Similarly to our previously published procedure, a solution of 1.4 g of sodium nitrite (20.3 mmol) was suspended in 15 ml (0.27 mol) of concentrated sulfuric acid, adding the nitrite in small portions at a rate slow enough to avoid the visible formation of brown nitrogen oxides. The solution was cooled in an ice bath. A solution of 1.0g of p-phenylenediamine (9.2 mmol) in 10 mL of glacial acetic acid was added dropwise over 10 min, and the reaction was allowed to proceed for one hour. After the addition was complete, the reaction was stirred for 30 min. During this time, the reaction mixture turned into a thick slush. Note: this material is likely the bisulfate salt, which should not be isolated due to a potential explosion hazard. 21.4 mmol of either sodium tetrafluoroborate (2.35 g) or sodium hexafluorophosphate (3.60g) was dissolved in a minimum amount of cold water, and this solution added dropwise with manual stirring using a glass rod to the reaction. 25 mL of icecold water was added. The thick off-white precipitate of bis(diazonium) salt was collected on a glass fritted funnel and washed with ice-cold water, cold methanol, and diethyl ether, and dried in a vacuum desiccator. When not in use, the salts were stored in a -20°C freezer. However, both tetrafluoroborate and hexafluorophosphate salts could be stored on the bench in a sealed vial for days without any detectable degradation in the case of the tetrafluoroborate and with only minor cosmetic darkening that did not yield an impurity measurable by NMR in the case of the hexafluorophosphate. Yield (PF 6-salt) 2.8 g (6.6 mmol, 72%). 1H NMR: 8.9 ppm (s) vs. TMS. UV-Vis (water) Lambda-max 254 nm.