In situ solvothermal syntheses of a heteronuclear copper(I)-alkaline metallic tetrazole-based coordination polymer

Abstract

Three novel heteronuclear coordination complexes, namely, [Cu₂K(Mtta)I₂] (1), [Cu₃K(Mtta)₃I] (2) and [Cu₂Na(Mtta)₂(CH₃CN)I] (3) (HMtta = 5-methyl-1H-tetrazole) have been solvothermal synthesized by CuI, MCl [M = K (1) and (2); M = Na (3)], sodium azide and acetonitrile generated via a [3 + 2] cycloaddition reaction of acetonitrile and sodium azide. Single crystal X-ray diffraction reveals that all complexes are three-dimensional (3D) frameworks, where complex 1 is constructed by connecting rare µ₅-N bridging Mtta ligands to Cu–I–K layers, complex 2 is built up by connecting inorganic cationic [Cu₆I₂]⁴⁺ clusters with numerous Mtta ligands, while complex 3 is constructed from open Cu(Mtta) subnets and Na⁺ centers. Furthermore, the luminescence properties of all complexes have been investigated. This work further supplements the mechanism of the Demko-Sharpless reaction in heteronuclear tetrazole-based coordination polymers.

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Introduction

In situ (meaning “in the original position” in Latin) ligand synthesis is of great interest in coordination chemistry and organic chemistry for the preparation of crystalline coordination compounds, and for the discovery of new organic reactions and understanding their mechanisms.¹ The advantage of the in situ solvo/hydrothermal method is making all the reactions take place just in one step from the reactants. The tetrazole functional groups have been extensively utilized in coordination chemistry, medicinal chemistry and materials science during the past two decades.² One of the main synthetic routes to achieve tetrazole systems is the [2 + 3] cycloaddition between organonitriles and azide salts, for instance, NaN₃ and also silyl, aliphatic, or aromatic azides RN₃.³ However, these traditional cycloaddition reactions have many disadvantages which involve expensive and toxic metal–organic azide complexes, or need for severe reaction condition.⁴ Recently, Demko and Sharpless pioneered a facile approach to the in situ synthesis of 5-substituted 1H-tetrazoles, namely through the [2 + 3] cycloaddition reaction of an azide with nitriles in water with the aid of a Lewis acid.⁵ Xiong and co-workers developed Sharpless' methods using Zn(II) to catalyze cycloaddition reactions of nitriles and azide to form zincterazole coordination polymers via in situ hydrothermal synthesis.⁶ Later, Li et al. reported two novel “Cu(I)-intermediate” products by replacement of zinc salts with Cu(I/II) salts as the catalyst to generate 5-substituted 1H-tetrazolate.⁷ Up to now, a great many coordination polymers containing tatrazole have been obtained via in siu solvo/hydrothermal syntheses.^1a,b,8

However, much attention has been focused on the preparation of 3D monometallic tetrazole coordination polymers, while the synthetic strategy toward heterometallic coordination polymers has received much less attention.⁹ It is well known that heterometallic coordination polymers have potentially interesting functions in sorption, conductivity, photolumiescence or magnetic ordering.¹⁰ Moreover, the structural preference of different metal centers in a mixed-metal system often leads to a broader palette of polymer structural motifs than can be achieved with monometallic systems, and thus makes the obtained structures more diverse.¹¹ In our continuing studies of in siu ligand solvo/hydrothermal synthesis of metal-tetrazolate complexes,¹² we report herein the syntheses and crystal structures of three novel 3D heteronuclear Cu–K/Na tetrazole-based coordination polymers, namely, [Cu₂K(Mtta)I₂] (1), [Cu₃K(Mtta)₃I] (2) and [Cu₂Na(Mtta)₂(CH₃CN)I] (3). (Scheme 1). Complex 1 is constructed by connecting rare µ₅-N bridging Mtta ligands to Cu–I–K layers, complex 2 is built up by connecting inorganic cationic [Cu₆I₂]⁴⁺ clusters with numerous Mtta ligands, while complex 3 is constructed from open Cu(Mtta) subnets and Na⁺ centers.

Scheme 1 In situ solvothermal syntheses of complexes 1–3.

