Syntheses , Crystal Structures and Thermal Behaviors of Two Supramolecular Salamo-Type Cobalt ( II ) and Zinc ( II ) Complexes

This paper reports the syntheses of two new complexes, [Co(L)(H2O)2] (1) and [{Zn(L2) (μ-OAc)Zn(n-PrOH)}2] (2), from asymmetric halogen-substituted Salamo-type ligands H2L and H3L, respectively. Investigation of the crystal structure of complex 1 reveals that the complex includes one Co(II) ion, one (L1)2− unit and two coordinated water molecules. Complex 1 shows slightly distorted octahedral coordination geometry, forming an infinite 2D supramolecular structure by intermolecular hydrogen bond and π–π stacking interactions. Complex 2 contains four Zn(II)ions, two completely deprotonated (L2)3− moieties, two coordinated μ-OAc− ions and n-propanol molecules. The Zn(II) ions in complex 2 display slightly distorted trigonal bipyramidal or square pyramidal geometries.

Salen-type N 2 O 2 compounds are capable of forming different types of complexes due to their several electron-rich donor centers [49] and the tautomerism effect of the enol and keto forms [50].Meanwhile, on account of the fact that the high electronegative oxygen atoms affect azomethine nitrogen and N 2 O 2 coordination plane on the basis of Salamo-type bisoxime ligands [45,[51][52][53][54][55], many complexes exhibit abundant interests, attractive structures and properties have been well documented [56][57][58][59].Furthermore, metal ions play key roles in wide range of differing biological processes and the interaction of the metal ion with drugs employed for therapeutic reasons is a subject of considerable interest [60].Herein, following our previous studies on the syntheses, structural characterizations and optical properties of Salamo-type complexes with the d 5 and d 10 group elements [49,61], we report the syntheses and structural characterizations of mononuclear Co(II) and tetranuclear Zn(II) complexes, [Co(L 1 )(H 2 O) 2 ] (1), [{Zn(L 2 )(µ-OAc)Zn(n-PrOH)} 2 ] (2) based on Salamo-type N 2 O 2 /N 2 O 3 ligands (H 2 L 1  3,5-Dichlorosalicylaldehyde, 3,5-dibromosalicylaldehyde, 5-chlorosalicylaldehyde and 3-hydroxysalicylaldehyde were purchased from a Acros Organics company (New York, NY, USA) and used without further purification.The other reagents and solvents were analytical grade acquired from Tanjin Chemical Reagent Factory (Tianjin, China).
C, H, and N analyses were obtained using a GmbH VarioEL V3.00 automatic elemental analysis instrument (Berlin, Germany).Elemental analyses for Co(II) and Zn(II) were detected by an IRIS ER/S•WP-1 ICP atomic emission spectrometer (Berlin, Germany).Molar conductance value measurements were carried out on a model DDS-11D type conductivity bridge (The United States CHI)using 1.0 × 10 −3 mol•L −3 solution in DMF at 18 • C. IR spectra were recorded on a Vertex70 FT-IR spectrophotometer (Bruker AVANCE, Billerica, MA, USA), with samples prepared as KBr (400-4000 cm −1 ) pellets.UV-Vis absorption spectra were recorded on a Shimadzu UV-2550 spectrophotometer (Shimadzu, Japan) in ethanolic solution. 1H NMR spectra were determined by a German Bruker AVANCE DRX-400 spectrometer (Bruker AVANCE, Billerica, MA, USA).DSC-TG analyses were carried out at a heating rate of 10 • C/min on a NETZSCH STA 449 F3 thermoanalyzer (NETZSCH Group, Germany).X-ray single crystal structures were determined on a Bruker Smart APEX CCD area detector (Bruker AVANCE, Billerica, MA, USA).Melting points were measured by the use of a microscopic melting point apparatus made in Beijing Taike Instrument Limited Company, and the thermometer was uncorrected.

