Terminal Oxo and Imido Transition-Metal Complexes of Groups 9-11

Metal-ligand cooperative transformation of alkyl azide to isocyanate occurring at a Co-Si moiety

A cobalt-silyl moiety reveals metal-ligand cooperative group transfer to generate isocyanate from the reaction of alkyl azide and CO. This reaction involves the reversible insertion of a nitrene group into a Co-Si bond. Photolysis leads to ligand substitution of a Co(CO)₂ species, allowing the successful catalytic conversion of AdN₃ to AdNCO under CO(g).

Selective Activation of Propane Using Intermediates Generated during Water Oxidation

Electrochemical conversion of light alkanes to high-value oxygenates provides an attractive avenue for eco-friendly utilization of these hydrocarbons. However, such conversion under ambient conditions remains exceptionally challenging due to the high energy barrier of C-H bond cleavage. Herein, we investigated theoretically the partial oxidation of propane on a series of single atom alloys by using active intermediates generated during water oxidation as the oxidant. We show that by controlling the potential and pH, stable surface oxygen atoms can be maintained under water oxidation conditions. The free energy barrier for C-H bond cleavage by the surface oxygen can be as small as 0.54 eV, which can be surmounted easily at room temperature. Our calculations identified three promising surfaces as effective propane oxidation catalysts. Our complementary experiments demonstrated the partial oxidation of propane to acetone on Ni-doped Au surfaces. We also investigated computationally the steps leading to acetone formation. These studies show that the concept of exploiting intermediates generated in water oxidation as oxidants provides a fruitful strategy for electrocatalyst design to efficiently convert hydrocarbons into value-added chemicals.

A Pseudotetrahedral Terminal Oxoiron(IV) Complex: Mechanistic Promiscuity in C-H Bond Oxidation Reactions

S=2 oxoiron(IV) species act as reactive intermediates in the catalytic cycle of nonheme iron oxygenases. The few available synthetic S=2 FeIV =O complexes known to date are often limited to trigonal bipyramidal and very rarely to octahedral geometries. Herein we describe the generation and characterization of an S=2 pseudotetrahedral FeIV =O complex 2 supported by the sterically demanding 1,4,7-tri-tert-butyl-1,4,7-triazacyclononane ligand. Complex 2 is a very potent oxidant in hydrogen atom abstraction (HAA) reactions with large non-classical deuterium kinetic isotope effects, suggesting hydrogen tunneling contributions. For sterically encumbered substrates, direct HAA is impeded and an alternative oxidative asynchronous proton-coupled electron transfer mechanism prevails, which is unique within the nonheme oxoiron community. The high reactivity and the similar spectroscopic parameters make 2 one of the best electronic and functional models for a biological oxoiron(IV) intermediate of taurine dioxygenase (TauD-J).

Structural and Oxidation State Alternatives in Platinum and Palladium Complexes of a Redox-Active Amidinato Ligand

Reaction of [Pt(DMSO)₂ Cl₂ ] or [Pd(MeCN)₂ Cl₂ ] with the electron-rich LH=N,N'-bis(4-dimethylaminophenyl)ethanimidamide yielded mononuclear [PtL₂ ] (1) but dinuclear [Pd₂ L₄ ] (2), a paddle-wheel complex. The neutral compounds were characterized through experiments (crystal structures, electrochemistry, UV-vis-NIR spectroscopy, magnetic resonance) and TD-DFT calculations as metal(II) species with noninnocent ligands L^- . The reversibly accessible cations [PtL₂ ]⁺ and [Pd₂ L₄ ]⁺ were also studied, the latter as [Pd₂ L₄ ][B{3,5-(CF₃ )₂ C₆ H₃ }₄ ] single crystals. Experimental and computational investigations were directed at the elucidation of the electronic structures, establishing the correct oxidation states within the alternatives [Pt^II (L^- )₂ ] or [Pt^. (L )₂ ], [Pt^II (L^0.5- )₂ ]⁺ or [Pt^III (L^- )₂ ]⁺ , [(Pd^II )₂ (μ-L^- )₄ ] or [(Pd^1.5 )₂ (μ-L^0.75- )₄ ], and [(Pd^2.5 )₂ (μ-L^- )₄ ]⁺ or [(Pd^II )₂ (μ-L^0.75- )₄ ]⁺ . In each case, the first alternative was shown to be most appropriate. Remarkable results include the preference of platinum for mononuclear planar [PtL₂ ] with an N-Pt-N bite angle of 62.8(2)° in contrast to [Pd₂ L₄ ], and the dimetal (Pd₂ ⁴⁺ →Pd₂ ⁵⁺ ) instead of ligand (L^- →L ) oxidation of the dinuclear palladium compound.

