Search

Ti-C homolysis is generally regarded as a catalyst deactivation step in olefin polymerization catalyzed by titanium complexes. The present work demonstrates that for industrially relevant Ti catalysts in toluene and related solvents a "detour" via radical activation of the solvent can prolong catalyst lifetime and productivity, leading to chain transfer to solvents (CTS). For differently substituted phosphinimide half-titanocenes and other Ti catalyst classes, i.e. constrained geometry (CGC) and amidinate catalysts, CTS leads to formation of up to ∼60% benzylated chains. The efficiency of the reactivation pathway depends mostly on steric factors and correlates well with the percentage of buried volume, %VBur, as well with as DFT predictors. Thus, the solvent is far from innocent, and catalyst behavior in such solvents may not be representative of polymerizations in more innocent saturated hydrocarbons. On the other hand, it might be the case that, in truly innocent solvents, the monomer itself can play a less innocent role.

A “flexible cluster” model approach to Ziegler− Natta catalysts for the production of isotactic polypropylene, allowing the use of realistically sized MgCl2 monolayer clusters (up to 38 MgCl2 units) without any constraints, was employed to investigate the formation of adducts between the MgCl2 support and three industrially relevant internal donor classes, namely phthalates, succinates, and 1,3-dimethoxypropanes. The calculated adsorption modes and thermochemical data for adducts of single-donor molecules confirmed earlier literature trends only in part. Results for adducts with multiple donor molecules, in turn, did not confirm the indications of periodic models about steric repulsion between neighboring adsorbates hampering high degrees of surface coverage; as a matter of fact, such repulsions seem to be largely traceable to unnecessary constraints inherent in periodic calculations.

In this paper we introduce a DFT‐D ‘flexible‐cluster’ approach to the computational modeling of MgCl2‐supported Ziegler‐Natta catalysts. The adsorption of TiCl4 and probe donor molecules on plain and defective lateral terminations of monolayered MgCl2 clusters was studied allowing for full structure relaxation by means of modern DFT methods including dispersion corrections. The approach offers a good compromise between speed and accuracy, and has great potential for realistic simulations of the catalytic species in industrially relevant formulations. In particular, compared with periodic DFT‐D descriptions, it is better able to capture the features of real nano‐sized and highly disordered primary catalyst particles with complex adsorbate compositions.

Alkyl benzoates, phthalates, and succinates are used as “internal donors” (ID) to modify the surface of MgCl2-supported Ziegler–Natta (ZN) catalysts for polypropylene production. Phthalates, in particular, are the working horses of this catalysis, but a REACH ban of these compounds for other applications has generated a desire from the market for phthalate-free ZN catalysts, which has in turn triggered a search for alternatives and revamped research in the area. It has been reported that under polymerization conditions certain ID classes undergo an extensive exchange with alkoxysilane “external donors” (ED), which represents an important opportunity for surface fine-tuning. ID clean-up has been attributed to ester group reduction by the AlR3 cocatalyst. We have now measured the activation parameters for the reactions of ethyl benzoate (EB), dibutyl phthalate (DBP), and diethyl 2,3-diisobutylsuccinate (DiBS) with AlEt3 in toluene solution by means of variable-temperature 1H NMR spectroscopy. Reduction is fast for EB and DBP, whereas it is slow for DiBS. Results of DFT calculations are in line with the observed reactivity. Therefore, we conclude that an irreversible reaction with AlEt3 is not the only option for ID/ED exchange. Possible alternatives are discussed.

