Régis M. Gauvin

17O NMR gives unprecedented insights into the structure of supported catalysts and their interaction with the silica carrier.

Flame silica was surface-labeled with (17)O, through isotopic enrichment of both siloxanes and silanols. After heat treatment at 200 and 700 °C under vacuum, the resulting partially dehydroxylated silica materials were investigated by high-field solid-state (1)H and (17)O NMR. More specifically, MQ MAS and HMQC sequences were used to probe the (17)O local environment. In a further step, these (17)O-tagged supports were used for the preparation of supported catalysts by reaction with perhydrocarbyl transition metal derivatives (zirconium tetraalkyl, tantalum trisalkyl-alkylidene, and tungsten trisalkyl-alkylidyne complexes). Detailed (17)O 1D and 2D MQ and HMQC MAS NMR studies demonstrate that signals in the Si-OH, Si-O-Si, and Si-O-metal regions are highly sensitive to local structural modifications, thanks to (17)O wide chemical shift and quadrupolar constant ranges. Experimental results were supported by DFT calculations. From the selective surface labeling, unprecedented information on interactions between supported catalysts and their inorganic carrier has been extracted.

Measurement of Aluminum-Carbon Distances Using S-RESPDOR NMR Experiments

It is demonstrated that reliable aluminum-carbon distances can be measured in samples with (13)C natural abundance by NMR spectroscopy. Overcoupled resonators, with only one radio-frequency synthesizer and one amplifier, are used to irradiate in the same pulse sequence (27)Al and (13)C nuclei, which differ by only 3.6 % in Larmor frequencies. The combination of (27)Al saturation pulse with heteronuclear dipolar recoupling yields dipolar dephasing of the (13)C signal, which only depends on the Al-C distance and the efficiency of the saturation pulse. Therefore, reliable distances can be obtained by rapid fitting of experimental data to an analytical expression. It is demonstrated that with natural isotopic abundance this approach allows recovery of Al-C distances of 216 pm for the covalent bond in lithium tetraalkyl aluminates, commonly used as a co-catalyst in olefin polymerization processes, and which range from 274 to 381 pm for the three carbon atoms in aluminum lactate. The accuracy of the measured internuclear distances is carefully estimated.

Well-Defined Silica-Supported Calcium Reagents : Control of Schlenk Equilibrium by Grafting

Calcium reagents Ca(alpha-Me(3)Si-2-Me(2)N-benzyl)(2) x 2 thf (1) and Ca[N(SiMe(3))(2)](2) x 2 thf (2) reacted with silica partially dehydroxylated at 700 degrees C to afford materials that bear ([triple bond]SiO)Ca(alpha-Me(3)Si-2-Me(2)N-benzyl) x 1.6 thf (SiO(2)-1) and ([triple bond]SiO)Ca [N(SiMe(3))(2)] x 1.3 thf (SiO(2)-2) fragments, respectively. Due to the bulk of the supported complexes, the silanol groups are only partially metalated: 50% in SiO(2)-1 and 70% in SiO(2)-2. In the case of SiO(2)-2, a parallel SiMe(3)-capping side reaction affords in fine a silanol-free surface. The materials were characterized by IR spectroscopy, 1D and 2D solid-state high-field NMR spectroscopy, and elemental analysis. Reaction of 2 with one equivalent of the bulky silanol (tBuO)(3)SiOH, a silica-surface mimic, afforded the homoleptic bis-silyloxide calcium derivative through ligand exchange (Schlenk equilibrium), and a derivative was isolated and structurally characterized. Preliminary studies have shown that both grafted benzyl and amide derivatives are active in olefin hydrosilylation, intramolecular hydroamination, and styrene polymerization, with evidence showing that catalysis occurs through supported species. In styrene polymerization, a marked influence of the surface acting as a ligand on the stereoselectivity of the reaction was observed, as syndiotactic-rich polystyrene (88% of r diads) was obtained. These results illustrate that grafting of calcium benzyl or amide compounds on a silica surface is a new concept to prevent ligand exchange through the Schlenk equilibrium. Heteroleptic calcium complexes that cannot be synthesized as stable molecular species in solution can be obtained as silica-supported species which have been shown to be catalytically active.

Catalytic Conversion of Alcohols into Carboxylic Acid Salts in Water: Scope, Recycling, and Mechanistic Insights

The catalytic conversion of alcohols into carboxylic acid salts in water was performed in the presence of ruthenium complexes supported by aliphatic PNP pincer ligands preformed or formed in situ. High activity toward a wide substrate scope was achieved with turnover number values of up to 4000. The air-stable catalytic system can be recycled by using toluene as a catalyst-immobilizing phase; the activity is maintained after five consecutive runs. Finally, mechanistic studies allowed some fundamental aspects related to water activation to be unveiled and to the mechanism postulated.

On the track to silica-supported tungsten oxo metathesis catalysts: input from 17O solid-state NMR.

The grafting of an oxo chloro trisalkyl tungsten derivative on silica dehydroxylated at 700 °C was studied by several techniques that showed reaction via W-Cl cleavage, to afford a well-defined precatalyst for alkene metathesis. This was further confirmed by DFT calculations on the grafting process. (17)O labeling of the oxo moiety of a series of related molecular and supported tungsten oxo derivatives was achieved, and the corresponding (17)O MAS NMR spectra were recorded. Combined experimental and theoretical NMR studies yielded information on the local structure of the surface species. Assessment of the (17)O NMR parameters also confirmed the nature of the grafting pathway by ruling out other possible grafting schemes, thanks to highly characteristic anisotropic features arising from the quadrupolar and chemical shift interactions.

