Thermal and structural characterization of copper(II) complexes with phenyl-2-pyridylketoxime (HPPK)

Copper(II) complexes of different nuclearity with phenyl-2-pyridylketoxime (HPPK), such as mononuclear [Cu(HPPK)(PPK)X], whereby X = NCS−(1) or NO3−(2), dinuclear [Cu(HPPK)Cl2]2(3) and trinuclear [Cu3(PPK)3(μ3–OH)(Cl)2]·nH2O (4) were synthesized, and their thermal properties were investigated. The studies involved gas-phase monitoring of decomposition products by infrared spectroscopy. Thermal decomposition performed in air revealed the multi-stage character of the process, the course of which depends on the compound composition. In all cases, CuO was found as the final product of the process. Furthermore, the crystal structure of 1 was revealed. It shows a square-pyramidal geometry around Cu(II), with the NCS− ion situated in the apical position and two HPPK ligands present in the base, one of which is deprotonated.


Introduction
The chemistry of metal complexes with oxime type of ligands has been investigated from the 1960s of the twentieth century due to their applications as analytical reagents [1][2][3][4]. Currently, a renewed interest in this class of compounds can be observed, as they are being used for the design of homo-/heterometallic clusters [5], as well as coordination polymers with attention-grabbing properties [6]. In general, ketoxime-based metal complexes show potential for the development of new oxygen activation catalysts and interesting magnetic properties [7][8][9][10]. More in particular, copper complexes of phenyl-2-pyridylketoxime are known for the formation of trinuclear clusters, which are the inorganic structural and functional analogs of crown ethers, and display interesting magnetic behaviour [11,12]. Some selective Ba 2? and Ca 2? receptors, based on site-selective transmetallation of polynuclear zinc(II)/ polyoxime complexes, have been discovered [13]. Furthermore, copper oximes can be utilized as suitable model compounds of the biologically active sites that can interact either covalently or non-covalently with DNA [14].
Despite the broad literature on the behaviour of copper complexes with phenyl-2-pyridylketoxime in solution, the number of reports concerning the solid state is rather limited and mostly treating about the trinuclear compounds mentioned above. Recently, we presented for the first time some structures of mononuclear Cu(II) complexes with HPPK [15]. It is worth noting that in each case one of the HPPKs, constituting these mononuclear complexes, was found deprotonated, whereas in the case of mononuclear Ni(II), Mn(II) and Cd(II) complexes, the HPPK ligands are neutral. In all mononuclear complexes, HPPK shows an N,N 0 -chelating coordination mode involving both the nitrogen atom of the oxime group and the nitrogen atom of & R. Szczęsny roszcz@umk.pl 1 Faculty of Chemistry, Nicolaus Copernicus University in Toruń, 87-100 Toruń, Poland the pyridyl group. However, depending on the pH and deprotonation level [1,16], coordination via the oxygen atom is also feasible, leading to the formation of higher nuclearity compounds, whereby the composition of the final product depends on many other factors, such as metal ion, counterions and molar ratio [17][18][19]. Studies focusing on the thermal behaviour of HPPK metal complexes are also rare to come by. There are some reports treating about the thermolysis of other types of transition metal complexes with oximes [20][21][22], but once again, excluding our recent results [15], there is no literature on the thermal behaviour of Cu(II) complexes with phenyl 2-pyridylketoxime. In continuation of our studies on this family of compounds, aimed towards the selection of precursors for the low-cost fabrication of semiconducting CuO thin films, we would like to report our preliminary results on the decomposition process in air for a range of Cu(II) complexes, monitored by IR spectroscopy.

Experimental Measurements
Thermal studies (TG, DTG, DTA) were performed on a SDT 2960 TA analyser. Decomposition processes were studied in dynamic atmosphere of dry air flowing at 40 mL min -1 with a heating rate of 10 and 5°C min -1 and a heating range up to 1000°C (sample mass varied between 2 and 5 mg). Gaseous products of thermal decomposition were detected by a FT-IR Bio-Rad Excalibur spectrophotometer equipped with a thermal connector for gases evolved from a TA SDT 2960 analyser. IR spectra were registered with a PerkinElmer 2000 FT-IR spectrometer in the range of 4000-400 cm -1 for KBr discs. Powder X-ray diffraction (PXRD) analysis was performed using a Philips (Almelo, The Netherlands) XPERT h-2h diffractometer with CuKa radiation. Elemental analyses were performed on a Vario MACRO CHN apparatus (Elementar Analysensysteme GmbH).