The design of stereoselective group 4 catalysts for the fine tuning of polyolefins microstructure is a challenging target thanks to the development of metallocene and postmetallocene systems. The main interactions leading to the enantioselective processes are predominantly steric in nature although several attempts to modify this general framework (e.g. by electronic factors) have been reported. Within the various strategies, the “directional site epimerization promoted electronically” published by Kol et al. immediately appeared as one of the most promising approaches, allowing for the synthesis of polypropylene samples with high isotacticities as well as high control of degree of isotacticity. It consists basically in modify the regular chain migration mechanism by placing donors of different trans influence trans to the two coordination sites in an octahedral environment. The system used is a salalen ligands, with halfsalan O,N,N donors bind in a fac mode and the half-salen O,N,N donors bind in a mer mode the metal active site (see Chart 1 A). The difference with respect to the salan ligands wrapping with a fac-fac geometry (see Chart 1 B) might allow to interplay with both steric and electronic factors. In this communication we will report about our extensive study by using DFT calculations on the propene polymerization mechanisms promoted by salan and salalen ligands. Our results offer an alternative interpretation of the experimental results.

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 some stage, intermediacy of the corresponding hydrido alkene complexes [Cp2TiH(CH2=CMeCMe3)]+ (VI) or [Cp2TiH(CH2=CMeSiMe3)]+ (or their D3 isotopologues). The major reason for the vastly different exchange behaviors of the two structurally very similar alkyltitanocene systems II and III is simply that, for steric and electronic reasons, the alkene in VI dissociates reversibly to the hydride, [Cp2TiH]+, and free alkene while that in [Cp2TiH(CH2=CMeSiMe3)]+ does not; CH2=CMeSiMe3 remains coordinated at the temperature and on the time scale of the experiments. DFT calculations generally support our mechanistic conclusions and furthermore point to a subtle ion pairing effect which hinders intramolecular η2-alkene rotation in VI and [Cp2TiH(CH2=CMeSiMe3)]+, thereby exerting a surprisingly important, heretofore unanticipated, influence on the nature of the chemistry involved.

The compound [Cp2Ti(Me)(CD2Cl2)][B(C6F5)4] reacts with trimethylvinylsilane (TMVS) to form the 1,2-insertion product [Cp2TiCH2CHMe(SiMe3)](+) (III), which exists in solution as equilibrating β- and γ-agostic isomers. In addition, while free rotation of the β-methyl group results in a single, averaged γ-H atom resonance at higher temperatures, decoalescence occurs below ~200 K, and the resonance of the γ-agostic hydrogen atom at δ ~ -7.4 is observed. Reaction of [Cp2Ti(CD3)(CD2Cl2)](+) with TMVS results in the formation of [Cp2TiCH2CH(CD3)(SiMe3)](+), which converts, via reversible β-elimination, to an equilibrium mixture of specifically [Cp2TiCH2CH(CD3)(SiMe3)](+) and [Cp2TiCD2CD(CH3)(SiMe3)](+). Complementing this conventional process, exchange spectroscopy experiments show that the β-H atom of [Cp2TiCH2CHMe(SiMe3)](+) undergoes exchange with the three hydrogen atoms of the β-methyl group (β-H/γ-H exchange) but not with the two α-H atoms. This exchange process is completely shut down when [Cp2TiCH2CH(CD3)(SiMe3)](+) is used, suggesting an H/D kinetic isotope effect much larger (apparently >16,000) than the maximum possible for an over-the-barrier process. It is proposed that β-H/γ-H exchange is facilitated by quantum mechanical proton tunnelling in which a hydrogen atom of the 2-methyl group of the alkene-hydride deinsertion product [Cp2TiHCH2═CMe(SiMe3)](+) undergoes reversible exchange with the hydride ligand via the allyl dihydrogen species [Cp2TiH2(η(3)-CH2C(SiMe3)CH2](+). Complementing these findings, DFT calculations were carried out to obtain energies and NMR parameters for all relevant species and thence to obtain better insight into the agostic preference(s) of complex III and the observed exchange processes. In all cases where comparisons between experimental and calculated data were possible, agreement was excellent.

