synthesis and crystal structures of copper(ii) …...was kept at room temperature to give green...

Synthesis of Copper(II) Complexes with Schiff Base Ligands Bull. Korean Chem. Soc. 2013, Vol. 34, No. 11 3233

http://dx.doi.org/10.5012/bkcs.2013.34.11.3233

Synthesis and Crystal Structures of Copper(II) Complexes with Schiff Base Ligands:

[Cu2(acpy-mdtc)2(HBA)(ClO4)]·H2O and [Cu2(acpy-phtsc)2(HBA)]·ClO4

Bon Kweon Koo

Department of Chemistry, Catholic University of Daegu, Gyeongbuk 712-702, Korea. E-mail: [email protected]

Received June 17, 2013, Accepted August 6, 2013

Two new Cu(II) complexes, [Cu2(acpy-mdtc)2(HBA)(ClO4)]·H2O (1) (acpy-mdtc− = 2-acetylpyridine S-

methyldithiocarbamate and HBA− = benzilic acid anion) and [Cu2(acpy-phtsc)2(HBA)]·ClO4 (2) (acpy-phtsc−

= 2-acetylpyridine 4-phenyl-3-thiosemicarbazate) have been synthesized and characterized by elemental

analysis, infrared spectroscopy, thermogravimetric analysis, and single crystal X-ray diffraction. The X-ray

analysis reveals that the structures of 1 and 2 are dinuclear copper(II) complexes bridged by two thiolate sulfur

atoms of Schiff base ligand and bidentate bridging HBA− anion. For 1, each of the two copper atoms has

different coordination environments. Cu1 adopts a five-coordinate square-pyramidal with a N2OS2 donor,

while Cu2 exhibits a distorted octahedral geometry in a N2O2S2 manner. For 2, two Cu(II) ions all have a five-

coordinate square-pyramidal with a N2OS2 donor. In each complex, the Schiff base ligand is coordinated to

copper ions as a tridentate thiol mode.

Key Words : Dinuclear Cu(II)-complexes, Schiff base, Benzilic acid, Crystal structures

Introduction

Thiosemicarbazones1 and Schiff bases derived from S-alkyl/aryl esters of dithiocarbazic acid2 are among the mostwidely studied sulfur-nitrogen chelating agents. Interest inmetal complexes of these ligands is stimulated by theirinteresting physico-chemical properties3 and significantbiological activities.4

We have reported Mo(VI), V(IV)O, Mn(II), Co(II), andZn(II) complexes of mono- or bis-Schiff base ligands result-ing from the condensation of salicylaldehyde or 2-acetyl-pyridine with dithiocarbamate or thiosemicarbazide.5 Recent-ly, dinuclear copper(II) complexes, [Cu2(L)2(CH3COO)](ClO4)(L = 2-benzoylpyridine S-methyldithiocarbazate)6 and[Cu2L2(SO4)] (L = di-2-pyridyl ketone N(4),N(4)-(butane-1,4-diyl)thiosemicarbazone)7 in which sulfur atom fromSchiff bases ligand together with acetate or sulfate oxygenatoms bridges the two copper(II) Ions, respectively, werereported. Although many Cu(II)-Schiff base complexes havebeen reported, dinueclear Cu(II) complexes consist of Schiffbase ligand and other the second ligand, especially, dinuclearcomplexes linked through the other second ligand except thesolvent molecule or the counter anion of metal salt asstarting material have been little published.8

In this work, the benzilic acid (H2BA) was taken as thesecond ligand. For metal ions, benzilic acid can provide avariety of chelating and/or bridging coordination modes dis-played by the carboxylic or hydroxy groups.9 Many frame-works constructed by BA2− or HBA− with transition metalions or rare earth ions have been reported, mainly usinghydrothermal synthetic method.10

As part of our long-standing interest in synthesizing andextending the dimensionality of coordination compoundswith mixed N/S coordination spheres we report herein a

dinuclear Cu(II) complexes of an acetylpyridine baseddithiocarbamate or 4-phenyl-3-thiosemicarbazide. Thermalproperties of the complexes are also discussed.

