Structures and Fluorescent and Magnetic Behaviors of Newly Synthesized Ni II and Cu II Coordination Compounds

Newly designed three trinuclear coordination compounds [Ni3(L)2(OAc)2(CH3OH)2] (1), [Ni3(L)2(OAc)2(CH3CH2CH2OH)2]·2CH3CH2CH2OH (2) and [Ni3(L)2(OAc)2(DMF)2]·1.71DMF (3) and one mononuclear coordination compound [Cu(L)2] (4) have been synthesized by H2L and nickel(II) and copper(II) acetate hydrates in different solvents. Single-crystal X-ray structure determinations revealed that the coordination compounds 1–3 have analogous molecular structures. The coordination compounds 1, 2, and 3 were affected by the coordinated methanol, n-propanol, and N,N-dimethylformamide molecules, respectively, and the various coordinated solvent molecules give rise to the formation of the representive solvent-induced NiII coordination compounds. All the NiII atoms are six-coordinated with geometries of slightly distorted octahedron. Obviously, in the coordination compound 4, the expected salamo-like monoor tri-nuclear CuII coordination compound has not been obtained, but a new CuII coordination compound [Cu(L)2] has been gained. The Cu1 atom is four-coordinated and possesses a geometry of slightly distorted planar quadrilateral. Furthermore, the fluorescence properties of coordination compounds 1–4 and magnetic behavior of coordination compound 1 were investigated.

Herein, following our previous research on the syntheses, crystal structures, and fluorescent and antimicrobial properties of Co II and Ni II coordination compounds with salamo-like bisoxime ligand H 2 L 1 (4,4 -dichloro-2,2 -[(propane-1,3-diyldioxy)bis(nitrilomethylidyne)]diphenol) [49,50], the alkoxy chain is increasing from two to three on the ligand H 2 L 1 , which can give rise to better flexibility and coordination ability.

Materials and Methods
5-Chlorosalicylaldehyde (98%) was obtained from Alfa Aesar and used without further purification.1,3-Dibromoprophane and other reagents and solvents were analytical grade reagents from Tianjin Chemical Reagent Factory (Tianjin, China).
C, H, and N analyses were performed using a GmbH VariuoEL V3.00 automatic elemental analysis instrument (Elementar, Berlin, Germany).Elemental analyses for Ni II or Cu II were measured with an IRIS ER/S-WP-1 ICP atomic emission spectrometer (Elementar, Berlin, Germany).Melting points were gained via a X 4 microscopic melting point apparatus made by Beijing Taike Instrument Company Limited and were uncorrected.IR spectra were measured on a Vertex 70 FT-IR spectrophotometer (Bruker, Billerica, MA, USA), with samples prepared as KBr (400-4000 cm −1 ) pellets.UV-vis absorption spectra were measured on a Shimadzu UV-3900 spectrometer (Shimadzu, Tokyo, Japan). 1 H NMR spectra were performed by German Bruker AVANCE DRX-400/600 spectroscopy (Bruker AVANCE, Billerica, MA, USA).X-ray single crystal structure determinations for the coordination compounds 1, 2, 3, and 4 were carried out on a Bruker Smart Apex CCD and SuperNova, Dual, Cu at zero, Eos four-circle diffractometers.Magnetic susceptibility data were collected on the powdered samples of the coordination compound 1 using a Quantum Design (San Diego, CA, USA) model MPMS XL7 SQUID magnetometer.Magnetic susceptibility measurements were performed at 1000 Oe in the 2-300 K temperature range.

Syntheses of the Coordination Compounds 1-4
Tri-and mono-nuclear coordination compounds 1-4 were synthesized by the reaction of H 2 L 1 with Ni(OAc)  A methanol solution (2 mL) of Ni(OAc) 2 •4H 2 O (5.07 mg, 0.015 mmol) was added dropwise to H 2 L 1 (3.83 mg, 0.010 mmol) in acetone (2 mL) and stirred for 30 min.The mixture was filtered, and the filtrate was allowed to stand at room temperature for ca.four weeks on slow evaporation of the solution in the dark.Several green block-like single crystals suitable for X-ray crystallographic analysis were collected and then filtrated and washed with n-hexane.The coordination compounds 2, 3, and 4 were prepared by an analogous procedure as for the coordination compound 1.

