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Square-planar cobalt(II)-systems have emerged as powerful carbene transfer catalysts for the synthesis of a variety of (hetero)cyclic compounds via redox non-innocent Co(III)-carbene radical intermediates. Spectroscopic detection and characterization of these reactive carbene radical intermediates has thus far been limited to a few scattered experiments, in part due to the fact that most studies have focused on mono-substituted carbene precursors. In this work, we demonstrate the unique formation of disubstituted cobalt(III)-carbene radicals in reactions between a cobalt(II)-porphyrin com-plex with acceptor-acceptor iodaneylidenes (iodonium ylides) as the carbene precursors. We report detailed spectroscopic characterization of the resulting reactive carbene radical species, and their application in styrene cyclopropanation. In particular, we demonstrate that iodonium ylides generate novel bis-carbenoid species leading to reversible substrate-promoted ligand modification of the commercially available [Co(TPP)]-catalyst. Two interconnected catalytic cycles are involved in the overall catalytic reaction with a mono-terminal carbene radical and an unprecedented N-enolate-carbene radical intermediate as the respective key species for the mono- and bis-carbene cycles. Notably, N-enolate formation is not a catalyst deactivation pathway, and both the N-enolate and the carbene radical moieties can be transferred as carbene units to styrene. The studies provide a detailed picture of the new [Co(TPP)]-catalyzed carbene transfer reactions from iodonium ylides. The findings are supported by detailed and unequivocal characterization of the reactive N-enolate & carbene radical intermediates and their deactivation products (EPR, UV-Vis, HR-MS, NMR, in-situ ATR-FT-IR, SC-XRD), Hammett analysis, mechanistic control experiments, DFT reaction pathway profiling and NEVPT2-CASSCF electronic structure calculations.

In search for cobalt replacements for alkyd paint curing we show that the photo-active complex [(Cp)Fe(C6H6)]+ (Cp = cyclopentadienyl) acts as a latent catalytic drier that allows for photochemical control over the onset of curing, without the need for anti-skinning agents such as the volatile MEKO normally used to prevent curing during paint storage. The highly soluble neutral complex [(Cp)Fe(Ch)] (Ch = cyclohexadienyl) readily converts to the photo-active complex [(Cp)Fe(C6H6)]+ upon oxidation in alkyd, allowing the latter to be dosed in a wide range of concentrations. Infrared and Raman studies show similar spectral changes of the alkyd paint matrix as have been observed in alkyd curing mediated by the known commercial cobalt- and manganese-based driers Durham NUODEX® Cobalt 10 Neo and NUODEX® DryCoat. The new [(Cp)Fe(Ch)] / [(Cp)Fe(C6H6)]+ system performs equally well as both commercial paint driers in terms of drying time, and outperforms NUODEX® DryCoat by showing a hardness development (increase) similar to the cobalt-based drier. Based on an observed light-dark on/off effect and EPR studies we propose that photolysis of [(Cp)Fe(C6H6)]+ generates short-lived active FeII species, explaining the excellent latency. The novel alkyd curing system [(Cp)Fe(Ch)] / [(Cp)Fe(C6H6)]+ presented herein is the first example of an intrinsically latent paint curing catalyst that is: (1) based on an abundant and harmless transition metal (Fe), (2) doesn’t require any anti-skinning agents, and (3) shows excellent performance in terms of drying times and hardness development.

Metal-catalyzed polymerization of carbon-carbon double bond containing monomers is most widely known and today people produce roughly 180 million tons of polyolefins every year. In this so-called olefin polymerization or vinyl insertion polymerization method, each monomer delivers two carbon atoms in each propagation step. The current source of C2 monomers is mainly crude oil, which is currently available in ample quantities and hence the monomers are cheaper at this moment. However, the supply of crude oil is finite and unsustainable, which calls for long-term sustainable strategies for predictable and uninterrupted supply of monomers. Boron-mediated C1 polymerization represents one of the oldest C1 polymerization methods. A variety of boron compounds such as organoboranes, haloboranes, and borohydrides are known to polymerize C1 monomers. Although nonfunctionalized polyolefins such as polyethylene, polypropylene, and polystyrene dominate the field of polymer science and technology, the demand for functionalized polyolefin was picked up in the past two decades.

