Outer-sphere Ion Pairs Research Articles

Benchtop-Stable Carbyl Iminopyridyl NiII Complexes for Olefin Polymerization.

Design of catalysts for Ni-catalyzed olefin polymerization predominantly focuses on ligand design rather than the activation process when attempting to achieve a broader scope of polyolefin micro- and macrostructures. Air-stable alkyl-or aryl-functionalized NiII precatalysts were designed which eliminate the need of in situ alkylating processes and are activated solely by halide abstraction to generate the cationic complex for olefin polymerization. These complexes represent an emerging class of olefin polymerization catalysts, enabling the study of various cocatalysts forming either inner- or outer-sphere ion pairs. It is demonstrated that an organoboron cocatalyst activation produces a well-defined ion pair, which in contrast to ill-defined organoaluminum cocatalysts, can directly activate the complex by halide abstraction to yield comparatively higher molecular weight homo/copolymers. Under high ethylene pressure, broader branching densities and the gradual incorporation of short-chain branches were achieved, circumventing the need for elaborate ligand design and copolymerization with α-olefins. The underlying chain-walking mechanism and ion pair interactions were further elucidated by DFT calculations. A phenyl group on the bridging carbon functioned as a rotational barrier, producing higher molecular weight polymers compared to methyl-substituted analogs. Here, we provide a perspective to manipulate the iminopyridyl NiII system, leveraging ion pair interactions and ligand design to govern polyolefin molecular weights and microstructures.

Diffusion Nuclear Magnetic Resonance Measurements on Cationic Gold (I) Complexes in Catalytic Conditions: Counterion and Solvent Effects.

The amount of free ions, ion pairs, and higher aggregate of the possible species present in a solution during the gold(I)-catalyzed alkoxylation of unsaturated hydrocarbon, i.e., ISIP (inner sphere ion pair) [(NHC)AuX] and OSIP (outer sphere ion pairs) [(NHC)Au(TME)X] [NHC 1,3-bis(2,6-di-isopropylphenyl)-imidazol-2-ylidene; TME = tetramethylethylene (2,3-bis methyl-butene); X- = Cl-, BF4-, OTf-; and OTs- BArF4- (ArF = 3,5-(CF3)2C6H3)], has been determined. The 1H and 19F DOSY NMR measurements conducted in catalytic conditions indicate that the dissociation degree (α) of the equilibrium ion pair/free ions {[(NHC)Au(TME)X] [(NHC)Au(TME)]+ + X-} depends on the nature of the counterion (X-) when chloroform is the catalytic solvent: while the compounds containing OTs- and OTf- as the counterion gave a low α (which means a high number of ion pairs) of 0.13 and 0.24, respectively, the compounds containing BF4- and BArF4- showed higher α values of 0.36 and 0.32, respectively. These results experimentally confirm previous deductions based on catalytic and theoretical data: the lower the α value, the greater the catalytic activity because the anion that can activate methanol during a nucleophilic attack, although the lower propensity to activate methanol of BF4- and BArF4-, as suggested by the DFT calculations, cannot be completely overlooked. As for the effect of the solvent, α increases as the dielectric constant increases, as expected, and in particular, green solvents with high dielectric constants show a very high α (0.90, 0.84, 0.80, and 0.70 for propylene carbonate, γ-valerolactone, acetone, and methanol, respectively), thus confirming that the moderately high activity of NHC-Au-OTf in these solvents is due to the specific effect of polar functionalities (O-H, C=O, O-R) in activating methanol. Finally, the DOSY measurements conducted in p-Cymene show the formation of quadrupole species: under these conditions, the anion can better exercise its 'template' and 'activating' roles, giving the highest TOF.

Cages versus Sheets: A Critical Comparison in the Size Range Expected for Methylaluminoxane (MAO).

