Reconstruction of a (6,3) Brick Wall Sheet Giving an Unprecedented

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Reconstruction of a (6,3) Brick Wall Sheet Giving an Unprecedented Shubnikov-type (5,34) Sheet in Luminescent Cuprous Cyanide Supramolecular Isomers of Pseudo-polyrotaxane Ying-Lian Qin, Jiang Liu, Juan-Juan Hou, Ru-Xin Yao, and Xian-Ming Zhang* School of Chemistry & Material Science, Key Laboratory of Magnetic Molecules and Magnetic Information Material, Ministry of Education, Shanxi Normal University, Linfen 041004, P. R. China S Supporting Information *

ABSTRACT: Reactions of CuCl2, K3[Fe(CN)6]/K4[Fe(CN)6], and pyrazine (pyz) in water solution at 160 °C/170 °C for 6 days led to two luminescent supramolecular isomers with stoichiometry [Cu2(pyz)(CN)2]·[CuCN] (1 and 2), which show 3D pseudopolyrotaxane structures with 1D [Cu(CN)]∞ chains penetrating 2D [Cu2(pyz)(CN)2] sheets. Isomer 1 contains linear, trigonal, and tetrahedral Cu(I) atoms, which are linked by pyz and cyanide groups into unprecedented (3,4)-connected Shubnikov-type (5,34) sheets, and these sheets are penetrated by [Cu(CN)]∞ chains via unsupported CuI−CuI interactions. In contrast, the [Cu2(pyz)(CN)2] sheets in isomer 2 can be viewed as a classical (6,3)-topological brick wall layer, which are also penetrated by [Cu(CN)]∞ chains in the absence of cuprophilicity. Schematically, the brick wall and Shubnikov-type (5,34) nets can be formed via arrangement of brick rows in ABAB and ABBA modes, respectively. It is worth noting that cyanide sources played a crucial role in the formation of supramolecular isomers. 1 and 2 show strong photoluminescence related to the various local coordination geometries of copper atoms and weak cuprophilic interactions.

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well as the supra- and intermolecular cuprophilic interactions.6 More noteworthy, when these energy equivalent metal−metal, metal−ligand, metal−π, and electrostatic interactions coexist, different packing motifs of these molecules tend to possess similar energy potentials. Therefore, minor changes of the crystal growth conditions might induce the formation of different crystalline phases. It is true that regular pentagons cannot tile the plane, but there are in fact 14 different types of convex pentagons that tile the plane including five (5,3) nets, eight (5,34) nets, and one (5, 3 6 ) net. 7 Although a sheetlike complex [Cu(4,4′bipyridine)1.5(PPh3)]BF4,8a which is composed of both pentagon and octagon and has a topology analogous to silicate mineral nekoite, was documented ten years ago, [{Cu2(O2CEt)4}5(HMTA)3]8b (HMTA = hexamethylenetetraamine) represents the sole example of a pentagon-only based metal−organic sheet, which shows a layered (5,34) Catalan network with μ3- and μ4-HMTA as nodes and paddlewheel -like [Cu2(O2CEt)4] clusters as linkers (Scheme 1b). To develop novel sheets with pentagonal tilings, we report herein the first example of Shubnikov-type (5,34) sheet based luminescent cuprous cyanide coordination polymer [Cu2(pyz)(CN)2]·[CuCN] (1) (pyz = pyrazine) (Scheme 1a). Parallel

INTRODUCTION The essence of supramolecular chemistry is the self-assembly of metal ions (or metal clusters) and organic ligands through coordination bonds, which is a spontaneous process that involves various driving forces (e.g., metal−ligand interactions, ion template, hydrogen bonding, π−π stacking) and chemical flexibility (facile rearrangement of the building blocks).1 Of particular note have been the isolation and characterization of two-dimensional (2D) networks that contain the most commonly encountered (6,3), (4,4), and Kagomé lattices, which show six-, four-, and three-sided polygons, respectively.2 However, the incorporation of pentagons into coordination polymers is an area that remains undeveloped.8 This may be explained as contrary to triangles, squares, and hexagons, regular pentagons cannot tile the plane. In terms of structural diversity, Cu(I)−cyanide−organoimine compounds are an attractive class of target compounds, in which [Cu(CN)]∞ chains can condense to create a vast array of structure types such as single and double chains, sheets, and 3D frameworks.3−5 Among 2D nets, the (6,3) topological architectures including honeycomb, brick wall, and herringbone are readily accessible,2a but pentagon based sheets are elusive. A question which arises here is how to form pentagons by this simple [Cu(CN)]∞ chain and feasible organodiimine ligand? The final formation of a superstructure in a isomeric system involves many factors, including small differences of thermodynamic and kinetic stabilities and unpredictable crystal packing effects as © 2012 American Chemical Society

