Copper(I) halide complexes such as CuI are routinely used in catalytic and material applications, such as catalysts in the Ullmann coupling and as p-type semiconductors, respectively.[1] However, these compounds typically suffer from poor solubility in organic solvents. Cationic copper(I) complexes with weakly coordinating anions are more soluble Cu(I) sources, and possess stabilizing ligands which are easily displaced for facile functionalization at the metal. Tetrakis(acetonitrile)copper(I) salts have been synthesized as early as 1923,[2] and derivatives with varied counter anions have been reported. The following procedure describes the synthesis of the tetrafluoroborate salt, in a modification to a 1990 Inorganic Syntheses protocol.[3]
Procedure: To a suspension of Cu2O (2.00g, 14.0mmol) and 50mL of acetonitrile in a 150mL Erlenmeyer flask was added 50% tetrafluoroboric acid (10mL, 57.0mmol). The mixture was heated, with stirring, to 60degC for 20min. which led to dissolution of the copper solids. The solution was subsequently filtered while hot, allowed to cool to room temperature, and then placed in the freezer for ca. 2h to allow product to crystallize. The crystals were quickly collected under vacuum filtration, washed with 30mL of diethyl ether, and then transferred into a Schenk flask to finish the drying process under high vacuum. Product was obtained as a white, crystalline solid (4.22g, 96%) which was stored in the glovebox.
Notes:
- The synthesis was carried out in air, using stock solvents. 50% w/w aqueous tetrafluoroboric acid was purchased from VWR.
- Care should be taken during addition of the acid, since the reaction is exothermic.
- Cu(MeCN)4 salts are stable in air when dissolved in acetonitrile, but degrade in the presence of oxygen/moisture in the solid state. Isolation of the crystals by filtration should be done quickly to avoid onset of decomposition (indicated by gradual conversion of the white product into a blue Cu(II) species).
- [Cu(MeCN)4][BF4] is soluble in toluene, DCM, acetonitrile, and insoluble in ether solvents. It has been used in a variety of literature transformations.[4]
- Cu(MeCN)4 salts with other counteranions can be prepared via analogous methods.
[1] a) Bertz, S. H.; Fairchild, E. H.; Guillaumet, G.; Suzenet, F. Copper(I) Iodide. EROS 2005. DOI: 10.1002/047084289X.rc229.pub2. b) Liu, A.; Zhu, H.; Kim, M.-G.; Kim, J.; Noh, Y.-Y. Adv. Sci. 2021, 8, 14, 2100546.
[2] Morgan, H. H. J. Chem. Soc., Trans. 1923, 123, 2901-2907.
[3] a) Kubas, G. J. Tetrakis(acetonitrile)copper(I) Hexafluorophosphate. Inorg. Synth. 1990, 28, 68-69. b) Li, C.; Li, W.; Henwood, A. F.; Hall, D.; Cordes, D. B.; Slawin, A. M. Z.; Lemaur, V.; Olivier, Y.; Samuel, I. D. W.; Zysman-Colman, E. Inorg. Chem. 2020, 59, 20, 14772-14784.
[4] Parish, E. J.; Qin, H.; Lipshutz, B. H.; Petersen, T. B. Tetrakis(acetonitrile)copper(I) Tetrafluoroborate. EROS 2007. DOI: 10.1002/9780470842898.rt040.pub2.