 3109m, 1710w, 1608w, 1567m, 1488s, 1478s, 1443s, 1386s, 1337w, 1243m, 1226s, 1150m, 1090m, 1021vw, 866w, 753vs, 738vs, 663s. 2.3. TGA analysis Thermogravimetric analysis was performed on a Perkin–Elmer Model TGA-7 Thermogravimetric

Data collection was carried out using a Siemens SMART CCD diffractometer at 173 K. Crystal data for C10 H16 Cl2 N4 Se2 Zn: monoclinic P21 =n, a ¼ 9:4437 , b ¼ 107:477 ð3Þ, b ¼ 13:6211ð5Þ, c3 ¼ 13:3532ð5Þ A  ð1Þ°, V ¼ 1638:4ð1Þ A , Z ¼ 4, Dcalc ¼ 1:972 g cm3 . Structure solution and refinement were based on 10,326 independent reflections, 3881 of which were unique. The structure was solved by direct methods, and all calculations were performed using the SHELXTL program package [9]. The final residuals ðI > 2rðIÞÞ were R1 ¼ 0:0291 and wR2 ¼ 0:0690 [10]. 2.5. Thermal decomposition study A 269.1 mg sample of ZnCl2 ðdmiseÞ2 was placed in a quartz tube fitted for N2 sweep in a Thermolyne 21100 Tube Furnace. The temperature was elevated to 400 °C over a one-hour period and then held for another hour at that temperature. The black residue (92.8 mg) was placed on an aluminum sample holder, and a powder diffraction pattern was collected using a Scintag X1 diffractometer equipped with a Cu-Ka radiation source and a Peltier cooled solid-state detector. Patterns were compared to published data from the International Centre for Diffraction Data (ICDD) [10].

3. Results and discussion Direct combination of ZnCl2 and dmise in hot acetonitrile leads to the formation of an air-stable crystalline 1:2 addition product in reasonably good percentage yield. X-ray crystallographic studies show the expected tetrahedral site symmetry about the zinc (Fig. 1) and no unusual structural anomalies are noted. The ring bond distances and angles agree well with previous studies for this ligand [7,10]. Given the chalcophilicity of zinc as evidenced in the natural minerals of zinc blende and wurzite, we were led to believe that this compound may be a potential synthon for zinc selenide. To substantiate this hypothesis, we performed two thermal decomposition experiments. TGA studies indicated a 67.22% weight loss from the original sample mass which corresponds to a residual molecular mass of 159.46 amu. This is 15 amu higher than the theoretical mass of ZnSe indicating possible

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residual contamination in addition to normal experimental error. The larger scale decomposition and subsequent powder diffraction study revealed the residue to be contaminated with a black powder, most likely carbon, although this was not confirmed. A powder diffraction pattern collected on the residue revealed broad peaks which matched perfectly with the ICDD pattern for hexagonal ZnSe. There appeared to be some cubic ZnSe as well since the observed relative peak intensities did not fit exactly with that of the pure hexagonal pattern. Observed peak broadening indicated the nanocrystalline nature of the residue. Residual carbon contamination could account for the observed errors in the TGA data. From the evidence above, we have confirmed our hypothesis that thermal decomposition of ZnCl2 ðdmiseÞ2 leads to the formation of ZnSe. From a precursor standpoint, ZnCl2 ðdmiseÞ2 makes an attractive alternative to others reported in the literature [4,5]. The starting ligand, dmise, is easily prepared with minimum exposure to noxious forms of selenium, and the adduct itself is easily prepared as well as being air-stable and non-odoriferous. With carbon contamination being possible, it is clear that simple thermal decomposition may not be the preferred route for decomposing ZnCl2 ðdmiseÞ2 , and an alternative thermal decomposition pathway such as the one reported by Jun et al. [4] could be explored by anyone wishing to pursue this problem further.

Acknowledgements The authors wish to thank Dr. David Collard at Georgia Institute of Technology for help in obtaining the TGA data.

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