Received: August 17, 2012 Revised: October 18, 2012 Published: October 23, 2012 6068

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graphic data are listed in Table 1; selected bond lengths and bond angles are given in Table S1 of the Supporting Information.

Scheme 1. Three Semiregular Dual Nets with Congruent Pentagons

Table 1. Crystallographic Data for Compound 1 and 2 formula fw cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρcalc (g cm−3) μ (mm−1) F(000) size (mm) reflections Tmax/Tmin data/parameters S R1a wR2b Δρmax/Δρmin (eA−3)

stacking of [Cu2(pyz)(CN)2] sheets generates 1D channels that are penetrated by [CuCN]∞ chains to give a 3D pseudopolyrotaxane. The unsupported CuI−CuI interactions in 1 are observed between two- and three-coordinate Cu(I) centers, which play an additional role for the stability of the Shubnikov-type sheet. By tuning reaction temperature and cyanide source, a related pseudopolyrotaxane supramolecular isomer 2 was synthesized, which contains thermodynamically favorable (6,3) brick wall layers penetrated by the [CuCN]∞ chains in the absence of CuI−CuI interaction. Interestingly, the pentagonal Shubnikov-type sheet can be viewed as a reconstruction of a brick wall layer.

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MATERIALS AND METHODS

All the starting materials were purchased commercially reagent grade and used without further purification. Elemental analyses were performed on a Perkin-Elmer 240 elemental analyzer. The FT-IR spectra were recorded from KBr pellets in the range 400−4000 cm−1 on a Nicolet 5DX spectrometer. XRPD data were recorded in a Bruker D8 ADVANCE diffractometer. Thermal analysis (TG) was carried out in air atmosphere using SETARAM LABSYS equipment with a heating rate of 10 °C/min. Photoluminescence analyses were performed on an Edinburgh FLS920 luminescence spectrometer. [Cu2(pyz)(CN)2]·[CuCN] (1). A mixture of CuCl2·2H2O (0.068 g, 0.4 mmol), K3[Fe(CN)6] (0.133 g, 0.4 mmol), and pyz (0.073g, 0.9 mmol) in a molar ratio of 1.0:1.0:2.25 and H2O (7 mL) was sealed in a 15-mL Teflon-lined stainless container (pH ≈ 6.5) and heated to 160 °C for 6 days. After it was cooled to room temperature at a rate of 5 °C/h, and subjected to filtration, orange block crystals of 1 in a yield of 40% were recovered. The measured pH value of the solution after reaction is about 8.5. Anal. Calcd for C7H4Cu3N5 (1): C 24.10, H 1.14, N 20.08. Found: C 24.20, H 1.35, N 20.05. IR (KBr, cm−1): 2124 s, 1415 s, 1042 s, 799 s, 1159 m, 1124 m, 2154 m, 1629 w, 3438 w. [Cu2(pyz)(CN)2]·[CuCN] (2). A mixture of CuCl2·2H2O (0.068 g, 0.4 mmol), K4Fe(CN)6·3H2O (0.168g, 0.4 mmol), and pyz (0.073g, 0.9 mmol) in a molar ratio of 1.0:1.0:2.25 and H2O (7 mL) was sealed in a 15-mL Teflon-lined stainless container (pH ≈ 6.0) and heated to 170 °C for 6 days. After it was cooled to room temperature at a rate of 5 °C/h, and subjected to filtration, a mixture of yellow lamellar crystals of 2 and orange crystals of 1 in a yield of 35% and 10% resulted and was separated by hand. The measured pH value of the solution after reaction is about 8.4. Anal. Calcd for C7H4Cu3N5 (2): C 24.1, H 1.14, N 20.08. Found: C 24.08, H 1.07, N 20.03. IR (KBr, cm−1): 2114 s, 1420 s, 1032 s, 801 s, 1059 m, 1124 m, 2134 m, 1609 w, 3440 w. X-ray Crystallographic Study. Data were collected at 298 K on a Bruker Apex diffractometer (Mo Kα, λ = 0.71073 Å). Lorentzpolarization and absorption corrections were applied. The structures were solved with direct methods and refined with a full-matrix leastsquares technique (SHELX-97).9 Analytical expressions of neutralatom scattering factors were employed, and anomalous dispersion corrections were incorporated. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms of organic ligands were geometrically placed and refined with isotropic temperature factors. The crystallo-