New cage models (MeAlO)n (Me3 Al)m (n=16, m=6 or 7) isomeric with previously reported sheet models for the principle activator found in hydrolytic MAO (h-MAO) are compared at M06-2X and MN15 levels of theory using density functional theory with respect to their thermodynamic stability. Reactivity of the neutrals or corresponding anions with formula [(MeAlO)16 (Me3 Al)6 Me]- towards chlorination, and loss of Me3 Al is explored while reactivity of the neutrals towards formation of contact- and outer-sphere ion pairs from Cp2 ZrMe2 and Cp2 ZrMeCl is examined. The results suggest on balance that a cage model for this activator is less consistent with experiment than an isomeric sheet model, although the latter are more stable based on free energy.

Evidence for Oxonium Ions in Ethereal "Hydrogen Chloride".

Although solutions of hydrogen chloride in ethereal solvents like diethyl ether or dioxane are commonly employed in the laboratory, the solution structure of these reagents has yet to be firmly established. Here, we analyze solutions of ethereal hydrogen chloride or deuterium chloride in toluene, in dichloromethane, or in the absence of a co-solvent by in situ infrared spectroscopy. The resulting spectra are inconsistent with free HCl or often-proposed 1:1 HCl-ether complexes but closely match the predicted spectra of oxonium ions generated via protonation of diethyl ether. Molecular dynamics simulation of the oxonium chloride complexes provides evidence for an outer-sphere contact ion pair. These results suggest new approaches for tuning the acidity of strong Brønsted acids in organic solvents and demonstrate the importance of conducting spectroscopic measurements under reaction-relevant conditions.

Ionization of Cp2 ZrMe2 and Lewis Bases by Methylaluminoxane: Computational Insights.

The interactions of the Lewis bases CO, octamethyltrisiloxane (OMTS) and 2,2'-bipyridine (bipy) with a sheet model for the principal activator (MeAlO)16 (Me3 Al)6 (16,6) in hydrolytic methylaluminoxane (MAO) were investigated by DFT. These studies reveal that OMTS and bipy form adducts with Me3 Al prior to methide abstraction by 16,6 to form the ion-pairs [Me2 Al(κ2 -L)][16,6] (5: L=OMTS, 6: L=bipy, [16,6]- =[(MeAlO)16 (Me3 Al)6 Me]- ) while CO simply binds to a reactive edge site without ionization. The binding and activation of Cp2 ZrMe2 with 16,6 to form both neutral adducts 1 Cp2 ZrMe2 ⋅16,6 and contact ion-pairs 4 and 7, both with formula [Cp2 ZrMe][μ-Me(MeAlO)16 (Me3 Al)6 ], featuring terminal and chelated MAO-anions, respectively was studied by DFT. The displacement of the anion with either excess Cp2 ZrMe2 or Me3 Al was also studied, forming outer-sphere ion-pairs [(Cp2 ZrMe)2 μ-Me][16,6] (2) and [Cp2 Zr(μ-Me)2 AlMe2 ][16,6] (3). The theoretical NMR spectra of these species were compared to experimental spectra of MAO and Cp2 ZrMe2 and found to be in good agreement with the reported data and assignments. These studies confirm that 16,6 is a very suitable model for the activators present in MAO but highlight the difficulty in accurately calculating thermodynamic quantities for molecules in this size regime.

Theoretical Elucidation of the Effect of Counteranions on the Olefin Polymerization Activity of (Pyridylamido)Hf(IV) Catalyst by QM and REMD Studies: MeB(C6F5)3– versus B(C6F5)4–

1-Octene insertion reaction by the cationic active species of (pyridylamido)Hf(IV) catalyst (HfCat+) was investigated, focusing on the effect of the counteranions (CAs), MeB(C6F5)3– and B(C6F5)4– on catalyst activity. Quantum mechanical (QM) calculation showed a remarkable difference in TS structures that HfCat+–MeB(C6F5)3– complex forms an “inner-sphere” ion pair (ISIP), while HfCat+–B(C6F5)4– forms an “outer-sphere” ion pair (OSIP). However, the activation free energies of 1-octene insertion by both complexes are almost identical to each other. Meanwhile, replica exchange molecular dynamics (REMD) calculation revealed that the time duration of the HfCat+ capturing 1-octene monomers throughout the REMD trajectories of HfCat+–B(C6F5)4– complex was ∼2.5 times that of HfCat+–MeB(C6F5)3–. This is because B(C6F5)4– is more likely to dissociate from HfCat+ to form an OSIP than MeB(C6F5)3– and thus allows monomers to approach HfCat+ more easily. Using these microscopic data, we have numerically evaluated the 1-octene polymerization reaction rate constants and succeeded in qualitatively reproducing the experimentally observed tendency of the polymerization reaction with B(C6F5)4– faster than with MeB(C6F5)3–. Finally, it is theoretically elucidated that the monomer capture process before the monomer insertion process determines the overall polymerization reaction rates in this catalytic system.

