The tautomerism of some substituted 2-hydroxypyridines is investigated by UV/Vis-and 1 H-, and 13 C-NMR spectroscopic methods, with the aid of the N-Me and O-Me fixed parents. NMR spectroscopic data do not allow discrimination between the two tautomeric forms (with the exception of the unsubstituted 2-hydroxypyridine), while UV/Vis-data permit the quantitative determination, in different solvents, of the amounts of the two forms. The electronic substituent effect and the change of solvent are discussed. An X-Ray diffraction study carried out on a crystal of 2-hydroxy-5-nitropyridine (7) reveals that this compound, in the solid state, is in the oxo-form.

Low sensitivity toward traces of moisture and high tolerance of different functional groups make indium tribromide suitable for carrying out multistep synthetic sequences. In particular, we have realized a 1,4-conjugated addition of indoles/thiols to alpha,beta-unsaturated ketones mediated by a catalytic amount (10 mol %) of InBr(3) obtaining the desired beta-substituted ketones in good yields. The Lewis acidity of indium salts was not affected by coordinating and acid nucleophiles; therefore, the subsequent catalytic 1,2-addition of Me(3)SiCN to carbonyl compounds can be performed in one pot. With the optimized atom-efficient protocol, several polyfunctionalized alpha-silyloxy cyanohydrins were synthesized in good to excellent yields (up to 97%) and a notable level of simple 1,3-diastereoselection (up to 84:16) was recorded in the case of 2-cyclohexen-1-one 2c.

The synthesis of new Fe(II) 5-aryl tetrazolate complexes [CpFe(CO)(L)(N4C−C6H4−CN)] (4) [L = CO (4a); PPh3 (4b); P(OCH3)3 (4c); CN-2,6-Me2C6H3 (4d)] is described. The target compounds were obtained by addition of sodium azide to the parent 1,4-dicyanobenzene complexes [CpFe(CO)(L)(NC−C6H4−CN)][O3SCF3] (2a−d). X-ray molecular structure of 4c confirms the predictions based on NMR (1H, 13C) studies concerning the coplanarity between the tetrazole and phenyl rings, with consequent interannular conjugation effect. The multidentate nature of the tetrazole ring involves the presence of different sites that can undergo electrophilic attack; reaction of complexes 4a−c with methyl triflate afforded the methylated cationic complexes [CpFe(CO)(L)(CH3−N4C−C6H4−CN)][O3SCF3], 5a−c, whose 1H and 13C NMR spectroscopy data suggested out-of-plane rotation of the phenyl ring and subsequent large reduction of interannular conjugation. Noteworthy, the same effect was obtained in a reversible way by addition of triflic acid to 4a, affording the protonated cationic complex [CpFe(CO)2(H−N4C−C6H4−CN)][O3SCF3], 6a, which was easily converted into its neutral precursor by treatment with a base.

Thermal treatment of [Os3(CO)12] with diphenyl(2‐thienyl)phosphane, Ph2P(C4H3S), results in the formation of [Os3(CO)12–x{Ph2P(C4H3S)}x] (x = 1–3, 1–3), but no C–H bond activation was observed. Reaction of [H2Os3(CO)10] with diphenyl(2‐thienyl)phosphane at ambient temperature affords [HOs3(μ‐H)(CO)10{Ph2P(C4H3S)}] (4), but when the samereaction is repeated at elevated temperatures, the cyclometallated species [(μ‐H)Os3(CO)9{μ3‐Ph2P(C4H2S)}] (5) and[(μ‐H)Os3(CO)8{μ3‐Ph2P(C4H2S)}{Ph2P(C4H3S)}] (6) are formed. In addition, two more products, tentatively assigned as [(μ‐H)Os3(CO)6{μ3‐Ph2P(C4H2S)}{μ‐Ph2P(C4H3S)}{Ph2P(C4H3S)}] (7) and [(μ‐H)Os3(CO)7{μ‐Ph2P(C4H2S)}{μ‐Ph2P(C4H3S)}{Ph2P(C4H3S)}] (8) are obtained. The dynamic behaviours of 2, 5 and 6 have been studied by variable‐temperature (VT) 1H and 31P{1H} NMR spectroscopy. The VT 31P{1H} NMR spectra of [Os3(CO)10{Ph2P(C4H3S)}2] (2) demonstrate that a mixture of two isomers, which are in rapid exchange at room temperature, is present and that the less common cis‐trans isomer, whose structure has been determined by X‐ray crystallography, is favoured for this cluster. The VT 1H NMR spectra of 5 indicate the presence of two isomers which are proposed to arise from an oscillation of the σ,η2‐vinyl group of the thienyl moiety between two metal atoms. A similar fluxional process is proposed to occur in 6 and the assignment of the room‐temperature structure(s) of this cluster was confirmed by 1H‐187Os 2D HMQC spectroscopy. In addition to 2, the solid‐state structures of 3, 5 and 6 have been determined by X‐ray crystallography. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

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