Experimental Section

Chemicals and Measurements. All chemicals are com-mercially available and were used as received without furtherpurification. The ligands, acpy-mdtcH and acpy-phtscH,were prepared as described in the literature.11,12 Elementalanalyses (CHN) were performed on a Vario EL EA-ElementarAnalyzer. Infrared spectra were recorded in the range from4000 to 400 cm−1 on a Mattson Polaris FT-IR Spectrophoto-meter using KBr pellets. Thermogravimetric (TG) and differ-ential thermal analysis (DTA) were performed on a ShimadzuDTG-60 instrument with a heating rate of 10 °C·min−1.

Preparation of [Cu2(acpy-mdtc)2(HBA)(ClO4)]·H2O (1).

To a methanol solution (20 mL) of acpy-mdtcH ligand(0.225 g, 1.00 mmol) was added Cu(ClO4)2·6H2O (0.371 g,1.00 mmol). To the resulting solution was added a methanolsolution (3 mL) of benzilic acid (0.228 g, 1.00 mmol) andtriethylamine (0.101 g, 1.00 mmol). The solution turned togreen and was refluxed for 3 h to yield green solid. The solidwas isolated by filtration and air-dried. The green filtratewas kept at room temperature to give green block crystals ingood quality for X-ray crystallography. Yield: 70% (0.329 g)based on Cu. Elemental Anal. Calcd. for C32H33N6O8S4ClCu2:C, 41.76; H, 3.61; N, 9.13; S,13.93. Found: C, 41.83; H,3.80; N, 9.02; S, 13.72%. Selected IR bands (KBr pellet,cm−1): 3467(w), 3063(w), 1590(s), 1562(m), 1486(w), 1431(s),1418(s), 1402(s), 1371(m), 1152(m), 1099(s), 1039(s), 948(w),767(w), 743(w), 702(w), 624(w).

Preparation of [Cu2(acpy-phtsc)2(HBA)]·ClO4 (2). Amethanol solution (20 mL) of 2-acetylpyridine (0.121 g,

3234 Bull. Korean Chem. Soc. 2013, Vol. 34, No. 11 Bon Kweon Koo

1.00 mmol), 4-phenyl-3-thiosemicarbazide (0.167 g, 1.00mmol), and a drop of conc-HCl was heated to reflux for 2 h.To the cooled solution Cu(ClO4)2·6H2O (0.371 g, 1.00mmol) was added with stirring. To the resulting solution wasadded a methanolic solution (3 mL) of benzilic acid (0.228g, 1.00 mmol) and triethylamine (0.101 g, 1.00 mmol). Thesolution turned immediately to green and was stirred furtherfor 3 h to yield a green solid. The solid was isolated byfiltration and air dried. The filtrate was kept at roomtemperature for a week to isolate green single crystals for x-ray crystallography. Yield 58% (0.273 g) based on Cu.Elemental Anal. Calcd. for C42H37N8O7S2ClCu2: C, 50.83;H, 3.76; N, 11.29; S, 6.46. Found: C, 51.09; H, 3.80; N,11.31; S, 6.57%. Selected IR bands (KBr pellet, cm−1):3315(m), 3056(w), 1598(m), 1575(m), 1563(m), 1541(m),1498(s), 1457(s), 1433(vs), 1392(m), 1157(m), 1110(sh),1063(s), 769(m), 748(s), 702(m), 690(m), 623(w).

X-ray Structure Determination. Single crystals of 1 and2 were obtained by the method described in the aboveprocedures. Structural measurement for the complexes wereperformed on a Bruker SMART APEX CCD diffractometerusing graphite monochromatized Mo-Kα radiation (λ =0.71073 Å) at the Korea Basic Science Institute. The struc-

tures were solved by direct method and refined on F2 by full-matrix least-squares procedures using the SHELXTL pro-grams.13 All non-hydrogen atoms were refined using aniso-tropic thermal parameters. The hydrogen atoms were includ-ed in the structure factor calculation at idealized positions byusing a riding model, but not refined. Images were createdwith the DIAMOND program.14 The crystallographic datafor complexes 1 and 2 are listed in Table 1.

Results and Discussion

The complexes of 1 and 2 were prepared from the meth-anolic solution of Cu(ClO4)2·6H2O, Schiff base ligand, andbenzilic acid. The molecular structures of ligands used areshown in Scheme 1. Our first aim in this work was to obtainthe coordination polymers which metal centers are bridgedby the benzilic acid as spacer. Unfortunately, attempts toobtain the high dimension network of the complexes byvarying stoichiometry, metal salts, and other reaction para-meters proved to be generally unsuccessful.

