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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-PNPN)]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(PNPNH)] (4) by protonation of the N-bridges. [ABSTRACT FROM AUTHOR]

A low-valent trinuclear iron complex with an unusual linear Fe(i)–Fe(ii)–Fe(i) unit is presented. It is accessed in a rational approach using a salt metathesis reaction between a new anionic Fe(i) containing heterocycle and FeCl2. Its electronic structure was studied by single crystal XRD analysis, EPR and Mössbauer spectroscopy, and magnetic susceptibility measurements. [ABSTRACT FROM AUTHOR]

The local coordination environment of single atom catalysts (SACs) often determines their catalytic performance. To understand these metal-support interactions, we prepared Pt SACs on cerium dioxide (CeO 2 ) cubes, octahedra and rods, with well-structured exposed crystal facets. The CeO 2 crystals were characterized by SEM, TEM, pXRD, and N 2 sorption, confirming the shape-selective synthesis, identical bulk structure, and variations in specific surface area, respectively. EPR, XPS, TEM and XANES measurements showed differences in the oxygen vacancy density following the trend rods > octahedra > cubes. AC-HAADF-STEM, XPS and CO-DRIFTS measurements confirmed the presence of only single Pt 2+ sites, with different surface platinum surface concentrations. We then compared the performance of the three catalysts in ammonia borane hydrolysis. Precise monitoring of reaction kinetics between 30-80 °C gave Arrhenius plots with hundreds of data points. All plots showed a clear inflection point, the temperature of which (rods > octahedra > cubes) correlates to the energy barrier of ammonia borane diffusion to the Pt sites. These activity differences reflect variations in the - facet dependent - degree of stabilization of intermediates by surface oxygen lone pairs and surface-metal binding strength. Our results show how choosing the right macroscopic support shape can give control over single atom catalysed reactions on the microscopic scale., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

We show that the incorporation of a biotinylated Co(TAML) cofactor within streptavidin enables asymmetric radical-type oxygen atom transfer catalysis with improved activity and enantioselectivity.

In this contribution, we describe a post-assembly modification approach to selectively coordinate transition metals in Pd 12 L 24 cuboctahedra. The herein reported approach involves the preparation of Pd 12 L 24 nanospheres with protonated nitrogen donor ligands that are covalently linked at the interior. The so obtained Pd 12 (LH + ) 24 nanospheres are shown to be suitable for coordinative post-modification after deprotection by deprotonation. Selective formation of tetra-coordinated M B in Pd 12 M B 6 L 24 , tri-coordinated M B in Pd 12 M B 8 L 24 nanospheres and two-coordinated M B in Pd 12 M B 12 L 24 nanospheres is achieved as a result of different nitrogen donor ligands. A combination of pulsed EPR spectroscopy (DEER) to measure Cu-Cu distances in the different spheres, NMR studies and computational investigations, support the presence of the complexes at precise locations of the Pd 12 M B 6 L 24 nanosphere. The general post-assembly modification methodology can be extended using other transition metal precursors or supramolecular systems and can guide precise formation and investigation of novel transition metal-complex containing nanospheres with well-defined composition., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

An azobenzene-bearing N-heterocyclic carbene-based gold catalyst is reported of which the reactivity in a cyclization reaction depends on the isomeric state of the azobenzene. The configurations of the catalyst can be reversibly switched by light and are stable during the reaction, effectively leading to a switchable catalyst system.

