ONO Dianionic Pincer-Type Ligand Precursors for the Synthesis of σ,π-Cyclooctenyl Iridium(III) Complexes: Formation Mechanism and Coordination Chemistry

Abstract

The σ,π-cyclooctenyl iridium (III) pincer compounds [Ir(κ 3-pydc-X)(1-κ-4,5-η-C 8 H 13)] (X = H (1), Cl, Br) have been prepared from [Ir(µ-OMe)(cod)] 2 and the corresponding 4substituted pyridine-2,6-dicarboxylic acids (H 2 pydc-X) or, alternatively, from their lithium salts (X = H) and [Ir(cod)(CH 3 CN) 2 ]PF 6. Deuterium labeling studies in combination with theoretical calculations have shown that formation of 1 involves a metal-mediated proton transfer in the reactive intermediate [Ir(κ 2-Hpydc)(cod)], through the solvent stabilized hydrido complex [IrH(κ 3-pydc)(cod)(CH 3 OH)], followed by olefin insertion. The formation of this hydrido intermediate results from the solvent-assisted proton transfer through a hydrogen-bonding network forming an eight-membered metallacycle. In contrast, reaction of [Ir(µ-OMe)(cod)] 2 with iminodiacetic acid derivatives, RN(CH 2 COOH) 2 , gave the stable iridium(I) mononuclear [Ir{κ 2-MeN(CH 2 COOH)(CH 2 COO)}(cod)] (R = Me) complex having a free carboxymethyl group, and the tetranuclear complex [Ir 4 {κ 4-PhN(CH 2 COO) 2 } 2 (cod) 4 ] (R = Ph) with doubly deprotonated ligands. The molecular structure of the related cyclooctene complex [Ir 4 {κ 4-PhN(CH 2 COO) 2 } 2 (coe) 8 ] has been determined by X-ray analysis. Reaction of 1 with monodentate N-and P-donor ligands gave the compounds [Ir(κ 3pydc)(1-κ-4,5-η-C 8 H 13)(L)] (L = py, BnNH 2 , PPh 3 and PMe 3). Reaction of 1 with the shortbite bis(diphenylphosphino)methane (dppm) afforded the mononuclear 1-dppm, with an uncoordinated P-donor atom, or the dinuclear 1 2-dppm complex as a function of the molar ratio used. Similarly, the dinuclear complexes 1 2-dppe and 1 2-dppp have been prepared using 1,2-bis(diphenylphosphino)ethane (dppe) and 1,3-bis(diphenylphosphino)propane (dppp) as bridging ligands. The diphosphine-bridged dinuclear assemblies have been obtained as two diastereoisomers in a 1:1 ratio due to the chirality of the mononuclear building block. The single crystal X-ray structures of 1-py and 1-dppm are reported.

Iridaoxacyclohexadiene-Bridged Mixed-Valence Iridium Cyclooctadiene Complex: Oxidative Addition and Hydrogen-Transfer to Coordinated Cyclooctadiene

“…The versatility of the different coordination modes of bonding and hapticities adopted by the COD ligands has been demonstrated in several reactions involving dinuclear Ir­(η 4 -COD) units as well as mononuclear Ir­(η 4 -COD) compounds. − Related dinuclear iridium compounds with hydride and phosphine ligands have also been prepared for a range of allyl and Cp* ligands, such as the thermolysis at higher concentrations of the allyl hydride compound Cp*IrH­(η 3 -C 3 H 5 ) in benzene, which leads to the oxidative addition product Cp*Ir­(η 1 -C 6 H 5 )­(η 1 -μ 2 -η 3 -CHCHCH 2 )­(μ 2 -H)­IrCp*, where the hydride and the η 1 ,η 3 -allyl ligands are bridging both iridium atoms. The consequent reductive elimination of benzene from the dinuclear compound in the presence of donor ligands L, such as PMe 3 and CO, affords the corresponding derivatives Cp*Ir­(L)­(η 1 -μ 2 -η 3 -CHCHCH 2 )­IrCp* .…”

Unsaturated Iridium(III) Complexes Supported by a Quinolato–Carboxylato ONO Pincer-Type Ligand: Synthesis, Reactivity, and Catalytic C–H Functionalization

“…In contrast, the formation of the methanol adduct [IrH(κ 2hqca)(cod)(CH 3 OH)] paves the way to the hydrido migration to the cod ligand, leading to 1. 16 Reactivity of [IrH(η 3 -hqca)(coe)]. The presence of a coordination vacant site and a replaceable coe ligand in [IrH(κ 3 -hqca)(coe)] ( 3) has prompted us to study its reactivity with N-and P-donor ligands.…”

“…Taking into account that, in general, carboxylic acids are more acidic than phenol derivatives, it is reasonable to propose the selective protonation of the methoxido bridges in [Ir(μ-OMe)(cod)] 2 by the acidic carboxyl group of H 2 hqca to give the mononuclear neutral iridium(I) intermediate [Ir(κ 2 -Hhqca)(cod)] having the monodeprotonated ligand κ 2 -N,O coordinated through the carboxylate moiety. By analogy with the mechanism operating in the formation of the related pyridine-2,6-dicarboxylate complex, the formation of 1a involves a metal-mediated proton transfer to the 1,5-cyclooctadiene ligand through the methanol-stabilized hydrido species [IrH(κ 2 -hqca)(cod)(CH 3 OH)], featuring a hydrogen bond with the uncoordinated carboxylate moiety of the 8-oxidoquinoline-2-carboxylato ligand κ 2 -N,O coordinated. Most probably, the formation of this key hydrido intermediate also results from solvent-assisted proton transfer through a hydrogen-bonding network instead of the less favorable direct protonation of the iridium center by the phenol moiety, formally an oxidative addition .…”

“…However, the reactivity studies on some iminodiacetic acid derivatives, RN-(CH 2 COOH) 2 (R = Me, Ph), has allowed us to identify the acidity (pK a ) and the rigidity of the potential tridentate ligand precursor as the key factors leading to the preparation of iridium(III) complexes. 16 In order to test whether this synthetic method could be applied to the direct synthesis of related dianionic tridentate pincer ONO iridium(III) complexes, we have explored the reactivity of 8-hydroxyquinoline-2-carboxylic acid. 17 This asymmetrical flat ONO pincer ligand precursor is more rigid than the symmetric pyridinedicarboxylato species due to the presence of the quinolinol moiety (Chart 1(ii)).…”

“…The application of this synthetic methodology to other dicarboxylic acid precursors for dianionic tridentate pincer ONO complexes has so far shown a narrow scope. However, the reactivity studies on some iminodiacetic acid derivatives, RN(CH 2 COOH) 2 (R = Me, Ph), has allowed us to identify the acidity (p K a ) and the rigidity of the potential tridentate ligand precursor as the key factors leading to the preparation of iridium(III) complexes …”