Complex Formation of Pyridine Oximes with Divalent Transition Metal Ions in Aqueous Soluition

The complex formation equilibria of pyridine-2-aldoxime and its methyl or amido derivatives (HL) with cobalt(II), zinc(II), and cadmium(II) ions, and the protonation and complex formation equilibria of pyridine-2,6-carboxamidoxime (H2L) with copper(II) and nickel(II) ions were studied in aqueous 0.1 M Na(Cl) solution at 25 C by potentiometric titrations with the use of glass electrode. The experimental data were analyzed with the least-squares computer program SUPERQUAD to determine the complexes formed and their stability constants. In addition, the structure of the crystallized pyridine-2,6-carboxamidoxime complex with the formula [Ni(HL)2]∙4H2O has been determined with X-ray measurements. The complexes of types Co(HL)2+ and Co(HL)22+ are mainly octahedral with a high spin d7 electron structure (t2g5eg2) and their oxidation states are stable. The deprotonated bis complexes of type Co(HL)L+ are often low spin (t2g6eg) and because of the easy loss of their only eg electron they are easily oxidized to very inert low spin cobalt(III) complexes (t2g6). Only small amounts of cobalt(III) complexes cause the very slow attainment of equilibrium often already in the pH range 2–5. Pyridine-2-carboxamidoxime and pyridine-2-aldoxime and probably also 1-(2-pyridinyl)ethanone oxime forms also tris complexes Co(HL)32+ and/or Co(HL)2L+. The complex formation of pyridine-2-aldoxime in the pH range 5–10 could be studied by using very small cobalt(II) ion concentrations. There, all the cobalt(II) form the low spin CoL2, which quantitatively displaces also the tris complex Co(HL)2L+. 6-methylpyridine-2-aldoxime forms complexes Co(HL)2+, CoL+, CoL2, Co2L2OH+, Co2L3+, and Co2L3OH, mainly in the pH range 6–10. The stabilities of the low spin CoL2, Co2L3+, and Co2L3OH and their oxidation reactions are decreased by the steric requirements of the 6-methyl groups of the ligands. Pyridine-2-acetamidoxime forms also a complex Co(H2L)3+ and pyridine-2-carboxamidoxime forms a complex Co2(HL)2H2L5+ with a positively charged ligand (H2L+). Zinc(II) and cadmium(II) ions form with 6-methylpyridine-2-aldoxime only Zn2L22+, Zn2L2OH+, and Zn2L2OH)2, and CdL+, CdL2, and Cd2L2OH+. With pyridine-2-acetamidoxime, they form only Zn(HL)2+, Zn2L2OH+, Cd(HL)2+, and CdL+. The other oximes form also Zn(HL)L+, ZnL2, Cd(HL)L+, and CdL2. Cd(HL)22+ reaches only with pyridine-2-carboxamidoxime and Zn(HL)22+ also with 1-(2-pyridinyl)- ethanone oxime measurable concentrations. Pyridine-2-carboxamidoxime forms also Zn4(L–H)2L22+ and Cd4(L–H)2L22+. The stability constants of the mono complexes M(HL)2+ increase with few exceptions in the order 6-methylpyridine-2-aldoxime < pyridine-2-acetamidoxime < pyridine-2-aldoxime < 1-(2-pyridinyl)ethanone oxime < pyridine-2-carboxamid-oxime < pyridine-2,6-dicarboxamidoxime and Cd < Zn < Co < Ni < Cu.

Grantor dc:publisher

Helsingin yliopisto

Year dc:date.issued

2020

Author dc:creator

• Salonen, Markku

• epäorgaaninen kemia

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Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/10138/321944

OAI record oai:identifier

https://helda.helsinki.fi/server/oai/request?verb=GetRecord&metadataPrefix=dim&identifier=oai%3Ahelda.helsinki.fi%3A10138%2F321944