Molecular oxofluorides OMFn of nickel, palladium and platinum: oxyl radicals with moderate ligand field inversion

High-valent late transition metal oxo compounds attracted attention because of their peculiar metal–oxygen bond. Their oxo ligands exhibit an electrophilic and distinct radical oxyl (O˙⁻) rather than the more common nucleophilic (O²⁻) character.

Fluoro Nitrenoid Complexes FN=MF2 (M=Co, Rh, Ir): Electronic Structure Dichotomy and Formation of Nitrido Fluorides N≡MF3

The fluoronitrenoid metal complexes FNCoF2 and FNRhF2 as well as the first ternary RhVI and IrVI complexes NIrF3 and NRhF3 are described. They were obtained by the reaction of excited Group-9 metal atoms with NF3 and their IR spectra, isolated in solid rare gases (neon and argon), were recorded. Aided by the observed 14/15 N isotope shifts and quantum-chemical predictions, all four stretching fundamentals of the novel complexes were safely assigned. The F-N stretching frequencies of the fluoronitrenoid complexes FNCoF2 (1056.8 cm-1 ) and FNRhF2 (872.6 cm-1 ) are very different and their N-M bonds vary greatly. In FNCoF2 , the FN ligand is singly bonded to Co and bears considerable iminyl/nitrene radical character, while the N-Rh bond in FNRhF2 is a strong double bond with comparatively strong σ- and π-bonds. The anticipated rearrangement of FNCoF2 to the nitrido CoVI complex is predicted to be endothermic and was not observed.

Computational Study on the Fate of Oxidative Directing Groups in Ru(II), Rh(III), and Pd(II) Catalyzed C-H Functionalization

Activation of C-H bonds assisted by a directing group is indispensable in organic synthesis. Among them, utilizing oxidative directing groups that can serve as an internal oxidant to drive the Mⁿ/Mⁿ⁺² catalytic cycle has recently become a promising strategy. A survey of published reactions involving N-alkoxyamides or N-acyloxyamides reveals that not all N-O bonds act as an internal oxidant. We have therefore systematically investigated the effect of the oxidative groups on a model reaction catalyzed by Ru(II), Rh(III), and Pd(II) complexes. DFT calculations show that N-methoxy and N-acyloxy groups oxidize Ru(II) to Ru(IV) and Rh(III) to Rh(V), but cannot oxidize a cyclo-Pd(II) intermediate to Pd(IV). The stability of the metal imido intermediate 7-M (M = Ru, Rh, and Pd) controls whether the oxidation occurs or not. N-Acyloxy groups show a more pronounced selectivity than N-methoxy to oxidize Ru(II) and Rh(III) species, while no distinctive effect is observed for Pd(II).

