Metal(ii) Complexes of Compartmental Polynuclear Schiff Bases Containing Phenolate and Alkoxy Groups

Five mono-nuclear Cu(II) and Ni(II) complexes and one dinuclear Zn(II) complex were synthesized from the Schiff bases

In ethanol, the reaction of H2L OEt -phda with Cu(OAc)2•H2O in a 1:1 or 1:2 molar ratio afforded the long needles olive green compound [Cu(L OEt -phda)(H2O)]•H2O (1).Also, the same product was obtained when Cu(OAc)2•H2O was added to the ligand followed by the addition of Zn(OAc)2•2H2O (1:1:1) and heated for about 15 min on a steam-bath or when the order of addition was reversed.The [Ni(L OEt -phda)]•H2O (2) complex was produced using a similar fashion as that described for 1.It was isolated regardless the use of excess Ni(OAc)2•4H2O and/or upon the addition of Zn(OAc)2•2H2O first followed by nickel(II) acetate.

Synthetic Aspects
The two crystalline yellowish-orange ligands H 2 L OEt -phda and H 2 L OEt -ambza were prepared in good yield (80%-90%) by the Schiff base condensation of an ethanolic solution containing 1,2-diaminobenzene or 2-aminobenzylamine with 3-ethoxy salicylaldehyde in a 1:2 molar ratio, respectively.These were characterized by elemental microanalysis, IR, 1 H and 13 C NMR as well as ESI-MS.
In ethanol, the reaction of H 2 L OEt -phda with Cu(OAc) 2 ¨H2 O in a 1:1 or 1:2 molar ratio afforded the long needles olive green compound [Cu(L OEt -phda)(H 2 O)]¨H 2 O (1).Also, the same product was obtained when Cu(OAc) 2 ¨H2 O was added to the ligand followed by the addition of Zn(OAc) 2 ¨2H 2 O (1:1:1) and heated for about 15 min on a steam-bath or when the order of addition was reversed.The [Ni(L OEt -phda)]¨H 2 O (2) complex was produced using a similar fashion as that described for 1.It was isolated regardless the use of excess Ni(OAc) 2 ¨4H 2 O and/or upon the addition of Zn(OAc) 2 ¨2H 2 O first followed by nickel(II) acetate.

General Characteristic Properties of the Complexes
The purity of the isolated complexes 1-6 was confirmed by elemental microanalyses (see experimental section).The complexes were found to be soluble in most common organic solvents: EtOH, MeOH, propan-2-ol, acetone, CH 2 Cl 2 , CHCl 3 and CH 3 CN and in some cases Et 2 O.This of course reflects their non-electrolytic nature and may explain why the complexes were obtained in relatively low yields (40%-60%).The non-electrolyte behavior of the complexes was also supported by measuring their molar conductivities in CH 3 CN; Λ M = 1-10 Ω ´1¨cm 2 ¨mol ´1.
The ESI-MS which was performed in MeOH on the 1-3 complexes clearly revealed the m/z peak of each complex is consistent with its given mononuclear formula [M(L OEt -X)] (M = Ni 2+ or Cu 2+ and X = phda 2´o r ambza 2´) .Also, in addition to the mononuclear m/z peak, complex [Cu(L OEt -phda)(H 2 O)]¨H 2 O (1) showed its major m/z peak at 933.016 corresponding to the protonated dimeric species [Cu 2 (C 24 H 22 N 2 O 4 ) 2 +H] + (calcd m/z = 933.182).This peak was not observed in complexes 2 and 3 nor in the dinuclear complex [Zn 2 (L-OEt -ambza)(µ-OAc)(OAc)] (6).In contrast, the ESI-MS of 6 did not show any m/z peak that corresponded to either the di-nor the mono-nuclear species, but instead the major peak was found to be for the ligand; [H 2 L OEt -ambza+H] + (calcd m/z = 419.54,Found: 419.197).This result may indicate the instability of the mononuclear [Zn(L OEt -ambza)] species compared to Cu(II) and Ni(II) complexes.

