1.INTRODUCTION:
Derivatives of hydrazine, particularly aminoguanidine, diaminoguanidine, diformylhydrazine and semicarbazide, find applications not only in different fields of chemistry but in various physiological studies also. Among these aminoguanidine and diaminoguanidine are very versatile and interesting potential bidentate ligands. Studies on aminoguanidine have contributed much to our knowledge of heterocyclic compounds, the higher hydro nitrogens, guanyl azyl, azides and nitrons.
Aminoguanidine and its derivatives play an important role in living organisms, molecular design and their physical and chemical properties1-5. A number of complexes in which neutral aminoguanidine, aminoguanidium mono cation and di cation act as ligands have been reported.
The coordination compounds of aminoguanidine with transition metal ions like Cu+2 have been reported. For example, [Cu (amgu)2] (NO3)2 has a square planar chelate structure like that of Ni(II)complexes6-7. Neutral aminoguanidine forms chelate complexes with both Pt (II) and Pd (II) in which it is coordinated to the metal ion through N4 of amino group and N1 of imino group forming a stable five membered ring.
In trans – [Pt (amgu) 2] (NO3)2 and trans – [Pd (Amgu) 2] Cl2 chelate complexes, aminoguanidine acts as a neutral ligand and the Pt complex is centrosymmetric. Complexes in which amino guanidine (+1) cation either in coordinated8 or non – coordinated9 to the metal ion have also been synthesized.
In aminoguanidinium (+1) pentaflouro zirconate reported by Bukvetskii et al9, aminoguanidinium (+1) ion exists as non - coordinated. Harrison et al10 has reported hydrothermal synthesis and crystal structure of (CN4H7)3Zn3(HPO3)4, a three dimensional frame work. Aminoguanidinium (+) ion is coordinated to the metal ion through hydrazine type of nitrogen as well as nitrogen atom of the amino group. In bis– aminoguanidinium tetra chloro cobaltate(II), [Co(amgu H)2 Cl4]11, amino guanidine monocation is coordinated to cobalt ion forming a trans - octahedral complex. The structure of bis–aminoguanidinium hexa chloro cobaltate (II) has been studied by Shvelashvili et al12.
In trans - [PtCl2 (amguH)2]8, two chlorine atoms compete inner ligand sphere with trans geometry and amguH+ is a monodentate ligand coordinated through hydrazine nitrogen. Aminoguanidinium (+2) hexafluoro zirconate12,14,15 is a technically interesting complex with piezo, pyroelectric and second harmonic generation (SHG) properties. Similarly aminoguanidinium hexachloro cuprate (II), (CH8N4)2[CuCl6], a novel thermochromic complex, has been synthesized by Romanenko et al17. Bromine analogue of this complex has been structurally characterised16. It is evident that aminoguanidine acts as an outer sphere di cation in these complexes rather a co-ordinating ligand.
Another novel behavior of aminoguanidine has been witnessed in the complex (CH7N4)(CH8N4)[SbCl6] in which aminoguanidine is present both as mono and di cation compensating the charge on the complex sphere. Salts of aminoguanidinium (+1) cation with inorganic acids18-22, with organic acids23-26 and salts of aminoguanidinium (+2) cations27,28 are well characterized which are mostly crystals.
Some Ni(II) complexes with amino guanidine and nitro amino guanidine have been characterized by chemical methods, thermal analysis, IR and diffuse reflectance spectra29. Studies on the magnetic behavior of these Ni(II) complexes reveal that they are diamagnetic in nature.
While it is evident that Pd and Pt complexes of aminoguanidine as a neutral ligand with inorganic ion as counter anion have been thoroughly characterized, the corresponding nickel complexes have not been studied in depth. Hence in this study, a systematic approach has been made to synthesize new aminoguanidine nickel complexes of various organic and inorganic counter anions and study them using analytical , IR, thermal reactivity and X- ray diffraction techniques.
2. Experimental:
2a.Materials:
Commercially available chemicals (AR or equal grade) were used without further purification. The solvents were distilled before use. Aminoguanidine was purchased from Aldrich and the remaining chemicals used were of commercial grade.
2b.Analytical methods:
(i). Estimation of Hydrazine:
The hydrogen content of the salt and complex was determined volumetrically using a standard KIO3 solution under Andrew’s Condition30.