On the Hydration of Heavy Rare Earth Ions: Ho3+, Er3+, Tm3+, Yb3+ and Lu3+-A Raman Study.

Raman spectra of aqueous Ho3+, Er3+, Tm3+, Yb3+, and Lu3+-perchlorate solutions were measured over a large wavenumber range from 50–4180 cm−1. In the low wavenumber range (terahertz region), strongly polarized Raman bands were detected at 387 cm−1, 389 cm−1, 391 cm−1, 394 cm−1, and 396 cm−1, respectively, which are fairly broad (full widths at half height at ~52 cm−1). These isotropic Raman bands were assigned to the breathing modes, ν1 Ln–O of the heavy rare earth (HRE) octaaqua ions, [Ln(H2O)8]3+. The strong polarization of these bands (depolarization degree ~0) reveals their totally symmetric character. The vibrational isotope effect was measured in Yb(ClO4)3 solutions in H2O and D2O and the shift of the ν1 mode in changing from H2O to D2O further supports the character of the band. The Ln–O bond distances of these HRE ions (Ho3+, Er3+, Tm3+, Yb3+, and Lu3+) follow the order of Ho–O > Er–O > Tm–O > Yb–O > Lu–O which correlates inversely with the band positions of the breathing modes of their corresponding octaaqua ions [Ln(OH2)8]3+. Furthermore, the force constants, kLn–O, were calculated for these symmetric stretching modes. Ytterbium perchlorate solutions were measured over a broad concentration range, from 0.240 mol·L−1 to 2.423 mol·L−1, and it was shown that with increasing solute concentration outer-sphere ion pairs and contact ion pairs were formed. At the dilute solution state (~0.3 mol·L−1), the fully hydrated ions [Yb(H2O)8]3+ exist, while at higher concentrations (CT > 2 mol·L−1), ion pairs are formed. The concentration behavior of Yb(ClO4)3 (aq) shows similar behavior to the one observed for La(ClO4)3(aq), Ce(ClO4)3(aq) and Lu(ClO4)3(aq) solutions. In ytterbium chloride solutions in water and heavy water, representative for the behavior of the other HRE ions, 1:1 chloro-complex formation was detected over the concentration range from 0.422–3.224 mol·L−1. The 1:1 chloro-complex in YbCl3(aq) is very weak, diminishing rapidly with dilution and vanishing at a concentration < 0.4 mol·L−1.

N,N′-dimethyl-N,N′-dicyclohexylsuccinamide: A novel molecule for the separation and recovery of Pd(II) by liquid-liquid extraction

