Ligand Binding Constants to Lithium Hexamethyldisilazide Determined by Diffusion-Ordered NMR Spectroscopy

Crystal structure of a three-coordinate lithium complex with monodentate phenyl-oxazoline and hexa-methyl-disilyl-amide ligands

The reaction of lithium hexa-methyl-disilyl-amide, [Li{N(Si(CH3)3)2}] (LiHMDS), with 4,4-dimethyl-2-phenyl-2-oxazoline (Phox, C11H13NO) in hexane produced colourless crystals of bis-(4,4-dimethyl-2-phenyl-2-oxazoline-κN)(hexa-methyl-disilyl-amido-κN)lithium, [Li(C6H18NSi2)(C11H13NO)2] or [Li{N(Si(CH3)3)2}(Phox)2] in high yield (89%). Despite the 1:1 proportion of the starting materials in the reaction mixture, the product formed with a 1:2 amide:oxazoline ratio. In the unit cell of the C2/c space group, the neutral mol-ecules lie on twofold rotation axes coinciding with the Li-N(amide) bonds. The lithium(I) centre adopts a trigonal-planar coordination geometry with three nitro-gen donor atoms, one from the HMDS anion and two from the oxazolines. All ligands are monodentate. In the phenyl-oxazoline units, the dihedral angle defined by the five-membered heterocyclic rings is 35.81 (5)°, while the phenyl substituents are approximately face-to-face, separated by 3.908 (5) Å. In the amide, the methyl groups assume a nearly eclipsed arrangement to minimize steric repulsion with the analogous substituents on the oxazoline rings. The non-covalent inter-actions in the solid-state structure of [Li{N(Si(CH3)3)2}(Phox)2] were assessed by Hirshfeld surface analysis and fingerprint plots. This new compound is attractive for catalysis due to its unique structural features.

Sodiated Oppolzer Enolates: Solution Structures, Mechanism of Alkylation, and Origin of Stereoselectivity

NMR spectroscopic studies reveal camphorsultam-derived sodium enolates known as Oppolzer enolates reside as monomers in neat THF and THF/HMPA solutions and as dimers in toluene when solvated by N,N,N',N'-tetramethylethylenediamine (TMEDA) and N,N,N',N'',N''-pentamethyldiethylenediamine (PMDTA). Density functional theory (DFT) computations attest to the solvation numbers. Rate studies show analogy with previously studied lithiated Oppolzer enolates in which alkylation occurs through non-chelated solvent-separated ion pairs. The origins of the selectivity trace to transition structures in which the alkylating agent is guided to the exo face of the camphor owing to stereoelectronic preferences imparted by the sultam sulfonyl moiety. Marked secondary-shell solvation effects are gleaned from the rate studies.

Lithiated Oppolzer Enolates: Solution Structures, Mechanism of Alkylation, and Origin of Stereoselectivity

Camphorsultam-based lithium enolates referred to colloquially as Oppolzer enolates are examined spectroscopically, crystallographically, kinetically, and computationally to ascertain the mechanism of alkylation and the origin of the stereoselectivity. Solvent- and substrate-dependent structures include tetramers for alkyl-substituted enolates in toluene, unsymmetric dimers for aryl-substituted enolates in toluene, substrate-independent symmetric dimers in THF and THF/toluene mixtures, HMPA-bridged trisolvated dimers at low HMPA concentrations, and disolvated monomers for the aryl-substituted enolates at elevated HMPA concentrations. Extensive analyses of the stereochemistry of aggregation are included. Rate studies for reaction with allyl bromide implicate an HMPA-solvated ion pair with a ⁺Li(HMPA)₄ counterion. Dependencies on toluene and THF are attributed to exclusively secondary-shell (medium) effects. Aided by density functional theory (DFT) computations, a stereochemical model is presented in which neither chelates nor the lithium gegenion serves roles. The stereoselectivity stems from the chirality within the sultam ring and not the camphor skeletal core.

