IRIDIUM COMPLEXES OF REDOX-ACTIVE LIGANDS: STRUCTURES AND MECHANISMS

Iminoxolenes and dioxolenes are redox-active ligands whose complex but quantifiable π interactions with metals defy simplistic π-donor/acceptor classifications. Tipsi, an iminoxolene with additional alkyne functionality, and 3,5-di-*tert*-butyl-1,2-benzoquinone react in sequence with an iridium(I) precursor to generate two stereoisomers of (κ²,η²-Tipsi)(3,5-^tBu₂Cat)IrCl (1a-b).Structural and spectroscopic analyses reveal that the redox-active ligands (especially Tipsi) dominate the π bonding framework, while the alkyne coordinates without significant π donation or backbonding. Its π-innocence is corroborated by ligand substitution with the σ-only pyridine ligand, which forms trans stereoisomers of (κ²-Tipsi)(3,5-^tBu₂Cat)Ir(py)Cl **(2a–b**) in which the iminoxolene and dioxolene structure and bonding are essentially unchanged. Isomerization from **1a** to **1b** and pyridine addition to the alkyne adducts to generate **2a–b** share common square pyramidal intermediates formed upon alkyne dissociation. Stereochemistry in the Tipsi-dioxolene system is governed by the stability of trans intermediate **3**, which is stabilized relative to the cis square pyramidal alternatives by an additional π bonding interaction. A similar interaction stabilizes an analogous bis(iminoxolene) complex, (Diso)₂IrCl, which quickly reacts with pyridine to establish an equilibrium with *trans*-(Diso)₂Ir(py)Cl. Elevated heating produces *cis*-(Diso)₂Ir(py)Cl as the thermodynamically favored product, providing a point of contrast with the Tipsi-dioxolene system. Kinetic and computational data suggest a mechanism in which pyridine, by trapping the stable trans (Diso)₂IrCl, inhibits formation of the thermodynamic product. This is because isomerization of (Diso)₂IrCl, a step with a higher free energy barrier, is necessary to generate the fleeting cis intermediate (Diso)₂IrCl^\* prior to pyridine associating to form *cis*-(Diso)₂Ir(py)Cl. The bis(Diso) system may find practical application through the reduction of (Diso)₂IrCl to (Diso)₂Ir, a potential hydrogenation catalyst. (Diso)₂Ir can react with dihydrogen to form (Diso)₂IrH, which selectively hydrogenates organic substrates across weak π bonds. Though kinetic measurements to gauge the favorability of hydrogen uptake are difficult to reproduce, experimental evidence confirms that the process of regenerating (Diso)₂IrH from (Diso)₂Ir can occur without assistance from iridium(0) nanoparticles, suggesting that a (Diso)₂Ir homogeneous hydrogenation catalyst is feasible.