Branches are an important aspect of the structure of real polyethylene. Branches can be short (Me, Et) or longer; long-chain branches (LCB, >100 carbons), in particular, are important because they can have a dramatic effect on polymer properties. In this review, we summarize mechanistic information from organometallic and computational chemistry and use this to examine the most probable sources of each type of branch. Short branches can be introduced deliberately by copolymerization with an α-olefin (possibly formed in situ from ethene). Me branches may be formed by one-carbon chain walking and propagation, and/or from insertion of an oligomer/macromer in an M Me bond formed via chain transfer to the cocatalyst [Me3Al or methylaluminoxane (MAO)]. Et branches are most likely formed through β-hydrogen transfer to ethene, followed immediately by reinsertion of the newly formed macromer. LCBs have usually been ascribed to reinsertion of macromers. However, certain catalysts exhibit LCB formation patterns that are hard to reconcile with this model, and a ‘two-monomer’ model was recently proposed to explain the observations for these systems. In this review, we present an alternative explanation (chain walking) that would fit the same facts for these catalysts.

Computational studies of a variety of polymn. catalyst models have revealed an unexpected fluidity in chain termination mechanisms.

The performance of correlated nb initio, methods and DFT methods was compared for the propagation and chain transfer steps of ethylene polymerization by a model aluminum-amidinate system, [{HC(NH)2}AlCH(2)CH(3)](+). All methods agree that the main chain transfer mechanism is beta-hydrogen transfer to the monomer (BHT), and that this is substantially easier than propagation; implications for the real Jordan system are discussed briefly. Counterpoise corrections are necessary to obtain reasonable olefin complexation energies. Activation energies are consistently lower at DFT (BP86, B3LYP) than at nb initio levels [MP2, MP3, MP4, CI, CCSD(T)]; the differences are particularly large (16 kcal/mol) for the BHT reaction. This is suggested to be related to the known problem of DFT in describing hydrogen bridged systems.

An unconventional mechanism for the stereoerror formation in propene polymerization catalyzed by C1-symmetric salalen-M systems (M = Zr, Hf) is suggested by DFT calculations. While propagation happens with the ligand in its fac-mer conformation, a change of ligand wrapping mode from fac-mer to fac-fac is the main source of the lower stereoselectivities obtained with Zr and Hf. This is different for the Ti analogues, where the ligand fac-mer wrapping mode does not play a role. Activation strain analysis indicates that the preference for a chain stationary mechanism of the Zr/Hf species is due to the energy required to distort the reactants (ΔEStrain) rather than to their mutual interaction (ΔEInt).

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.

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.

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.

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.

Accurate in silica prediction of copolymerization performance of olefin polymerization catalysts is demonstrated. It is shown by the example of 19 metallocene and post-metallocene group IV metal (Ti, Zr, Hf) systems that DFT (M06-2X(PCM)/TZ//TPSSTPSS/DZ) can accurately describe the copolymerization factor r(e): i.e., the competition of ethene and propene for insertion in metal n-alkyl bonds. Experimental r(e) values were computationally reproduced with a mean average deviation (MAD) and maximum deviation of only 0.2 and 0.5 kcal/mol, respectively. Both dispersion and solvent corrections play a crucial role in achieving this accuracy. Ethene insertion is found to be entropically favored for all catalysts due to a combination of symmetry factors and less congested insertion geometries. The enthalpic preference for either ethene or propene is catalyst dependent. The predictions are based on straightforward calculation of relevant insertion transition state energies; there are no indications for a shift in rate-limiting step from insertion to e.g. olefin capture or chain rotation.

The reaction of phenazine with standard aluminum alkyls AlR3 (R = Me (a), Et (b), Et/Cl (c), i-Bu (d)) afforded, after initial coordination and formation of [AlMe3]2(μ-η1:η1-C12H8N2) (1), a further slow and complex transformation. As a result, the dinuclear and diamagnetic [AlR2]2(μ-η1:η1-C12H8N2)2 (R = Me (2a), Et (2b), Et/Cl (2c), i-Bu (2d)) were formed in parallel to the paramagnetic and monomeric (AlR3)[η1-C14H13NN(R)] (3). The diamagnetism of 2 is obtained via dimerization and implementation of an efficient intramolecular antiferromagnetic coupling. In the case of the reaction with Et2AlCl, the corresponding (AlEtCl2)[η1-C14H13NN(Et)] (3c) was isolated and fully characterized. In all the other cases, only compelling ESR evidence was obtained for the formation of isostructural species. On the basis of the isolation of both 2c and 3c from the same reaction, a mechanism is proposed involving ligand-assisted homolysis of Al−C bonds.