Abstract N,N′-dimethyl-N,N′-dicyclohexylsuccinamide (DMDCHSA) is investigated as a potential molecule for the liquid-liquid extraction of Pd(II) from chloride solutions for the first time. The effect of several parameters on Pd(II) extraction, such as the contact period between both phases, hydrochloric acid, extractant and hydrogen ion concentrations, is evaluated. Pd(II) extraction equilibrium is very fast (30 s) and the extraction percentage (%E) increases with the HCl concentration in the aqueous phases, being higher than 60% for [HCl] > 5 M. The loading capacity of DMDCHSA for Pd(II) is reasonable (molar ratio extractant/metal higher than 16). Several stripping agents (e.g. distilled water, 1 M HCl, seawater and 20 g/L chloride solution as NaCl) were successfully used to transfer Pd(II) to a new aqueous phase, and data obtained from five successive extraction-stripping cycles suggest a good DMDCHSA stability pattern. Attempts to replace 1,2-dichloroethane (1,2-DCE) by commercial and more environmentally friendly diluents showed much worse %E for Pd(II). Selectivity tests with binary, ternary and more complex metal ion solutions were carried out to evaluate the performance of DMDCHSA towards Pd(II) recovery from 6 M HCl, when in presence of Pt(IV), Fe(III), Zn(II), Al(III) and Ce(III), metal ions usually present in solutions that may result from the hydrometallurgical treatment of spent automobile catalytic converters. It was generally observed that the additional metal ions do not affect the recovery of Pd(II) by DMDCHSA, although Fe(III) and Pt(IV) were co-extracted in a great extent. A solvent extraction (SX) scheme is proposed, based on a previous separation of Fe(III) with tributylphosphate (TBP) and on the selective and sequential stripping of Pt(IV) and Pd(II) from the loaded DMDCHSA with 0.01 M thiourea in 0.5 M HCl and seawater, respectively. The dependence of the Pd(II) distribution ratios on DMDCHSA and acidity, complemented with UV–Visible spectroscopy data, points out to DMDCHSA:Pd(II) extracted species with a 2:1 molar ratio and suggests the occurrence of an outer-sphere ion pair reaction, in which both [PdCl4]2− and HCl are extracted.

Properties of lithium under hydrothermal conditions revealed by in situ Raman spectroscopic characterization of Li2O-SO3-H2O (D2O) systems at temperatures up to 420 °C

Lithium (Li) is an important component of hydrothermal fluids, especially submarine hydrothermal fluids. Investigation of the species and ion complexation of Li+ at elevated temperature and pressure can improve our knowledge on the behavior of Li under hydrothermal conditions. In this study, in situ optical and Raman spectroscopic experiments were conducted on the Li2SO4-H2SO4-H2O system and its D2O analogue at temperatures≤420°C. An unexpected liquid-liquid phase separation (immiscibility) was observed at temperatures above 336.5°C; the aqueous phase was separated into a sulfate-rich heavy liquid phase and a sulfate-poor light liquid phase at vapor-saturation pressures. The liquid-liquid phase separation temperature decreased as the Li2SO4 concentration increased in dilute solutions (≤1.25 m), increased as the Li2SO4 concentration increased in concentrated solutions (>1.25 m), and exhibited a lower critical solution temperature (LCST) at ~336.5°C. The presence of excess H2SO4 (or D2SO4) increased the liquid-liquid phase separation temperature at a constant Li2SO4 concentration. Liquid-liquid phase separation is common in organic-bearing solutions, and LCST is considered to be a macro-scale property of polymer solutions, indicating complicated ion pairing between Li+ and SO42− at high temperature. In situ Raman spectra of the v1(SO42−) band indicated the presence of “free” SO42−, LiSO4−, possible Li2SO40 and other poly-ion associations. The Li+-SO42− association increased with increasing temperature at a constant Li2SO4 concentration. These results indicate that Li+ can form contact ion pairs with ligands despite its strong hydration tendency.In submarine hydrothermal systems, various contact ion pairs should be important Li species during the high-temperature leaching of Li from basalts (e.g., >200°C). The formation of strong and various Li+-SO42− pairs can enhance the leaching of Li because the mobility and diffusion of the Li+-SO42− complex has been reported to be stronger than that of “free” ions. During the low-temperature (e.g., <150°C) uptake of Li+ into secondary minerals (e.g., clays), free Li+ and outer-sphere ion pairs are the dominant Li+ species. Contact ion pairs are also important Li species during the hydrothermal alteration/precipitation of Li-rich minerals in pegmatite systems. Moreover, liquid-liquid phase separation was found to play important roles in the formation of low-temperature minerals (e.g., calcite). However, similar crystallization pathways have rarely been documented for inorganic systems under high temperatures. Anhydrous Li2SO4 was observed to precipitate within the dense liquid phase at ≥360°C in 1.5 m Li2SO4, confirming that the immiscible dense liquid phase can serve as a precursor for the crystallization of sulfates and other minerals under hydrothermal conditions. The possible processes for the dense-liquid-based crystallization by particle attachment can be summarized as (1) strong and reversible ion association induces liquid-liquid phase separation, (2) nucleation and crystallization within the dense liquid phase, and (3) continued growth on the surface of early formed solid phase. Submarine hydrothermal fluids often contain low-dielectric components such as methane and other organic components, which lower the dielectric constant of hydrothermal fluids and promote ion association. Therefore, liquid-liquid phase separation may occur in submarine hydrothermal fluids and can play important roles in the precipitation of sulfates and other minerals.

