Crystal structure of the thermochromic bis(diethylammonium) tetrachloridocuprate(II) complex

The low-temperature structure of bis(diethylammonium) tetrachloridocuprate is reported. The complex exhibits thermochromism and has a two-dimensional hydrogen-bonded network through N—H⋯Cl hydrogen bonds.


Chemical context
Thermochromic compounds exhibit a reversible change in color corresponding to a change in temperature. This change can occur in the solid state or in solution and is typically due to geometry rearrangement at the molecular level. Several mechanisms have been proposed for this rearrangement, including phase transitions, changes in solvation, changes in ligand geometry, coordination number, and finally crystal packing (White & LeBlanc, 1999). There are two generally accepted classes of thermochromism: (i) continuous; used to describe a gradual change in color, most likely due to breaking or rearrangement of the crystal structure (Roberts et al., 1981), and (ii) discontinuous; used to describe a dramatic change in color over a specific or small temperature range (Van Oort, 1988). Two classes of thermochromic compounds that have practical applications today include liquid crystals and leuco dyes. Liquid crystals exist on the boundary between the liquid and solid states. They are classified as discontinuous due to the chemistry of their transitions (Amberger & Savji, 2008). As a result, thermochromic liquid crystals have been used to make 'mood rings', thermometers, and game pieces (Chandler, 2012). Although color changes in liquid crystals are more sensitive to external stimuli such as temperature changes, they have a highly specialized manufacturing process and are difficult to make. For this reason, new thermochromic compounds such as leuco dyes are highly sought after. Leuco dyes are easier to work with and less sensitive to temperature changes. They have been used in advertising labels, textiles, and packaging for microwaveable syrup bottles and beverage cans that indicate content temperature changes (Muthyala, 1997). Given the intriguing applications of thermochromic compounds, we report the synthesis and structural characterization of a bis(diethylammonium) tetrachloridocuprate complex (I) that displays thermochromic properties.

Structural commentary
The asymmetric unit of the thermochromic complex (Et 2 NH 2 ) 2 [CuCl 4 ] consists of four unique diethylammonium cations and one full and two half tetrachloridocuprate anions (Fig. 1). The diethylammonium cations and the complete anion (Cu1) occupy general positions within the unit cell. The two half-tetrachloridocuprate anions are located on crystallographic twofold axes at [ 1 2 , 1 2 , z] and [ 1 2 , 0, z]. Each copper cation exhibits different coordination geometries. Cu2, located on a twofold rotation axis, has close to ideal square-planar geometry, with trans Cl-Cu-Cl angles close to 180 (Table 1). Analysis of these angles through the 4 metric developed by Yang et al. (2007) yields a 4 value of 0.02 for Cu2. A value of zero (0) is indicative of an ideal square-planar geometry while a value of one (1) indicates an ideal tetrahedral geometry. In contrast, Cu1 and Cu3 adopt distorted square-planar geometries, tending to a disphenoidal (or 'see-saw') type geometry with 4 = 0.27 and 0.48, respectively. The 4 value is calculated from: [360 À ( + )]/141; where and are the two largest angles about the four-coordinate copper(II) atom in question. However, these distortions are solely in the bond angles about the copper(II) atoms: all of the Cu-Cl bond lengths are similar (Table 1). A mean-plane analysis of each copper(II) atom shows the gradual change from the atoms being nearly co-planar (Cu2), through an intermediate distortion (Cu1) to a more pronounced out-of-plane arrangement of chlorine atoms around Cu3, in which the chlorine atoms are located 0.68 Å from the mean plane (Table 2). These distortions, along with the hydrogen-bonded network described below, are likely the cause for the thermochromism observed within the sample.

Supramolecular features
The extended structure consists of the diethylammonium cations forming an extended hydrogen-bonded network with the chlorine atoms of the tetrachloridocuprate anions. All of the ammonium cations serve as hydrogen-bond donors; the ammonium cation hydrogen atoms were located in difference Fourier maps and refined freely. Ammonium cations involving N1, N2 and N3 all serve as donors of a single hydrogen-bond to one chlorine and as a donor of a bifurcated hydrogen bond to a pair of chlorine atoms on one copper(II) atom. The hydrogen atoms on N4 both form bifurcated interactions, albeit weakly ( Table 3). All of the chlorine atoms serve as hydrogen-bond acceptors (Table 3, Fig. 2). While some of the reported interactions are quite long (NÁ Á ÁCl > 3.2 Å ), and could be classified as weak interactions (Jeffrey, 1997), they are observed where the hydrogen atom is interacting with two chlorine atoms that are adjacent to each other/bonded to the Atom labelling scheme for bis(diethylammonium) tetrachloridocuprate. Atomic displacement ellipsoids are depicted at the 50% probability level and H atoms as spheres of an arbitrary radius. [Symmetry codes: (i) Àx + 1, Ày + 1, z; (ii) Àx + 1, Ày, z.] Table 1 Selected geometric parameters (Å , ).
same copper (II) atom and are considered by us to be bifurcated hydrogen bonds. The Cu2 anion is notable because all four chlorine atoms are acceptors of bifurcated hydrogen bonds from N1 and N4; Cu2 is located on a twofold rotation axis. N1 also donates a single hydrogen bond to Cl1. N2 has a bifurcated hydrogen bond to chlorine atoms Cl2 and Cl3 on Cu1 and also forms a single donor hydrogen bond to Cl4 of an adjacent Cu1 anion. The diethylammonium cation that includes N3 has both a bifurcated hydrogen bond to Cl3 and Cl4 (Cu1) and a single donor hydrogen bond to Cl7 (Cu3). The hydrogen atoms on N4 are donor atoms of bifurcated hydrogen bonds to Cl5/Cl6 on Cu2 and Cl7/Cl8 on Cu3. The ultimate result of this prolific hydrogen-bond bridging of [CuCl 4 ] 2À anions is a two-dimensional sheet extending parallel to the ab plane (Fig. 2). Inspection of this plane along the crystallographic a axis reveals a gentle corrugation of the sheet (Fig. 2b). This hydrogen-bonded sheet is likely the driving force for crystallization (Desiraju, 2002).