The formally Co(0) complex LCo(N2) (L =2,6-bis(2,6-dimethylphenyliminoethyl)pyridine) can be prepared via either Na/Hg reduction of LCoCl2 or hydrogenolysis of LCoCH2SiMe3. In the latter reaction, LCoH could be trapped by reaction with NCC6H4-4-Cl to give LCoN=CHC6H4-4-Cl. LCo(N2) reacts with many alkyl and aryl halides RX, including aryl chlorides, to give a mixture of LCoR and LCoX in a halogen atom abstraction mechanism. Intermediacy of free alkyl and aryl radicals is confirmed by the ring-opening of cyclopropylmethyl to crotyl, and the rearrangement of 2,4,6-tBu3C6H2 to 3,5-tBu2C6H3CMe2CH2, before binding to Co. The organocobalt species generated in this way react further with activated halides R′X (alkyl iodides; allyl and benzyl halides) to give cross-coupling products RR′ in what is most likely again a halogen abstraction mechanism. DFT studies support the proposed radical pathways for both steps. MeI couples smoothly with LCoCH2SiMe3 to give LCoI and CH3CH2SiMe3, but the analogous reaction of tBuI leads in part to radical attack at the 3 and 4 positions of the pyridine ring to form (tBu2-L)CoI and (tBu2-L)CoI2.

Reaction of [Cp2Ti(Me)(CD2Cl2)]+ (I) with 3,3-dimethyl-1-butene in CD2Cl2 at 205 K produces the α-agostic insertion product [Cp2TiCH2CHMetBu]+ (II), in which the chirality at C(2) induces preferential agostic binding of one of the diastereotopic α-H atoms; subsequent coordination of ethyl ether to II leaves the α-agostic interaction weakened but intact. On warming, II undergoes β-hydrogen elimination and the resulting hydride reacts further with excess 3,3-dimethyl-1-butene to form the β-agostic species [Cp2TiCH2CH2tBu]+. NMR data and calculated (DFT) energies support the assignments.

The adsorption of TiCl4 on the surfaces of MgCl2 crystals has been investigated by means of state-of-the-art periodic hybrid DFT methods, as the first step of a comprehensive study aiming to elucidate the structure of the active species in industrial MgCl2-supported Ziegler-Natta catalysts for ethene and propene polymerization. A first distinctive feature of the approach was the thorough evaluation of dispersion forces, crucial because the binding of TiCl4 on MgCl2 surfaces turned out to be essentially dispersion-driven. Also important was a careful investigation of the effects of different choices on basis set and density functional (DF) on the quantitative aspects of the results; this allowed us to trace the unusually large disagreement in the previous literature and identify unambiguous trends. In particular, three full sets of calculations were run adopting the B3LYP(-D), PBE0(-D) and M06 approximations: to the best of our knowledge, the last represents the first case of M06 functional implementation in a periodic code (CRYSTAL) of widespread use. The results consistently indicated that the adsorption of TiCl4 on well-formed MgCl2 crystals under conditions relevant for catalysis can only occur on MgCl2(1 1 0) or equivalent lateral faces, whereas the interaction with MgCl2(1 0 4) - for decades claimed as the most important surface in stereoselective catalysts - is too weak for the formation of stable adducts. The implications of these findings for catalysis are discussed. (C) 2011 Elsevier Inc. All rights reserved.