Iron and manganese oxo complexes, oxo wall and beyond

High-valent metal-oxo species with multiply-bonded M-O groups have been proposed as key intermediates in many biological and abiological catalytic oxidation reactions. These intermediates are implicated as active oxidants in alkane hydroxylation, olefin epoxidation and other oxidation reactions. For example, [Fe^ivO(porphyrinato^•-)]⁺ cofactors bearing π-radical porphyrinato^•- ligands oxidize organic substrates in cytochrome P450 enzymes, which are common to many life forms. Likewise, high-valent Mn-oxo species are active for H₂O oxidation in photosystem II. The chemistry of these native reactive species has inspired chemists to prepare highly oxidized transition-metal complexes as functional mimics. Although many synthetic Fe-O and Mn-O complexes now exist, the analogous oxo complexes of the late transition metals (groups 9-11) are rare. Indeed, late-transition-metal-oxo complexes of tetragonal (fourfold) symmetry should be electronically unstable, a rule commonly referred to as the 'oxo wall'. A few late metal-oxos have been prepared by targeting other symmetries or unusual spin states. These complexes have been studied using spectroscopic and theoretical methods. This Review describes mononuclear non-haem Fe-O and Mn-O species, the nature of the oxo wall and recent advances in the preparation of oxo complexes of Co, Ni and Cu beyond the oxo wall.

C-H Bond Activation by an Imido Cobalt(III) and the Resulting Amido Cobalt(II) Complex

The 3d-metal mediated nitrene transfer is under intense scrutiny due to its potential as an atom economic and ecologically benign way for the directed amination of (un)functionalised C-H bonds. Here we present the isolation and characterisation of a rare, trigonal imido cobalt(III) complex, which bears a rather long cobalt-imido bond. It can cleanly cleave strong C-H bonds with a bond dissociation energy of up to 92 kcal mol-1 in an intermolecular fashion, unprecedented for imido cobalt complexes. This resulted in the amido cobalt(II) complex [Co(hmds)2 (NHt Bu)]- . Kinetic studies on this reaction revealed an H atom transfer mechanism. Remarkably, the cobalt(II) amide itself is capable of mediating H atom abstraction or stepwise proton/electron transfer depending on the substrate. A cobalt-mediated catalytic application for substrate dehydrogenation using an organo azide is presented.

Pogo-Stick Iron and Cobalt Complexes: Synthesis, Structures, and Magnetic Properties

The synthesis, structures, and magnetic properties of monomeric half-sandwich iron and cobalt imidazolin-2-iminato complexes have been comprehensively investigated. Salt metathesis reactions of [Cp'M(μ-I)]₂ (1-M, M = Fe, Co; Cp' = η⁵-1,2,4-tri-tert-butylcyclopentadienyl) with [Im^DippNLi]₂ (Im^DippN = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-iminato) furnishes the terminal half-sandwich compounds [Cp'M(NIm^Dipp)] (2-M, M = Fe, Co), which can be regarded as models for elusive half-sandwich iron and cobalt imido complexes. X-ray diffraction analysis confirmed the structure motif of a one-legged piano stool. Complex 2-Co can also be prepared by an acid-base reaction between [Cp'Co{N(SiMe₃)₂}] (3-Co) and Im^DippNH. The electronic and magnetic properties of 2-M and 3-Co were probed by ⁵⁷Fe Mössbauer spectroscopy (M = Fe), X-band EPR spectroscopy (M = Co), and solid-state magnetic susceptibility measurements. In particular, the central metal atom adopts a high-spin (S = 2) state in 2-Fe, while the cobalt complex 2-Co represents a rare example of a Co(II) species with a coordination number different from six displaying a low-spin to high-spin spin-crossover (SCO) behavior. The experimental observations are complemented by DFT calculations.

