New Coordination Compounds of Cu II with Schi ﬀ Base Ligands—Crystal Structure, Thermal, and Spectral Investigations

: The new mono-, di- and tetranuclear coordination compounds [Cu(HL 1 )] · H 2 O (1), [Cu 2 (L 1 )(OAc)(MeOH)] · 2H 2 O · MeOH (2), [Cu 4 (L 2 ) 2 (OAc) 2 ] · 4MeOH (3), and [Cu 4 (L 2 ) 2 (OAc) 2 ] · 4H 2 O · 4MeOH (4) were synthesized by the direct reaction of 2,2 (cid:48) -{(2-hydroxypropane-1,3-diyl)bis [nitrilomethylidene]}bis(4-bromo-6-methoxyphenol) (H 3 L 1 ) or 2,2 (cid:48) -{(2-hydroxypropane-1,3-diyl)bis (nitriloeth-1-yl-1-ylidene)}diphenol (H 3 L 2 ) and the Cu(II) salt. They were characterized by elemental analysis, X-ray fluorescence (XRF), Fourier transform infrared (FTIR) spectroscopy, simultaneous thermal analysis and differential scanning calorimetry (TG / DSC), and thermal analysis coupled with Fourier transform infrared spectroscopy (TG-FTIR) techniques and the single crystal X-ray diffraction study. In the dinuclear complex 2, the copper(II) ions are bridged by an alkoxo- and a carboxylato bridges. The tetranuclearcomplexes3and4areformedfromdinuclearspecieslinkagethroughthephenoxooxygen atoms of the fully deprotonated H 3 L 2 . Compounds 1–4 are stable at room temperature. During heating in air, at first, the solvent molecules (water and / or methanol) are lost and after that, the organic part undergoes defragmentation and combustion. The final decomposition solid product is CuO. The main gaseous products resulting from the thermal degradation of 1–4 in a nitrogen atmosphere were: H 2 O, MeOH, CH 3 COOH, CH 4 , C 6 H 5 OH, CO 2 , CO, and NH 3 .

We are interested in the synthesis of Schiff bases based on aromatic aldehydes or ketones with flexible diamines (e.g., 1,3-diaminopropane) and the study of their coordination ability toward transition metal ions. Most Schiff bases, formed in reaction of 1,3-diaminopropane with o-hydroxy aromatic aldehydes/ketones (Scheme 1a), coordinate a single metal(II) ion in the N 2 O 2 tetradentate cavity (Scheme 1b). The coordination environment of the metal ion consists of two oxygen and two nitrogen atoms of one Schiff base and occasionally, additional donor atoms derived from other molecules In this paper, we present Cu II complexes with similar types of ligands as depicted in Scheme 2a. The Schiff bases contain an additional hydroxyl group, which enhances their ability to coordinate more than one metal ion (see Scheme 2b-h and Table S1). According to the information found in the CSD database, two types of copper(II) ions coordination are most common (i.e., one ion is coordinated in the N2O2 cavity of the Schiff base (Scheme 2b) or when additional deprotonation of the alcoholic group occurs, two copper atoms are bound by N2O3 donor atoms of the ligand (Scheme 2c). Tetranuclear copper(II) complexes, where there is an additional oxygen atom either from the (Scheme 2d) alkoxide or phenoxide (Scheme 2e) group of the second Schiff base in the coordination environment of the central ion, are also quite often observed. Structures with a mixed type of coordination (Scheme 2g,h) are less common. Mukherjee et al. [29] synthesized cationic complexes consisting of a trinuclear copper(II) core (CSD codes: ILAPEQ [27] and ILAPIU [27]) where the coordination geometry of two terminal copper atoms was distorted square planar while the central ones had a distorted square-pyramidal. Yang and co-workers [30] also described a hexanuclear copper(II) complex (Scheme 2h).
In this paper, we present Cu II complexes with similar types of ligands as depicted in Scheme 2a. The Schiff bases contain an additional hydroxyl group, which enhances their ability to coordinate more than one metal ion (see Scheme 2b-h and Table S1). According to the information found in the CSD database, two types of copper(II) ions coordination are most common (i.e., one ion is coordinated in the N 2 O 2 cavity of the Schiff base (Scheme 2b) or when additional deprotonation of the alcoholic group occurs, two copper atoms are bound by N 2 O 3 donor atoms of the ligand (Scheme 2c). Tetranuclear copper(II) complexes, where there is an additional oxygen atom either from the (Scheme 2d) alkoxide or phenoxide (Scheme 2e) group of the second Schiff base in the coordination environment of the central ion, are also quite often observed. Structures with a mixed type of coordination (Scheme 2g,h) are less common. Mukherjee et al. [29] synthesized cationic complexes consisting of a trinuclear copper(II) core (CSD codes: ILAPEQ [27] and ILAPIU [27]) where the coordination geometry of two terminal copper atoms was distorted square planar while the central ones had a distorted square-pyramidal. Yang and co-workers [30] also described a hexanuclear copper(II) complex (Scheme 2h).