Reduction of the tetranuclear {((t-Bu)NPPh2)- Cr(μ-(t-Bu)NPPh2)2Cr}2(μ-Cl)2 (1) with either KC8 or vinyl Grignard afforded the dinuclear, mixed-valence (Me3P)Cr(μ- (t-Bu)NPPh2)3Cr (2) with the two metals possessing distinctively different coordination environments. According to the formulation of 2 as Cr(I)/Cr(II) mixed-valence species, 2 acts as a self-activating catalyst, producing under pressure of ethylene a mixture of 1-butene and 1-hexene. Activation with three different activators selectively produced three different products, namely, 1-butene with TEAL, 1-hexene with DMAO/TEAL, and LAO-free highly linear HMWPE with DMAO. Mixtures of 1-hexene and 1-butene were also obtained upon activation with vinyl Grignard. In this case it was possible to isolate, albeit in very low yield, an intriguing butadiene/butadiene-diyl cluster, {((η 4 -butadiene)Cr (μ,η 4 -butadienediyl)(μ- NP)Mg)2(μ-Cl)4Mg(THF)2}{((THF)3Mg)2(μ-Cl)3}2 (3), which is also a highly selective self-activating trimerization catalyst.

The activity of two bis(phenoxy-amine)ZrR2 precatalysts (bis(phenoxy-amine) = N,N′-(3-tBu-5-OMe-2-C6H2OCH2)2-N,N′-Me2-(NCH2CH2N); R = Me (1), Bn (2, benzyl)) toward propene polymerization has been evaluated using different activators and cocatalysts: MAO, MAO/TBP, B(C6F5)3/TIBA, and [CPh3][B(C6F5)4]/TIBA (MAO = methylalumoxane, TBP = 2,6-di-tert-butylphenol, TIBA = triisobutylaluminum). It was found that the nature of the activator affects the activity only to a small extent. NMR studies in solution and DFT calculations on the 3a–c and 4a–c (a, MeB(C6F5)3–; b, BnB(C6F5)3–; c, B(C6F5)4–) ion pairs deriving from the activation processes of 1 and 2, respectively, showed that three isomers can form. All of them have the anion in the second coordination sphere, whereas the binding modality of the ligand leads to the mer-mer most stable isomer, fac-mer isomer of intermediate stability, and fac-fac least stable isomer. Notably, the energy of the fac-fac isomer, which is supposed to be the active species in the p...

Reaction of decamethylmetallocene cations [Cp-2*M](+) (M = Sc, Ti, V) with acetone and benzophenone resulted in the formation of the corresponding acetone adducts [Cp-2*M(OCMe2)(n)](+) (M = Sc, n = 2; M = Ti, n = 1; M = V, n = 1) and benzophenone adducts [Cp-2*M(OCPh)](+). The stoichiometry of these adducts is determined by both the electronic configuration of the metal center as well as steric pressure imparted by the large Cp*-ligands. In addition, the M-O-C angle is controlled by the number of free valence orbitals of the Cp-2*M unit. (C) 2011 Elsevier B.V. All rights reserved.

A detailed DFT study of Pd(PMe3)/NMe3-catalyzed butadiene telomerization of methanol predicts that the rate- and selectivity-determining step of the catalytic cycle is the external nucleophilic att...

