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)2 C(R), Ar* = 2,6-bis(diphenylmethyl)-4-tert-butylphenyl, R = NCt 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 [LAr* ]FeN3 (py) (3-py) at 0 °C cleanly generates the Fe(IV) nitride [LAr* ]FeN(py) (1). The 15 N NMR spectrum of the 115 N (50% Fe≡15 N) isotopomer shows a resonance at 1016 ppm (vs externally referenced CH3 NO2 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 -(t Bu2 CN)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 PMe2 Ph 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═PMe2 Ph)(py) (5) and [LAr* ]Fe(N═C═NAr)(py) (6). Similarly, 1 reacts with PhSiH3 to give [LAr* ]Fe[N(H)(SiH2 Ph)](py) (7) which Fukui analysis shows to proceed via electrophilic insertion of the nitride into the Si-H bond.