Highly Reactive CoIII,IV2(μ-O)2 Diamond Core Complex That Cleaves C-H Bonds

The selective activation of strong sp³ C-H bonds at mild conditions is a key step in many biological and synthetic transformations and an unsolved challenge for synthetic chemists. In nature, soluble methane monooxygenase (sMMO) is one representative example of nonheme dinuclear iron-dependent enzymes that activate strong sp³ C-H bonds by a high-valent diiron(IV) intermediate Q. To date, synthetic model complexes of sMMO-Q have shown limited abilities to oxidize strong C-H bonds. In this work, we generated a high-valent Co^III,IV₂(μ-O)₂ complex 3 supported by a tetradentate tris(2-pyridylmethyl)amine (TPA) ligand via one-electron oxidation of its Co^III₂(μ-O)₂ precursor 2. Characterization of 2 and 3 using X-ray absorption spectroscopy and DFT calculations showed that both species possess a diamond core structure with a short Co···Co distance of 2.78 Å. Furthermore, 3 is an EPR active species showing an S = 1/2 signal with clearly observable hyperfine splittings originated from the coupling of the ⁵⁹Co nuclear spin with the electronic spin. Importantly, 3 is a highly reactive oxidant for sp³ C-H bonds, and an oxygenation reagent. 3 has the highest rate constant (1.5 M^-1 s^-1 at -60 °C) for oxidizing 9,10-dihydroanthracene (DHA) compared to diamond core complexes of other first-row transition metals including Mn, Fe and Cu reported previously. Specifically, 3 is about 4-5 orders of magnitude more reactive than the diiron analogs Fe^III,IV₂(μ-O)₂ and Fe^IV₂(μ-O)₂ supported by TPA and related ligands. These findings shed light on future development of more reactive approaches for C-H bond activation by bioinspired dicobalt complexes.

O-Functionalization of a cobalt carbonyl generates a terminal cobalt carbyne

Despite efforts toward extending multiple bonding motifs to late metal systems, examples of late transition metal carbynes remain scarce. Herein, we describe the synthesis of a series of L3Co(CO) complexes supported by a trisphosphine ligand framework, with the most reduced of these complexes being amenable to O-functionalization. This transformation provides access to the second reported example of a terminal Co-carbyne complex, in this case stabilized in a pseudotetrahedral geometry (i.e., L3Co[triple bond, length as m-dash]C-OSiR3). Its geometry makes its electronic structure suitable for comparison to structurally-related examples of terminal Co-imido and oxo species.

Metal-Oxyl Species and Their Possible Roles in Chemical Oxidations

Metal-oxyl (Mⁿ⁺-O^•) complexes having an oxyl radical ligand, which are electronically equivalent to well-known metal-oxo (M⁽ⁿ⁺¹⁾⁺═O) complexes, are surveyed as a new category of metal-based oxidants. Detection and characterization of Mⁿ⁺-O^• species have been made in some cases, although proposals and characterization of the species are mostly done on the basis of density functional theory (DFT) calculations. The reactivity of Mⁿ⁺-O^• complexes will provide a way to achieve potentially difficult oxidative conversion of substrates. This Viewpoint will provide state-of-the-art knowledge on the Mⁿ⁺-O^• species in terms of the formation, characterization, and DFT-based proposals to shed light on the characteristics of the intriguing oxidatively active species.

M-O Bonding Beyond the Oxo Wall: Spectroscopy and Reactivity of Cobalt(III)-Oxyl and Cobalt(III)-Oxo Complexes

Terminal oxo complexes of late transition metals are frequently proposed reactive intermediates. However, they are scarcely known beyond Group 8. Using mass spectrometry, we prepared and characterized two such complexes: [(N4Py)CoIII (O)]+ (1) and [(N4Py)CoIV (O)]2+ (2). Infrared photodissociation spectroscopy revealed that the Co-O bond in 1 is rather strong, in accordance with its lack of chemical reactivity. On the contrary, 2 has a very weak Co-O bond characterized by a stretching frequency of ≤659 cm-1 . Accordingly, 2 can abstract hydrogen atoms from non-activated secondary alkanes. Previously, this reactivity has only been observed in the gas phase for small, coordinatively unsaturated metal complexes. Multireference ab-initio calculations suggest that 2, formally a cobalt(IV)-oxo complex, is best described as cobalt(III)-oxyl. Our results provide important data on changes to metal-oxo bonding behind the oxo wall and show that cobalt-oxo complexes are promising targets for developing highly active C-H oxidation catalysts.

