Synthesis, Structure, and Properties of a Dinuclear Cu(II) Coordination Polymer Based on Quinoxaline and 3,3-Thiodipropionic Acid Ligands

The coordination polymer [Cu2(TDPH)4(QNX)].DMF, (QNX = Quinoxaline; TDPH = 3,3-thiodipropionic acid), has been prepared by reaction of copper acetate, TDPH, and quinoxaline. The compound was characterized by elemental analysis, FTIR spectroscopy, and single-crystal X-ray diffraction. The crystal is monoclinic with a P21/n space group and dimensions of a = 12.889(3) Å, b = 14.983(4) Å, c = 14.091(3) Å, α = 90°, β = 90.200(11)°, γ = 90°, V = 2721.18 (2) Å3, Z = 4. The ligands are hexagonally coordinated to the Cu(II) centre in the form of Cu2O4N with one nitrogen atom from the quinoxaline ligand, and four oxygen atoms from four TDPH molecules in a monodentate fashion. The Cu–Cu bond length was 2.642(1) and 2.629(1) Å for the Cu1–Cu1 and Cu2–Cu2 bonds. The QNX ligand bridged the two copper atoms. The catalytic reduction of 4-nitrophenol to 4-aminophenol using NaBH4 in the presence of [Cu2(TDPH)4(QNX)].DMF, as catalyst was completed within 11 min. The 4-aminophenol product was confirmed using 1H NMR spectroscopy.


Introduction
Coordination polymers have attracted a lot of attention over the decades because of their interesting properties, synthetic routes and rich chemistry. Synthesis through the solventfree and solvent-based techniques have been reported [1,2]. Research into the preparation of coordination polymers/ networks have led to the discovery of materials such as metal-organic frameworks which have advanced functionalities [2][3][4][5][6].
Porous coordination polymers have evolved as a class of versatile and attractive functional porous materials. They are made up of organic molecules connected by metal ions in a uniform porous network which incorporates sufficient stability and chemical resistance to most solvents [6,7]. Porous coordination polymers have over the years emerged as an important group of porous hybrid materials with diverse potential applications including catalysis [8], gas storage [9], luminescence and molecular sensing [10,11], including drug loading and delivery [12]. Coordination polymers are usually prepared under conditions which allow for tunable structure and incorporation of building blocks with desired functions thereby making them suitable for specific purposes [8]. These class of compounds are also characterized by large surface area and well-defined reaction environments which increases their potential applications in the area of heterogeneous catalysis [13]. Chiral ligands such as 1,1′-bi-2-naphthol, porphyrins, and their derivatives have been used as building blocks in the preparation of coordination polymers for specific usage as asymmetric catalysts [11][12][13]. The incorporation of molecular structures with catalytic attributes enables 1 3 the possibility of the spatial control to obtain coordination polymers with synergistic catalytic effect [7,14,15]. Their stability and chemical resistance in most solvents have made them materials of great interest in chemical separation and heterogeneous catalysis [15][16][17][18][19].
Coordination polymers provide an interesting platform for the preparation of luminescent solid-state materials due to their well-defined structural morphology [14,17]. Coordination polymers with luminescent properties have been isolated for some time with the first report appearing in 2002 [13,16]. Thereafter, various researches have been focused on the exploitation of their light emission properties to generate functional materials [18][19][20]. The possibility of incorporating permanent porosity into their structures is responsible for the luminescence features of coordination polymers. The immobilization of guest molecules, in the pores, close to luminescent centres may lead to a shift in wavelength, which affects their emission properties [18]. Coordination polymers developed with luminescent properties have been applied in sensing and detection of molecules. The observed luminescence could originate from the metal ion or the organic ligand with a conjugated system [19]. Lanthanide coordination polymers have been reported lately with fascinating photoluminescence properties. Polymetallic lanthanide coordination polymers reported by White et al. [19], were shown to simultaneously emit numerous near-infrared signals due to the different lanthanide ions present. Strong luminescent emission peaks observed in a tetranuclear Cu(II)-based coordination polymer were attributed to the presence of conjugated ligand systems in the structure [20]. Conjugation upon coordination with metal ions enhance greatly the luminescence properties of coordination polymers [19,22]. Those with structures constructed from carboxylate ligands have been reported to exhibit conjugation upon coordination with metal ions [21]. Several coordination polymers based on carboxylate ligands have therefore been reported with exciting catalytic and luminescence properties.
In this study, we utilized a combination of carboxylate and nitrogen donor linkers to prepare a dinuclear Cu(II) coordination polymer. To the best of our knowledge, there is no report of such dinuclear Cu(II) coordination polymer having a combination of quinoxaline and 3,3-thiodipropionic acid ligand system. The conjugated system of the quinoxaline ligand was exploited to incorporate additional luminescence property to the material along with the Cu(II) ion while the carboxylate linker was chosen to achieve a porous network structure. Thus, we herein report the synthesis, crystal structure, luminescence, and catalytic property of a coordination polymer assembled using Cu(II) ion, quinoxaline, and 3,3-thiodipropionic acid, and formulated as [Cu 2 (TDPH) 4 (QNX)].DMF.

