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 HLAr*‑R (LAr*‑R = [(Ar*N)2C(R)]−, Ar* = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl),
R = Me (LAr*‑Me), tBu (LAr*‑tBu), Ph (LAr*‑Ph),
NiPr2 (LAr*‑iPr2N), and Pip (LAr*‑Pip)) gives access to the three-coordinate iron–mesityl complexes (LAr*‑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 (LAr*‑Me)Fe(dbabh)(Hdbabh), while three-coordinate (LAr*‑Pip)Fe(dbabh) is
synthesized reproducibly. Complexes (LAr*‑Me)Fe(dbabh)(Hdbabh) and (LAr*‑Pip)Fe(dbabh) are thermally
insensitive in solution to temperatures of up to 100 °C. On the
other hand, both (LAr*‑Me)Fe(dbabh)(Hdbabh) and (LAr*‑Pip)Fe(dbabh) show sensitivity to blue LED light
(395 nm), undergoing photochemical transformations. For instance,
the photolysis of (LAr*‑Me)Fe(dbabh)(Hdbabh) leads to N–C bond scission
and C–C bond coupling across the –dbabh moieties to
give four-coordinate (LAr*‑Me)Fe(N=dbabh–dbabhNH2). Photolyzing pyridine-d5 (py-d5) solutions of (LAr*‑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 (LAr*‑Pip)FeCl(py-d5) 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.