Yttrium catalysts for syndioselective β-butyrolactone polymerization: on the origin of ligand-induced stereoselectivity

Synthesis of aliphatic polyesters has been studied intensively due to their biocompatible and biodegradable properties and their applications in medical and agricultural fields. Among them, polyhydroxybutyrate (PHB), which is a naturally occurring polymer, is synthesized by a variety of bacteria and algae. However, this polymer has poor mechanical properties due to its brittleness. Herein is presented a practical route to a highly syndiotactic PHB by way of a one-pot reaction. We report the results of a comprehensive investigation of the newly discovered stereoselective and controlled polymerization of racemic β-butyrolactone (rac-BBL) using an initiator prepared in situ from yttrium(III) isopropoxide Y(OiPr)3, and a bisphenoxide ligand. DFT studies on the alkoxide catalyst reveal that the R and S monomers are almost equivalent for the first ring-opening reaction. The selectivity of the next insertion was scrutinized, demonstrating that the polymerization process is predicted to be stationary (back-side insertion).

Heteronuclear NMR spectroscopy as a surface-selective technique: a unique look at the hydroxyl groups of γ-alumina.

The surface hydroxyl groups of γ-alumina dehydroxylated at 500 °C were studied by a combination of one- and two-dimensional homo- and heteronuclear (1)H and (27)Al NMR spectroscopy at high magnetic field. In particular, by harnessing (1)H-(27) Al dipolar interactions, a high selectivity was achieved in unveiling the topology of the alumina surface. The terminal versus bridging character of the hydroxyl groups observed in the (1)H magic-angle spinning (MAS) NMR spectrum was demonstrated thanks to (1)H-(27) Al RESPDOR (resonance-echo saturation-pulse double-resonance). In a further step the hydroxyl groups were assigned to their aluminium neighbours thanks to a {(1)H}-(27) Al dipolar heteronuclear multiple quantum correlation (D-HMQC), which was used to establish a first coordination map. Then, in combination with (1)H-(1) H double quantum (DQ) MAS, these elements helped to reveal intimate structural features of the surface hydroxyls. Finally, the nature of a peculiar reactive hydroxyl group was demonstrated following this methodology in the case of CO2 reactivity with alumina.

The D-HMQC MAS-NMR Technique: An Efficient Tool for the Editing of Through-Space Correlation Spectra Between Quadrupolar and Spin-1/2 (31P, 29Si, 1H, 13C) Nuclei

The D-HMQC (dipolar heteronuclear multiple-quantum coherence) technique is a recently developed NMR pulse sequence particularly suitable for the investigation of spatial proximity between quadrupolar and spin-1/2 nuclei. Compared to the cross-polarisation magic-angle spinning technique applied to a quadrupolar nucleus, D-HMQC does not require time-consuming optimisations and exhibits on the quadrupolar spin a better robustness to irradiation offset and to Cq values and radiofrequency field. Furthermore, the high robustness to irradiation offset makes of the D-HMQC sequence the technique of choice for the structural characterisation of materials especially at high magnetic field. We show here how the D-HMQC can be easily implemented and optimised to give access to the structural analysis of silicate-, phosphate-, carbon- and proton-containing materials. An emphasis will be on describing the most popular dipolar recoupling schemes that can be used in that sequence and providing their advantages and drawbacks.

A Study of Transition-Metal Organometallic Complexes Combining 35Cl Solid-State NMR Spectroscopy and 35Cl NQR Spectroscopy and First-Principles DFT Calculations

A series of transition-metal organometallic complexes with commonly occurring metal-chlorine bonding motifs were characterized using (35)Cl solid-state NMR (SSNMR) spectroscopy, (35)Cl nuclear quadrupole resonance (NQR) spectroscopy, and first-principles density functional theory (DFT) calculations of NMR interaction tensors. Static (35)Cl ultra-wideline NMR spectra were acquired in a piecewise manner at standard (9.4 T) and high (21.1 T) magnetic field strengths using the WURST-QCPMG pulse sequence. The (35)Cl electric field gradient (EFG) and chemical shielding (CS) tensor parameters were readily extracted from analytical simulations of the spectra; in particular, the quadrupolar parameters are shown to be very sensitive to structural differences, and can easily differentiate between chlorine atoms in bridging and terminal bonding environments. (35)Cl NQR spectra were acquired for many of the complexes, which aided in resolving structurally similar, yet crystallographically distinct and magnetically inequivalent chlorine sites, and with the interpretation and assignment of (35)Cl SSNMR spectra. (35)Cl EFG tensors obtained from first-principles DFT calculations are consistently in good agreement with experiment, highlighting the importance of using a combined approach of theoretical and experimental methods for structural characterization. Finally, a preliminary example of a (35)Cl SSNMR spectrum of a transition-metal species (TiCl4) diluted and supported on non-porous silica is presented. The combination of (35)Cl SSNMR and (35)Cl NQR spectroscopy and DFT calculations is shown to be a promising and simple methodology for the characterization of all manner of chlorine-containing transition-metal complexes, in pure, impure bulk and supported forms.

A well-defined silica-supported dinuclear tungsten(III) amido species: synthesis, characterization and reactivity.

Grafting of [W(2)(NMe(2))(6)] onto dehydroxylated silica affords the well-defined surface species [([triple bond, length as m-dash]Si-O)W(2)(NMe(2))(5)], characterized by elemental analysis, and infrared, Raman and NMR spectroscopies, and the catalytic reactivity of this supported tungsten(III) d(3)-d(3) dimer and of its alkoxide derivatives towards alkynes has been probed.