Experimental
Crystallographic data have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication, No. CCDC 1511309 and 1511310 for complexes 1 and 2. Copies of these data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (Telephone: (44) 01223 762910; Fax: +44-1223-336033; E-mail: deposit@ccdc.cam.ac.uk).These data can also be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html.

Materials and Methods
3,5-Dichlorosalicylaldehyde, 3,5-dibromosalicylaldehyde, 5-chlorosalicylaldehyde and 3hydroxysalicylaldehyde were purchased from a Acros Organics company (New York, NY, USA) and used without further purification.The other reagents and solvents were analytical grade acquired from Tanjin Chemical Reagent Factory (Tianjin, China).
C, H, and N analyses were obtained using a GmbH VarioEL V3.00 automatic elemental analysis instrument (Berlin, Germany).Elemental analyses for Co(II) and Zn(II) were detected by an IRIS ER/S•WP-1 ICP atomic emission spectrometer (Berlin, Germany).Molar conductance value measurements were carried out on a model DDS-11D type conductivity bridge (The United States CHI)using 1.0 × 10 −3 mol•L −3 solution in DMF at 18 °C.IR spectra were recorded on a Vertex70 FT-IR spectrophotometer (Bruker AVANCE, Billerica, MA, USA), with samples prepared as KBr (400-4000 cm −1 ) pellets.UV-Vis absorption spectra were recorded on a Shimadzu UV-2550 spectrophotometer (Shimadzu, Japan) in ethanolic solution. 1H NMR spectra were determined by a German Bruker AVANCE DRX-400 spectrometer (Bruker AVANCE, Billerica, MA, USA).DSC-TG analyses were carried out at a heating rate of 10 °C/min on a NETZSCH STA 449 F3 thermoanalyzer (NETZSCH Group, Germany).X-ray single crystal structures were determined on a Bruker Smart APEX CCD area detector (Bruker AVANCE, Billerica, MA, USA).Melting points were measured by the use of a microscopic melting point apparatus made in Beijing Taike Instrument Limited Company, and the thermometer was uncorrected.

Synthesis of Complex 1
A pink, transparent n-propanol solution (2 mL) of cobalt(II) acetate tetrahydrate (3.30 mg, 0.013 mmol) was added dropwise to a colorless mixed solution of CH 2 Cl 2 and CH 3 CN (4 mL) of H 2 L 1 (3.85 mg, 0.007 mmol) at room temperature.The color of the mixed solution turned to yellow immediately, the filtrate was allowed to stand at room temperature for about two weeks, after which light-yellow prismatical single crystals suitable for X-ray structural determination were obtained by the slow evaporation from mixed solution.Anal.Calcd for C 16

Synthesis of Complex 2
A colorless, transparent n-propanol solution (2 mL) of zinc(II) acetate tetrahydrate (4.51 mg, 0.021 mmol) was added dropwise to a colorless acetonitrile solution (4 mL) of H 3 L 2 (3.49mg, 0.010 mmol) at room temperature.The color of the mixing solution was immediately turned to yellow, and the filtrate was left to stand at room temperature for about two weeks, after which light-yellow prismatical single crystals suitable for X-ray structural determination were obtained by the slow evaporation from mixed solution.Anal.Single crystal X-ray diffraction data of complexes 1 and 2 were collected at 293(2) and 228(3) K, respectively, on a BRUKER SMART APEX II CCD diffractometer with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å).The LP factor semi-empirical absorption corrections were applied using the SADABS program.The structures were solved by direct methods using SHELXS and refinement was done against F 2 using SHELXL.The non-hydrogen atoms were refined anisotropically; hydrogen atoms were positioned geometrically (C-H = 0.93, 0.96 and 0.97 Å) and were refined as riding, with U iso (H) = 1.20 or 1.50 U eq (C).The crystal data and experimental parameters relevant to the structure determinations are listed in Table 1 and the final positional and thermal parameters are available as Supplementary Materials.

