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The dinuclear aluminum salt {[iBu2(DMA)Al]2(μ-H)}+[B(C6F5)4]− (AlHAl; DMA = N,N-dimethylaniline) is the prototype of a new class of molecular cocatalysts for catalytic olefin polymerization, its modular nature offering easy avenues for tailoring the activator to specific needs. We report here, as proof of concept, a first variant (s-AlHAl) bearing p-hexadecyl-N,N-dimethylaniline (DMAC16) units, which enhances solubility in aliphatic hydrocarbons. The novel s-AlHAl was used successfully as an activator/scavenger in ethylene/1-hexene copolymerization in a high-temperature solution process.

Solvents constitute around 80% of the total volume of chemicals used in fine-chemical processes and contribute significantly to their environmental impact and hazard profile. Thus, there is a strong driving force towards the replacement of traditional fossil-based solvents by alternatives that are more benign in terms of their origin, availability, convenience of synthesis, handling, biodegradability and environmental impact. In the class of polar aprotic solvents, the most successful “green” replacement is γ-valerolactone (GVL). Here, we propose the use of a structurally related compound, 5-methyl-1,3-dioxolane-4-one (LA-H,H), as a reaction medium. It is easily prepared from lactic acid and formaldehyde and satisfies the criteria for a green solvent. It is stable under neutral or basic conditions. Despite the presence of a ketal functionality, it even survives mildly acidic conditions. Evaluation of the Kamlet–Taft and Hansen solvent parameters shows that indeed LA-H,H and GVL are closely similar, suggesting that LA-H,H is an effective new green entry in the class of polar aprotic solvents. We have tested its performance in Pd-catalyzed Heck arylation of methyl acrylate and in the Menschutkin reaction of N-methylimidazole with 1-iodobutane. LA-H,H is also the parent of a whole family of potential solvents easily prepared from two green precursors: α-hydroxy carboxylic acids (lactic, mandelic, and α-hydroxyisobutyric acids) and aldehydes/ketones (formaldehyde, acetaldehyde, and acetone); five such variations were briefly examined. Interestingly, LA-H,H has the unusual property of forming a three-phase system when combined with water and hexane, which may allow technological variations that are not possible with the more normal one- and two-phase systems. As a curiosity, a rare four-phase system is achievable combining LA-H,H with octane, water and perfluorodecaline.

σ-Bond metathesis between zirconocene cations and Cp2WH2 affords a bimetallic bridging hydride with an out of plane configuration and no M–M interaction. Small molecules react predominantly at the Zr-C bond in the absence of bimetallic cooperativity.

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The oxidative addition of α-glucosyl and α-galactosyl halides to the platinum(0) complex [Pt(2,9-dimethyl-1,10-phenanthroline)(ethene)] has been investigated. In all cases the main reaction product is the five-coordinate Pt(II) complex containing in axial position an α-glycosyl fragment in the unusual 1C4 conformation. In the case of glucose, a minor product could be identified as the β isomer, which retains the typical 4C1 chair. The single crystal structures of two diastereomers are compared. The outcome of the reaction does not depend on the nature of the halide and is only slightly affected by the solvent. The addition mechanism is discussed, also in the light of literature data. Experimental data, along with DFT calculations, point towards a radical chain mechanism.

The combination of an LAu(I) fragment with a potentially chelating ligand (Formula presented.) can result in different coordination modes of (Formula presented.) : strictly monodentate, symmetrically bidentate, or intermediate with asymmetric bidentate binding of (Formula presented.). Density Functional calculations indicate that for π-acceptor ancillary ligands L (C2H4, CO) and bis(nitrogen) donors (Formula presented.) (bipyridine, phenanthroline, β-diiminate) symmetric chelate structures are obtained. With primarily σ-donating ancillary ligands L (Me−, Cl−, MeCN) the asymmetric coordination mode is the norm. Phosphine ancillary ligands L are on the edge and display the highest sensitivity to ligand variation. Asymmetry increases when (a) going from anionic (β-diiminate) to neutral (bipyridine, phenanthroline) bidentates (Formula presented.); (b) making (Formula presented.) less electron-rich e. g. through having aryl instead of alkyl groups at N or through introduction of CF3 substituents. Inversion of the asymmetry through “gold hopping” is remarkably facile (barrier mostly

Highly accurate high-throughput experimentation (HTE) data for a set of 21 silicon-bridged C2-symmetric ansa-zirconocenes in propene homopolymerization were collected and were used to develop quant...

A series of 20 bis(phenolate-ether) ligands, in combination with Zr and Hf, was tested for performance in propene polymerization, focusing on molecular weight, stereoregularity and regioregularity of the resulting polymer. Ligand variation covers length of the aliphatic linker between the ligand halves, as well as steric bulk of the groups ortho to the phenolate oxygen. The linker length has a dramatic effect on MW: two-carbon linkers produce oligomers (Mn < 2.5 kDa) while three- and four-carbon linkers generate much higher MW (Mn typically 50–500 kDa). Stereoselectivity can be tuned using large, flat substituents in the o-phenolate position; tuning of regioselectivity is much harder. Hf catalysts are slower than their Zr analogs and do not work well with MAO/BHT (BHT = 2,6-di-tert-butyl-4-methylphenol); they are generally more selective (MW, stereo and regio). Density functional calculations agree fairly well with observed selectivities, supporting the involvement of a fac/fac coordinated active species. These O4 catalysts are considerably more flexible than e.g. metallocenes, making accurate prediction of PP microstructure a significant challenge.

