Solvent-Induced Unsymmetric Salamo-Like Trinuclear Ni II Complexes : Syntheses , Crystal Structures , Fluorescent and Magnetic Properties

Solvent-induced trinuclear NiII complexes, [{Ni(L)(MeOH)}2(OAc)2Ni]·2MeOH (1), [{Ni(L) (EtOH)}2(OAc)2Ni]·2H2O (2), [{Ni(L)(n-PrOH)}2(OAc)2Ni]·2H2O (3) and [{Ni(L)(i-PrOH)}2(OAc)2Ni] (4), have been prepared with an unsymmetric Salamo-like ligand H2L, and characterized via X-ray crystallography, FT-IR, UV-Vis and fluorescence spectra. In complexes 1, 2, 3 and 4, there are two ligand (L)2− moieties, two acetato ligands, two coordinated methanol, ethanol, n-propanol or i-propanol molecules, respectively, as well as other crystallizing solvent molecules. Two acetato ligands coordinated to the three NiII ions via usual Ni-O-C-O-Ni bridges, and four μ-phenoxo oxygen atoms coming from two [NiL(solvent)] units coordinate to the central NiII ions. Although different solvents are induced in the complexes, all the NiII ions are six-coordinated and adopt geometries of distorted octahedron. Magnetic measurements were performed on complex 2, an intramolecular antiferromagnetic interaction was observed between NiII ions and a simulation of the experimental data gives J = −2.96 cm−1 and g = 2.30.


Material and General Methods
All chemicals were used without further purification, and were of analytical reagent grades.C, H, and N analyses were acquired by an Elementar GmbH VarioEL V3.00 automatic elemental analysis instrument (Hanau, Germany).Elemental analyses for Ni II ions were detected by an IRIS ER/S-WP-1 ICP atomic emission spectrometer (Elementar, Berlin, Germany).Keep accounts of the IR spectra data using the VERTEX70 FT-IR spectrophotometer (Bruker, Karlsruhe, Germany), and the samples were prepared as KBr (500-4000 cm −1 ) pellets. 1 H NMR spectra were recorded using a Mercury-400BB spectrometer (Varian, Palo Alto, CA, USA) at 400 MHz.Melting points were measured via an X 4 microscopic melting point apparatus produced by Beijing Taike Instrument Limited Company (Beijing, China) and were uncorrected.Fluorescent spectra were performed on a LS-55 fluorescence photometer (Perkin-Elmer, Norwalk, America).X-ray single crystal structure determinations were performed on a Bruker Smart Apex CCD diffractometer (Karlsruhe, Germany).Magnetic susceptibility data were measured on powdered samples of complex 2 using a Quantum Design model MPMS XL7 SQUID magnetometer (Quantum Design, San Diego, CA, America).Magnetic susceptibility measurements were performed at 1000 Oe in the 2-300 K temperature range.Complex 2: Complex 2 was prepared by a method similar to that of complex 1 except substituting methanol with ethanol and acetonitrile with acetone.The color of the mixture continued to be pale-green but cooled off immediately and clear light green, block-like crystals were gained after two weeks following the solvent was slowly evaporated.Anal.

X-ray Crystallography
Because of the similar structures of the complexes 1-4, only the details of the data collection and refinements of Ni II complex 1 are presented in Table 1 (That of 2, 3 and 4 are listed in Table S1).A single crystal of Ni II complexes 1, 2, 3 and 4 was put on a Bruker Smart 1000 CCD area detector.The reflections were collected by a graphite monochromated Mo Ka radiation (λ = 0.71073 Å) at 294K for Ni II complexes 1, 2 and 4, and that of Ni II complex 3 was collected by a graphite monochromated Cu Ka radiation (λ = 1.54184Å) at 293 K.The structures were solved by the program SHELXL-97 and Fourier difference techniques, and were refined by the full-matrix least-squares method on F 2 .All hydrogen atoms were added in calculated positions.The non-hydrogen atoms were refined anisotropically.Supplementary crystallographic data for this paper have been deposited at Cambridge Crystallographic Data Centre (1480197, 1480194, 1480196 and 1480195 for complexes 1, 2, 3 and 4) and can be gained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html.