Materials and Methods
Quinoxaline and 3,3-thiodipropionic acid were commercially sourced from Sigma Aldrich and were used as received. The hydrated Cu(II) salt (Cu(CH 3 COO) 2 ·H 2 O), dimethylformamide (DMF), and ethanol were purchased from British Drug House (BDH) Poole, England. The melting point of [Cu 2 (TDPH) 4 (QNX)].DMF was determined using a Gallen-Kamp melting point apparatus. This is to determine the stability of the compound under increased temperature. The compound [Cu 2 (TDPH) 4 (QNX)].DMF was observed to have a melting point > 400 °C. Elemental analysis to determine the percentage of C, H, and N was carried out with a Carlo Erba Model EA1108 elemental analyzer. Thermogravimetric analysis was carried out on a TGA Q500 V6.7 Build 203 thermogravimetric analyser in the presence of nitrogen. FTIR analysis of the compound was carried out using KBr pellets on FTIR 8400s Shimadzu spectrophotometer. Brunauer-Emmett-Teller surface area measurements of [Cu 2 (TDPH) 4 (QNX)].DMF was carried out by N 2 adsorption experiments on a NOVA 4200e BET instrument. The fluorescence spectra of the compound and the ligands were recorded on a Perkin Elmer LS45 Fluorescence spectrophotometer using DMSO as a solvent. The 1 H NMR spectra were obtained using a Bruker Avance 400 MHz NMR instrument with D 2 O as a solvent. Powder X-ray diffraction (PXRD) was carried out on a Siemens D5000 diffractometer.

Crystallographic Data Collection and Structural Analysis
The X-ray data were collected on graphite-monochromated MoKα radiation (λ = 0.71073 Å) Bruker Kappa Apex II 1 3 CCD diffractometer. The crystals were mounted using cryoloops with Paratone-N oil. By intrinsic phasing using SheLXS structure solution program, [Cu 2 (TDPH) 4 (QNX)]. DMF structure was solved using Olex2 and the ShelXL was used as the refinement package by Least Squares minimization. The crystal parameters, collection data and the refinements details are presented in Table 1.

Description of the Crystal Structure of [Cu 2 (TDPH) 4 (QNX)].DMF
ORTEP diagram of [Cu 2 (TDPH) 4 (QNX)].DMF is presented in Fig. 1 and the polymeric view showing the paddle-wheel structure is given in Fig. 2. The interatomic bond lengths and angles around the Cu(II) ion are given in Table 1 Fig. 2 showed the formation of a paddle-wheel structure by the Cu atoms with the quinoxaline and 3,3-thiodipropionic acid ligands.

PXRD Results
The PXRD pattern (experimental and simulated) of [Cu 2 (TDPH) 4 (QNX)].DMF presented in Fig. 3 indicates the purity of the [Cu 2 (TDPH) 4 (QNX)].DMF compound prepared. Orientation preferences is proposed as a reason for the differences observed in the peak intensities, where the simulated pattern represents diffraction from the [Cu 2 (TDPH) 4 (QNX)].DMF crystal planes while the experimental pattern may not include all crystal planes.