Crystal Structure Determinations of the Coordination Compounds 1-4
The crystal diffractometers with a monochromatic beam of Mo Kα radiation (λ = 0.71073 Å) produced using Graphite monochromator from a sealed Mo X-ray tube was used for gaining crystal data for the coordination compounds 1-4 at 293(2), 294.39 (10) 294.29 (10), and 293(2) K, respectively.The program(s) used to solve structure were SHELXS-2008 (Sheldrick, 2008) [52]; the program(s) used to refine structure were SHELXL-2008 (Sheldrick, 2008) [53].The crystal data and experimental parameters relevant to the structure determinations are listed in Table 1.Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre (1588647, 1588644, 1588646, and 1588645 for the coordination compounds 1, 2, 3, and 4) and can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

IR Spectra
The FT-IR spectral results of H 2 L 1 and its corresponding coordination compounds 1-4 exhibited different bands in the 4000-400 cm −1 region (Figure 1).
A typical C=N stretching band of H 2 L 1 appeared at 1606 cm −1 , and that of the coordination compounds 1-4 appeared at 1612, 1616, 1629, and 1623 cm −1 , respectively [54,55].The C=N stretching bands are shifted to high wavenumber, exhibiting that the Ni II and Cu II atoms are bonded by oxime nitrogen atoms of deprotonated (L 2 ) 2− and (L 1 ) − moieties.Hence, conclusion could be given that the ligands H 2 L 1 and H 2 L 2 have bonded with Ni II and Cu II atoms [56,57].The free ligand H 2 L 1 exhibited a Ar-O stretching band at 1263 cm −1 , while the Ar-O stretching bands of the coordination compounds 1-4 appeared at 1197, 1195, 1197, and 1198 cm −1 , respectively.The Ar-O stretching bands are waved to low wavenumber, which could be presence of the coordination of phenolic oxygen to Ni II and Cu II atoms [58,59].The free ligand H 2 L 1 showed a desired absorption band at 3101 cm −1 , which as an evidence for the presence of phenolic OH groups.The expected OH stretching absorption bands in the coordination compounds 1 and 2 are observed at 3230 and 3354 cm −1 , exhibiting the existence of coordinated methanol and n-propanol molecules, respectively [60].Furthermore, a N-H stretching band is observed at 3353 cm −1 in the coordination compound 4.

UV-vis Absorption Spectra
UV-vis absorption spectral results of H 2 L 1 and its coordination compounds 1-4 were determined in 2.5 × 10 −5 M chloroform solution and are shown in Figure 2. The absorption spectrum of H 2 L 1 showed three absorption peaks at ca. 223, 267, and 325 nm, the peaks at 223 and 267 nm can be assigned to the π-π* transitions of the phenyl rings, and the peak at 325 nm can be attributed to the π-π* transitions of the oxime group [61,62].Compared to H 2 L 1 , with the appearance of the first peaks at about 233 nm are observed in the coordination compounds 1-4, These absorption peaks are shifted bathochromically, exhibiting coordination of the (L 1 ) 2− and (L 2 ) -moieties with Ni II and Cu II atoms.The absorption peak at ca. 267 nm is absent in the coordination compounds 1-4.Meanwhile, new absorption peaks are observed at about 345-367 nm in the coordination compounds 1-4 that might be owing to L→M charge-transfer transitions, which are characteristic of the transition metallic coordination compounds with N 2 O 2 coordination spheres [63][64][65].

