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

The reaction of Fe 2 (mes) 4 with the super-bulky amidines and guanidines HL Ar*-R (L Ar*-R = [(Ar*N) 2 C(R)] - , Ar* = 2,6-bis(diphenylmethyl)-4- tert -butylphenyl), R = Me ( L Ar*-Me ), t Bu ( L Ar*-tBu ), Ph ( L Ar*-Ph ), N i Pr 2 ( L Ar*-iPr2N ), and Pip ( L Ar*-Pip )) gives access to the three-coordinate iron-mesityl complexes (L Ar*-R )Fe(mes) only where L Ar*-R = L Ar*-Me , L Ar*-Ph , or L Ar*-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 Me 3 SiCl to solutions of [P] produces the iron(II) complex (L Ar*-Pip )FeCl(py- d 5 ) as shown by 1 H 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 C 14 H 11 N 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.