Methods
Elemental analysis for C, H, and N was carried out using a CHN 2400 Perkin Elmer analyzer. The content of the copper was determined using an ED XRF spectrophotometer Canberra-Packard. The Fourier-transform infrared spectroscopy (FTIR) (4000-400 cm −1 ) of 1, 2, and H 3 L 1 was performed on a M-80 spectrophotometer Carl Zeiss Jena using KBr pellets. In the case of 3, 4, and H 3 L 2 , the infrared spectra (4000-520 cm −1 ) were recorded on a Thermo Scientific Nicolet 6700 FTIR equipped with a Smart iTR diamond ATR accessory with a resolution of 4 cm −1 for 16 scans. Thermal analysis of 1-4 was carried out by the thermogravimetric (TG, DTG) and differential scanning calorimetry (DSC) methods using the SETSYS 16/18 analyzer (Setaram). The experiments were carried out under air flow in the temperature range of 20-700 • C at a heating rate of 10 • C·min −1 . The samples of 5.99 mg (1), 7.74 mg (2), 5.24 mg (3), and 6.31 mg (4) were heated in Al 2 O 3 crucibles. The TG-FTIR analysis of 1-4 was carried out using a TGA Q5000 analyzer TA Instruments, New Castle, Delaware, USA, interfaced to the Nicolet 6700 FTIR spectrophotometer (Thermo Scientific, Waltham, MA, USA). The samples of 1-4 were placed in an open platinum crucible and heated from ambient temperature to 700 • C with a heating rate of 20 • C·min −1 . Gas analysis was performed by matching the spectra against those from the spectrum library Nicolet TGA Vapor Phase of the software Ominic. The Schiff base H 3 L 1 was synthesized from 5-bromo-2-hydroxy-3-methoxybenzaldehyde (2.31 g, 10 mmol) and 1,3-diamino-2-propanol (0.45 g, 5 mmol) in hot methanol (100 mL) according to the literature [31]. Yield 79%. Anal. (%) for C 19 (2) Complex 2 was obtained in the same manner as 1. The synthesis was different by the ratio of the used reagents (copper(II) salt to the Schiff base H 3 L 1 ). In the case of 1, it was 1:1 and in the case of 2, it was 2:1, respectively. The amount of used substance was as follows: 0.8 mmol (0.1596 g) of copper(II) acetate monohydrate and 0.4 mmol (0.2065 g) of H 3 L 1 . Deep green colored crystals 2 resulted from the slow evaporation of the solution at low temperature (~6 • C).

Synthesis
Yield 52%, Anal. (%) for C 23 (3) The synthesis of complex 3 was also performed in a methanol solution. The ratio of the copper(II) acetate monohydrate to the Schiff base H 3 L 2 was 1:1. To a hot solution of the Schiff base (1 mmol, 0.3444 g), the stoichiometric amount of the copper(II) acetate monohydrate (1 mmol, 0,1997 g) was added. The reaction mixture was refluxed for 1 h. The green solution was left at room temperature (20 • C) for evaporation of the excess solvent. After a few days, green crystals started to form in the reaction mixture.
Yield  (4) The synthesis of complex 4 was carried out under similar conditions as complex 3 with the difference in the stoichiometric ratio of the copper(II) ions to the Schiff base H 3 L 2 (2:1). The Schiff base was also dissolved in methanol at 55 • C and then the solution of the copper(II) acetate was added dropwise. After one hour of heating, the mixture was cooled and left at room temperature to evaporate the solvent. Dark, green crystals formed after a few days. Yield

X-ray Crystal Structure Determination
Data collections of complexes (1-4) were performed on an Oxford Diffraction Xcalibur CCD diffractometer (programs CrysAlis CCD [33]) by using graphite-monochromated MoKα (λ = 0.71073 Å). Datasets were collected at 100 K or 293 K using the ω scan technique, with an angular scan width of 1.0 • . CrysAlis Red [33] was used for cell refinement and data reduction. All the data were corrected for Lorentz and polarization effects. The structures were solved by direct methods using the SHELXS-2018 program and refined on F 2 by full-matrix least-squares methods using SHELXL-2018 [34] (both implemented in the WinGX software package [35]). The carbon atom (C(10)) linked to hydroxyl oxygen atom (protonated or deprotonated) (for complexes 1, 2 and 4) and atom C(9) (for 1) exhibited disorder and were modeled over two sites with occupancies of 0.343/0.657 (1), 0.435/0.565 (2) and 0.808/0.192 (4). Additionally, in the case of 1, the distances of O(10A)−C(11), O(3)−C(10A), and O(3)−C(10B) were restrained using the DFIX command. All C-bound H atoms were placed at calculated positions and refined using a riding model with U is O(H) = 1.2 or 1.5 U eq (C). The water-hydrogen atoms for complexes 1 and 4 (H3W and H4W) were located on the Fourier map and refined using the DFIX command with a fixed O-H distance of 0.84/0.87 Å. Next, when water hydrogen atoms with appropriate position were determined, the DFIX commands were removed and the hydrogen atoms were refined again using the AFIX 3 command with an appointed temperature factor data of −1.5000. The water hydrogen atoms for complexes 2 and 4 (two other hydrogens H1W and H2W) were positioned geometrically and refined using a riding model (AFIX 6). The OH groups were idealized and refined using rigid groups allowed to rotate about the O-H bond (AFIX 147). A summary of crystal data, experimental details, and refinement results is provided in Table 1. The molecular plots were drawn with ORTEP3 for Windows [35] and Mercury [36] The PLATON program was used to perform the geometrical calculations [37] The CIF file refinement can be retrieved from the Cambridge Crystallographic Data Center (CCDC).

