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)₂C­(R), Ar* = 2,6-bis(diphenylmethyl)-4-<i>tert</i>-butylphenyl, R = NC^<i>t</i>Bu₂) 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*]­FeN₃(py) (<b>3-py</b>) at 0 °C cleanly generates the Fe­(IV) nitride [L^Ar*]­FeN(py) (<b>1</b>). The ¹⁵N NMR spectrum of the <b>1</b>^{<b>¹⁵N</b>} (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 <b>1</b> 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 <b>1</b> shows a relatively small HOMO/LUMO gap with the LUMO comprised by Fe(d_{<i>xz</i>,<i>yz</i>})N(p_{<i>x</i>,<i>y</i>}) π*-orbitals, a splitting that is manifested in the electronic absorption spectrum of <b>1</b> (λ = 610 nm, ε = 1375 L·mol^–1·cm^–1; λ = 613 nm (calcd)). Complex <b>1</b> persists in low-temperature solutions of pyridine but becomes unstable at room temperature, gradually converting to the Fe(II) hydrazide product [κ²-(^<i>t</i>Bu₂CN)C(η⁶-NAr*)­(<i>N</i>-NAr*)]Fe (<b>4</b>) 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 <b>1</b> 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 [L^Ar*]Fe(NPMe₂Ph)(py) (<b>5</b>) and [L^Ar*]Fe(NCNAr)(py) (<b>6</b>). Similarly, <b>1</b> reacts with PhSiH₃ to give [L^Ar*]­Fe­[N­(H)­(SiH₂Ph)]­(py) (<b>7</b>) which Fukui analysis shows to proceed via electrophilic insertion of the nitride into the Si–H bond