Conformationally dynamic titanium and zirconium cationic complexes of bis(naphthoxy)pyridine ligands: structure, "oscillation" and olefin polymerization catalysis.

Discrete ionic complexes {ONOSiMe2tBu}M(CH2Ph)((η6-Ph)CH2B(C6F5)3) (M = Ti, 2-Ti; Zr, 2-Zr) have been prepared from the parent neutral dibenzyl precursors and B(C6F5)3. Also, a neutral dichloro complex {ONOSiMe2tBu}ZrCl2(Me2NH) (3) was synthesized by the reaction of a proligand {ONOSiMe2tBu}H2 (1-H2) with (Me2N)2ZrCl2(DME). The compounds were characterized by NMR spectroscopy and X-ray crystallography (for 2-Zr and 3). In the solid state, both 2-Zr and 3 feature meso-like binding of the ligand. VT NMR studies of 2-Ti and 2-Zr revealed that the former species dissociates in solution to form an outer-sphere ion pair (OSIP) that features a complicated dynamic behavior, while the latter compound retains an inner-sphere ion pair (ISIP) structure that interconverts between Cs-symmetric (meso-like) and C1-symmetric (rac-like) conformations. The mechanism of this interconversion was assessed by DFT calculations and the corresponding barrier for the straightforward interconversion was calculated ().

Discrete Ionic Complexes of Highly Isoselective Zirconocenes. Solution Dynamics, Trimethylaluminum Adducts, and Implications in Propylene Polymerization

Discrete ionic complexes belonging to two main families of highly isoselective polymerization zirconocene systems, C1-symmetric {Cp/Flu} (a,b) and C2-symmetric {SBI} (c) systems, have been scrutinized. The ion pair reorganization processes for inner-sphere ion pairs (ISIPs) 3a,b-MeB(C6F5)3, 3c-MeB(C6F5)3, and 3c-MeAl(C6F5)3 and for the outer-sphere ion pair (OSIP) AlMe3 adducts [6b,c]+[B(C6F5)4]–, quantified by dynamic NMR analysis, were found to feature lower activation barriers for the {SBI}-based systems in comparison to those for the {Cp/Flu}-based congeners. The higher electrophilic character of the {Cp/Flu}-based cationic systems was corroborated by UV/vis spectroscopy studies coupled with TD-DFT calculations. These fundamental differences between the ionic systems of the two metallocene families are discussed in light of their respective propylene polymerization performances, and reasons for the higher productivity of {SBI} systems in comparison to {Cp-Flu} systems are proposed.

Selective Ethylene Trimerization by Titanium Complexes Bearing Phenoxy-Imine Ligands: NMR and EPR Spectroscopic Studies of the Reaction Intermediates

The catalyst systems (FI)TiCl3/MAO (FI = phenoxyimine ligand with an additional aryl–O–CH3 donor) display exceptionally high activity in selective ethylene trimerization. By means of NMR and EPR spectroscopy, the nature of the Ti species formed in the catalyst systems (FI)TiCl3/MAO, (FI)TiCl3/MMAO, and (FI)TiCl3/AlR3/[Ph3C]+[B(C6F5)4]− (R = Me, Et, iBu) has been studied. It was shown that outer-sphere ion pairs of the type [(FI)TiIVMe2]+[A]− ([A]− = [MeMAO]−, [MeMMAO]−, [B(C6F5)4]−) are formed at the initial stage of the reaction of (FI)TiCl3 with MAO, MMAO, and AlMe3/[Ph3C]+[B(C6F5)4]−. These ion pairs further partially convert into TiIII and TiII species. In the systems (FI)TiCl3/MAO and (FI)TiCl3/AlMe3/[Ph3C]+[B(C6F5)4]−, complexes with the proposed structures (FI)TiIIIMe2, (FI)TiIICl, and [(FI)TiII(S)]+[A]− ([A]− = [MeMAO]−, [B(C6F5)4)]−, S = solvent, vacancy) were observed (concentrations of TiIII species was lower than those of the TiII congeners). In contrast, in the system (FI)TiCl3/MMAO, the conc...