compd 1

compd 2

C7H4Cu3N5 348.77 orthorhombic Pmmn 9.6056(10) 21.931(2) 4.8504(5) 90 90 90 1021.78(18) 4 2.267 6.154 672 0.52 × 0.24 × 0.18 5086/1214 0.4038/0.1421 1214/0/84 1.230 0.0311, 0.0826 0.0344, 0.0921 0.518/−0.652

C7H4Cu3N5 348.77 monoclinic C2/c 15.958(8) 9.652(5) 27.058(13) 90 97.955(9) 90 4128(4) 16 2.245 6.094 2688 0.20 × 0.12 × 0.10 9656/4490 0.5809/0.3754 4490/0/271 1.021 0.0445, 0.1071 0.0846, 0.1279 0.681/−0.493

R 1 = ∑||F o | − |F c ||/∑|F o |. ∑[w(Fo2)2]]1/2. a

b

wR 2 = [∑[w(F o 2 − F c 2 ) 2 ]/

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RESULTS AND DISCUSSION Compound 1 crystallizes in the orthorhombic space group Pmmn, and the asymmetric unit consists of three crystallographically independent Cu(I) centers, three cyanides, and one pyz, as shown in Figure 1a. Compound 1 is EPR-silent, which indicates that all copper atoms are monovalent. All atoms except for C(1) and C(2) localize in special positions. The indistinguishable C(3), N(3), C(4), and N(4) atoms occupy the same sites and have a site occupancy of 0.25. The N(3)− N(4) distance of 1.141(7) Å is the typical CN bond length of cyanide. The Cu(1) and Cu(3) atoms localize in special positions with a site occupancy of 0.25. Cu(2), Cu(4), C(5), N(5), C(6), and N(6) atoms are within a crystallographic mirror and thus have site occupancy of 0.5. The Cu(1) adopts a distorted tetrahedral geometry, coordinated by N(4) and C(4a)/N(4a) from two different cyanides and N(1) and N(1a) from a different pyz ligand. The Cu(2) atom shows a trigonal environment, coordinated by N(5) and C(5b)/N(5b) from two cyanides and one nitrogen atom from pyz ligand. The coordination geometry of Cu(3) is close to linear, ligated by N(3) and C(3c)/N(3c) from two cyanides. The L−Cu(3)−L (L = C, N) bond angle is 177.6(4)°. The coordination geometry of Cu(4) is similar to that of Cu(3) but slight distorted, coordinated by N(6) and C(6d) from two cyanides with a C(6d)−Cu(4)−N(6) bond angle of 173.0(2)°. The slight distortion from linearity may arise from a weak interaction between the Cu(4)chain···Cu(2)sheet. The Cu2−Cu4 distance of 2.8238(10) Å is close to the sum of the van der Waals radii of two copper atoms (2.80 Å), which is not associated with ligand-bridged, hydrogen-bonded, electrostatic6069