Results and Discussion
H 2 L is white powder, stable in air and soluble in some organic solvents such as ethanol, methanol, CH 2 Cl 2 , CHCl 3 , acetonitrile, acetone, DMF and THF, but insoluble in Et 2 O, n-hexane.Ni II complexes 1, 2, 3 and 4 are all soluble in CHCl 3 and DMF, but not soluble in n-hexane, Et 2 O, acetone and acetonitrile.

Crystal Structure Descriptions of Ni II Complexes 1, 2, 3 and 4
Selected bond lengths and angles for Ni II complexes 1, 2, 3 and 4 are listed in Table 2. Crystal structures and atom numberings of complexes 1, 2, 3 and 4 are depicted in Figures 1-4, respectively.
X-ray crystal structure analyses revealed that the Ni II complexes 1, 2, 3 and 4 take on similar crystal structures, which all are symmetric trinuclear structures.The crystals of Ni II complexes are solved as tetragonal space group I4 1 /a (1), triclinic space group P-1 (2), monoclinic space group C2/c (3) and monoclinic space group P2 1 /n (4), respectively.All the Ni II complexes consist of three Ni II ions, two (L) 2− moieties, two µ-acetato ligands, and two coordinated methanol, ethanol, n-propanol or i-propanol molecules in Ni II complexes 1, 2, 3 and 4, respectively, as well as crystalizing solvent molecules (two MeOH in 1 and two H 2 O in 2 and 3, respectively).As depicted in

IR Spectra
The IR spectra of H 2 L and its corresponding Ni II complexes 1, 2, 3 and 4 exhibited several distinguishable resonances in the region of 400-4000 cm −1 and are depicted in Table 4 and Figure S1.The free ligand H 2 L showed a broad typical O-H band at 3165 cm −1 .The vanishing of these bands in the FT-IR spectra of Ni II complexes 1, 2, 3 and 4 indicated that the O-H groups of H 2 L have been deprotonated and coordinated to the Ni II ions [89].The free ligand H 2 L exhibited a typical C=N stretching band at 1632 cm −1 , which is moved to 1608, 1608, 1612 and 1609 cm −1 in Ni II complexes 1, 2, 3 and 4, respectively, exhibiting a weak π-accepting ability of the deprotonated (L) 2− ligand [82].The strong Ar-O stretching band within the 1263-1213 cm −1 range always appears for the Salen-like ligands.This band occurred at 1260 cm −1 for H 2 L, and at 1219, 1217, 1219 and 1215 cm −1 for Ni II complexes 1, 2, 3 and 4, respectively.The Ar-O stretching band is waved to lower frequency, exhibiting that Ni-O (phenolic) bonds are formed in Ni II complexes 1, 2, 3 and 4.
The far-infrared spectra of Ni II complexes 1, 2, 3 and 4 are gained in the region of 500-100 cm −1 in order to identify Ni-N and Ni-O bonds.The spectrum showed ν(Ni-N) and ν(Ni-O) frequencies of Ni II complex 1 at 588 and 420 cm −1 , Ni II complex 2 at 592 and 410 cm −1 , Ni II complex 3 at 592 and 409 cm −1 , and Ni II complex 4 at 590 and 419 cm −1 , indicating that the Ni II ions are bonded by N 2 O 2 donor atoms of the (L) 2− moieties.Hence, it gives evidence for the coordination of H 2 L with the Ni II ions.These assignments are consistent with the literature frequency values [90].

