Synthesis and Characterization of [Ni(IV)α-

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Synthesis and Characterization of [Ni(IV)-(benzoinoxime)] Complexes Generated by Ligand-Metal Redox Reaction

Saptarsi Mondal Dept. of Chemistry M.Sc. 1st year Roll No-115030013 Indian Institute of Technology, Bombay

Brief Introduction:The complexes of Nickel-(-benzoinoxime) can be prepared in solid state with or without potassium persulphate and they leads to complex-B and complex-A respectively. Two different synthesis also leads to the same oxidation state of Ni i.e. (IV)

Complex-A

Complex-B

Solid state synthesis of the complexes:The complexes are isolated by precipitation from an ethanol-water mixture(1:1) Complex-A:

In the absence of oxidant the process consists of mixing 0.291 g(1 mmole) of Ni(NO3)2.6H2O, in 20 ml of distilled water and 0.682 g(3 mmoles) of -benzoinoxime dissolved in 20 ml of absolute ethanol. The pH is fixed to 10 by ammonia. A yellow precipitate appears(complex-A)

Complex-B:0.81 g(3 mmoles) of K2S2O8 taken in 20 ml of water are mixed with 0.291 g(1 mmole) of Ni(NO3)2 , 6H2O in the presence of ammonia(pH=11) when a black product precipitates(NiO2.3H2O). It disappears when 20 ml of the ethanolic -benzoinoxime solution (3 mmoles) is added and a yellow orange precipitate appears (complex-B).It is filtered and washed with water and ethanolwater mixture and dried at 800C.

Characterization:1.Elemental Analysis : The following formulas Ni(C14H11NO)2(OH)2.H2O and Ni(C14H12NO2)2(OH)2.1.5 H20 are found from the table.

Complex A B

Formula C28H26N205Ni C28H29N2O7.5Ni

Formula weight 528.71 571.71

Calculated(Found) % C H 63.55 (63.25) 58.77 (58.91) 4.91 (4.75) 5.07 (4.73)

N 5.29 (5.25) 4.89 (5.19)

Ni 11.10 (10.80) 10.26 (10.24)

2. Magnetic Study : The magnetic moment of complex-B is 0.867 B.M. though theoretically it is diamagnetic. So it undergoes decomposition to Ni(III) complex which can be detected by the EPR study. The magnetic moment of complex-A varies until it reaches 0.835 B.M. So it can be accounted as a progressive oxidation towards (IV) state probably with an intermediate paramagnetic state. 3.Conductometric Study : The weak values of the molar conductivities carried out in DMSO show a non electrolytic behaviour of the complexes.

Table-2

Complex ColourA Yellow

Yield(%)90

M.P.(OC)135

M (-1 cm2mole-1)20.83

(M.B)0.435

B

Yellow orange 85

1582T 1g 1T 1g

20.37

0.870

UV-Visible spectroscopic study : d6 low spin

1A

1g

Complex-A: Band -430nm (=145M-1cm-1) , shoulder peak-520nm(=50M-1cm-1) Complex-A after 5 days/complex-B: 430 nm band disappears. Well defined band 520nm(=240M-1cm-1), shoulder peak-600nm (=100 M-1cm-1) . The same band positions are obtained with the chloroform solution of extracting Ni in the presence of oxidant . The spectra of the dissolved complexes(A & B) are also exactly the same.

Complex-A

Complex-A after 5 days/complex-B

IR Study :Characteristic IR Bands of the ligand and the complexes as KBr Pellets (cm-1)

H20(free) Ligand Complex-A 3600 Complex-B 3600Inference :

Bound(OH)

C=N 1491

N-O 1242

C-O 986

Ni-O

Ni-N

3200 3200

1493 1500 1228

1015 580 1010 590

475 480

1. The shift of C=N and C=O vibrations and appearance of two new metal-ligand bands in the IR spectra prove that coordination occurs by both N and O atoms . It also implies deprotonation of OH group.

