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A study of the structural and magnetic properties of some transition metal complexes of pyridine and pyrazine carboxylates Stevens, Cecilia Leigh

Abstract

In this work, a variety of transition-metal complexes incorporating anionic ligands with certain similar structural characteristics were synthesised and examined. The compounds were characterised by IR and UV/VIS/NIR spectroscopy, thermogravimetric analysis, elemental analysis, variable-temperature magnetic susceptibility, and, where possible, also by single-crystal X-ray analysis or X-ray powder diffraction. The neutral ligand precursors employed were 2,3-pyridine dicarboxylic acid (2,3-pydcH₂), 2,4- pyridine dicarboxylic acid (2,4-pydcH₂), 2,5-pyridine dicarboxylic acid (2,5-pydcH₂), pyrazine carboxylic acid (pyzcH), 2,5-pyrazine dicarboxylic acid (2,5-pyzdcH₂), and benzimidazole-2-carboxylic acid (bicH₂), all of which have a carboxylic acid group adjacent to a heteroaromatic ring nitrogen. The metal ions used were manganese (II), cobalt (II), nickel (II), copper (II), and cadmium (II). Both benchtop and hydrothermal preparations were performed. The isolated products include monometallic, polymetallic, and heterobimetallic coordination compounds. Magneto-structural correlations were performed for the various paramagnetic compounds prepared. The majority of the compounds were magnetically dilute or weakly antiferromagnetic. One polymetallic short-range ferromagnet was prepared (Cu(2,4-pydc)•0.5H₂O), one heterobimetallic coordination polymer proved to be ferromagnetic ([Cu(2,4-pydc)₂Mn(H₂O)4][sub x]), and one heterobimetallic species proved to have long-range ferrimagnetic ordering (Co(2,5-pydc)₂Mn-H₂O). Compounds incorporating ligated water were subjected to thermal dehydration, and the derivatives of lesser hydration were characterized and their magnetic activities compared to that of the parent compounds. In most cases, the dehydrated compounds were weakly antiferromagnetic, with slightly stronger exchange than the weakly antiferromagnetic or magnetically dilute parent compounds. Some showed no measurable change. Three showed an alteration in the type of magnetic coupling taking place: from weakly antiferromagnetic to ferromagnetic for [Cu(2,4-pydc)(H₂O) ₂][sub x] → Cu(2,4-pydc)-0.5H₂O , from ferromagnetic to weakly antiferromagnetic for [Cu(2,4-pydc)₂Mn(H₂O)₄][sub x] → Cu(2,4-pydc)₂Mn(H₂O), and from weakly antiferromagnetic to strongly ferrimagnetic for [Co(2,5-pydc)₂(H₂O)₂Mn(H₂O)₄][sub x] 4xH₂O → Co(2,5-pydc)₂Mn(H₂O).

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