Synthesis, Characterization, and Reaction Chemistry of a Family of 1-Iridacyclohexa-2,4-diene Complexes

The reaction of (Cl)Ir(PEt₃)₃ with potassium 2,4-dimethylpentadienide produces the 1-iridacyclohexa-2,4-diene complex (IrCH═C(Me)CH═C(Me)CH₂)(PEt₃)₃(H) (1) via an iridium-centered activation of a pentadienyl C—H bond. The kinetic product of this reaction is the fac isomer (la), which slowly isomerizes to the thermodynamically preferred mer isomer (lb). Treatment of lb with methyl tri-fluoromethanesulfonate results in hydride abstraction and production of [(IrCH═C(Me)CH═C(Me)C-H₂)(PEt₃)₃]⁺O₃SCF₃^- (2), in which one of the ring double bonds coordinates to iridium. Compound 2 can be converted back to 1b by treating with K⁺BEt₃H^- in tetrahydrofuran. In solution at 25 °C, 2 decomposes to [(η⁵-1,3-dimethylcyclopentadienyl)Ir(PEt₃)₂(H)]⁺O₃SCF₃^- (3). However, treatment of 2 at low temperature with a series of ligands (L), including I^-, CO, NCMe, and CNCMe₃, produces a family of adducts having the formula [(IrCH═C(Me)CH═C(Me)CH₂)(PEt₃)₃(L)]⁺O₃SCF₃^- (4-7, respectively). The iodide and acetonitrile adducts are formed as pure mer isomers. In contrast, the carbonyl and tert-butyl isocyanide adducts are initially produced as mixtures of fac and mer isomers but slowly convert to the pure mer isomers upon stirring at 25 °C. When 2 is treated with PMe₃, adduct formation is accompanied by exchange of the two axial PEt₃ ligands for smaller PMe₃ ligands. The product of this reaction, [(IrCH═C(Me)CH═C(Me)CH₂)(PEt₃)(PMe₃)₃]⁺O₃SCF₃^- (8), crystallizes in the monoclinic space group P2₁/c (No. 14) with a = 14.77 (1) Å, b = 11.588 (9) Å, c = 20.29 (1) Å, β = 109.91 (5)°, V = 3265 (5) ų, and Z = 4. Lithium diisopropylamide reacts with 2 at -30 °C in acetone to abstract a proton, producing the metallabenzene complex (IR–CH–C(Me)–CH–C(Me)–CH)(PEt₃)₃ (9). Compound 9 exhibits the ring current and thermal stability expected for an aromatic species.