DOI: 10.7274/28792598
title: Iridium-Carbon, Iridium-Nitrogen and Iridium-Oxygen Bond Formation in Bis(iminoxolene)iridium Complexes

description: The reactions of bis(4,6-di-tert-butyl-2-(2,6-diisopropylphenyliminoxolene)iridium chloride ((Diso)2IrCl) with oxygen atom transfer reagents have been previously reported to form the iridium alkoxy chloride complex, (Diso)(DisoO)IrCl. This reaction is hypothesized to proceed through an oxoiridium complex that undergoes C–H bond activation to form an iridium alcohol chloride complex, (Diso)(DisoOH)IrCl, as an intermediate which has now been observed in reactions at low conversion. This complex can be prepared independently from the hydrogen atom transfer reaction of (Diso)(DisoO)IrCl with hydrazobenzene. To discriminate between a terminal oxo or bridging oxo complex in the reaction to form the alkoxy complex, a KIE competition experiment was proposed from a reaction mixture of (Diso)2IrX (X = Cl, Br, or I). While the mixture of (Diso)2IrCl and (Diso)2IrI proved unusable due to the chloride’s propensity towards oxidation, a more reactive (Diso)2IrBr can be synthesized by salt metathesis of (Diso)2IrCl with LiBr or oxidation of (Diso)2Ir with CBr4. However, oxidation of isotopically labeled (Diso)(Diso-d2)IrBr gives KIE values that are too similar to those shown by (Diso)2IrCl.
To allow for potential characterization of a putative Ir=O species, the oxidation-resistant N-(2,6-dibromophenyl)-4,6-di-tert-butyl-o-iminobenzoquinone (Briq) ligand is used to synthesize (Briq)2Ir complexes. Metalation of Briq with the iridium precursor [(coe)2IrCl]2 leads to precipitation of a mixture of (Briq)2IrClx (x = 0, 1 or 2). Treatment with excess oxidant or reductant allows separation of single iridium complexes. Exhaustive oxidation of the mixture with PhICl2 affords paramagnetic trans-(Briq)2IrCl2. Exhaustive reduction with either NaC10H8 or CoCp2 allows the isolation of (Briq)2IrNa(THF)2 or [CoCp2][(Briq)2Ir], respectively. (Briq)2IrNa(THF)2 is the first known iridium complex containing a crystallographically characterized Ir–Na bond. Subsequent one-electron oxidation of the anionic complex yields (Briq)2Ir, while oxidation by elemental iodine produces (Briq)2IrI. The organoiridium complex (Briq)2IrCH3 can be prepared by reaction of [(Briq)2Ir]– with CH3I.
Four-coordinate (Briq)2Ir reacts with ethylene to form diiridaethane in a 10:1 ratio of (A,A):(A,C) diastereomers. This reaction likely proceeds through the formation of a (Briq)2Ir(C2H4) adduct that reacts with another (Briq)2Ir to form the diiridaethane complex. When prepared from the reactions of [(Briq)2Ir]– with 1,2-dibromoethane or 1,2-bis(tosyloxy)ethane, [(Briq)2Ir]2(µ-C2H4) is isolated as a roughly 3:1 mixture (A,A) to (A,C)-isomers, presumably through a polar pathway involving a [(Briq)2Ir(C2H4)]+. (A,C)-[(Briq)2Ir]2(µ-C2H4) demonstrates dynamics at low temperatures which interchanges the environments of the two (Briq)2Ir fragments in the C1-symmetric staggered structure by a ratcheting motion of the two fragments past one another. The iminoxolene ligand environments on each iridium are equivalent even at low temperature due to fast Ir–C rotation that is akin to the motion of an eggbeater. 
Trans-(Briq)2IrF(CH3CN) can be prepared by reaction of (Briq)2IrNa(THF)2 with the cationic F+ source 1-fluorocollidinium tetrafluoroborate in the presence of pyridine in acetonitrile. Addition of trimethylsilyl azide to the iridium fluoride complex leads to the formation of a novel [(Briq)2Ir]2(µ-N) complex. In this complex, the iminoxolene ligands are reduced relative to their state in the precursor, and iridium adopts an oxidation state of +5, which is strongly stabilized by p donation from the nitride. Reduction and oxidation of the µ-nitride complex with CoCp2 or AgOTf respectively affords the corresponding anionic and cationic µ-nitrido complexes. Like (A,C)-[(Briq)2Ir]2(µ-C2H4), (A,C)-{[(Briq)2Ir]2(µ-N)}– demonstrates dynamics at low temperatures due to slowed Ir–N bond rotation. The (A,A)-isomer is D2-symmetric, and thus, does not show fluxionality. {[(Briq)2Ir]2(µ-N)}OTf is has a low-lying triplet state, as judged by its temperature-dependent, paramagnetically shifted 1H NMR spectrum.
Reactions of (Briq)2IrI with oxygen atom transfer reagents allow the preparation of an (iminoxolene)iridium complex with a gem-diol chelate. (Briq)(Briq-1,1-diolate)IrI is capable of performing oxygen atom transfers of phosphine to phosphine oxide as well as hydrogen atom abstractions of hydrazobenzene to azobenzene. In each case, (Briq)2IrI is reformed at the end of the reaction. The nature of the oxidant in these reactions is unclear as it is possible that the substrates react directly with the 1,1-diolate complex. Alternatively, a terminal oxo complex that is in equilibrium with the 1,1-diolate complex could be the active oxidant in these reactions. Xanthene oxidation with (Briq)(Briq-1,1-diolate)IrI gives primarily xanthone early on in the reaction, but 9-hydroxyxanthene is formed as the reaction progresses. The changing product distribution suggests that the nature of the oxidant may be changing over the course of the reaction, perhaps because an intermediate such as (Briq)2Ir(OH) could also react with hydrocarbons.