Synthesis and Structure of a Ladder-like Co-crystal CuICl with 3,5-Dipropyl-4-amino-1,2,4-triazole①

YAN Juan-Zhi LU Li-Ping

SU Fenga ZHU Miao-Lia②

a (Institute of Molecular Science, Shanxi University, Taiyuan 030006, China)

b (Taiyuan University, Taiyuan 030001, China)

1 INTRODUCTION

Copper complexes and clusters are of great interest for their intriguing architectures and potential applications[1]. Particular attention has been paid to the photochemical and photophysical pro- perties of copper(I)complexes in light of the d10electronic configuration with diversified luminescent behavior[2-4]. The univalent and closed-shell Cu(I)ions are a soft Lewis acid and can readily coordinate with halides, pseudohalides and N-containing li- gands as a result of the soft-soft bonding pre- ference[5]. Many species, such as rhomboid dimers, cubane tetramers,four- or six-membered rings, and zigzag-like or staircase chains, can be further connected by neutral nitrogen-containing ligands or hydrogen interactions to give diversiform structures[6,7]. Some in situ generated Cu(I)X source routes have been developed as new approaches to synthesize copper(I)halide based complexes including halide substitution, copper oxidation and copper reduction routes. The reduction of cupric salts has proven to be the most promising approach for in situ generation of Cu(I)X sources[6]. Hydrogen phosphonate, H3PO3, a moderate-intensity reducing agent, is unstable. In the acid condition, Cu2+ions can be reduced to Cu+in the presence of H3PO3, based on the potentials:

1,2,4-Triazole and its derivatives are very interesting five-membered N-heterocycle aromatic ligands,which can bridge metal ions with μ-1,2, μ-2,4or μ-1,2,4binding modes[8,9]. The μ-1,2bridging mode is the coordination mode usually found for many triazoles,in which the N4-position is substituted[10]. The size and shape of substituent groups on the triazolate ring are crucial for the structures of metal triazolate frameworks. Under hydrothermal (solvothermal)conditions, a large number of mononuclear, polynuclear, and multi-dimensional coordination polymers of Cu(I)and 1,2,4-triazolates with interesting optical properties have been prepared and characterized[11], which involves in situ generated 1,2,4-tirazolates by cycloaddition of organic nitriles with ammonia in the presence of divalent copper[4,12].Moreover, the ligands 4-amino-1,2,4-triazole derivatives seem to be unstable under hydro(solvo)thermal conditions at high temperature. Thus, different approaches, such as the reductive deaminization reactions[13]and direct synthesized organic/inorganic hybrid complexes incorporating copper halides with 4-aminotriazole as organic linkers[14], have already been observed in the synthesis of copper(I)-halide with 1,2,4-triazolate compounds. In this study,compound 1 was received by employing divalent copper, H3PO3, and 3,5-dipropyl-4-amino-1,2,4-triazole under mild reaction conditions and its IR,single-crystal X-ray diffraction, elemental analysis and luminescent property have been investigated.

2 EXPERIMENTAL

2. 1 Materials and measurements

All reagents were purchased commercially and used without further purification. Ligand dpatrz was synthesized according to the literature[15]. Elemental analysis (C, H, and N)was obtained on an Elementar Vario EL III analyzer. The FT-IR spectrum was recorded from KBr pellets in the range of 4000～400 cm-1on a Brukep Tensor 27 spectrometer.UV-Vis spectra (in H2O)were recorded on a CARY 50 Bio UV-visible spectrophotometer. Luminescence spectra were recorded on a CARY Eclipse(Varian, USA)fluorescence spectrophotometer at room temperature.

2. 2 Synthesis of [Cu2(dpatrz)2Cl2](1)

CuCl2·2H2O (0.17 g, 1 mmol)was added to a stirred solution of dpatrz (0.168 g, 2 mmol)in 20 mL of water. H3PO3(0.164 g, 2 mmol)in water (2 mL)was slowly dropwise added. The reacting solution was stirred for 6 h, and then filtered to remove the insoluble substance. The filtrate was left in air to evaporate. Light green bar crystals were obtained after 14 days (yield: 45%). Anal. Calcd. (%)for C16H32N8Cl2Cu2(Mr= 534.48): C, 35.96; H, 6.0; N,20.97. Found (%): C, 35.72; H, 6.06; N, 21.05. IR(KBr cm-1): 3261(s), 3190(m), 2968(s), 2897(m),1630(m), 1543(s), 1373(w), 1221(w), 1089(w),956(m), 902(w), 750(w), 536(w).

