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[email protected] Current Catalysis, 2015, 4, 43-56
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Synthesis, Spectral and Thermo Analytical Studies of Nickel (II) and Ruthenium (II) Complexes of Schiff Base Ligands for Hydrogenation of Benzene Tejpal Singh Chundawata,b, Poonam Kumaria, Subash Chandra Mohapatraa, Sunita Bhagata,*, Sanjay Bawejab, Sachin U. Nandanwarb and Mousumi Chakrabortyb,# a
Organic Synthesis Research Laboratory, Department of Chemistry, A.R.S.D. College, University of Delhi, New Delhi-110021, India
b
Department of Applied Sciences, ITM University, Gurgaon-122017, Haryana, India
c
Department of Chemical Engineering, S.V. National Institute of Technology, Surat395007, Gujarat, India Abstract: Novel Nickel(II) and Ruthenium(II) complexes of schiff base ligands were synthesized and studied using various spectral and thermoanalytical techniques. One of the metal complexes [NiII (L-2)2(CH3OH)2] has been structurally characterized on the basis of X-ray data and was found to adopt a helical structure stabilized by strong intermolecular H-bonding to form a dimeric structure. The complex crystallizes in the triclinic space group P-1 with unit cell dimensions a=11.1863(13) Å, b=12.0526(16) Å, c =12.5639(14) Å, = 112.216°, =95.065°, = 105.289° and Z=2. The coordination environment around the Ni(II) ion is in distorted octahedral geometry with four donor atoms (N2O2) coming from the two ligands providing four coordination sites (two by each ligand) through two azomethine nitrogen, N1 and N2 (Ni(1)–N(1) = 2.080(5) and Ni(1)–N(2) = 2.090(5)Å and two deprotonated hydroxyl oxygen, O1 and O2 (Ni(1)–O(1) = 2.008(3) and Ni(1)–O(2) = 1.998(3)Å. The remaining two coordination sites O3 and O4 are provided by two coordinated methanol molecules (Ni(1)–O(3) = 2.093(4) and Ni(1)–O(4) = 2.154(4)Å. Stability of the complexes was determined using TG-DTA.The fluorescence quantum yield of complexes was found to be lower than that of the (L-2) ( =0.032). [NiII (L-2)2(CH3OH)2] was used as a catalyst in hydrogenation.
Keywords: Selective hydrogenation, schiff base complex, dimeric crystal structure, hydrogen bonding network. INTRODUCTION The design and strategic synthesis of transition metal complexes of Schiff bases is an active area of research, as they offer opportunity for including substrate chirality, tuning metal centered electronic factor and enhancing solubility and stability of either homogenous or heterogenous catalyst. Schiff base complexes play very important role in coordination chemistry mainly due to their facile syntheses, solubility and easily tunable steric and *Address correspondence to this author at the Organic Synthesis Research Laboratory, Department of Chemistry, A.R.S.D. College, University of Delhi, New Delhi-110021, India; Tel: +91-9891351935; Fax: +91-011-24111390; E-mail:
[email protected] # Both corresponding authors contribute equally
2211-54/15 $58.00+.00
electronic properties [1]. Among these oxygen and nitrogen donor Schiff’s base ligands and their metal complexes are of particular interest because of their unusual configurations, structural lability and their sensitivity to molecular environments [2, 3]. Planar ligands with amine or imine donor groups and bridging phenolic oxygens are usually obtained by condensation of appropriate formyl and amine precursors [4]. Schiff base ligands have been proved to be very effective in constructing supramolecular architectures such as coordination polymers, double helixes and triple helicates [5]. Schiff bases can accommodate different metal centers involving various coordination modes allowing successful synthesis of homo- and hetero metallic complexes [6]. ©2015 Bentham Science Publishers
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Current Catalysis, 2015, Vol. 4, No. 1
Chundawat et al. NH2
R1 O R2 1
R1 MW, Heat N
+ Solvent R2 R3 2
R3
3 L-1: R1=OH, R2=H, R3=H L-2: R1=OH, R2=H, R3=Br L-3: R1=H, R2=CH3, R3=Br
Scheme (1). Synthesis of (L-1) &(L-2): Methanol, MW, 800C and (L-3): Toluene, PTSA, Molecular sieves, MW, 140 °C.