Single-crystal X-ray crystallography
The single-crystal X-ray diffraction data for 1 were collected on an Oxford Sapphire CCD diffractometer, MoKa radiation (k = 0.71073 Å ). The CrysAlis package of programs was used for the data collection (CrysAlis CCD) [23], cell refinement and data reduction (CrysAlisPro Red) [24]. Empirical absorption correction was applied. The structure was solved by direct method using SHELXS-97 and refined by full-matrix least square method based on F2 using SHELXL-97 [25]. The program Mercury was used to prepare the molecular graphics image [26]. All non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were placed in calculated positions with displacement factors fixed at 1.2 times Ueq of the parent C, O atoms with C-H = 0.95 (aromatic) and O-H = 0.82 Å . One of the phenyl rings (C25-C30) was found disordered over two positions with refined site occupancies of 0.59(2):0.41 (2). Consequently, a few geometrical restraints were placed on the bond lengths of this ring. Details of the diffraction experiment and structure refinement are summarized in Table 1. CCDC-975557 (for 1) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.
Attempts were undertaken to determine the crystal structure of 2; however, the poor quality of the crystals did not allow for full characterization. It was shown that it crystallizes in the Fdd2 space group of an orthorhombic system with unit cell parameters: a = 34.7 Å , b = 29.2 Å , c = 8.9 Å . The crystal structures of the two other compounds (3 and 4) have previously been described [27,28].

), [Cu(HPPK)(PPK)(NO 3 )] (2) and [Cu(HPPK)Cl 2 ] 2 (3)
Compounds 1-3 were synthesized as presented earlier [1,29]. It was found that the compounds take up water over time, which might explain some discrepancies in the elemental analyses (the expected results are given, taking into account the presence of 0.5 water molecule for each compound as well). Anal. Calc. for C 25  [ Compound 4 was obtained according to an alternative method to the one presented in [28], using complex (3) for its preparation. [Cu(HPPK)Cl 2 ] 2 (3.4 mmol) was dissolved in 50 mL of methanol, and an excess of (CH 3 ) 2 N-CH 2 CH 2 -N(CH 3 ) 2 (TMEDA) was added, under continuous stirring. The mixture was filtered and dried in air stream. Then, acetone was added, resulting in a green solution with a white precipitate of TMEDA-HCl salt. After filtration, the solution was evaporated to dryness and the dark green residue was dried in vacuo.

Results and discussion
All complexes are stable in air and dissolve much better in methanol or ethanol than in water. In comparison with the free ketoxime ligand (1591 and 948 cm -1 ) [30][31][32], the IR absorption bands such as those for m(C=N) pyridyl and m(N-O) groups are slightly shifted towards lower frequencies, for all compounds. The intensity of the acyclic m(C=N) band from the oxime group, which appears at 1628 cm -1 for HPPK, is decreased or covered by other bands after coordination for 1, 2, 4, whereas in 3 this band is visible and is shifted to 1656 cm -1 .
Crystal structure of complex 1 The mononuclear, neutral Cu(II) complex 1 crystallizes in the monoclinic P2 1 /c space group with one independent molecule in the asymmetric unit-see Fig. 1. The Cu(II) ion shows a distorted square-based pyramidal coordination environment, formed by five N atoms (for geometrical parameters, see Table 2). The pyridine and imine N atoms originating from two N,N 0 -chelating ketoxime and ketoximato ligands, respectively, are situated in the base plane, whereas the N atom coming from the NCSanion is located in the apical position. The geometric parameter s, introduced by Addison et al. [33] as an index of the degree of trigonality for five coordinated systems, indicates quite a high distortion of the geometry around the metal centre, with s = 0.44 (whereby s = 0 for a perfect square-based pyramidal geometry). The value of the bond length Cu1-N31 Up till now, our previous study is the only one dealing with crystal structures of mononuclear Cu(II) complexes with phenyl-2-pyridylketoxime/ato ligands (see Table 3 for the unit cells parameters). Even the molecular structures are pretty much the same (distorted square-pyramidal geometry around the Cu(II) ions, coordination number 5), though the packing of all the compounds differs significantly. It is worth mentioning that the Cu(II) ions in complexes 3 and 4 show a similar type of geometry around the metal ion.