Experimental

Materials and physical measurements

All materials and reagents were obtained commercially and used without further purification. Elemental (C, H, N) analyses were performed on a Perkin–Elmer 2400 element analyzer. Infrared (IR) samples were prepared as KBr pellets, and spectra were obtained in the 400–4000 cm⁻¹ range using a Nicolet Avatar 360 FT-IR spectrophotometer. X-Ray data collection were recorded on a Bruker Apex II CCD diffractometer operating at 50 kV and 30 mA using Mo Kα radiation (λ = 0.71073 Å). Fluorescence spectra were recorded with an Edinburgh FLS920 Spectrophotometer analyzer.

Syntheses of complexes 1–3

A mixture of CuI (0.095 g; 0.5 mmol),{ [ KI (0.083g; 0.5 mmol) and CH₃CN (10 mL) for 1]; [KI (0.083g; 0.5 mmol), CH₃CN (5 mL) and benzene (5 mL) for 2]; [NaCl (0.025 g; 0.5 mmol) and CH₃CN (10 mL) for 3]}, NaN₃ (0.033 g; 0.5 mmol), was sealed in a 23 mL Teflon reactor and kept under autogenous pressure at 170 °C for 3 d. The mixture was cooled down to room temperature at a rate of 5 °C h⁻¹, and colorless crystals were obtained in yield 25% based on Cu for 1 [35% based on Cu for 2; 30% based on Cu for 3]. 1: Cu₂KC₂N₄H₃I₂ (504): calcd.: C 4.77, H 0.60, N 11.13; found: C 4.50, H 0.65, N 10.80; IR (KBr, cm⁻¹): 2924(w), 2358(m), 1500(s), 1385(s), 1156(s), 1245(s), 1041(m), 721(w), 694(m); 2: Cu₃KC₆N₁₂H₉I (605.87): calcd.: C 12.01, H 0.50, N 28.01; found: C 12.50, H 0. 55, N 27.85; IR (KBr, cm⁻¹): 2948(w), 2100(m), 1620(s), 1490(s), 1384(s), 1153(m), 1132(s), 1039(w), 702(s); 3: Cu₂NaC₆N₉H₉I (484.21): calcd.: C 14.87, H 1.86, N 26.01; found: C 15.00, H 1.90, N 25.80; IR (KBr,cm⁻¹): 2928(w), 2066(s), 1500(s), 1389(s), 1145(s), 1124(s), 1043(m), 696(s).

X-Ray crystallography

Suitable single crystals of complexes 1–3 were selected and mounted in air onto thin glass fibers. Accurate unit cell parameters were determined by the least-squares fit of 2θ values, and intensity data were measured on a Bruker Apex II CCD diffractometer operating at 50 kV and 30 mA using Mo Ka radiation (λ = 0.71073 Å). Data collection and reduction were performed using the APEX II software.¹³ Multi-scan absorption corrections were applied for all data sets using the APEX II program.¹³ All structures were solved by direct methods and refined by full-matrix least squares on F² using the SHELXTL program package.¹³ All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms on water molecules were located from difference Fourier maps and refined using a riding model. The C4 atom in compound 3 is disordered. The crystallographic data for 1–3 are listed in Table 1 and the selected bond lengths and bond angles of the three complexes are listed in Table 2.

Table 1 Crystallographic data and structure refinement summary for complexes 1–3

	1	2	3
a R 1 = ∑||F0| − |Fc||/∑|F0|. b wR 2 = [∑w (F0^2 − Fc^2)^2]/∑w(F0^2)^2]^1/2.
Formula	Cu2KC2N4H3I2	Cu3KC6N12H9I	Cu2NaC6N9H9I
Mr	503.06	605.87	484.21
Crystal system	Monoclinic	Monoclinic	Orthorhombic
Space group	C2/m	C2/m	Pnma
a/Å	19.446(2)	9.3946(14)	9.5131(13)
b/Å	4.2397(5)	13.260(2)	15.302(2)
c/Å	12.4437(14)	12.7927(19)	9.2209(5)
α/°	90	90	90
β/°	107.177(2)	92.674(2)	90
γ/°	90	90	90
V/Å^3	980.15(19)	1591.9(4)	1438.1(3)
Z	4	4	4
D c/g cm^−3	3.409	2.528	2.236
µ/mm^−1	11.008	6.189	5.134
F(000)	904	1152	920
Parameters	68	113	96
Goodness-of-fit	1.019	1.054	1.072
R 1 [I > 2σ(I)]^a	0.0284	0.0372	0.0284
wR 2 (all data)^b	0.0620	0.1005	0.0620

