Square-planar iridium(II) and iridium(III) amido complexes stabilized by a PNP pincer ligand

C-H Bond Activation by Iridium(III) and Iridium(IV) Oxo Complexes

Oxidation of an iridium(III) oxo precursor enabled the structural, spectroscopic, and quantum-chemical characterization of the first well-defined iridium(IV) oxo complex. Side-by-side examination of the proton-coupled electron transfer thermochemistry revealed similar driving forces for the isostructural oxo complexes in two redox states due to compensating contributions from H+ and e- transfer. However, C-H activation of dihydroanthracene revealed significant hydrogen tunneling for the distinctly more basic iridium(III) oxo complex. Our findings complement the growing body of data that relate tunneling to ground state properties as predictors for the selectivity of C-H bond activation.

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.

Quadratic Spin-Orbit Mechanism of the Electronic g-Tensor

Understanding how the electronic g-tensor is linked to the electronic structure is desirable for the correct interpretation of electron paramagnetic resonance spectra. For heavy-element compounds with large spin-orbit (SO) effects, this is still not completely clear. We report our investigation of quadratic SO contributions to the g-shift in heavy transition metal complexes. We implemented third-order perturbation theory in order to analyze the contributions arising from frontier molecular spin orbitals (MSOs). We show that the dominant quadratic SO term─spin-Zeeman (SO²/SZ)─generally makes a negative contribution to the g-shift, irrespective of the particular electronic configuration or molecular symmetry. We further analyze how the SO²/SZ contribution adds to or subtracts from the linear orbital-Zeeman (SO/OZ) contribution to the individual principal components of the g-tensor. Our study suggests that the SO²/SZ mechanism decreases the anisotropy of the g-tensor in early transition metal complexes and increases it in late transition metal complexes. Finally, we apply MSO analysis to the investigation of g-tensor trends in a set of closely related Ir and Rh pincer complexes and evaluate the influence of different chemical factors (the nuclear charge of the central atom and the terminal ligand) on the magnitudes of the g-shifts. We expect our conclusions to aid the understanding of spectra in magnetic resonance investigations of heavy transition metal compounds.

Mono- and Bis(iminoxolene)iridium Complexes: Synthesis and Covalency in π Bonding

N-(2,6-Diisopropylphenyl)-4,6-di-tert-butyl-o-iminobenzoquinone (Diso) reacts with the (cyclooctadiene)iridium chloride dimer to form a monoiminoxolene complex, (Diso)Ir(cod)Cl. Reaction of 2 equiv of the iminoquinone with chlorobis(cyclooctene)iridium dimer affords the bis-iminoxolene (Diso)₂IrCl. This five-coordinate complex adopts a distorted square pyramidal structure with an apical chloride ligand and undergoes halide exchange to form an air-stable iodide complex. (Diso)₂IrCl can be reduced by one electron to form neutral, square planar (Diso)₂Ir, while oxidation with PhICl₂ gives octahedral trans-(Diso)₂IrCl₂. The cis isomer can be prepared by air oxidation of (Diso)₂IrCl; cis/trans isomerization is not observed even on prolonged heating. Structural and spectroscopic features of the complexes are consistent with the presence of strong, covalent π bonding between the metal and the iminoxolene ligands, with structural data suggesting between 45 and 60% iridium character in the π bonding orbitals, depending on the ancillary ligands. The spectroscopic similarity of (Diso)₂Ir and (Diso)₂IrCl to their cobalt congeners suggests that the first-row metal complexes likewise have appreciably covalent metal-iminoxolene π bonds.

Transfer hydrogenation catalysis in cells

Hydrogenation reactions in biology are usually carried out by enzymes with nicotinamide adenine dinucleotide (NAD(P)H) or flavin mononucleotide (FAMH2)/flavinadenine dinucleotide (FADH2) as cofactors and hydride sources. Industrial scale chemical transfer hydrogenation uses small molecules such as formic acid or alcohols (e.g. propanol) as hydride sources and transition metal complexes as catalysts. We focus here on organometallic half-sandwich RuII and OsII η6-arene complexes and RhIII and IrIII η5-Cp x complexes which catalyse hydrogenation of biomolecules such as pyruvate and quinones in aqueous media, and generate biologically important species such as H2 and H2O2. Organometallic catalysts can achieve enantioselectivity, and moreover can be active in living cells, which is surprising on account of the variety of poisons present. Such catalysts can induce reductive stress using formate as hydride source or oxidative stress by accepting hydride from NAD(P)H. In some cases, photocatalytic redox reactions can be induced by light absorption at metal or flavin centres. These artificial transformations can interfere in biochemical pathways in unusual ways, and are the basis for the design of metallodrugs with novel mechanisms of action.