Thermal studies
Thermal analyses of 1-4 were carried out to give information about the decomposition processes of the Cu(II) complexes, depending on the counterion involved, nuclearity of the compound and coordination mode of the ketoxime ligand. The summarized results are listed in Table 4.
The decomposition of 1 proceeds in two main stages. The process starts at 150°C and is completed at 663°C (Fig. 2a). The first multi-step consists of at least four processes and is connected with a weight loss (WL) of ca. 53%. The second one with a mass loss of ca. 27% consists of two exothermic processes on DTA, namely a weak one at 463°C and a strong one at 523°C. The plot of the intensities of volatile species arising during the thermolysis versus time (see ESI, Fig. S1a) (1) shows peaks at about 17 and 53 min, corresponding to temperatures of 190 and 550°C, respectively, which correlates to the most intensive peaks on the DTG curve. At lower temperature, mainly the evolution of CO 2 and H 2 O was observed [34]. At this temperature, there are also very weak peaks present, with maxima at 2070/2046 cm -1 , originating most likely from the oxime groups [34][35][36][37], and at 2239/2204, which could be attributed to the pyridyl ring's decomposition with N 2 O release [38]. Spectra registered at higher temperature indicate the release of CO 2 and H 2 O. Moreover, a peak occurring in the range of 1300-1400 cm -1 could be assigned to SO 2 release, originating from detachment of the NCSion (ESI, Fig. S1c) [39]. Powder diffraction analysis of the residue indicates that CuO is the final product.
In the case of 2, the thermolysis consists of at least eight overlapping processes (Fig. 2b). The complex is stable up to 94°C, and no further changes were observed above 392°C. The result of the TG-IR measurement performed for 2 is presented in Fig. 3. Two distinct peaks appear in the plot of the intensity profiles of the evolved gases ( Fig. 3a). At the first stage, the intensity of the peaks is rather low, but the strongest one in the range of 2240-2200 cm -1 and the weak one at 1200-1300 cm -1 (Fig. 3b) could be evidence of NO 3 detachment and the formation of nitrogen oxides [39]. The release of NO 2 at this temperature was also apparent from its characteristic smell, noticed upon heating of the compound on the hot stage. The presence of water and carbon dioxide indicates the simultaneous decomposition of the organic ligand.  Carbon dioxide and traces of water are the only products registered at higher temperature spectra, whereby a second increase in the intensity of the evolved gases was observed (Fig. 3c). Thermal decomposition of complex [Cu(HPPK)Cl 2 ] 3 is also a multi-step process (Fig. 4a). This compound is stable up to 180°C. Between this temperature and 400°C, the mass loss is about 39%. The processes occurring at higher temperatures (until ca. 770°C) are associated with a mass loss of about 48%. The total mass loss of the sample, amounting to 87%, is significantly higher (11%) than calculated for the formation of the CuO residue. This phenomenon can be caused by the intensive evaporation of formed CuCl 2 or CuCl, for which the melting points are 498 and 426°C, respectively [40]. This would also mean that chloride is leaving the compound only at higher temperatures. The measurement was repeated with a heating rate of 5°C min -1 , resulting in a similar mass change (WL = 87.9%). During the TG-IR measurement performed for 3, the most intensive set of bands was detected at a temperature corresponding to the maximum on the DTG curve above 700°C, indicating the presence of CO 2 and H 2 O. T init initial temperature, T max maximum temperature, T f final temperature; * calculated for the unhydrated complex Thermal and structural characterization of copper(II) complexes with phenyl-2-pyridylketoxime… Complex 4 is thermally stable up to 120°C, and its decomposition process is connected with three main exothermic effects on the DTA curve, with maxima at 172, 230 and 352°C (Fig. 4b). In the DTG curve, two main stages are visible. The first one is completed at about 270°C and connected with a mass loss of ca. 54%. The second very strong effect, with peak maximum at 346°C (DTG), is connected with a mass loss of about 32%. The final product is formed at around 400°C. Similarly to the previous chloride complex, the total mass loss is higher than calculated for the remaining residue of copper oxide (more than 10%). As the decomposition of the compound is finished at much lower temperature than in the case of 3, the phenomenon could be caused here by the formation and volatilization of double salts of copper chloride that melt below 400°C [41]. The TG-IR results indicate three maxima in the plot of the intensities of the gas evolution that correspond with temperatures 220, 250 and 360°C (Fig. 5a). At the first stage of thermolysis, the bands from carbon dioxide show the highest intensity and are followed by medium-and low-intensity peaks situated at 1224, 1180, 1133, 1033 cm -1 and 1391, 1338 cm -1 , respectively, attributed to HPPK decomposition. At 250°C, new sharp peaks at 1793 (mC=O) and 1304 cm -1 appear in the spectrum, being the volatile products (Fig. 5b), whereas above 360°C the primary gaseous product is carbon dioxide (Fig. 5c). Thermal decomposition of the studied complexes in the presence of oxygen takes place for all complexes in two principal steps, leading to the formation of Cu(II) oxide (Fig. S2), which was expected under these conditions, whereas in the case of N 2 atmosphere, the final products can differ and residues such as Cu, CuO or Cu 2 O have been observed [42][43][44][45]. The thermal stability, as well as the temperature of reaction completion, differs depending on the compound composition [46][47][48]. For mononuclear complex 1 with NO 3 counterion, and its previously reported analogous compounds with fluorinated carboxylates, such as CF 3 COOand C 3 F 7 COO -, the decomposition process shows a rather similar pattern. The first and second steps of decomposition occur rather close to each other, and the process is finished relatively quickly, namely Thermal and structural characterization of copper(II) complexes with phenyl-2-pyridylketoxime… below 500°C. It takes a different course in the case of the analogous compounds with Cl - [15] and NCS -, whereby the two decomposition steps of are more separated and the process is completed only above 650°C. It can be assumed that this is due to the presence of the counterions, which in the case of the mononuclear complexes with NO 3 and fluorinated ions are already being released at the first stage of the decomposition process, whereas in the case of NCSand Clions they are being released during the second stage of the process. Comparing the thermal stability of dinuclear compound 3 and trinuclear copper(II) 9-metallacrown-3 complex 4, it is clear that the first one is much more stable. The presence of double chloride bridges in 3 can surely be one of the reasons, but also the packing arrangement might have some impact. Whereas the molecules are efficiently packed in 3, complex 4 is characterized by supramolecular layers, kept together by C-H … p, C-H … Cl and p-p interactions in the bc plane, with voids in between, that were initially taken up by solvent molecules (see Fig. S3).

