Multiple N–H and C–H Hydrogen Atom Abstractions Through Coordination-Induced Bond Weakening at Fe-Amine Complexes

We report the use of the reported
Fe-phthalocyanine complex, PcFe
(1; Pc = 1,4,8,11,15,18,22,25-octaethoxy-phthalocyanine),
to generate PcFe–amine complexes 1-(NH3)2, 1-(MeNH2)2,
and 1-(Me2NH)2. Treatment of 1 or 1-(NH3)2 to
an excess of the stable aryloxide radical, 2,4,6-tritert-butylphenoxyl radical (tBuArO•), under NH3 resulted in catalytic H atom abstraction
(HAA) and C–N coupling to generate the product 4-amino-2,4,6-tritert-butylcyclohexa-2,5-dien-1-one (2) and tBuArOH. Exposing 1-(NH3)2 to an excess
of the trityl (CPh3) variant, 2,6-di-tert-butyl-4-tritylphenoxyl radical (TrArO•), under NH3 did not lead to catalytic ammonia oxidation
as previously reported in a related Ru-porphyrin complex. However,
pronounced coordination-induced bond weakening of both α N–H
and β C–H in the alkylamine congeners, 1-(MeNH2)2 and 1-(Me2NH)2, led to multiple HAA events yielding the unsaturated cyanide
complex, 1-(MeNH2)­(CN), and imine complex, 1-(MeNCH2)2, respectively. Subsequent
C–N bond formation was also observed in the latter upon addition
of a coordinating ligand. Detailed computational studies support an
alternating mechanism involving sequential N–H and C–H
HAA to generate these unsaturated products.