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connected and 3-connected nodes, respectively. Pyrazine and metalloligand “CN−Cu(3)−CN” can be considered as linkers. The topology of the sheet in 1 is an unprecedented (3,4)connected Shubnikov-type (5,34) net (Scheme 1a). The ratio of 3-connected nodes to 4-connected nodes is 2:1, which results in a Shubnikov-type topological plane with a short Schlafli symbol {53}2{54·82}.11 Selectively, the basic building blocks in the Shubnikov-type (5, 3 4 ) sheet in 1 are hexameric [Cu6(CN)4(pyz)2] brick units that stand side by side to form a brick row. The adjacent brick rows are arranged in ABBA mode to finish a (3,4)-connected Shubnikov-type (5,34) net (Figure 1b). Each hexameric [Cu6(CN)4(pyz)2] brick unit is sized ca. 9.60 × 8.57 Å2 and contains three different types of copper atoms (one bicoordinate Cu3 atom, two tetracoordinate Cu1, and three tricoordinate Cu2 atoms) as well as four cyanides and two pyz groups. The layers are stacked along the a-axis direction in a parallel fashion with the sequence of AB (Figure S2 of the Supporting Information), and this gives rise to channels of ca. 4.0 × 4.0 Å2 along the c-axis direction, and these channels are penetrated by 1D [Cu(CN)]∞ chains with unsupported CuI−CuI interactions in an inclined way to form the 3D pseudopolyrotaxane (Figure 1c). Although pentagonal features such as cyclopentane are chemically accessible, crystal engineering has not yet demonstrated an ability to persistently control the self-assembly of components.8,12 Compound 2 crystallizes in the monoclinic space group C2/ c, and the asymmetric unit consists of six crystallographically independent Cu(I) centers, six cyanides, and three pyz as shown in Figure 2a. All atoms except for N7, N8, N10, N12, C11, C12, C14, and C16 localize in general positions. The indistinguishable (N7), (C11), (N8), (C12), (N10), (C14), (N12), and (C16) atoms occupy the same sites and have a site occupancy of 0.5; the C(11)−N(7h) and C(12)−N(8i) distances of 1.110(10) Å and 1.132(10) Å are the typical CN bond length of cyanide. Cu1, Cu2, Cu3, and Cu4 show trigonal geometries, each coordinated by one pyrazine N and one C and one N from two cyanides. Both Cu5 and Cu6 atoms of the [Cu(CN)]∞ chain show a linear geometry, coordinated by one C/N and one N/C from two CN groups. The X− Cu(5)−X bond angle is 179.7(2)°; the X−Cu(6)−X bond angle is 178.2(2)°. Compared with 1, the L−Cu−L bond angles of 179.7(2)° and 178.2(2)° in [Cu(CN)]∞ chains are almost linear, which may be due to the absence of Cu I −Cu I interactions between Cu(chain)···Cu(sheet) (Figure S3 of the Supporting Information). The structure of 2 is a 3D pseudocatenate-like network constructed by [Cu(CN)]∞ chains and 2D (6,3) topological brick wall sheets of [Cu2(pyz)(CN)2]. The 2D [Cu2(pyz)(CN)2] sheets contain only three-coordinate Cu(I) atoms and are also constructed by hexameric [Cu6(CN)4(pyz)2] brick units with sized ca. 9.65 × 8.86 Å2. Different from the Shubnikov-type (5,34) sheet in 1, the brick wall net can be schematically viewed as an arrangement of brick rows in ABAB mode rather than ABBA mode (Figure 2b). It is worth noting that the brick wall sheets of 2 involve three different types of cuprous coordination hexameric units, respectively, [Cu(1)3Cu(2)3(CN)4(pyz)2], [Cu(3)6(CN)4(pyz)2], and [Cu(4)6(CN)4(pyz)2]. Adjacent brick wall sheets stack in an offset fashion to generate 1D channels ca. 4.1 × 4.1 Å2 (Figure S4 of the Supporting Information). Similar to 1, there are 1D [Cu(CN)]∞ chains, which are almost linear. The channels are penetrated by [Cu(CN)]∞ chains in an inclined way to form the 3D pseudopolyrotaxane (Figure 2c). Some CuCN

Figure 1. View of the coordination environments of copper atoms (a), the (3,4)-connected Shubnikov-type [Cu2(pyz)(CN)2] sheet showing an ABBA arrangement of brick rows (b), and the 3D pseudopolyrotaxane showing [Cu(CN)]∞ chains penetrating channels and unsupported CuI−CuI interactions in 1 (c).