The compound Cp(2)TiMe(2) reacts with [Ph(3)C][B(C(6)F(5))(4)] in CD(2)Cl(2) at 205 K to give, inter alia, [Cp(2)TiMe(CD(2)Cl(2))][B(C(6)F(5))(4)]. This solvent-separated ion pair reacts in turn with 2,4-dimethyl-1-pentene (DMP) to give a series of cationic species, the first being the alkene complex [Cp(2)TiMe(DMP)](+), which undergoes ready migratory insertion to form the σ-alkyl complex [Cp(2)Ti(CH(2)CMe(2)CH(2)CHMe(2))](+). The latter, which does not contain a β-hydrogen atom, rearranges rapidly via an unprecedented 1,5-σ bond metathesis reaction involving two isomeric ε-agostic species to give the σ-alkyl species [Cp(2)Ti(CH(2)CHMeCH(2)CMe(3))](+); this does contain a β-hydrogen atom and, in concurrent processes, eliminates H(2) or 2,4,4-trimethyl-1-pentene (a major product) to form respectively the allylic complex [Cp(2)Ti{η(3)-(CH(2))(2)CCH(2)CMe(3)}](+) (a major product) or the hydride complex [Cp(2)TiH](+). The latter reacts reversibly with free DMP to give the insertion product [Cp(2)Ti(CH(2)CHMeCH(2)CHMe(2))](+) (V, a major product), in which the italicized hydrogen atom engages in a β-agostic interaction with the metal atom. Compound V is a rare example of both a β-agostic derivative of a group 4 metallocene and a β-agostic compound of any metal in which the (1)H resonance of the agostic hydrogen can be identified in the (1)H NMR spectrum (δ -3.43). Interestingly, a NOESY experiment on V indicates slow mutual exchange between the agostic hydrogen atom, the hydrogen atoms on C(1), and those of Me(2). These observations are consistent with the intermediacy of the allylic dihydrogen species [Cp(2)Ti(H(2)){η(3)-(CH(2))(2)CCH(2)CHMe(2)}](+), which loses H(2) to form [Cp(2)Ti{η(3)-(CH(2))(2)CCH(2)CHMe(2)}](+) (a minor product). Support for all steps of the proposed reaction scheme comes from product distributions, from labeling studies utilizing [Cp(2)Ti(CD(3))(CD(2)Cl(2))](+), and from extensive DFT calculations. The observed titanocene-based chemistry stands in stark contrast to that of the analogous zirconium system, in which the unusual but well-characterized cationic methyl alkene complex [Cp(2)ZrMe(DMP)](+) does not undergo migratory insertion and subsequent reactions.

(Py)2CoR2 (R = CH2SiMe3) is easily prepared from (Py)4CoCl2 and RLi. It is fairly stable at room temperature and serves as a convenient source of CoR2 for transfer to other ligands. Unfortunately, (Py)2CoR2 was obtained only as an oil, but the structure of the related complex (Py)2CoR′2 (R′ = CH2CMe2Ph) could be confirmed by a single-crystal X-ray diffraction study. Transfer of the CoR2 fragment from (Py)2CoR2 or (TMEDA)CoR2 to diiminepyridine-type ligands (1−6) was studied as a function of ligand steric and electronic properties. Reaction with N-2,6-dimethylphenyl (1) and N-2,4,6-trimethylphenyl (2) ligands produced diamagnetic monoalkyl complexes; the structure of (1)CoR was confirmed by X-ray diffraction. With the less shielding N-phenyl (3) and N-benzyl (4) ligands, 1H NMR indicated formation of diamagnetic CoI alkyl species, but they were not stable enough to allow isolation. Fluorinated ligand 5 appears to be less reactive and−despite its supposedly stronger π-acceptor character−also does not lead t...

Most of the commonly used approximate density functionals have systematic errors in the description of the stability of hydrocarbons. This poses a challenge for the realistic modeling of reactions involving hydrocarbons, such as olefin polymerization. Practical remedies have been proposed, including the application to usual black-box DFT of additional empirical correction CR(-6) terms for the van der Waals interaction (termed DFT-D), or introducing additional pseudopotentials that introduce some medium-to-long-range attraction (C-Pot). In this Article, we use the DFT-D scheme as realized in our BOptimize package to evaluate the performance of a range of commonly used DFT functionals (combinations of xPBE, B88, OPTX with LYP and cPBE GGAs and hybrids) for the modeling of the thermodynamics of reactions of the growth of common polyolefins. We also review and reproduce some of the previously done benchmarks in the area: alkane branching and relative stability of C12H12 and C10H16 isomers. In addition to the common DFT methods, computations with correlated wave function methods (MP2) and the new functionals B97-D and M06-L were performed. The performance of the special density functionals B97-D and M06-L is, in general, similar to the best DFT-D corrected regular functionals (BPBE-D and PBE-D). The results show that (1) the DFT-D correction is sufficient to describe alkane branching, but its performance depends on the parametrization; (2) inclusion of the correction is essential for a proper description of the thermodynamics of reactions of polymer growth; and (3) not all approximate density functionals perform effectively for the description of hydrocarbons even with the correction. The C-Pot method for the B3LYP functional shows quantitatively correct results for our test cases. The enthalpies of hydrocarbon reactions were analyzed in terms of the repulsion characteristics of a given DFT method. PBE is the least repulsive, while OLYP is the most. However, there are cases where the failure of a DFT method cannot be correlated with its repulsive character. A striking example is the performance of B3LYP and BLYP for caged molecules with small carbocycles, such as the [D3d]-octahedrane. The stability of [D3d]-octahedrane is underestimated by the B3LYP, BLYP, and B97-D functionals, but not by DFT methods that contain either B88 exchange or LYP correlation functionals separately. While DFT-D cannot amend the performance of the former functionals for the octahedrane, C-Pot for B3LYP does.