Results and Discussion
Complexes 1 and 2 with the Salamo-type bisoxime chelating N 2 O 2 /N 2 O 3 ligands (H 2 L 1 and H 3 L 2 ) have been duly synthesized, and structurally characterized by molar conductance, IR spectra, UV-Vis spectra, DSC-TG and X-ray crystallography analyses.

Molar Conductance
Complexes 1 and 2 are both soluble in DMF and DMSO, but insoluble in CHCl 3 , CH 2 Cl 2 , EtOH, MeOH, MeCN, THF, acetone, ethyl acetate and n-hexane.Only complex 1 displays good stability in air at room temperature while complex 2 shows relative instability.Moreover, the ligands (H 2 L 1 and H 3 L 2 ) are both soluble in the above solvents.The molar conductance values of complexes 1 and 2 in 1.0 × 10 −3 mol•L −3 DMF solutions are 2.2 and 2.9 Ω −1 •cm 2 •mol −1 , respectively, with the implication that complexes 1 and 2 are non-electrolytes.

Crystal Structure of Complex 1
The results of the X-ray structural study reveal that complex 1 crystallizes in the monoclinic system and P2(1)/c space group.As depicted in Figure 1, complex 1 concludes one Co(II) ion together with one deprotonated ligand (L 1 ) 2− unit (in the form of enol), and two coordinated water molecules.The Co(II) ion is hexa-coordinated by two oxime nitrogen (N1 and N2) atoms and two deprotonated phenoxo oxygen (O1 and O4) atoms, the four atoms are all from one deprotonated (L 1 ) 2− unit, and two oxygen (O5 and O6) atoms from two coordinated water molecules.The coordination environment around the Co(II) ion is best described as a slightly distorted octahedral geometry (Figure 1).This shows a correlation to the reported trinuclear [Co 3 (5-NO 2 salamo) 2 (OAc) 2 (MeOH) 2 ]•2MeOH, in which the Co(II) ions of the trinuclear complex are all slightly distorted octahedral geometries with hexacoordination [54].Unlike the mononuclear complex [Co(salen)], the Co(II) ion is a distorted square planar geometry with tetracoordination [56]

Molar Conductance
Complexes 1 and 2 are both soluble in DMF and DMSO, but insoluble in CHCl3, CH2Cl2, EtOH, MeOH, MeCN, THF, acetone, ethyl acetate and n-hexane.Only complex 1 displays good stability in air at room temperature while complex 2 shows relative instability.Moreover, the ligands (H2L 1 and H3L 2 ) are both soluble in the above solvents.The molar conductance values of complexes 1 and 2 in 1.0 × 10 −3 mol•L −3 DMF solutions are 2.2 and 2.9 Ω −1 •cm 2 •mol −1 , respectively, with the implication that complexes 1 and 2 are non-electrolytes.

