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)(2)Fe(S(t)Bu)(2)](-) ? Fe(μ-S(t)Bu)(NO)(2)](2)(2-) ? Fe(μ-S(t)Bu)(NO)(2)](2)(-) ? Fe(μ-S(t)Bu)(NO)(2)](2) and cation](NO)(2)Fe(SEt)(2)] ? cation](2)(NO)(2)Fe(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 Roussin's 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)(2)Fe(SR)(2)] upon reduction of the {Fe(NO)(2)}(9) DNICs cation](NO)(2)Fe(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 ν(NO) spectra may also be adopted to characterize and discriminate (NO)(2)Fe(μ-S(t)Bu)](2) IR ν(NO) 1809 vw, 1778 s, 1753 s cm(-1) (KBr)], Fe(μ-S(t)Bu)(NO)(2)](2)(-) IR ν(NO) 1674 s, 1651 s cm(-1) (KBr)], Fe(μ-S(t)Bu)(NO)(2)](2)(2-) IR ν(NO) 1637 m, 1613 s, 1578 s, 1567 s cm(-1) (KBr)], and K-18-crown-6 ether](2)(NO)(2)Fe(SEt)(2)] IR ν(NO) 1604 s, 1560 s cm(-1) (KBr)].