Reduction of the bis(iminopyridine) FeCl(2) complex {2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}FeCl(2) using NaH has led to the formation of a surprising variety of structures depending on the amount of reductant. Some of the species reported in this work were isolated from the same reaction mixture, and their structures suggest the presence of multiple pathways for dinitrogen activation. The reaction with 3 equiv of NaH afforded {2-[2,6-(iPr)(2)PhN=C(CH(3))]-6-[2,6-(iPr)(20PhN-C=CH(2)](C(5)H(3)N)}Fe(micro,eta(2)-N(2))Na (THF) (1) containing one N(2) unit terminally bound to Fe and side-on attached to the Na atom. In the process, one of the two imine methyl groups has been deprotonated, transforming the neutral ligand into the corresponding monoanionic version. When 4 equiv were employed, two other dinitrogen complexes {2-[2,6-(iPr)(2)PhN=C(CH(3))]-6-[2,6-(iPr)(2)PhN-C=CH(2)](C(5)H(3)N)}Fe(micro-N2)Na(Et(2)O)(3) (2) and {2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}Fe(micro-N(2))Na[Na(THF)(2)] (3) were obtained from the same reaction mixture. Complex 2 is chemically equivalent to 1, the different degree of solvation of the alkali cation being the factor apparently responsible for the sigma-bonding mode of ligation of the N(2) unit to Na, versus the pi-bonding mode featured in 1. In complex 3, the ligand remains neutral but a larger extent of reduction has been obtained, as indicated by the presence of two Na atoms in the structure. A further increase in the amount of reductant (12 equiv) afforded a mixture of {2-[2,6-(iPr)(2)PhN=C(CH(3))]-6-[2,6-(iPr)(2)PhN-C=CH(2)](C(5)H(3)N)}Fe-N(2) (4) and [{2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}Fe-N(2)](2)(micro-Na) [Na(THF)(2)](2) (5) which were isolated by fractional crystallization. Complex 4, also containing a terminally bonded N(2) unit and a deprotonated anionic ligand bearing no Na cations, appears to be the precursor of 1. The apparent contradiction that excess NaH is required for its successful isolation (4 is the least reduced complex of this series) is most likely explained by the formation of the partner product 5, which may tentatively be regarded as the result of aggregation between 1 and 3 (with the ligand system in its neutral form). Finally, reduction carried out in the presence of additional free ligand afforded {2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}Fe(eta(1)-N(2)){2,6-[2,6-(iPr)(2)PhN=C(CH(3))](20(NC(5)H(2))}[Na(THF)(2)] (6) and {2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(C(5)H(3)N)}Fe{2,6-[2,6-(iPr)(2)PhN=C(CH(3))](2)(NC(5)H(2))}Na(THF)(2)) (7). In both species, the Fe metal is bonded to the pyridine ring para position of an additional (L)Na unit. Complex 6 chemically differs from 7 (the major component) only for the presence of an end-on coordinated N(2).