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] (L = (ArN)C(R), Ar = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl, R = NCBu) 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]FeN(py) (3-py) at 0 °C cleanly generates the Fe(IV) nitride [L]FeN(py) (1). The N NMR spectrum of the 1 (50% Fe≡N) isotopomer shows a resonance at 1016 ppm (vs externally referenced CHNO 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 = 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)N(p) π*-orbitals, a splitting that is manifested in the electronic absorption spectrum of 1 (λ = 610 nm, ε = 1375 L·mol·cm; 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-(BuCN)C(η-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 PMePh and Ar-N≡C (Ar = 2,6-dimethylphenyl) leads to partial N-atom transfer and formation of the Fe(II) addition products [L]Fe(N═PMePh)(py) (5) and [L]Fe(N═C═NAr)(py) (6). Similarly, 1 reacts with PhSiH to give [L]FeN(H)(SiHPh) (7) which Fukui analysis shows to proceed via electrophilic insertion of the nitride into the Si-H bond.