Conclusions
Thermal analyses of a range of copper(II) phenyl-2pyridylketoxime complexes in dry air atmosphere indicate the influence of counterions and compound composition on the course of the decomposition process. The mononuclear Cu(II) complex 1 (with NCS -) behaves similarly to a previously reported analogous compound with Cl -, whereas complex 2 (with NO 3 -) is comparable to analogous complexes with CF 3 COOand C 3 F 7 COOcounterions. The difference is connected with the course of the decomposition, which for the complexes with NCSand Clis related to decomposition of the counterions in the second step of the process, whereas NO 3 and the fluoro-carboxylates are being released in the first step of the process. This is also the reason why the process is finalized faster in the latter case and explains the lack of clear separation between the two decomposition steps, which is apparent for the former. The dinuclear compound 3, consisting of double chloride bridges, decomposes fully only at 765°C, meaning at a temperature almost 100°C higher than observed for the mononuclear compound with monodentate Cl -, whereas the decomposition of the trinuclear metallacrown 4 is completed at around 400°C, similarly to the mononuclear compound 2. The most thermally stable is dinuclear complex 3, consisting of Cl bridges (ca. 180°C), then mononuclear compound 1 with NCScounterions (ca. 160°C), followed by trinuclear complex 4 (ca. 150°C) and mononuclear compound 2 with NO 3 counterion (ca. 95°C). Heat treatment under the selected conditions leads to the formation of CuO. Further investigations into this family of compound and their potential applications are ongoing.