New Members of a Class of Dinitrosyliron Complexes (DNICs): Interconversion and Spectroscopic Discrimination of the Anionic {Fe(NO)₂}⁹ [(NO)₂Fe(C₃H₃N₂)₂]⁻ and [(NO)₂Fe(C₃H₃N₂)(SR)]⁻ (C₃H₃N₂ = Deprotonated Imidazole; R = <i>^t</i>Bu, Et, Ph)

The anionic {Fe(NO)2}9 DNIC [(NO)2Fe(C3H3N2)2]− (2) (C3H3N2 = deprotonated imidazole) containing the deprotonated imidazole-coordinated ligands and DNICs [(NO)2Fe(C3H3N2)(SR)]− (R = tBu (3), Et (4), Ph (5)) containing the mixed deprotonated imidazole−thiolate coordinated ligands, respectively, were synthesized by thiol protonation or thiolate(s) ligand-exchange reaction. The anionic {Fe(NO)2}9 DNICs 2−5 were characterized by IR, UV–vis, EPR, and single-crystal X-ray diffraction. The facile transformation among the anionic {Fe(NO)2}9 DNICs 2−5 and [(NO)2Fe(StBu)2]−/[(NO)2Fe(SEt)2]−/[(NO)2Fe(SPh)2]− was demonstrated in this systematic study. Of importance, the distinct electron-donating ability of thiolates serve to regulate the stability of the anionic {Fe(NO)2}9 DNICs and the ligand-substitution reactions of DNICs. At 298 K, DNIC 2 exhibits the nine-line EPR signal with g = 2.027 (aN(NO) = 2.20 and aN(Im-H) = 3.15 G; Im-H = deprotonated imidazole) and DNIC 3 displays the nine-line signals with g = 2.027 (aN(NO) = 2.35 and aN(Im-H) = 4.10 G). Interestingly, the EPR spectrum of complex 4 exhibits a well-resolved 11-line pattern with g = 2.027 (aN(NO) = 2.50, aN(Im-H) = 4.10 G, and aH = 1.55 G) at 298 K. The EPR spectra (the pattern of hyperfine splitting) in combination with IR νNO spectra (ΔνNO = the separation of NO stretching frequencies, ΔνNO = ∼62 cm−1 for 2 vs ∼50 cm−1 for 3−5 vs ∼43 cm−1 for [(NO)2Fe(StBu)2]−/[(NO)2Fe(SEt)2]−/[(NO)2Fe(SPh)2]−) may serve as an efficient tool for the discrimination of the existence of the anionic {Fe(NO)2}9 DNICs containing the different ligations [N,N]/[N,S]/[S,S].