Efficient and economical plastic waste upcycling relies on the development of catalysts capable of polymer degradation. A systematic high-throughput screening of twenty-eight polymerization catalyst precursors, belonging to the catalyst families of metallocenes, ansa-metallocenes, and hemi- and post-metallocenes, in cis-1,4-polybutadiene (PB) degradation reveals, for the first time, important structure–activity correlations. The upcycling conditions involve activation of the catalysts (at 0.18 % catalyst loading) with tri-iso-butyl aluminum at 50 °C in toluene. The data indicate the ability to degrade PB is a general reactivity profile of neutral group 4 metal hydrides. A simple quantitative-structure activity relationship (QSAR) model utilizing two descriptors for the distribution of steric bulk in the active pocket and one measuring the metal ion electrophilicity reveals the degradation ability improves with increased but not overbearing steric congestion and lower electrophilicity.

This work describes the application of a library of iron(III)-salen catalysts in the production of biodiesel from vegetable oils. The conversion of neutral soybean oil is complete within two hours at 160–180 °C with low catalyst loading (0.10 mol%). A comparative screening reveals that the catalysts containing acetate as a fifth ligand are the most performing, and these have been conveniently used to convert acidic and waste cooking oils (WCO). WCOs were used as received without further purification to produce biodiesel in high yield (85–90%) under optimized conditions (2 h at 180 °C, catalyst loading 0.1 mol%, oil to alcohol molar ratio 1:20). The iron content in the lipophilic and hydrophilic phases of the crude mixture was investigated and the residual concentration in biodiesel was found to be in the order of 10–14 ppm, comparable to that contained in biodiesels from other sources. Graphical Abstract

The performance of C2-symmetric ansa-hafnocene catalysts for isotactic polypropylene typically deteriorates at increasing temperature much faster than that of their zirconium analogues. Herein, we analyze in detail a set of five Hf/Zr metallocene pairs—including some of the latest generation catalysts—at medium- to high-polymerization temperature. Quantitative structure–activity relationship (QSAR) models for stereoselectivity, the ratio allyl/vinyl chain ends, and 2,1/3,1 misinsertions in the polymer indicate a strong dependence of polymerization performance on electrophilicity of the catalyst, which is a function of the ligand framework and the metal center. Based on this insight, the stronger performance decline of hafnocenes is ascribed to electrophilicity-dependent stabilization effects.

Catalytic activity in olefin polymerization depends not only on the catalyst but also, crucially, on activator/alkylator/scavenger “packages.” Along with binary mixtures containing Lewis or Bronste...

Absolute rigidity is rare in the "soft" world of organometallics. Here we introduce two cyclopenta[a]triptycyl ansa-zirconocene catalysts for isotactic-selective propene polymerization, designed by means of an integrated high-throughput experimentation/quantitative structure-activity relationship modeling approach. An ultrarigid ligand precisely wrapped around the Zr center enforces an enzyme-like lock and key fit, effectively hampering undesired reactive events, even at high temperature. Stereodefective units are hardly detectable by 13C NMR in the polymer produced at 120 °C; this corresponds to an enantioselectivity exceeding 6-7 kcal/mol: i.e., less than 1 propene misinsertion every 4000 (and at room temperature, one every ∼40000!).

For a set of 40 silicon-bridged C2-symmetric ansa-zirconocenes, reactivity ratios in ethene/1-hexene copolymerization were experimentally determined by means of an accurate high-throughput experimentation (HTE) approach, and used to develop quantitative structure-activity relationship (QSAR) models for comonomer affinity using chemically meaningful descriptors. These QSAR models rely almost exclusively on steric descriptors, with the single most important descriptor being the 'openness' of the open quadrants. Catalysts with an unobstructed main insertion pathway, i.e. without substituents affecting the open quadrant, show a remarkable insensitivity to further substituent effects, be it in 2-, 4-, 5-, 6- or 7-position or the bridge. We attribute this insensitivity to a shift in rate-limiting step for the comonomer incorporation, from insertion to olefin capture, with the latter being much less sensitive to modulation of the active pocket than the former. This indicates that our best incorporators are already close to the upper limit for comonomer affinity within this catalyst class. This journal is

Compared to heterogenous Ziegler–Natta systems (ZNS), ansa-metallocene catalysts for the industrial production of isotactic polypropylene feature a higher cost-to-performance balance. In particular, the C2-symmetric bis(indenyl) ansa-zirconocenes disclosed in the 1990s are complex to prepare, less stereo- and/or regioselective than ZNS, and lose performance at practical application temperatures. The golden era of these complexes, though, was before High Throughput Experimentation (HTE) could contribute significantly to their evolution. Herein, we illustrate a Quantitative Structure – Activity Relationship (QSAR) model trained on a robust and highly accurate HTE database. The clear-box QSAR model utilizes, in particular, a limited number of chemically intuitive 3D geometric descriptors that screen various regions of space in and around the catalytic pocket in a modular way thus enabling to quantify individual substituent contributions. The main focus of the paper is on the methodology, which should be of rather broad applicability in molecular organometallic catalysis. Then again, it is worth emphasizing that the specific application reported here led us to identify in a comparatively short time novel zirconocene catalysts rivaling or even outperforming all previous homologues which strongly indicates that the metallocene story is not over yet.

