Synthesis of eta(2):sigma(2)-diene complexes of iridium(III) by the reaction of eta(4):pi(2)-diene iridium(I) species with Lewis bases

Treatment of the eta(4)-diene complex Tp(Me2)Ir(eta(4)-CH2=C(Me)C(Me)=CH2) (1) with a variety of soft, and hard Lewis bases induces an unprecedented c hange in the binding mode of the diene Ligand from eta(4):pi(2) to eta(2):s igma(2) and formation of Ir(III) derivatives of composition TpMe(2)Ir-(eta( 2):sigma(2)-CH2C(Me)=C(Me)CH2)(L) (4) (L = CO, PMe3, MeCN, C5H5N, etc.). Si milar reactivity is exhibited by the analogous compounds Tp(Me2)Ir(eta(4)-C H2=C(Me)CH=CH2) (2) and Tp(Me2)Ir(eta(4)-CH2=CHCH=CH2) (3); the facility wi th which the transformation occurs follows the sequence 1 > 2 > 3. The stud y of these processes under kinetic and thermodynamic control suggests that metal-based Lewis acid centers of composition Tp(Me2)Ir(R)(R') (R and R' = alkyl groups) are of intermediate soft-hard nature.