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Table 2 Selected bond distances (Å) and angles (°)for complexes 1–3. Symmetry transformations used to generate equivalent atoms are given as footnotes

a i: x, y + 1, z; ii: −x + 1, −y + 3, −z + 1; iii: x − 1/2, y − 1/2, z; iv: x − 1/2, y + 1/2, z; v: −x + 1, −y + 2, −z + 1; vi: x, y − 1, z; vii: −x + 3/2, −y + 5/2, −z + 2; viii: −x + 3/2, −y + 3/2, −z + 2; ix: x + 1/2, y + 1/2, z; x: x + 1/2, y − 1/2, z. b i: −x, −y + 1, −z + 1; ii: x, −y + 1, z; iii: x − 1/2, −y + 1/2, z; iv: −x + 1/2, −y + 1/2, −z + 2; v: −x + 1/2, y + 1/2, −z + 1; vi: −x + 1/2, −y + 1/2, −z + 1; vii: x + 1/2, −y + 1/2, z; viii: −x + 1, y, −z + 2. c i: x, −y + 3/2, z, ii: −x + 1/2, −y + 1, z + 1/2; iii: −x, −y + 1, −z + 1; iv: −x + 1/2, −y + 1, z − 1/2; v: x, −y + 1/2, z.
Complex 1^a
I2–Cu1^i	2.662(6)	N2–K1	2.922(4)	K1^ii–I2–K1^iv	98.32(4)
K1–I1^i	3.656(16)	N2–K1^vi	2.922(4)	N3–N2–K1	114.6(3)
I2–K1^ii	3.493(19)	N3–Cu1^v	2.061(6)	N1–N2–K1	113.4(3)
I2–K1^iii	3.509(15)	K1–N2^i	2.922(4)	N3–N2–K1^vi	114.6(3)
I2–K1^iv	3.509(15)	K1–I2^ii	3.493(19)	N1–N2–K1^vi	113.4(3)
Cu1–N4	2.023(5)	K1–I2^ix	3.509(15)	K1^iii–I2–K1^iv	74.34(4)
Cu1–N3^v	2.061(6)	I2^x–K1–K1^i	127.17(18)	N4–Cu1–N3^v	114.9(2)
Cu1–I2^vi	2.6616(6)	I1^i–K1–K1^i	54.57(18)	N4–Cu1–I2^vi	108.8(10)
I1–Cu2	2.636(11)	Cu1^i–I2–Cu1	105.58(4)	N3^v–Cu1–I2^vi	109.1(8)
I1–Cu2^vii	2.707(8)	Cu1^i–I2–K1^ii	86.82(3)	Cu2–I1–Cu2^vii	67.36(2)
I1–Cu2^viii	2.707(8)	Cu1–I2–K1^ii	86.82(3)	Cu2^vii–I1–Cu2^viii	103.11(4)
I1–K1^vi	3.656(16)	Cu1^i–I2–K1^iii	163.96(3)	Cu2–I1–K1^vi	80.99(3)
I1–K1	3.656(16)	Cu1–I2–K1^iii	89.92(2)	Cu2–I1–K1	80.99(3)
Cu2–N1	2.025(6)	K1^ii–I2–K1^iii	98.32(4)	I1^vii–Cu2–I1^viii	103.11(4)
Cu2–I1^vii	2.707(8)	Cu1^i–I2–K1^iv	89.92(2)	N1–Cu2–Cu2^vii	127.58(8)
Cu2–I1^viii	2.707(8)	Cu1–I2–K1^iv	163.96(3)	N1–Cu2–Cu2^viii	127.58(8)
Complex 2^b
I1–Cu2	2.648(12)	K1–N2^viii	3.455(5)	N3–K1–N3^viii	170.3(2)
I1–Cu1	2.770(9)	N1–Cu2^vi	2.037(5)	N4–Cu1–K1^iv	90.57(15)
Cu1–N2^iii	2.001(5)	Cu2–I1–Cu2^i	61.76(4)	I1–Cu1–K1^iv	164.97(4)
Cu1–N5	1.985(5)	Cu2–I1–Cu1^ii	130.74(3)	N1^v–Cu2–I1	113.30(12)
Cu1–N4	2.013(5)	Cu1^ii–I1–Cu1	71.72(3)	N3^viii–K1–N6	107.19(15)
Cu2–N1^v	2.037(5)	N2^iii–Cu1–N5	122.7(2)	N1^v–Cu2–I1^i	101.11(13)
N6–K1	2.995(6)	N2^iii–Cu1–N4	107.1(2)	N3–K1–N2^viii	151.14(15)
K1–N3^viii	2.767(4)	N2^iii–Cu1–I1	100.08(13)
K1–N6^viii	2.995(6)	N2^iii–Cu1–K1^iv	68.44(13)
Complex 3^c
I1–Cu1^i	2.671(7)	Na1–N3^v	2.436(4)	N1–Cu1–N2^iii	111.78(15)
I1–Na1^ii	3.270(3)	Na1–I1^iv	3.270(3)	N1–Cu1–N4^ii	112.95(16)
Cu1–N1	2.009(4)	Na1–I1^iii	3.433(3)	N2^iii–Cu1–N4^ii	106.78(16)
Cu1–N2^iii	2.032(4)	Cu1^i–I1–Cu1	150.98(4)	N1–Cu1–I1	110.33(12)
Cu1–N4^ii	2.035(4)	Cu1^i–I1–Na1^ii	85.92(2)	N5–Na1–N3	109.63(12)
Na1–N5	2.344(11)	Cu1^i–I1–Na1^iii	76.197(18)	N5–Na1–I1^iv	97.8(3)
I1–Cu1	2.6709(7)	N2^iii–Cu1–I1	106.71(11)	N3–Na1–I1^iv	86.99(12)
N4–Cu1^iv	2.035(4)	N4^ii–Cu1–I1	107.99(11)	I1^iv–Na1–I1^iii	168.32(11)