“Structural” AlMe2(bht) was found to be an integral building block of the “TMA-free” olefin polymerization cocatalyst MAO/BHT (TMA = trimethylaluminum, MAO = methylaluminoxane, BHT = 2,6-di-tert-butyl-4-methylphenol). NMR studies show that treatment of MAO/BHT with the neutral N-donor pyridine (py) leads to selective generation of neutral AlMe2(bht)(py). The reaction of MAO/BHT with 2,2'-bipyridine (bipy) generates cationic [AlMe(bht)(bipy)]+ fragments (bht = BHT phenolate), analogously to what happens when unmodified MAO is treated with bipy, causing [AlMe2(bipy)]+ formation. This suggests that the activation of an olefin polymerization precatalyst LnMX2 (X = Me or Cl) can occur via an indirect pathway involving [AlMeR]+ (R = Me or bht) transient species (rather than direct Cl/Me abstraction by the Al-cages) not only when unmodified MAO is exploited but also in the case of “TMA-free” BHT-modified MAO. The high chemoselectivity of py for neutral Al adducts however presents a distinct difference to unmodified MAO. These observations indicate that a) “structural” TMA is converted into “structural” AlMe2(bht) upon reaction of MAO with BHT and that b) MAO/BHT is harder to ionize than unmodified MAO. A refined formula and cluster size estimation [(AlOMe)0.87(AlMe2bht)0.13]n (n = 57–84) is proposed for MAO/BHT based on this new experimental evidence, accounting for the presence of “structural” AlMe2(bht) units.

Zirconaziridinium ions [Cp2Zr(η2-CH2NR2]+ can potentially activate H2 by two routes: σ-bond metathesis, or FLP reactivity. We show here that Zr-C hydrogenolysis by σ-bond metathesis precedes and enables subsequent heterolytic H2 cleavage by FLP pathways. DFT calculations show the involvement of transition states with approximately linear N···H···H and bent Zr···H···H arrangements without any direct Zr-amine interaction.

The established model cluster (AlOMe)16(AlMe3)6 for methylaluminoxane (MAO) cocatalyst has been studied by density functional theory, aiming to rationalize the different behaviors of unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di-tert-butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites A–C to release neutral Al fragments (i.e., AlMe2R; R = Me or bht) or transient aluminum cations (i.e., [AlMeR]+) has been investigated both in the absence and in the presence of neutral N-donors. Sites C are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites A and B, albeit of lower Lewis acidity, should be still able to release cationic Al fragments when TMA-depleted modified MAOs are treated with N-donors (e.g. [AlMe(bht)]+ from MAO/BHT). These findings provide tentative interpretations for earlier observations of donor-dependent ionization tendencies of MAO and MAO/BHT and how TMA depleted MAOs can still be potent activators., A model cluster for methylaluminoxane (MAO) was used to study the tendency of model Lewis acidic sites to release neutral Al fragments or transient aluminum cations both for unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di-tert-butyl-4-methylphenol). Albeit only sites of lower Lewis acidity are found in TMA depleted modified MAOs, they can release cationic Al fragments when treated with certain N-donors. This shows how TMA depleted MAOs can still be potent activators.

Compared to heterogenous Ziegler&ndash, Natta systems (ZNS), ansa-metallocene catalysts for the industrial production of isotactic polypropylene feature a higher cost-to-performance balance. In particular, the C2-symmetric bis(indenyl) ansa-zirconocenes disclosed in the 1990s are complex to prepare, less stereo- and/or regioselective than ZNS, and lose performance at practical application temperatures. The golden era of these complexes, though, was before High Throughput Experimentation (HTE) could contribute significantly to their evolution. Herein, we illustrate a Quantitative Structure &ndash, Activity Relationship (QSAR) model trained on a robust and highly accurate HTE database. The clear-box QSAR model utilizes, in particular, a limited number of chemically intuitive 3D geometric descriptors that screen various regions of space in and around the catalytic pocket in a modular way thus enabling to quantify individual substituent contributions. The main focus of the paper is on the methodology, which should be of rather broad applicability in molecular organometallic catalysis. Then again, it is worth emphasizing that the specific application reported here led us to identify in a comparatively short time novel zirconocene catalysts rivaling or even outperforming all previous homologues which strongly indicates that the metallocene story is not over yet.

A set of 19 silicon-bridged C2-symmetric zirconocenes rac-R′2Si(2-Me-4-R-indenyl)2ZrCl2 of varying steric demand in position 4 were synthesized and screened in propene homopolymerization in a high-throughput experimental setup. The size and accuracy of the experimental data set allow to identify surprisingly good correlations among stereoselectivity, regioselectivity, and molecular weight capability (R2 ≈ 0.8−0.9) over a broad range. We rationalize this trend by assuming that steric tuning in the 4-position affects both preferred insertion and stereoerror formation similarly but leaves other barriers largely unaffected. A quantitative structure−activity relationship based on one single computational descriptor, Δ%VBur using the difference in the percent of buried volume between the “blocked” and “open” quadrants of the catalyst precursoris established. Provided that a large sphere of 5.0 Å is used, stereoselectivity can be predicted with unprecedented accuracy, i.e., a mean average deviation (MAD) of 0.18 kcal/mol (ΔΔG‡ enantio), 0.0007 (σ, probability that the preferred propene enantioface is selected at an active site of given chirality), or 0.3% (mmmm pentads). On the basis of this empirical model, we predicted that the catalyst with R = o-tolyl is an ideal candidate for high stereoselectivity/high MW capability. Ad hoc synthesis and testing of the precursor confirmed the expectations: the catalyst shows the highest stereoselectivity reported so far (σ = 0.9999) for metallocenes at 60 °C, while maintaining a high MW capability (Mw > 1 MDa) and relatively high regioselectivity.