Crystal Structure of Complex1
The results of the X-ray structural study reveal that complex 1 crystallizes in the monoclinic system and P2(1)/c space group.As depicted in Figure 1, complex 1 concludes one Co(II) ion together with one deprotonated ligand (L 1 ) 2− unit (in the form of enol), and two coordinated water molecules.The Co(II) ion is hexa-coordinated by two oxime nitrogen (N1 and N2) atoms and two deprotonated phenoxo oxygen (O1 and O4) atoms, the four atoms are all from one deprotonated (L 1 ) 2− unit, and two oxygen (O5 and O6) atoms from two coordinated water molecules.The coordination environment around the Co(II) ion is best described as a slightly distorted octahedral geometry (Figure 1).(L 1 ) 2− unit of complex 1 acts as a tetradentate agent through two phenolic oxygen and oxime nitrogen atoms, which are in the equatorial positions.The four N 2 O 2 donor atoms of (L 1 ) 2− unit are approximately coplanar, and the dihedral angle of N1-N2-O4 and N2-O1-O4 is about 0.58(2) • .The axial sites are filled up by two water molecules in a relatively large angle of O5-Co(II)-O6 Crystals 2017, 7, 217 6 of 15 (175.33(12)• ).It is explicit that the bond distance Co1-O5 (2.141(2) Å) shows a more significant length than that of Co1-O6 (2.124(2) Å), and this reveals the varied coordination abilities of the two coordinated water molecules (Table 2).TheCo(II)ion of complex 1 is almost coplanar with the mean plane through the N 2 O 2 core, diverging from the mean plane by 0.028 Å and the four donor atoms (N1, N2, O1 and O4) from their mean plane by 0.008, −0.008, −0.008 and 0.008 Å.Thus, the octahedral Co(II) center is composed of the N 2 O 2 coordination sphere made up of (L 1 ) 2− unit in the equatorial plane and two coordinated water molecules.The seven-membered chelate ring (Co-N1-O2-C8-C9-O3-N2) in complex 1 is in a gauche conformation with the ethylene carbon atoms above the N 2 O 2 coordination plane (C8, 0.502 Å and C9, 1.148 Å).The Co-O(phenolic) bonds of 2.083(3) Å and 2.062(3) Å and Co-N(oxime) bonds of 2.088(4) Å and 2.123(4) Å are in conformity with the average bond lengths observed for the corresponding bonds in the Co(II) complexes that anchors tetradentate Salamo-type ligands [62].3 and 4) [17], which perform a crucial role in constructing and stabilizing supramolecular structure.The oxygen O2 (O3) atoms of the N 2 O 2 molecules are hydrogen bonded to the C8-H8B (C9-H9B) groups of another complex molecule linking adjacent complex molecules into an infinite 2D supramolecular structure (Figure 2) [63][64][65][66][67].

Crystal Structure of Complex 2
Complex 2 crystallizes in the triclinic system and P-1 space group.As shown in Figure 3, the structure of complex 2 is made up of four Zn(II) ions, two deprotonated (L 2 ) 3− units (in the form of enol), two μ-acetate ions and two coordinated n-PrOH molecules, showing consistency with the analytical data.The terminal Zn(II) (Zn1) ion is pentacoordinated by two oxime nitrogen (N1 and N2) atoms and two phenoxo oxygen (O1 and O5) atoms of one deprotonated (L 2 ) 3− unit, and one oxygen (O6) atom of one μ-acetate ion, displaying a slightly distorted trigonal bipyramidal coordination motif (τ = 0.704) (Figure 3).Meanwhile, the other Zn(II) (Zn2) ion is pentacoordinated by three phenoxo oxygen (O4, O4i and O5) atoms of two tetradentate deprotonated (L 2 ) 3− units, one carbonyl oxygen (O7) atom from the μ-acetate ion as well as one hydroxyl oxygen (O8) atom from one coordinated n-Pr-OH molecule.The coordination geometry around the Zn(II) (Zn2) ion can be

