Molecular Structures of THF‐Solvated Alkali‐Metal 2,2,6,6‐Tetramethylpiperidides Finally Revealed: X‐ray Crystallographic, DFT, and NMR (including DOSY) Spectroscopic Studies

Abstract

The often studied THF solvates of the utility alkali-metal amides lithium and sodium 2,2,6,6-tetramethylpiperidide are shown to exist in the solid state as asymmetric cyclic dimers containing a central M(2)N(2) ring and one molecule of donor per metal to give a distorted trigonal planar metal coordination. DFT studies support these structures and confirm the asymmetry in the ring. In C(6)D(12) solution, the lithium amide displays a concentration-dependent equilibrium between a solvated and unsolvated species which have been shown by diffusion-ordered NMR spectroscopy (DOSY) to be a dimer and larger oligomer, respectively. A third species, a solvated monomer, is also present in very low concentration, as proven by spiking the NMR sample with THF. In contrast, the sodium amide displays a far simpler C(6)D(12) solution chemistry, consistent with the solid-state dimeric arrangement but with labile THF ligands.

Electronic, Steric, and Ligand Influence on the Solid‐State Structures of Substituted Sodium and Potassium Anilides

“…[23] In all these alkali metal amides deaggregation can be achieved by Lewis bases depending on their denticity and donor strengths. The vastly growing importance of the organometallic chemistry of the heavier alkali metals [24] has led to a renewed interest in their amide chemistry, which can be seen in several recent investigations of Na(tmp), [25] NaNPh 2 , and KNPh 2 . [21] Herein, light is shed on the influence of Lewis acidity (comparing the derivatives of sodium and potassium), the electronic and steric factors induced by the N-substituted anilide ions [N(Ph)R with R = Me, Ph, iPr], and the neutral coligands (ethers, amines) on the solid-state structures, which focus on coordination number, degree of aggregation, and binding mode.…”

Sequential Synthesis of Organosilicon‐Linked 2‐Methoxypyridines by Non‐Cryogenic ortho‐Metallation Using the nBu2TMPMg·Li Reagent and Hydrosilylation

“…Following the excellent structural studies of bimetallic complexes conducted by Mulvey and Robertson,39 we proposed a structure for the intermediate magnesate ( n Bu[2‐MeO‐3‐pyridyl]TMPMg Li) optimised at the semi‐empirical PM3 level of theory40 (Figure 1). Because one THF solvent molecule usually takes part in the coordination to the lithium atom,41 we calculated the proposed structure with one molecule of THF. The results show relatively short distances between Li and the surrounding atoms in the vicinity of the reactive site.…”

Method of Continuous Variation: Characterization of Alkali Metal Enolates Using 1H and 19F NMR Spectroscopies

“…Sodium tetramethylpiperidide reported by Mulvey may work well but was not tested. 4,31 We settled on two bases. The highly soluble NaHMDS is easily prepared and purified.…”