(ii).Estimation of Nickel:
The Ni content in the complex was estimated by EDTA complexometric titration. A known weight of the complex sample was dissolved in HNO3 and made up to a standard volume. A known volume of this solution was titrated against standardized EDTA using murexide indicator maintaining the pH of the reaction mixture as 10 using ammonia basic buffer.
2c.Physico – chemical techniques:
The instrumental techniques employed in the present study are melting point measurements, infra red spectroscopy, thermal analysis like differential thermal analysis (DTA) and thermogravimetry (TG), powdered and single crystal X –ray diffraction.
(i). Infrared Spectrum:
The infrared spectra of the solid samples in the range 4000 – 400 cm-1 were recorded on a Shimadzu FTIR 8000 Spectrophotometer and Perkin – Elmer 597 model Spectrophotometer using KBr pellets.
(ii). Thermal analysis:
The simultaneous TG-DTA studies were done on a STA 1500 thermal analyzer and the curves obtained in air using platinum cups as sample holders with 5 - 10 mg of the sample at a heating rate of 10ºC per minute up to 700 – 800ºC.
(iii). X – ray Powder Diffraction:
The XRD patterns of the prepared complexes in the present study were recorded on a JEOL JDX 8030 and Phillips X- ray diffractometer using Cu – Kα radiation with nickel filters. The XRD data were collected at low temperatures using Mo – Kα radiation in Kappa CCD diffractometer. Structure solution and refinement were done by using SHELX97 programs. For molecular graphics ORTEP package was used.
3.Preparation of complexes:
3a. Preparation of [Ni(amgu)2H2O]SO4 and [Ni (amgu)2]X2 where X = NO3,Cl and CH3COO-
10ml aqueous solution containing 0.001 mole of respective Ni(II) salts namely NiSO4.6H2O, NiCl2.6H2O, Ni (NO3)2.6H2O and Ni(CH3COO)2 was added to 30 ml of aqueous solution of aminoguandine bicarbonate containing 0.272g (or 0.002mole).The turbidity formed while heating over a water bath was cleared by adding a few drops of respective 2N acid. After maintaining pH of the solution as 7, it is concentrated over a water bath to one half of its volume and kept for crystallization at room temperature. Red single crystals were isolated from green mother liquor in case of NiSO4, Ni(NO3) and NiCl2 after three days. Ni(CH3COO)2 gave a red polycrystalline product. The complexes were washed with distilled ethanol and air dried.
3b. Preparation of [Ni(amgu)2]C3H3O3.Cl.3H2O:
10 ml of aqueous solution of NiCl2.6H2O containing 0.238 g (or 0.001 mole) of it was mixed with 20ml of maleic acid containing 0.118g (or 0.001mole) and 0.272g of aminoguanidine bicarbonate at a pH 9. The resulting solution was concentrated over a water bath to half of its volume. On keeping the concentrated solution for crystallization at room temperature, a red colour solid product from green colour solution was formed after three days. The product was washed with distilled ethanol and dried in air.
3c. Preparation of [Ni (amgu)2]C6H4COO)2.4H2O:
0.116g (or 0.001 mole) of terepthalic acid was added to 0.544g aminoguanidine bicarbonate (0.004 mole) dissolved in 30ml of distilled water and kept over a hot water bath. The pH of this solution was maintained at 9. To this solution 10 ml of NiCl2.6H2O (0.237g, 0.001 moles) solution was added. The resulting clear solution was concentrated over a hot water bath to half of its volume. The concentrated solution was kept for crystallization. The reddish orange soft mushy compound formed within a day from the green colour solution. It was washed with distilled ethanol and air dried.
4. RESULTS AND DISCUSSIONS:
Many of the properties of the aminoguanidine are more close to those of hydrazine than those of guanidine. Hence aminoguanidine may be regarded as a substituted hydrazine i.e., guanyl hydrazine. The presence of guanyl group modifies the properties of hydrazine especially in basicity, consequently the aminoguanidine is a stronger base than hydrazine.
This diacidic base forms (amgu H)+ and (amgu H2)+2 ionic salts with inorganic and carboxylic acids. This basic property of amino guanidine suggests that this ligand can appear in metal complexes in different forms depending on the medium. In the present studies during the preparation of the complexes of amgu with Ni(II), the pH of the medium is maintained either 7 or above, so that the neutral amgu may get coordinated to the metal ion. Further this helps for the chelation of amino nitrogen N1 and imino nitrogen N4 being in unprotonated condition.