There is an increasing interest in the application of metal-organic cages (MOCs) in a biomedicinal context, as they can offer non-classical distribution in organisms compared to molecular substrates, while revealing novel cytotoxicity mechanisms. Unfortunately, many MOCs are not sufficiently stable under in vivo conditions, making it difficult to study their structure-activity relationships in living cells. As such, it is currently unclear whether MOC cytotoxicity stems from supramolecular features or their decomposition products. Herein, we describe the toxicity and photophysical properties of highly-stable rhodamine functionalized platinum-based Pt 2 L 4 nanospheres as well as their building blocks under in vitro and in vivo conditions. We show that in both zebrafish and human cancer cell lines, the Pt 2 L 4 nanospheres demonstrate reduced cytotoxicity and altered biodistribution within the body of zebrafish embryos compared to the building blocks. We anticipate that the composition-dependent biodistribution of Pt 2 L 4 spheres together with their cytotoxic and photophysical properties provides the fundament for MOC application in cancer therapy., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Single-atom catalysts often show exceptionally high performance per metal loading. However, the isolated atom sites tend to agglomerate during preparation and/or high-temperature reaction. Here we show that in the case of Rh/Al 2 O 3 this deactivation can be prevented by dissolution/exsolution of metal atoms into/from the support. We design and synthesise a series of single-atom catalysts, characterise them and study the impact of exsolution in the dry reforming of methane at 700-900 °C. The catalysts' performance increases with increasing reaction time, as the rhodium atoms migrate from the subsurface to the surface. Although the oxidation state of rhodium changes from Rh(iii) to Rh(ii) or Rh(0) during catalysis, atom migration is the main factor affecting catalyst performance. The implications of these results for preparing real-life catalysts are discussed., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Homogeneous ruthenium catalysed methanol dehydrogenation could become a key reaction for hydrogen production in liquid fuel cells. In order to improve existing catalytic systems, mechanistic insight is paramount in directing future studies. Herein, we describe what computational mechanistic research has taught us so far about ruthenium catalysed dehydrogenation reactions. In general, two mechanistic pathways can be operative in these reactions: a metal-centered or a metal-ligand cooperative (Noyori-Morris type) minimum energy reaction pathway (MERP). Discerning between these mechanisms on the basis of computational studies has proven to be highly input dependent, and to circumvent pitfalls it is important to consider several factors, such as solvent effects, metal-ligand cooperativity, alternative geometries, and complex electronic structures of metal centres. This Frontiers article summarizes the reported computational research performed on ruthenium catalyzed dehydrogenation reactions performed in the past decade, and serves as a guide for future research.

Several metalloenzymes, including [FeFe]-hydrogenase, employ cofactors wherein multiple metal atoms work together with surrounding ligands that mediate heterolytic and concerted proton-electron transfer (CPET) bond activation steps. Herein, we report a new dinucleating PNNP expanded pincer ligand, which can bind two low-valent iron atoms in close proximity to enable metal-metal cooperativity (MMC). In addition, reversible partial dearomatization of the ligand's naphthyridine core enables both heterolytic metal-ligand cooperativity (MLC) and chemical non-innocence through CPET steps. Thermochemical and computational studies show how a change in ligand binding mode can lower the bond dissociation free energy of ligand C(sp 3 )-H bonds by ∼25 kcal mol -1 . H-atom abstraction enabled trapping of an unstable intermediate, which undergoes facile loss of two carbonyl ligands to form an unusual paramagnetic ( S = ) complex containing a mixed-valent iron(0)-iron(i) core bound within a partially dearomatized PNNP ligand. Finally, cyclic voltammetry experiments showed that these diiron complexes show catalytic activity for the electrochemical hydrogen evolution reaction. This work presents the first example of a ligand system that enables MMC, heterolytic MLC and chemical non-innocence, thereby providing important insights and opportunities for the development of bimetallic systems that exploit these features to enable new (catalytic) reactivity., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Supramolecular self-assemblies are used across various fields for different applications including their use as containers for catalysts, drugs and fluorophores. M 12 L 24 spheres are among the most studied, as they offer plenty of space for functionalization, yielding systems with unique properties in comparison to their single components. Detailed studies on the formation of M 12 L 24 structures using palladium cornerstones (that have generally dynamic coordination chemistry) aided in the development of synthetic protocols. The more robust platinum-based systems received thus far much less attention. The general use of platinum-based assemblies remains elusive as parameters and design principles of the ligand building blocks are not fully established. As platinum-based nanospheres are more robust due to the kinetically more stable nitrogen-platinum bond, we studied the sphere formation process in detail in order to develop descriptors for the formation of platinum-based nanospheres. In a systematic study, using time-dependent mass spectrometry, 1 H-NMR and DOSY NMR, we identified new kinetically trapped intermediates during the formation of Pt 12 L 24 spheres and we developed key parameters for selective formation of Pt 12 L 24 spheres. Molecular mechanics calculations and experimental result support the importance of charge and steric bulk placed at the endo -site of the ditopic linker for selective sphere formation. Applicability of these principles is demonstrated by employing various ditopic ligands with different bend-angles for the synthesis of a range of Pt 2 L 4 , Pt 3 L 6 , Pt 4 L 8 and Pt 12 L 24 polyhedra with platinum cornerstones in excellent yields, thus paving the way for future applications of well-defined robust platinum nanospheres of different shapes and sizes with the general composition Pt n L 2 n ., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Herein, we report a supramolecular rhodium complex that can form dimeric or monomeric Rh-species catalytically active in hydroformylation, depending on the binding of effectors within the integrated DIM-receptor. X-ray crystal structures, in situ (high-pressure (HP)) spectroscopy studies, and molecular modelling studies show that in the absence of effectors, the preferred Rh-species formed is the dimer, of which two ligands coordinate to two rhodium metals. Importantly, upon binding guest molecules, -effectors-, to the DIM-receptor under hydroformylation conditions, the monomeric Rh-active species is formed, as evidenced by a combination of in situ HP NMR and IR spectroscopy studies and molecular modelling. As the monomeric complex has different catalytic properties from the dimeric complex, we effectively generate a catalytic system of which the properties respond to the presence of effectors, reminiscent of how the properties of proteins are regulated in nature. Indeed, catalytic and kinetic experiments show that both the selectivity and activity of this supramolecular catalytic system can be influenced in the hydroformylation of 1-octene using acetate as an effector that shift the equilibrium from the dimeric to monomeric species.

