Insight into the dinuclear {Fe(NO) 2} 10{Fe(NO) 2} 10 and mononuclear {Fe(NO) 2} 10 dinitrosyliron complexes

The reversible redox transformations [(NO) 2Fe(S tBu) 2] - ⇌ [Fe(μ-S tBu)(NO) 2] 2 2- ⇌ [Fe(μ-S tBu)(NO) 2] 2 - ⇌ [Fe(μ-S tBu)(NO) 2] 2 and [cation][(NO) 2Fe(SEt) 2] ⇌ [cation] 2[(NO) 2Fe(SEt) 2] (cation = K +-18-crown-6 ether) are demonstrated. The countercation of the {Fe(NO) 2} 9 dinitrosyliron complexes (DNICs) functions to control the formation of the {Fe(NO) 2} 10{Fe(NO) 2} 10 dianionic reduced Roussins red ester (RRE) [PPN] 2[Fe(μ-SR)(NO) 2] 2 or the {Fe(NO) 2} 10 dianionic reduced monomeric DNIC [K +-18-crown-6 ether] 2[(NO) 2Fe(SR) 2] upon reduction of the {Fe(NO) 2} 9 DNICs [cation][(NO) 2Fe(SR) 2] (cation = PPN +, K +-18- crown-6 ether; R = alkyl). The binding preference of ligands [OPh] -/[SR] - toward the {Fe(NO) 2} 10{Fe(NO) 2} 10 motif of dianionic reduced RRE follows the ligand-displacement series [SR] - > [OPh] -. Compared to the Fe K-edge preedge energy falling within the range of 7113.6-7113.8 eV for the dinuclear {Fe(NO) 2} 9{Fe(NO) 2} 9 DNICs and 7113.4-7113.8 eV for the mononuclear {Fe(NO) 2} 9 DNICs, the {Fe(NO) 2} 10 dianionic reduced monomeric DNICs and the {Fe(NO) 2} 10{Fe(NO) 2} 10 dianionic reduced RREs containing S/O/N-ligation modes display the characteristic preedge energy 7113.1-7113.3 eV, which may be adopted to probe the formation of the EPR-silent {Fe(NO) 2} 10-{Fe(NO) 2} 10 dianionic reduced RREs and {Fe(NO) 2} 10 dianionic reduced monomeric DNICs in biology. In addition to the characteristic Fe/S K-edge preedge energy, the IR v NO spectra may also be adopted to characterize and discriminate [(NO) 2Fe(μ-S tBu)] 2 [IR v NO 1809 vw, 1778 s, 1753 s cm -1 (KBr)], [Fe(μ-S tBu)(NO) 2] 2 - [IR v NO 1674 s, 1651 s cm -1 (KBr)], [Fe(μ-S tBu)(NO) 2] 2 2- [IR v NO 1637 m, 1613 s, 1578 s, 1567 s cm -1 (KBr)], and [K-18-crown-6 ether] 2[(NO) 2Fe(SEt) 2] [IR v NO 1604 s, 1560 s cm -1 (KBr)].