Synthesis and Physicochemical Studies of Nickel ( II ) Complexes of Various 2-Alkyl-1-phenyl-1 , 3-butanediones and Their 2 , 2 ′-Bipyridine and 1 , 10-Phenanthroline Adducts

Some 2-substituted-1-phenyl-1,3-butanedionato nickel(II) complexes and their 2,2′-bipyridine (bipy) and 1,10-phenanthroline (phen) adducts have been synthesized and characterized by elemental analyses, infrared, electronic spectral studies, conductance, and magnetic susceptibility measurements. The electronic spectral data have been interpreted in terms of the π3→π4 and other transitions and the effect of the substituents at β-position on the different transitions determined. The infrared spectra of the nickel(II) complexes showed that the frequencies of the asymmetric C=O + C=C stretching vibrations were lowered from their ligand values.


Introduction
β-diketone and their metal complexes are among the most widely studied coordination compounds since they have wide application in the industries as catalyst (Schwieger et al, 2009;Xingbang et al, 2009;Ferreira et al, 2002;Poncelet et al, 2005;Lassahn et al, 2005;Campelo et al, 2006) and also as precursors for chemical vapour deposition (Zhang et al, 2006;Nable et al, 2003;Banger et al, 2001;Dela Rosa et al, 2003).Thermal transfer printing materials containing metal β-diketonates exhibit good whiteness and image stability (Miura et al, 1993) and it has been found that toner containing metal complexes of β-diketones are stable, controllable and capable of producing clear colour images even at high temperature and high humidity without producing copier stain (Hiroshi and Katsuhiko, 1987).

Preparation of Ni(Me-bzac) 2 .2H 2 O
3 mL 2-Me-1-phenyl-1,3-butanedione(3.39 g, 19.2 mmol) dissolved in 10 mL methanol was added to nickel(II) chloride hexahydrate (2.28 g, 9.60 mmol) in 3 mL water, while stirring and the pH was raised to 8. The Precipitated solids were washed with 40% methanol and dried over silica gel.Similar procedure was used for preparation of the other nickel(II) complexes.

Physical Measurements
Elemental analyses for C, H and N were determined by the analytical laboratory of Geological Survey of Ethiopia while nickel was determined using a complexometric method (Vogel, 1986).The analytical and physical data are presented in Tables 1 and 2. The room temperature magnetic susceptibilities of the compounds were measured by MSB-AUTO (Sherwood scientific).
The electrolytic conductivities of the soluble compounds in nitromethane at room temperature were determined using Digital conductivity meter (Labtech).
The solution spectra of the compounds in methanol and chloroform were recorded on a Unicam UV-Visible Spectrophotometer using 1cm glass cell.
The reflectance spectra of the nickel(II) complexes were recorded on a Perkin Elmer Lambda 950 UV/VIS spectrophotometer at the Department of Chemical Engineering, Faculty of Technology, Addis Ababa University, Ethiopia using calcium carbonate as reference.The infrared spectra of the compounds as pressed KBr disc were recorded on Perkin Elmer Spectrophotometer BX FT-IR.

