Studies on selected beta-diketones and their metal chelates.

Purified samples of sixteen compounds of the type RCOCHR'COR2 have been prepared by standard synthetic procedures. Twelve correspond to R = R2 = CH3 with R' representing the substituent groups -H,-CH3,-C2H5, -nC3H7,-iC3H7,O2N-, -OCOC2H5,-OCCH3,-Br,-Cl,allyl, butylidene, and a second series of four in which R = CH3, R' = H, R2 = C2H5; R = R' = CH3, R2 = C2H5; R = R2 C2H5, R' = H; R = R2 = C2H5, R' = CH3. Using a potentiometric technique, acid dissociation constants (pKa) in water and 50 per cent aqueous dioxan at 25 have been obtained for a number of these compounds. Variation of pKa with temperature has been studied for the second series, together with pentane-2,4-dione, its 3-methyl derivative, l-phenylbutane-l,3-dione and 1,3-diphenylpropane-l,3-dione. The stability constants (logK1, logK2) of their chelates with divalent cobalt, copper, magnesium, manganese, nickel and zinc have been determined at 25 , and also at 5 degrees, 15 degrees, 35 degrees and 45 degrees in the case of cobalt (II). This variation of pKa and logK with temperature has enabled enthalpies ([delta]H) and entropies ([delta]S) of protonation and chelation to be obtained for these ligands and their cobalt (II) chelates. Nearly forty metal chelates have been prepared for the divalent metals cobalt, nickel and copper, and the trivalent metals iron, chromium and cobalt, and infra-red studies carried out from 400-4000cm-1. Diffuse reflectance measurements have been made on the nickel (II) chelates. The transmission spectra of five ligands and several metal chelates have been measured in chloroform solution. During spectroscopic pKa studies, a number of the 3-substituted pentane-2,4-diones have been found to decompose, the decomposition appearing to be dependent upon the acidity (pH) of the solution. The decomposition of 3-methylpentane-2,4-dione has been studied in some detail.