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.

Reported here are several new adducts of the type (RBDI)M(HE)2 where RBDI is the β-diiminate ligand [2,6-R2C6H3N═CMe]2CH (R = Me, Et, MeO; M = Rh, Ir; HE = HSiEt3, HGeEt3, HSnnBu3, HBPin). DFT calculations are used to analyze the bonding in these and related Cp* complexes, in particular with respect to the degree of oxidative addition (OA) of the H–E bonds: this increases in the order H2 < HBPin < HSiEt3 ≈ HGeEt3 ≈ HSnnBu3, it proceeds further for Ir than for Rh, and Cp* promotes OA to a larger degree in comparison to the BDI ligand. The first metal–HE dissociation energy is rather sensitive to steric effects and increases in the order CH4 ≪ H2 ≈ HBPin ≈ HSiMe3 ≈ HGeMe3 < HSnMe3, with Ir uniformly binding HE more strongly than Rh by about 10 kcal/mol. M–HE dissociation is the first step of the intramolecular ligand functionalization first reported for (MeBDI)Rh(HSiEt3)2. Such functionalization was not observed for any of the new complexes reported here. On the basis of the idea that stannane dissociation ...

Both Ni(0) and Ni(I) compounds are believed to exhibit cross-coupling catalytic properties under various conditions, and the compounds Ni(PPh3)4 and NiCl(PPh3)3 are compared as catalysts for representative Suzuki−Miyaura and Heck−Mizoroki cross-coupling reactions. The Ni(0) compound exhibits catalytic activities, for cross-coupling of chloro and bromoanisole with phenylboronic acid and of bromobenzene with styrene, yielding results which are comparable with those of many palladium-based catalysts, but our findings with NiCl(PPh3)3 are at this point unclear. It seems to convert to catalytically active Ni(0) species under Suzuki−Miyaura reaction conditions and is ineffective for Heck−Mizoroki cross-coupling. The paramagnetic Ni(I) compounds NiX(PPh3)3 (X = Cl, Br, I) are characterized for the first time by 1H NMR spectroscopy and are found to exhibit broad meta and para resonances at δ 9−11 and 3−4, respectively, and very broad ortho resonances at δ 4−6; these resonances are very useful for detecting Ni(I) species in solution. The chemical shifts of NiCl(PPh3)3 vary with the concentration of free PPh3, with which it exchanges, and are also temperature-dependent, consistent with Curie law behavior. The compound trans-NiPhCl(PPh3)2, the product of oxidative addition of chlorobenzene to Ni(PPh3)4 and a putative intermediate in cross-coupling reactions of chlorobenzene, is found during the course of this investigation to exhibit entirely unanticipated thermal lability in solution in the absence of free PPh3. It readily decomposes to biphenyl and NiCl(PPh3)2 in a reaction relevant to the long-known but little-understood nickel-catalyzed conversion of aryl halides to biaryls. Ni(I) and biphenyl formation is initiated by PPh3 dissociation from trans-NiPhCl(PPh3)2 and formation of a dinuclear intermediate, a process which is now better defined using DFT methodologies.

A computational exploration of the predicted molecular weights for Ti- and Zr-catalyzed olefin polymerizations shows that there are considerable opportunities for electronic tuning. Ligand variation mainly affects the propagation rate, whereas chain transfer to the monomer is hardly affected by electronic factors. The results are analyzed in terms of the effects of ligand variation on the relative energies of “connected couples” of reactant local minima and the corresponding transition states on the basis of the Hammond postulate and the Curtin–Hammett principle. For the constrained-geometry catalysts (CGCs) and bis(amido) systems studied, better donating ligands increase the preference for a “planar” metal environment and produce higher molecular weights.

Salalen−Ti systems for propene polymerization catalysis were originally developed under the assumption of a “directional site epimerization promoted electronically” strategy and allowed the synthesis of polypropylene samples with extraordinarily high isotacticities. Our DFT calculations revealed an alternative and more intriguing interpretation of polymerization behavior of such systems. Ligand coordination around the metal center driven by the monomer and the polymer chain dictates the formation of active species different from the ones expected from X-ray structures of precursors. The “synergic” role of R1 and R3 substituents is responsible of the extraordinarily high isotacticities reported in propene polymerization.

Quantitative agreement between experimentally determined M–C bond dissociation energies (BDE) and DFT predictions (M06-2X/TZ//TPSSTPSS/DZ) can be reached by choosing the correct anchor for experimentally derived BDE. For the example of the archetypical metallocene catalyst Cp 2 TiCl 2 , it is shown that titanium–carbon bonds are very weak under polymerization conditions and fluctuate; steric strain is introduced after 2,1 insertion and via olefin capture. Thus, homolysis can become competitive with chain propagation. Depending on the catalyst and temperature, 2,1 insertion can be only a temporary inconvenience (dormancy) or a definitive decay event. It is then shown for a set of nine common Ti and Zr polymerization catalysts how ligand variation affects the metal–carbon BDE. Predicted stabilities of the M(IV) oxidation state with respect to homolysis are in nice agreement with the experimentally observed temperature tolerance of the various catalysts: homolysis is easier for Ti than for Zr, and cyclopentadienyl groups in particular facilitate homolysis, especially in bis-cyclopentadienyl systems.

