The desire to rationally design and synthesize useful solid materials through the controlled assembly of molecular components, has led to a recent surge of interest in the concept of crystal engineering    . So far, considerable progress has been achieved in tuning and predicting the mode of assembly and orientation of individual building blocks into structures with specific topologies and properties   , but accurate prediction and control of the 3-D structure of molecular crystals and coordination polymers remains difficult. Reduced Schiff base ligands tend to form a suitable conformational and coordination environment. They have been found to form more flexible and multidentate network complexes because of the reduction of the C=N bond of the Schiff base which helps to overcome ligand stability   . In recent years, significant research interests have been devoted to coordination polymers with chiral helical structural motifs . The preparation of chiral coordination complexes is a very well-established procedure and it dates back to Werner, who showed that the complex chirality depends on the formation of cis-coordination geometry around the metal ion with appropriate chelating/monodentate ligands. This can be understood based on the symmetry criteria that the absence of improper axis of rotation results in chirality . Metal complexes of N-(2-pyridylmethyl)-amino acid derivatives with different spacers have been reported to have multiple attractive architectures including multidimensional and oligomeric structures with cavities, channels and chiral capsules owing to their robust hydrogen-bonding ability and inherent chirality  - . In the pioneering work in these fields, N-(2-pyridylmethyl)-alanine has been used to build the copper, cobalt and zinc complexes . There have been few reports on new nickel (II) with N-(2-pyridylmethyl)-alanine in literature . Furthermore, nickel (II) Schiff-base complexes have been reported to have promising applications for their luminescence  . Herein, we report on a novel nickel (II) complex with pyridyl reduced Schiff-base ligand of amino acid derivatives [Ni(II)(L-pyala)2]∙4H2O, L-Hpyala = N-(2-pyridylmethyl)-L-alanine, its synthesis, structural and photoluminescence properties. This article brings out some structural diversity as compared to a similarly reported nickel (II) complex in relation to the role of lattice water due to modified experimental conditions such as change in pH in directing the formation of supramolecular structures in the solid state . In addition, its photoluminescence properties have been evaluated.
All chemicals and solvents used for the synthesis were of reagent grade. Nickel acetate tetrahydrate (Aldrich), 2-pyridine aldehyde (Prolabo) and L-alanine (Prolabo) were used as received. The solvents, ethanol was dried and distilled according to standard methods.
2.2. Analytical Methods
Elemental analysis for carbon, nitrogen and hydrogen were carried out on a Fisons instrument 1108 CHNS-O. Infrared spectra were recorded on a Perkin-Elmer model IR-457 spectrometer and a spectrum 100 FT-IR Perkin Perkin-Elmer spectrometer, while X-ray diffraction was carried out with a CCD bidimensional diffractometer using monochromatic radiation, λ (Mo − Kα) = 0.71073 Å, operating at 50 kV and 40 mA. TGA was recorded using a Mettler Toledo TGA/DSC1 SATR system while UV-visible spectrophotometer, HACH DR 3900, MARK Brucker.
2.3.1. Synthesis of Ligand
The synthesis of the ligand was as described in literature .
2.3.2. Synthesis of [Ni(C9H11N2O2)2]∙4H2O (1)
Pyala (0.36 g, 2 mmol) in 10 mL water/ethanol mixture (1:1) was added drop wise to a 5 ml aqueous solution of Ni(OOCH3)2∙4H2O (0.245 g, 1 mmol), while stirring magnetically at room temperature. Stirring continued for two hours. Suitable purple crystals for X-ray analysis were obtained from the solution by slow evaporation after two weeks. Yield 70%; Anal. Calcd. For C18H30NiN4O8; C, 41.52; H, 4.12; N, 22.01; Found: C, 41.46; H, 4.07; N, 22.59.
2.4. X-Ray Crystal Structure Determination
The crystal structure of the complex was determined by single crystal X-ray diffraction on a Brucker APEX-II using monochromatic MoKα radiation (λ = 0.71073 Å) at a temperature of 198 K and integrated with SAINT-Plus program , and absorption corrections were carried out by multi-scan method by SADABS . The Structure was solved by direct methods and refined against F2 by full-matrix least-squares techniques with SHELTL . All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were included from calculated positions and refined riding their respective parent atoms with isotropic displacement parameters.
3. Results and Discussion
3.1. Infra-Red Spectra
The IR spectra of the ligand showed a characteristic weak absorption band at 2982 cm−1 due to ν (N-H) stretching of amines . This band confirms the formation of imine (-C=N) group of Schiff bases which was reduced during the synthesis by NaBH4/H2O to the amine group -N-H. Furthermore the IR spectra of the ligand revealed a strong absorption band at 1672 cm−1 due to ν (C=C, C=N) stretching of the pyridyl group . This band in the metal complex shifted to 1577 cm−1 and 1597 cm−1. The decrease in wave number as a result of reduction in electron density around the ν (C=C, C=N) bond of the ring, suggests the par ticipation of the pyridyl ν (C=N) bond in the complexation through the nitrogen atom . In addition the Ni(II) complex exhibited a very weak absorption band around 3288 cm−1 which suggests the deprotonation of the ν (OH) bond and the participation of the oxygen atom in complexation .