Table 1. Crystal Data and Structure Refinement for Complexes 1and 2

Complex 1 2

Empirical formula C32H33ClCu2N6O8S4C42H37ClCu2N8O7S2Formula weight 920.45 992.45

T (K) 200(2) 296(2)

Crystal system Triclinic Monoclinic

Space group P-1 C2/c

a (Å) 11.3283(4) 20.1936(9)

b (Å) 13.0052(5) 30.7713(15)

c (Å) 13.6178(5) 15.3860(7)

α (°) 96.3090(10) 90

β (°) 95.3530(10) 107.2780(10)

γ (°) 92.2560(10) 90

V (Å3) 1982.92(13) 9129.2(7)

Z 2 8

μ (mm−1) 1.405 1.139

F(000) 936 4064

θ (°) 1.51 to 26.00 1.25 to 26.00

Limiting indices −13 ≤ h ≤ 13, −16 ≤ k ≤ 14, −16 ≤ l ≤ 15

−18 ≤ h ≤ 24, −33 ≤ k ≤ 37, −18 ≤ l ≤ 18

Reflections collected 12494 28751

Independent reflections 7697 [R(int) = 0.0241]

8979 [R(int) = 0.0764]

Observed reflections [I ≥ 2σ(I)]

5824 4863

Goodness-of-fit on F2 1.147 1.006

Final R indices [I ≥ 2σ(I)] R1 = 0.0567, wR2 = 0.1445

R1 = 0.0514, wR2 = 0.1170

R indices (all data) R1 = 0.0858, wR2 = 0.2130

R1 = 0.1241,wR2 = 0.1698

Largest peak and hole (e Å−3) 1.303 and −1.200 0.666 and −0.590

Table 2. Selected Bond Lengths (Å) and Angles ( o ) for Complexes1 and 2

Complex 1

Cu1–N5 1.948(5) Cu1–N4 2.017(5)

Cu1–S3 2.273(2) Cu1–S1 2.797(2)

Cu1–O1 1.950(4) Cu2–N1 2.012(5)

Cu2–N2 1.952(5) Cu2–S1 2.257(2)

Cu2–O2 1.943(4) Cu2–S3 3.036(2)

Cu2–O5 2.743(8)

N5–Cu1–O1 171.8(2) N5–Cu1–N4 81.0(2)

O1–Cu1–N4 94.1(2) N5–Cu1–S3 84.9(2)

O1–Cu1–S3 98.6(1) N4–Cu1–S3 163.1(2)

O2–Cu2–N2 171.3(2) O2–Cu2–N1 93.9(2)

N2–Cu2–N1 80.5(2) O2–Cu2–S1 99.8(2)

N2–Cu2–S1 86.0(2) N1–Cu2–S1 166.2(2)

Complex 2

Cu1–N2 1.944(4) Cu1–O1 1.950(3)

Cu1–N1 2.004(4) Cu1–S1 2.257(2)

Cu1–S2 2.931(1) Cu2–N6 1.945(4)

Cu2–O2 1.945(3) Cu2–N5 1.994(4)

Cu2–S2 2.254(1) Cu2–S1 3.113(2)

N2–Cu1–O1 174.1(2) N2–Cu1–N1 80.7(2)

O1–Cu1–N1 93.5(2) N2–Cu1–S1 84.8(1)

O1–Cu1–S1 101.1(1) N1–Cu1–S1 160.4(1)

N6–Cu2–O2 171.9(2) N6–Cu2–N5 80.5(2)

O2–Cu2–N5 95.5(2) N6–Cu2–S2 85.3(1)

O2–Cu2–S2 99.8(1) N5–Cu2–S2 162.5(1)

Scheme 1. Ligands used.