Ross et al 41 has established the planar nature of aminoguanidinium (+1) cation except two hydrogen atom bonded to two terminal hydrazine nitrogen atom.
4a. Analytical Data:
The composition of the complexes formed was fixed by hydrazine and metal estimation. The analytical data of the complexes are presented in Table:1. These data are in good agreement with the proposed molecular formulae.
4b. IR Data:
The IR spectra of all the Ni(II) square planar complexes prepared are given in Table 2. The IR spectrum of complexes shows a broad band in the region of 3354 – 3000 cm-1 and this range may be assigned to ʋN-H stretching vibrations. The ʋC=N stretching frequency of free aminoguanidine is observed at1691 cm-1. The C=N stretching frequency in the complexes are noticed at 1650 cm-1 which reveals that imino nitrogen atom is involved complexation.
The N-N stretching frequency of free aminoguanidine is observed at 1114 cm-1 and a slight increase in this frequency to 1139 cm-1 for the complexes prepared suggests the hydrazine nitrogen is bonded to nickel ion.
4c. Thermal Data:
It is interesting to note that the aminoguanidine nickel complexes of nitrate, chloride, and sulphate are sensitive to heat and they spurt on heating where as the carboxylate complexes show relatively mild spurting. Thermogram(TG) of these complexes are recorded and listed in Table 3. The composition of the intermediates and final products are those which best fit with the observed mass loss in TG. DTA results are in good agreement with thermo gravimetric data.
(i). [Ni(amgu)2](CH3COO)2. 2 H2O
The TG of this complex shows three distinct steps of decomposition. The first endotherm at 150ºC in DTA is in accordance with the loss of two molecules of water corresponding to 10% weight loss in TG. The exotherm at 264ºC is due to the elimination of two amgu molecules as seen from TG. Finally the nickel acetate intermediate decomposes exothermally at 364ºC to yield metallic nickel as the final product.
(ii). [Ni(amgu)2]C4H3O4Cl.3 H2O
This complex also exhibits three stages during its decomposition. The first endotherm at 242ºC in DTA is due to the elimination of three water molecules. Relatively high temperature of dehydration indicates that the water molecule are probably involved in strong hydrogen bonding or coordinated to nickel. The broad endotherm at 471ºC is attributed to the removal of two molecules of aminoguanidine in the coordination sphere and chloride ion which exists as counter ion resulting in the formation of nickel maleate. Finally the broad exothermic peak observed at 574ºC is attributed to the decomposition of this nickel maleate to give metallic nickel.
(iii). [Ni(amgu)2]C8H4O4Cl.4 H2O
The thermogram of this complex shows four distinct peaks in DTA. The first endotherm at 106ºC corresponds to the elimination of two water molecules. The second endotherm at 244 ºC is due to the loss of another two water molecule which may be strongly bonded to the metal. The endotherm at 363 ºC followed by exothermic decomposition at 550 ºC provides the end product either nickel or nickel oxide.
The percentage weight loss observed in TG – DTA is very much lower than the expected value in case of this final product formation namely nickel. This decrease may be attributed to the spurting of these complexes during the analysis.
4d. X- ray Powder Diffraction:
To understand the isomorphic nature between nickel complexes of maleate and terephthalate, their X- ray powder diffraction patterns have been compared. From the XRD pattern, it is concluded that these two complexes do not exhibit isomorphism.
Structural Study of [Ni(Amgu)2 H2O]SO4:
Among
all the complexes prepared the complex with the sulphate anion yielded single
crystals highly suitable for X-ray studies. This complex crystallizes in
monoclinic space group p21/m with cell dimensions a= 5.9988(3)
, b = 14.4843(7)
, c = 6.8974(3) , α = 90º, β = 112.6630º (10), γ = 90º
and V = 553.03. The R value is 0.0614.
The complex is a monomer in which nickel is six coordinated with an octahedron structure (Fig). The four nitrogen atoms from two chelate bidentate aminoguanidine ligands occupy the square planar corner position and the apical positions of the octahedron are occupied by two oxygen atoms from two water molecules. But it is interesting to note that water-oxygen is having an occupancy factor of 0.5 each. Therefore, as a whole, only one water molecule is present in the complex.