Effect of structural variability and exibility of aliphatic diamines on self-assembly of host-guest coordination polymers constructed of copper cyanide networks: Structure study and catalytic activity

A new 3D-host-guest supramolecular coordination polymer (SCP); ∞ 3 [(Cu 3 (CN) 3 ) 2 .(DAHP)], 1 [1,7-diaminoheptane=.(DAHP)] had been synthesized by self-assembly at ambient conditions. X-ray single crystal diffraction of SCP 1 indicated the formation of two-fold [Cu 3 (CN) 3 ] 2 units containing tetrahedral copper(I) atoms which are arranged in unique way to create 3D-network. The neutral [Cu 3 (CN) 3 ] 2 building blocks create unique complex structure containing the minicycle [Cu 2 (μ 3 -CN) 2 ] motif with wide cavities enable to capsulate the long chain DAHP as guest molecule. The topology of 1 had been studied by elemental analysis, IR-spectra and thermogravimetric analyses. The topology of 1 had been compared with the prototype SCP containing different aliphatic diamines which indicated the effect of structural variability and exibility of aliphatic diamines on the network structure of these SCP. The catalytic and photo-catalytic activity of 1 was studied for mineralization of methylene blue (MB) utilizing H 2 O 2 as an oxidant.


Introduction
The most interesting research areas in coordination chemistry is the design of multi-dimensional supramolecular coordination polymers (SCPs) which nd wide applications in many different areas such as of catalysis, photocatalysis, sensors, luminescence and medicinal chemistry [1][2][3][4][5][6][7][8]. Also, one of the most interesting family of coordination polymers is cyano-bridged SCPs owing to their intriguing structural topologies and interesting properties [9][10][11][12][13][14][15][16]. The attractive phenomenon of the [CuCN] n building blocks from the viewpoint of structural engineering comes from their versatility in accommodating various ligand-imposed geometric requirements [9, 11,14,17,18]. The bridging of CN in the quadro-[M 2 (m 3 -CN) 2 ] motif represents an exceptional case which looks almost like "carbon monoxide". The bifurcated [M 2 (m 3 -CN) 2 ] fragment represented a basic building block of an increasing number of SCPs [16][17][18][19][20][21][22][23][24][25]. Organo-diimines had been widely utilized to synthesize different SCPs using copper(I) cyanide [14,21,25], while aliphatic diamines had only been used with silver thiocyanate, silver(I) cyanide and an example with silver azide [26][27][28]. Also, the [Cu x (CN) x+n ] nanion creates fantastic topologies starting from ethylenediamine (en) to 1,6-diaminohexane (DAHX) [11,29,30] as far as we known. The present work represents continuation to our previous work on design of different architectures based on aliphatic diamines and copper cyanide in presence of Me 3 SnCl. Here, long chain aliphatic diamine; 1,7heptanediamine (DAHP) was used as a template to construct new 3D-SCP based copper cyanide in presence of Me 3 SnCl. The effect of structural variability and exibility of aliphatic diamines on selfassembly of 3D-host-guest networks had been discussed. The catalytic and photo-catalytic activities of the SCP 1 towards methylene blue (MB) dye as a pollutant as well as mechanism of oxidation are also reported.

Materials and Physical Measurements
Solvents and chemicals were of analytical grade. The complex K 3 [Cu(CN) 4 ] was synthesized according to the literature procedures [30]. Spectrometers and equipment used in this study are the same as reported . SHELX-97 and SIR92 programs had been utilized for structure solution, re nement by full-matrix least-squares techniques on F 2 and data out-put [31]. Mercury 3.8 software was applied for visualizations of the structure.

Measurements of catalytic activity of SCP1
Experiments for MB dye mineralization via the catalysts SCP1 were carried out by using 25 mL reactors which contain a suitable volume of MB dye solution with a concentration of (7.50 × 10 −5 M) with certain amount of SCP1 catalyst (25 mg) which were put in a water shaker thermostat to obtain constant temperature at 28±1 °C in dark for 1 h. After that to each reactor a solution of H 2 O 2 (0.1 M) was added and the time had been recorded. De nite amount of each reactor was taken at interval times then after the absorbance had been recorded at 610 nm. The photo-catalytic activity was measured using UV-light radiation emitted from a 300W medium pressure mercury lamp at (λ=365 nm). The e ciency catalytic degradation (D%) was calculated using equation (1) The colorless needles of SCP 1 had been synthesized by the reaction of the components K 3 [Cu(CN) 4 ], Me 3 SnCl and NH 2 (CH 2 ) 7 NH 2 (DAHP) applying the self-assembly method at ambient conditions. Crystal structure analysis data indicated the molecular formula C 13 H 18 N 8 Cu 6 and the structure is tin free. The crystal lattice of 1 exhibits the monoclinic system with a space group C2/c, Table 1. Asymmetric unit of 1 shows the simple neutral [Cu 3 (CN) 3 Fig. 1c. This unique ring system seems to be essential for constructing such wide voids of box like structure to accommodate the guest long chain DAHP. DAHP adopts bent structure at C14 where the angle C13-C14-C13 equals 117.24 o . SCP 1 can be considered the second example containing two perpendicular mini-cycles with one (Cu2) atom connecting two (Cu1) atoms producing cluster like structure [11]. The Cu1-Cu2 distance equals to 2.51(5) Å which represents a remarkable feature of the [Cu 2 (μ 3 -CN) 2 ] motifs. Thus, SCP 1 can be considered as a new compound which belongs to a growing up family of prototype SCP displaying the bifurcated [Cu 2 (μ 3 -CN) 2 ] motif [10, 14,16,18,19,23,32,33]. In the bifurcated minicycle rings, the distances between the Cu atoms (Cu1 and Cu2) and the CN groups vary from 1.93(5) Å to 2.43(4) Å and angles vary from 96.6(13) o to 124.0(2) o suggesting that the Cu-CN bonds are secondary with a much wider or shorter range of distances and angles, Table 2. The observed deviations of the distances of Cu1-C8 = 2.43(4) Å and Cu2-C7 = 2.21(4) Å than the distances of the usual Cu-CN bonds, Cu1-C7 = 1.97 (5) Å and Cu2-C8 = 1.96(5) Å, and the narrow angles of N-Cu1-C=101.0(2) o -104.79(2) o and C7-CU2-C7=96.6(12) o , Table 2, suppose pronounced copper-carbon contact which was postulated via production of two three-center Cu-C-Cu bonds like the diborane bonding [16]. Thus, [Cu 3 (CN) 3 ] 2 units are arranged in a pretty way to form fused ring system of hexagonal-like structures in addition to the minicycle motifs creating 1D-chains, Fig. 2. This fused ring system seems essential for constructing such wide voids of box like structure to accommodate the long chain DAHP (8.23 Å). The 3D-structure of SCP1 contains two facing DAHP molecules in each box like structure, Fig 3. DAHP molecules form H-bonds with the cyanide groups (2.36-2.79 Å), Table 3. The H-bonds cause further stabilization for the host-guest structure of SCP 1.
Alternatively, the 3D-polymeric network of 1 consists of two [Cu 3

