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

Saptarsi MondalDept. of Chemistry

M.Sc. 1st yearRoll No-115030013

Indian Institute of Technology, Bombay

Brief Introduction:

The complexes of Nickel-(α-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 α-benzoinoxime dissolved in 20 ml of absolute ethanol. The pH is fixed to 10 by ammonia. A yellow precipitate appears(complex-A)

C

N

HC

O

Ni

O N

CCH

OHOH

C

N

HC

O

Ni

O N

CCH

OHOH

HO

OH

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 α-benzoinoxime solution (3 mmoles) is added and a yellow orange precipitate appears (complex-B).It is filtered and washed with water and ethanol-water 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.

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.

Complex Formula Formula weight

Calculated(Found) %C H N Ni

A C28H26N205Ni 528.71 63.55 4.91 5.29 11.10(63.25) (4.75) (5.25) (10.80)

B C28H29N2O7.5Ni 571.71 58.77 5.07 4.89 10.26(58.91) (4.73) (5.19) (10.24)

Table-2

UV-Visible spectroscopic study : d6 low spin 2T1g

1T1g

1A1g

Complex-A: Band -430nm (ε=145M-1cm-1) , shoulder peak-520nm(ε=50M-1cm-1)

Complex-A after 5 days/complex-B: 430 nm band disappears. Well defined band -520nm(ε=240M-1cm-1), shoulder peak-600nm (ε=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 Colour Yield(%) M.P.(OC) χM (Ω-1 cm2mole-1)

Μ(M.B)

A Yellow 90 135 20.83 0.435

B Yellow orange 85 158 20.37 0.870

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)

Inference :

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.

H20(free) Bound(OH) C=N N-O C-O Ni-O Ni-N

Ligand 1491 1242 986

Complex-A 3600 3200 1493 1015 580 475

Complex-B 3600 3200 1500 1228 1010 590 480

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

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.

δ(ppm) Assignment Coupling const.(Hz)

α-benzoinoxime 7.13-7.16(m)7.28-7.33(m)5.59(d)3.72(d)

H(aromatic)H(aromatic)H-C-OH*H*-C-OH

5.25.3

Complex-A 6.78-8.58(m)3.65(d)2.91(d)2.17(s)1.26(s)

H(aromatic)H*-C-O-H*-C-NH2Bound OH-NH2

20.018.0

Complex-B 6.54-8.76(m)3.75(s)1.86(s)

H(aromatic)H*-C-OHBound OH

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 [CR2=NOH]H+ R2=NH2

+ CR2CHNH3 +

simply-

iii) So, this NMR spectrum is superposition of the two types of ligands (imine and amine).

Conclusion :

1. Ni complexed by α-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)[α-benzoin oxime to +II state:1 mole of L-ascorbic acid is reacted with one mole of the complex to produceto 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)- α-benzoin oxime

+ Ni(IV) complex + Ni(III) + H+

Reductant Oxidant Primary radical(R*)

+ R* (Initiation) RM* RM* + nM RM*n+1 (Propagation)

Mn*+ Mn* Polymer (Termination)

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

C

NHO

HC

O

Ni

O

CH

C

NOH

C

NHO

HC

O

Ni

O

CH

C

NO

H2C C

CH3

COOCH3

R CH2 C

CH3

COOCH3

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 α-Benzoinoxime Complexes Generated by Ligand-Metal Redox ReactionZ. Hank a , S. Boutamine a , M. Meklati a & O. Vittori .2. Polymerization of Methyl Methacrylate with Nickel (II)a-Benzoinoxime ComplexHafida Harrar-Ferfera, Farouk AmraniFaculty of Chemistry, University of Sciences and Technology, Houari Boumediene, Algiers 16111,Algeria3. Kinetics and Mechanisms of Reduction of a Nickel(iv) Complex by Ascorbate IonBy 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.