Ring Slippage and Dissociation of Pentamethylcyclopentadienyl Ligand in an (η5-Cp*)Ir Complex with a κ3-O,C,O Tridentate Calix[4]arene Ligand under Mild Conditions

Post-Synthetic Modification of Calix[4]arene Framework by Iridium-Mediated Alkyne Insertion into an Inert C-C Bond: A Novel Strategy for Unsymmetrical Macrocycles

As a novel synthetic method for unsymmetrical macrocycles, we herein developed a post-synthetic modification of calix[4]arenes by insertion of a terminal alkyne into an inert C(methylene)-C(aryl) bond of the macrocyclic framework. In this transformation, C-iridated calix[4]arenes, readily synthesized through C-H bond activation of the parent calix[4]arene, undergoes C(methylene)-C(aryl) bond cleavage followed by insertion of an alkyne to provide a ring-expanded calix[4]arene complex. Removal of the iridium metal from the resulting complex was readily accomplished by a simple treatment with an acid. The developed sequential method affords novel unsymmetrical, monofunctionalized macrocyclic compounds via 3 steps from the parent calix[4]arene in good yield. The unique unsymmetrical structures of the alkyne-inserted macrocycles were evaluated by X-ray single crystal analyses. On the basis of theoretical calculations, we propose a reaction mechanism involving an uncommon C-C bond cleavage step through δ-carbon elimination for the ring enlargement process.

Iridium-Catalyzed Hydrogenation of a Phenoxy Radical to the Phenol: Overcoming Catalyst Deactivation with Visible Light Irradiation

Piano-stool iridium hydride complexes bearing phenylpyridine ligands are effective precatalysts for promoting the formation of element-hydrogen bonds using H2 as the stoichiometric H-atom source. Irradiation with blue light resulted in a profound enhancement of catalyst turnover for the iridium-catalyzed hydrogenation of the aryloxyl radical 2,4,6-tBu3-C6H2O• to the corresponding phenol. Monitoring the progress of the reaction revealed the formation of an iridium 3,3-dimethyl-2,3-dihydrobenzofuranyl compound arising from two C-H activation events following the proton-coupled electron transfer (PCET) step. Under thermal conditions, this compound was inactive for catalytic aryloxide hydrogenation, representing a deactivation pathway. Irradiation with blue light under H2 released the free heterocycle and regenerated the piano-stool iridium hydride precatalyst, establishing a pathway for catalyst recovery and overall enhanced turnover.

One-pot Syntheses of Benzo- and Benzofuran-fused Iridaoxabenzenes via CH Bond Activations of Alkyl-bridged Diphenol Derivatives

One-pot syntheses of new π-extended metallaaromatic compounds have been developed by utilizing Ir-mediated CH bond activation of ethylene- or ethylidene-bridged diphenol derivatives. Depending on the bridging alkyl groups, two types of iridaoxabenzenes, both of which are doubly fused with benzo and benzofuran units, have been obtained. Studies on their structures and electronic characters indicate that both complexes have an aromatic character on the iridaoxacycles, and their π-conjugated systems are fully delocalized over the whole molecular skeletons. These novel metallaaromatic complexes exhibited some reactivities which are distinct from those reported for the non-fused metallaaromatic compounds.

Cp* non-innocence and the implications of (η4-Cp*H)Rh intermediates in the hydrogenation of CO2, NAD+, amino-borane, and the Cp* framework - a computational study

In hydrogenation mediated by half-sandwich complexes of Rh, Cp*Rh(III)-H intermediates are critical hydride-delivery agents. For bipyridine-supported complexes, a unique transformation named 'Cp* non-innocence' leads to the appearance of (Cp*H)Rh(I) intermediates, which are purported to exhibit enhanced hydride-delivery capabilities. In this work, DFT calculations performed to compare the role of these complexes in hydrogenation reveal that (Cp*H)Rh(I) intermediates do not compete with the conventional pathway (involving Cp*Rh(III)-H); instead they can lead to sequential hydrogenation of the Cp* framework, and potentially, catalyst degradation. Thus, caution is warranted when invoking the truly homogeneous nature of hydrogenation catalysis mediated by this popular class of complexes.

Inherently dinuclear iridium(III) meso architectures accessed by cyclometalation of calix[4]arene-based bis(aryltriazoles)

Conventional cyclometalation of calix[4]arene bis(aryltriazoles) with iridium(III) chloride hydrate leads to unique meso architectures in which the Ir₂Cl₂ core is cross-bound by two (C^N)₂ ligands, which allows further replacement of the chloride bridges with ancillary ligands while maintaining the dinuclear structures of the complexes having independent or coupled iridium pairs.

Synthesis and structures of diaryloxystannylenes and -plumbylenes embedded in 1,3-diethers of thiacalix[4]arene

New types of diaryloxystannylenes and -plumbylenes have been synthesized and structurally characterized. Lappert's diaminostannylene and -plumbylene E[N(SiMe3)2]2 (E = Sn, Pb) react with 1,3-diethers of tetrathiacalix[4]arene bearing benzyl or SiiPr3 groups to give the corresponding diaryloxystannylenes and -plumbylenes embedded in thiacalix[4]arene, respectively. X-ray diffraction analysis revealed that these tetrylenes exist as monomers in the solid-state. The structure of the thiacalix[4]arene scaffold is highly dependent on the substituents on the phenolic oxygen atoms. The benzyl derivatives adopt apparently C2-symmetric structures with a strong intramolecular SnOBn coordination, whereas the SiiPr3 derivatives have a distorted thiacalix[4]arene skeleton with relatively weak intramolecular EOSiiPr3 and ES interactions. On the basis of the 119Sn{1H} and 207Pb NMR studies in solution, the tin and lead atoms in the benzyl derivatives are strongly coordinated by the ethereal oxygen atoms, whereas those in the SiiPr3 derivatives are weakly coordinated by the bridging sulfur atoms or ethereal oxygen atoms. The tetrylene complexes of thiacalix[4]arene presented here do not show any isomerization in a temperature range from 20 to 110 °C, which is in sharp contrast to the fact that related diaryloxygermylenes and -stannylenes embedded in calix[4]arene underwent isomerization under heating conditions. This difference could be attributable to the ring size of thiacalix[4]arene larger than that of calix[4]arene.

A hemilabile manganese(i)–phenol complex and its coordination induced O–H bond weakening

Synthesis and characterization of [(HPO)(PO)Mn(CO)₂ (^H1), a phenol bound first-row transition metal complex, is reported. Thermochemical analysis of ^H1 indicated the presence of coordination induced O–H bond weakening.

Catalytic recycling of NAD(P)H

A large number of industrially relevant enzymes depend upon dihydronicotinamide adenine dinucleotide (NADH) and dihydronicotinamide adenine dinucleotide phosphate (NADPH) cofactors, which are too expensive to be added in stoichiometric amounts. Existing NAD(P)H-recycling systems suffer from low activity, or the generation of side products. This review focuses on NAD(P)H cofactor regeneration catalyzed by transition metal complexes such as rhodium, ruthenium and iridium complexes using cheap reducing agents such as hydrogen (H₂) and ethanol, which have attracted increasing attention as sustainable energy carriers. The catalytic mechanisms for the regioselective reduction of NAD(P)⁺ are discussed with emphasis on identification of catalytically active intermediates such as transition metal hydride complexes. Applications of NAD(P)H-recycling systems to develop artificial photosynthesis are also discussed.