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 [L ^Ar* ] ^- (L ^Ar* = (Ar*N) ₂ C(R), Ar* = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl, R = NC ^t Bu ₂ ) 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 [L ^Ar* ]FeN ₃ (py) (3-py) at 0 °C cleanly generates the Fe(IV) nitride [L ^Ar* ]FeN(py) (1). The ¹⁵ N NMR spectrum of the 1 ^{15 N} (50% Fe≡ ¹⁵ N) isotopomer shows a resonance at 1016 ppm (vs externally referenced CH ₃ NO ₂ 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, τ ₄ = 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(d _xz,yz )N(p _x,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 [κ ² -( ^t Bu ₂ CN)C(η ⁶ -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 PMe ₂ Ph and Ar-N≡C (Ar = 2,6-dimethylphenyl) leads to partial N-atom transfer and formation of the Fe(II) addition products [L ^Ar* ]Fe(N═PMe ₂ Ph)(py) (5) and [L ^Ar* ]Fe(N═C═NAr)(py) (6). Similarly, 1 reacts with PhSiH ₃ to give [L ^Ar* ]Fe[N(H)(SiH ₂ Ph)](py) (7) which Fukui analysis shows to proceed via electrophilic insertion of the nitride into the Si-H bond.