Description of the Structures. The molecular structuresof 1 and 2 were determined using single crystal X-raydiffraction techniques. Selected bond parameters are listed inTable 2. The molecular structure of complex 1 containsdinuclear [Cu2(acpy-mdtc)2(HBA)(ClO4)] in which twothiolate sulfur atoms and bidentate bridging HBA− anionbridge the two copper(II) and two hemihydrate moleculecenters (Figure 1(a)). Each of the two copper atoms in[Cu2(acpy-mdtc)2(HBA)(ClO4)] has different coordinationenvironments. Cu1 adopts a five-coordinate square-pyrami-dal (τ = 0.145, the geometric parameter τ = |β − α |/60, whereβ and α are the two largest angles around the central atom; τ= 0 and 1 for the perfect square pyramidal and trigonalbipyramidal geometries, respectively.15) with a N2OS2 donor.The pyridine nitrogen (N4), the azomethine nitrogen atom(N5) and the thiolate sulfur atom (S3) together with thecarboxyl oxygen atom (O1) from HBA− ligand comprise thebasal plane of the square-pyramid whereas the thiolate sulfuratom (S1) of another ligand occupies the apical position. Themaximum displacement of them from the coordination planeis −0.059(5) Å (N4). Cu1 atom displaces 0.157(1) Å out ofthe plane. The behavior of acpy-mdtc− results in the formationof two five-membered rings around Cu1 atom. Two planes[Cu1–N4–C14–C15–N5 and Cu1–N5–N6–C17–S3] arenearly planar with mean deviation of 0.045(6) and 0.058(4)

Å, respectively, the dihedral angle between them being8.043(1)°.

The environment around Cu2 atom can be best describedas a distorted octahedral geometry in a N2O2S2 manner. Onethiolate sulfur atom (S1), one azomethine nitrogen atom(N2) and one pyridine nitrogen atom (N1) from one acpy-mdtc− ligand and one oxygen atom (O2) from HBA− ligandoccupy the basal positions, the two remaining positions inthe octahedral geometry are the axial ones which areoccupied by one thiolate sulfur atom (S3) from the anotherligand and one perchlorate oxygen atom (O5). The bridgingthiolate sulfur atoms (S1 or S3) connect the two copper atomcenters. The large difference between the two Cu–S distances(Cu1–S3 2.273(2)/Cu2-S1 2.257(2) Å) in the basal planeand Cu1–S1 2.797(2)/Cu2-S3 3.036(2) Å in the apicalposition) can be ascribed to a Jahn-Teller distortion.16 Similarthiolate sulfur-bridging has also been observed in thecomplexes [Cu(apsme)(NCS)]2 (apsme = 2-acetylpyrazineS-methyldithiocarbazate)17 and in [Cu(fpytsc)X]2 (fpytsc =anionic form of 2-pyridinecarbaldehydethiosemicarbazone;X = Cl, Br).18

The C–S bond length in acpy-mdtc− increases from thetypical of thione linkage 1.671(4) Å7 to C8−S1 1.750(6) andC17−S3 1.757(6) Å, respectively. Similarly, C8–N3/C17-N6suffer a significant decrease from the normal single bond of1.52 Å19 to 1.298(8)/1.311(8) Å, respectively. A comparisonof the N2–N3/N5–N6 distance [1.389(7)/1.374(7) Å] withthat in S-methyldithiocarbazate20 shows that the bond isshorter than a single N–N bond (1.44 Å) indicating that asignificant π-charge delocalization occurs along the C–N–N–C moiety. These changes indicate the ligand in the presentcomplex is coordinated in its deprotonated thiolate form asobserved in most complexes derived from carbamate/semi-carbazone.21 Also, the two acpy-mdtc− ligands have slightlydifferent Cu–N(pyridine) bond distances and they are longerthan the Cu–N(azomethine) distances, this may be attributedto the fact that the azomethine nitrogen is a stronger basecompared with the pyridine nitrogen.6,22

The HBA− ligand together with each thiolate sulfur atombridges two Cu(II) centers. The Cu…Cu distance is 3.265(1)Å which is smaller than 3.453 Å23 of dimeric Cu(II) complexwith only the μ-thiolate bridge of the thiosemicarbazoneligand. The Cu2–O5 bond length (2.743(8) Å) is significant-ly longer than the distance of Cu2−O2 1.943(4) Å in theHBA−, denoting the strength of the perchlorate oxygencoordination.

