Photoinduced Cleavage of a Strained N–C Bond in an Iron Complex Supported by Super-Bulky Amidinate and Guanidinate Ligands

The reaction of Fe2(mes)4 with the super-bulky amidines and guanidines HL Ar*‑R (LAr*‑R = [(Ar*N)2C­(R)]−, Ar* = 2,6-bis­(diphenylmethyl)-4-tert-butylphenyl), R = Me (L Ar*‑Me ), t Bu (L Ar*‑tBu ), Ph (L Ar*‑Ph ), NiPr2 (L Ar*‑iPr2N ), and Pip (L Ar*‑Pip )) gives access to the three-coordinate iron–mesityl complexes (L Ar*‑R )­Fe­(mes) only where LAr*‑R = LAr*‑Me, LAr*‑Ph, or LAr*‑Pip. Subsequent protonolysis with the N-atom transfer reagent Hdbabh (Hdbabh = 2,3:5,6-dibenzo-7-azabicyclo[2.2.1]­hepta-2,5-diene) is limited in success, providing in one instance a few crystals of four-coordinate (L Ar*‑Me )­Fe­(dbabh)­(Hdbabh), while three-coordinate (L Ar*‑Pip )­Fe­(dbabh) is synthesized reproducibly. Complexes (L Ar*‑Me )­Fe­(dbabh)­(Hdbabh) and (L Ar*‑Pip )­Fe­(dbabh) are thermally insensitive in solution to temperatures of up to 100 °C. On the other hand, both (L Ar*‑Me )­Fe­(dbabh)­(Hdbabh) and (L Ar*‑Pip )­Fe­(dbabh) show sensitivity to blue LED light (395 nm), undergoing photochemical transformations. For instance, the photolysis of (L Ar*‑Me )­Fe­(dbabh)­(Hdbabh) leads to N–C bond scission and C–C bond coupling across the –dbabh moieties to give four-coordinate (L Ar*‑Me )­Fe­(N=dbabh–dbabhNH 2 ). Photolyzing pyridine-d 5 (py-d 5) solutions of (L Ar*‑Pip )­Fe­(dbabh) at −5 °C produces a new paramagnetic photoproduct, [P]. Due to the thermal sensitivity of compound [P], it has eluded structural characterization; yet, Evans’ method measurements suggest that the iron­(II) oxidation state is maintained, thereby pointing to the –dbabh moiety as the locus of chemical change. In line with this assessment, addition of excess Me3SiCl to solutions of [P] produces the iron­(II) complex (L Ar*‑Pip )­FeCl­(py-d 5 ) as shown by 1H NMR spectroscopy. Gas chromatography/mass spectrometry analysis of the solutions of [P] shows a peak in the chromatogram with a molecular mass corresponding to a formulation of C14H11N that cannot be attributed to Hdbabh. This provides evidence for the photochemical-induced isomerization of the –dbabh ligand, revealing a heretofore unknown photochemical sensitivity of this N atom transfer reagent.