A Sodium Sodate as Precursor for Lanthanide Bis(4-R-benzoxazol-2-yl)methanide Single-Molecule Magnets

From the sodium sodate precursor [(Na(thf)₆][Na{(4-Me-NCOC₆H₃)₂CH}₂] (1) three isostructural dinuclear lanthanide complexes [(μ-Cl)Ln^III{(4-MeNCOC₆H₃)₂CH}₂]₂ with Ln = Gd (2), Dy (3), and Er (4) based on the N,N'-chelating monoanionic bis(4-methylbenzoxazol-2-yl)methanide ligand (titled "Mebox") were synthesized and characterized by X-ray diffraction and magnetic measurements. The sodium precursor 1 was analyzed via X-ray diffraction and diffusion-ordered NMR spectroscopy experiments (DOSY-NMR) in order to investigate its aggregation in solution and the solid state. The sodium analog [(thf)₃Na(NCOC₆H₄)₂CH] (1') based on the bis(benzoxazol-2-yl)-methanide ligand (titled "box") was prepared and analyzed for comparison reasons. From the lanthanide derivatives 2-4, the Dy^III complex 3 displays slow relaxation of magnetization at zero field, with a relaxation barrier of U = 315.7 cm^-1. The coupling strength between the two lanthanide centers was estimated with the Gd^III equivalent 2, giving a weak antiferromagnetic coupling of J = -0.035 cm^-1.

Metalated Porous Phenanthroline-Based Polymers as Efficient Heterogeneous Catalysts for Regioselective C-H Activation of Heteroarenes

Direct C-H bond activation of heterocycles as a step-economical and environmentally friendly approach to build the heterobiaryls motifs is highly attractive, but it still has a challenge to design and prepare a cheap and regioselective heterogeneous catalyst. To tackle this challenge, we have introduced Ni species into a porous phenanthroline-based organic polymer donated as POP-Phen@Ni. This heterogeneous catalyst shows excellent catalytic performances in regioselective C-H activation of heterocycles, even better than those of the corresponding homogenous catalyst. H/D exchange experiments show that the lithium bis(trimethylsilyl)amide (LiHMDS), a base added in the reaction, play a very important role during the reaction processes. We believe that this heterogeneous catalyst would open a new door for design of heterogeneous catalysts to efficiently catalyze the regioselective C-H activation of heterocycles.

Alkali-Metal Mediation: Diversity of Applications in Main-Group Organometallic Chemistry

Organolithium compounds have been at the forefront of synthetic chemistry for over a century, as they mediate the synthesis of myriads of compounds that are utilised worldwide in academic and industrial settings. For that reason, lithium has always been the most important alkali metal in organometallic chemistry. Today, that importance is being seriously challenged by sodium and potassium, as the alkali-metal mediation of organic reactions in general has started branching off in several new directions. Recent examples covering main-group homogeneous catalysis, stoichiometric organic synthesis, low-valent main-group metal chemistry, polymerization, and green chemistry are showcased in this Review. Since alkali-metal compounds are often not the end products of these applications, their roles are rarely given top billing. Thus, this Review has been written to alert the community to this rising unifying phenomenon of "alkali-metal mediation".

The interpretation of small molecule diffusion coefficients: Quantitative use of diffusion-ordered NMR spectroscopy

Measuring accurate molecular self-diffusion coefficients, D, by nuclear magnetic resonance (NMR) techniques has become routine as hardware, software and experimental methodologies have all improved. However, the quantitative interpretation of such data remains difficult, particularly for small molecules. This review article first provides a description of, and explanation for, the failure of the Stokes-Einstein equation to accurately predict small molecule diffusion coefficients, before moving on to three broadly complementary methods for their quantitative interpretation. Two are based on power laws, but differ in the nature of the reference molecules used. The third addresses the uncertainties in the Stokes-Einstein equation directly. For all three methods, a wide range of examples are used to show the range of chemistry to which diffusion NMR can be applied, and how best to implement the different methods to obtain quantitative information from the chemical systems studied.