Reactivity of a Tp–Iridacyclopentene Complex

The known iridacyclopentene [TpIr­{κ2(C,C)-CH2CH2C­(R)C­(R)}­(THF)]
(1; R = CO2Me) is unstable in C6H6 solution (t1/2 ≈
4 h at 25 °C), evolving, following dissociation of THF, to the
Ir­(I) diene [TpIr­{η4-CH2CHC­(R)CH­(R)}]
(3) as a ca. 9:1 mixture of trans and cis isomers. However,
studies carried out in C6D6 demonstrate that
this is not an inert solvent in the reaction, as partial deuteration
of 1 to give 1-d2 (in the presence of THF) or trans-3-d2 (in C6D6 alone)
easily occurs. Complex 1 is also photochemically unstable
and under UV radiation affords a dimeric product resulting from a
2 + 2 addition process of two Ir–C­(R)C­(R)– moieties.
The reactions of complex 1 with olefins (ethylene, propene,
methyl acrylate, dimethyl maleate, and dimethyl fumarate) also take
place under very mild conditions, but in comparison with the TpMe2IrIII system, the resulting adducts are quite
thermally stable and most of them only experience dissociation, under
forcing conditions, with the exception of that derived from methyl
acrylate, which partially evolves by insertion into both of the Ir–C
bonds. Dimethyl maleate instead reacts via C–H activation to
give the complex [TpIr­(cis-C­(R)CEt­(R))­(cis-C­(R)CH­(R))­(H2O)], having two differently
substituted alkenyl ligands. The disubstituted alkynes R′CCR′
(R′ = Me, Ph) react with 1, giving very stable
iridacyclopentadienes with the formula TpIr­{κ2(C,C)-C­(R′)C­(R′)­C­(R)C­(R)}­(C2H4)] (R′ = Me, Ph). Methyl propiolate and
acetylene also enter this reaction manifold but are also able to insert
selectively into the Ir–alkenyl bond to give interesting bis-allyl
species with the formula [TpIr­(CH2CHC­(R)-η3-C­(R)­CHCH2)] (20) and [TpIr­(CH2CHC­(R)-η3-C­(R)­CHCH­(R))]. Finally,
the deuterium atoms of complex 20-d2, obtained from the reaction of 1 with DCCD,
are exclusively located in the η3-allyl functionality
and surprisingly the complex does not experience interconversion between
the otherwise compositionally identical η1- and η3-allyl functionalities even upon prolonged heating in C6H6 at 120 °C.