We have studied the use of amino acid histidine as a precursor for N-heterocyclic carbene (NHC) ligands. This natural amino acid possesses an imidazole substituent, which makes it an interesting NHC precursor that contains both an acid and an amino functionality. These functionalities may be used for further tuning of NHC complexes. We have developed routes for the synthesis of symmetric and dissymmetric alkyl, benzyl, and aryl-substituted histidinium salts. Subsequently, the corresponding Ag and Pd histidylidenes were synthesized and the palladium complexes were tested in the Z-selective transfer semihydrogenation of alkynes. Histidylidene palladium complexes that contain additional donor functionalities were found to display good selectivities. The best catalytic results were obtained with a Pd-histidylidene complex that contains two picolyl functional groups.

We herein report on the application and structural investigation of a new set of complexes that contain bidentate N-heterocyclic carbenes (NHCs) and primary amine moieties of the type [M(arene)Cl(L)] [M=Ru, Ir, or Rh; arene=p-cymene or pentamethylcyclopentadienyl; L=1-(2-aminophenyl)-3-(n-alkyl)imidazol-2-ylidine]. These complexes were tested and compared in the hydrogenation of acetophenone with hydrogen. Structural variations in the chelate ring size of the heteroditopic ligand revealed that smaller chelate ring sizes in combination with ring conjugation in the ligand are beneficial for the activity of this type of catalyst, favoring an inner-sphere coordination pathway. Additionally, increasing the steric bulk of the alkyl substituent on the NHC aided the reaction, showing almost no induction period and formation of a more active catalyst for the n-butyl complex relative to complexes with smaller Me and Et substituents. As is common in hydrogenation reactions, the activity of the complexes decreases in the order Ru>Ir>Rh. The application of [Ru(p-cym)Cl(L)]PF6, which outperforms its reported analogues, has been successfully extended to the hydrogenation of more challenging biomass-inspired substrates.

The self-assembly of poly- A acetate) (st-PEA) into van der Waals-stabilized liquid-crystalline (LC) aggregates is reported. The LC behavior of these materials is unex- pected, and unusual for flexible sp 3 - carbon backbone polymers. Although the dense packing of polar ester func- tionalities along the carbon backbone of st-PEA could perhaps be expected to lead directly to rigid-rod behavior, molecular modeling reveals that indi- vidual st-PEA chains are actually highly flexible and should not reveal rigid-rod induced LC behavior. None- theless, st-PEA clearly reveals LC be- havior, both in solution and in the melt over a broad elevated temperature range. A combined set of experimental measurements, supported by MM/MD studies, suggests that the observed LC behavior is due to self-aggregation of st-PEA into higher-order aggregates. According to MM/MD modeling st- PEA single helices adopt a flexible hel- ical structure with a preferred trans- gauche syn-syn-anti-anti orientation. Unexpectedly, similar modeling experi- ments suggest that three of these heli- ces can self-assemble into triple-helical aggregates. Higher-order assemblies were not observed in the MM/MD sim- ulations, suggesting that the triple helix is the most stable aggregate configura- tion. DLS data confirmed the aggrega- tion of st-PEA into higher-order struc- tures, and suggest the formation of rod- like particles. The dimensions derived from these light-scattering experiments correspond with st-PEA triple-helix formation. Langmuir-Blodgett surface pressure-area isotherms also point to the formation of rod-like st-PEA ag- gregates with similar dimensions as st- PEA triple helixes. Upon increasing the st-PEA concentration, the viscosity of the polymer solution increases strongly, and at concentrations above 20 wt % st-PEA forms an organogel. STM on this gel reveals the formation of helical aggregates on the graphite surface-solution interface with shapes and dimensions matching st-PEA triple helices, in good agreement with the structures proposed by molecular mod- eling. X-ray diffraction, WAXS, SAXS and solid state NMR spectroscopy studies suggest that st-PEA triple heli- ces are also present in the solid state, up to temperatures well above the melting point of st-PEA. Formation of higher-order aggregates explains the observed LC behavior of st-PEA, em- phasizing the importance of the "terti- ary structure" of synthetic polymers on their material properties.