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Results and discussions

Syntheses consideration and general characterizations

The tetrazole with the four nitrogen atoms of the functional group have excellent coordination ability to act as either a multidentate or a bridging building block in supramolecular assemblies. However, they usually adopt 1-coordination (Scheme 2a) or act as bidentate ligands in most of the reported monometallic crystals.¹⁴ If the other non-coordinated nitrogen atoms in tetrazole can be bound, this provides a chance for other different metal ions to construct new heteronuclear tetrazole coordination polymers? With this in mind, we selected CuI as a suitable metal salt with the consideration that I⁻ anions can strongly bind to Cu⁺ cations to generate secondary building units (SBU),¹⁵ as well as adding alkaline metal salts in situ solvothermal conditions. Fortunately, as we expected, the three novel heteronuclear Copper(I)-Alkaline metallic tetrazole-based coordination polymers were successfully synthesized by a solvothermal reaction of CH₃CN/bezene, NaN₃, CuI and different alkaline metal salts. IR spectroscopic measurements were conducted to verify the emergence of a peak at 1400–1500 cm⁻¹ (Fig. S1–S3)† in the corresponding spectra of the products, suggesting that tetrazole groups are present.¹⁶ Single crystal X-ray diffraction analysis reveals that the tetrazole groups were deprotonated and every nitrogen atom has coordinated to metal ions, which further demonstrates strong the coordinating capacity of the tetrazole groups acting as multifunctional ligands.¹⁷

Scheme 2 (a) A detailed representation of 1-coordination mode. Selected partly coordination modes of tetrazole group: (b) observed in complex 1; (c) observed in complex 2; (d) observed in complex 3 and {[Cu₂(µ₃-Mtta)₂(CN)][Na(CH₃CN)]}_n; (e) observed in complex [CuNa₂(tza)₂(H₂O)] (red and black arrows representing Cu⁺and K⁺/Na⁺, respectively).

