The relevance of low-valent iron complexes bearing sulfur-containing ligands with the reactive iron intermediates in enzymatic dinitrogen fixation has stimulated great synthetic efforts toward them. Low-valent iron thiolate complexes have been mostly known for the thiolate-bridged diiron(I) carbonyl compounds. In contrast, isolable mononuclear iron(I) thiolate complexes are exceedingly rare. Herein, we report the synthesis and characterization of a mononuclear iron(I) complex [(IMes)Fe(SAr*)] (1, IMes=1,3-bis(mesityl)imidazole-2-ylidene, Ar*=2,6-(2',4',6'-Prⁱ₃C₆H₂)₂C₆H₃) as well as its catalytic performance in dinitrogen silylation reaction. Complex 1 was synthesized from the reaction of (IMes)₂FeBr with KSAr* in Et₂O in 81% isolated yield. Single-crystal X-ray diffraction studies revealed that the thiolate ligand in 1 is coordinating to the iron center in an σ:η⁶-fashion via the sulfur atom and one of the flanking Prⁱ₃C₆H₂ ring with the Fe—S distance of 0.2228(1) nm and the Fe—C(Trip) distances ranging from 0.2076(2) to 0.2205(2) nm. Zero-field ⁵⁷Fe Mössbauer spectrum at 80 K (δ=0.56 mm•s^-1; ΔE_Q=0.98 mm•s^-1) and X-band electron paramagnetic resonance data (g=[2.20, 2.03, 1.99]) at 103 K of 1 point to its low-spin iron(I) nature (S=1/2). These data are comparable with those of Holland's iron(I) thiolate complex K[(2-S-1,3-(6-F-3-(2,4,6-Prⁱ₃C₆H₂)C₆H₂)₂C₆H₄)Fe] (Nature 2015, 526, 96). Density functional theory studies support the S=1/2 ground spin-state for 1 and reveal that the interaction between the iron atom and the sulfur atom is essentially σ-bonding in character. In addition, π- and δ-type interactions between the iron atom and the η⁶-Prⁱ₃C₆H₂ ring in 1 also exist. Complex 1 can catalyze the reaction of N₂ with KC₈ and Me₃SiCl in Et₂O at room temperature, affording N(SiMe₃)₃ with a turn-over number up to 73 in 24 h and 87 in 48 h. The reaction employing (IMes)₂FeBr as catalyst has a turn-over number up to 73 in 24 h and elongating the reaction time to 48 h does not lead to further increase of the turn-over number. The fine catalytic performance with 1 over (IMes)₂FeBr implies that the thiolate ligand in 1 might render the catalytically active intermediate a relatively long lifetime.

Key words: N-heterocyclic carbene, iron, sulfur ligand, N₂ activation, low valent