Catalytic Studies of Complexes of Organic Compounds Part-4: Synthesis, Characterization, Catalytic activity of Cd(II) Complex of Chiral Schiff base

Chiral Schiff base ligand from 3,5-Diiodo-salicylaldehyde with a chiral amine (1S,2S)(+)-1,2Diaminocyclohexane is synthesized, and its Cd (II) complex was synthesized. These were analysed by the physical constant, TLC, Colour, UV-Vis, FTIR and 1H NMR method. Also, efforts were made to study the catalytic activity of Cd(II) chiral Schiff base complex. The oxidation of benzyl alcohol was used as model reaction using acetonitrile as solvent. The present reaction system was heterogeneous system of catalysis.


INTROdUCTION
Oxidation of alcohol to the corresponding carbonyl compound (aldehyde, ketone or acid) is the key step in many organic synthetic methods. The catalytic oxidation of alcohol to aldehyde or ketone has been developed employing aqueous hydrogen peroxide (H 2 O 2 ) in presence of Cd complex as catalyst is a safe process. In particular, benzyl alcohol (primary aromatic) was oxidized selectively to their corresponding aldehyde can be optimised in the present conditions. The oxidative transformation of primary alcoholic compounds to the aldehydes remains very problematic due to the oxidation of primary alcohols the other possible product (uncontrolled oxidation process) is the analogous carboxylic acids [1][2][3][4] . Last but not the least, tandem oxidation may be achieved without the need to isolate the any intermediate or change in solvent. Schiff bases and their metal complexes are of prime importance in the field of synthetic chemistry. Literature reports that these complexes are studied in the view of organic intermediates, metal complexation 5 also voltammetric 6 reduction and oxidation processes. The potential of chiral schiff base complex as catalyst towards oxidations, ring closer, epoxidation, hydrogenations, polymerizations and various coupling reactions [7][8] . Chiral Schiff bases are the reported as a catalyst viz. in asymmetric cyclopropanation 9 and also in varied organic reactions.
The oxidative transformation of primary and secondary alcohols selectively into the respective aldehydes and ketones without forming undesired product is the most important reaction for both industrial applications and academic interest [10][11] , still many processes of oxidation in use are nonenvironmentally friendly. (where X = Cl or Br; E = P or As; L = monobasic bidentate Schiff base ligands) have been synthesized and characterized. The catalytic efficiency of one of the ruthenium complex was determined in the case of oxidation of primary and secondary alcohols into their corresponding aldehydes and ketones in the presence of NMO (N-methylmorpholine-N-oxide) as co-oxidant 17 . An enantiopure GO (galactose oxidase) enzyme model has been used synthesized from readily available (R)-binam and Cu(OTf) 2 (TEMPO = 2,2,6,6-Tetramethyl-piperidin-1-oxyl) and has been effectively used as an efficient chiral catalyst for the oxidative kinetic resolution of secondary alcohols. 18 The dilute H 2 O 2 is an oxidant of ideal choice. It is cheap, readily available and gives water as the only by product. Many systems using aqueous H 2 O 2 as oxidant and Ligand-based catalysts under catalytic conditions have been reported by J. F. Larrow et al., 19 . Also literature contains synthesis of ligand from (1R,2R)-(+)-1,2-Diaminocyclohexane and their complexes, However, literature reveals that there is scare or no publication on use of Cd complexes of chiral Schiff bases, an asymmetric catalyst, for the oxidative transformation of benzyl alcohol. As per the literature survey done by us there is no literature available on heterogenous catalytic oxidative transformation of alcohols by these synthesized complex N,N'-(S,S)-1,2-Cyclohexylenebis-(3,5-diiodosalicylidene-iminato) cadmium (III) chloride, complex using N,O-donors along with H 2 O 2 as oxidant.
In continuation of our reported studies 14 of complex of organic compound with respect to behaviour, here we have reported the synthesis of chiral schiff base from 3,5-Diiodo-salicylaldehyde with chiral cyclodiamine, viz. (1S,2S)-(+)-1,2-Diaminocyclohexane in 2:1 proportion. The metal complex was formed from chiral Schiff base with cadmium chloride monohydrate, it was characterized and its catalytic effects were studied, on oxidation of a model compound, benzyl alcohol (Ph-CH 2 -OH).