Descriptions and comments on the crystal structures

[Cu₂K(Mtta)I₂] (1). The architecture of 1 is a three-dimensional coordination framework with monoclinic space group C₂/m. As illustrated in Fig. 1a, the asymmetric unit resides on a crystallographically imposed inversion center, and two crystallographically independent Cu(I) and I⁻ ions exist. Although the two unique Cu(I) ions are four-coordinated, the difference between them is that the Cu(1) center presents a slightly distorted tetrahedron defined by two nitrogen atoms from two Mtta ligands and two µ₅-I ions, while the Cu(2) center adopts a distorted tetrahedral environment coordinated by one nitrogen atom from one Mtta ligand and three µ₅-I anions. The distances of Cu–N and Cu–I bonds fall in the range of 2.017–2.061 Å and 2.636–2.707 Å, respectively, which is in good agreement with the corresponding distances found in the two reported complexes.¹⁵ The K center features a seven-coordinated model, formed by two nitrogen atoms from two Mtta ligands and five µ₅-I anions with K–N distances of 2.918–2.922 Å (Table 2). The K–I distances ranging from 3.493 to 3.509 Å are slightly shorter than those reported in the literature [K–I = 3.62–3.71 Å].¹⁸ Each tetradentate Mtta ligand acting as a rare µ₅-bridging ligand binds to two Cu(1), one Cu(2) and two K cations, leading to form a regular planar Cu–N2–Cu–N2 six-membered ring with Cu(1)⋯Cu(1) separation of 3.496 Å. To the best of our knowledge, no similar Mtta ligand involving µ₅-bridging mode has been reported. Along the b-axis, Cu(2) ions are connected through µ₅-I ions to give rise to a mildly zigzag chain, which is identical to that of structure [(CuI)₂(µ₂-pyrimidine].¹⁹ Meanwhile, K ions and µ₅-I anions adopt the same mode to generate another infinite chain in the direction of the crystallographic b-axis. It is noted that the heteronuclear Cu–I–K layer is formed parallel to the bc plane via an alternate assembly of [CuI]_n and [KI]_n chains (Fig. 1b). In general, the Cu–Mtta units link neighboring [CuIK]_n layers into an unusual 3D structural motif (Fig. 1c).

Fig. 1 (a) Ball and stick plot showing the asymmetric unit of 1. All H atoms are omitted for clarity. (b) Perspective view of the heteronuclear Cu–I–K layers with [CuI]_n and [KI]_n chains in 1. (c) Perspective view of an unusual 3D structural motif by the linkage of the Cu–Mtta units and [CuIK]_n layers of 1.

[Cu₃K(Mtta)₃I] (2). Complex 2 crystallizes in a monoclinic system with space group C₂/m. X-Ray crystallographic analysis shows that it also exhibits a three-dimensional framework constructed of inorganic cationic [Cu₆I₂]⁴⁺ clusters, Mtta linkers and K centers. As shown in Fig. 2a, it contains two crystallographically independent Cu(I) (1 + 0.5) ions, one K ion, half an iodine ion, one and a half Mtta ligand per asymmetric unit. The coordination environment of Cu(1) is different from that of Cu(2), where Cu(1) is four-coordinated by three nitrogen atoms from three Mtta ligands and one µ₄-I ion, while the Cu(2) lies on a centre of symmetry and is coordinated by two nitrogen atoms from two Mtta ligands, two µ₄-I anions and one Cu⋯Cu interaction. The bond distances of Cu–N and Cu–I fall in the range of 1.983–2.039 Å and 2.648–2.769 Å, respectively. In contrast with complex 1, the coordination geometry of each K center adopts four-coordinated but not seven-coordinated geometry, which can be described as a distorted tetrahedral geometry by four nitrogen atoms from four Mtta ligands. Meanwhile, I⁻ ions have no coordination to K ions, which is also different from that of complex 1. Intriguingly, the 3D structure of 2 contains numerous inorganic cationic [Cu₆I₂]⁴⁺ clusters formed though Cu–I bonds with two symmetrically-related I⁻ ions adopting µ₄-bridging mode to link six Cu⁺ ions. The distance of the opposite Cu⋯Cu separation is 2.772 Å, which is slightly shorter than the sum of the van der Waals radii of two copper atoms (2.8 Å), suggesting a significant copper–copper interaction. Each hexanuclear Cu cluster connects to four adjacent clusters through Mtta ligands and extends in the bc-plane, which are further interlinked by K ions to complete the 3D framework (Fig. 2b). Although numerous Cu_n [n = 3, 4, 5, 6, 7, 8, 10] clusters have been documented, this hexanuclear [Cu₆I₂]⁴⁺ cluster is rare with [Cu₆I₅]⁺ cluster representing the known examples.^8a,20 If the hexanuclear Cu cluster is regarded as a node, the intricate 3D framework of 2 can simply be viewed as the linkage of the K centers and [Cu₆I₂]⁴⁺ clusters though Mtta ligands (Fig. 2c).