2. 3 Crystal structure determination

Crystallographic data of compound 1 were collected at 298 K on an imaging plate type diffractometer(Rigaku RAXIS-RAPID)with graphite-monochromated Mo-Kα radiation (λ = 0.71069 ?). The crystal with suitable size (0.22mm × 0.13mm × 0.12mm)was selected for data collection. The structure was solved by direct methods with the program package[16]. After all non-H atoms were refined anisotropically, hydrogen atoms attached to C and N atoms were added theoretically and treated as riding on the concerned atoms. The final cycle of fullmatrix least-squares refinement was based on the observed reflections and variable parameters. For compound 1, a total of 10245 reflections were obtained in the range of 2.97＜θ＜25.05° with 4197 unique ones (Rint= 0.0434), of which 3162 were observed (I ＞ 2σ(I)). The final R = 0.0445, wR =0.116 (w = 1/[σ2(Fo2)+ (0.0412P)2+ 1.4284P],where P = (Fo2+ 2Fc2)/3), (Δρ)max= 0.52, (Δρ)min=–0.33 e/?3, (Δ/σ)max= 0.001 and S = 1.050. The selected bond lengths, bond angles and H-bonds for 1 are listed in Tables 1 and 2, respectively.

Table 1. Selected Bond Distances (?)and Bond Angles (o)for Compound 1

Table 2. Hydrogen Bond Geometry (?, °)of Compound 1

3 RESULTS AND DISCUSSION

3. 1 Crystal structure of compound 1

Compound 1 consists of two independent molecules and has two chemically identical entities with different conformations, namely, cis- and trans-side chain complexes relative to the triazole ring (Fig. 1,A for trans-[Cu(I)(μ2-dpatrz)2Cu(I)Cl2]and B for cis-[Cu(I)(μ2-dpatrz)2Cu(I)Cl2]). There are two halves of molecules in the asymmetric unit and each molecule shows a dimeric structure with an inversion center. The complete molecule contains two univalent copper ions, two dpatz ligands and two chlorine anions. Both Cu ions adopt slightly distorted trigonal geometries via two nitrogen atoms from two symmetry-related dpatrz ligands and one chlorine atom. In molecules A and B (Fig. 1 and Table 2), the bond lengths of Cu–N and Cu–Cl are similar and located in the ranges of 1.968(3)～1.976(4)? for Cu–N and 2.203(1)～2.214(1)? for Cu–Cl, respectively. The angles around Cu ions are close to 120° (for A 118.3(1), 120.2(1)and 121.4(1)°;for B 117.0(1), 119.0(1)and 123.4(1)°), which indicates a distorted trigonal-planar geometry in Cu ion environment. The values of bond lengths and bond angles agree with those of [Cu2Cl2(admtz)2]and [Cu2Br2(admtz)2][14]. The contact of Cu··Cu with 3.428(1)? in molecule B is slightly longer than that in molecule A with 3.415(1)?.

The difference between molecules A and B is that the propyl side chains are in trans-position relative to the triazole ring mean planes (N(1)/N(2)/N(3)/C(5)/C(9))for A with the distances from plane of 1.420(5)for C(7A)and –1.472(6)? for C(11A)and cis- for B with the values of 1.198(5)for C(7B)and 1.295(7)? for C(11B). The angles of propyl chain C(7)(C(11))in both entities have slight changes (Table 2). In each dimeric structure, the dpatrz ligands adopt a μ1,2-bridging mode to bridge two Cu(I)ions. The chlorine ions slightly deviate from the plane of the dimer formed by two Cu ions and two hydrazines with the distances of 0.127(1)for A and –0.640(1)? for B. As shown in Fig. 2 and Table 3, the weak interactions play an important role in stabilizing the structure of 1. In molecules A(B),hydrogen bonding interactions N(4A)··Cl(1A)iv(3.343(4)?)(N(4B)··Cl(1B)v= 3.393(5)?, symmetry codes: iv –x+1, –y+2, –z+2; v x+1, y, z))form chains C(7)and rings R22(14)[17]on the first graphset level. These chains and rings assemble the molecules A(B)forming one-dimensional stair-step chains (Fig. 2)running parallel to the [01–1]direction. Furthermore, chains B are built by C–H··π(C(7B)··Cg(1)i(3.766(6)?, Cg(1)ifor a centre of N(1B)/N(2B)/N(3B)/C(5B)/C(9B), symmetry code: i–x+2, –y+2, –z+2)(Fig. 2B)interactions with centrosymmetric R22(14)motifs. Thus, weak C–H··π interactions make the distance of N–H··Cl in tape B much shorter than that in tape A.