D-H• • • A d(D-H) d(H-A) d(D-A) ∠ ∠ ∠D-X-A
Coordination compound 1 O5-H5  The coordination compound 1 belongs to the triclinic system, with space group P − 1, which includes of three Ni II atoms, two completely deprotonated (L 1 ) 2− units, two µ 2 -acetato ligands, and two coordinated methanol molecules.The terminal Ni II atoms (Ni1 or Ni1 #1 ) are six-coordinated by two oxime nitrogen atoms (N1, N2 or N1 #1 , N2 #1 ) and two phenoxo oxygen atoms (O5, O6 or O5 #1 , O6 #1 ), the four atoms mentioned above are all from one deprotonated (L 1 ) 2− units, one oxygen atom (O4 or O4 #1 ) from the µ 2 -acetato ligands and one oxygen atom (O7 or O7 #1 ) from the coordinated methanol molecules (Figure 3).The central Ni II atom (Ni2) is also six-coordinated via four phenoxo oxygen atoms from two (L 1 ) 2− units, and two oxygen atoms from µ 2 -acetato ligands (O3 or O3 #1 ), The Ni1 and Ni2 atoms (Ni1 #1 and Ni2) are connected through µ 2 -acetato ligands in an usual M-O-C-O-M fashion [66,67].The coordination sphere around all the Ni II atoms are best described as slightly distorted octahedral geometries [68,69].For the synthesis of Ni II coordination compounds must be under dark conditions, the conditions are different from the previously reported Ni II coordination compounds [50], the synthesis conditions are different, and the structures of resulting coordination compounds will be changed.The molecular structures and atom numberings of the coordination compounds 2 and 3 are shown in Figures 4 and 5, respectively.Contributions to scattering these highly disordered solvent molecules were removed using the SQUEEZE routine of PLATON.The structure of the coordination compounds 1 was then refined again using the data generated.In the coordination compound 1, there are six pairs of the intramolecular C8-H8B and C16-H16• • • O7 hydrogen bonds are formed (Figure 6) [68,69], and the weak hydrogen bonds existing in the coordination compound 1 has been described in graph sets (Figure 7) [70].In the coordination compound 2, there are three intramolecular hydrogen bonds [68,69] (Figure 8).C8-H8A• • • O5, O6-H6• • • O7, and O7-H7A• • • O8 hydrogen bonds are formed.The donor (C8-H8A) from the (L 1 ) 2-unit forms hydrogen bond with oxygen atom (O5) of the µ 2 -acetato ligand as hydrogen bond receptor.The donors (O6-H6 and O7-H7A) from coordinated n-propanol molecule and crystalline n-propanol molecule form hydrogen bonds with oxygen atoms (O7 and O8) of crystalline n-propanol molecule and the µ 2 -acetato ligand as hydrogen bond receptors.The weak hydrogen bonds existing in the coordination compound 2 have been described in graph sets (Figure 9) [70].

Crystal Structure of the Coordination Compound 4
Molecular structure of the coordination compound 4 is shown in Figure 14, The structure revealed that the coordination compound 4 crystallizes in the monoclinic system, space group C 2/c.Clearly, the expected mono-or tri-nuclear Cu II coordination compound has not been gained.Catalysis of Cu II ions results in undesigned cleavages of two C-C and two N-O bonds in H 2 L 1 , and a new coordination compound [Cu(L 2 ) 2 ] has been obtained.In C=N bond, the electronegativity of N atom is higher than C atom, so the electron cloud density of C atom is lower.In addition, due to the high electronegativity of Cl atom, the electron cloud density of C atom in C=N bond will be further reduced in this conjugated system, and it is positively charged.If the electronegativity of O atom in the O-C bond is high, it will attack the C atom in C=N bond and form the new ligand H 2 L 2 .Finally, a new mononuclear Cu II coordination compound has been gained.This phenomenon is not similar to the cleavage of Cu II coordination compounds reported previously [72][73][74].In the coordination compound 4, Cu1 atom is four-coordinated and possesses a geometry of slightly distorted planar quadrilateral with two nitrogen atoms of imino and the two phenolic oxygen atoms of the (L 2 ) − units [72][73][74][75].These angles of N1-Cu1-N1 #4 and O1-Cu1-O1 #4 are all 180.0 • .There are two pairs of the intramolecular N1-H1• • • O1 and C5-H5• • • O2 hydrogen bonds are formed (Figure 15) [72][73][74][75].The weak hydrogen bonds existing in the coordination compound 4 have been described in graph sets (Figure 16) [70].As illustrated in Figure 17, the coordination compound 4 is linked by a pair of inter-molecular hydrogen bond interactions forming a 1D hydrogen bonded chain.
The ligand H 2 L 1 demonstrates a strong emission peak at ca. 507 nm upon excitation at 324 nm, which could be attributed to the intraligand π-π* transition [72,73].The coordination compounds 1, 2, 3, and 4 demonstrate weak photoluminescence with maximum emissions at ca. 519, 523, 522 and 521 nm upon excitation at 378 nm (based on global maxima determined from three-dimensional fluorescence spectra), respectively, and the peaks are bathochromically shifted, which should be attributed to ligand-to-metal charge transfer (LMCT) [74,75].Compared with H 2 L 1 , the emission intensities of the coordination compounds 1, 2, 3, and 4 are reduced, which indicate that the Ni II and Cu II ions have the behaviors of fluorescent quenching, and the quenching of Cu II ion is more obvious than that of Ni II ion.