Fig. 2 (a) Ball and stick plot showing the asymmetric unit of 2. All H atoms are omitted for clarity. (b) Perspective view of the hexanuclear [Cu₆I₂]⁴⁺ clusters bridged by Mtta ligands along the a-axis in 2. (c) Topological view of the intricate 3D framework of 2 by a linkage of the K centers and [Cu₆I₂]⁴⁺ clusters as nodes.

[Cu₂Na(Mtta)₂(CH₃CN)I] (3). X-Ray analysis of complex 3 reveals that it is also a three-dimensional framework constructed of open Cu(Mtta) subnets and Na⁺ centers, crystallizing in an orthorhombic system with space group Pnma. As shown in Fig. 3a, it comprises one unique Cu(I) ion, half a Na ion, half a I ion, one Mtta ligand and half an acetonitrile per asymmetric unit. Each Mtta ligand is attached to one Na ion and three Cu ions. The Na ion is five-coordinated and defined by three nitrogen atoms from two Mtta ligands and one acetonitrile. The Na–I bond lengths spanning the range 3.271–3.437 Å are unexceptional, and the Na–N bond lengths ranging from 2.364 to 2.442 Å are comparable to those found in a 1D sodium tetrazole coordination polymer [2.467–2.528 Å].^8c Each tetrahedral Cu centre is coordinated by three nitrogen atoms from three Mtta ligands and one µ₄-I anion. The Cu–N and Cu–I distances vary from 2.011 to 2.037 Å and 2.650 to 2.671 Å, respectively. In the structure of 3, two symmetrically-related copper atoms with Cu⋯Cu separation of 3.594 Å, are linked by four neighboring N atoms from two Mtta ligands, giving rise to a distorted Cu–N2–Cu–N2 six-membered ring. The torsion angle of N1–N2–Cu1–N1ⁱ (i: −x, −y + 1, −z + 1) in the six-membered ring is 14.5°, which is different from that of complex 1 (0° in complex 1) but similar to the reported complexes {[Cu(Mtta)]·0.17H₂O}_n^7a and {[Cu₂(CN)₆(dmtrz)₃]}_n.^18a In the structural assembly, each Mtta ligands interconnects Cu⁺ tetrahedrons, forming an open three-dimensional network (Fig. 3b). The unit of Na⁺ ion binding to one acetonitrile is encapsulated into the cavities via coordination to nitrogen and iodine atoms, therefore the formation of Na⁺ is trigonal bipyramid (Fig. 3c).

Fig. 3 (a) Ball and stick plot showing the asymmetric unit of 3. All H atoms were omitted for clarity. (b) Perspective view of an open 3D network of 3 by the linkage of Mtta ligands and Cu⁺ tetrahedrons. (c) Perspective view of the cavities in 3 formed by Na⁺ trigonal bipyramid.

