Switching the Spin State of Diphenylcarbene via Halogen Bonding

The interactions between diphenylcarbene DPC and the halogen bond donors CF₃I and CF₃Br were investigated using matrix isolation spectroscopy (IR, UV-vis, and EPR) in combination with QM and QM/MM calculations. Both halogen bond donors CF₃X form very strong complexes with the singlet state of DPC, but only weakly interact with triplet DPC. This results in a switching of the spin state of DPC, the singlet complexes becoming more stable than the triplet complexes. CF₃I forms a second complex (type II) with DPC that is thermodynamically slightly more stable. Calculations predict that in this second complex the DPC···I distance is shorter than the F3C···I distance, whereas in the first (type I) complex the DPC···I distance is, as expected, longer. CF₃Br only forms the type I complex. Upon irradiation I or Br, respectively, are transferred to the DPC carbene center and radical pairs are formed. Finally, on annealing, the formal C-X insertion product of DPC is observed. Thus, halogen bonding is a powerful new principle to control the spin state of reactive carbenes