Results
The reaction of copper(II) acetate and Schiff bases gives rise to mono-, di-, or tetranuclear derivatives, depending on the reaction conditions: ratio of reagents and the nature of the substituent in the benzene ring of the N,O-donor ligands. The results are summarized in Scheme 4.

Fourier Transform Infrared (FTIR) Spectra
The used Schiff bases: 2,2'-{(2-hydroxypropane-1,3-diyl)bis[nitrilomethylidene]}bis(4-bromo-6methoxyphenol) H3L 1 and H3L 2 , respectively, are potentially capable of forming coordinate bonds with various metal ions through the azomethine, phenolic, hydroxyl, or methoxy groups. The FTIR spectra of the Cu(II) coordination compounds 1-4 were compared with the spectra of H3L 1 and H3L 2 (Figures 1 and 2, Figures S1-S6) in order to obtain information about the binding mode of the Schiff base ligands to copper(II) ions. The position and/or the intensities of the peaks are expected to be changed upon complexation.

Fourier Transform Infrared (FTIR) Spectra
The used Schiff bases: 2,2 -{(2-hydroxypropane-1,3-diyl)bis[nitrilomethylidene]}bis(4-bromo-6methoxyphenol) H 3 L 1 and H 3 L 2 , respectively, are potentially capable of forming coordinate bonds with various metal ions through the azomethine, phenolic, hydroxyl, or methoxy groups. The FTIR spectra of the Cu(II) coordination compounds 1-4 were compared with the spectra of H 3 L 1 and H 3 L 2 (Figures 1 and 2, Figures S1-S6) in order to obtain information about the binding mode of the Schiff base ligands to copper(II) ions. The position and/or the intensities of the peaks are expected to be changed upon complexation.  In the spectra of the free ligands H3L 1 and H3L 2 , strong sharp absorption bands present at around 1644 and 1607 cm −1 , respectively, can be assigned for the imine stretching frequency ν(C = N). Upon complexation of H3L 1 , this band is redshifted to 1624 (1) and 1632 cm −1 (2). A similar downward shift was observed in the peak positions of ν(C = N) stretch vibrations of 3 (1601 cm −1 ) and 4 (1600 cm −1 ) having in the crystal structure deprotonated H3L 2 as a ligand (1607 cm −1 ). This feature may be explained by the withdrawing of electrons from nitrogen to Cu(II) ions due to the coordination. The strong phenolic stretching vibration ν(C-O) at 1236 (1 and 2) and 1230 cm −1 (3 and 4) confirmed that oxygen atoms coordinate to metal ions. A medium or weak broad band with a maximum at around 3452 (1), 3432 (2), 3490 (3), and 3180 cm −1 (4) can be assigned to the OH stretching vibrations, ν(O-H) of OH groups with different chemical environments (i.e., water/methanol molecules and/or hydroxyl groups of the Schiff base ligand that remain protonated in compound 1, as confirmed by X-ray analysis). In the FTIR spectra of complexes 2-4, the strong bands characteristic of asymmetric νas(OCO) and symmetric νs(OCO) stretching vibrations of the carboxylate group were observed at 1564 and 1424 (2), 1532 and 1417 (3), and 1531 and 1409 cm −1 (4), respectively [38][39][40]. The differences between values of asymmetric νas(OCO) and symmetric νs(OCO) stretching vibrations (115-140 cm −1 ) of a bridging carboxylate group of the acetic ion corresponded with those reported in the literature [32,41]. In the spectra of the free ligands H 3 L 1 and H 3 L 2 , strong sharp absorption bands present at around 1644 and 1607 cm −1 , respectively, can be assigned for the imine stretching frequency ν(C = N). Upon complexation of H 3 L 1 , this band is redshifted to 1624 (1) and 1632 cm −1 (2). A similar downward shift was observed in the peak positions of ν(C = N) stretch vibrations of 3 (1601 cm −1 ) and 4 (1600 cm −1 ) having in the crystal structure deprotonated H 3 L 2 as a ligand (1607 cm −1 ). This feature may be explained by the withdrawing of electrons from nitrogen to Cu(II) ions due to the coordination. The strong phenolic stretching vibration ν(C-O) at 1236 (1 and 2) and 1230 cm −1 (3 and 4) confirmed that oxygen atoms coordinate to metal ions. A medium or weak broad band with a maximum at around 3452 (1), 3432 (2), 3490 (3), and 3180 cm −1 (4) can be assigned to the OH stretching vibrations, ν(O-H) of OH groups with different chemical environments (i.e., water/methanol molecules and/or hydroxyl groups of the Schiff base ligand that remain protonated in compound 1, as confirmed by X-ray analysis).

