Slicing the π in Three Unequal Pieces: Iridium Complexes with Alkyne, Iminoxolene, and Dioxolene Ligands

The formally isoelectronic alkyne, iminoquinone, and benzoquinone ligands can all exhibit significant metal–ligand π bonding. Condensation of 2,6-bis­(triisopropylsilylethynyl)­aniline with 3,5-di-tert-butyl-1,2-benzoquinone affords a ligand with two such moieties, the alkyne-containing iminoquinone Tipsi. All three groups are installed around a single iridium center by successive treatment of [(coe)2IrCl]2 with Tipsi and 3,5-di-tert-butyl-1,2-benzoquinone to give (κ2,η2-Tipsi)­(3,5-tBu2Cat)­IrCl in two isomeric forms with cis iminoxolene and dioxolene groups and a bound alkyne. Structural, spectroscopic, and computational data indicate that the dioxolene and especially the iminoxolene are strongly engaged in π bonding but that the alkyne is not an effective π donor or acceptor. Pyridine displaces the bound alkyne in (κ2,η2-Tipsi)­(3,5-tBu2Cat)­IrCl to give two isomers of (κ2-Tipsi)­(3,5-tBu2Cat)­Ir­(py)­Cl, both with the iminoxolene and dioxolene trans to each other. The reaction rates and modest stereospecificity of the dissociative substitution reaction are consistent with a mechanism with competitive, stereochemically distinct, pathways for forming square pyramidal intermediates, which rapidly isomerize to the trans isomer before irreversible trapping by pyridine. The five-coordinate intermediates funnel down to the trans isomer because it is stabilized by an additional metal–ligand π interaction that is not possible in the cis isomers.