Although simple nickel dihalides may seem as attractive Ni(II) sources in organometallic/transition-metal chemistry, they exhibit poor solubility in organic solvents. Furthermore, the use of nickel complexes in synthetic applications often requires anhydrous conditions. The dimethoxyethane complex of nickel dibromide (NiBr2(dme)) is a useful source of soluble NiBr2 which I have repeatedly used in nickel chemistry research due to its practical advantages. The complex can be purchased from chemical vendors (Millipore-Sigma, 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 starting from NiCl2 hydrate that can be tedious.[1] However, a more convenient, modified synthesis was reported in 2018 starting from NiBr2 trihydrate.[2]
Procedure:[2] A 500mL two-neck round-bottom flask equipped with a swivel-frit apparatus was loaded with NiCl2 trihydrate (26.2g, 96.1mmol) and the glassware was attached to a dual-manifold vacuum line, where it was kept under argon. 60 mL of non-distilled absolute ethanol were added via syringe or cannula and the nickel solution heated, with stirring, at 60degC for 1.5h. To the resulting green solution was then added non-distilled dimethoxyethane (DME) without cooling. The solution was subsequently stirred at 70degC overnight, after which the volatile materials were removed in vacuo. Solvent removal led to formation of a viscous green gum that turned into an orange solid upon further drying. 300mL of anhydrous DME were added via syringe or cannula and the mixture was heated to 85degC for 2h, causing the salmon-colored powder of [NiBr2(DME)] to precipitate. Mixture was then allowed to cool to room temperature, and the product was further crystallized by cooling the reaction vessel to -78degC (dry-ice/acetone). Filtration, washing the filtrand with anhydrous diethyl ether (3x20mL), and drying under high vacuum, yielded the product as a salmon-orange powder (reported yield: 29.3g, 98.8%). Product was subsequently transferred into the glovebox for storage.
Notes:
- Anhydrous DME can be dried with molecular sieves for 48h prior to synthesis. Nickel(II) bromide trihydrate can be purchased from chemical vendors (Millipore-Sigma, Fischer Scientific).
- The employed procedure circumvents water removal with triethyl orthoformate because the equilibrium is driven towards the target compound upon the first solvent removal, which gradually removes water. Second addition of DME (anhydrous) fully forms the product after heating, which precipitates and is isolated by filtration.
- Since the complex isn’t oxygen-sensitive, I have stored crude product mixtures in air by submerging them in heptane. The product can be purified by Soxhlet extraction of the crude material with DME overnight. It precipitates in the round-bottom flask.
- Stirring a mixture of anhydrous NiBr2 in DME for several days seems to afford a mixture of compounds of the type NiBr2(dme)n with n varying between 0-2.[3] This is therefore, not a reliable way for making the target compound.
- NiBr2(dme) is stable to oxygen, but degrades to a green solid in the presence of moisture. It must be handled and stored under inert atmosphere.
[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] Gomes, C. S. B.; Costa, S. I.; Silva, L. C.; Jimenez-Tenorio, M.; Valerga, P.; Puerta, M. C.; Gomes, P. T. Eur. J. Inorg. Chem. 2018, 5, 597-607.
[3] Davis, L. M. Syntheses, Properties, and Reactions of Transition Metal Complexes of Di(tert-butyl)amide and 2,2,6,6-tetramethylpiperidide. Ph.D. Dissertation, University of Illinois: Urbana-Champaign, Urbana, IL, 2014.