Synthesis, Physicochemical Properties and Antimicrobial Activities of Nickel(II) Complexes of 4,4,4-Trifluoro-1-(2-Naphthyl)-1,3-Butanedione and Their Adducts

Nickel(II) complex of 4,4,4-trifluoro-1-(2-Naphthyl)-1,3-butanedione(tfnb) and its 2,2′-bipyridine (bipy), ethylenediamine (en) and 1,10-phenanthroline (phen) adducts were synthesized and characterized by elemental analysis, infrared, electronic spectral, magnetic susceptibility measurements and antimicrobial activities. Three other nickel(II) compounds were synthesized along with these compounds. The bands in the 1651-1513 cm have been assigned as ν(C=O) + ν(C=C). The infrared and electronic spectral measurements are indicative of a probable six-coordinate octahedral geometry for all the nickel(II) complexes and adducts. The antimicrobial activity shows that [Ni(tfnb)2]2H2O complex and its adducts have no antibacterial and antifungal activities.


Introduction
β-diketones are highly versatile reagents for organic synthesis.They have been found to have remarkable application in metal extractions (Wenzel et al., 1985), as catalysts (Bose et al., 2016) and as precursors for Chemical Vapour Deposition (CVD) application (Banger et al., 2001).β-diketones and their complexes have attracted the attention of researchers because of their biological activities (Omoregie, 2012;Omoregie et al;2016;Omoregie et al., 2017) and electrochemical features (Ahumada et al., 2018).

Methodology
Ten microorganisms were used in the screening using a disc diffusion test.This test was performed to screen and determine the complex potentials according to their inhibition zone diameter.A mass of the culture medium (Nutrient agar and Sabouraud Dextrose agar) was weighed and dissolved in distilled water.The mixing was heated to homogenize and sterilized in an autoclave for 15 minutes before the mixture was then poured into Petri dishes and left to solidify.The compounds were weighed and dissolved in 2mL of methanol, making a concentration of 50 mg/mL, from which serial dilution of about four more concentrations were prepared.
A colony from a 24-hour fresh culture of the various organisms was dissolved in 5mL of sterile distilled water which was spread on the prepared culture agar.Sizeable hole was bored on the culture plate and the different concentrations of the complex were added into each hole.The Petri dishes were then incubated for 24 hours for the bacteria and 48 hours for fungi before the inhibition zone diameter was measured.The results are presented in Table 5.

Physical Measurements
Elemental analysis for CHN was determined at the School of Chemistry and Physics, University of KwaZulu-Natal South Africa.Percentage metal in the nickel(II) compounds was determined titrimetrically using EDTA.The magnetic susceptibilities of the compounds at room temperature were measured using Sherwood magnetic susceptibility balance.
Infrared spectra were measured using nujol on Perkin Elmer Spectrophotometer 11 FT-IR.The electronic spectra of the compounds in methanol and chloroform were recorded on a Perkin Elmer Lambda double beam UV/VIS spectrophotometer, using 1cm glass cell at the Department of Chemistry, University of Ibadan, Nigeria.

Results and Discussion
Table 1 shows the analytical data, colour, percentage yield and room temperature magnetic moments (μ eff ) of the prepared nickel(II) complexes.Percentage metal analysis was in significant agreement with those calculated for the proposed formula.Nickel(II) compounds were obtained as various shades of green.

Magnetic Moment
Literature has indicated that the magnetic moment of nickel(II) complexes have values between 2.8-4.2B.M. The expected spin only (μ s.o ) value for nickel(II) complexes is 2.83 B.M. but there are always deviations which are attributed to both orbital contributions and spin-orbit coupling of the 3 A 2g and 3 T 2g (F) terms depending on the stereochemistry (Patel and Woods, 1990;Greenwood and Earnsaw, 1997).Experimental moments of 2.9-3.3B.M. were observed for octahedral nickel 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. were observed for tetrahedral nickel due to orbital contributions giving higher moments than the spin-only value of 2.83 B.M. (Patel and Woods, 1990).
The room temperature magnetic moment of the synthesized nickel(II) compounds were in the range 2.86-3.07B.M. which is indicative of octahedral geometries except for [Ni(tfnb) 2 Phen]H 2 O with lower moment of 2.36 B.M. which may be due to interconversion of stereochemistries and/or dimerization (Osowole et al, 2000).

Infrared Spectra
The relevant infrared bands are presented in Table 2. Assignments of the infrared bands were made by comparing the spectra of the compounds with those already reported in the literature on similar systems (Omoregie, 2012;Omoregie et al., 2018).Literature review has shown that infrared spectra of β-diketones shows an appreciable coupling of different vibrational modes due to overlap of absorption frequencies (Nakamoto et al., 1961, Woods et al;2009a;Woods et al., 2009b;Omoregie et al., 2014;Omoregie and Woods, 2011).The metal-free ligand, HL, which consists of the uncoordinated C=O+C=C stretching vibrations has a double band in the 1603-1516 cm -1 region.Hypsochromic shifts of  as (C=O)  as (C=C) vibrations were observed in all the adducts relative to the parent complex except in [Ni(tfnb) 2 Bipy] which had a bathochromic shift.
CH deformation band of 2,2′-bipyridine was observed as strong band at 759 cm -1 region while the phenanthroline adducts bands were observed around 726-729 cm -1 and 847 cm -1 region.The presence of bands due to (M-O) and (M-N) in the range 420-696 cm -1 in the 2,2′-bipyridine and 1,10-phenanthroline adducts is further evidence of coordination (Patel and Woods, 1990b).

Antimicrobial Activities
The antimicrobial screening of the synthesized complexes was investigated using Gram-positive bacteria, Staphylococcus aureus and Bacillus subtilis, Gram-negative bacteria, Pseudomonas aeruginosa, Salmonella typhi, Escherichia coli and Klebsiella pneumoniae and four fungi (Candida albicans, Aspergillus niger, Penicillium notatum, Rhizopus stolonifer).The results of antibacterial activity are presented in Table 5.The ligand indicated a significant activity against the tested organisms except Aspergillus niger, Penicillium notatum, Rhizopus stolonifer in relation to which it indicated moderate activity.