attracted, or π−π stacked effects, indicating genuine unsupported CuI−CuI contacts.10 To date, the best evidence for cuprophilic interactions has been found in compounds [tBuCu(CN)Li·(OEt 2 ) 2 ], 1 0 b [{(Me 3 SiCH 2 ) 2 CuLi} 2 (Et2O)3],10c and ([CuCl2]−)2 dimer,10d in which two nearly linear [R−Cu−R] (R = Me, H, Cl) anions form almost perpendicular dimers with CuI−CuI separations of 2.71, 2.83, and 2.92 Å. Differently, cuprophilic interaction in 1 is formed between two- and three-coordinate copper centers, and a similar cuprophilic interaction but with the shortest distance 2.651(4) Å was found in [Cu 2 (4,4′-bpy)(CN) 2 ]·[Cu(SCN)].10a The overall structure of 1 is a 3D pseudocatenate-like network constructed by threading of 2D [Cu2(pyz)(CN)2] sheets with 1D [Cu(CN)]∞ chains via unsupported CuI−CuI interactions. Within the [Cu2(pyz)(CN)2] sheets, tetrahedral Cu(1) and trigonal Cu(2) atoms can be considered as 46070

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supramolecular isomers have already been reported: brick wall, herringbone, and bilayer.2 In terms of topology, it should be noted that both brick wall and herringbone are (6,3) topological nets because one can be converted to the other by distortions but they do not break the links. It is interesting to note that if one calculates the possible tilting patterns that are possible for T-shaped nodes, three (6,3) topological nets have already been realized. However, to date, the dislocation arrangement of the T-shaped node giving the (5,34) net has never been realized. In 1 and 2, the reconstruction of the frameworks can be completed by cleavage of the original junctions and connection of new junctions between two closest neighbor ribbons. When 3-connected cuprous centers in sheets of 2 are replaced by 2-, 3-, and 4-connected cuprous centers, an unprecedented Shubnikov-type (5,34) 2D sheet in 1 is formed. On the other hand, the stack fashion of adjacent sheets is different. Theoretically, the sheets in supramolecular isomer 1 stack in a parallel fashion to produce a larger channel than that of isomer 2 with an offset fashion stack. However, the existence of CuI−CuI interactions in isomer 1 contributes to the channels in 1 being smaller than those in 2. In order to check the bulk purity, XRPD of supramolecular isomers 1 and 2 was recorded in a Bruker D8 X-ray powdered diffractometer. As can be seen from Figure S6 of the Supporting Information, the recorded and simulated XRPD patterns in 1 is quite similar, confirming the bulk purity of supramolecular isomer 1. However, the intensities of some peaks show minor differences in 2, which is possibly due to the preferred orientation of crystals. The IR spectra show intense stretching absorptions of cyanides at 2124 cm−1 for 1 and at 2114 cm−1 for 2, respectively (Figure S5 of the Supporting Information). Cyanide Source and Temperature/Time Effect. To further understand the cyanide source and temperature/time effect involved in the syntheses of two supramolecular isomers, some comparative and repeated experiments have been performed. It should be mentioned that 1 and 2 are also available with various ratios of cyanide to copper, but the ratio of 1.0:1.0 in starting materials is the best experimental condition to get high yield and quality of single crystals. Besides, we found that reaction temperature and time are also crucial to the formation of 1 and 2. High quality single crystals of 1 and 2 can be synthesized at the temperature of 160−170 °C. A lower reaction temperature at 150 °C resulted in uncharacterized red powder, and a higher reaction temperature at 180 °C gave charry product. Moreover, we have done some further experiments by controlling reaction time. In the synthesis of supramolecular isomer 1 at 160−170 °C, the increase of the reaction time gave only minor difference in yield and quality of crystals. In the synthesis of supramolecular isomer 2, shorter reaction time (below 144 h) resulted in a mixture of the metastable isomer 1 and the stable isomer 2. Increase of the reaction time resulted in increase of the yield of stable 2 and decrease of the yield of the metastable 1. When the reaction time is larger than 192 h, only the stable isomer 2 was obtained (Table S2 of the Supporting Information). As can be seen from the above experiments, although the cyanide resource (K3[Fe(CN)6] or K4[Fe(CN)6]) plays a crucial role in the formation of isomers 1 and 2, the temperature and time effect could not be ignored. Compared with traditional cyanide sources of KCN and CuCN, environmentally friendly K3[Fe(CN)6] or K4[Fe(CN)6] can slowly release cyanide upon heat treatment, which is a key for the formation of diverse cuprous

Figure 2. Coordination environments of copper atoms (a), the (6,3) brick wall [Cu2(pyz)(CN)2] sheet showing an ABAB arrangement of brick rows (b), and a 3D pseudopolyrotaxane showing brick wall sheets threaded by [Cu(CN)]∞ chains in 2 (c).

