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Magnetic properties of ultrafine clusters of Fe HO ·4HO (ferrihydrite, FH), isolated in pores of polysorb, were studied by Mossbauer spectroscopy and by thermodynamical analysis. Thermodynamical analysis allowed the conclusion that magnetic properties of ultrafine clusters cannot be interpreted in terms of a secondorder magnetic phase transition or of superparamagnetic behavior alone but require the consideration of a jumplike first order magnetic phase transition (JMT). The critical radius R below which the JMT is to be expected in clusters was derived from thermodynamic criteria. It was determined as R "2 /(1!¹ /¹ ), where , and are constants derived from surface energy, magnetostriction, compressibility and ¹ "3/2 Nk ¹ (N is the number of iron atoms, k is the Boltzmann constant, ¹ is the Curie temperature of the clusters). For the smallest FH clusters isolated in pores of polysorb, the critical radius and the JMT temperature were estimated by Mossbauer spectroscopy to be R :1.5-2.0 nm and ¹ (+2 :4.2-6 K, respectively. Satisfactory agreement between the value R , estimated from the experimental data and the one derived by thermodynamical analysis was achieved. Interfacial (cluster-surface) and intercluster interactions were found to destroy the JMT effect and to give rise to a second-order magnetic phase transition.

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The reaction of cobalt(II) and iron(III) pivalates with anions of 2furancarboxylic acid (pyromucic acid, Hfur) yielded trinuclear homometallic complexes with linear ([Co 3 (fur) 6 (phen) 2 ] (1, phen = 1,10-phenanthroline)) and triangular ([Co 3 O(fur) 6 (H 2 O) 3 ] (3), [Fe 3 O(fur) 6 (THF) 3 ] ⋅ 3THF (4)) structures of the metal moiety. The hexanuclear mixed carboxylate cobalt complex [Co 6 (piv) 8 (Hpiv) 4 (fur) 2 (OH) 2 ] (2, piv À = (CH 3) 3 COO À) was isolated as an intermediate in the synthesis of 3. According to CVA data, complex 3 contains bivalent and trivalent cobalt ions. According to Mossbauer spectroscopy data, all the iron(III) atoms in 4, which is an isostructural analogue of 3, are solely in the highspin state. The antibacterial activity of compounds 1-4, K(fur) (5) and Na(fur) (6) was determined in vitro against a nonpathogenic Mycolicibacterium smegmatis strain. The results of in vitro biological tests against Mycolicibacterium smegmatis showed a high bioactivity of complexes 1 and 4.

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