Magnetic Behavior
The magnetic behaviors of the coordination compound 1 were measured and discussed individually represented the coordination compounds 2 and 3, owing to these coordination compounds have analogous structures, there are little difference in magnetic behavior.The temperature dependence of magnetic susceptibilities of the coordination compound 1 is depicted in Figure 19.
The χ M T value at 300 K for the coordination compound 1 is 3.28 cm 3 K mol −1 , which is higher than the value of 3 cm 3 K mol −1 expected for three Ni II (S = 1) magnetically isolated ions [77][78][79].Upon lowering the temperature, the χ M T value of the coordination compound 1 gradually decreases to reach a minimum value of 0.60 cm 3 K mol −1 at 2 K, which shows that a weak antiferromagnetic interaction exists in such a coordination compound [80].Moreover, the magnetic susceptibilities (1/χ M ) obey the Curie-Weiss law (χ M = C/(T − θ)) in the 2-300 K temperature range, giving a negative Weiss constant θ = -14.220K and C = 3.466 cm 3 K mol −1 (Figure 19, inset) and confirming the antiferromagnetic interaction presented again by the coordination compound 1 [80].

Conclusions
Different solvent molecules were introduced, three new homotrinuclear Ni II coordination compounds 1, 2, and 3 with a salamo-like bisoxime ligand H 2 L 1 were designed and synthesized.Catalysis of Cu II ions results in undesigned cleavage of two C-C and two N-O bonds in H 2 L 1 .A new mono-nuclear Cu II coordination compound [Cu(L 2 ) 2 ] has been obtained.Single-crystal X-ray crystal structure analyses revealed that the coordination compounds 1-3 have analogous molecular structures and were affected by the coordinated methanol, n-propanol, and N,N-dimethylformamide molecules, respectively, and the virous solvent molecules observed give rise to the formation of the characteristic solvent-induced Ni II coordination compounds.All the Ni II atoms are six-coordinated with geometries of slightly distorted octahedron.Obviously, the expected mono-or tri-nuclear Cu II coordination compound has not been gained, but a new Cu II coordination compound [Cu(L 2 ) 2 ] has been obtained.In coordination compound 4, the Cu1 atom is four-coordinated with a geometry of slightly distorted planar quadrilateral.The fluorescent behavior of H 2 L 1 and the coordination compounds 1-4 were studied.Compared with H 2 L 1 , the emission intensities of the coordination compounds 1-4 decreases clearly, which exhibits that the Ni II and Cu II ions bear the qualities of fluorescence quenching, and the quenching of Cu II ion is more obvious than that of Ni II ion.In addition, the magnetic behavior showed that there is a relatively weak antiferromagnetic interaction in coordination compound 1.

Figure 3 .
Figure 3. (a) Molecular structure and atom numberings of the coordination compound 1 with 30% probability displacement ellipsoids; (b) Coordination polyhedrons for Ni II atoms.

Figure 4 .
Figure 4. (a) Molecular structure and atom numberings of the coordination compound 2 with 30% probability displacement ellipsoids; (b) Coordination polyhedrons for Ni II atoms.

Figure 5 .
Figure 5. (a) Molecular structure and atom numberings of the coordination compound 3 with 30% probability displacement ellipsoids; (b) Coordination polyhedrons for Ni II atoms.

Figure 6 .
Figure 6.View of the intramolecular hydrogen bond interactions of the coordination compound 1.

Figure 7 .
Figure 7. (a) Graph set assignments for coordination compound 1; (b) Partial enlarged drawing of hydrogen bonds.

Figure 8 .
Figure 8.View of the intramolecular hydrogen bond interactions of the coordination compound 2.

Figure 9 .
Figure 9. (a) Graph set assignments for coordination compound 2; (b) Partial enlarged drawing of hydrogen bonds.

Figure 10 .
Figure 10.View of the intramolecular hydrogen bond interactions of the coordination compound 3.

Figure 11 .
Figure 11.(a) Graph set assignments for coordination compound 3; (b) Partial enlarged drawing of hydrogen bonds.

Figure 12 .
Figure 12.View of an infinite 1D hydrogen bonded chain of the coordination compound 3.

Figure 14 .
Figure 14.(a) Molecular structure and atom numberings of the coordination compound 4 with 30% probability displacement ellipsoids; (b) Coordination polyhedrons for Cu II atom.

Figure 15 .
Figure 15.View of the intramolecular hydrogen bond interactions of the coordination compound 4.

Figure 16 .
Figure 16.(a) Graph set assignments for coordination compound 4; (b) Partial enlarged drawing of hydrogen bonds.

Figure 17 .
Figure 17.View of an infinite 1D hydrogen bonded chain motif of the coordination compound 4 along the b axis.

Figure 19 .
Figure 19.Plots of χ M T vs. T for the coordination compound 1 between 2 to 300 K. Inset: Temperature dependence of χ M −1 .The red solid lines show the best fitting results.