in metallabenzenes-in which a metal atom formally replaces a CH
group in benzene-lies in their aromaticity and ability to mediate
organic reactions. A team led by Guochen
Jia at the Hong Kong University of Science & Technology
has recently developed a convenient way to prepare osmabenzenes
(shown; Ph = phenyl). By treating dichloro- or dibromotris(triphenylphosphine)osmium
with readily accessible 1,4-pentadiyne-3-ol, Jia's team has made
the first air-stable phosphonium salts of metallabenzenes [J.
Am. Chem. Soc., 126, 6862 (2004)]. The
researchers suggest that the reaction is initiated by coordination
of one terminal carbon-carbon triple bond of the dialkyne to osmium,
replacing one of the triphenylphospine ligands. Successive nucleophilic
attacks on the two carbon-carbon triple bonds by two triphenylphosphines
followed by dissociation of the hydroxyl group in the original
diyne lead to formation of an osmabenzene phosphonium salt.