The dinuclear ruthenium complex [Ru 2 H(μ-H)(Me 2 dad)(dbcot) 2 ] contains a 1,4-dimethyl-diazabuta-1,3-diene (Me 2 dad) as a non-innocent bridging ligand between the metal centers to give a [Ru 2 (Me 2 dad)] core. In addition, each ruthenium is bound to one dibenzo[ a , e ]cyclooctatetraene (dbcot) ligand. This Ru dimer converts H 2 to protons and electrons. It also catalyzes reversibly under mild conditions the selective hydrogenation of vitamins K 2 and K 3 to their corresponding hydroquinone equivalents without affecting the C[double bond, length as m-dash]C double bonds. Mechanistic studies suggest that the [Ru 2 (Me 2 dad)] moiety, like hydrogenases, reacts with H 2 and releases electrons and protons stepwise., (This journal is © The Royal Society of Chemistry 2019.)

The dinuclear ruthenium complex [Cp*Ru(μ-C10H8)RuCp*] (1; Cp* = η5-C5Me5) was prepared by reduction of the cationic precursor [Cp*Ru(η6-C10H8)]PF6 with KC8. Diamagnetic 1 has a symmetric molecular structure with an anti-facial configuration of the Cp*Ru moieties coordinating to naphthalene. Density Functional Theory (DFT) studies showed an electronic structure similar to that of the analogous diiron complex [Cp*Fe(μ-C10H8)FeCp*]. Cyclic voltammetry and UV-vis spectroelectrochemistry showed that 1 can be reversibly oxidized to 1+ and 12+. Chemical oxidation with [Cp2Fe]BArF4 afforded the paramagnetic compound [1]BArF4, which was investigated by EPR, single-crystal X-ray diffractometry and DFT calculations. Reaction of 1 with Brookhart's acid gave the hydride complex [3]BArF4, which was characterized spectroscopically and crystallographically. Cyclic voltammetry showed that [3]+ is converted back to 1 upon reduction and oxidation.

The iridium(iii/iv/v) imido redox series [Ir(N t Bu){N(CHCHP t Bu 2 ) 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-N t Bu bonding. Reduction gives the neutral imide [Ir(N t Bu){N(CHCHP t Bu 2 ) 2 }] as the first example of an iridium complex with a triplet ground state. Its reactivity with respect to nitrene transfer to selected electrophiles (CO 2 ) and nucleophiles (PMe 3 ), respectively, is reported.

