The mechanisms for the trimerization of acetylene in the
presence of (PH2CH2CH2PH2)IrCl as an active catalyst were
investigated by density functional (B3LYP and M06) theory
calculations. The reactant bis(acetylene)Ir
complex has a
distorted trigonal bipyramid structure, and thus, there are three
types of equilibrium structures connected by pseudorotation.
The first oxidative coupling takes place between the acetylene
molecule coordinating at the axial position and that in the
equatorial plane in these reactant complexes to afford square
pyramidal iridacyclopentadiene complexes through one of
the two favorable transition states. Then, the third acetylene
molecule attacks the iridacyclopentadiene intermediate, accompanied
by Cl migration, to afford the iridacyclopentadieneacetylene
complex. The formation of a benzene complex from
this acetylene complex involves reaction pathways based on
intramolecular [4+2] cycloaddition and the Schore mechanism
through iridacycloheptatriene, as well as the bicycle mechanism.
The most favorable pathway for the formation of the
benzene complex is through intramolecular [4+2] cycloaddition
to form the η4-benzeneIr
complex, and finally, to the η2-
benzeneIr
complex. This reaction mechanism is compared with
that for reactions catalyzed by TpIr and CpIr systems.