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.