New, stable heterobidentate phosphane-olefin ligands based on the dibenzo[b,f]phosphepine backbone are reported together with their redox properties and coordination chemistry to rhodium(I). The X-ray crystal structures and DFT calculations show different conformations for the P-phenyl (6a) and P-mesityl (6b) derivatives. Cyclic voltammetry (vs Fc/Fc(+)) of 6a supported by UV-vis spectroelectrochemistry showed two cathodic waves, a reversible one at E-1/2 = -2.62 V (I-f/I-b, = 1.0) and a quasi-reversible (I-f/I-b approximate to 1.2) one at E-1/2 = -3.03 V. Reduction with sodium afforded a mixture of the radical anion [6a(center dot)](-), characterized by EPR spectroscopy, and dianion [6a](2-), for which an X-ray crystal structure was obtained. Both 6a and 6b bind to Rh-1 centers, giving rise to 3:1 (8a) and 2:1 (8b) ligand:Rh complexes, respectively. Two dibenzo[b,f]phosphepines in 8a and 8b act as heterobidentate ligands in which both the phosphorus atom and the olefinic double bond coordinate to rhodium, but the third ligand in 8a binds as a monodentate P-donor. The cyclic voltammogram of 8b showed two close-lying waves, a one-electron reversible wave at -1.45 V and a two-electron quasi-reversible wave at -1.80 V (I-f/I-b approximate to 1.3). 8a showed a reversible wave at -1.71 V and irreversible waves at lower potentials.

The synthesis of the novel C-3-symmetric tripodal, tetradentate ligand 1, bearing only phosphorus atoms as donor groups, is described, starting from commercially available o-tolyldiphenylphosphine, and its molecular structure has been determined by X-ray crystallographic analysis. Coordination to the cationic Ir-1 precursor [Ir(COE)(2)(acetone)(2)]PF6 led to a highly unsymmetrical species (90% yield) with four inequivalent phosphorus atoms, as evidenced by P-31 NMR spectroscopy. The corresponding H-1 NMR spectrum exhibited a pseudo doublet of quartets at delta -5.9 ppm with one large trans P-H coupling ((2)J(P-H) = 115.4 Hz) and a much smaller cis coupling ((2)J(P-H) = 10.8 Hz). X-ray crystallography confirmed the formation of complex 2, [Ir(H)(kappa P-5,P,P,P,C-1)]PF6, which is a rare example of a structurally characterized mononuclear Ir hydride species bearing an ortho-metalated phosphine ligand. This species does not react with hydride sources, but addition of 1 equiv of CF3COOH resulted in facile overall formal protonation of the Ir-C bond. DFT calculations support a pathway involving initial reductive elimination, forming the highly distorted four-coordinate species 2', followed by protonation at iridium to give the dicationic monohydride species 3, with an activation barrier Delta G(dagger) of 28.2 kcal mol(-1). Deuteration experiments support this mechanism. Reductive elimination can also be induced by reaction of 2 with carbon monoxide, yielding the monocationic carbonyl complex [Ir-1(CO)(1)]PF6 as the sole product.