Description of the Molecular Structure
The results of X-ray analysis are presented in Figures 3-10, Figures S7 and S8. The selected bond distances and bond angles are given in Table 2. The hydrogen bond parameters are listed in Table S2 (Supplementary Materials). and the Cu II is coordinated by a pair of phenolate oxygen (O(1) and O(4)) and imine nitrogen (N(1) and N(2)) atoms ( Figure 3). The alcohol group of the aliphatic chain of the Schiff base ligand remains protonated and acts as a hydrogen bond donor to a molecule of water. The water molecule further acts as a hydrogen bond donor, interacting with two phenolate and one methoxy oxygen atoms of the Schiff base ligand (Table S2). Consequently, this results in the formation of the hydrogen bonded chain that propagates parallel to the crystallographic a-axis ( Figure 4). The central atom has distorted square planar coordination with a significant tetrahedral distortion as indicated by the less than linear bond angles (156.13 (12)° O(4)-Cu(1)-N(1) and 156.70(12)° O(1)-Cu(1)-N(2)) and the dihedral angle between the two N-Cu(1)-O planes (θ = 32.42°). This can be confirmed by the structural index parameters τ4 [42] and τ4 ' [43], which were equal to 0.33. The Cu-O and Cu-N bonds were within normal values and comparable to those observed in the related copper(II) complexes (Table S1).   In contrast to 1, complex 2 crystalizes in the triclinic system, space group P-1 as a binuclear unit [Cu2(L 1 )(OAc)MeOH]·2H2O·MeOH ( Figure 5). In its crystal structure, the metal centers Cu(1) and Cu(2) are double bridged by the oxygen atoms of the alkoxide group and the carboxylate group of the acetate ion, respectively. Compound 2 crystallizes with two methanol and two water molecules, one methanol molecule is axially coordinated to one copper atom Cu (1) [44]. In the case of the Cu(2) center, the NO3 atoms showed less tetrahedral distortion from the square planar geometry than that observed for N2O2 core around the Cu II center in complex 1, as indicated by the structural index parameters (τ4 = 0.13 and τ4' = 0.12). The presence of O-H···O and C-H···O bonds (Table S2) expanded the structure of complex 2 into the 3D network presented in Figure 6.  In contrast to 1, complex 2 crystalizes in the triclinic system, space group P-1 as a binuclear unit [Cu2(L 1 )(OAc)MeOH]·2H2O·MeOH ( Figure 5). In its crystal structure, the metal centers Cu(1) and Cu(2) are double bridged by the oxygen atoms of the alkoxide group and the carboxylate group of the acetate ion, respectively. Compound 2 crystallizes with two methanol and two water molecules, one methanol molecule is axially coordinated to one copper atom Cu (1) [44]. In the case of the Cu(2) center, the NO3 atoms showed less tetrahedral distortion from the square planar geometry than that observed for N2O2 core around the Cu II center in complex 1, as indicated by the structural index parameters (τ4 = 0.13 and τ4' = 0.12). The presence of O-H···O and C-H···O bonds (Table S2) expanded the structure of complex 2 into the 3D network presented in Figure 6.  Complex 3 crystallizes in the centrosymmetric monoclinic space group P21/n. Single-crystal Xray analysis showed that the asymmetric unit of 3 contains two crystallographically independent Cu(II) centers, one deprotonated Schiff base ((L 2 ) 3-), the acetate ion, and two methanol molecules (Figure 7). Both Cu II ions are coordinated by one phenoxy, alkoxy, and carboxylate oxygen and imine nitrogen atoms. Additionally, the Cu(2) ion is bound by the phenolate oxygen of second (L 2 ) 3-ions, which leads to the formation of a tetranuclear complex. The Cu(1) atom resides in an almost perfect square planar environment that was confirmed by the values of the τ4 and τ4' parameters [42,43], which were equal to 0.05 and 0.04, respectively. In contrast to complex 2, the coordination polyhedron around the Cu(2) ion was a significant distorted square pyramid (τ5 = 0.26) [44]. The Cu-O and Cu-N bonds were within the normal values and were comparable to those observed in the related copper(II) complexes [3,32,45]. The dimeric units [Cu4(L 2 )2(OAc)2] and methanol molecules were consolidated by the O-H···O hydrogen bonds (Figure 8). The presence of weak C-H···O interactions connects the discrete units into a layered structure ( Figure S8).  Complex 3 crystallizes in the centrosymmetric monoclinic space group P21/n. Single-crystal Xray analysis showed that the asymmetric unit of 3 contains two crystallographically independent Cu(II) centers, one deprotonated Schiff base ((L 2 ) 3-), the acetate ion, and two methanol molecules (Figure 7). Both Cu II ions are coordinated by one phenoxy, alkoxy, and carboxylate oxygen and imine nitrogen atoms. Additionally, the Cu(2) ion is bound by the phenolate oxygen of second (L 2 ) 3-ions, which leads to the formation of a tetranuclear complex. The Cu(1) atom resides in an almost perfect square planar environment that was confirmed by the values of the τ4 and τ4' parameters [42,43], which were equal to 0.05 and 0.04, respectively. In contrast to complex 2, the coordination polyhedron around the Cu(2) ion was a significant distorted square pyramid (τ5 = 0.26) [44]. The Cu-O and Cu-N bonds were within the normal values and were comparable to those observed in the related copper(II) complexes [3,32,45]. The dimeric units [Cu4(L 2 )2(OAc)2] and methanol molecules were consolidated by the O-H···O hydrogen bonds ( Figure 8). The presence of weak C-H···O interactions connects the discrete units into a layered structure ( Figure S8).  Complex 4 crystallizes in the monoclinic crystal system in space group P21/c with the asymmetric unit shown in Figure 9. Similar to complex 3, the asymmetric dimeric units are linked through the phenoxo oxygen atom of the Schiff base. The coordination environments around metal centers are also similar to those observed for complex 3. In complex 4, the structural index parameters τ4 and τ4' were equal to 0.03, indicating less significant tetrahedral distortion from the square planar geometry than observed for 3. The coordination polyhedrons around Cu(2) ions had a distorted square pyramidal geometry that was confirmed by the value of τ5 parameter (0.18). The structure of complex 4 also differed from 3 in the number and type of solvent molecules (i.e., it contains four additional water molecules). The presence of methanol and water molecules cause the stabilization of a structure by the formation of O-H···O hydrogen bonds and consolidates tetranuclear units into a threedimensional supramolecular network ( Figure 10).   Complex 4 crystallizes in the monoclinic crystal system in space group P21/c with the asymmetric unit shown in Figure 9. Similar to complex 3, the asymmetric dimeric units are linked through the phenoxo oxygen atom of the Schiff base. The coordination environments around metal centers are also similar to those observed for complex 3. In complex 4, the structural index parameters τ4 and τ4' were equal to 0.03, indicating less significant tetrahedral distortion from the square planar geometry than observed for 3. The coordination polyhedrons around Cu(2) ions had a distorted square pyramidal geometry that was confirmed by the value of τ5 parameter (0.18). The structure of complex 4 also differed from 3 in the number and type of solvent molecules (i.e., it contains four additional water molecules). The presence of methanol and water molecules cause the stabilization of a structure by the formation of O-H···O hydrogen bonds and consolidates tetranuclear units into a threedimensional supramolecular network ( Figure 10).