2. Disappearance of N-O band (for complex-A) establishes the oxime reduction to an imine and consequently Ni oxidation to (IV) state.

3. The coordinated H2O molecules in the complexes are identified by the broad absorption around 3200 cm-1 due to OH grouping.

Table: 1H NMR data for the ligand and the complexes in CDCl3(ppm)

(ppm)-benzoinoxime 7.13-7.16(m) 7.28-7.33(m) 5.59(d) 3.72(d) 6.78-8.58(m) 3.65(d) 2.91(d) 2.17(s) 1.26(s) 6.54-8.76(m) 3.75(s) 1.86(s)

AssignmentH(aromatic) H(aromatic) H-C-OH* H*-C-OH H(aromatic) H*-C-OH*-C-NH2 Bound OH -NH2 H(aromatic) H*-C-OH Bound OH

Coupling const.(Hz)

5.2 5.3 20.0 18.0

Complex-A

Complex-B

Inference :1. Ligand : The value of the coupling constant indicates the existence of an interaction between two protons. OH peak is absent due to rapid exchange with the solvent CDCl3. 2.Complex-B : Spectrum is very similar to that of the oxime.

3.Complex-A :i) Very unique spectrum compared to oxime and complex-B. Exhibits a peak at 1.26 ppm for amine. So oximes are reduced to amines in which the N-O bond is cleaved before hydrogenation of the C=N bond. The reaction is : CR2=NOH simplyiii) So, this NMR spectrum is superposition of the two types of ligands (imine and amine). [CR2=NOH]H+ R2=NH2+ CR2CHNH3 +

Conclusion :1. Ni complexed by -benzoinoxime is oxidized by the ligand without any oxidants. This oxidation occurs by an electron transfer from the metal to the ligand. 2.The existence of the higher oxidation states of Ni and the structural change of the ligand has been exposed on the basis of UV-Visible,1H NMR and magnetic data.

Reactivity and Application :i)Reduction of Ni(IV)[-benzoin oxime to +II state:1 mole of L-ascorbic acid is reacted with one mole of the complex to produce to produce +II oxidation state in the pH range 1-6. Reaction follows first order kinetics.

[NiL]+2 +H2A

[Ni(H2L)]2+ + A

ii) Polymerisation of MMA with Ni(II)- -benzoin oxime

+ Ni(IV) complex

+ Ni(III) + H+

Reductant + R*

Oxidant

Primary radical(R*) (Initiation)RM*

RM* +

nM

RM*n+1 (Propagation) Polymer (Termination)

Mn*+ Mn*

Proposed mechanism of MMA polymerization initiated by Ni(II)a-benzoinoxime complex.

Acknowledgement :I am very thankful to Prof. C.P.Rao , Prof.Maity for encouraging me to deliver a presentation on a particular topic which would be helpful for me in future. I also want to thank Rajashree madam, Amal Sir , Puspendra Sir, and Akanksha Madam for their constant help and precious words in the lab. Thank you all.

References :1. Synthesis and Characterization of High Oxidation State Nickel -Benzoinoxime Complexes Generated by Ligand-Metal Redox Reaction Z. Hank a , S. Boutamine a , M. Meklati a & O. Vittori . 2. Polymerization of Methyl Methacrylate with Nickel (II)a-Benzoinoxime Complex Hafida Harrar-Ferfera, Farouk Amrani Faculty of Chemistry, University of Sciences and Technology, Houari Boumediene, Algiers 16111,Algeria 3. Kinetics and Mechanisms of Reduction of a Nickel(iv) Complex by Ascorbate Ion By A. Graham Lappin,' Mauro C. M. Laranjeira, and Lawson Youde-Owei, Department of Chemistry, University of Glasgow, Glasgow G12 8QQ. 4. R.P.Sarcar-PART-II, page no-769. General and Inorganic Chemistry.