The previous mechanistic studies of the Rh-mediated polymerization of carbenes suggests the involvement of organometallic compounds that have been derived from Rh(diene) species that contain a five-membered chelate ring of the type Rh{κ2-C,O-[-CH(COOR)-CH(Pol)-C(OR)=O-]} (Pol = polymer chain), which are characterized by the coordination of the β-ester group of the growing polymer chain to the metal. Herein we present our efforts to characterize the possibly related RhI(cod){κ2-C,O-[-CH(COOR)-CHR′-C(OR)=O-]} (cod = 1,5-cyclooctadiene) species. These structures can be generated by means of olefin exchange with an allyl complex and with the concomitant release of the corresponding 1,3-diene, which is derived from the allyl ligand. The product is unstable and further reacts by means of a bimolecular C-H activation process to form a related dinuclear complex [(cod)Rh(μ-{-CH(COOR)-CH-C(OR)=O-})2Rh(cod)], in which the carbon dianion of the dimethyl or diethyl succinate bridges two Rh(cod) moieties. The complex was synthesized in an independent way and was structurally characterized. We investigated the importance of these complexes in carbene polymerization.

The preparation and comprehensive characterization of a series of homoleptic sandwich complexes containing diphosphacyclobutadiene ligands are reported. Compounds [K([18]crown-6)(thf)(2)][Fe(η(4)-P(2)C(2)tBu(2))(2)] (K1), [K([18]crown-6)(thf)(2)][Co(η(4)-P(2)C(2)tBu(2))(2)] (K2), and [K([18]crown-6)(thf)(2)][Co(η(4)-P(2)C(2)Ad(2))(2)] (K3, Ad = adamantyl) were obtained from reactions of [K([18]crown-6)(thf)(2)][M(η(4)-C(14)H(10))(2)] (M = Fe, Co) with tBuC[triple bond]P (1, 2), or with AdC[triple bond]P (3). Neutral sandwiches [M(η(4)-P(2)C(2)tBu(2))(2)] (4: M = Fe 5: M = Co) were obtained by oxidizing 1 and 2 with [Cp(2)Fe]PF(6). Cyclic voltammetry and spectro-electrochemistry indicate that the two [M(η(4)-P(2)C(2)tBu(2))(2)](-)/[M(η(4)-P(2)C(2)tBu(2))(2)] moieties can be reversibly interconverted by one electron oxidation and reduction, respectively. Complexes 1-5 were characterized by multinuclear NMR, EPR (1 and 5), UV/Vis, and Mossbauer spectroscopies (1 and 4), mass spectrometry (4 and 5), and microanalysis (1-3). The molecular structures of 1-5 were determined by using X-ray crystallography. Essentially D(2d)-symmetric structures were found for all five complexes, which show the two 1,3-diphosphacyclobutadiene rings in a staggered orientation. Density functional theory calculations revealed the importance of covalent metal-ligand π bonding in 1-5. Possible oxidation state assignments for the metal ions are discussed.

The high-yielding synthesis and detailed characterization of two well-defined, linkage isomeric tripodal, tetradentate all-phosphorus ligands 1-3 is described. Coordination to Cu(I) resulted in formation of complexes 4-6, for which the molecular structures indicate overall tridentate coordination to the copper atom in the solid state, with one dangling peripheral phosphine. The solution studies suggest fast exchange between the three phosphine side-arms. For these new Cu(I) complexes, preliminary catalytic activity in the cyclopropanation of styrene with ethyldiazoacetate (EDA) is disclosed. The anticipated well-defined tetradentate coordination in a C(3)-symmetric fashion was achieved with Rh(I) and Ir(I), leading to the overall five-coordinated complexes 7-12. Complex 11 has the norbornadiene (nbd) ligand coordinated in an unprecedented monodentate 2,3-eta(2) mode to Rh. Furthermore, unexpected but very interesting redox-chemistry and reactivity was displayed by the Rh(Cl)-complexes 7 and 8. Oxidation resulted in the formation of stable Rh(II) metalloradicals [7]PF(6) and [8]PF(6) that were characterized by X-ray crystallography, magnetic susceptibility measurements, cyclic voltammetry, and electron paramagnetic resonance (EPR) spectroscopy. Subsequent redox-reactivity of these metalloradicals toward molecular hydrogen is described, resulting in the formation of Rh(III) hydride compounds.