Ir And 1h-nmr Spectra
The ν CN band represents a ngerprint absorption band in IR-spectrum of SCP1. It is well known that the cyanide frequencies close to those of genuine salt are attributed to Cu-CN-Cu linear bridging while bands higher than those of the salt exhibit terminal or nonlinearly bridging CN groups [34][35][36]. The IR-spectrum of SCP1 shows two strong bands at 2084 and 2108 cm -1 due to ν CN which support nonlinear bridging [(Cu 3 (CN) 3 ) 2 ] system compared to the band at 2076 cm -1 of the free ion of [Cu(CN) 4 ] 3- [34]. The presence of two ν CN bands in the IR-spectrum of SCP1 can be attributed to the presence of two types of nonlinear cyanide ligands in the structure of SCP1 as was supported by X-ray crystallography. The ν NH , δ NH and γ NH bands of amino groups appear at 3212 and 3229 cm -1 ,1582-1585 cm -1 and 697-719 cm -1 , respectively which re ect their participation in H-bonds as they locate at lower wavenumbers than those of free amines as indicated previously by X-ray crystallography. The CH 2 groups of DAHP absorb at 2933 and 2855 cm -1 corresponding to νCH and at 1474 cm -1 due to δCH while γCH bands appear at 774-870 cm -1 . The νCu-C bond locates as weak band at 483 cm -1 .

Catalytic Studies
Considerable amounts of dyes in wastewater causes harmful to the environment system owing to their resistance to be biodegradable due to their chemical stability [37,38]. So, elimination of such hazardous materials is an important environmentally target. Here, methylene blue (MB) as one of the wide used dye was used as a selective pollutant for mineralization experiments to estimate the catalytic e ciency of 1 utilizing the eco-friendly H 2 O 2 . MB is usually utilized for dyeing wood, cotton and silk which suffers hard decomposition under light irradiation [39]. To address the catalytic activity of SCP1 for degradation of MB, the absorption spectra of MB at 664 nm were measured at regular time intervals.  Fig.5. Irradiation of the catalytic solution by UV-light enhances the reaction rate giving 95% degradation e ciency within 40 min, Fig.6. It is worth mentioning that the absorbance bands at 250, 296 and 664 nm of MB dye disappear supporting mineralization of MB dye via destruction of the phenyl ring and the hetero-poly-aromatic system. The observed rate constant, K obs ., was calculated from the slope of the rst-order plot which veri es: A o and At are the absorbance at time t= 0 and time= t, respectively. The plot exhibits pseudo rst-order rate with respect to MB dye concentration, K obs = 0.0189 min -1 (R = 0.994) for normal degradation and K obs = 0.0252 min -1 (R = 0.980) for degradation under UV-light radiation. On the other hand, the catalyst 1 reserved the catalytic activity for at least ve cycles, Fig. 7. The catalyst SCP1 was washed several times with double distilled water and was dried after each catalytic cycle. After that the catalyst was utilized in starting a new experiment. The degradation e ciencies of the rst three cycles are the same while after that they suffer very slight decrease indicating that the SCP1 catalyst maintained its e cient catalytic activity, Fig. 7. The IR spectra of the catalyst 1 had been carried out after each cycle which displays the same bands of the fresh prepared SCP indicating that the catalyst was not oxidized by H 2 O 2 , the case which was further supported by elemental analyses indicating that the structure of the catalyst 1 retains its identity.

Hydroxyl radical determination
Applying the disodium salt of terephthalic acid (NaTA) technique the • OH radicals can be detected since NaTA reacts selectively with • OH radicals giving a bright stable uorescence [40]. This reaction represents selective probe for detecting •

General Discussion
One of the most important characteristics of supramolecular coordination polymers (SCP), of course, is the structural variability which can be affected by many factors as the combination of the type and number of ligands and metal ions, coordination geometry and the presence of ionic charge. Also, structural exibility is usually accompanied with structural variability when approaching supramolecular coordination polymer solids. Self-assembly of R 3   motifs which construct wide cavities enough for the longer H 2 DAP guest and the water molecules to locate as guests into the network structure of 7. The guest H 2 O molecules are present in the structures of 4, 6 and 7 to ll the space since nature tends to avoid empty space [29]. As the chain of the diamine extends to 6 (DAHX) and 7 (DAHP) CH 2 fragments, the structures consist of 3D-(Cu 3