IR and UV-VIS Spectra of the Complexes
Although the IR spectra of the two ligands under investigation did not clearly reveal the stretching frequencies of the phenolic O-H and the C=N imine groups, the structures of the ligands were confirmed by 1 H and 13 C NMR as well as with ESI-MS (experimental section).In general, the structural features of the complexes were strongly dominated by the ligands and hence the IR did not provide conclusive evidence about the coordination modes of the O-H and C=N bonds.However, the IR spectra of the complexes show some general features for example, they display a weak band around 3050 cm ´1 and a weak series over the frequency range 2980-2870 cm ´1 attributable to ν(C-H) of the aromatic and aliphatic groups, respectively.Also, the complexes showed a strong band at The UV-Vis spectral data of the complexes 2-5 were recorded in CH 3 CN, whereas 1 was recorded in CH 2 Cl 2 .The spectrum of copper(II) complex [Cu(L OEt -phda)(H 2 O)]¨H 2 O (1) revealed the presence of a broad band at 600 nm and a less intense low energy band at 976 nm.This feature often indicates a distorted square pyramidal (SP) environment around the central Cu 2+ ion, which is characteristic with the appearance of a broad band in the 550-650 nm region (dxz,dyz Ñ dx 2 -y 2 transition) and may be associated with a low energy shoulder at λ > 800 nm [43][44][45][46].The very strong band observed at 500 nm can be assigned to ligand-metal charge transfer transition (CT MÑL).The visible spectra of Cu(II) complexes 3 and 4 displayed single absorption bands at 613 and 619 nm, respectively.This band could be attributed to 2 B 1g Ð 2 A 1g transition in square planar Cu(II) geometry [47][48][49].
The visible spectrum of Ni(II) compound 2 exhibited very strong band at 398 (ε = 1920 M ´1cm ´1) which can be assigned to LMCT transition as in the case of Cu(II) complex 1 for the same dianion ligand, L OEt -phda 2´a nd another band at 422 nm due 1 A 1g Ð 1 B 1g transition [47].On the other hand, complex 5 displayed only one band at a much lower energy (624 nm) compared to 2. The very strong red shift of this band is most likely attributed to the reduced ligand field strength of L OEt -ambza 2´.

Description of the Structures
Compound 1 consists of neutral and mononuclear [Cu(L OEt -phda)(H 2 O)] units and twofold disordered lattice water molecules with split occupancy 0.5.It crystallizes in the tetragonal space group P-42 1 m (no.113) with Z = 4.The Cu II center, aqua ligand (O3) and lattice water molecules (O4, O5) are located at special positions with site symmetry m.A perspective view together with a partial atom numbering scheme of 1 is depicted in Figure 1.The Cu1 center is penta-coordinated by N1, N1', O1, O1' donor atoms of the tetradentate L OEt -phda 2´S chiff base dianion, and O3 of aqua ligand.The CuN 2 O 3 chromophore may be described as tetragonal pyramid (SP) (τ = 0.00) [48]   The UV-Vis spectral data of the complexes 2-5 were recorded in CH3CN, whereas 1 was recorded in CH2Cl2.The spectrum of copper(II) complex [Cu(L OEt -phda)(H2O)]•H2O (1) revealed the presence of a broad band at 600 nm and a less intense low energy band at 976 nm.This feature often indicates a distorted square pyramidal (SP) environment around the central Cu 2+ ion, which is characteristic with the appearance of a broad band in the 550-650 nm region (dxz,dyz → dx 2 -y 2 transition) and may be associated with a low energy shoulder at λ > 800 nm [43][44][45][46].The very strong band observed at 500 nm can be assigned to ligand-metal charge transfer transition (CT M→L).The visible spectra of Cu(II) complexes 3 and 4 displayed single absorption bands at 613 and 619 nm, respectively.This band could be attributed to 2 B1g← 2 A1g transition in square planar Cu(II) geometry [47][48][49].
The visible spectrum of Ni(II) compound 2 exhibited very strong band at 398 (ε = 1920 M −1 cm −1 ) which can be assigned to LMCT transition as in the case of Cu(II) complex 1 for the same dianion ligand, L OEt -phda 2− and another band at 422 nm due 1 A1g← 1 B1g transition [47].On the other hand, complex 5 displayed only one band at a much lower energy (624 nm) compared to 2. The very strong red shift of this band is most likely attributed to the reduced ligand field strength of L OEt -ambza 2− .

Materials and Physical Measurements
2-Aminobenzylamine and 2-aminobenzene were purchased from TCI-America (Portland, OR, USA), whereas 3-ethoxy salicylaldehyde was purchased from Alfa Aesar (Ward Hill, MA, USA).All other chemicals were commercially available and used without further purification.Infrared spectra were recorded on a Cary 630 (ATR) spectrometer (Foster City, CA, USA).Electronic spectra were recorded using an Agilent 8453 HP diode array UV-Vis spectrophotometer (Santa Clara, CA, USA). 1 H and 13 C NMR spectra for the ligands were obtained at room temperature on a Varian 400 NMR spectrometer (Santa Clara, CA, USA) operating at 400 MHz ( 1 H) and 100 MHz ( 13 C). 1 H and 13 C NMR chemical shifts (δ) are reported in ppm and were referenced internally to residual solvent resonances (DMSO-d 6 : δH = 2.49, δC = 39.4 ppm).ESI-MS were measured in MeOH on LC-MS Varian Saturn 2200 Spectrometer (Santa Clara, CA, USA).The conductivity measurements were performed using a Mettler Toledo Seven Easy conductivity (Columbus, OH, USA), calibrated by the aid of a 1413 µS/cm conductivity standard.Elemental analyses were carried out by the Atlantic Microlaboratory (Norcross, GA, USA).