Formation of Cationic Intermediates upon the Activation of Bis(imino)pyridine Nickel Catalysts

The activation of bis(imino)pyridine nickel(II) dichloride complexes [LNiCl2] with methylalumoxane (MAO) and AlMe3/[CPh3][B(C6F5)4] was studied by multinuclear NMR spectroscopy. Formation of similar outer-sphere ion pairs of the types [LNiMe]+[MeMAO]− and [LNiMe]+[B(C6F5)4]−, capable of promoting norbornene polymerization, was observed.

Probing the interactions between all components of the catalytic pool for homogeneous olefin polymerisation by diffusion NMR spectroscopy

Diffusion NMR spectroscopy was applied to investigate all individual components and combinations thereof for the Cp2ZrMe2/MAO (DMAO)/TBP (MAO = methylaluminoxane, DMAO = AlMe3 depleted MAO, TBP = 2,6-di-tert-butylphenol) ternary system, selected as a prototypical catalytic pool for homogeneous olefin polymerisation. Both MAO and DMAO were found to self-aggregate in C6D6 with the latter having a higher propensity. TBP reacts with DMAO affording MeAl(2,6-di-tert-butylphenoxide)2 and causing a structural modification of DMAO, whose aggregates become much larger. The actual dimensions and self-aggregation tendency of (D)MAO, which depend on Al concentration and the possible presence of TBP, turned out to carry over to [Cp2Zr(μ-Me)2AlMe2]MeMAO (1) OSIP (outer sphere ion pair) and [Cp2Zr(+)Me···MeMAO(-)] (2) ISIP (inner sphere ion pair) that form upon activation of Cp2ZrMe2. Once the intrinsic self-aggregation tendency of MAO has been subtracted, OSIP 1 and ISIP 2 behave exactly as analogous ion pairs with borate ions: ISIP 2 does not self-aggregate, whereas OSIP 1 exhibits the same self-aggregation trends of zirconocene OSIPs with borate counterions.

Mechanistic Investigations into the Behavior of a Labeled Zirconocene Polymerization Catalyst

Kinetics associated with the [(SBI)Zr(CH2SiMe2(C6H4)NMe2)][MeB(C6F5)3] (1a)-catalyzed polymerization of 1-hexene in a mixed toluene-d8/chlorobenzene-d5 solvent at −33 °C were investigated via 1H NMR and compared to the kinetics associated with the (SBI)ZrMe(MeB(C6F5)3) (1c)-catalyzed polymerization of 1-hexene under identical conditions. In the presence of 1-hexene, both catalysts form an identical propagating species, (SBI)Zr(poly-1-hexyl)(MeB(C6F5)3) (1b), but the concentration of 1b during 1a-catalyzed polymerization is only ca. 40% of the anticipated value. Under reaction conditions, 1b reacts reversibly with the model complex p-TMS-C6H4-NMe2 (2) to yield the outer-sphere ion pair tentatively identified as [(SBI)Zr(poly-1-hexyl)(2)][MeB(C6F5)3] (1e), which acts as an essentially dormant site during 1-hexene polymerization. Warming of 1b in the absence of additives generates the well-defined hydridoborate complex (SBI)ZrMe(HB(C6F5)3) (1d), which does not reinitiate in the presence of 1-hexene. β-Hydrid...