Thermal Analysis
The thermal properties of complexes 1-4 were studied in an air atmosphere and the thermogravimetric (TG), the differential thermal analysis (DTG) and the differential scanning calorimetric (DSC) curves and TG-FTIR spectra are presented in Figures 11-16, S9 and S10.
The shape of the TG curves indicates that 1-4 are stable at room temperature. In 1 (Figure 11), the initial decomposition step seen in the range of 45-70 °C (centered at ca. 59 °C) concerns the release of one water molecule with a mass loss of 2.60% (calcd. 3.02%). This step is accompanied by the small endothermic effect recorded on the DSC curve related to the dehydration process. The obtained product is stable until ca. 290 °C, and then decomposes until stabilization at a temperature about 700 °C. In the DSC plot, two exothermic processes were detected. The final product mass loss was 13.40% for the initial compound 1 sample, being consistent with CuO (calcd. 13.35%). Similar mass losses were found on the TG curve recorded under nitrogen. The FTIR spectra of the emitted gases confirmed that during this stage, molecules of water were removed from the structure of 1. The main gaseous products resulting from thermal degradation of 1 were mainly: H2O, CO2, CO, and NH3 ( Figure 12).     The mononuclear complex [Cu(HL 1 )]·H 2 O (1) crystallizes in the centrosymmetric triclinic system, space group P-1. The doubly deprotonated Schiff base ((HL 1 ) 2− ) acts as a tetradentate ligand and the Cu II is coordinated by a pair of phenolate oxygen (O(1) and O(4)) and imine nitrogen (N(1) and N(2)) atoms ( Figure 3). The alcohol group of the aliphatic chain of the Schiff base ligand remains protonated and acts as a hydrogen bond donor to a molecule of water. The water molecule further acts as a hydrogen bond donor, interacting with two phenolate and one methoxy oxygen atoms of the Schiff base ligand (Table S2). Consequently, this results in the formation of the hydrogen bonded chain that propagates parallel to the crystallographic a-axis (Figure 4). The central atom has distorted square planar coordination with a significant tetrahedral distortion as indicated by the less than linear bond angles (156.13 (12) • O(4)-Cu(1)-N(1) and 156.70 (12) • O(1)-Cu(1)-N(2)) and the dihedral angle between the two N-Cu(1)-O planes (θ = 32.42 • ). This can be confirmed by the structural index parameters τ 4 [42] and τ 4 ' [43], which were equal to 0.33. The Cu-O and Cu-N bonds were within normal values and comparable to those observed in the related copper(II) complexes (Table S1). In contrast to 1, complex 2 crystalizes in the triclinic system, space group P-1 as a binuclear unit [Cu 2 (L 1 )(OAc)MeOH]·2H 2 O·MeOH ( Figure 5). In its crystal structure, the metal centers Cu(1) and Cu(2) are double bridged by the oxygen atoms of the alkoxide group and the carboxylate group of the acetate ion, respectively. Compound 2 crystallizes with two methanol and two water molecules, one methanol molecule is axially coordinated to one copper atom Cu (1) [44]. In the case of the Cu(2) center, the NO 3 atoms showed less tetrahedral distortion from the square planar geometry than that observed for N 2 O 2 core around the Cu II center in complex 1, as indicated by the structural index parameters (τ 4 = 0.13 and τ 4 ' = 0.12). The presence of O-H···O and C-H···O bonds (Table S2) expanded the structure of complex 2 into the 3D network presented in Figure 6.
Complex 3 crystallizes in the centrosymmetric monoclinic space group P2 1 /n. Single-crystal X-ray analysis showed that the asymmetric unit of 3 contains two crystallographically independent Cu(II) centers, one deprotonated Schiff base ((L 2 ) 3-), the acetate ion, and two methanol molecules (Figure 7). Both Cu II ions are coordinated by one phenoxy, alkoxy, and carboxylate oxygen and imine nitrogen atoms. Additionally, the Cu(2) ion is bound by the phenolate oxygen of second (L 2 ) 3ions, which leads to the formation of a tetranuclear complex. The Cu(1) atom resides in an almost perfect square planar environment that was confirmed by the values of the τ 4 and τ 4 ' parameters [42,43], which were equal to 0.05 and 0.04, respectively. In contrast to complex 2, the coordination polyhedron around the Cu(2) ion was a significant distorted square pyramid (τ 5 = 0.26) [44]. The Cu-O and Cu-N bonds were within the normal values and were comparable to those observed in the related copper(II) complexes [3,32,45]. The dimeric units [Cu 4 (L 2 ) 2 (OAc) 2 ] and methanol molecules were consolidated by the O-H···O hydrogen bonds (Figure 8). The presence of weak C-H···O interactions connects the discrete units into a layered structure ( Figure S8). Complex 4 crystallizes in the monoclinic crystal system in space group P2 1 /c with the asymmetric unit shown in Figure 9. Similar to complex 3, the asymmetric dimeric units are linked through the phenoxo oxygen atom of the Schiff base. The coordination environments around metal centers are also similar to those observed for complex 3. In complex 4, the structural index parameters τ 4 and τ 4 ' were equal to 0.03, indicating less significant tetrahedral distortion from the square planar geometry than observed for 3. The coordination polyhedrons around Cu(2) ions had a distorted square pyramidal geometry that was confirmed by the value of τ 5 parameter (0.18). The structure of complex 4 also differed from 3 in the number and type of solvent molecules (i.e., it contains four additional water molecules). The presence of methanol and water molecules cause the stabilization of a structure by the formation of O-H···O hydrogen bonds and consolidates tetranuclear units into a three-dimensional supramolecular network (Figure 10).