Summary of the structures

According to the mechanism of Demko-Sharpless reaction,⁵ we can deem the N-metal coordination complexes favoring to obey 1-coordination mode (Scheme 2a) due to a lower energy barrier in the initial coordination to the nitrile functionality.^1a,5b On the basis of the reported structural analysis about copper(I)-Mtta frameworks, it is apparent that different steric hindrance from substituted groups in tetrazole results in different topological networks.^7d Furthermore, among these three and two analogous heteronuclear copper(I)-alkaline metallic tetrazole-based coordination polymers, [CuNa₂(tza)₂(H₂O)]^9c and {[Cu₂(µ₃-Mtta)₂(CN)][Na(CH₃CN)]}_n,^9d Cu(I)-salts along with other metal salts (like K and Na), mostly adopt 1-coordination mode to link to the tetrazole group as shown in Scheme 2(b)–(e). It may ascribe to Cu(I) salts to act as catalyst with a priority to occupy the lowest reaction energy barrier position. Therefore, it is natural to come up with such an idea that this pre-occupied effect of the metal cation should be a strategy for the preparation of N-metal coordination compounds with certain mode in heteronuclear tetrazole-based complexes.

Thermal stability

To examine the thermal stabilities of the three compounds, thermogravimetric analyses were carried out at a heating rate of 10 °C min⁻¹ under an air atmosphere. As shown in Fig. 4, the TGA curves of complexes 1 and 2 indicate that they are stable up to about 259 and 315 °C, while beyond these temperatures, they begin to decompose and lose their crystallinity. The TGA trace of 3 exhibits two main steps of weight loss. The first step occurs from 223 to 265 °C, which corresponds to the loss of one coordinating acetonitrile molecule. The observed weight loss of 8.15% is close to the calculated value of 8.47%. The second step covers temperature from 332 to 374 °C, during which the organic groups are burned, and then upon further heating decomposes to unidentified products.

Fig. 4 TGA curves of complexes 1–3.

Luminescence properties

It is universally acknowledged that many Cu(I) compounds exhibit excellent luminescence properties.^21–23 Herein, the photoluminescence measurements of complexes 1–3 were carried out in the solid state at room temperature. The emission spectra have a maximum at 532, 461 and 460 nm and fluorescence lifetime approximate to 0.96, 1.02 and 1.12 ns for complexes 1–3, respectively, with an excitation upon 361, 303 and 345 nm (Fig. 5 and Fig. S4–S6†). As we know, it is usually believed that the entire transition of Cu(I) compounds can be assigned as metal-to-ligand charge-transfer, copper-centered emission, ligand-centeterd and interlight charge-transfer. Using reported literature examples,^22,23 this leads us to the conclusion that the transition of Cu(I) complexes 1–3 may be attributed to [Cu → Mtta] metal-to-ligand charge-transfer [MLCT]. The large difference in the emission maximum of complex 1 as compared to the maxima observed in 2 and 3 implies that the excited-state energy level is related to related copper (I) coordination environments. The red shift of emission spectrum of 1 relative to that of 2 and 3 may be ascribed to the higher d* energy level of copper(I) in 1 than that in 2 and 3.^7b,7c,9d

Fig. 5 Solid-state emission spectrum of complexes 1–3 at room temperature (excited at 361, 303, 345 nm).

Conclusion

We have synthesized three novel coordination heteronuclear complexes 1–3 by in situ solvothermal methods. All complexes exhibit intriguing heteronuclear 3D frameworks constructed by diverse clusters or chains and Mtta linkers. This work further supplements the mechanism of Demko-Sharpless reaction in heteronuclear tetrazole-based coordination polymers that Cu(I) salts acting as lewis acid along with other metal salts (just like K/Na in this work), mostly adopt containing 1-coordination mode to link tetrazole group. By locating the position of the certain metal center in advance, it can be a promising strategy for the self-assembly of the N-metal coordination compounds confirming the certain coordination mode with potential applications. Furthermore, these heteronuclear copper(I)-alkaline metallic coordination polymers extend the supramoleclar chemistry of tetrazole in the multi-nuclear domain.

In summary, the heteronuclear copper(I)-alkaline metallic tetrazole-based coordination polymers formed by [2 + 3] in situ solvothermal cyclo-addition reaction will open a new avenue for exploring novel heteronuclear metal–organic coordination polymers containing alkaline metals which is very rare in supramolecular chemistry.

Acknowledgements

This work was supported by the National Natural Science Foundation of China, Grant No. 20871048.

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Footnote

† Electronic supplementary information (ESI) available: IR spectra and fluorescence decay curves for compounds 1–3. CCDC reference numbers 713551(1), 713552(2) and 713553 (3). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/b910418f

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