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In this work, we report on a non-solvate and on a new solvate forms of rifamycin S, demonstrating that another stable conformation of the active molecule exists and it may be isolated by planning the crystallization conditions. In fact, in these structures rifamycin S has a previously uncharacterised, bent conformation: implications of these findings on the models used to explain rifamycin activity are discussed. Polymorphism has also been reported for the rifamycin-based antibiotic rifaximin, but without single-crystal X-ray diffraction characterization. Here the structure of a tetrahydrate form of rifaximin is discussed; confirming the flexibility of the drug, the two independent molecules in the unit cell are an example of conformational isomorphism. [ABSTRACT FROM AUTHOR]

Azide complexes [M(RN3)(CO)3P2]BPh41–3[M = Mn, Re; R = C6H5CH2, 4-CH3C6H4CH2, C6H5, 4-CH3C6H4, C5H9; P = PPh(OEt)2, PPh2(OEt)] were prepared by allowing tricarbonyl MH(CO)3P2 hydride complexes to react first with Brønsted acid (HBF4, CF3SO3H) and then with organic azide in the dark. In sunlight the reaction yielded tetraazabutadiene [M(η2-1,4-R2N4)(CO)2P2]BPh44–6 complexes or, with benzyl azide, imine [M{η1-NH=C(H)Ar}(CO)3P2]BPh47(Ar = C6H5, 4-CH3C6H4) derivatives. Tetraazabutadiene [M(η2-1,4-R2N4)(CO)2P2]BPh4 complexes were also prepared by reacting dicarbonyl MH(CO)2P3 species first with Brønsted acid and then with an excess of organic azide. Complexes were characterised spectroscopically (IR, 1H, 31P, 13C, 15N NMR data) and by the X-ray crystal structure determination of complex [Re{η2-1,4-(C6H5CH2)2N4}(CO)2{PPh(OEt)2}2]BPh4(5a). Strong evidence for coordination of the organic azide was obtained from the 15N NMR spectra of labelled [M(C6H5CH215NN15N)(CO)3P2]BPh4 derivatives. [ABSTRACT FROM AUTHOR]