Two planes [C21–C26 and C27–C32] in HBA− are nearlyplanar with mean deviation of 0.003(7) and 0.005(8) Å,respectively, the dihedral angle between them being 87.3(2).The Cu1-S1-Cu2/Cu1-S3-Cu2 bridging angle is 79.71(5)/74.34(5)o, respectively, which is much shorter than the valueof 87.22o reported for the complex having the μ-phenolatoand μ-acetate bridging groups as the smallest value found inthis series of complexes.24

The dinuclear units are further stabilized and consolidatedinto a three-dimensional network by the intra- and inter-molecular hydrogen bonds (Table 3). The perchlorate ion

Figure 1. (a) Molecular structure of complex 1 with atomiclabeling. (b) The 2D layer framework of complex 1 formed by H-bond. All H-atoms in (b) have been omitted for clarity.


coordinated to the Cu(II) ion links the adjacent dinuclearunit by H-bonding (C29–H29…O6) through HBA− ligand togive 1D chain network along a-axis. The chains constructs2D framework by the H-bonds (C12-H12…O4) betweenperchlorate oxygen atoms and adjacent pyridine rings(Figure 1(b), and finally, 3D network is accomplished by H-bonds between pyridine ring and water molecule (C11-H11…O8), between perchlorate and water (O6…O9 3.43(3) Å),and between water molecules (O8…O9 2.69(3) Å).

In contrast to the complex 1, in complex 2 two Cu(II) ionsall have a five-coordinate square-pyramidal (τ = 0.23 forCu1 and 0.16 for Cu2) with a N2OS2 donor (Figure 2(a)).The perchlorate ion is not coordinated to the Cu(II) ion. Theshortest distance between Cu(II) ion and perchlorate oxygenatoms is 4.750(5) Å, compared to complex 1 (Cu2-O52.743(8) Å). There is no other significant structural differ-ences between complexes 1 and 2, except for the acpy-phtsc− ligand coordinated to Cu(II) ion. As the complex 1,acpy-phtsc− ligand acts as thiolate form. Complex 2 is alsofurther stabilized through intra- and inter-molecular H-bonds(Table 3) and it constructs 1D chains along c-axis by H-bonds between perchlorate oxygen atoms and pyridine ring(C4-H4…O4, C15-H15…O7, C16-H16…O5) or hydroxyloxygen atom from HBA− (O3-H3…O7) (Figure 2(b)). The1D chains form double chain by H-bond between perchlorateoxygen and phenyl amine nitrogen atom (N8-H8…O6). Thedouble chains are also interlinked through H-bonds (N4-H4A…O5 and C4-H4…O4) between perchlorate oxygen

atom and phenyl amine nitrogen or pyridine rings to form2D network along a-axis (Figure 2(c)).

The IR spectra of the free ligands have prominent bandsappearing at ca. 3232, 1622, and 1059 cm−1 due to ν(N-H),ν(C=N), and ν(C=S) for Schiff base ligands,25 and thebands at ca. 1700, 1347, 1246, 1173, and 1051 cm−1 due toν(C=O), δ(O-H)COH, δ(O-H)COOH, ν(C-O)COH, ν(C-O)COOHstretching modes for H2BA, respectively.26 On complexationthese all bands for the Schiff base, except the ν(N-H) band at3315 cm−1 for phenyl amine of complex 2 disappeared andthe ν(C=N) bands shifted to 1590 and 1598 cm−1, respective-ly. These results indicate NNS coordination mode of theligand in the thiol form. In addition, for all complexes theC=O stretching and O-H bending of the carboxylic acidgroup for the H2BA disappeared and the O-H bendings ofCOH (1371 and 1392 cm−1 for 1 and 2, respectively) and theC–O stretching bands (1152, 1039 for 1, and 1157, 1110cm−1 for 2) of the COH and COO− are observed, respectively,supporting that HBA− is coordinated to Cu(II) throughcarboxyl group. The IR spectra also show the bands at 3461cm−1 (for 1) corresponding to the vibration absorption oflattice water.27 The intense bands at 1099 (for 1) and 1063cm−1 (for 2) can be assigned to the ClO4 group.6,28

