Amide nitrogen co-ordination of CoII and NiII in ternary 2,2′-bipyridine-containing systems. A solution and solid-state study

The ternary systems 2,2′-bipyridine (bipy)-M-amino acid [M = CoII or NiII; amino acid (H2L) = N-p-tolyl-sulfonylglycine (tsgly), -β;-alanine (ts-β-ala) or N-benzoylglycine (H2hip)] were investigated in aqueous solution by means of polarography, electronic spectroscopy and potentiometry, in order to identify the type, number and stability of complex species, as a function of pH and bipy : M : amino acid ratio. With tsgly the prevailing species are [MII(bipy)n(HL)]+ and [MII(bipy)nL] (n = 1 or 2), with pKNH for the deprotonation of sulfonamide nitrogen 7.8(2) and 7.4(2), for CoII and NiII, respectively and the crystal and molecular structure of [Co(bipy)2(tsglyNO)]·2H2O (NO indicates binding as a N,O-chelating dianion) was determined. The CoII is six-co-ordinated by four nitrogen atoms of the bipy moieties, the deprotonated sulfonamide nitrogen and one carboxylic oxygen of the tsgly dianion. On standing in air, an alkaline solution of bipy-CoII-tsgly turned from orange to wine-red in a few days, indicating the oxidation of CoII to CoIII. With ts-β-ala and H2hip metal hydrolysis prevents the formation of deprotonated complexes.