To Be Bridgehead or Not to Be? This is a Question of Metallabicycles on the Interplay between Aromaticity and Ring Strain

Transition-metal-containing
metallaaromatics have attracted considerable
interest from both experimental and computational chemists because
they can display properties of both organometallic compounds and aromatic
organic compounds. In general, the transition metal in a metallabicycle
prefers a nonbridged position to the bridgehead one because of the
larger ring strain caused by the rigidity in the bridgehead position,
as exemplified by metallanaphthalene and metallanaphthalyne. On the
contrary, the osmium atoms in recently synthesized osmapentalyne and
osmapentalene are located at the bridgehead position. To probe the
origin of the difference between these metallabicycles, we carried
out density functional theory calculations. The metal-bridgehead osmanaphthalene
and osmanaphthalyne are computed to be less stable by 17.9 and 26.3
kcal mol–1 than the non-metal-bridged ones, respectively.
In addition, the metal-bridgehead osmapentalene and osmapentalyne
are more stable by 11.8 and 22.8 kcal mol–1 than
the non-metal-bridged isomers, respectively. Further study revealed
that the ring strains in these paired isomers are comparable to each
other. Thus, it is aromaticity rather than ring strain that determines
the relative thermodynamic stabilities of these complexes.