The gold(III) methoxide complex (C∧N∧C)AuOMe (1) reacts with tris(p-tolyl)phosphine in benzene at room temperature under O abstraction to give the methylgold product (C∧N∧C)AuMe (2) together with OP(p-tol)3 ((C∧N∧C) = [2,6-(C6H3tBu-4)2pyridine]2−). Calculations show that this reaction is energetically favorable (ΔG = −32.3 kcal mol−1). The side products in this reaction, the Au(II) complex [Au(C∧N∧C)]2 (3) and the phosphorane (p-tol)3P(OMe)2, suggest that at least two reaction pathways may operate, including one involving (C∧N∧C)Au• radicals. Attempts to model the reaction by DFT methods showed that PPh3 can approach 1 to give a near-linear Au−O−P arrangement, without phosphine coordination to gold. The analogous reaction of (C∧N∧C)AuOEt, on the other hand, gives exclusively a mixture of 3 and (p-tol)3P(OEt)2. Whereas the reaction of (C∧N∧C)AuOR (R = But, p-C6H4F) with P(p-tol)3 proceeds over a period of hours, compounds with R = CH2CF3, CH(CF3)2 react almost instantaneously, to give 3 and OP(ptol) 3. In chlorinated solvents, treatment of the alkoxides (C∧N∧C)AuOR with phosphines generates [(C∧N∧C)Au(PR3)]Cl, via Cl abstraction from the solvent. Attempts to extend the synthesis of gold(III) alkoxides to allyl alcohols were unsuccessful; the reaction of (C∧N∧C)AuOH with an excess of CH2 CHCH2OH in toluene led instead to allyl alcohol isomerization to give a mixture of gold alkyls, (C∧N∧C)AuR′ (R′ = −CH2CH2CHO (10), −CH2CH(CH2OH)OCH2CHCH2 (11)), while 2-methallyl alcohol affords R′ = CH2CH(Me)CHO (12). The crystal structure of 11 was determined. The formation of Au−C instead of the expected Au−O products is in line with the trend in metal−ligand bond dissociation energies for Au(III): M−H > M−C > M−O.

The reaction of 1,1'-Li

Zr and Hf complexes of bis(phenolate-ether) (“O4”) ligands feature high activity, stereoselectivity and molecular weight capability for propene polymerization at high temperature. Here we report a simplified ligand synthesis and several new examples of O4 ligands. The formation of precatalysts LMR2 (M = Zr, Hf; R = Bn, Me) from LH2 and MR4 was found to be accompanied in some cases by the formation of dimers (μ-L)2[MR2]2, and X-ray structures of two such dimers have been determined. Treatment of LMMe2 with [Ph3C]+[B(C6F5)4]– produces fairly clean cationic species LMMe+ which were studied by 1H NMR. 2D ROESY data, in particular, suggest that for “smaller” O4 ligands the LMMe+ cation reversibly rearranges from the active (fac/fac) form to a presumably inactive fac/mer or mer/mer form; more bulky substituents appear to suppress this rearrangement. Implications for polymerization catalysis are discussed.

MAO​/BHT (MAO = methylalumoxane; BHT = 2,​6-​di-​tert-​butyl-​4-​methylphenol) cocatalyst for olefin polymn. has been investigated by NMR spectroscopy. It has been found that it consists of oligomeric [AlOMe0.9(bht)​0.1]​n cages and monomeric MeAl(bht)​2. Diffusion NMR indicates an av. n for Al-​clusters of 62-​96, i.e. 2-​3 times higher than that estd. for unmodified MAO under analogous conditions (n ≈ 26-​41)​. The reactivity of MAO​/BHT has been explored by monitoring the activation of the Cp*-​phosphinimide titanium dichloride precatalyst Cp*(tBu3P=N)​TiCl2. Comparison with independently synthesized model species and DFT modeling allowed characterization of the reaction mixts. obtained at varying aluminum to titanium ratios. Homodinuclear adducts [Cp*(tBu3P=N)​TiX]​2(μ-​Y)​+ (X, Y = Me or Cl) forming Outer Sphere Ion Pairs (OSIPs) with MAO​/BHT-​derived anions are dominant at low Al​/Ti ratios, whereas mononuclear Inner Sphere Ion Pairs (ISIPs) [Cp*(tBu3P=N)​Ti-​X]​+[MAO​/BHT]​-​ are formed at high Al​/Ti ratios; both types of species are found to be viable precursors for the cationic active species. Activation of dibenzyl analog Cp*(tBu3P=N)​TiBn2 results in the clean formation of [Cp*(tBu3P=N)​Ti-​Bn]​+[MAO​/BHT]​-​ OSIP, giving sharp 1H and 31P NMR signals; this reaction was exploited to quantify the amt. of strongly acidic sites on Al-​clusters, shedding further light on the structure and properties of MAO​/BHT.