UV-Vis Absorption Spectra
The UV-Vis spectra of H 2 L and its Ni II complexes 1, 2, 3 and 4 were determined in 1.0 × 10 −5 mol• L −1 ethanol solution (Figure 9).The spectrum of H 2 L includes two relatively intense bands at 274 and 309 nm, attributed to the π-π* transitions of the benzene rings in the salicylaldehyde and oxime groups [91].Upon coordination of H 2 L, the band at ca. 309 nm disappears, indicating that the oxime nitrogen atoms are coordinated to the Ni II ions.The intraligand π-π* transition of the benzene rings of salicylaldehyde is slightly waved, and appears at 278, 276, 277 and 279.5 nm in Ni II complexes 1, 2, 3 and 4, respectively.The new bands observed at 349, 348, 350 and 349 nm for Ni II complexes 1, 2, 3 and 4, respectively, are attributed to typical bands that are a result of mixing of L → M charge-transfer transitions with the d-d transitions of 3 A 2g → 3 T 1g (P) for octahedral Ni(II) ions, which are characteristic of transition metal N 2 O 2 complexes and that of the spin-allowed d-d transition 3 A 2g → 3 T 1g (F) and the 3 A 2g → 3 T 2g (F) transition which usually appear after 600 nm.

Fluorescence Properties
The emission spectra of H 2 L and its Ni II complexes in dilute ethanol solution (c = 1.0 × 10 −5 mol• L −1 ) at room temperature are depicted in Figure 10.The ligand H 2 L has no intense photoluminescence upon excitation at 350 nm.Ni II complexes 1, 2, 3 and 4 showed photoluminescence with maximum emissions at ca. 407.9, 411.8, 415, 392.9 nm upon excitation at 350 nm, respectively.Ni II complexes 1, 2, 3 and 4 showed intense photoluminescence which includes that fluorescence characteristics that have been affected by the introductions of the Ni II ions.

Magnetic Properties
Because of the similar structure of the composites 1-4, there is difference in their magnetic properties.Only the magnetic property of complex 2 is discussed.Magnetic analysis of complex 2 was measured under the applied magnetic field of 1000 Oe, and magnetic susceptibility data of complex 2 were measured within the 2-300 K temperature range.Samples were measured with the single crystals of complex 2. The temperature dependence of magnetic susceptibilities of Ni II complex 2 is depicted in Figure 11, as a plot of χ M T against T. The χ M T value of 3.88 cm 3 •K•mol −1 at 300 K for trinuclear Ni II complex 2 is slightly higher than the value of 3.00 cm 3 •K•mol −1 expected for three Ni II (3d 8 , S = 1) isolated ions.When decreasing the temperature, the χ M T plot decreases very slowly till about 125 K and then begins to drop down sharply and reaches a minimum value of 0.58 cm 3 •K•mol −1 at 2 K, suggesting that the dominant antiferromagnetic interactions are propagated between Ni II ions.There is no peak in the χ M vs. T plot.Therefore, no long-range magnetic ordering is found.)] to treat exchange interaction (J) between Ni II ions, so the resulting magnetic susceptibility equation is: x = e −J/kT J is the intramolecular exchange integral between Ni II ions, and the other symbols have their usual meanings.The best fitting for the experimental data of complex 2 gives J = −2.96cm −1 , g = 2.30 and the agreement factor R = ∑[ (χ M T) obsd − (χ M T) calc ] 2 / ∑ (χ M T) 2 obsd is 2.51 × 10 −4 .The small negative value of J also indicates that a weak antiferromagnetic interaction is operative between the Ni II ions.The antiferromagnetic parameters of complex 2 are close to other Ni II complex [92].

Solvent Effect
The structures revealed that the structural features of complexes 1, 2, 3 and 4 are found to be similar except for the distinction of coordinated and/or crystallizing solvent molecules.Most notably, the solvent has an effect on the four complexes and causes their slight distinctions in the structures.The influence of solvent effect is obviously exhibited in bond distances and angles for complexes 1, 2, 3 and 4 (Table 2).Notably, the bond lengths from the O6 atom of coordinated methanol, ethanol, n-propanol or i-propanol molecules to the terminal Ni II  is 2.51 × 10 −4 .The small negative value of J also indicates that a weak antiferromagnetic interaction is operative between the Ni II ions.The antiferromagnetic parameters of complex 2 are close to other Ni II complex [92].