Results and Discussion
The reaction of 2-alkyl-1-phenyl-1,3-butanedione with metal salt is represented by equation 1 while that of the metal complex and base is represented by equation 2. The adducts prepared directly from metal salt are represented by equation 3. Reasonable yield of the precipitates were obtained.The analytical data, colours, %yields, melting points/decomposition temperatures and room temperature magnetic moments (μ eff ) of the complexes are given in Table 1.All the compounds were obtained as various shades of green except few that were pinkish in colour.They exhibited good solubility in chloroform and methanol except few.The microanalytical data for the compound agree with the calculated values (Table 2).
The principal infrared bands are presented in Table 3. Infrared studies on diketones have shown that electron releasing substituents give rise to low  as (C=O) as (C=C) (Hancock and Thornton, 1969;Patel and Adimado, 1979).Infrared spectra of 2-substituted-1-phenyl-1,3-butanediones studied showed a decrease in frequency as the electron in the system increases except Me-bzacH and i-Pr-bzacH which showed an increase in frequency.Reduction of the frequency of  as (C=O) as (C=C) is an indication of the positive inductive effect of the alkyl group on the system.Single bands of  s (C-O)C-H were observed in all the ligands except i-Pr-bzacH had two distinct bands.Reduction of the coupled  s (C-O)C-H vibrations of 2-substituted-1-phenyl-1,3-butanedione was observed as the length of the alkyl group increases except i-Pr-bzacH which had increased frequency.The reduction in frequency also indicates the positive inductive effect of the alkyl group on the system.The methyl deformation bands of the ligands studied were observed in the 1421-1359 cm -1 region (Wood and Patel, 1994;Patel and Woods, 1990;Tanaka et al, 1969;Koshimura et al, 1973;Nakamoto et al, 1961).These bands were observed as double bands except n-Bu-bzacH which had a single absorption band.The infrared spectra of the nickel(II) complexes showed that the frequencies of the asymmetric C=O + C=C stretching vibrations were lowered from the ligand values in the order: The order shows that the substituted Ni(II) complexes have larger bathochromic shift than the unsubstituted complex except Ni(n-Bu-bzac) 2 .2H 2 O which had hypsochromic shift.The larger bathochromic shift probably indicates the positive inductive effect of the alkyl group on the system.The  as (C=O)  as (C=C) stretching vibrations of these nickel(II) complexes were observed in the range 1560-1726 cm -1 while  s (C-O)C-H were observed in the 1450-1492 cm -1 region with Ni(bzac) 2 .2H 2 O Ni(Et-bzac) 2 .2H 2 O, and Ni(n-Bu-bzac) 2 .2H 2 O having multiple bands.A decrease in  s (C-O)C-H of 2-substituted-1-phenyl-1,3-butanedionato nickel(II) complexes was observed as the length of the alkyl group increases except Ni(Et-bzac) 2 .2H 2 O and Ni(n-Bu-bzac) 2 .2H 2 O which had increased frequency.The decrease in frequency indicates the positive inductive effect of the alkyl group on the system (Bamkole and Ogunkoya, 1981).The frequency of the methyl deformation band decreases as the length of the alkyl group increases with the exception of Ni(n-Pr-bzac) 2 .2H 2 O and Ni(n-Bu-bzac) 2 .2H 2 O which had increased frequency.
The phenyl-substituted ligands had bands in the 38,000-46,000 cm -1 region, which may be attributed to primary bands in the benzene π system (Ogden and Selbin, 1968).The synthesized ligands showed similar bands in the region 39,526-44,248 cm -1 which have been assigned to π-π* transition of the phenyl ring.
Hypsochromic shifts of the π 3 -π 4 * transition bands were 1,10-observed on substituting the 2-position of 1-phenyl-1,3-butanedione with methyl, ethyl, n-propyl and i-propyl in chloroform and methanol.Hypsochromic shifts of the π 3 -π 4 * transition bands could be attributed to the positive inductive effect of the alkyl groups leading to higher frequency shift (Graddon and Schulz, 1965).
The solution spectra in methanol showed that the π 3 -π 4 * transitions of the nickel(II) complexes are lowered from the ligand values in the order: Ni ).The order shows that the substituted Ni(II) complexes have larger bathochromic shift than the unsubstituted complex, Ni(bzac) 2 .2H 2 O except Ni(n-Bu-bzac) 2 .2H 2 O which had π 3 -π 4 * hypsochromic shifts in chloroform and methanol.
The room temperature magnetic moments of the synthesized nickel(II) complexes were in the range 3.02-3.26B.M. which is indicative of octahedral geometries except Ni(n-Pr-bzac) 2 .2H 2 O with moment of 4.11 B.M. The spin only (μ s.o ) value of 2.83 B.M is expected for nickel(II) complexes.Experimental moments of 2.9-3.3B.M. are normally observed for octahedral nickel(II) complexes due to spin-orbit coupling of the 3 A 2g and 3 T 2g (F) terms while moments of 3.2-4.1 B.M. are observed for tetrahedral nickel due to orbital contributions.In the synthesized complexes, lower moments were observed on substituting the 2-position of Ni(bzac) 2 with alkyl groups except Ni(Et-bzac) 2 .2H 2 O and Ni(n-Pr-bzac) 2 .2H 2 O which had increased moments.Reduction in moment may probably be due to increase in electron density around the metal leading to reduction in orbital contribution and therefore lower moment.The adducts studied displayed effective magnetic moments (μ eff ) in the range 1.81-3.31B.M. The magnetic moments between 1.81-2.65B.M. which are lower than the spin only value were observed for Ni(Me-bzac) 2 phen and Ni(n-Bu-bzac) 2 phen.The lowering of the moment observed in the compounds studied may be attributed to interconversion of stereochemistries and /or dimerization (Osowole et al, 2000).A decrease in moment was observed on comparing the 2,2′-bipyridine adduct of 2-substituted-1-phenyl-1,3-butanedionato nickel(II) complexes with Ni(bzac) 2 bipy.The 1,10-phenanthroline adduct of nickel(II) 2-substituted-1-phenyl-1,3-butanediones also exhibited decreased moment.Reduction in moment is attributed to increase electron density around the nickel leading to reduction in orbital contribution and therefore lower moments are obtained.

Figure 1 .
Fig. 1.Proposed structures for Nickel adducts Figure 1.Proposed structures for Nickel adducts