We report the coordination behavior of ureaphosphane ligand 1[2-(diphenylphosphanyl)ethyl]-3-phenylurea (L1) towards different rhodium precursor complexes. Depending on the nature of the anion and the ligand/metal ratio, L1 acts either as a hybrid P, O-coordinating chelate, as an anionic P,N-coordinating chelate or as a supramolecular (hydrogen-bonded) bidentate ligand. The different types of complexes were investigated in the asymmetric hydrogenation of three olefinic substrates using ureaphosphanes L2 and L3, which are chiral analogues of L1. For methyl 2-acetamidoacrylate (Si) and dimethyl itaconate (S2) the neutral complex [Rh(L2-kappa P-2,N)(cod)], containing an anionic RN-coordinating ureaphosphane ligand, provided the best enantioselectivity (69.2 and 24.3 %, respectively). For N-(3,4-dihydro-2-naphth-alenyl)acetamide (S3) the best enantioselectivity was obtained with the cationic complex [Rh(L3-kappa P-2,O)(nbd)]BF4, containing a hybrid PO-coordinating ureaphosphane ligand (86.1 %).

This chapter gives an overview of the different spin-state crossing scenarios affecting the reactivity of organometallic compounds. It focuses on the effects of crossing spin states in a number of elementary reactions typically observed for organometallic compounds, such as ligand exchange, oxidative addition, reductive elimination, migration/insertion, β-hydride elimination, C-H bond activation and intramolecular electron transfer between a transition metal and a coordinated redox active ligand. These processes are highly relevant for catalysis. To address the influence of the spin-crossing on the reaction rate, Schroder et al. introduced the two-state reactivity (TSR) concept, which applies if the overall kinetics for product formation results from an interplay between the barriers for the spin-inversion and the barriers on the high- and low-spin surfaces. The chapter discusses examples of homogeneous first-row transition metal systems that catalyze (cyclo)oligomerization reactions of alkynes and alkenes, and which undergo a spin-crossing during the catalytic turnover.

The design of ancillary ligands used to modify the structural and reactivity properties of metal complexes has evolved into a rapidly expanding sub-discipline in inorganic and organometallic chemistry. Ancillary ligand design has figured directly in the discovery of new bonding motifs and stoichiometric reactivity, as well as in the development of new catalytic protocols that have had widespread positive impact on chemical synthesis on benchtop and industrial scales. Ligand Design in Metal Chemistry presents a collection of cutting-edge contributions from leaders in the field of ligand design, encompassing a broad spectrum of ancillary ligand classes and reactivity applications. Topics covered include: Key concepts in ligand design; Redox non-innocent ligands; Ligands for selective alkene metathesis; Ligands in cross-coupling. Ligand design in polymerizationThe design of ancillary ligands used to modify the structural and reactivity properties of metal complexes has evolved into a rapidly expanding sub-discipline in inorganic and organometallic chemistry. Ancillary ligand design has figured directly in the discovery of new bonding motifs and stoichiometric reactivity, as well as in the development of new catalytic protocols that have had widespread positive impact on chemical synthesis on benchtop and industrial scales. Ligand Design in Metal Chemistry presents a collection of cutting-edge contributions from leaders in the field of ligand design, encompassing a broad spectrum of ancillary ligand classes and reactivity applications. Topics covered include: Key concepts in ligand design; Redox non-innocent ligands; Ligands for selective alkene metathesis; Ligands in cross-coupling; Ligand design in polymerization.

Reversible cyclometalation is demonstrated as a strategy for the activation of small protic molecules, with a proof-of-principle catalytic application in the dehydrogenation of formic acid in the absence of an exogenous base. The well-defined RhI complex Rh(CO)(L) 1, bearing the reactive cyclometalated PN(C) ligand L (LH = PNCH = 2-di(tert-butylphosphinomethyl)-6-phenylpyridine), undergoes protonolysis of the Rh–CPh bond with weak protic reagents, such as thiols and trifluoromethanesulfonamide. This system also displays bifunctional metal–ligand protonolysis reactivity with formic acid and subsequent decarboxylation of the formate complex. Density functional theory (DFT) calculations show that H2 evolution from putative Rh(CO)(H)(LH) complex A is very facile, proposedly encompassing formal C–H oxidative addition at Rh to give Cvia agostic intermediate B and subsequent reductive elimination of H2. Complex 1 is a catalytically competent species for base-free formic acid dehydrogenation, with the intermediacy of formate complex 4. DFT calculations reveal accessible barriers for involvement of a flanking phenyl group for both initial activation of formic acid and release of H2, supporting a cooperative pathway. Reversible C–H activation is thus a viable mechanism for metal–ligand bifunctional catalysis.