Synthesis of the Complexes
A general method was used to synthesize the complexes 1-3 and 5 by heating an ethanolic solution (15 mL) containing a mixture of M(OAc) 2 ¨nH 2 O (M = Cu, n = 3; M = Ni, n = 4) or M(NO 3 ) 2 ¨nH 2 O (M = Cu, n = 3, M = Ni, n = 6) with pH adjusted to ~9 with Et 3 N and the corresponding ligand (1:1 molar ratio and typically 0.25 mmol of each) for 5-10 min.The resulting solution was then filtered through celite and allowed to crystallize at room temperature for 1-3 days.The precipitate obtained was collected by filtration and dried in air.
[Cu(L OEt -phda)(H 2 O)]¨H 2 O (1).Tiny long olive green crystals suitable for X-ray structure determination were obtained from dilute ethanolic solution (overall yield 48% [Ni(L OEt -phda)]¨H 2 O (2).The complex was separated as shiny brick-red crystals of X-ray quality (overall yield: 53%).Characterization: Anal.calcd for C 24 H 24 N 2 O 5 Ni (MM = 479.[Cu(L OEt -ambza)]¨H 2 O (4).A mixture of the ligand H 2 L OEt -ambza (0.085 g, 0.20 mmol) and Cu(hfacac) 2 (0.096, 0.20 mmol) dissolved in EtOH (15 mL) and pH of the solution was adjusted to ~9 by Et 3 N.This was followed by heating on a water-bath for 5 min, then filtered through celite and allowed to stand at room temperature.The resulting brownish green crude precipitate was collected by filtration and recrystallized from CH 3 CN, where green plates of the complex of X-ray quality were isolated after 3 days.These were collected by filtration and dried in air (overall yield: 47% [Ni(L OEt -ambza)] (5).This complex was isolated as a brownish-yellow crystalline compound upon further crystallization from EtOH (overall yield: 44% [Zn 2 (L OEt -ambza)(µ-OAc)(OAc)] (6).A mixture of H 2 L OEt -ambza (0.085 g, 0.20 mmol) and Zn(OAc) 2 ¨2H 2 O (0.088, 0.40 mmol) dissolved in EtOH (15 mL) was heated for 5 min, followed by filtration through celite and then allowed to stand at room temperature.After 3 days, the resulting crude precipitate was collected by filtration and recrystallized from CH 3 CN to afford tiny yellow single crystals of suitable for X-ray structure determination (overall yield: 64%

X-Ray Crystal Structure Analysis
The X-ray single-crystal data of the six compounds were collected on a Bruker-AXS APEX CCD diffractometer at 100(2) K (Madison, WI, USA).The crystallographic data, conditions retained for the intensity data collection and some features of the structure refinements are listed in Tables 1 and 2. The intensities were collected with Mo-Kα radiation (λ = 0.71073 Å).Data processing, Lorentz-polarization and absorption corrections were performed using SAINT, APEX and the SADABS computer programs [51][52][53].The structures were solved by direct methods and refined by full-matrix least-squares methods on F 2 , using the SHELXTL [54] program package.All non-hydrogen atoms were refined anisotropically.The hydrogen atoms were located from difference Fourier maps, assigned with isotropic displacement factors and included in the final refinement cycles by use HFIX (parent C atoms) or DFIX (i.e., O-H distance restraints for parent O atoms) utility of the SHELXTL program package.In case of 1, split occupancy of 0.5 was applied for O4 and O5 atoms of disordered water molecule, and their H atoms were omitted.High R1 (I > 2σ(I)) values of 0.0818 (for 3) and 0.0934 (for 6) are caused by low crystal quality of samples.Molecular plots were performed with the Mercury program [55].
The unsuccessful isolation of dinuclear complexes with H 2 L OEt -ambza where the alkoxy groups are not participating in the coordination with a second metal ion is most likely attributed to: (1) the large dihedral angles (32-129 ˝in complexes 3-6, see X-ray section) between the phenolate rings and the plane containing the central benzene ring which make the alkoxy groups pointing away and coming in an in-appropriate position to coordinate to a second metal into the O 2 -O 2 bonding site and (2) the large bite angle (bond distances in O 2 -O 2 are within the range 5.2-5.9Ǻin these complexes).This data is summarized in Table 3 for L OEt -ambza-metal(II) complexes.As a result, no complexes with N 2 -O 2 + O 2 -O 2 were isolated.An alternative approach to synthesize such dinuclear complexes is to use large 4d or 5d transition metal ions and alkali/alkaline earth metal ions by incorporating them into the precursor synthesized mononuclear Cu(II) and Ni(II) complexes (3-5).