Crystal Structure of Complex 2
Complex 2 crystallizes in the triclinic system and P-1 space group.As shown in Figure 3, the structure of complex 2 is made up of four Zn(II) ions, two deprotonated (L 2 ) 3− units (in the form of enol), two µ-acetate ions and two coordinated n-PrOH molecules, showing consistency with the analytical data.The terminal Zn(II) (Zn1) ion is pentacoordinated by two oxime nitrogen (N1 and N2) atoms and two phenoxo oxygen (O1 and O5) atoms of one deprotonated (L 2 ) 3− unit, and one oxygen (O6) atom of one µ-acetate ion, displaying a slightly distorted trigonal bipyramidal coordination motif (τ = 0.704) (Figure 3).Meanwhile, the other Zn(II) (Zn2) ion is pentacoordinated by three phenoxo oxygen (O4, O4i and O5) atoms of two tetradentate deprotonated (L 2 ) 3− units, one carbonyl oxygen (O7) atom from the µ-acetate ion as well as one hydroxyl oxygen (O8) atom from one coordinated Crystals 2017, 7, 217 8 of 15 n-Pr-OH molecule.The coordination geometry around the Zn(II) (Zn2) ion can be described as a slightly distorted square pyramidal coordination sphere (τ = 0.178) (Figure 3).This tetranuclear Zn(II) complex is rare case for Salen-or Salamo-type complexes, showing that the complexation of 3-hydroxy Salamo-type ligands with Zn(II) acetate occurs cooperatively [57], unlike in the case of other substituted Salen- [32][33][34] or Salamo-type ligands [35,36].In fact, this tetranuclear Zn(II) complex exhibits a similar structure to the Zn(II) complex previously reported [35].The description of the tetranuclear structure is two [Zn(L 2 )(µ-OAc)Zn(n-PrOH)] units connected with two diphenoxy-bridges.In each of the units, the Zn1 center was coordinated via N 2 O 2 donors.The central Zn2 was coordinated by three deprotonated µ-phenolic oxygen atoms in two [Zn(L 2 )] chelates and one oxygen atom of coordinated n-propanol molecule.The acetate ion was coordinated to two Zn(II) ions through Zn1-O-C-O-Zn2 bridge.Thus, the complex is composed of four five-coordinated Zn(II) centers, alike with the coordination geometries of Zn(II) ions in the literature predecent [57].This is attributed to the difference in the bond lengths and angles between the central ion, and the coordination groups, as well as the distortions of the geometries of the Zn(II) centers.
Crystals 2017, 7, 217 8 of 15 described as a slightly distorted square pyramidal coordination sphere (τ = 0.178) (Figure 3).This tetranuclear Zn(II) complex is rare case for Salen-or Salamo-type complexes, showing that the complexation of 3-hydroxy Salamo-type ligands with Zn(II) acetate occurs cooperatively [57], unlike in the case of other substituted Salen- [32][33][34]or Salamo-type ligands [35,36].In fact, this tetranuclear Zn(II) complex exhibits a similar structure to the Zn(II) complex previously reported [35].The description of the tetranuclear structure is two [Zn(L 2 )(μ-OAc)Zn(n-PrOH)] units connected with two diphenoxy-bridges.In each of the units, the Zn1 center was coordinated via N2O2 donors.The central Zn2 was coordinated by three deprotonated μ-phenolic oxygen atoms in two [Zn(L 2 )] chelates and one oxygen atom of coordinated n-propanol molecule.The acetate ion was coordinated to two Zn(II) ions through Zn1-O-C-O-Zn2 bridge.Thus, the complex is composed of four five-coordinated Zn(II) centers, alike with the coordination geometries of Zn(II) ions in the literature predecent [57].This is attributed to the difference in the bond lengths and angles between the central ion, and the coordination groups, as well as the distortions of the geometries of the Zn(II) centers.

IR Spectra
The FT-IR spectra of H2L 1 and H3L 2 and their corresponding complexes 1 and 2 exhibit various bands in the 4000-400 cm −1 region.Figure 4 shows the most important FT-IR bands for H2L 1 and H3L 2 and complexes 1 and 2.
In general, the O-H stretching frequency of Salen-type ligand appears at approximately 3433 cm −1 owing to the intramolecuar OH•••N=C hydrogen bonding [59], the absorption commonly appears in the FT-IR spectrum as a broad or double band and occasionally, the band seems to be undetectable.Disappearance of the band is expected for the metal complexes as a result of the substitution reaction when the OH hydrogen is substituted or replaced by the metal ions, thus leading to the complex formation [60].
At 1613 and 1612 cm −1 , the free ligands H2L 1 and H3L 2 show typical C=N stretching bands, while complexes 1 and 2 exhibit the characteristic C=N bands at 1609 and 1604 cm −1 , respectively.Upon complexation, shifts to lower frequencies by ca. 4 and 8 cm −1 [52] are observed in the C=N stretching frequencies for complexes 1 and 2, respectively, indicating a decrease in the C=N bond order owing to the effects of the coordinated bonds of Co(II) and Zn(II) ions with the lone pair of oxime nitrogen electrons [61].As reported, the Ar-O stretching frequency in the 1216-1213 cm −1 range appears as a