A series of mononuclear Fe(II) triflate complexes based on the 3,3-bis(1-alkylimidazole-2-yl)propionate ester (BAIP) ligand scaffold are reported. I, these complexes, the tripodal N,N,O-BAIP ester ligand is varied by (i) changing the ester moiety (i.e., n-Pr, tert-Bu esters, n-Pr amide), (ii) changing the methylimidazole moieties to methylbenzimidazole moieties, and (iii) changing the methylimidazole moieties to 1-ethyl-4-isopropylimidazole moieties. The general structure of the resulting complexes comprises two facially capping BAIP ligands around a coordinatively saturated octahedral Fe(II) center, with either a transoid or cisoid orientation of the N,N,O-donor manifold that depends on the combined steric and electronic demand of the ligands, In the ligands. In the case of the sterically most encumbered ligand, a four-coordinate all N-coordinate complex is formed as well, which cocrystallizes with the six-coordinate complex. In combination with the catalytic properties of the new complexes in the epoxidation/cis-dihydroxylation of cyclooctene with H2O2, in terms of turnover number and cis-diol formation, these studies provide a number of insights for further ligand design and catalyst development aimed at Fe-mediated cis-dihydroxylation.

The first example of an iridium(I) species bearing a terminal phosphido (PR(2)(-)) ligand is reported. This stable compound shows well-behaved reactivity toward various electrophiles, owing to its exposed phosphorus lone pair, allowing reversible protonation, selective alkylation, isolation of a phosphidoborane of iridium, and generation of a phosphido-bridged iridium(I)-gold(I) dinuclear species.

Hydrogen holds the potential to be one of the major energy carriers for the future. However, a hydrogen-based economy requires technology that allows efficient and safe storage and release of H2. In this light, the reversible storage of dihydrogen in the form of formic acid (HCOOH) provides an interesting H2 storage-release system. Formic acid can be obtained from the catalytic hydrogenation of CO2 [1a–c] or by direct hydrothermal oxidation of biomass.[1d–f] Since the early nineties, substantial developments in the production of HCOOH have stimulated interest in the reverse reaction for hydrogen release on demand.[2] Important recent breakthroughs are the highly active Fe-based dehydrogenation catalyst reported by Laurenczy, Beller, and coworkers[ 2h] and work by Fujita and co-workers on an iridium catalyst that is highly active for CO2 hydrogenation or HCOOH dehydrogenation, depending on pH.[2k] The dehydrogenation of HCOOH to H2 and CO2 is typically performed in the presence of substoichiometric amounts of base, which drastically decreases the hydrogen content (from the theoretical maximum of 4.4 wt% in pure HCOOH to 2.3 wt% based on the typical 5:2 HCOOH/ NEt3 mixture). In the context of developing efficient hydrogen- based fuel cells, HCOOH/base mixtures were recently optimized for H2 release using a ruthenium-based catalysts.[3] Ideally, catalytic dehydrogenation should be performed in the absence of base and other additives, thereby maximizing the overall hydrogen storage capacity (4.4 wt%). Besides this, hydrogen produced from HCOOH/NEt3 mixtures requires purification to remove traces of volatile amines prior to application in fuel cells.[