Thermal Analysis
The thermal properties of complexes 1-4 were studied in an air atmosphere and the thermogravimetric (TG), the differential thermal analysis (DTG) and the differential scanning calorimetric (DSC) curves and TG-FTIR spectra are presented in Figures 11-16, Figures S9 and S10.

Thermal Analysis
The thermal properties of complexes 1-4 were studied in an air atmosphere and the thermogravimetric (TG), the differential thermal analysis (DTG) and the differential scanning calorimetric (DSC) curves and TG-FTIR spectra are presented in Figures 11-16, S9 and S10.
The shape of the TG curves indicates that 1-4 are stable at room temperature. In 1 (Figure 11), the initial decomposition step seen in the range of 45-70 °C (centered at ca. 59 °C) concerns the release of one water molecule with a mass loss of 2.60% (calcd. 3.02%). This step is accompanied by the small endothermic effect recorded on the DSC curve related to the dehydration process. The obtained product is stable until ca. 290 °C, and then decomposes until stabilization at a temperature about 700 °C. In the DSC plot, two exothermic processes were detected. The final product mass loss was 13.40% for the initial compound 1 sample, being consistent with CuO (calcd. 13.35%). Similar mass losses were found on the TG curve recorded under nitrogen. The FTIR spectra of the emitted gases confirmed that during this stage, molecules of water were removed from the structure of 1. The main gaseous products resulting from thermal degradation of 1 were mainly: H2O, CO2, CO, and NH3 ( Figure 12). Figure 11. The thermogravimetric (TG), the differential thermal analysis (DTG), and the differential scanning calorimetric (DSC) curves of the thermal decomposition of complex 1 in air. Figure 11. The thermogravimetric (TG), the differential thermal analysis (DTG), and the differential scanning calorimetric (DSC) curves of the thermal decomposition of complex 1 in air. The TG, DTG, and DSC curves of compound 2 are shown in Figure 13. The first step recorded on the TG curve in the temperature range of 70-110 °C (centered at ca. 97 °C) was related to the dehydration process and concerned the release of two water molecules with a mass loss of 4.90% (calcd. 4.50%). This step as also accompanied by an endothermic effect. The rapid mass loss of 15.00% at 270 °C can be accounted for by the loss of the methanol molecules and the acetic acid (calcd. 15.40%). The next step of the decomposition occurred in the range of 320-630 °C and can be due to the Schiff base ligand decomposition. The final product mass was 19.80%, which is comparable to the calculated value of 19.90% for CuO. The TG-FTIR of the gaseous product released during thermal analysis under an inert atmosphere is presented in Figure S9.
In the case of complex 3 (Figure 14), the mass changes on the TG curve at a temperature up to 185 °C can be assigned to the desolvation processes. During this stage, two methanol molecules are gradually removed from the structure (calcd. mass loss 5.58%, found 6.32%). The remaining solvent molecules are lost in the next step together with the acetate ions. This step also relates to the partial defragmentation, which results in the release of volatile fragments of the organic ligand. The formed intermediate, unstable organic matrix undergoes combustion in the last stage, which was confirmed by the large exothermic effect on the DSC curve. The final decomposition product formed at 558 °C was CuO. The found overall mass loss was 71.83% while the calculated one was 72.26%. The TG, DTG, and DSC curves of compound 2 are shown in Figure 13. The first step recorded on the TG curve in the temperature range of 70-110 °C (centered at ca. 97 °C) was related to the dehydration process and concerned the release of two water molecules with a mass loss of 4.90% (calcd. 4.50%). This step as also accompanied by an endothermic effect. The rapid mass loss of 15.00% at 270 °C can be accounted for by the loss of the methanol molecules and the acetic acid (calcd. 15.40%). The next step of the decomposition occurred in the range of 320-630 °C and can be due to the Schiff base ligand decomposition. The final product mass was 19.80%, which is comparable to the calculated value of 19.90% for CuO. The TG-FTIR of the gaseous product released during thermal analysis under an inert atmosphere is presented in Figure S9.
In the case of complex 3 (Figure 14), the mass changes on the TG curve at a temperature up to 185 °C can be assigned to the desolvation processes. During this stage, two methanol molecules are gradually removed from the structure (calcd. mass loss 5.58%, found 6.32%). The remaining solvent molecules are lost in the next step together with the acetate ions. This step also relates to the partial defragmentation, which results in the release of volatile fragments of the organic ligand. The formed intermediate, unstable organic matrix undergoes combustion in the last stage, which was confirmed by the large exothermic effect on the DSC curve. The final decomposition product formed at 558 °C was CuO. The found overall mass loss was 71.83% while the calculated one was 72.26%. The thermal behavior of complex 3 was also studied in the inert atmosphere of nitrogen. The first stage was similar to that observed in air (i.e., desolvation process). In contrast to the oxidizing atmosphere, a smaller amount of solvent (one and half molecule, found mass loss 4.1%) was released. The remaining methanol molecules were removed from the complex together with the acetate ions, which was confirmed in the FTIR spectra of the gaseous products. The characteristic vibration bands The remaining methanol molecules were removed from the complex together with the acetate ions, which was confirmed in the FTIR spectra of the gaseous products. The characteristic vibration bands of CH3OH were observed in the wavenumber ranges between 2750-3150 and 950-1100 cm −1 , together with those assigned to the acetic acid and their products of the pyrolysis (i.e., CO2 and H2O) ( Figure  15). The further heating of the sample was related to the pyrolysis of the Schiff base ligand, which led to the formation of the following gaseous products: CO2, NH3, H2O, C6H5OH, and CO [46]. Complex 4 undergoes similar processes on heating as compound 3 ( Figure 16). The desolvation process starts ca. 45 °C. In the first step, three water molecules were probably lost (calcd. mass loss 4.43%, found 4.36%). The same process was also observed on the TG curve recorded in a nitrogen atmosphere (the found initial mass loss was 4.06%). Analysis of the FTIR spectra of the evolved gas phase confirmed that only water molecules were released during first step of the desolvation process ( Figure S10). The remaining water molecule was given off in the next stage, during which the methanol was also gradually removed from the structure. At 175 °C, the mass loss found on the TG curve was 6.94%, which corresponded to a loss of four water molecules and a half molecule of methanol (calcd. mass loss 7.22% in an oxidizing and 6.85% in an inert atmosphere). The other methanol molecules were lost along with the release of the acetate ions. The acetic ions were registered in the FTIR spectra of gaseous products as acetic acid ( Figure S10). During this stage, at higher temperature, the decomposition and the combustion/pyrolysis of the Schiff base was also observed. The final product of the thermal decomposition of 4 in air was CuO, which formed at 525 °C. In contrast, under nitrogen stream, the decomposition process of ligand is not finished; the residue remaining after the analysis at 700 °C was 44.40%. Pyrolysis of the Schiff base ligand was associated with the emission of gases similar to those mentioned for complex 3 as well as CH4.