3.2. Crystal Structure of [Ni (C9H11N2O2)2]∙4H2O(1)
The structure of 1 with atomic numbering scheme is shown in Figure 1. The Crystal data and structure refinement details for the title compound are summarized in Table 1 while selected bond lengths and angles are listed in Table 2. The X-ray single crystal diffraction analysis reveals that complex 1 crystallizes in the
Table 1. Crystal data and structure refinement for 1.
Table 2. Selected bond lengths (Å) and Bond angles (˚) of 1.
Figure 1. Coordination environment of Ni(II) center with atom labeling in 1.
orthorhombic system P212121 space group. The Ni(II) centre lying on the C2 axis (x, 1, 1), has a distorted octahedral geometry, which is surrounded by two oxygen and four nitrogen donors from two monodentate carboxylate Ni1-O1, 2.0232 (3) Ǻ and Ni1-O3, 2.0381 (3) Ǻ, two amino nitrogen atoms Ni1-N1, 2.0947 (4) Ǻ, Ni1-N1, 2.0950 (4) Ǻ and two pyridyl nitrogens Ni1-N19, 2.1030 (3) Ǻ; Ni1-N8, 2.1177 (3) Ǻ. The O1-Ni1-O3, O1-Ni1-N12, O3-Ni1-N12, O1-Ni1-N1 and O3-Ni1-N1 bond angles are 90.916 (13)˚, 90.916 (13)˚, 93.328 (14)˚, 92.749 (14)˚ and 95.509 (13)˚ respectively. These values are comparable to previously reported results of a similar nickel (II) complex with the same ligand having Ni-O bond distances in the range of 2.0496 (12) - 2.035 (2) Ǻ and Ni-N bond distances in the range 2.0849 (13) - 2.110 (2) Ǻ . Furthermore the crystallographic studies revealed a different packing pattern with chiral channels runing parallel to the c-axis relative to that of a similar compund previously reported. This packing in 1 differs notably as evident in Figure 2 which could be attributed to the presence of four moelcules of water of crystallisation in the compound that facilitate hydrogen bonding formation.
3.3. Thermal Stability
The thermogravimetric analysis (TGA) of the complex was recorded with the TG curve shown in Figure 3. The first step decomposition at 25˚C [7.88% weight loss (calculated, 11.92%)] is attributed to the loss of three molecules of lattice water and the second decomposition at 110˚C [2.50% weight loss (calculated, 3.60%)] which suggests the final loss of one molecule of lattice water, making a total loss of four water molecules of crystallization associated to the nickel (II) complex. The third decomposition at 270˚C corresponding to 29.23% weight loss (calculated, 36.38%) and the fourth decomposition at 330˚C corresponding to 27.50% weight loss (calculated, 29.05%), suggests the decomposition of the ligands. The final residue of 32.89% (calculated, 30.58) is attributed to the mixture of nickel oxides.
Figure 2. Packing of 1 along c-axis, showing a zigzag Ni(II) array of chiral channels.
Figure 3. TGA thermogram for 1.
3.4. UV-Visible Spectra
The UV-visible spectra of 1 shows a small energy transition band at 320 - 331 nm and a weak broad absorption band in the range of 488 - 614 nm as shown in Figure 4. The small energy transition band at 320 - 331 nm in the nickel (II) spectra is attributed to the metal to ligand charge transfer (MLCT) transition in the nickel (II) complex and the weak broad band observed in the range of 488 - 614 nm is presumably due to the spin allowed d-d transition (3A2g → 3T2g (F) which is characteristic of an octahedral geometry around the nickel (II) atom .
3.5. Photoluminescent Behavior of Compound
The photoluminescent (PL) property of the complex (1) was investigated in the solid state at room temperature. The PL curve shown in Figure 5 showed strong fluorescent emission at 442 nm. This was obtained when the complex was excited at 325 nm. In order to understand the nature of the emission, we examined the PL property of the free ligand found that the strongest emission peak is at 391 and 432 nm. Therefore the luminescence of the complexes May be attributed to metal to ligand charge transfer (MLCT) or ligand to metal charge transfer (LMCT)   . These observations suggest that the compound may be good a candidates for potential photoactive and in the preparation of optical.
Figure 4. UV-visible spectra of Ni(II) complex.
Figure 5. Solid state emission spectra of complex 1.
A novel Ni(II) coordination polymer of N-(2-pyridylmethyl)-(L)-alanine (Hpyala) has been obtained and the structure compared with that of a similar compound previously reported. The structural diversity of the compound compared with that previously reported is as a result of a difference in crystal water due to the modification of the experimental conditions such as pH in directing the formation of supramolecular structures in the solid state. Compound (1) crystallizes in the orthorhombic system with chiral space group, P212121. The diversity in the structure is seen in its packing along various axes.
The authors gratefully acknowledge Dr. Namanga Jude Eko, Scientist at OSRAM for the photoluminescence analysis.
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