Abstract
Addition of the stronger □-donating and □-accepting phosphine ligands, PPh3, PPh3-3-SO3Na or Ph2P(CH2)5PPh2, triggered the reductive elimination of bridged thiolates of RRE [Fe(μ-SC6H4-o-COOH)(NO)2]2 (1) to yield the EPR-silent neutral {Fe(NO)2}10 [(PPh3)2Fe(NO)2] (2), the water-soluble {Fe(NO)2}10 [(PPh3-3-SO3Na)2Fe(NO)2] (4), and the dimeric {Fe(NO)2}10-{Fe(NO)2}10 DNICs [Fe2(-μ-PPh2(CH2)5PPh2-)2(NO)4] (3), respectively. Complex 2-4 were characterized by IR, UV-vis, and single-crystal X-ray diffraction. Oxidation of complex 3 by 2 equiv of [NO][BF4] resulted in the formation of EPR-active cationic {Fe(NO)2}9-{Fe(NO)2}9 dimeric complex [Fe2(-μ-PPh2(CH2)5PPh2-)2(NO)4][BF4]2 (32+). Reaction of complex 32+ with thiolates ([PPN][SC7H4SN], [PPN][SPh] or [PPN][SEt]) yields anionic {Fe(NO)2}9 complex [PPN][(SC7H4SN)2Fe(NO)2] (6) and neutral {Fe(NO)2}10 complex 3. These results unambiguously illustrate one aspect of how the reducing ability and the binding mode of the thiolate ligands functions to regulate the conversion of cationic {Fe(NO)2}9 into anionic {Fe(NO)2}9 or neutral {Fe(NO)2}10 DNICs upon the reaction of complex 32+ and thiolates. Dimeric {Fe(NO)2}9 dinitrosyl iron complex (DNIC) [Fe(μ-SC7H4SN)(NO)2]2 (5) with S and N atoms of the anionic [-SC7H4SN-]– ligand bound to two separate {Fe(NO)2}9 cores, respectively, shows the Fe…Fe distance of 4.0 Å. A straightforward reaction of complex 5 and nucleophiles (PMe3, imidazole, [PhCOO]-, [OPh]-, [SC7H4SN]- or [N3]-) led to the EPR-active {Fe(NO)2}9 DNICs in THF. In addition, the precursor [(CO)2Fe(NO)2][BF4] (12) provided a new method for synthesizing several kinds of DNICs with different ligations, such as [(SC5H5N)2Fe(NO)2]- (13), [(NO2)2Fe(NO)2]- (15), [(NO3)2Fe(NO)2]- and [(OPh)2Fe(NO)2]- (16). All these {Fe(NO)2}9 DNICs can be classified into the anionic {Fe(NO)2}9 DNICs with S/N/O ligation, the neutral {Fe(NO)2}9 DNIC with one thiolate and one neutral imidazole ligation, and the cationic {Fe(NO)2}9 DNICs with the neutral P-containing coordinated ligands. Moreover, the biotinylated ligand, N,N'-[dithiobis(4,1-phenylene)]bis{5-[(3aS,6aR)-2-oxohexa-hydro-1H-thieno[3,4-d]imidazol-4-yl]pentanamide} ([(SC6H4-o-NHCO(CH2)4Biotin)2]; Ligand 1) and biotinylated RRE [Fe(μ-SC6H4-o-NHCO(CH2)4Biotin)(NO)2]2 (17) were synthesized and identified by NMR, IR, UV-vis. EA and ESI-MS. The HABA assay shows that the average 3.19 (Ligand 1) and 3.64 (complex 17) biotinylated parts bind to avidin. The water-soluble iron-thiolate NO trapping agent/scavenger [Na•3THF][Fe(SC6H4-o-S)2]2 (18) was synthesized and characterized. Reaction of complex 18 and [PPN][Cl] resulted in the formation of [PPN]2[Fe(SC6H4-o-S)2]2 (20) via cation exchange. Nitrosylation and the subsequent sulfur oxygenation of complex 20 yielded the dimeric disulfinate {Fe(NO)}6 complex [PPN]2[(NO)Fe(SO2C6H4-o-SO2)(μ-SC6H4-o-S)]2 (25). Reduction of complex 25 by [PPN][SC6H4-o-NH2] resulted in the formation of {Fe(NO)}7 complex [PPN]2[(NO)Fe(SO2C6H4-o-SO2)(SC6H4-o-S)] (26) with a bent Fe-N-O bond angle of 165.7o. Complex 20 was adopted as the precursor to synthesize complexes [PPN][(Cl)Fe(S-C6H4-o-S)(S,SCN(CH3)2)] (27), [(NO)Fe(S-C6H4-o-S)(S,SCN(CH3)2)] (30) and [PPN][(NO)Fe(S-C6H4-o-S)(S, SCN(CH3)2)] (31) with mixed [S-C6H4-o-S]2- and [S,SCN(CH3)2]- coordinated ligands. Nitrosylation of complex 27 led to the formation of complex 30. Photolysis of complex 30 in the presence of [PPN][Cl] led to the formation of complex 27. Reduction of {Fe(NO)}6 complex 30 by [PPN][SPh] resulted in the formation of {Fe(NO)}7 complex 31 with a bent Fe-N-O bond angle of 150.8o. Upon addition of O2 into a CH2Cl2 solution of complex 30 or 31, respectively, the yellow decomposition occurred for complex 30, while, complex 30 was reobtained from O2 oxidation of complex 31. Obviously, the deficient electronic density surrounding the iron center resulting form the less electron-donating anionic dithiocarbamate ligand retards the sulfur oxygenation of 1,2-benzenedithiolate to yield the iron-thiolate sulfinate nitrosyl complex. Moreover, the solvent dependent complexes 18 and 20 can convert acetonenitrile to acetamide in the presence of NaOH. This result implicates that the substrate nitrile coordinating to the catalytic metal center of active site is a key step of nitrile hydrolysis to amide. In the animal model studies, the aqueous solution of complex 18 was injected to the right femoral vein as the NO trapping agent/scavenger to male Wistar rats, which increased the blood pressure around 10 mmHg. The neutral trinuclear iron-thiolate nitrosyl, [(ON)Fe(μ-S,S-C6H4)]3 (23), and its oxidation product, [(ON)Fe(μ-S,S-C6H4)]3[PF6] (24), were synthesized and characterized by IR, X-ray diffraction, X-ray absorption, EPR and magnetic measurement. The five-coordinated square pyramidal geometry around each iron atom in complex 23 remains intact when complex 23 is oxidized to yield complex 24. Magnetic measurements and EPR results show that there is only one unpaired electron in complex 23 (Stotal = 1/2) and no unpaired electron (Stotal = 0) in 24. The detailed geometric comparisons between complexes 23 and 24 provide the understanding of the role that the unpaired electron plays in the chemical bonding of this trinuclear complex. Significant shortening of the Fe-Fe, Fe-N and Fe-S distances around Fe(1) is observed when complex 23 is oxidized to 24. This result implicates that the removal of the unpaired electron does induce the strengthening of the Fe-Fe, Fe-N and Fe-S bonds in Fe(1) fragment. A significant shift of the □NO stretching frequency from 1751 cm-1 (23) to 1821, 1857 cm-1 (24) (KBr) also indicates the strengthening of the N-O bonds in complex 24. The EPR, X-ray absorption and magnetic measurements lead to the conclusion that the unpaired electron in complex 23 is mainly allocated in Fe(1) fragment and was best described as {Fe(1)NO}7.