Reaction of MBn4 (M = Zr, Hf ) with diiminepyridine ligands MeDIP, EtDIP and iPrDIP [RDIP = 2,6-(2,6-R2C6H3N=CMe)2- C5H3N] results in the transfer of two benzyl groups to the DIP imine carbon atoms, generating mixtures of rac and meso isomers of [RDIP + 2Bn]MBn2. The diastereomers could in several cases be separated by fractional crystallization, and the X-ray structures of rac and meso [EtDIP + 2Bn]ZrBn2, meso- [MeDIP + 2Bn]ZrBn2 and meso-[EtDIP + 2Bn]HfBn2 are reported. On heating either diastereomer of [EtDIP + 2Bn]ZrBn2 in solution, the other isomer grows back in, demonstrating reversibility of the alkyl transfer. Transfer to other positions of the DIP skeleton was not observed. The reaction of [EtDIP + 2Bn]ZrBn2 with two equivalents of TEMPO produced structurally characterized [EtDIP + 2Bn]Zr(Bn)(TEMPO) which shows no tendency to isomerize.

Four Cl/Me substituted [ONNO] Zr-catalysts have been tested in ethene/&alpha, olefin polymerization. Replacing electron-donating methyl with isosteric but electron-withdrawing chlorine substituents results in a significant increase of comonomer incorporation. Exploration of steric and electronic properties of the ancillary ligand by DFT confirm that relative reactivity ratios are mainly determined by the electrophilicity of the metal center. Furthermore, quantitative DFT modeling of propagation barriers that determine polymerization kinetics reveals that electronic effects observed in these catalysts affect relative barriers for insertion and a capture-like transition state (TS).

Reaction of (TBBP)AlMe⋅THF with [Cp*2 Zr(Me)OH] gave [(TBBP)Al(THF)-O-Zr(Me)Cp*2 ] (TBBP=3,3',5,5'-tetra-tBu-2,2'-biphenolato). Reaction of [DIPPnacnacAl(Me)-O-Zr(Me)Cp2 ] with [PhMe2 NH](+) [B(C6 F5 )4 ](-) gave a cationic Al/Zr complex that could be structurally characterized as its THF adduct [(DIPPnacnac)Al(Me)-O-Zr(THF)Cp2 ](+) [B(C6 F5 )4 ](-) (DIPPnacnac=HC[(Me)C=N(2,6-iPr2 -C6 H3 )]2 ). The first complex polymerizes ethene in the presence of an alkylaluminum scavenger but in the absence of methylalumoxane (MAO). The adduct cation is inactive under these conditions. Theoretical calculations show very high energy barriers (ΔG=40-47 kcal mol(-1) ) for ethene insertion with a bridged AlOZr catalyst. This is due to an unfavorable six-membered-ring transition state, in which the methyl group bridges the metal and ethene with an obtuse metal-Me-C angle that prevents synchronized bond-breaking and making. A more-likely pathway is dissociation of the Al-O-Zr complex into an aluminate and the active polymerization catalyst [Cp*2 ZrMe](+) .

The hydroauration of internal and terminal alkynes by gold(III) hydride complexes [(C^N^C)AuH] was found to be mediated by radicals and proceeds by an unexpected binuclear outer‐sphere mechanism to cleanly form trans‐insertion products. Radical precursors such as azobisisobutyronitrile lead to a drastic rate enhancement. DFT calculations support the proposed radical mechanism, with very low activation barriers, and rule out mononuclear mechanistic alternatives. These alkyne hydroaurations are highly regio‐ and stereospecific for the formation of Z‐vinyl isomers, with Z/E ratios of >99:1 in most cases.

Rhodium β-diiminate complexes of conjugated dienes, (RBDI)Rh(diene) (RBDI = [2,6-R2C6H3-NCMe]2CH; R = Me, Et; diene = 1,3-hexadiene, 1,3-pentadiene, 1,3-butadiene, 2-Me3SiO-1,3-cyclohexadiene) as well as (MeBDI)-Ir(1,3-butadiene) show fluxional behavior caused by squareplanar− tetrahedral interconversion where the diene rotates relative to the diiminate ligand. The mechanism involves neither (partial) ligand dissociation nor a change in spin state. Density functional calculations support the rotation mechanism and indicate that it is assisted by a partial valence isomerization of the diene to an enediyl ligand at the tetrahedral geometry. Nonconjugated dienes, which cannot undergo such valence isomerization, are predicted to have much larger barriers to rotation.

Quenched-flow data for propene polymerization with rac-Me2Si(2-Me-4-Ph-1-indenyl)(2)ZrCl2/MAO support a picture where removal of MAO qualitatively changes the kinetic profile from a mainly enthalpic to a mainly entropic barrier. DFT studies suggest that a not previously recognized singly-bridged end-on coordination mode of Me6Al2 to catalytically active centers may be kinetically relevant as a resting state. In contrast, the more traditional doubly-bridged complex of Me3Al is proposed to be more relevant to chain transfer to cocatalyst.