The purpose of this review is to show that (cooperative) ligand radical reactivity can be effectively employed in synthetic organometallic chemistry and catalysis to achieve selectivity in radical-type transformations. The ‘redox non-innocence’ of ligands, and the controlled reactivity of ‘ligand radicals’ – giving rise to new, intriguing substrate transformations – allow unusual and selective radical-type substrate coupling reactions, ligand rearrangements and C-Y bond formations. In this review, several examples of fast and selective ligand-centered radical transformations in the open-shell organometallic chemistry of transition metals are described, focussing on radical-type reactions of olefin-, vinylidene-, vinyl-, alkyne-, allyl-, propargyl-, carbonyl-, and carbene ligands. Intriguing and selective substrate-substrate coupling reactions, (covalent) carbon-metal bond formations and hydrogen atom transfer reactions from and to ligand radicals are summarized. To conclude this chapter, an overview of recently disclosed new insights in the catalytic mechanism of CoII(por) catalysed olefin cyclopropanation is presented, showing that enzyme-like cooperative metal-ligand-radical reactivity is no longer reserved to real enzymes, but is a useful new concept to steer and control radical-type transformations in future bio-inspired organometallic catalysis.

The supramolecular interactions between a Rh phosphine catalyst equipped with an anion-binding pocket and alkenes that contain anionic functionalities (see picture) provide an excellent design concept to achieve remote control of the regioselectivity in hydroformylation reactions. The 4-pentenoate and 3-butenylphosphonate, which fit tightly between the Rh center and the pocket, were hydroformylated with unprecedented selectivity.

The binuclear iridium complex [(cod)(Cl)Ir(bpi)Ir(cod)]PF6 (bpi = (pyridin-2-ylmethyl)(pyridin-2-ylmethylene)amine; cod = 1,5-cyclooctadiene) reveals a noteworthy asymmetric binuclear coordination geometry, wherein the bpi ligand acts as a heteroditopic ligand and has an unusual π-coordinated imine moiety. This species is an effective precatalyst for water oxidation. After a short incubation time the catalyst reveals a turnover frequency of 3400 mol mol-1 s-1 with an overall turnover number >1000. © 2011 American Chemical Society., Financial support from the European Research Council (ERC Grant Agreement 202886-CatCIR), NWO-CW (VIDI grant 700.55.426, VENI grant 700.59.410), the MEC/FEDER (Project CTQ2008-03860, Spain), and the University of Amsterdam is gratefully acknowledged.

Cationic rhodium and iridium complexes with dibenzo[a,e]cyclooctatetraene (dbcot) and bislutidylamine (bla) were synthesized from the respective [M(μ-Cl)(dbcot)]2 dimers and the bla ligand and were characterized by NMR, X-ray diffraction, and cyclic voltammetry. Both [MI(dbcot)(bla)]+ complexes are easily oxidized by one electron to form paramagnetic dicationic analogues [MII(dbcot)(bla)]2+ (M = Rh, Ir). In the solid state, the geometry changes from a distorted trigonal bipyramid to square pyramidal upon oxidation of the metal from MI to MII, as evidenced by X-ray diffraction studies of the iridium complexes. The paramagnetic complexes undergo facile one-electron reduction in the presence of base, most likely involving oxidation of the solvent or hydroxide anions. A control experiment using the corresponding [RhII(dbcot)(Bn-bla)]2+ complex, wherein Bn-bla is the N-benzyl-protected bla ligand, gives similar results, which strongly suggests that deprotonation of the N−H moiety of the [MII(dbcot)(bla)]2+ plays a minor role (if any at all) in the observed base-mediated reduction processes. Reaction of bla with [Ir(μ-Cl)(coe)]2 (coe = cis-cyclooctene) results in oxidative addition of the vinylic C−H bond of coe to the metal center, as evidenced by X-ray diffraction.

The synthesis of a new, all-phosphorus pincer PPNEt2P ligand L3NEt2, which is derived from 2-indolylphosphine and features a central N2P(NEt2) core, is described. This ‘PPP’ species shows coordination toward Rh as a neutral trisphosphine ligand. Tridentate diphenylphosphine-derived PPHP ligands L1H and L2H, 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 PNHP 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.

