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Ionic crystals consist of two or more different kinds of ions that usually have different sizes. The packing of these ions into a crystal structure is more complex than the packing of metal atoms that are the same size.
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Atoms and molecules interact through bonds (or forces): intramolecular and intermolecular. The forces are electrostatic as they arise from interactions (attractive or repulsive) between charged species (permanent, partial, or temporary charges) and exist with varying strengths between ions, polar, nonpolar, and neutral molecules. The different types of intermolecular forces are ion–dipole, dipole–dipole, hydrogen bonds, and dispersion; among these, dipole–dipole, hydrogen...
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Solubility is the measure of the maximum amount of solute that can be dissolved in a given quantity of solvent at a given temperature and pressure. Solubility is usually measured in molarity (M) or moles per liter (mol/L). A compound is termed soluble if it dissolves in water.
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Solution, Solubility, and Solubility Equilibrium
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A type of Lewis acid-base chemistry involves the formation of a complex ion (or a coordination complex) comprising a central atom, typically a transition metal cation, surrounded by ions or molecules called ligands. These ligands can be neutral molecules like H2O or NH3, or ions such as CN− or OH−. Often, the ligands act as Lewis bases, donating a pair of electrons to the central atom. These types of Lewis acid-base reactions are examples of a broad subdiscipline called coordination...
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Potassium Hexamethyldisilazide (KHMDS): Solvent-Dependent Solution Structures.

Jesse A Spivey1, David B Collum1

  • 1Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States.

Journal of the American Chemical Society
|June 20, 2024
PubMed
Summary
This summary is machine-generated.

Potassium hexamethyldisilazide (KHMDS) solution structures were studied using NMR and DFT. Ligand coordination influences KHMDS aggregation, forming dimers, monomers, and ion pairs depending on ligand type and concentration.

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Area of Science:

  • Organometallic Chemistry
  • Solution State Chemistry
  • Spectroscopy

Background:

  • Potassium hexamethyldisilazide (KHMDS) is a widely used strong, non-nucleophilic base in organic synthesis.
  • Understanding the solution-state aggregation and speciation of KHMDS is crucial for controlling its reactivity.
  • Previous studies have suggested various aggregation states, but a comprehensive analysis in the presence of different ligands is lacking.

Purpose of the Study:

  • To elucidate the solution structures and aggregation behavior of KHMDS and its isotopically labeled variant ([15N]KHMDS).
  • To investigate the influence of various coordinating ligands on KHMDS speciation (monomers, dimers, ion pairs).
  • To correlate experimental findings with theoretical calculations for a deeper understanding of KHMDS solvation.

Main Methods:

  • 29Si NMR spectroscopy and 15N-29Si couplings were employed to probe KHMDS structures.
  • The method of continuous variations was used to determine aggregation states.
  • Density functional theory (DFT) computations were utilized to model solvation effects and energetics.

Main Results:

  • KHMDS exists as dimers, monomers, and ion pairs in solution, with speciation dictated by ligand type and concentration.
  • Weakly coordinating ligands favor dimers, while chelating ligands can lead to monomers or ion pairs under specific conditions.
  • DFT calculations generally align with experimental observations regarding aggregates and solvates, though temperature dependencies differed.

Conclusions:

  • The aggregation state of KHMDS is highly tunable through coordination with various ligands.
  • This study provides a detailed map of KHMDS solution behavior, essential for optimizing its use in synthesis.
  • The interplay between ligand coordination, aggregation, and solvation significantly impacts the structure and reactivity of KHMDS.