Carbon Dioxide Activation by a Palladium Terminal Imido Complex

We recently reported the first example of a palladium(ii) terminal imido complex. We proposed that this complex features exceptional high nucleophilicity at the nitrogen atom and a peculiar zwitterionic electronic structure with an anti-bonding highest-occupied molecular orbital (HOMO). This complex swiftly activated moderately acidic CH, OH, and NH bonds and also reacted with dihydrogen. However, unambiguous nucleophilic reactivity with substrates not featuring a hydrogen atom could not be observed. Herein, we now show that this nucleophilic complex also reacts with CO2 to give a ring-strained four-membered palladium(ii) carbamate complex. Remarkably, the same product is obtained in the reaction of the related bisamido complex, albeit at a slower reaction rate. Density functional theory calculations indicate that the addition of CO2 does not proceed via initial 1,2-addition across the Pd–N bond, but instead through nucleophilic attack by the imido (amido respectively) nitrogen atom.

Molecular and electronic structure of an azidocobalt(iii) complex derived from X-ray crystallography, linear spectroscopy and quantum chemical calculations

X-ray diffraction, UV/Vis electronic spectroscopy and Fourier-transform infrared spectroscopy are used with DFT and CAS calculations to explore the molecular and electronic structure of the cationic complex (cyclam)(diazido)cobalt(iii).

Pentamethylcyclopentadienyl ruthenium “pogo stick” complexes with nitrogen donor ligands

The preparation and reactivity of an imidazolin-2-iminato ruthenium complex with a rare one-legged piano-stool (“pogo stick”) geometry is reported.

Oxygen-Oxygen Bond Cleavage and Formation in Co(II)-Mediated Stoichiometric O2 Reduction via the Potential Intermediacy of a Co(IV) Oxyl Radical

In reactions of significance to alternative energy schemes, metal catalysts are needed to overcome kinetically and thermodynamically difficult processes. Often, high-oxidation-state, high-energy metal oxo intermediates are proposed as mediators in elementary steps involving O-O bond cleavage and formation, but the mechanisms of these steps are difficult to study because of the fleeting nature of these species. Here we utilized a novel dianionic pentadentate ligand system that enabled a detailed mechanistic investigation of the protonation of a cobalt(III)-cobalt(III) peroxo dimer, a known intermediate in oxygen reduction catalysis to hydrogen peroxide. It was shown that double protonation occurs rapidly and leads to a low-energy O-O bond cleavage step that generates a Co(III) aquo complex and a highly reactive Co(IV) oxyl cation. The latter was probed computationally and experimentally implicated through chemical interception and isotope labeling experiments. In the absence of competing chemical reagents, it dimerizes and eliminates dioxygen in a step highly relevant to O-O bond formation in the oxygen evolution step in water oxidation. Thus, the study demonstrates both facile O-O bond cleavage and formation in the stoichiometric reduction of O₂ to H₂O with 2 equiv of Co(II) and suggests a new pathway for selective reduction of O₂ to water via Co(III)-O-O-Co(III) peroxo intermediates.

An Isolable Terminal Imido Complex of Palladium and Catalytic Implications

Herein, we report the isolation and a reactivity study of the first example of an elusive palladium(II) terminal imido complex. This scaffold is an alleged key intermediate for various catalytic processes, including the amination of C-H bonds. We demonstrate facile nitrene transfer with H-H, C-H, N-H, and O-H bonds and elucidate its role in catalysis. The high reactivity is due to the population of the antibonding highest occupied molecular orbital (HOMO), which results in unique charge separation within the closed-shell imido functionality. Hence, N atom transfer is not necessarily associated with the high valency of the metal (Pd^III , Pd^IV ) or the open-shell character of a nitrene as commonly inferred.