MATERIAL ANd METHOdS
The chemicals used were of synthesis grades, for the present work. TLC plates of aluminium material of Merck make (silica gel 60 F254) were used with TLC grade solvents for monitoring the progress and reaction completion. Iodine chamber was used to highlight the starting and final products spot identification in TLC. The estimation of elemental halogen and cadmium in complex was estimated by reported methods 20-21 . FTIR spectral determination were made in 4000-600 cm -1 frequency on a Bruker Spectrum 2000 FT-IR spectrophotometer at SAIF, Kochi for the ligands while FTIR of complexes were recorded on Perkin FT-IR Spectrophotometer, at IISER, Bhopal in the in the range 4000-450 nm. The other instruments used for analysis are mentioned in earlier reports 14 .

Synthesis of Ligand
The ligand, L was synthesized as per previously reported work procedure 14 . TLC using mobile phase Ethyl Acetate: n-Hexane 2.5:0.5 was performed, to monitor the completion of reaction. The yellow-brown coloured crude product was filtered, dried and recrystallized from ethanol, dried and kept in vacuum desiccator containing calcium carbonate as desiccant. On further drying the yellow-brown pure product was obtained, m.p. = 123-126 o C.

Catalytic oxidation
The oxidation process of benzyl alcohol by use of CdL, used to study the catalytic parameters are as per previous report. 14

RESULTS ANd dISCUSSION
Ligand (S,S)-N,N'-Bis-(3,5-diiodosalicylidene)-1,2-diaminocyclohexane and the Cd(II) complex was synthesized in two steps. Firstly, the selected 3,5-Diiodosalicylaldehyde and the Chiral amine were condensed to get (S,S)-N,N-Bis-(3,5diiodo-salicylidene)-1,2-diaminocyclohexane. In the second step, cadmium (II) chloride monohydrate (CdCl 2 .H 2 O) was used to prepare its Cd(II) complex i.e. [ (DiiodoSalcyclo)Cd (II)Cl]. As mentioned in our reported work 14 reports that 2 nd mole may react separately after the 1 st mole reacted or both moles reacts in the same step. The route of synthesis of ligand and the Cd(II) complex is depicted respectively in Scheme 1 and Scheme 2. Synthesized chiral Schiff base and its Cd(II) complex were analysed by colour, TLC, m.p., elemental (C, H, N, Cl and Cd) and spectral data viz. UV-Viz, FTIR and 1 H NMR data (400 MHz, DMSO) results. The results are as displayed below.
The elemental analysis data of the ligand and complex are satisfied in agreement for proposed molecular formulas. UV-Vis spectra for the Ligand, L and its Complex were recorded using ethanol as solvent. The obtained values of elemental analysis for the schiff base ligand and its metal chelates are consistent with the calculated values. The complexation of chiral Schiff base with cadmium (III) ion showed UV-Vis differences in π → π* transition was shifted from 258 nm to 265 nm, and for the n→π* transition was changed from 343 nm to 347 nm. The UV-Vis spectra for ligand, L before and after the complex, CdL formation is depicted in Figure 1.
The FTIR spectra of ligand, L before and after complex, CdL formation depicted respectively in Figure 2A and Figure 2B. The ligand exhibited a band around 2930 cm -1 weak band due to intramolecular H-bond between H of -OH group and N of C=N group, which is in agreement with earlier reports [23][24] . The stretching vibrations of phenolic OH and phenolic C=O in the free ligand appear at 2930 and 1210 cm -1 respectively and the stretching bands of phenolic C=O have been found at higher values (1245 cm -1 ) in Cd chelate confirming the coordination through phenolic oxygen (M=O) [23][24] . In the FTIR spectrum of ligand, the absorption frequency at 1625 cm -1 appeared because of Schiff base which on chelation, it is shifted to lower 23-27 wave numbers upto 15 cm -1 in metal chelates indicating the participation of azomethine nitrogen in coordination with central metal ion (M-N) which is due to the reduction of double bond character in C=N bond. The complexation of Schiff base with cadmium ion understood with significant differences in FTIR frequency for >C=N and >C-O groups.
The notice of absence 28 of phenolic . The extra agreement of formation of complex has been provided due to band at 550 cm -1 with weak-intensity, which is attributed to n Cd-O of phenolic part of Schiff base in the complex 30 . The FTIR spectral characteristics frequencies are in concurrence with that reported for the similar compounds as reported 29-30 .
The 1 H NMR (in DMSO) of Chiral Schiff bases, L and its complex, CdL are depicted in Figure  3A and Figure 3B respectively.   The results of oxidation catalysis of benzyl alcohol by using H 2 O 2 as oxidant and the cadmium complex of chiral schiff base as catalyst in acetonitrile to form benzaldehyde are summarized in Table 2.
The Table 2 indicated a comparison of the present oxidation catalysis with the results available in literature which indicated that, time wise it is less time than required by other Heterogeneous catalysis and it matches with the conversion of the benzaldehyde formation 2,14,[17][18] . In the present work, % conversion of oxidation of benzyl alcohol to benzaldehyde by H 2 O 2 oxidation in presence of catalytic CdL, using solvent acetonitrile is 51.33% which is slightly less as compared to that with the Cr(II) catalyst from chiral Schiff base with dibromo substituent which is small in size, and hence it is more efficient catalyst. As because the size of iodine makes the ligand more bulkier than when it is bromo substituent the rate of oxidation is less than the Cr(III) L catalyst recently reported 14 .
Literature showed that the many ruthenium catalytic reactions were reported with a oxidants of wide variety viz., hydrogen peroxide 15

