Tropylium fluoborate (CAS NO.: ), which is known as Cycloheptatrienylium, tetrafluoroborate (1-), could be produced through the following synthetic routes.

A suspension of 100 g. (0.48 mole, 33% excess) of phosphorus pentachloride in 800 ml. of carbon tetrachloride is prepared in a 1-l. flask equipped with an efficient stirrer and an exit valve for the hydrogen chloride that is evolved. Tropilidene (cycloheptatriene; 24.2 g. of 91% material; 0.24 mole) is added all at once, and the mixture is stirred for 3 hours at room temperature.

Absolute ethanol (400 ml.) is vigorously stirred in a 1-l. wide-necked Erlenmeyer flask immersed in an ice bath. The tropylium hexachlorophosphate-tropylium chloride double salt2 is separated from the reaction mixture by suction filtration, washed briefly with fresh carbon tetrachloride, and transferred as rapidly as possible into the cold, well-stirred ethanol. The salt dissolves rapidly and exothermally to give a reddish solution. Fifty milliliters (0.39 mole) of 50% aqueous fluoboric acid is added rapidly to the cold stirred solution. The dense white precipitate of tropylium fluoborate that forms is separated by suction filtration, washed with a little cold ethanol and with ether, and air-dried at room temperature; weight 34–38 g. (80–89%); decomposition point about 200°; λ_max0.1N HCl 218 mμ (log ε 4.70), 274 mμ (log ε 3.61). The product is 98–100% pure.