Database survey
There are 59 structures that incorporate the bis-diethylammonium ligand moiety with a tetrachloridocuprate complex (Groom & Allen 2014; CSD Version 5.36). Of those 59 structures, 23 incorporate bridging chloride ligands, while 36 have independent tetrachloridocuprate complexes present. Thirteen structures incorporate the bis-ethylammonium ligand as a linear structure as presented in this manuscript. In addition, of the 59 structures, eleven show the tetrachloridocuprate complex adopting a distorted square-planar geometry as presented in complex (I).

Synthesis and crystallization
The synthetic procedure is outlined in Fig. 3.
General Procedure: Bis-diethylammonium tetrachloridocuprate was synthesized according to literature procedures (Choi & Larrabee, 1989). Reagents and solvents used were purchased from commercial sources (Sigma-Aldrich and Fisher Scientific). A Perkin Elmer FT-ATR spectrometer was used to collect IR spectra with three scans from 200 nm to 800 nm at a resolution of 1 cm À1 . The melting point was recorded on a Fluka Mel-Temp melting point apparatus (Electrothermal) equipped with 51 II thermometer.
Synthesis of bis-diethylammonium tetrachloridocuprate: Diethylammonium hydrochloride (2.22 g, 20.3 mmol) was dissolved in 15 mL of 2-propanol to afford a clear solution. Copper(II) chloride dihydrate (1.75 g, 10.1 mmol) was dissolved in 3 ml ethanol producing a dark green solution. Both solutions were mixed, generating a brownish-black colored product that was heated in a water bath for 3 min. Upon removal from the water bath, a 10 ml solution of 20% v/v 2-propanol and ethyl acetate was added to the mixture. The mixture was placed in an ice bath, which gave a brightgreen precipitate. The precipitate was filtered, washed with three 10 ml aliquots of ethyl acetate, then air dried to produce the desired product as a bright green thermochromic solid Hydrogen-bonding scheme for bis(diethylammonium) tetrachloridocuprate viewed (a) along the c axis and (b) along the a axis. Atomic displacement ellipsoids are depicted at the 50% probability level and H atoms as spheres of an arbitrary radius. Ethyl H atoms have been omitted for clarity. Hydrogen bonds are shown as blue dashed lines. Table 3 Hydrogen-bond geometry (Å , ).  (3) 125 (2) Symmetry codes: (iii) Àx þ 1; Ày þ 1; z þ 1; (iv) x; y; z þ 1; (v) x À 1 2 ; Ày þ 1 2 ; Àz þ 1; (vi) x À 1 2 ; Ày þ 1 2 ; Àz; (vii) Àx þ 1 2 ; y þ 1 2 ; Àz.

Figure 3
The synthetic scheme.
Thermochromic properties: Green-colored solid at temperatures lower than 327 K and bright-yellow colored solid at temperatures greater than 328 K.

Refinement
Details of the refinement are found in Table 4. All nonhydrogen atoms were refined with anisotropic atomic displa-cement parameters. Hydrogen atoms bonded to carbon were included in geometrically calculated positions with U iso (H) = 1.2U eq (C methylene ) and 1.5U eq (C methyl ). Methyl groups were allowed a torsional degree of freedom and C-H distances were set to 0.99 Å (methylene) and 0.98 Å (methyl). Ammonium hydrogen atoms were located in difference Fourier maps and refined freely. The structure was refined as an inversion twin, with a 0.52:0.48 twin ratio. Because this ratio is close to 0.5, data were inspected carefully for signs of missed inversion symmetry; no higher symmetry was found. One reflection (0 0 1) was obscured by the beamstop and was omitted from the refinement. program(s) used to solve structure: SHELXS (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: publCIF (Westrip, 2010).