Conclusions
The deprotonated heptadentate and pentadenate Schiff base ligands H3L 1 and H3L 2 in all compounds coordinated Cu(II) ions through the azomethine as well as phenolate groups. The presence of an additional hydroxyl group in the structure of the Schiff bases allowed us to synthesis di-and tetranuclear complexes. In the crystal structure of polynuclear compounds, the metal ions are additionally linked via acetate ions and the alkoxy group. In 2, 3, and 4, two types of metal center were present (i.e., four-and five-coordinated). The coordination environment around the Cu II centers can be described as a distorted square pyramid (C.N. = 5) or square planar (C.N. = 4).
Thermal analysis indicated that the coordination compounds 1-4 are thermally stable at room temperature. Their decomposition processes in air appear to follow a similar trend, starting from losing the crystallization solvent molecules, followed by the ligand decomposition, and finally forming CuO. In contrast to an oxidizing atmosphere, under a nitrogen atmosphere, the pyrolysis The shape of the TG curves indicates that 1-4 are stable at room temperature. In 1 (Figure 11), the initial decomposition step seen in the range of 45-70 • C (centered at ca. 59 • C) concerns the release of one water molecule with a mass loss of 2.60% (calcd. 3.02%). This step is accompanied by the small endothermic effect recorded on the DSC curve related to the dehydration process. The obtained product is stable until ca. 290 • C, and then decomposes until stabilization at a temperature about 700 • C. In the DSC plot, two exothermic processes were detected. The final product mass loss was 13.40% for the initial compound 1 sample, being consistent with CuO (calcd. 13.35%). Similar mass losses were found on the TG curve recorded under nitrogen. The FTIR spectra of the emitted gases confirmed that during this stage, molecules of water were removed from the structure of 1. The main gaseous products resulting from thermal degradation of 1 were mainly: H 2 O, CO 2 , CO, and NH 3 ( Figure 12).
The TG, DTG, and DSC curves of compound 2 are shown in Figure 13. The first step recorded on the TG curve in the temperature range of 70-110 • C (centered at ca. 97 • C) was related to the dehydration process and concerned the release of two water molecules with a mass loss of 4.90% (calcd. 4.50%).
This step as also accompanied by an endothermic effect. The rapid mass loss of 15.00% at 270 • C can be accounted for by the loss of the methanol molecules and the acetic acid (calcd. 15.40%). The next step of the decomposition occurred in the range of 320-630 • C and can be due to the Schiff base ligand decomposition. The final product mass was 19.80%, which is comparable to the calculated value of 19.90% for CuO. The TG-FTIR of the gaseous product released during thermal analysis under an inert atmosphere is presented in Figure S9.
In the case of complex 3 (Figure 14), the mass changes on the TG curve at a temperature up to 185 • C can be assigned to the desolvation processes. During this stage, two methanol molecules are gradually removed from the structure (calcd. mass loss 5.58%, found 6.32%). The remaining solvent molecules are lost in the next step together with the acetate ions. This step also relates to the partial defragmentation, which results in the release of volatile fragments of the organic ligand. The formed intermediate, unstable organic matrix undergoes combustion in the last stage, which was confirmed by the large exothermic effect on the DSC curve. The final decomposition product formed at 558 • C was CuO. The found overall mass loss was 71.83% while the calculated one was 72.26%.
The thermal behavior of complex 3 was also studied in the inert atmosphere of nitrogen. The first stage was similar to that observed in air (i.e., desolvation process). In contrast to the oxidizing atmosphere, a smaller amount of solvent (one and half molecule, found mass loss 4.1%) was released. The remaining methanol molecules were removed from the complex together with the acetate ions, which was confirmed in the FTIR spectra of the gaseous products. The characteristic vibration bands of CH 3 OH were observed in the wavenumber ranges between 2750-3150 and 950-1100 cm −1 , together with those assigned to the acetic acid and their products of the pyrolysis (i.e., CO 2 and H 2 O) ( Figure 15). The further heating of the sample was related to the pyrolysis of the Schiff base ligand, which led to the formation of the following gaseous products: CO 2 , NH 3 , H 2 O, C 6 H 5 OH, and CO [46].
Complex 4 undergoes similar processes on heating as compound 3 ( Figure 16). The desolvation process starts ca. 45 • C. In the first step, three water molecules were probably lost (calcd. mass loss 4.43%, found 4.36%). The same process was also observed on the TG curve recorded in a nitrogen atmosphere (the found initial mass loss was 4.06%).
Analysis of the FTIR spectra of the evolved gas phase confirmed that only water molecules were released during first step of the desolvation process ( Figure S10). The remaining water molecule was given off in the next stage, during which the methanol was also gradually removed from the structure. At 175 • C, the mass loss found on the TG curve was 6.94%, which corresponded to a loss of four water molecules and a half molecule of methanol (calcd. mass loss 7.22% in an oxidizing and 6.85% in an inert atmosphere). The other methanol molecules were lost along with the release of the acetate ions. The acetic ions were registered in the FTIR spectra of gaseous products as acetic acid ( Figure S10). During this stage, at higher temperature, the decomposition and the combustion/pyrolysis of the Schiff base was also observed. The final product of the thermal decomposition of 4 in air was CuO, which formed at 525 • C. In contrast, under nitrogen stream, the decomposition process of ligand is not finished; the residue remaining after the analysis at 700 • C was 44.40%. Pyrolysis of the Schiff base ligand was associated with the emission of gases similar to those mentioned for complex 3 as well as CH 4 .