Fig. 1. ORTEP view with 30% probability level of 1 (A for trans-[Cu(I)(μ2-dpatrz)2Cu(I)Cl2]and B for cis-[Cu(I)(μ2-dpatrz)2Cu(I)-Cl2]). Open dashed line is for N–H··Cl hydrogen bond.Symmetry codes: (i)–x+2, –y+2, –z+2; (ii)–x+1, –y+2, –z+1

Fig. 2. Perspective view of the 2D network in 1. Tape A: stair-step assembly built from N–H··Cl hydrogen bonds;Tape B: N–H··Cl hydrogen bonds and C–H··π weak interactions. Uninvolved hydrogen atoms are omitted for clarity. Cg(1)for centre of N(1B)/N(2B)/C(5B)/N(3B)/C(1B). Blue, N; gray, C; cyan, Cu and green, Cl

Between molecules A and B, the intermolecular hydrogen bonding interactions N(4A)··Cl(1B)(3.273(4)?)and N(4B)··Cl(1A)iii(3.374(5)?,symmetry code: iii x, –y+3/2, z–1/2)form layers with chains C22(17)and rings R34(18)second graphset level. These chains and rings extend stair-step tapes to build two-dimensional networks along the[031]direction (Fig. 2). Ladder-like packing of crystal 1 is stabilized by rings R34(18)and weak C–H··Cl(C(10B)··Cl(1A)iii)(3.741(5)?, symmetry code: iii x, –y+3/2, z–1/2)hydrogen bonds parallel to the [01–2]direction. Although C··Cl and C··π interactions are very weak in comparison to the N··Cl hydrogen bonding interactions, they still contribute to stabilizing the three-dimensional network of 1.

3. 2 Bond valence expression and UV-Vis absorption

In the prescription of compound 1 synthesis, we employed chemicals CuCl2·2H2O, but the trigonal planar geometries of Cu(I)cations were observed.Here, we have carried on the calculation based on the BVS model[18]to assign the oxidation states of copper ions in 1. In this method, the empirical expressions below are employed.

Based on the similar bond distances of molecules A and B, only the oxidation state of copper centers of molecule A is calculated. Taking the initial R0value of 1.525 ? for Cu(I)–N and 1.858 ? for Cu(I)–Cl, R0value of 1.751 ? for Cu(II)–N and 2.000 ? for Cu(II)–Cl[19], when following Eqs. (3)and (4), the BVS values for copper ions come out to be 0.97 and 1.66, respectively. Thus, oxidation state of +1 is assigned to copper cation in the solid phase of 1. The exclusivity of Cu(I)phases may reflect the stabilization of Cu(I)oxidation state through π-backbonding to the aromatic amine ligand[20]. Redox reaction of compound 1 synthesis is shown below:

The presence of monovalent copper in 1 is further supported by the UV-Vis absorption spectra of 1 in H2O. Cu(II)complexes generally exhibit a broad and very weak copper-centered d-d transition band in the region of 500～1000 nm, related to the coordination environment of central Cu(II)[21]. As described in Fig. 3, the characteristic of Cu(II)d-d transition is being strengthened as time goes on, indicating there are monovalent copper cations in this complex going with an oxidating process under air O2. The features of absorption spectra of ligand dpatrz and compound 1 are similar with absorption bands at 260 nm,which may be attributed to the π-π* transition of the coordinated dpatrz ligand[22].

Fig. 3. UV-Vis spectra of the ligand (dpatrz)and compound 1 in H2O. The inset is for the transformation of UV-Vis spectra of 1 in H2O along 90 minutes ([I]= 2.0 × 1.0-3 M and 3 minutes spacing interval)

Fig. 4. Solid-state photoluminescence spectra of ligand and compound 1 at room temperature

3. 3 Photoluminescence property

The fluorescence properties of free ligand and compound 1 have been studied at room temperature in the solid state. As given in Fig. 4, the free dpatrz ligand and complexes display a similar shoulder peak at ca. 423 nm with 237 and 253 nm excitation,respectively. The emissions are neither metal-toligand charge transfer nor ligand-to-metal charge transfer in nature and probably can be assigned to the intraligand fluorescent π-π* emission[14,23].

4 CONCLUSION

One new Cu(I)Cl compound with dpatrz containing two co-crystallized centrosymmetric isomers has been constructed successfully. The Cu(I)centers are coordinated in triangle geometries and the hydrogen-bond networks are constructed from infinite 2D layers formed by N–H··Cl hydrogen bonds between 1D ladder-like chains assembled by N–H··Cl hydrogen bonds and C–H··π weak interactions. Bond valence sum (BVS)and UV-Vis absorption spectra support the existence of Cu(I).Compound 1 exhibits extensive green blue phosphorescence in the solid state at room temperature.

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