Synthesis, Characterization, Interionic Structure, and Self-Aggregation Tendency of Zirconaaziridinium Salts Bearing Long Alkyl Chains

Zirconaaziridinium inner sphere ion pairs (ISIPs) bearing alkyl chains of different lengths, [Cp2Zr(η2-CH2NR1R2)+···X−] (R1 = Me, R2 = C2H5, X− = MeB(C6F5)3−, ZrNC2BN; R1 = Me, R2 = C16H33, X− = MeB(C6F5)3−, ZrNC16BN; R1 = R2 = C18H37, X− = MeB(C6F5)3−, ZrNC18BN; R1 = Me, R2 = Ph, X− = MeB(C6F5)3−, ZrNPhBN; R1 = R2 = C18H37, X− = B(C6F5)4−, ZrNC18BT; R1 = Me, R2 = Ph, X− = B(C6F5)4−, ZrNPhBT), have been synthesized by the reaction of [Cp2ZrMe+···MeB(C6F5)3−] or [Cp2ZrMe+···B(C6F5)4−] with the suitable tertiary amine followed by selective C−H activation of one Me group of the coordinated amine and methane elimination. Subsequent reaction of ISIPs with THF afforded the corresponding outer-sphere ion pairs (OSIPs) ([ZrNC2THF][BN], [ZrNC16THF][BN], [ZrNC18THF][BN], [ZrNPhTHF][BN], [ZrNC18THF][BT], [ZrNPhTHF][BT]) in which the anion has been displaced into the second coordination sphere. The interionic structure in solution (i.e., the relative cation−anion position) and the self-aggregation level of both ISIPs...

Ions and Hydrogen Bonding in a Hydrophobic Environment: CCl4

It is generally expected that ions in an aqueous ionic solution in contact with a hydrophobic phase enter the hydrophobic phase accompanied by a hydration shell. This expectation suggests that the ion mole fraction in the hydrophobic phase is less than, or at most, equal to that of water. Both gravimetric and spectroscopic evidence shows that for a model hydrophobic phase, carbon tetrachloride, this is not the case: In contact with a 1 M simple salt solution (sodium or potassium halide), the salt concentration in carbon tetrachloride ranges from 1.4 to nearly 3 times that of water. Infrared spectra of the OH stretch region support a model in which water associates with the cation, primarily as water monomers. Salts containing larger, more polarizable anions can form outer-sphere ion pairs that support water dimers, giving rise to a spectral signature at 3440 cm(-1). In CCl(4), the infrared spectral signature of the normally strongly ionized acid HCl clearly shows the presence of molecular HCl. Additionally, the presence of a Q branch for HCl indicates restricted rotational motion. The spectral and gravimetric data provide compelling evidence for ion clusters in the hydrophobic phase, which is a result that may have implications for hydrophobic matter in both biological and environmental systems.

Activation of a bis(phenoxy-amine) precatalyst for olefin polymerisation: first evidence for an outer sphere ion pair with the methylborate counterion

A bis(phenoxy-amine)ZrMe(2) complex (1) was synthesized and activated with B(C(6)F(5))(3) obtaining an ion pair (2) having the MeB(C(6)F(5))(3)(-) counterion in the second coordination sphere as deduced by NMR studies. Both DFT calculations and NMR investigations suggest that the activation is coupled with a change of the binding modality of the tetradentate ligand from cis(N,N)-trans(O,O) in the octahedral dimethyl precursor to 1 cis(N, N)-cis(O,O) in the ion pair 2.

Intra- and Intermolecular NMR Studies on the Activation of Arylcyclometallated Hafnium Pyridyl-Amido Olefin Polymerization Precatalysts