Catalytic synthesis of substituted 1,2-dihydronaphthalenes via metalloradical activation of o -styryl N -tosyl hydrazones (( E )-2-(prop-1-en-1-yl)benzene- N -tosyl hydrazones) is presented, taking advantage of the intrinsic reactivity of a cobalt(iii)-carbene radical intermediate. The method has been successfully applied to a broad range of substrates with various R 1 substituents at the aromatic ring, producing the desired ring products in good to excellent isolated yields for substrates with an R 2 = COOEt substituent at the vinylic position (∼70-90%). Changing the R 2 moiety from an ester to other substituents has a surprisingly large influence on the (isolated) yields. This behaviour is unexpected for a radical rebound ring-closure mechanism, and points to a mechanism proceeding via ortho -quinodimethane ( o -QDM) intermediates. Furthermore, substrates with an alkyl substituent on the allylic position reacted to form E -aryl-dienes in excellent yields, rather than the expected 1,2-dihydronaphthalenes. This result, combined with the outcome of supporting DFT calculations, strongly points to the release of reactive o -QDM intermediates from the metal centre in all cases, which either undergo a 6π-cyclisation step to form the 1,2-dihydronaphthalenes, or a [1,7]-hydride shift to produce the E -aryl-dienes. Trapping experiments using TEMPO confirm the involvement of cobalt(iii)-carbene radical intermediates. EPR spectroscopic spin-trapping experiments using phenyl N-tert -butylnitrone (PBN) confirm the radical nature of the catalytic reaction.

We report the synthesis of the trifluoromethyl cobalt(iii)tetraphenylporphyrinato complex [Co(TPP)CF3], which loses fluoride upon one-electron reduction and transfers a difluorocarbene moiety to n-butyl acrylate to produce the corresponding gem-difluorocyclopropane. Catalytic CF 2 transfer from Me 3 SiCF 3 to n-butyl acrylate becomes possible when directly using the divalent cobalt(ii) porphyrin catalysts in the presence of NaI.

The synthesis and tautomeric nature of three xanthene-based bisMETAMORPhos ligands ( La-Lc ) is reported. Coordination of these bis(sulfonamidophosphines) to Ir(acac)(cod) initially leads to the formation of Ir I ( L H ) species ( 1a ), which convert via formal oxidative addition of the ligand to Ir III ( L ) monohydride complexes 2a-c . The rate for this step strongly depends on the ligand employed. Ir III complexes 2a-c were applied in the base-free dehydrogenation of formic acid, reaching turnover frequencies of 3090, 877 and 1791 h -1 , respectively. The dual role of the ligand in the mechanism of the dehydrogenation reaction was studied by 1 H and 31 P NMR spectroscopy and DFT calculations. Besides functioning as an internal base, bisMETAMORPhos also assists in the pre-assembly of formic acid within the Ir-coordination sphere and aids in stabilizing the rate-determining transition state through hydrogen-bonding.

The reaction of the 17e nickel(I) radical [CpNi(IDipp)] (1, IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with P4 results in a nickel tetraphosphide [{CpNi(IDipp)}2(μ-η(1):η(1)-P4)] with a butterfly-P4(2-) ligand; related chalcogenides [{CpNi(IDipp)}2(μ-E2)] (E = S, Se, Te) and [{CpNi(IDipp)}2(μ-E3)] (E = S, Se) are formed with S8, Se∞ and Te∞.

Copolymerisation of carbenes and olefins (ethene), mediated by Rh-based catalyst precursors, is presented as a new, proof-of-concept methodology for the controlled synthesis of functional polymers. The reactions studied show that olefin-carbene polymerisation reactions provide a viable alternative to more traditional olefin polymerization techniques. Rh(III)-catalyst precursors, while active in the homopolymerisation of either olefins or carbenes, proved to be virtually inactive in olefin-carbene copolymerization. Conversely, the use of Rh(I)(cod) catalyst precursors allows the synthesis of high molecular-weight, highly functionalized copolymers. The reactions yield a mixture of copolymers and some carbene homopolymers, which proved to be difficult to separate. Polyethylene was not formed under the applied reaction conditions. The average ethene content in this mixture could be increased up to 11%, although analysis of the mixture revealed that the ethene content in fractions of the copolymer mixture can be as high as 70%. Attempts to increase the ethene content by increasing the ethene pressure unexpectedly led to lower average ethene contents, which is most likely due to changes in the ratio of copolymers vs. carbene homopolymer. This behaviour is most likely a result of the reactivity difference of different active Rh-species formed under the applied reaction conditions. Apparently, higher ethene concentrations slow down the copolymerisation process (mediated by yet unidentified Rh-species) compared to the formation of homopolymers (mediated by different Rh-catalysts; most likely (allyl)Rh(III)-alkyl species), thereby changing the product ratio in favour of the homopolymer. The average ethene content in the copolymer mixture therefore decreases, while the ethene content within the copolymer fraction has likely increased at higher ethene concentrations (but simply less copolymer is formed). The obtained copolymers exhibit a blocky microstructure, with the functional blocks being highly stereoregular. Branching does occur and the functional groups are present in the polymer backbone as well as at the branches. Formation of copolymers was confirmed by Maldi-ToF analysis, which revealed incorporation of several ethene units into the copolymers.

