Abstract
The reversible redox transformations [(NO)2Fe(StBu)2]– [Fe(-StBu)(NO)2]22– (2) [Fe(-StBu)(NO)2]2– [Fe(-StBu)(NO)2]2 are demonstrated. 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]–, rationalizing that most of the DNICs and RREs characterized nowadays are bound to protein via cysteinate side chains. Compared to the Fe K-edge pre-edge 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 reduced DNICs and the {Fe(NO)2}10-{Fe(NO)2}10 dianionic reduced RREs containing S-/O-/N-ligation modes display the characteristic pre-edge 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 pre-edge energy, the IR νNO spectra may also be adopted to characterize and discriminate [(NO)2Fe(-StBu)]2 (IR νNO 1809 vw, 1778 s, 1753 s cm-1 (KBr)), [Fe(-StBu)(NO)2]2– (IR νNO 1674 s, 1651 s cm-1 (KBr)), and [Fe(-StBu)(NO)2]22– (IR νNO 1637 m, 1613 s, 1578 s, 1567 s cm-1 (KBr)). Additionally, The fluxional terminal and semibridging NO-coordinate ligands of DNIC [Fe4(-S)2(-NO)2(NO)6]2– (3), a precursor of Roussin’s black salt (RBS), are characterized by IR NO), 15N(NO) NMR, single-crystal X-ray diffraction, and DFT calculations. Compared to the {Fe(NO)2}9 and {Fe(NO)2}10 DNICs/RREs displaying 15N (NO) NMR chemical shift (23 ~ 76 ppm) and (-7.8 ~ 25 ppm), respectively, the first semibridging nitroxyl of complex 3 exhibits the distinct 15N (NO) NMR chemical shift (200.8 and 200.1 ppm), suggesting the 15N (NO) NMR technique can serve as an efficient tool to discriminate the binding fashions of NO. In the last part, the stable {Fe(NO)2}10 reduced DNICs [(NO)2Fe(S(CH2)nS)]2– [ n = 3 (4); n = 2 (5) ] containing chelate dithiolate were synthesized. On the basis of the electrochemistry, and DFT calculations of 4 and 5, the bite angle or ring strain inherent in the chelate-dithiolate-bound DNICs will function to tune the Fe-S bonding level, so as to modulate the configurations and electrochemical properties of DNICs (E1/2 = -1.64 V for 4 and E1/2 = -1.33 V for 5). The dithiolate ligands are also introduced to build [(NO)2Fe(-2-SC2H4S)(-NO)Fe(NO)]2– (7) bearing a bridging NO as well as the fluxional isomers of [(NO)2Fe(-2-SC3H6S)(-NO)Fe(NO)]2– (8-a) and [(NO)2Fe(-SC3H6S)Fe(NO)2]2– (8-b). Intriguingly, the electrochemical studies demonstrate that complex 7, resembling to the calculated double-reduced intermediate [(CO)3Fe(-2-edt)(-CO)Fe(CO)2]2– of [FeFe]-H2ase model, act as an active molecular electrocatalyst for proton reduction from weak acid with low overpotential.