A Terminal Iron(IV) Nitride Supported by a Super Bulky Guanidinate Ligand and Examination of Its Electronic Structure and Reactivity

Utilizing
the bulky guanidinate ligand [LAr*]− (LAr* = (Ar*N)2C­(R), Ar* = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl, R = NCtBu2) for kinetic stabilization, the synthesis of a rare terminal
Fe­(IV) nitride complex is reported. UV irradiation of a pyridine solution
of the Fe­(II) azide [LAr*]­FeN3(py) (3-py) at 0 °C cleanly generates the Fe­(IV) nitride [LAr*]­FeN(py) (1). The 15N NMR spectrum
of the 115N (50% Fe15N) isotopomer shows a resonance at 1016 ppm (vs externally
referenced CH3NO2 at 380 ppm), comparable to
that known for other terminal iron nitrides. Notably, the computed
structure of 1 reveals an iron center with distorted
tetrahedral geometry, τ4 = 0.72, featuring a short
FeN bond (1.52 Å). Inspection of the frontier orbital
ordering of 1 shows a relatively small HOMO/LUMO gap
with the LUMO comprised by Fe(dxz,yz)N(px,y) π*-orbitals,
a splitting that is manifested in the electronic absorption spectrum
of 1 (λ = 610 nm, ε = 1375 L·mol–1·cm–1; λ = 613 nm (calcd)).
Complex 1 persists in low-temperature solutions of pyridine
but becomes unstable at room temperature, gradually converting to
the Fe(II) hydrazide product [κ2-(tBu2CN)C(η6-NAr*)­(N-NAr*)]Fe (4) upon standing via intramolecular N-atom
insertion. This reactivity of the FeN moiety was assessed
through molecular orbital analysis, which suggests electrophilic character
at the nitride functionality. Accordingly, treatment of 1 with the nucleophiles PMe2Ph and Ar–NC
(Ar = 2,6-dimethylphenyl) leads to partial N-atom transfer and formation
of the Fe(II) addition products [LAr*]Fe(NPMe2Ph)(py) (5) and [LAr*]Fe(NCNAr)(py) (6). Similarly, 1 reacts with PhSiH3 to give [LAr*]­Fe­[N­(H)­(SiH2Ph)]­(py)
(7) which Fukui analysis shows to proceed via electrophilic
insertion of the nitride into the Si–H bond.