compounds with similar [Cu6(CN)4(L)2] layers penetrated by [Cu(CN)]∞ chains have been documented,4,3b and these include (CuCN) 3 [(CuCN) 2 (4,4′-bpy)] 2 , (CuCN)[(CuCN) 2 (pyz)], 4 d (CuCN) 2 [(CuCN) 2 (4,4′-bpy)], 4 a (CuCN)7(MePip)2, (CuCN)4(Me2Pip), (CuCN)7(EtPip)2, and (CuCN)4(Et2Pip).3b It is interesting to note that the reported compound (CuCN)[(CuCN)2(pyz)] is also a supramolecular isomer of 1 and 2, which is constructed by [Cu(CN)]∞ chains and 2D (6,3) topological brick wall sheets. Supramolecular isomerism in coordination polymers has been widely encountered and studied.13,14 The orthorhombic and monoclinic crystal structures show significant supramolecular arrangements of (6,3) and (5,34) layers in 1 and 2. The (6,3) net is accessible by self-assembly of T-shaped nodes. In the context of 2D coordination polymers, three distinct 2D 6071

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coordination geometries. From a thermodynamic/kinetic point of view, K3[Fe(CN)6] and K4[Fe(CN)6] have the following two types of different hydrolysis process, which may contribute to the formation of supramolecular isomers. K3[Fe(CN)6 ] + 3H 2OFe(OH)3 + 3KCN + 3HCN

K4[Fe(CN)6 ] + H 2OK3[Fe(CN)5 H 2O] + KCN

In this context, the hydrolysis of K4[Fe(CN)6] is slower than that of K3[Fe(CN)6] because the activation energy of K4[Fe(CN)6] is larger than the energy of K3[Fe(CN)6].15 As for the synthesis of supramolecular isomer 1, thermal hydrolysis of K3[Fe(CN)6] would produce insoluble iron hydroxide, KCN, and hydrogen cyanide. This analysis is in line with our observed byproduct of brown powder iron hydroxide in the preparation of 1. With formation of insoluble iron hydroxide, K3[Fe(CN)6] can slowly release cyanide. Cyanide may act as reducing agent to reduce Cu(II) into Cu(I) and as bridging ligand to coordinate copper(I) ions with different geometries, which is a key factor in the formation of a (5,34) Shubnikovtype network. Isomer 1 could be synthesized as a single product at 160 °C with a 1.0:1.0:2.5 ratio of CuCl2/K3[Fe(CN)6]/pyz in water. Keeping the same molar ratio and replacing the cyanide resource by K4[Fe(CN)6], supramolecular isomer 2 as well as minor 1 were obtained at 170 °C. Kinetically, the dissociation of K4[Fe(CN)6] to release cyanide is slower due to higher activation energy. Crystallization under hydrothermal conditions possibly follows a nonequilibrium course, and thus, a metastable phase may be preferentially isolated. After considering several pathways, the most stable phase can be isolated. Compared with a (6,3) sheet in 2, a (5,34) Shubnikovtype network in 1 shows a lower symmetry. In addition, metastable isomer 1 can be converted into stable isomer 2 via further hydrothermal treatment. This is occasionally consistent with an entropic effect. From the entropic point of view, a reaction at a higher temperature may favor the formation of a structure with higher symmetry. Similar temperature induced isomerism has been previously observed by us in H5O2[Cu2(CN)3].3d Thermogravimetry and Photoluminescence. Thermogravimetric analyses for 1 and 2 in an air atmosphere and under 1 atm of pressure at the heating rate of 10 °C min−1 were performed on polycrystalline samples, which showed similar thermal stability of 1 and 2. This may be attributed to their similar layered structures. As shown in Figure S7 of the Supporting Information, TGA traces of 1 and 2 exhibit two main steps of weight loss. The initial weight losses of 23.5% for 1 and 24.0% for 2 occur in the temperature ranges of 170−280 °C and 180−280 °C, which correspond to the loss of pyrazine ligand (calc 22.96%). The second weight losses of 22.6% for 1 and 22.0% for 2 occur in the temperature ranges of 450−710 °C and 460−710 °C, which correspond to the removal of cyanides, and an intermediate of copper is formed by deduction from the empirical composition (calc weight loss 22.4%). The intermediate of copper is slowly oxidized in air upon further heating, and the stable residue is not formed up to 900 °C. The room temperature emission spectra of powdered 1 and 2 are shown in Figure 3. As can be seen, 1 shows a weak blue emission centered at 465 nm (τ = 2.49 μs) and a strong red emission band centered at 618 nm (τ = 5.05 μs) upon photoexcitation at 328 nm while 2 shows only the blue emission band centered at 450 nm (τ = 3.28 μs) upon photoexcitation at 322 nm. On the basis of a microsecond-

