[Cu(MeCN)₄]BF₄

[Cu(MeCN)₄]BF₄ (tetrakis(acetonitrile)copper(I) tetrafluoroborate) is a copper(I) salt. Four acetonitrile molecules bind the copper(I) ion, and tetrafluoroborate is the counterion. The solid is sensitive to air and moisture, so it is stored under inert gas. It is used as a source of copper(I), both to prepare copper(I) complexes and as a catalyst.

Copper(I) complexes. The salt is combined with ligands to make copper(I) complexes. Reported examples include complexes with bis-phosphine ligands and with carbene ligands. Other reports describe copper clusters, including copper hydride clusters. In one study, cationic copper(I) carbene complexes made from the salt were efficient catalysts for the hydrosilylation of cyclohexanone. In another study, copper(I) from the salt served as a template for a molecular link called a Solomon link. It held bipyridine-based ligands together while platinum(II) linked them.

Catalysis with chiral ligands. Combined with a chiral ligand, the salt gives copper(I) catalysts for enantioselective reactions. Reported chiral ligands include SEGPHOS, DM-BINAP and bisoxazolines. Reported reactions include 1,3-dipolar cycloadditions of azomethine ylides. These reactions build pyrrolidine rings and can create several stereocentres in one step. In one study, the salt with the chiral ligand DTBM-SEGPHOS catalysed [3 + 2] cycloadditions of isocyanoacetates with β-trifluoromethyl enones, which gave 2-pyrrolines. Two other studies describe an enantioselective (3 + 3) cycloaddition of diazo compounds and an enantioselective Friedel–Crafts alkylation of indoles with N-sulfonylaziridines.

Sulfonylative cross-coupling. In one study, the salt with a bipyridine ligand catalysed a sulfonylative Suzuki–Miyaura coupling. Aryl boronic acids, sulfur dioxide and aryl iodides were joined in one step to give diaryl sulfones. The sulfur dioxide came from the surrogate reagent DABSO, and the reaction ran in the polar aprotic solvent DMPU. Under changed conditions, the same study stopped the reaction at a presumed copper sulfinate. The sulfinates were trapped as sodium salts and then converted into sulfones, sulfonamides and sulfonyl fluorides.

Click chemistry. The salt is used as a copper(I) source in the copper-catalysed azide–alkyne cycloaddition (CuAAC), a click reaction. CuAAC joins an azide and a terminal alkyne and gives only the 1,4-disubstituted 1,2,3-triazole. In one study, the salt with the ligand TBTA closed two rings in the synthesis of a molecular cage. In another, the salt with the ligand THPTA joined ubiquitin to the histone protein H1.2 in aqueous buffer. Other single studies used the salt to make biocompatible copper(I) microdevices for bioorthogonal click and uncaging reactions, and in a light-driven reaction that builds triazoles from carboxylic acids, potassium azide and alkynes.

Materials. In one study, copper(I) complexes made from the salt and chelating bis-phosphine ligands were used to prepare light-emitting devices. In another study, copper was electrodeposited from mixtures of supercritical carbon dioxide and acetonitrile. The salt gave better-quality copper deposits than the copper(II) complex tested.