Contains fulltext : 58516.pdf (Publisher’s version ) (Open Access) The epitaxial lift- off process allows the separation of a thin layer of III/ V material from the substrate by selective etching of an intermediate AlAs layer with HF. In a theory proposed for this process, it was assumed that for every mole of AlAs dissolved three moles of H-2 gas are formed. In order to verify this assumption the reaction mechanism and stoichiometry were investigated in the present work. The solid, solution and gaseous reaction products of the etch process have been examined by a number of techniques. It was found that aluminum fluoride is formed, both in the solid form as well as in solution. Furthermore, instead of H-2 arsine (AsH3) is formed in the etch process. Some oxygen- related arsenic compounds like AsO, AsOH, and AsO2 have also been detected with gas chromatography/ mass spectroscopy. The presence of oxygen in the etching environment accelerates the etching process, while a total absence of oxygen resulted in the process coming to a premature halt. It is argued that, in the absence of oxygen, the etching surface is stabilized, possibly by the sparingly soluble AlF3 or by solid arsenic. (C) 2004 The Electrochemical Society.

Reaction of the Rh(II) dimer [LRh]2(μ-Br)2 (L = ([2,6-Me2C6H3NCMe]2CH) with the bulky amide LiNH(2,6-iPr2C6H3) leads to a monomeric Rh(II) amide with a T-shape N3Rh arrangement and a pronounced agostic interaction. Reaction with the less bulky LiNH(2,6-Me2C6H3) results in benzylic C-H activation to a binuclear complex with a unique aza-xylylene bridging mode. With LiNHC6H5 we observe initial formation of [LRh]2(μ-NHPh)2 followed by N,C2′ coupling of two anilines. These reactions can be explained based on aminyl radical character of the Rh(II) amides (supported by EPR evidence) and/or involvement of nitrene intermediates.

A series of complexes ML2(x+) (M = Mn-Zn, L = 2,6-bis(iminomethyl)pyridine) was investigated by theoretical methods. Electron transfer from the metal "t(2g)" orbitals to the ligand pi orbitals is reflected in the elongation of ligand C-N bonds and shortening of the C(py)-C(imine) bonds. Using zinc complexes as references, these deformations could be used to quantify the number of electrons transferred. Strong transfer is found in low-spin MnL2(+) (ca. 2 e) and in high-spin MnL2(+) and low-spin MnL2(2+), FeL2(2+), and CoL2(+) (ca. 1 e each). Smaller transfer is found in CoL(2)(2+), and the transfer is insignificant in high-spin MnL2(2+), NiL2(2+), and CuL2(2+). Analysis of the unpaired electron density on the metal (using the Staroverov-Davidson method) shows that the contribution of a biradical description, in which ligand radical anions are antiferromagnetically coupled to the metal center, is significant in most cases. In the case of CoL2(+) and high-spin MnL2(+), where the metal-ligand bond is weakened, it amounts to over 50% of the total transfer.

Contains fulltext : 35468.pdf (Publisher’s version ) (Open Access) Unprecedented rhodium-catalyzed stereoselective polymerization of "carbenes" from ethyl diazoacetate (EDA) to give high molecular mass poly(ethyl 2-ylidene-acetate) is described. The mononuclear, neutral [(N, O-ligand) M-I(cod)] (M) Rh, Ir) catalytic precursors for this reaction are characterized by (among others) single-crystal X-ray diffraction. These species mediate formation of a new type of polymers from EDA: carbon-chain polymers functionalized with a polar substituent at each carbon of the polymer backbone. The polymers are obtained as white powders with surprisingly sharp NMR resonances. Solution and solid state NMR data for these new polymers reveal a highly stereoregular polymer, with a high degree of crystallinity. The polymer is likely syndiotactic. Material properties are very different from those of atactic poly(diethyl fumarate) polymer obtained by radical polymerization of diethyl fumarate. Other diazoacetates are also polymerized. Further studies are underway to reveal possible applications of these new materials.