Structures of 1, 2, 3a and 5a (cf. Fig. ) were taken from a previous investigation, but the QM region was extended and re-optimized. A comparison between the QM regions employed here and in ref. 37 is shown in Fig. in the SI. To obtain the reaction and activation energies, linear transit calculations with a single restrained reaction coordinate were performed. The point with the highest energy is considered to be the transition state (TS). In case the most stable spin-state for an intermediate changed between reactant and product, the highest point can be the crossing point between the two spin states. Singlepoint calculations for the final total energies were performed using the def2-TZVPP basis set (and the corresponding auxiliary basis). Zero-point vibrational energies (ZPVE) and thermochemical corrections were generally neglected, except for one reaction step (6b→7 in Fig. ) where we evaluate this approximation critically by verifying the nature of the TS through a frequency calculation (more details are given in the SI). All calculations include a D3 dispersion correction with Becke-Johnson damping. Energies were checked employing the B3LYP-D3 66-68 functional using the same structures and triple-zeta basis sets. For simplicity, the functionals are referred to as TPSS and B3LYP in the following, but all calculations in this work include dispersion corrections. QM/MM energies are reported and in some cases a breakdown into energy parts (MM as well as QM with or without point charges) is provided. In cases where spin densities are discussed, they origin from Mulliken population analyses (at the B3LYP/def2-TZVPP level).