Solvent Effect
The structures revealed that the structural features of complexes 1, 2, 3 and 4 are found to be similar except for the distinction of coordinated and/or crystallizing solvent molecules.Most notably, the solvent has an effect on the four complexes and causes their slight distinctions in the structures.The influence of solvent effect is obviously exhibited in bond distances and angles for complexes 1, 2, 3 and 4 (Table 2).Notably, the bond lengths from the O6 atom of coordinated methanol, ethanol, n-propanol or i-propanol molecules to the terminal Ni II ions in complexes 1, 2, 3 and 4 are 2.121(3), 2.126(3), 2.118(3) and 2.352(4) Ǻ, respectively, which give a basic regular elongation except npropanol when the steric hindrance successively becomes larger from methanol, ethanol, n-propanol to i-propanol.Furthermore, in complexes 1, 2, 3 and 4, two imino nitrogen and two phenolic oxygen atoms form the square base with the Ni-N bonds being slightly longer than the corresponding Ni-O bonds.The elongation of the Ni-N bonds is probably owing to the weaker coordination abilities of the nitrogen atoms than of phenolic oxygen atoms.Meanwhile, the angle N1-Ni1-N2 (106.18 (16), 99.88 (17), 106.24 (12) and 98.0(2)°) and O1-Ni1-O4 (79.56(13), 81.13 (11),79.16(9)and 85.73(12)°) formed two imino nitrogen with the terminal Ni II ions as well as two phenolic oxygen atoms with the terminal Ni II ions in Ni II complexes 1, 2, 3 and 4 are all different, respectively.That is to say, there is a difference between the degree of distortion of the octahedral geometries of the terminal Ni II ions in complexes 1, 2, 3 and 4 because of the solvent effect.In addition, in spite of the supra-molecular structures, complexes 1, 2, 3 and 4 also have a similar 0-dimensional structure, which linked by different intra-molecular and/or inter-molecular hydrogen bond interactions.The solvent effects also lead to the changes in UV-Vis, IR spectra, and fluorescence properties in complexes 1, 2, 3 and 4.

Conclusions
Four new synthesized Ni II complexes 1, 2, 3 and 4 have been designed and characterized structurally.X-ray crystal structure determinations revealed that the structural features of complexes 1, 2, 3 and 4 are similar except for the differences in the coordinated and/or crystallizing solvent molecules.They are all tri-nuclear structures with three Ni II ions, two ligand (L) 2− moieties, two acetate ligands and two coordinated solvent molecules.Although different solvent molecules are induced in the four Ni II complexes, it is worth noting that all of the Ni II ions in complexes 1, 2, 3 and 4 are six-coordinated and possess slightly distorted octahedrons, with different distortion degrees of the octahedral geometries around the Ni II ions.Interestingly, the existence of the solvent effect in complexes 1, 2, 3 and 4 may be responsible for the slight differences in their crystal and fluorescence properties.In addition, magnetic susceptibility research performed for complex 2 indicated that the magnetic exchange between the Ni II ions exhibited antiferromagnetic interactions.

Supplementary Materials:
The following are available online at www.mdpi.com/xxx/s1, Figure S1: The IR spectra of H2L and its corresponding Ni II complexes 1, 2, 3 and 4, Table S1: Crystal data and structure refinements for Ni II complexes 2, 3 and 4.

Figure 5 .
Figure 5. Intra-and inter-molecular hydrogen bonds of Ni II complex 1.

Figure 6 .
Figure 6.Intra-and inter-molecular hydrogen bonds of Ni II complex 2.

Figure 8 .
Figure 8. Intramolecular hydrogen bonds of the Ni II complex 4.

Figure 11 . 1 M
Figure 11.Temperature dependence of χ M T and χ M versus T of 2. Inset: temperature dependence of χ M −1 ; the solid line represents the best fit of the Curie−Weiss law χ M = C/(T − θ).

Table 1 .
Crystal data and structure refinements for Ni II complex 1.