The Effect of 4-Acyl Substituents on the Infrared Stretching Frequencies Of Some 1-Phenyl -3- Methyl -4- Acylpyrazol  -5-Ones And Their Magnesium (Ii) ,Cobalt(Ii), Copper (Ii) And Zinc (Ii) Chelates.


The divalent metal chelates of Mg,Co,Cu and Zn with 4-acetyl (hpmap), 4- benzoyl(hpmbp),4-butyryl(hpmbup),4-capyroyl(hpmcp),4-propiony  (hpmprp)    and   4-palmitoyl(hpmpp)   derivatives   of    1-phenyl   -3-methyl pyrazol-5-one  have been   synthesized   and  characterized  by UV ,IR,   and conductivity measurements.

It is shown that the ligands behaved like bidentate enols, all forming neutral chelates with the metal ions, bonding through oxygen of the enolic hydroxyl group and  /or the oxygen atom of  the carbonyl group of the ligands keto-enol tautomer.

The i.r spectra of the ligands and their chelates have been measured between 4000cm-1 and 400cm-1and assignments proposed for observed frequencies.

The effect of 4- acyl substituents on the carbonyl stretching frequencies of the complexes was also investigated and the results showed that there was an increase in the carbonyl stretching frequency bands as the length of the alkyl substituent increased for magnesium (II),cobalt(II) and copper (II) chelates and the reverse trend was observed for zinc (II) chelates.

The infrared carbonyl and metal oxygen stretching frequencies of the transition metal chelates were also compared with the Irving and Williams stability order for transition metal complexes(Cu  >  Ni  >Co  >Mn >Zn)

and it was observed that the magnitude of the  M-O  stretching frequencies followed closely the Irving Williams stability order while the C=O stretching frequencies did not. This has been attributed to electronic and steric effects.


Background of Study

There has been a lot of interest in the chemistry and stereochemistry of metal  complexes in recent years because of its growing applications in both biological and chemical processes.

The chemistry of these groups of compounds was first proposed in  18931 by a Swiss chemist, Alfred Werner who used his coordination theory of primary and secondary valences to account for the phenomenon by which apparently all stable saturated molecules combine to form molecular complexes.

Werner showed that the properties of many complexes formed by various transition metals could be explained by the postulate that the metal atoms have a ligancy of six or four, with the attached groups arranged about the central atom at the corners of a  circumscribed  regular  octahedron or tetrahedron.

Almost  every  kind of metal atom can serve as a central atom in a complex , although some metals like the transition metals do so more readily than others.

When a metal atom coordinates with two or more donor groups of a single ligand called the chelating agent , a chelate is formed.

One of  the significant features of these chelating agents is that whereas complex formation may involve more than one intermediate step, Chelation is a one step process. 6,7

Since Urbain,s work on the structure and reactivity of β-diketones  in 1896,  these groups of chelating agents have been of utmost importance to chemist and research workers alike.

These β-diketones are ligands bearing two carbonyl groups separated by a methylene group. The intervening methylene group bears an active hydrogen atom.

The acidity of the hydrogen atom is caused by the electron withdrawing powers of the two carbonyl groups that flank them. Owning to electronic and field effects ,


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