Gold(iii) forms spectroscopically detectable H–B and H–Si σ-complexes; experiments and DFT calculations demonstrate heterolytic H–Si, H–H and H–C bond cleavage., The coordinatively unsaturated gold(iii) chelate complex [(C^N–CH)Au(C6F5)]+ (1+) reacts with main group hydrides H–BPin and H–SiEt3 in dichloromethane solution at –70 °C to form the corresponding σ-complexes, which were spectroscopically characterized (C^N–CH = 2-(C6H3But)-6-(C6H4But)pyridine anion; Pin = OCMe2CMe2O). In the presence of an external base such as diethyl ether, heterolytic cleavage of the silane H–Si bond leads to the gold hydrides [{(C^N–CH)AuC6F5}2(μ-H)]+ (2+) and (C^N–CH)AuH(C6F5) (5), together with spectroscopically detected [Et3Si–OEt2]+. The activation of dihydrogen also involves heterolytic H–H bond cleavage but requires a higher temperature (–20 °C). H2 activation proceeds in two mechanistically distinct steps: the first leading to 2 plus [H(OEt2)2]+, the second to protonation of one of the C^N pyridine ligands and reductive elimination of C6F5H. By comparison, formation of gold hydrides by cleavage of suitably activated C–H bonds is very much more facile; e.g. the reaction of 1·OEt2 with Hantzsch ester is essentially instantaneous and quantitative at –30 °C. This is the first experimental observation of species involved in the initial steps of gold catalyzed hydroboration, hydrosilylation and hydrogenation and the first demonstration of the ability of organic C–H bonds to act as hydride donors towards gold.

Isocyanides insert into gold(iii)-hydrogen bonds to give the first examples of gold iminoformyl complexes. The reaction is initiated by catalytic amounts of radicals; DFT calculations indicate that this is an equilibrium reaction driven forward by isocyanide in sufficient excess to trap the Au(ii) intermediate.

The factors affecting the rates of reductive C@C cross-coupling reactions in gold(III) aryls were studied by using complexes that allow easy access to a series of electronically modified aryl ligands, as well as to gold methyl and vinyl complexes, by using the pincer compounds [(C^N^C)AuR] (R=C6F5, CH=CMe2, Me and p-C6H4X, where X=OMe, F, H, tBu, Cl, CF3, or NO2) as starting materials (C^N^C=2,6-(4’-tBuC6H3)2pyridine dianion). Protodeauration followed by addition of one equivalent SMe2 leads to the quantitative generation of the thioether complexes [(C^NCH) AuR(SMe2)]+. Upon addition of a second SMe2 pyridine is displaced, which triggers the reductive aryl@R elimination. The rates for these cross-couplings increase in the sequence k(vinyl)>k(aryl)@k(C6F5)>k(Me). Vinyl@aryl coupling is particularly fast, 1.15V10@3 Lmol@1 s@1 at 221 K, whereas both C6F5 and Me couplings encountered higher barriers for the C@C bond forming step. The use of P(p-tol)3 in place of SMe2 greatly accelerates the C@C couplings. Computational modelling shows that in the C^N-bonded compounds displacement of N by a donor L is required before the aryl ligands can adopt a conformation suitable for C@C bond formation, so that elimination takes place from a four-coordinate intermediate. The C@C bond formation is the rate-limiting step. In the non-chelating case, reductive C(sp2)@C(sp2) elimination from three-coordinate ions [(Ar1)(Ar2)AuL]+ is almost barrier-free, particularly if L=phosphine.

Suzuki cross-coupling of haloalkenes is generally assumed to occur with retention of the alkene stereochemistry. While studying Suzuki cross-couplings on E-1,2-dichlorovinyl phenyl ketone, we were surprised to observe extensive isomerization. More surprisingly, the ligand employed strongly influenced the degree of isomerization: DPEphos and Xantphos led to 96% isomerized cross-coupled product whereas reactions in the absence of a phosphine ligand, or reactions employing t-BuXantphos, gave 94% retention of stereochemistry. While E–Z isomerization in Pd-catalyzed vinylic couplings has previously been attributed to events within the cross-coupling catalytic cycle, we present experimental and computational evidence for a separate Pd-catalyzed isomerization process in these reactions.

Reactions of the nickel(0) compound NiL4 (L = PPh3) with alkyl halides RX involve initial inner-sphere halogen atom abstraction from the alkyl halides to form alkyl radicals R· and halonickel(I) metalloradical species NiX(PPh3)2,3. The radical pairs then undergo combination within the solvent cage to give the square planar nickel(II) compounds NiRX(PPh3)2. Radical intermediacy is demonstrated persuasively by observations that the relative rates vary in the orders tert-butyl > sec-butyl > n-butyl and RI > RBr > RCl, while density functional theory calculations indicate that the radical mechanism provides a lower energy pathway than do alternative, more conventional pathways. The product of the reaction of Ni(PPh3)4 with methyl iodide, NiMeI(PPh3)2, decomposes in solution to ethane and NiI(PPh3)2,3, but when RX = EtI, n-BuI, sec-BuI, tert-BuI, the alkyl-nickel products undergo rapid β-hydrogen elimination to give the hydride NiHI(PPh3)2 plus the corresponding alkene(s). Reactions also occur in which a portion of the alkyl radicals diffuses from the solvent cage and abstracts hydrogen from NiHI(PPh3)2 to form alkanes RH and Ni(I) species NiI(PPh3)2. As a result, NiHI(PPh3)2 is invariably a minor product while the major products are alkanes RH, alkenes R−H, and NiI(PPh3)2. Hydride NiHI(PPh3)2 is found to decompose to H2 and NiI(PPh3)2 but is stable at low temperatures where it exhibits unusual NMR behavior because of exchange involving free PPh3 and the bis- and trisphosphine species, NiHI(PPh3)2 and NiHI(PPh3)3. Present in all of the reactions are paramagnetic, substitution-labile Ni(I) metalloradical species. As a result, resonances of PPh3, ethylene, and the smaller iodoalkenes are generally broad and shifted because of exchange between free and coordinated ligands.