Conclusions
The deprotonated heptadentate and pentadenate Schiff base ligands H 3 L 1 and H 3 L 2 in all compounds coordinated Cu(II) ions through the azomethine as well as phenolate groups. The presence of an additional hydroxyl group in the structure of the Schiff bases allowed us to synthesis di-and tetranuclear complexes. In the crystal structure of polynuclear compounds, the metal ions are additionally linked via acetate ions and the alkoxy group. In 2, 3, and 4, two types of metal center were present (i.e., four-and five-coordinated). The coordination environment around the Cu II centers can be described as a distorted square pyramid (C.N. = 5) or square planar (C.N. = 4).
Thermal analysis indicated that the coordination compounds 1-4 are thermally stable at room temperature. Their decomposition processes in air appear to follow a similar trend, starting from losing the crystallization solvent molecules, followed by the ligand decomposition, and finally forming CuO. In contrast to an oxidizing atmosphere, under a nitrogen atmosphere, the pyrolysis process of Schiff base ligands is not finished.

Supplementary Materials:
The following are available online at http://www.mdpi.com/2073-4352/10/11/1004/s1, Table S1: List of chosen mono-and polynuclear Schiff base complexes of Cu II found in the CSD search. Table S2: Hydrogen bonding and C-H···π interactions geometry [Å] for complexes 1-4. Figure S1: FTIR spectra of the Schiff base ligand H 3 L 1 and complex 1. Figure. S2: FTIR spectra of the Schiff base ligand H 3 L 1 and complex 2. Figure S3: FTIR spectra of the complexes 1 and 2. Figure S4: FTIR spectra of the Schiff base ligand H 3 L 2 and complex 3. Figure S5: FTIR spectra of the Schiff base ligand H 3 L 2 and complex 4. Figure S6: FTIR spectra of the complexes 3 and 4. Figure S7: The packing of structure 1 along the a direction. Figure S8: The crystal structure packing of complex 3 showing formed layers via C-H···O interactions. Figure S9: FTIR spectra of gaseous products of complex 2, decomposition in nitrogen. Figure S10: FTIR spectra of gaseous products of complex 4 decomposition in nitrogen.