Rh-mediated carbene (co)polymerisation of diazomethane works best in the presence of Rh(III) catalyst precursors, the use of which leads to a significant increase in polymer yield and molecular weight. Chain termination via β-hydride elimination is severely suppressed for these species, although this process does still occur leading to unsaturated chain ends. Subsequent chain walking leading to the formation of branched polymers seems not to occur. Computational studies describing pathways for both chain propagation and chain termination using a (cycloocta-2,5-dien-1-yl)Rh(III)(alkyl) species as a representative model for the active species revealed that chain propagation is favoured for these species, although β-hydride elimination is still viable at the applied reaction temperatures. The computational studies are in excellent agreement with all experimental results, and further reveal that chain propagation via carbene insertion (leading to linear poly-methylene) occurs with a much lower energy barrier than insertion of 1-alkenes into either the Rh-H bond after β-hydride elimination or into the Rh-C bond of the growing polymer chain (leading to branched polymers). These energetic differences conveniently explain why experimentally the formation of branches is not observed in (co)polymerisation reactions employing diazomethane. The formation of substantial amounts of low-M(w) oligomers and dimers in the experimental reactions can be ascribed to the presence of (1,5-cyclooctadiene)Rh(I) species in the reaction mixture, for which chain termination via β-hydride elimination is clearly favoured over chain propagation. These two species stem from a non-selective catalyst activation process during which the catalyst precursors are in situ activated towards carbene polymerisation, and as such the results in this paper might contribute to further improvements of this reaction.

This article describes a new strategy for the recycling of a homogeneous hydroformylation catalyst, by selective adsorption of the catalyst to tailor-made supports after a batchwise reaction. The separation of the catalyst from the product mixture is based on selective non-covalent supramolecular interactions between a ligand and the support. Changing the solvent releases the active catalyst back into the reactor and allows a subsequent batch reaction with the recycled active catalyst. For this purpose, the bidentate NixantPhos ligand has been equipped with a pyridine group. The corresponding rhodium pre-catalyst [Rh(Nix-py)(acac)] (acac = acetylacetonate) forms a very selective, active and highly stable catalyst, and able to reach a turnover number (TON) of 170 000 in a single run (reaction performed in nearly neat 1-octene, S/C ratio of 200 000, at 140 °C, 20 bars syngas pressure). Various commercially available supports have been explored in binding studies and recycling experiments. The end-capped silica-alumina performs the best so far with respect to ligand-adsorbing properties for the current purpose. Although this system has not been fully optimized, four recycling runs could be performed successfully.

DNA-based catalysis can be used to control the enantioselectivity of copper-catalysed Diels-Alder and Friedel-Crafts reactions to produce either enantiomer of the product by changing the denticity of the ligand coordinated to the Cu(II) ion, even though the DNA adopts a right handed helical conformation only., (This journal is © The Royal Society of Chemistry 2012)

The synthesis of a new, all-phosphorus pincer PP(NEt2)P ligand L3(NEt2), which is derived from 2-indolylphosphine and features a central N(2)P(NEt(2)) core, is described. This 'PPP' species shows coordination toward Rh as a neutral trisphosphine ligand. Tridentate diphenylphosphine-derived PP(H)P ligands L1(H) and L2(H), featuring a secondary phosphine core, show 'ambivalent' coordination, acting as persistent neutral triphosphine ligands with Rh, and as easily-formed monoanionic phosphido(bisphosphine) pincer ligands toward Pd. These subtle differences, which might be more general for group 9 and 10 metal complexes with this ligand set, are explained by comparative DFT calculations (BP86; def2-TZVP level of theory) for the Rh and Pd species involved, including those with the structurally related PN(H)P ligands. The optimized structure for complex PdCl(L2) indicates minimal overlap of available Pd d-orbitals with the lone pair of the central, deprotonated phosphorus atom (formally a phosphido fragment), suggesting that it behaves predominantly like a bulky phosphine instead of a phosphido fragment., (This journal is © The Royal Society of Chemistry 2011)

