Iron(III) Bound by Hydrosulfide Anion Ligands: NO-Promoted Stabilization of the [FeIII–SH] Motif

Spontaneous transformation of the thermally stable [HS]−-bound {Fe­(NO)2}9 dinitrosyl iron complex (DNIC) [(HS)2Fe­(NO)2]− (1) into [(NO)2Fe­(μ-S)]22– (Roussin’s red salt (RRS)) along with release of H2S, probed by NBD-SCN (NBD = nitrobenzofurazan), was observed when DNIC 1 was dissolved in water at ambient temperature. The reversible transformation of RRS into DNIC 1 (RRS → DNIC 1) in the presence of H2S was demonstrated. In contrast, the thermally unstable hydrosulfide-containing mononitrosyl iron complex (MNIC) [(HS)3FeIII(NO)]− (3) and [FeIII(SH)4]− (5) in THF/DMF spontaneously dimerized into the first structurally characterized FeIII–hydrosulfide complexes [(NO)­(SH)­Fe­(μ-S)]22– (4) with two {Fe­(NO)}7 motifs antiferromagnetically coupled and [(SH)2Fe­(μ-S)]22– (6) resulting from two FeIII (S = 5/2) centers antiferromagnetically coupled to yield an S = 0 ground state with thermal occupancy of higher spin states, respectively. That is, the greater the number of NO ligands bound to [2Fe2S], the larger the antiferromagnetic coupling constant. On the basis of DFT computation and the experimental (and calculated) reduction potential (E1/2) of complexes 1, 3, and 5, the NO-coordinate ligand(s) of complexes 1 and 3 serves as the stronger electron-donating ligand, compared to thiolate, to reduce the effective nuclear charge (Zeff) of the iron center and prevent DNIC 1 from dimerization in an organic solvent (MeCN).