Pyridyl-amido catalysts have emerged recently with great promise for olefin polymerization. Insights into the activation chemistry are presented in an initial attempt to understand the polymerization mechanisms of these important catalysts. The activation of C1-symmetric arylcyclometallated hafnium pyridyl-amido precatalysts, denoted Me2Hf{N(-),N,C(-)} (1, aryl = naphthyl; 2, aryl = phenyl), with both Lewis (B(C6F5)3 and [CPh3][B(C6F5)4]) and Brønsted ([HNR3][B(C6F5)4]) acids is investigated. Reactions of 1 with B(C6F5)3 lead to abstraction of a methyl group and formation of a single inner-sphere diastereoisomeric ion pair [MeHf{N(-),N,C(-)}][MeB(C6F5)3] (3). A 1:1 mixture of the two possible outer-sphere diastereoisomeric ion pairs [MeHf{N(-),N,C(-)}][B(C6F5)4] (4) is obtained when [CPh3][B(C6F5)4] is used. [HNR3][B(C6F5)4] selectively protonates the aryl arm of the tridentate ligand in both precatalysts 1 and 2. A remarkably stable [Me2Hf{N(-),N,C2}][B(C6F5)4] (5) outer-sphere ion pair is formed when the naphthyl substituent is present. The stability is attributed to a hafnium/eta(2)-naphthyl interaction and the release of an eclipsing H-H interaction between naphthyl and pyridine moieties, as evidenced through extensive NMR studies, X-ray single crystal investigation and DFT calculations. When the aryl substituent is phenyl, [Me2Hf{N(-),N,C2}][B(C6F5)4] (10) is originally obtained from protonation of 2, but this species rapidly undergoes remetalation, methane evolution, and amine coordination, giving a diastereomeric mixture of [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (11). This species transforms over time into the trianionic-ligated [Hf{N(-),C(-),N,C(-)}NR3][B(C6F5)4] (12) through activation of a C-H bond of an amido-isopropyl group. In contrast, ion pair 5 does not spontaneously undergo remetalation of the naphthyl moiety; it reacts with NMe2Ph leading to [MeHf{N(-),N}NMe2C6H4][B(C6F5)4] (7) through ortho-metalation of the aniline. Ion pair 7 successively undergoes a complex transformation ultimately leading to [Hf{N(-),C(-),N,C(-)}NMe2Ph][B(C6F5)4] (8), strictly analogous to 12. The reaction of 5 with aliphatic amines leads to the formation of a single diastereomeric ion pair [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (9). These differences in activation chemistry are manifested in the polymerization characteristics of these different precatalyst/cocatalyst combinations. Relatively long induction times are observed for propene polymerizations with the naphthyl precatalyst 1 activated with [HNMe3Ph][B(C6F5)4]. However, no induction time is present when 1 is activated with Lewis acids. Similarly, precatalyst 2 shows no induction period with either Lewis or Brønsted acids. Correlation of the solution behavior of these ion pairs and the polymerization characteristics of these various species provides a basis for an initial picture of the polymerization mechanism of these important catalyst systems.

Synthesis, Reactivity, and Computational Studies of [η5-C5H5-(η5-C5H4CMe2C6H4Me)TiMe]+: Aromatic C−H Bond Activation at −50 °C

The titanocene compound [η5-C5H5-(η5-C5H4CMe2C6H4Me)TiMe2] (1) was prepared from the titanocene dichloride precursor [η5-C5H5-(η5-C5H4CMe2C6H4Me)TiCl2] (1a). The solid-state structures of 1a and 1 were determined and are similar to other titanocene compounds. The reaction of compound 1 with B(C6F5)3 in CD2Cl2 was monitored by NMR spectroscopy. At −60 °C the expected cationic compound [η5-C5H5(η5:η1-C5H4CMe2C6H4Me)TiMe]+ (2) with a coordinated tolyl group was observed. Warming the samples to −50 °C leads to evolution of methane, indicating C−H bond activation of the coordinated tolyl moiety. The so-formed titanacycles form with the anion [MeB(C6F5)3]− the inner sphere ion pair (ISIP) [η5-C5H5-(η5:σ1-C5H4CMe2C6H3Me)Ti-μ-MeB(C6F5)3] (3) and is in exchange with an outer sphere ion pair η5-C5H5-(η5:σ1-C5H4CMe2C6H3Me)Ti]+[MeB(C6F5)3]− (4), with the free coordination site probably occupied by a solvent molecule. The structure of 4 could not be determined unambiguously. The homoleptic bimetallic compound [{η5-C5H5-(η5-C5H4CMe2C6H4Me)Ti}2-μ-Me]+ (5) was prepared by reaction of 2 equiv of 1 with 1 equiv of B(C6F5)3 in CD2Cl2 at −50 °C and monitored by NMR spectroscopy. Detailed density functional theory (DFT) studies of the formation of 3 and 4 from 2 corroborate the NMR results.