The mechanism of cobalt(II) porphyrin-mediated aziridination of styrene with PhSO(2)N(3) was studied by means of DFT calculations. The computations clearly indicate the involvement of a cobalt 'nitrene radical' intermediate in the Co(II)(por)-catalyzed alkene aziridination. The addition of styrene to this species proceeds in a stepwise fashion via radical addition of the 'nitrene radical'C to the C=C double bond of styrene to form a γ-alkyl radical intermediate D. The thus formed tri-radical species D easily collapses in an almost barrierless ring closure reaction (TS3) to form the aziridine, thereby regenerating the cobalt(II) porphyrin catalyst. The radical addition of the 'nitrene radical'C to the olefin (TS2) proceeds with a comparable barrier as its formation (TS1), thus providing a good explanation for the first order kinetics in both substrates and the catalyst observed experimentally. Formation of C is clearly accelerated by stabilization of C and TS1 via hydrogen bonding between the S=O and N-H units. The computed radical-type mechanism agrees well with all available mechanistic and kinetic information. The computed free energy profile readily explains the superior performance of the Co(II)(porAmide) system with H-bond donor functionalities over the non-functionalized Co(TPP).

We present new ureaphosphine ligands that coordinate in a P,O-bidentate fashion to rhodium(i). The ureaphosphine-Rh(i)-complexes were effectively used in the asymmetric hydrogenation of cyclic enamides giving high conversions and enantioselectivity.

Unprecedented regio- and enantioselectivities (>91%) are reported for the Rh-catalyzed asymmetric hydroformylation of 2,3- and 2,5-dihydrofuran using tunable hybrid phosphine-phosphonite ligands.

Facile oxidation of a sterically encumbered Rh(I) complex generates a stable Rh(II) metalloradical species; the latter is able to activate H(2) under formation of the corresponding Rh(III) complex.

In contrast to the recently reported cyclodimerisations of phosphaalkynes, alkynes preferably trimerise at the same low-valent iron(-I) centre.

New Rh(I)(cod) complexes of N-heterocyclic carbene ligands containing a pending pyridyl moiety for subsequent binding of Zn(II)-templates were designed for supramolecular catalyst control in 'carbene polymerization' reactions. Zn(II)-templates indeed have an influence on the yields and the polymer molecular weights (and weight distributions) produced by these 'supramolecular assemblies'. However, control experiments reveal that the effect of the Zn(II)-template on the polymerization results is general and not due to the assembly formation.

Treatment of [(Me3tpa)IrII(ethene)]2+ (Me(3)tpa =N,N,N-tri(6-methyl-2-pyridylmethyl)amine)(1(2+)) with dioxygen in weakly coordinating solvents results in formation of [(Me3tpa)IrIII(ethene)(superoxo)]2+ (4a2+). In the presence of DMPO (DMPO = 5,5-dimethyl-2-pyrrolidine-1-oxide) DMPO is substituted for ethene, and subsequently oxidized to DMPOX by the superoxo fragment to give [(Me3tpa)IrIII(DMPOX)]2+ (7(2+); DMPOX = 5,5-dimethyl-2-pyrrolidone-1-oxide). In acetonitrile, in the absence of DMPO, oxygenation of 1(2+) to [(Me3tpa)IrIII(formylmethyl)(MeCN)]2+ (2(2+)) is observed. In the presence of DMPO the formation of 2(2+) and 7(2+) is competing. Oxygenation of 1(2+) to 2(2+) may proceed via 4a(2+), involving an insertion mechanism at the metal. However, a mechanism based on olefin ligand non-innocence seems a reasonable alternative. This involves formation of acetonitrile adduct [(Me3tpa)Ir(ethene)(MeCN)]2+ (3(2+)), which has a significant metalla-ethyl radical (IrIII-CH2CH2*) character, allowing attack of 3O2 directly at the ethene ligand. Both pathways are discussed on the basis of experimental observations and DFT geometry optimizations.

[RhI(t-Bu2-boxate)(C2H4)2] spontaneously disproportionates to the mononuclear [RhII(t-Bu2-boxate)2], whereas [RhI(Ph2-boxate)(C2H4)2] is stable against disproportionation.