A slightly modified literature procedure was used to synthesize Me IPr tBu •HBF4. In a 100 mL three-necked, round bottom flask bisimine a4 (1800 mg, 3.48 mmol, 1.0 eq) was dissolved in THF (35 mL) and the solution was heated to 70 °C. To this solution ZnCl2 (498.4 mg, 3.66 mmol, 1.05 eq) was added and the mixture was stirred for 5 minutes. A 4M solution of HCl (1.1 mL, 4.35 mmol, 1.25 eq) and paraformaldehyde (115 mg, 3.83 mmol, 1.1 eq) were added successively and rapidly and stirred overnight at 70 °C. The mixture was cooled to ambient temperature and concentrated and the resultant residue was then dissolved in DCM (50mL) and washed with 2M HCl (3x50mL) and brine (1x50mL) and brine of pH = 8 (1 x 50ml), with 2 times back extracting with DCM. The organic solution was dried over MgSO4 and concentrated. The black crude oil was suspended in 40 ml 100/1 v/v water/THF mixture and stirred for 15 mins. Then the mixture was treated with excess HBF4 (48% w/w in water) at room temperature, resulting a black suspension. After stirring for 24 more hours, the mixture was extracted with DCM (3 x50mL), dried over anhydrous sodium sulfate, and filtered. The filtrate was concentrated before adding pentane, which caused precipitation of the pure imidazolium tetrafluoroborate salt; which was isolated by filtration and dried under vacuum as a pale-yellow solid, yield: 480 mg (22%). 1 H NMR (400 MHz, CDCl3) δ 8.37 (s, 1H), 7.35 (s, 4H), 2.34 (hept, J = 6.5 Hz, 4H), 2.20 (s, 6H), 1.38 (s, 18H), 1.30 (d, J = 6.8 Hz, 12H), 1.17 (d, J = 6.9 Hz, 12H). A slightly modified literature procedure 9,10 was used to synthesize SIPr tBu •HBF4. Diamine a6 (1.88g, 3.82 mmol, 1.0 equiv) and NH4Cl (306.1 mg, 5.72 mmol, 1.5 equiv) were dissolved in HC(OEt)3 (0.5 M) under nitrogen atmosphere. The mixture was stirred at 115 °C for 20 h and then cooled to room temperature. The crude product was filtered, washed with Et2O (3 x 10 mL) and dried in vacuo to afford the saturated imidazolium chloride salt as a white solid. The white solid was suspended in 40 ml 100/1 v/v water/THF mixture and stirred for 15 min. Then treated with excess HBF4 (48% w/ w in water) at room temperature. After the mixture was stirred for a further 24 hours, the mixture was extracted with DCM (3 x50mL), dried over anhydrous sodium sulfate, and filtered. The filtrate was concentrated before adding pentane, which caused precipitation of the pure imidazolium tetrafluoroborate salt; which was isolated by filtration and dried under vacuum as a white solid, yield: (1.20g, 2.02 mmol, 60%). A slightly modified literature procedure was used to synthesize Me IPr* tBu •HBF4. In a 100mL three-necked, roundbottom flask bisimine a5 (2100 mg, 2.07 mmol, 1.0 eq) was dissolved in THF (20 mL) and the solution was heated to 70 °C. To this solution ZnCl2 (296.6 mg, 2.18 mmol, 1.05 eq) was added and the mixture was stirred for 5 minutes. A 4 M solution of HCl (0.65 mL, 2.59 mmol, 1.25 eq) and paraformaldehyde (68.5 mg, 2.28 mmol, 1.1 eq) were added succesively and rapidly and stirred overnight at 70 °C. The mixture was cooled to ambient temperature and concentrated and the resultant residue was then dissolved in DCM (40 mL) and washed with 2M HCl (3x40mL) and brine (1x40 mL) and brine of pH=8 (1x40 ml), with 2 times back extracting with DCM. The organic solution was dried over MgSO4 and concentrated. The black crude oil was suspended in 40 ml 100/1 v/v water/THF mixture and stirred for 15 min. Then treated with excess HBF4 (48% w/ w in water) at room temperature, which resulted in the formation of a black suspension. After the mixture was stirred for a further 24 hours, the mixture was extracted with DCM (3 x50mL), dried over anhydrous sodium sulfate, and filtered. The filtrate was concentrated before adding pentane, which caused precipitation of the pure imidazolium tetrafluoroborate salt; which was isolated by filtration and dried under vacuum as a pale-yellow solid, yield: 1300 mg (57%). Free carbene IPr, SIPr and SIPr OMe were synthesized according to the literature .