Reactions of Ir(acac)(cyclooctene)(PCy₃) with H₂, HC⋮CR, HSiR₃, and HSnPh₃: The Acetylacetonato Ligand as a Stabilizer for Iridium(I), Iridium(III), and Iridium(V) Derivatives

Abstract

The acetylacetonato complex Ir(acac)(cyclooctene)(PCy3) (1) reacts with molecular hydrogen in the presence of 1 equiv of PCy3 to give Ir(acac)H2(PCy3)2 (2). The addition of 1 equiv of PhC⋮CH to a benzene-d6 solution of 1 causes the displacement of the coordinated olefin and the formation of Ir(acac)(η2-PhC⋮CH)(PCy3) (3). The addition of hexane to this solution reverses the reaction and precipitates 1. The treatment of 1 with 3 equiv of PhC⋮CH affords Ir{κ3-CHC(Ph)CH[C(O)CH3]2}(C2Ph)(CPhCH2)(PCy3) (4), which is a result of the oxidative addition of the HC⋮ bond of an alkyne, the insertion of a second alkyne into an Ir−C3(acac) bond, and the subsequent insertion of a third alkyne into the Ir−H bond, previously formed. The structure of 4 was determined by an X-ray investigation. The coordination geometry around the iridium atom can be rationalized as a distorted octahedron with the bicyclic ligand occupying three coordination sites of a octahedral face. In the presence of PCy3 the reaction of 1 and PhC⋮CH leads to Ir(acac)H(C2Ph)(PCy3)2 (5). Under the same conditions, CyC⋮CH and Me3SiC⋮CH afford the corresponding hydrido−alkynyl derivatives Ir(acac)H(C2R)(PCy3)2 (R = Cy (6), Me3Si (7)). The addition of silanes HSiR3 to 1 gives Ir(acac)H(SiR3)(PCy3) (SiR3 = SiEt3 (8), SiPh3 (9), SiHPh2 (10)). The structure of 8 was also determined by an X-ray analysis. The coordination geometry around the metallic center of 8 can be rationalized as a square pyramid with the triethylsilyl group located at the apex. In the presence of PCy3, the reactions of 1 with silanes lead to Ir(acac)H(SiR3)(PCy3)2 (SiR3 = SiEt3 (11), SiHPh2 (12), SiH2Ph (13)). Complex 1 also reacts with HSnPh3. In the absence of PCy3 the reaction product is Ir(acac)H(SnPh3)(PCy3) (14), while in the presence of PCy3 the six-coordinate derivative Ir(acac)H(SnPh3)(PCy3)2 (15) is obtained. Under atmospheric pressure of hydrogen, complex 8 is converted into the trihydrido−silyl−iridium(V) derivative Ir(acac)H3(SiEt3)(PCy3) (32). In solution, this complex is fluxional with values of ΔH⧧ and ΔS⧧ of 12.23 (± 0.76) kcal mol-1 and 1.45 (± 1.84) cal K-1 mol-1, respectively. Complex 8 has also been found to be an active catalyst for the addition of HSiEt3 to PhC⋮CH. In all experiments, PhCHCH2, PhC⋮CSiEt3, cis-PhCHCH(SiEt3), trans-PhCHCH(SiEt3), and Ph(SiEt3)CCH2 were obtained. The major product in all cases is the thermodynamically less stable cis-PhCHCH(SiEt3), resulting from the anti-addition of the silane to the alkyne. This product is selectively formed in approximately 70% yield.