Scheme 1 .
Scheme 1.Some of the very common compartmental Schiff base ligands based on 3alkoxysalicylaldehyde.
with O3 in the apical site [Cu1-O3 = 2.360(3) Å] The basal Cu-O/N bond distances are 1.9337(14) and 1.9711(16) Å, and the O1-Cu1-N1' bond angle is 170.15(7)˝.Cu1 deviates by 0.160 Å from the basal O 2 N 2 plane.The dihedral angle between the two N-Cu-O coordination planes is 13.7 ˝and the dihedral angle of the two benzene rings of the phenolate moieties is 6.0 ˝, whereas the dihedral angle of the phenolate ring with the central benzene ring is 4.1 ˝.The N1¨¨¨N1' and O1¨¨¨O1' separations within the N 2 -O 2 unit are 2.613 and 2.707 Å, the O2¨¨¨O2' separation of the O 2 -O 2 unit is 5.605 Å.The shortest metal-metal separation is 4.9513(5) Å.Along the c-axis of the unit cell a supramolecular 1D system is formed via bifurcated hydrogen bonds of type O-H¨¨¨(O,O) from aqua donor ligands to neighboring O1 and O2 acceptor atoms of L OEt -phda 2´( with O3 in the apical site [Cu1-O3 = 2.360(3) Å] The basal Cu-O/N bond distances are 1.9337(14) and 1.9711(16) Å, and the O1-Cu1-N1' bond angle is 170.15(7)°.Cu1 deviates by 0.160 Å from the basal O2N2 plane.The dihedral angle between the two N-Cu-O coordination planes is 13.7° and the dihedral angle of the two benzene rings of the phenolate moieties is 6.0°, whereas the dihedral angle of the phenolate ring with the central benzene ring is 4.1°.The N1•••N1' and O1•••O1' separations within the N2-O2 unit are 2.613 and 2.707 Å, the O2•••O2' separation of the O2-O2 unit is 5.605 Å.The shortest metal-metal separation is 4.9513(5) Å.Along the c-axis of the unit cell a supramolecular 1D system is formed via bifurcated hydrogen bonds of type O-H•••(O,O) from aqua donor ligands to neighboring O1 and O2 acceptor atoms of L OEt -phda 2− (Table

Figure 1 .
Figure 1.Perspective view of 1. Disordered water molecule is omitted.

Figure 1 .
Figure 1.Perspective view of 1. Disordered water molecule is omitted.
-O1 bond angles: 175.70(6) and 176.37(6)°].Ni1 deviates by 0.006 Å from the O2N2 plane.The dihedral angle between the two N-Ni-O coordination planes is 5.1° and the dihedral angle of the two phenolate rings is 1.5°, whereas the dihedral angle of the phenolate ring with the central benzene ring is 15.4°.The N1•••N2 and O1•••O2 separations within the N2-O2 unit are 2.540 and 2.461 Å, the O3•••O4 separation of the O2-O2 unit is 5.088 Å.The shortest metal-metal separation is 5.6773(4) Å.The water H atoms form bifurcated O-H•••(O,O) intermolecular hydrogen bonds with the O atoms of the phenolate and ethoxy groups (Table

N
-Ni-O coordination planes is 144.9° and the dihedral angle of the two phenolate rings is 132.3°,whereas the dihedral angles of the phenolate rings with the central benzene ring are 118.2(ring-O1) and 35.5° (ring-O2), respectively.The N1•••N2 and O1•••O2 separations within the N2-O2 unit are 2.858 and 2.653 Å, the O3•••O4 separation of the O2-O2 unit is 5.873 Å.The shortest metal-metal separation is 4.8273(11) Å.The water H atoms form bifurcated O-H•••(O,O) intermolecular hydrogen bonds with the O atoms of the phenolate and ethoxy groups, and EtOH forms a hydrogen bond of type O-H•••O to the water molecule (Table
´1 due to the C-H out of plane bending.Complexes 1-4 displayed one or two weak band(s) over the 3520-3870 cm ´1 region due to the stretching frequency, ν(O-H) of H 2 O/EtOH molecules of crystallization and/or coordinated H 2 O in [Cu(L OEt -phda)(H 2 O)]¨H 2 O (1) where the two bands were located at 3099 and 3467 cm ´1.

Table 1 .
Crystallographic Data and Processing Parameters for Compounds 1-3.

Table 3 .
The O 2 -O 2 Bond Distances and the Dihedral Angle(s) between the Phenolate Rings and Benzene Ring in the Complexes 3-6.The following are available online at http://www.mdpi.com/2073-4352/6/8/91/s1.CCDC 1489003-1489008 contain the supplementary crystallographic data for 1-6, respectively.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/ data_request/cif.Hydrogen bonds are listed in TableS1, packing views for crystal structures 1-6 are given in FiguresS1-S6, respectively.