The most common source of nickel dihalide reagents are the hexahydrates, which tend to be poorly soluble in organic solvents. Furthermore, the use of nickel complexes in synthetic applications often requires anhydrous conditions. The dimethoxyethane complex of nickel dichloride (NiCl2(dme)) is a useful source of soluble NiCl2 which I have repeatedly used in nickel chemistry research due to its practical advantages. The complex can be purchased from chemical vendors (Aldrich, STREM), but can become expensive if bought in large quantities. If large amounts are needed, it is better to prepare it in-house. There is a two-step Inorganic Synthesis procedure from NiCl2 hexahydrate.[1] However, a more convenient synthesis was reported in 2007 starting from anhydrous NiCl2.[2]
Procedure:[2] A 500mL two-neck round-bottom flask equipped with a reflux condenser was loaded with anhydrous NiCl2 (100 g, 771.6 mmol) in the glovebox and then transferred to the Schlenk line. 300 mL of pre-dried methanol and 50 mL of trimethylorthoformate were added via syringe or cannula and the mixture stirred under argon, at reflux overnight. Unreacted NiCl2 was then removed by filtration and roughly 80% of the solution was removed under reduced pressure to obtain a green gel. A minimum of pre-dried methanol was added to solubilize the gel, followed by 300 mL of anhydrous dimethoxyethane (DME). The solution was stirred at reflux overnight. A yellow powder of [NiCl2(DME)] precipitated and was isolated by filtration through a canula or swivel frit. The powder was washed with anhydrous pentane, dried in vacuo, and subsequently transferred into the glovebox for storage.
Notes:
- Methanol solvent and DME were pre-dried with molecular sieves for 48h prior to synthesis.
- The procedure requires an initial dehydration step because DME is unable to displace the water ligands from the metal hydrate. Triethyl orthoformate is an ortho ester commonly used as dehydrating agent. In the presence of metal halide hydrates, it forms the corresponding ethanolic metal solvates. Herein, the first stage solubilizes the highly insoluble NiCl2 by forming a solvate complex with alcohols (methanol and ethanol). This step prepares the metal salt for the second stage, where DME can displace the alcohol ligands and precipitate the product.
- After the first reflux, solvent should only be partially removed in vacuo (roughly 80%). If too much solvent is removed, the metal center will begin to lose its coordinated alcohols.
- NiCl2(dme) is stable to oxygen, but degrades in the presence of moisture. It must be handled and stored under inert atmosphere.
- For some applications of NiCl2(dme) see this EROS article.[3]
[1] Ward, L. G. L. Anhydrous nickel(II) halides and their tetrakis(ethanol) and 1,2-dimethoxyethane complexes. Inorg. Synth. 1971, 13, 154-164.
[2] Kermagoret, A.; Braunstein, P. Organometallics 2008, 27, 1, 88-99.
[3] Roman, D. S. Nickel(II) Chloride Ethylene Glycol Dimethyl Ether Complex. EROS 2012. DOI: 10.1002/047084289X.rn01428.