The formation of Schiff base intermediates in reactions of biological importance [7-9] viz. transamination, racemization and decarboxylation is well documented [10]. The -system in a Schiff base that often imposes geometrical constraints affecting electronic structure [11]. The chemistry of Schiff base complexes with nickel has a strong role in bioinorganic chemistry and redox enzyme systems [12]. Morrow and Kolasa reported the cleavage of plasmid DNA by square planar nickelsalen [bis-(salicylidene) ethylenediamine] in the presence of either magnesium mono peroxyphthalic acid (MPPA) or iodosylbenzene [13]. Further, the chemistry of ruthenium Schiff base complexes also play a major role in inorganic chemistry due to its photoinduced [14, 15] and various catalytic properties [16, 17]. Ruthenium complexes act as catalysts due to their reversible and accessible oxidation states. In addition, Schiff base complexes of transition metals are efficient catalysts in carrying out asymmetric reduction of organic compounds [18-21]. Hydrogenation of aromatic compounds to aliphatic cyclic products is an important reaction with potential applications in chemical industry [22-24]. Reduction of benzene is of immense application in the industry, as the product, cyclohexene is used as a raw material for the production of adipic acid and caprolactam, both of which are intermediates used in the production of Nylon 6 and Nylon 66 [25]. Various metal-based catalysts including those of nickel and ruthenium have been extensively used for the partial and complete reduction of benzene [26-28]. The catalytic reduction of benzene nucleus generally requires more severe conditions than that of simple olefins [29] Even though a lot of success has been achieved
with regard to transition metal complex catalyzed hydrogenation of olefins [30], there are only few reports on studies involving hydrogenations of arenes using homogeneous metal complex catalysts [31-33]. Considering all these findings on Schiff base chemistry, we have synthesized some novel Nickel (II) and Ruthenium (II) complexes of Schiff bases and studied their spectral and thermo analytical properties. In this paper, we describe the synthesis of Nickel (II) and Ruthenium (II) complexes with various Schiff base ligands viz. 2-phenyliminomethyl-phenol (L-1), 2-[(4-bromo-phenylimino)methyl]-phenol (L-2) and (4-bromo-phenyl)-(1phenyl-ethylidene)-amine (L-3) and studies of their thermal and thermoanalytical properties. The synthesized complex of Schiff bases has been used as catalyst for hydrogenation reaction of benzene under mild condition. The activity of the catalyst was checked by varying the different parameters like effect of hydrogen pressure, reaction time and reaction temperature. RESULT AND DISCUSSION Synthesis of ligands L-1, L-2 and L-3 was done by microwave method and the complexation was carried out by condensation in methanol (Scheme 1). The Nickel complex separates as a yellowish solid and ruthenium complex separates as a red violet solid. The complexes were stable in air and are soluble in methanol, acetonitrile, DMF and DMSO (Scheme 2). Confirmation of the complex formation was determined by IR, NMR and X-ray analyses. Spectral data are mentioned in supporting information.
Synthesis, Spectral and Thermo Analytical Studies of Nickel (II)
Current Catalysis, 2015, Vol. 4, No. 1
45
Br
OH
MeOH +
H3C
NiCl2.6H2O or RuCl3.3H2O
H
N
O
O
Reflux, 10-12hrs
N
M N
O
2 1
H O CH3
Br
Br 3
M=Ni, Ru
Scheme (2). Synthesis of nickel and ruthenium complex of ligand (L-2).
Description of crystal structure [NiII (L2)2(CH3OH)2] CH3OH X-ray structure determination of the Nickel complex of L-2 has been carried out and mentioned in Table 1.
Empirical Formula
C29H30Br2N2NiO5
Theta range for data collection
2.95 to 25.00°
Index ranges
-13