Trapping Iron(III)-Oxo Species at the Boundary of the "Oxo Wall": Insights into the Nature of the Fe(III)-O Bond

Terminal non-heme iron(IV)-oxo compounds are among the most powerful and best studied oxidants of strong C-H bonds. In contrast to the increasing number of such complexes (>80 thus far), corresponding one-electron-reduced derivatives are much rarer and presumably less stable, and only two iron(III)-oxo complexes have been characterized to date, both of which are stabilized by hydrogen-bonding interactions. Herein we have employed gas-phase techniques to generate and identify a series of terminal iron(III)-oxo complexes, all without built-in hydrogen bonding. Some of these complexes exhibit ∼70 cm^-1 decrease in ν(Fe-O) frequencies expected for a half-order decrease in bond order upon one-electron reduction to an S = 5/2 center, while others have ν(Fe-O) frequencies essentially unchanged from those of their parent iron(IV)-oxo complexes. The latter result suggests that the added electron does not occupy a d orbital with Fe═O antibonding character, requiring an S = 3/2 spin assignment for the nascent Fe^III-O^- species. In the latter cases, water is found to hydrogen bond to the Fe^III-O^- unit, resulting in a change from quartet to sextet spin state. Reactivity studies also demonstrate the extraordinary basicity of these iron(III)-oxo complexes. Our observations show that metal-oxo species at the boundary of the "Oxo Wall" are accessible, and the data provide a lead to detect iron(III)-oxo intermediates in biological and biomimetic reactions.

Rh(III)-Catalyzed C-H Activation of Boronic Acid with Aryl Azide

A Rh(III)-catalyzed C-H activation of boronic acid with aryl azide to obtain unsymmetric carbazoles, 1 H-indoles, or indolines has been developed. The reaction constructs dual distinct C-N bonds via sp²/sp³ C-H activation and rhodium nitrene insertion. Synthetically, this approach represents an access to widely used carbazole derivatives. The practical application to CBP and unsymmetric TCTA derivatives has also been performed. Mechanistic experiments and DFT calculations demonstrate that a five-membered rhodacycle species is the key intermediate.

Catalytic Nitrene Homocoupling by an Iron(II) Bis(alkoxide) Complex: Bulking Up the Alkoxide Enables a Wider Range of Substrates and Provides Insight into the Reaction Mechanism

The reaction of HOR' (OR' = di-t-butyl-(3,5-diphenylphenyl)methoxide) with an iron(II) amide precursor forms the iron(II) bis(alkoxide) complex Fe(OR')₂(THF)₂ (2). 2 (5-10 mol %) serves as a catalyst for the conversion of aryl azides into the corresponding azoarenes. The highest yields are observed for aryl azides featuring two ortho substituents; other substitution patterns in the aryl azide precursor lead to moderate or low yields. The reaction of 2 with stoichiometric amounts (2 equiv) of the corresponding aryl azide shows the formation of azoarenes as the only organic products for the bulkier aryl azides (Ar = mesityl, 2,6-diethylphenyl). In contrast, formation of tetrazene complexes Fe(OR')₂(ArNNNNAr) (3-6) is observed for the less bulky aryl azides (Ar = phenyl, 4-methylphenyl, 4-methoxyphenyl, 3,5-dimethylphenyl). The electronic structure of selected tetrazene complexes was probed by spectroscopy (field-dependent ⁵⁷Fe Mössbauer and high-frequency EPR) and density functional theory calculations. These studies revealed that Fe(OR')₂(ArNNNNAr) complexes contain high-spin ( S = 5/2) iron(III) centers exchange-coupled to tetrazene radical anions. Tetrazene complexes Fe(OR')₂(ArNNNNAr) produce the corresponding azoarenes (ArNNAr) upon heating. Treatment of a tetrazene complex Fe(OR')₂(ArNNNNAr) with a different azide (N₃Ar') produces all three possible products ArNNAr, ArNNAr', and Ar'NNAr'. These experiments and quantum mechanics/molecular mechanics calculations exploring the reaction mechanism suggest that the tetrazene functionality serves as a masked form of the reactive iron mono(imido) species.