Thermal Analysis of [Cu 2 (TDPH) 4 (QNX)].DMF
Thermal analysis of compound [Cu 2 (TDPH) 4 (QNX)].DMF presented in Fig. 5 shows that the first mass loss occurred between the temperature of 145-160 °C which corresponds to the loss of a DMF molecule present outside the coordination sphere. Thereafter, a major loss in mass was observed at a temperature of about 358 °C up to 390 °C attributable to the decomposition of one molecule of TDPH (calc./found: 26.0/26.4%). The next decomposition stage was observed between 390 and 420 °C which resulted

Photoluminescence Properties
Coordination polymers having the hydroxide motifs have been reported to exhibit photoluminescence properties [29,33]. The photoluminescence spectra showing the fluorescence emission property of compound [Cu 2 (TDPH) 4 (QNX)].DMF is presented in Fig. 6. It was observed that the excitation wavelength of 388 nm resulted in emission peaks observed at 401 nm and 547 nm, while the free ligands (quinoxaline and TDPH) exhibit photoluminescence emissions at 545 and 409 nm respectively [29]. The emission bands of 401 and 547 nm observed for compound [Cu 2 (TDPH) 4 (QNX)].DMF may be derived from the different coordination environment of quinoxaline and TDPH and also ligand to metal charge transfer (LMCT) [29,34]. The free ligands displayed luminescence characters at 409 and 545 nm respectively. Fluorescent intensity of quinoxaline (λ emi = 545 nm) diminished upon coordination to the Cu(II) ion, this may be due to ligand centred electronic transition and may be a useful indicator for the use of [Cu 2 (TDPH) 4 (QNX)].DMF as sensors in applications in which Cu(II) ions are being evaluated [33,35].

SEM Results
The morphology of [Cu 2 (TDPH) 4 (QNX)].DMF observed by scanning electron microscope (SEM) depicts the compounds as uniform crystalline block-like aggregates of regular shaped rectangles Fig. 7a. The particle size distribution of [Cu 2 (TDPH) 4 (QNX)].DMF illustrated in Fig. 7b shows a somewhat uniform size distribution with a minimum particle size of 149 µm while the maximum was observed to be 159 µm. The uniform crystallinity and homogeneous nature of the material as seen in the SEM images positively influences the catalytic performance of the material.

Catalytic Activity Study
The UV-Vis spectra of the 4-NP at 317 nm was red-shifted to 400 nm (Fig. 8) upon introduction of NaBH 4 into the reaction medium, the colour of the solution changed to bright yellow from light yellow attributable to the presence of 4-nitrophenolate ion [34,35]. The reduction process started with the introduction of [Cu 2 (TDPH) 4 (QNX)].DMF into the reaction mixture and the progress of the reaction was monitored using UV-Vis spectrophotometer. Absorption band was observed at 300 nm upon addition of the catalyst, this is due to the conversion of 4-NP to 4-AP, which continued to increase in intensity while the band at 400 nm decreased in intensity with increase in reaction time [32][33][34]. The UV-Vis spectra of the reaction in which NaBH 4 is absent and present is presented in Fig. 8a and b, while Fig. 9 shows the UV-Vis spectra of the reaction upon addition of [Cu 2 (TDPH) 4 (QNX)].DMF into the reaction medium. The characteristic peak of 4-nitrophenol which was red-shifted to 400 nm disappeared completely after a reaction time of 11 min (Fig. 9) in the presence of [Cu 2 (TDPH) 4 (QNX)].DMF as a catalyst. The colour of the reaction mixture faded from bright yellow to colourless at the end of the reaction.

Reduction Kinetics
The reduction kinetics for the reaction is represented by Eq. (1) [34,36]: r = reactant reduction rate; t = reaction time; k app = kinetic rate constant; c t = concentration of 4-NP at time t; and c o = initial concentration of 4-NP. Equation (1) above can also be written as [36]: A t and A o represents absorbance at time t and 0 respectively. A plot of ln  Figure 10 presents the pseudo-firstorder kinetic plots for the reduction of 4-nitrophenol using NaBH 4 , in the presence of [Cu 2 (TDPH) 4 (QNX)].DMF as catalysts.
The value of the rate constant (k) for the reduction process was calculated from the slope of the plot of ln (A t /A o ) as 8.4 × 10 −3 s −1 . Reports have indicated that reduction of 4-nitrophenol by NaBH 4 occurs by a transfer of an electron from the BH − 4 ion, facilitated by the catalyst, to the 4-nitrophenol, overcoming the kinetic barrier of the reaction, in the process, which would normally not have commenced in the absence of a catalyst [34][35][36][37][38]. This is supported by the control experiment which was carried out in the absence of [Cu 2 (TDPH) 4 (QNX)].DMF. No observable change was observed in the reaction mixture of 4-nitrophenol and NaBH 4 in the absence of [Cu 2 (TDPH) 4 (QNX)].DMF as catalysts.