Recent Progress with Pincer Transition Metal Catalysts for Sustainability

Our planet urgently needs sustainable solutions to alleviate the anthropogenic global warming and climate change. Homogeneous catalysis has the potential to play a fundamental role in this process, providing novel, efficient, and at the same time eco-friendly routes for both chemicals and energy production. In particular, pincer-type ligation shows promising properties in terms of long-term stability and selectivity, as well as allowing for mild reaction conditions and low catalyst loading. Indeed, pincer complexes have been applied to a plethora of sustainable chemical processes, such as hydrogen release, CO2 capture and conversion, N2 fixation, and biomass valorization for the synthesis of high-value chemicals and fuels. In this work, we show the main advances of the last five years in the use of pincer transition metal complexes in key catalytic processes aiming for a more sustainable chemical and energy production.

(Electro-)chemical Splitting of Dinitrogen with a Rhenium Pincer Complex

The splitting of N₂ into well‐defined terminal nitride complexes is a key reaction for nitrogen fixation at ambient conditions. In continuation of our previous work on rhenium pincer mediated N₂ splitting, nitrogen activation and cleavage upon (electro)chemical reduction of [ReCl₂(L2)] {L2 = N(CHCHPtBu₂)₂^–} is reported. The electrochemical characterization of [ReCl₂(L2)] and comparison with our previously reported platform [ReCl₂(L1)] {L1 = N(CH₂CH₂PtBu₂)₂^–} provides mechanistic insight to rationalize the dependence of nitride yield on the reductant. Furthermore, the reactivity of N₂ derived nitride complex [Re(N)Cl(L2)] with electrophiles is presented.

A Terminal Iridium Oxo Complex with a Triplet Ground State

A terminal iridium oxo complex with an open-shell (S=1) ground state was isolated upon hydrogen atom transfer (HAT) from the respective iridium(II) hydroxide. Electronic structure examinations support large spin delocalization to the oxygen atom. Selected oxo transfer reactions indicate the ambiphilic reactivity of the iridium oxo moiety. Calorimetric and computational examinations of the HAT revealed a bond dissociation free energy for the IrO-H bond that is sufficient for hydrogen atom abstraction towards C-H bonds and small contributions from entropy and spin-orbit coupling to the HAT thermochemistry.

An iridium(iii/iv/v) redox series featuring a terminal imido complex with triplet ground state

The iridium(iii/iv/v) imido redox series [Ir(NtBu){N(CHCHPtBu2)2}]0/+/2+ was synthesized and examined spectroscopically, magnetically, crystallographically and computationally. The monocationic iridium(iv) imide exhibits an electronic doublet ground state with considerable 'imidyl' character as a result of covalent Ir-NtBu bonding. Reduction gives the neutral imide [Ir(NtBu){N(CHCHPtBu2)2}] as the first example of an iridium complex with a triplet ground state. Its reactivity with respect to nitrene transfer to selected electrophiles (CO2) and nucleophiles (PMe3), respectively, is reported.

A square-planar osmium(ii) complex

Reduction of the pincer complex [Os^IIICl₂(PNP)] (PNP = N(CHCHPtBu₂)₂) affords the isolation and full characterization of an osmium(ii) complex with square-planar coordination geometry, i.e. [Os^IICl(PNP)]. Spectroscopic, structural and magnetic data in combination with multireference computations indicate strong temperature independent paramagnetism, which arises from an energetically well separated ground state that mixes with excited states through spin-orbit coupling.

First-row transition metal complexes of ENENES ligands: the ability of the thioether donor to impact the coordination chemistry

The reactions of two variants of ENENES ligands, E(CH₂)₂NH(CH)₂SR, where E = 4-morpholinyl, R = Ph (a), Bn (b) with MCl₂ (M = Mn, Fe, Co, Ni and Cu) in coordinating solvents (MeCN, EtOH) affords isolable complexes, whose magnetic susceptibility measurements suggest paramagnetism and a high-spin formulation.

Coupling of terminal iridium nitrido complexes

The homo- and heterocoupling of molecular terminal iridium(iv) and iridium(v) nitrides is examined. The experimental coupling rates are discussed based on a computational analysis of the transition states.

Oxygen Reduction with a Bifunctional Iridium Dihydride Complex

The iridium dihydride [Ir(H)2 (HPNP)](+) (PNP=N(CH2 CH2 PtBu2 )2 ) reacts with O2 to give the unusual, square-planar iridium(III) hydroxide [Ir(OH)(PNP)](+) and water. Regeneration of the dihydride with H2 closes a quasi-catalytic synthetic oxygen-reduction reaction (ORR) cycle that can be run several times. Experimental and computational examinations are in agreement with an oxygenation mechanism via rate-limiting O2 coordination followed by H-transfer at a single metal site, facilitated by the cooperating pincer ligand. Hence, the four electrons required for the ORR are stored within the two covalent MH bonds of a mononuclear metal complex.