A theoretical study on the oxidation of alkenes to aldehydes catalyzed by ruthenium porphyrins using O2 as the sole oxidant

Density functional theory (DFT) calculations were used to study the ruthenium porphyrin-catalyzed oxidation of styrene to generate an aldehyde. The results indicate that two reactive oxidants, dioxoruthenium and monooxoruthenium-superoxo porphyrins, participate in the catalytic oxidation. In the mechanism, the resultant monooxoruthenium porphyrin acts in the tandem epoxide isomerization (E-I) to selectively yield an aldehyde and generate a dioxoruthenium porphyrin, thereby triggering new oxidation reaction cycles. In this calculation, several key elements responsible for the observed oxidative ability have been established by using Frontier molecular orbital (FMO) theory, natural bond orbital (NBO) analysis, etc., which include the reaction energy, the spin exchange effect, the spin-state conversion process, and the energy level of the lowest unoccupied molecular orbitals (LUMOs) of the reactive oxidants. The comparative oxidative abilities of the ruthenium-oxo/superoxo compounds with different axial ligands are also investigated. The results suggest that the ruthenium-oxo/superoxo species featuring a chlorine axial ligand is more reactive than that substituted with oxygen. This tuneable reactivity can be understood when considering the different electronic characters of the two ligands and the effective atomic number rule (EAN).

Oxygen radical character in group 11 oxygen fluorides

Transition metal complexes bearing terminal oxido ligands are quite common, yet group 11 terminal oxo complexes remain elusive. Here we show that excited coinage metal atoms M (M = Au, Ag, Cu) react with OF₂ to form hypofluorites FOMF and group 11 oxygen metal fluorides OMF₂, OAuF and OAgF. These compounds have been characterized by IR matrix-isolation spectroscopy in conjunction with state-of-the-art quantum-chemical calculations. The oxygen fluorides are formed by photolysis of the initially prepared hypofluorites. The linear molecules OAgF and OAuF have a ³Σ ⁻ ground state with a biradical character. Two unpaired electrons are located mainly at the oxygen ligand in antibonding O−M π* orbitals. For the ²B₂ ground state of the OM^IIIF₂ compounds only an O−M single bond arises and a significant spin-density contribution was found at the oxygen atom as well.

While transition metal complexes bearing terminal oxido ligands are common, those of group 11 elements have yet to be experimentally observed. Here, Riedel and colleagues synthesise molecular oxygen fluorides of copper, silver and gold, and show that the oxo ligands possess radical character.

N-Atom transfer via thermal or photolytic activation of a Co-azido complex with a PNP pincer ligand

Thermal or photolytic activation of well-defined mononuclear [Co(N₃)(PNP)] (PNP = 2,2'-bis(diisopropylphosphino)-4,4'-ditolylamido) results in the structurally characterized dinuclear species [Co(μ-N;κ³-P,N,N-PN^PN)]₂ (3), with two N-bridging phosphiniminato bridgeheads. Density Functional Theory (DFT) calculations indicate the intermediacy of a mononuclear cobalt-nitrido complex, followed by N-migratory insertion into a Co-P bond. Reaction of 3 with two equiv. HCl leads to rupture of the dimer with formation of mononuclear [CoCl(PN^PN^H)] (4) by protonation of the N-bridges.

How to tame a palladium terminal oxo

The isolation of terminal oxo complexes of the late transition metals promises new avenues in oxidation catalysis like the selective and catalytic hydroxylation of unreactive CH bonds, the activation of water, or the upgrading of olefins. While terminal oxo ligands are ubiquitous for early transition metals, well-characterized examples with group 10 metals remain hitherto elusive. In search for palladium terminal oxo complexes, the relative stability/reactivity of such compounds are evaluated computationally (CASSCF/NEVPT2; DFT). The calculations investigate only well-known ligand systems with established synthetic procedures and relevance for coordination chemistry and homogeneous catalysis. They delineate and quantify, which electronic properties of ancillary ligands are crucial for taming otherwise highly reactive terminal oxo intermediates. Notably, carbene ligands with both strong σ-donor and strong π-acceptor properties are best suited for the stabilization of palladium(ii) terminal oxo complexes, whereas ligands with a weaker ligand field lead to highly reactive complexes. Strongly donating ligands are an excellent choice for high-valent palladium(iv) terminal oxo compounds. Low coordinate palladium(ii) as well as high-valent palladium(iv) complexes are best suited for the activation of strong bonds.