Photochemical Production of Ethane from an Iridium Methyl Complex

Photoinduced Group Transposition via Iridium-Nitrenoid Leading to Amidative Inner-Sphere Aryl Migration

We herein report a fundamental mechanistic investigation into photochemical metal-nitrenoid generation and inner-sphere transposition reactivity using organometallic photoprecursors. By designing Cp*Ir(hydroxamate)(Ar) complexes, we induced photo-initiated ligand activation, allowing us to explore the amidative σ(Ir-aryl) migration reactivity. A combination of experimental mechanistic studies, femtosecond transient absorption spectroscopy, and density functional theory (DFT) calculations revealed that the metal-to-ligand charge transfer enables the σ(N-O) cleavage, followed by Ir-acylnitrenoid generation. The final inner-sphere σ(Ir-aryl) group migration results in a net amidative group transposition.

Dinitrogen activation at chromium by photochemically induced CrII-C bond homolysis

The synthesis of the organochromium(II) complexes [POCOPtBu]Cr(R) (R = p-Tol, Bn) is reported. Exposure of [POCOPtBu]Cr(Bn) to visible light promoted homolytic Cr-CBn bond cleavage and formed {[POCOPtBu]Cr}2(η1:η1μ-N2) via a putative [POCOPtBu]Cr(I) species.

Reactivity umpolung (reversal) of ligands in transition metal complexes

The success and power of homogeneous catalysis derives in large part from the wide choice of transition metal ions and their ligands. This tutorial review introduces examples where the reactivity of a ligand is completely reversed (umpolung) from Lewis basic/nucleophilic to acidic/electrophilic or vice versa on changing the metal and co-ligands. Understanding this phenomenon will assist in the rational design of catalysts and the understanding of metalloenzyme mechanisms. Labelling a metal and ligand with Seebach donor and acceptor labels helps to identify whether a reaction involving the intermolecular attack on the ligand is displaying native reactivity or reactivity umpolung. This has been done for complexes of nitriles, carbonyls, isonitriles, dinitrogen, Fischer carbenes, alkenes, alkynes, hydrides, methyls, methylidenes and alkylidenes, silylenes, oxides, imides/nitrenes, alkylidynes, methylidynes, and nitrides. The electronic influence of the metal and co-ligands is discussed in terms of the energy of (HOMO) d electrons. The energy can be related to the pKLACa (LAC is ligand acidity constant) of the theoretical hydride complexes [H-[M]-L]+ formed by the protonation of pair of valence d electrons on the metal in the [M-L] complex. Preliminary findings indicate that a negative pKLACa indicates that nucleophilic attack by a carbanion or amine on the ligand will likely occur while a positive pKLACa indicates that electrophilic attack by strong acids on the ligand will usually occur when the ligand is nitrile, carbonyl, isonitrile, alkene and η6-arene.

P,N-type phosphaalkene-based Ir(i) complexes: synthesis, coordination chemistry, and catalytic applications

Iridium P,N phosphaalkene complexes show a rich coordination chemistry with unusual twofold C–H activation. The Ir(i) chloride complex can be applied for C–N coupling and alcohol upgrading reactions.

Merging Two Functions in a Single Rh Catalyst System: Bimodular Conjugate for Light-Induced Oxidative Coupling

A single molecular rhodium catalyst system (PC2-Cp^#Rh^III) bearing two functional domains for both photosensitization and C-H carbometalation was designed to enable an intramolecular redox process. The hypothesized charge-transfer species (PC2^•--Cp^#Rh^IV) was characterized by spectroscopic and electrochemical analyses. This photoinduced internal oxidation allows a facile access to the triplet state of the key post-transmetalation intermediate that readily undergoes C-C bond-forming reductive elimination with a lower activation barrier than in its singlet state, thus enabling catalytic C-H arylation and methylation processes.

Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins

Many photoactive metal complexes can act as electron donors or acceptors upon photoexcitation, but hydrogen atom transfer (HAT) reactivity is rare. We discovered that a typical representative of a widely used class of iridium hydride complexes acts as an H-atom donor to unactivated olefins upon irradiation at 470 nm in the presence of tertiary alkyl amines as sacrificial electron and proton sources. The catalytic hydrogenation of simple olefins served as a test ground to establish this new photo-reactivity of iridium hydrides. Substrates that are very difficult to activate by photoinduced electron transfer were readily hydrogenated, and structure-reactivity relationships established with 12 different olefins are in line with typical HAT reactivity, reflecting the relative stabilities of radical intermediates formed by HAT. Radical clock, H/D isotope labeling, and transient absorption experiments provide further mechanistic insight and corroborate the interpretation of the overall reactivity in terms of photo-triggered hydrogen atom transfer (photo-HAT). The catalytically active species is identified as an Ir(ii) hydride with an IrII-H bond dissociation free energy around 44 kcal mol-1, which is formed after reductive 3MLCT excited-state quenching of the corresponding Ir(iii) hydride, i.e. the actual HAT step occurs on the ground-state potential energy surface. The photo-HAT reactivity presented here represents a conceptually novel approach to photocatalysis with metal complexes, which is fundamentally different from the many prior studies relying on photoinduced electron transfer.

Mechanistic basis for tuning iridium hydride photochemistry from H2 evolution to hydride transfer hydrodechlorination

The photochemistry of metal hydride complexes is dominated by H2 evolution, limiting access to reductive transformations based on photochemical hydride transfer. In this article, the innate H2 evolution photochemistry of the iridium hydride complexes [Cp*Ir(bpy-OMe)H]+ (1, bpy-OMe = 4,4'-dimethoxy-2,2'-bipyridine) and [Cp*Ir(bpy)H]+ (2, bpy = 2,2'-bipyridine) is diverted towards photochemical hydrodechlorination. Net hydride transfer from 1 and 2 to dichloromethane produces chloromethane with high selectivity and exceptional photochemical quantum yield (Φ ≤ 1.3). Thermodynamic and kinetic mechanistic studies are consistent with a non-radical-chain reaction sequence initiated by "self-quenching" electron transfer between excited state and ground state hydride complexes, followed by proton-coupled electron transfer (PCET) hydrodechlorination that outcompetes H-H coupling. This unique photochemical mechanism provides a new hope for the development of light-driven hydride transfer reactions.

A coordinatively unsaturated iridium complex with an unsymmetrical redox-active ligand: (spectro)electrochemical and reactivity studies

Metal–ligand cooperativity can be used in iridium complexes with an unsymmetrically substituted redox-active diamidobenzene ligand for bond activation reactions.

Excited-State Switching between Ligand-Centered and Charge Transfer Modulated by Metal-Carbon Bonds in Cyclopentadienyl Iridium Complexes

Three series of pentamethylcyclopentadienyl (Cp*) Ir(III) complexes with different bidentate ligands were synthesized and structurally characterized, [Cp*Ir(tpy)L] ⁿ⁺ (tpy = 2-tolylpyridinato; n = 0 or 1), [Cp*Ir(piq)L] ⁿ⁺ (piq = 1-phenylisoquinolinato; n = 0 or 1), and [Cp*Ir(bpy)L] ^m+ (bpy = 2,2'-bipyridine; m = 1 or 2), featuring a range of monodentate carbon-donor ligands within each series [L = 2,6-dimethylphenylisocyanide; 3,5-dimethylimidazol-2-ylidene (NHC); methyl)]. The spectroscopic and photophysical properties of these molecules and those of the photocatalyst [Cp*Ir(bpy)H]⁺ were examined to establish electronic structure-photophysical property relationships that engender productive photochemical reactivity of this hydride and its methyl analogue. The Ir(III) chromophores containing ancillary CNAr ligands exhibited features anticipated for predominantly ligand-centered (LC) excited states, and analogues bearing the NHC ancillary exhibited properties consistent with LC excited states containing a small admixture of metal-to-ligand charge-transfer (MLCT) character. However, the molecules featuring anionic and strongly σ-donating methyl or hydride ligands exhibited photophysical properties consistent with a high degree of CT character. Density functional theory calculations suggest that the lowest energy triplet states in these complexes are composed of a mixture of MLCT and ligand-to-ligand CT originating from both the Cp* and methyl or hydride ancillary ligands. The high degree of CT character in the triplet excited states of methyliridium complexes bearing C^N-cyclometalated ligands offer a striking contrast to the photophysical properties of pseudo-octahedral structures fac-Ir(C^N)₃ or Ir(C^N)₂(acac) that have lowest-energy triplet excited states characterized as primarily LC character with a more moderate MLCT admixture.