IR Spectra
The FT-IR spectra of H 2 L 1 and H 3 L 2 and their corresponding complexes 1 and 2 exhibit various bands in the 4000-400 cm −1 region.Figure 4 shows the most important FT-IR bands for H 2 L 1 and H 3 L 2 and complexes 1 and 2.
In general, the O-H stretching frequency of Salen-type ligand appears at approximately 3433 cm −1 owing to the intramolecuar OH•••N=C hydrogen bonding [59], the absorption commonly appears in the FT-IR spectrum as a broad or double band and occasionally, the band seems to be undetectable.Disappearance of the band is expected for the metal complexes as a result of the substitution reaction when the OH hydrogen is substituted or replaced by the metal ions, thus leading to the complex formation [60].
At 1613 and 1612 cm −1 , the free ligands H 2 L 1 and H 3 L 2 show typical C=N stretching bands, while complexes 1 and 2 exhibit the characteristic C=N bands at 1609 and 1604 cm −1 , respectively.Upon complexation, shifts to lower frequencies by ca. 4 and 8 cm −1 [52] are observed in the C=N Crystals 2017, 7, 217 9 of 15 stretching frequencies for complexes 1 and 2, respectively, indicating a decrease in the C=N bond order owing to the effects of the coordinated bonds of Co(II) and Zn(II) ions with the lone pair of oxime nitrogen electrons [61].As reported, the Ar-O stretching frequency in the 1216-1213 cm −1 range appears as a strong band [60].These bands occur at 1270 and 1264 cm −1 for the ligands H 2 L 1 and H 3 L 2 , and at 1209 and 1203 cm −1 for complexes 1 and 2, respectively.The Ar-O stretching frequencies are shifted to a lower frequency, indicating that the Co(II)-O and Zn(II)-O bonds were formed between the Co(II) and Zn(II) ions and oxygen of phenolic group [61].
The typical absorption bands at 3413, 1643, and 519 cm −1 in complex 1 are assigned to the coordinated water molecules as are substantiated by crystal structure.The IR spectrum of complex 2 shows the expected strong absorption band due to νO-H at ca. 3450 cm −1 , which is evident for the existence of n-propanol molecule.and Zn(II) ions and oxygen of phenolic group [61].
The typical absorption bands at 3413, 1643, and 519 cm −1 in complex 1 are assigned to the coordinated water molecules as are substantiated by crystal structure.The IR spectrum of complex 2 shows the expected strong absorption band due to νO-H at ca. 3450 cm −1 , which is evident for the existence of n-propanol molecule.

UV-Vis Spectra
The absorption spectra of ligands H 2 L 1 and H 3 L 2 and their corresponding complexes 1 and 2 were determined in 5 × 10 −5 mol•L −1 ethanolic solution (Figure 5).Obviously, the absorption peaks of the ligands H 2 L 1 and H 3 L 2 differ from those of complexes 1 and 2 upon complexation.The electronic absorption spectrum of the Salamo-type ligand H 2 L 1 consists of three relatively intense bands centered at 225, 268 and 325 nm (for H 3 L 2 , 219, 270 and 321 nm), which may be assigned to the π-π* transitions of the phenyl of salicylaldehyde and the oxime group [58].The absorption bands at 268 and 325 nm (for H 3 L 2 , 270 nm) disappear from the UV-vis spectrum upon complexation of the ligand with the metal ions, and indicates that the oxime nitrogen atoms are involved in coordination.The phenyl of salicylaldehyde intraligand π-π transition is a little shifted to 236 and 214 nm in the corresponding complexes 1 and 2, respectively.Besides, the newly emerged band observed at 377 nm for complex 1 is assigned to the L→M charge-transfer transition, which is typical of the transition metal complexes with N 2 O 2 coordination spheres [68].
Crystals 2017, 7, 217 10 of 15 the ligands H2L 1 and H3L 2 differ from those of complexes 1 and 2 upon complexation.The electronic absorption spectrum of the Salamo-type ligand H2L 1 consists of three relatively intense bands centered at 225, 268 and 325 nm (for H3L 2 , 219, 270 and 321 nm), which may be assigned to the π-π* transitions of the phenyl of salicylaldehyde and the oxime group [58].The absorption bands at 268 and 325 nm (for H3L 2 , 270 nm) disappear from the UV-vis spectrum upon complexation of the ligand with the metal ions, and indicates that the oxime nitrogen atoms are involved in coordination.The phenyl of salicylaldehyde intraligand π-π transition is a little shifted to 236 and 214 nm in the corresponding complexes 1 and 2, respectively.Besides, the newly emerged band observed at 377 nm for complex 1 is assigned to the L→M charge-transfer transition, which is typical of the transition metal complexes with N2O2 coordination spheres [68].