The TG-DTA analysis was performed to examine the

Table 3. Parameters (Å, o) for Hydrogen-Bonding Interaction ofComplex 1 and 2

Donor–H···A D–H H···A D–A D–H···A

Complex 1

O3–H3A···O7 0.84 2.13 2.90(1) 153.4

C1–H1···O2 0.95 2.59 3.10(1) 113.4

C7–H7A···N3 0.98 2.42 2.82(1) 104.2

C10–H10···O1 0.95 2.62 3.12(1) 113.3

C11–H11···O8i 0.95 2.66 3.59(2) 168.1

C12–H12···O4i 0.95 2.41 3.36(1) 171.5

C16–H16A···N6 0.98 2.39 2.82(1) 105.7

C28–H28···O1 0.95 2.63 3.26(1) 123.8

C29–H29···O6ii 0.95 2.57 3.47(1) 158.9

Complex 2

O3–H3A···O7 0.82 2.18 2.91(1) 147.9

N4–H4A···O5iii 0.86 2.24 3.09(1) 174.5

N8–H8···O6iv 0.86 2.20 3.04(1) 165.6

C1–H1···O1 0.93 2.62 3.10(1) 112.4

C4–H4···O4v 0.93 2.51 3.09(1) 120.3

C7–H7A···N3 0.96 2.47 2.79(1) 99.3

C10–H10··N3 0.93 2.30 2.88(1) 119.7

C15–H15···O7 0.93 2.44 3.27(1) 149.2

C16–H16···O5 0.93 2.61 3.33(1) 135.0

C21–H21A···N7 0.96 2.40 2.79(1) 103.9

C24–H24···N7 0.93 2.29 2.87(1) 119.8

Symmetry codes: i) 1−x, 1−y, 2−z; ii) 1−x, 1−y, 1−z; iii) 0.5−x, 0.5−y, 1−z; iv) 2−x, 1−y, 1−z; v) 1+x, 1−y, −0.5+z.

Figure 2. (a) Molecular structure of complex 2 with atomiclabeling. (b) 1D-chain networks of cmplex 2 along with c-axis. (c)2D layer in complex 2. All H atoms in (b) and (c) have beenomitted for clarity.


stability and decomposition pattern of compounds. Theirthermal decomposition behaviors are similar to each other(Figure 3). For 1, the weight loss of 2.05% from 37 to 70 oCdisplays the release of 1 mole of lattice water molecule performula unit (calc. 1.96%). The release of lattice water wasaccompanied by endothermic effect on the DTA curve observedat 65 oC. The solvent-free species, in general, decompose intwo steps. The first weight loss (25.74%) takes place in thenarrow temperature range of 190-220 °C with an exothermicpeak at 214 °C, followed by the weight loss of 54.66% in abroader temperature range (220-800 °C). The total weightloss of 80.4% corresponds to the loss of all organic compo-nents and perchlorate ion (calc. 84.24%). However, theobserved weight loss is less than the calculated value. Thisindicates the decomposition ends above 800 °C. Complex 2is stable up to 220 °C and then it undergoes a rapid weightloss of 30.95% in the temperature range of 220-252 oC withan exothermic peak at 246 oC. The second gradual weightloss of 54.74% take place up to 782 °C. This weight loss wasaccompanied by exothermic effect on the DTA curve withmaximun at 456 and 566 °C, respectively. The total weightloss of 85.69% corresponds to the loss of all organic compo-nents and perchlorate anion (calc. 87.2%). The remainingresidue of 14.7% presumably corresponds to the formationof CuO (calc. 16.03%).

Conclusion

Two new dinuclear copperl(II) complexes, [Cu2(acpy-mdtc)2(HBA)(ClO4)](H2O) (1) and [Cu2(acpy-phTsc)2(HBA)(ClO4)] (2) have been synthesized and characterized byelemental analysis, infrared spectroscopy, thermogravimetric

analysis, and single crystal X-ray diffraction. In each com-plex, two copper(II) ions are bridged by HBA− anion togetherwith thiolate sulfur atom from Schiff base ligand. Also, thestructures are further extended into supramolecular frame-work by hydrogen bonds. It is noteworthy that two copper(II)centers are connected through benzilic acid. To our know-ledge, these structures are not common.

Supplementary Materials. Crystallographic data in CIFformat have been deposited with the Cambridge StructuralDatabase CCDC-944764(1) and -944765(2), respectively.These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CambridgeCrystallographic Data Centre, 12, Union Road, CambridgeCB2 1EZ, UK; Fax: +44-1223/336-033; E-mail: [email protected]).

Acknowledgments. This research was supported by theResearch Grants of Catholic University of Daegu in 2012.The author also acknowledges the Korea Basic ScienceInstitute for providing the crystal structure results.

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synthesis and crystal structures of copper(ii) …...was kept at room temperature to give green...

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