Applied Science and Convergence Technology 2021; 30(4): 95-97
Published online July 30, 2021
Copyright © The Korean Vacuum Society.
Department of Applied Chemistry, Andong National University, Andong 36729, Republic of Korea
Correspondence to:E-mail: firstname.lastname@example.org
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
A new way to prepare single crystals of ligand-coordinated chromium (II) acetate with quadruple bonds is presented. Coordination of water and pyridine with crystal growth occur by seed-mediated growth. The product was obtained as crystals with well-defined facets. Raman spectroscopy confirmed the high quality and purity of the products. Mechanism studies showed that the growth reaction is driven by heterogeneous crystallization at the interface of the seeds. Seed-mediated growth results in the high quality and large size of the crystals with improved yield for shorter reaction time.
Keywords: Chromium, Quadruple bonds, Seed-mediated growth, Crystallization, Solid-liquid interface
Many coordination complexes have unique quadruple bonds between the metal centers . The electronic configuration of the quadruple bonds is σ2π4δ2. The δ orbitals are formed by the overlap of the d orbitals that are in eclipsed positions. Paddlewheel ligands are often employed to interlock the metal centers so that the
Among various complexes with quadruple bonds, chromium (II) acetate [Cr2(OAc)4] is a representative example that can resist oxidation in the air for a couple of hours [1, 4, 5] [Fig. 1(a)]. And there are numerous ligands that can be coordinated at the axial position of the complexes. The correlation between the basicity of the ligands and the chromium acetates is well studied in previous reports [1, 2, 6, 7]. Therefore, chromium (II) acetates are eligible for various applications such as electronics in that they can be handled in air and their properties can be controlled by choosing proper ligands.
Ligand-coordinated chromium (II) acetates are prepared by two-step reactions: synthesis of anhydrous chromium (II) acetate and coordination of the ligands to the anhydrous complex. In the case of the former, Levy
Herein, an improved way to synthesize ligand-coordinated chromium (II) acetate is presented. Unlike the previous methods, this new method features seed-mediated growth and rapid cooling of the solution during the crystallization in a freezer [Fig. 1(b)]. This process promotes the crystallization and provides protection against oxidation. The properties of the products were then analyzed by Raman spectroscopy, which clearly tells the quality and purity of the samples. Also, the crystal growth mechanism at the solid-liquid interface of the seed and the solution was investigated based on the observations.
Anhydrous chromium (II) acetate was synthesized by the method after Levy
The ligands were then coordinated by dissolving the anhydrous chromium (II) acetate powder into the target ligand solvents. Before the dissolution, the ligand solvents (120 ml) were degassed by heating the solvent near the boiling temperature with supplying Ar gas. After 30 min, 1 g of the anhydrous powder was put into the degassed solution while being stirred. When the dissolution was completed, the stirring process was immediately stopped and taken out of the heating bath. The solution was then rapidly cooled down to –20 °C in a freezer under inert gas condition; a balloon of Ar gas was attached to the vessel to prevent the infusion of oxygen in the air. After 3 h, red crystals precipitated at the bottom were retrieved by thawing and filtering the frozen solution at room temperature (20 °C).
For Raman spectroscopy, Alpha 300R (WITec) was used with excitation laser of Nd:YAG (532.2 nm, 0.5 mW). All measurements were carried out in air at room temperature (20 °C).
Anhydrous chromium (II) acetate was received as red powder [Figs. 2(a) and 2(d)]. Water-coordinated chromium (II) acetate [chromium (II) acetate hydrate] was prepared by dissolving anhydrous chromium (II) acetate into degassed water. Anhydrous chromium (II) acetate can be effectively converted to ligand-coordinated complexes by both liquid- and vapor-phase ligand, unless it is oxidized during the ligand [2,9]. After cooled down to –20 °C, red crystals were found at the bottom of the flask. Note that the aqueous solution, once frozen, provides protection against oxidation [Fig. 2(b)]. The diameter of crystal is submillimeter (~0.5 mm), which is comparable or slightly larger to the direct dissolution by Song
In addition, pyridine can be used for the reaction [Fig. 2(c)]. Both water and pyridine have lone pairs that can form coordination bonding with chromium (II) acetate. However, the cooling temperature, –20 °C, is not enough to freeze pyridine, because its melting point is –42 °C. Indeed, after the cooling process, the solution was not frozen. But small red crystals were also precipitated at the bottom of the vessel. The size of the crystals reached 50 µm in diameter, which also comparable to the previous report  [Fig. 2(f)]. Its Raman spectrum also matches with the previously reported spectrum, and no other contaminants or residues were found (Fig. 3). The yield was estimated 19.07 %, which is also higher than the previous report. This shows that the current method can be applied to ligands other than water. And freezing the solvent is not necessary for the improved crystallization; the temperature gradient is the crucial factor.
When hexane was used for the reaction, small crystals were dispersed right after putting the anhydrous powder. The dispersed powder rapidly sank down at the bottom of the vial. And no coordination product was obtained. These results match with previous reports. [1, 2, 9] Hexane is not capable of forming coordination bonds with chromium (II) acetate, because it does not have any lone pairs to donate. Hexane therefore does not dissolve chromium (II) acetate. This shows that crystallites can be dispersed right after putting the anhydrous powder regardless of the dissolution or coordination.
One of the major differences of this new method from the previous methods is the rapid cooling of the “unclear” solution. In general, rapid cooling results in the small size of the crystals. The retention of the crystal size and the higher yield, despite faster reaction, imply that the reaction mechanism is different from the previous methods. The opaqueness of the solution right before the crystallization implies that there are particulates floating in the solution, forming a (quasi-) colloidal suspension. To assure the presence of the small particulates in the solution, a small portion of the solution was drop-casted onto a substrate. And optical microscopy revealed the presence of small crystals in the solution (Fig. 4). The size of the crystallites was approximately 10 µm, which is considerably smaller than the size of the final product. Since chromium (II) acetate can undergo ligand coordination by simple exposure of the ligand vapor, such dispersion in solvent can facilitate the ligand coordination . This implies that these small crystallites serve as nucleation sites during the growth reaction. The previous report shows that the growth without the seeds, i.e., using clear supernatant only, requires longer time (up to 24 h) to achieve the comparable size of the crystals .
Therefore, the presence of tiny crystallites before the cooling process is the key to the improvement. In general, crystallization is cominvolves the generation of nucleation seeds, and the growth step accompanies the increase in the size of pre-existing seeds. In pristine solution, the nucleation occurs by the self-agglomeration of the solutes in supersaturated solution, but this process is kinetically slow. In the previous method by Song
In conclusion, the heterogeneous crystallization of ligand-coordinated chromium (II) acetate is achieved by rapid cooling of the solution. When compared with the products obtained by Song
This work was supported by a grant from 2019 Research Fund of Andong National University.