Figure 3. Photoluminescent emissions of 1 and 2 upon excitation at 328 and 322 nm.

order decay lifetime, the emissions in 1 and 2 suggest some character of phosphorescence. In general, possible assignments for the excited states of Cu(I)-complexes are ligand centered π → π* (LC), ligand-to-metal (LMCT), metal-cluster-centered (CC), metal-to-ligand (MLCT), or metal centered d10 → d9s1 (MC) transitions.16 Copper(I) cyanide itself exists in two polymorphs, and it shows a weak emission band centered at 392 nm, mainly attributed to a metal centered transition of the type 3d → (4p, 4s).3c Compared with CuCN itself, 1 and 2 have essentially unchanged excitation energy, but they emit at lower and broader energies. According to reported CuCN itself and CuCN−diimine complexes' photoluminescences,3a,b the emission bands at 465 nm for 1 and 450 nm for 2 are possibly assigned to a metal centered transition (MC) and copper-tocyanide charge-transfer (MLCT). The increase of 73 nm for 1 and 58 nm for 2 in the Stokes shift are possibly associated with the increased vibrational modes through incorporation of the Pyz linkers. In addition, it should be noted that there are unsupported CuI−CuI interactions in 1, and according to related literature, metallophilicity may affect the luminescence properties of complexes.15 A search of luminescent Cu(I) complexes with cuprophilicity reveals that they often show a distinct high energy emission peak ascribed to MLCT and a low energy emission peak ascribed to CC excitations, which coincides with 1. Thus, it is possible to assign the strong red emission band centered at 618 nm as CC excited states mixing with metal to pyrazine charge transfer. In conclusion, the very large difference in the emission spectra of these two isomers implies that the properties of the phosphorescence are mainly related to the diversified local coordination geometries of metals and different metallophilicity.

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CONCLUSION We have successfully synthesized two 3D pseudopolyrotaxane supramolecular isomers of [Cu2(pyz)(CN)2]·[CuCN] (1 and 2) under hydrothermal conditions by changing cyanide source and reaction temperature. Supramolecular isomer 1 is the first example containing pentagonal a Shubnikov-type sheet [Cu2(pyz)(CN)2], which consists of linear, trigonal, and tetrahedral Cu(I) atoms. Compared with a (6,3) brick wall net, the Shubnikov-type sheet is a metastable phase. As can be seen from structures, polyrotaxane penetrated by [CuCN]∞ chains as well as unsupported cuprophilic interactions are responsible for the stability of the Shubnikov-type sheets. 6072

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Crystal Growth & Design

Article

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Under the hydrothermal nonequilibrium crystallization conditions, the different releases of cyanide via hydrolysis of environmentally friendly K3[Fe(CN)6] and K4[Fe(CN)6] may play a crucial role in the formation of different supramolecular isomers. Interestingly, from a crystal engineering point of view, the Shubnikov-type sheets can be viewed as reconstructions of brick wall layers. The different photoluminescences of isomers 1 and 2 are associated with diverse local coordination geometries of metal and unsupported CuI−CuI interactions. The self-assembly of metal ions and organic ligands through coordination bonds and weak supramolecular interactions can be expected to produce more and more superstructures.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures, and IR, TG, XRPD, and crystallographic cif data for 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax and telephone: +86 357 2051402. E-mail: zhangxm@dns. sxnu.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program 2012CB821701), the Ministry of Education of China (No. IRT1156), the National Science Fund for Distinguished Young Scholars (20925101), and the Fund for Returned Oversea Scholars in Shanxi.

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REFERENCES

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dx.doi.org/10.1021/cg301192y | Cryst. Growth Des. 2012, 12, 6068−6073