Possible mechanism for the Benzyl alcohol to Benzaldehyde oxidation reaction
A possible route of oxidation mechanism based on the above discussed data can be proposed as below, The product of oxidation catalysis obtained was dried over sodium sulphate and was concentrated in microflask under vacuum (carefully) and resulting liquid product was purified by eluting the reaction mass using hexane:ethyl acetate on silica gel column to obtain the benzylaldehyde as a colourless liquid showed FTIR (in cm  Conceptually, above sequence can be written in the form of Scheme 4 as below... Typical mechanism of catalytic reaction for Cd 2+ -moiety is discussed here. But, on the basis of colour change through catalytic system of reaction and depending on mechanisms assigned for reactions of similar nature [39][40][41][42]45 , one could predict that important process is oxo-cadmium (II) moiety presence in catalytic transformation through alcohols oxidation, as depicted in Scheme 3, which is scarely discussed.

CONCLUSION
The chiral Schiff base was synthesized from chiral amine and its mononuclear Cd(II) complex was synthesized. The prepared Cd(II) complex was characterized by different analytical and spectral methods. Cd(II)-complex was tested as catalysts for one pot conversion of benzyl alcohol to aldehyde by the oxidation by use of H 2 O 2 in mild conditions. The possible oxidized products of benzyl alcohol were benzaldehyde and or an impurity, benzoic acid. It is heterogeneous system of catalysis. In the visible region, complex exhibited a peculiar absorption band responsible for the metal-to-ligand charge transfer (MLCT). The prepared CdL is a promising catalyst in oxidation of alcohol using hydrogen peroxide as terminal oxidant. The main benefit of this catalytic system are the easy synthesis of ligand and its Cd complex and also modification of ligand is easy process. In present work, % conversion (oxidation) benzyl alcohol to benzaldehyde by H 2 O 2 oxidation in presence of catalytic CdL, using solvent acetonitrile is less than the Cr(III)L catalyst with comparatively less bulkier ligand (substituent) recently reported 19 . This work leads to the development of new class of catalyst system for oxidative transformation of benzyl alcohol to benzaldehyde and related work is now underway.

ACKNOwLEdgEMENT
Authors are thankful to the Management of Smt. G. G. Khadse College, Muktainagar for providing required facilities. Authors are thankful for recording UV-Vis spectra and HPLC analysis by Shree Reliable's Industrial laboratories, Jalgaon (MS). We are also thankful to SAIF, Kochi, Kerla for FTIR and 1 H NMR spectra recording.

Conflicts of Interest
The authors declare no conflict of interest.