Homolytic N-H activation of ammonia: hydrogen transfer of parent iridium ammine, amide, imide, and nitride species

The redox series [Ir(n)(NHx)(PNP)] (n = II-IV, x = 3-0; PNP = N(CHCHPtBu2)2) was examined with respect to electron, proton, and hydrogen atom transfer steps. The experimental and computational results suggest that the Ir(III) imido species [Ir(NH)(PNP)] is not stable but undergoes disproportionation to the respective Ir(II) amido and Ir(IV) nitrido species. N-H bond strengths are estimated upon reaction with hydrogen atom transfer reagents to rationalize this observation and are used to discuss the reactivity of these compounds toward E-H bond activation.

The effect of nitrido, azide, and nitrosyl ligands on magnetization densities and magnetic properties of iridium PNP pincer-type complexes

The magnetisation density of different iridium PNP pincer-type complexes depends on the chosen quantum chemistry method.

Square-planar ruthenium(II) complexes: control of spin state by pincer ligand functionalization

Functionalization of the PNP pincer ligand backbone allows for a comparison of the dialkyl amido, vinyl alkyl amido, and divinyl amido ruthenium(II) pincer complex series [RuCl{N(CH2 CH2 PtBu2 )2 }], [RuCl{N(CHCHPtBu2 )(CH2 CH2 PtBu2 )}], and [RuCl{N(CHCHPtBu2 )2 }], in which the ruthenium(II) ions are in the extremely rare square-planar coordination geometry. Whereas the dialkylamido complex adopts an electronic singlet (S=0) ground state and energetically low-lying triplet (S=1) state, the vinyl alkyl amido and the divinyl amido complexes exhibit unusual triplet (S=1) ground states as confirmed by experimental and computational examination. However, essentially non-magnetic ground states arise for the two intermediate-spin complexes owing to unusually large zero-field splitting (D>+200 cm(-1) ). The change in ground state electronic configuration is attributed to tailored pincer ligand-to-metal π-donation within the PNP ligand series.

Organoiridium complexes: anticancer agents and catalysts

Iridium is a relatively rare precious heavy metal, only slightly less dense than osmium. Researchers have long recognized the catalytic properties of square-planar Ir^I complexes, such as Crabtree’s hydrogenation catalyst, an organometallic complex with cyclooctadiene, phosphane, and pyridine ligands. More recently, chemists have developed half-sandwich pseudo-octahedral pentamethylcyclopentadienyl Ir^III complexes containing diamine ligands that efficiently catalyze transfer hydrogenation reactions of ketones and aldehydes in water using H₂ or formate as the hydrogen source. Although sometimes assumed to be chemically inert, the reactivity of low-spin 5d⁶ Ir^III centers is highly dependent on the set of ligands. Cp* complexes with strong σ-donor C^∧C-chelating ligands can even stabilize Ir^IV and catalyze the oxidation of water. In comparison with well developed Ir catalysts, Ir-based pharmaceuticals are still in their infancy. In this Account, we review recent developments in organoiridium complexes as both catalysts and anticancer agents.

Initial studies of anticancer activity with organoiridium complexes focused on square-planar Ir^I complexes because of their structural and electronic similarity to Pt^II anticancer complexes such as cisplatin. Recently, researchers have studied half-sandwich Ir^III anticancer complexes. These complexes with the formula [(Cp^x)Ir(L^∧L′)Z]^0/n+ (with Cp* or extended Cp* and L^∧L′ = chelated C^∧N or N^∧N ligands) have a much greater potency (nanomolar) toward a range of cancer cells (especially leukemia, colon cancer, breast cancer, prostate cancer, and melanoma) than cisplatin. Their mechanism of action may involve both an attack on DNA and a perturbation of the redox status of cells. Some of these complexes can form Ir^III-hydride complexes using coenzyme NAD(P)H as a source of hydride to catalyze the generation of H₂ or the reduction of quinones to semiquinones. Intriguingly, relatively unreactive organoiridium complexes containing an imine as a monodentate ligand have prooxidant activity, which appears to involve catalytic hydride transfer to oxygen and the generation of hydrogen peroxide in cells. In addition, researchers have designed inert Ir^III complexes as potent kinase inhibitors. Octahedral cyclometalated Ir^III complexes not only serve as cell imaging agents, but can also inhibit tumor necrosis factor α, promote DNA oxidation, generate singlet oxygen when photoactivated, and exhibit good anticancer activity. Although relatively unexplored, organoiridium chemistry offers unique features that researchers can exploit to generate novel diagnostic agents and drugs with new mechanisms of action.

[IrCl{N(CHCHPtBu2)2}]−: a versatile source of the IrI(PNP) pincer platform

The iridium(ii) complex [IrCl{N(CHCHPtBu₂)₂}] is reduced by KC₈ to give the anionic iridium(i) pincer complex [IrCl{N(CHCHPtBu₂)₂}]⁻ which was isolated and fully characterized upon stabilization of the counter cation with crown ether as [K(15-cr-5)₂][IrCl{N(CHCHPtBu₂)₂}].