Thermodynamic Studies
The reduction reactions were carried out at four different temperatures of 30, 40, 50, and 60 °C. The results showed a decrease in the rate constant as temperature increases. An increase in rate constant was, however, observed at 60 °C. This is attributed to the increase in disorderliness at that  [23]. Figure 11 represents a plot of ln (A t /A o ) against time (s) at different temperatures while the corresponding rate constants are presented in Table 2.

Effect of Initial 4-NP Concentration
The dependence of the reaction rate on different concentrations of 4-nitrophenol was investigated and the rate constants were calculated at different concentrations of 4-NP. The results are presented in Fig. 12 and the parameters obtained from the plots in Table 3. The data in Table 3 indicates that the rate of the reduction process decreases as the concentration increases. This is due to the higher adsorption affinity for 4-NP by the catalyst compared to the BH − 4 ions giving rise to fewer adsorption sites for the BH − 4 [34,35].

Catalytic Activity, Conversion Efficiency, and Turnover Frequency
The activity of the catalyst was estimated by taking the ratio of the rate constants to the weight of the catalyst used [38]. Conversion efficiency for the process was obtained using Eq. 3 while the turn over frequency (TOF) was calculated using Eq. 4 [38][39][40][41][42] where A t = absorbance at time t, A o = initial absorbance, m i = initial no of moles of 4-NP, X = percentage conversion, x = molecular weight of 4-NP, w = mass of catalyst (g) and t = reaction time (h).
The catalytic activity was estimated as 2.1 g −1 s −1 while the conversion efficiency was 99.4%, these values are comparable with the results obtained for other catalysts employed for the same reaction [41,42]. The observed turnover frequency for the catalyst is 188.6 molecules g −1 s −1 .

Reusability Experiment
Reusability of [Cu 2 (TDPH) 4 (QNX)].DMF as a catalyst for the reduction process was investigated by regenerating the spent catalyst. After each catalytic experiment, the catalyst was recovered from the solution by filtration, washed several times with distilled water, dried in the oven at 80 °C for 24 h and then reused. The results in Fig. 13 indicate that a substantial amount of [Cu 2 (TDPH) 4 (QNX)].DMF can be recovered from the spent catalyst, an indication that it is reusable. The recovered catalyst is reusable over at least three cycles except that an increase in time required for the reduction reaction to complete was observed after each catalytic cycle.   ].DMF as a catalyst for the reduction of 4-nitrophenol to 4-aminophenol was compared with previously reported catalysts for the same reduction process using NaBH 4 as the reducing agent. As observed from Table 4, compound [Cu 2 (TDPH) 4 (QNX)].DMF compared favourably well with the other catalysts. The rate constant obtained for the reduction process in the present study is higher (8.4 × 10 -3 s −1 ) when compared with the most recently reported catalysts [43,44]. This result, therefore, indicates that compound [Cu 2 (TDPH) 4 (QNX)].DMF is a better alternative to other effective catalysts reported for this reaction. The 1 H NMR spectra of the 4-aminophenol product formed presents signal shifts: 1 H NMR (δ ppm in D2O) (δ, ppm) 1.8319 and 3.3575 ascribable to the amino-group chemical shifts [45]. These were observed to be absent in the 1 H NMR spectra of the 4-nitrophenol (Fig. S2) (Table 5).

Conclusions
This paper presents a new dinuclear Cu(II) coordination polymer, [Cu 2 (TDPH) 4 (QNX)].DMF. The structure of the compound was confirmed by single-crystal X-ray diffraction analysis. The two Cu(II) ions are present in the structure with each in the octahedral coordination. The compound was investigated as a catalyst for the conversion of 4-nitrophenol to the less toxic 4-aminophenol using NaBH 4 as the reducing