Growing chain orientational preference is one of the key factors determining the stereoselectivity of olefin polymerization and related reactions such as carboalumination; alpha-agostic interactions are an important component of this preference. Using insertion of ethene in the M-CH3 bond as a model, the intrinsic orientational preference was evaluated for a number of important catalyst types through determination of the energy profile for methyl group rotation at the insertion TS. Ti and Zr metallocenes show a pronounced preference (Ti, 10 kcal/mol; Zr, 6 kcal/mol) for a classical a-agostic arrangement with a single short M···CH contact and an elongated C-H bond; on CH3 rotation the agostic elongation mostly disappears. In contrast, for all non-metallocene systems studied the orientational preference is much smaller or even opposite that of metallocenes. Moreover, on CH3 rotation the agostic C-H bond elongation gets spread out over two C-H bonds rather than disappearing. These results point to greater chain orientation flexibility for non-metallocene catalysts.

The α-agostic alkyltitanocene(IV) complex [Cp2TiCH2CH(CH3)(CMe3)]+ (II) and its D3 isotopologue [Cp2TiCH2CH(CD3)(CMe3)]+ (II-CD3) undergo reversible, intramolecular H–H(D) exchange among the α-, β-, and γ- (β-Me) positions of the alkyl ligands in addition to concomitant intermolecular H–H(D) exchange with the vinylic and 2-methyl sites of the product of β-hydrogen elimination, CH2═CMeCMe3, results which are in strong contrast to the exchange behavior of the γ-agostic silyl analogues [Cp2TiCH2CH(CH3)(SiMe3)]+ (III) and [Cp2TiCH2CH(CD3)(SiMe3)]+ (III-CD3). As has been previously shown, III undergoes tunnelling-expedited exchange of the β-H (but not α-H) with the hydrogen atoms of the β-methyl group (β-H/γ-H exchange) while III-CD3 isomerizes reversibly but specifically to the isotopomer [Cp2TiCD2CD(CH3)(SiMe3)]+, forgoing β-H/γ-H exchange completely. In this paper we show that all of the exchange processes of II/II-CD3 and III/III-CD3 initially involve conventional β-H elimination processes and thus, at som...

This paper describes the synthesis, properties and crystal structure of a new phosphonium-1-indenylide ligand, 4,7-dimethyl-1-C9H4PMePh2, and the syntheses of complexes of rhodium(I) and iridium(I) containing both the parent PHIN ligand, 1-C9H6PMePh2 (I) and 4,7-dimethyl-1-C9H4PMePh2 (II). The complexes [M(Z4-COD)(Z5-PHIN)]X (M = Rh, Ir; PHIN = I, II; COD = 1,5-cyclooctadiene; X = Cl, BF4) and [Rh(Z5-I)(PPh3)2]BF4 have been prepared and characterized by HR-ESM, elemental analyses and NMR spectroscopy. While [Ir(Z4-COD)(Z5-I)]Cl undergoes one or more chloride-induced exchange processes involving the COD ligand, the analogous complexes of rhodium and iridium with the more weakly coordinating BF4 counter anion exhibit no exchange processes. In contrast, [Rh(Z5-I)(PPh3)2]BF4 undergoes an exchange process involving the inequivalent PPh3 phosphorus atoms and which is rationalized using DFT calculations.

Reaction of the diiminepyridine ligand EtDIP (2,6-Et2-C6H3N═CMe)2C5H3N) with TiCl3(THF)3 gave the corresponding Ti(III) complex (EtDIP)TiCl3 (1). Reduction of 1 with 1 equiv of KC8 produced the formally Ti(II) complex (EtDIP)TiCl2 (2). From this, (EtDIP)TiClR complexes (R = Me (3a), Me3SiCH2 (3b), Ph (3c)) were obtained by addition of 1 equiv of RLi. Similarly, dialkyl complexes (EtDIP)TiR2 (R = Me (4a), Me3SiCH2 (4b)) were obtained with 2 equiv of RLi. All new complexes except 3b were characterized by single-crystal X-ray diffraction. EPR studies indicate that complex 1 is best regarded as a true Ti(III) complex with an “innocent” DIP ligand. Complexes 2–4 are all diamagnetic. In contrast to DIP complexes of the late transition metals Fe and Co, the new complexes 2–4 show strong upfield 1H NMR shifts for the pyridine β and γ protons caused by transfer of negative charge to the DIP ligand. On the basis of this and the C═N and Cimine–CPy bond lengths, a description involving Ti(IV) and a dianionic ligand seems most appropriate, and DFT calculations support this interpretation. This means that reduction of Ti(III) complex 1 results in oxidation of the metal center to Ti(IV). VT-NMR studies of 4a suggest a small and temperature-dependent thermal population of a triplet state, and indeed calculations indicate that 4a has the lowest singlet–triplet energy difference of the systems studied.

Selective Au-C bond cleavage and arene-C-H activation in (C^N^C)Au(III) pincer complexes are reversible, leading to a solvent-dependent proton shuttling process. The ether-free cleavage products are non-fluxional and show weak gold(III)-arene interactions commensurate with intermediates postulated previously for CMD-type arene activation.