1. Preparation of 1 [Ir(IMes)(H)2(Py)] + Cl -, 2 [Ir(IMes)(Cl)(H)2(Py)2] and 3 [Ir(IMes)(CD 3 OD)(H) 2 (Py) 2 ] in methanol-d 4 : Pre-catalyst [IrCl(IMes)(COD)] (2.25 mM) was dissolved in 600 µL of methanol-d4 and 13.8 mM of pyridine was added, yielding a light yellow. The solution was bubbled with hydrogen at 3 bar pressure until it turned transparent and full conversion of the pre-catalyst was confirmed by PASADENA measurements as described before. The presence of 1, 2 and 3 is confirmed by the data presented in the main text. A solution was prepared according to 7 and 20 µL of a 38 % concentrated DCl solution was added. The appearance of 2 was confirmed by NMR (see the main text).