Thermal Properties
Thermal stability studies were performed for complexes 1 and 2. The TG curve of complex 1 occurs in two stages.The first stage is between 132 and 176 • C. The TG curve shows a 6.0% weight loss in this temperature range, which is roughly similar to the 5.8% value calculated for the loss of two coordinated water molecule from the outer coordination sphere of complex 1.Then, the remaining solid residue was stable up to around 207 • C, after which the compound begins to decompose.At 800 • C, the TG curve shows an approximately 88.3% total mass loss, indicating the complete removal of the (L 1 ) 2− unit.The main residual product was CoO, with a value of 11.7% (theoretical residual value was 12.1%).
The TG curve of complex 2 occurred in three stages.The first stage occurs in the 187-215 • C range.The TG curve indicates that the weight loss corresponding to this temperature range is 10.4%, which is roughly similar to the 10.1% value estimated for the loss of two coordinated n-PrOH molecules from the inner coordination sphere of complex 2. The second stage starts from 256 • C with a weight loss of 9.7%, which corresponds to the loss of two µ-acetate (theoretical mass loss, 9.9%) ions.Finally, the third weight loss starts at around 283 • C, leading to the full decomposition of the compound.At 650 • C, the TG curve shows about 92.8% total mass loss, indicating the complete removal of the (L 2 ) 3− unit.The main residual product was ZnO, with a value of 7.2% (theoretical residual value was 6.8%).

Figure 1 .
Figure 1.(a) Molecular structure and atom numberings of complex 1 with 30% probability displacement ellipsoids(hydrogen atoms are omitted for clarity); (b) Coordination polyhedron for Co(II) ion of complex 1.

Figure 1 .
Figure 1.(a) Molecular structure and atom numberings of complex 1 with 30% probability displacement ellipsoids(hydrogen atoms are omitted for clarity); (b) Coordination polyhedron for Co(II) ion of complex 1.

Figure 3 .
Figure 3. (a) Molecular structure and atom numberings of complex 2 with 30% probability displacement ellipsoids (hydrogen atoms are omitted for clarity).(b) Coordination polyhedra for Zn1 and Zn2 ions of complex 2.

Figure 3 .
Figure 3. (a) Molecular structure and atom numberings of complex 2 with 30% probability displacement ellipsoids (hydrogen atoms are omitted for clarity).(b) Coordination polyhedra for Zn1 and Zn2 ions of complex 2.
60].These bands occur at 1270 and 1264 cm −1 for the ligands H2L 1 and H3L 2 , and at 1209 and 1203 cm −1 for complexes 1 and 2, respectively.The Ar-O stretching frequencies are shifted to a lower frequency, indicating that the Co(II)-O and Zn(II)-O bonds were formed between the Co(II)

Table 1 .
Crystal data and structure refinement parameters for complexes 1 and 2. Formula C 16 H 14 Br 2 Cl 2 CoN 2 O 6 C 42 H 46 Cl 2 N 4 O 16 Zn 4