Neutral and reduced Roussin's red salt ester [Fe₂(μ-RS)₂(NO)₄] (R = n-Pr, t-Bu, 6-methyl-2-pyridyl and 4,6-dimethyl-2-pyrimidyl): synthesis, X-ray crystal structures, spectroscopic, electrochemical and density functional theoretical investigations

Abstract

A series of Roussin's red salt esters [Fe 2 (μ-RS) 2 (NO) 4 ] (R = n-Pr (1), t-Bu (2), 6-methyl-2-pyridyl (3) and 4,6-dimethyl-2-pyrimidyl (4)) were synthesized by the reaction of Fe(NO) 2 (CO) 2 with thiols or thiolates. Complexes 1-4 were characterized by IR, UV-vis, 1 H-NMR, electrochemistry, and single-crystal X-ray diffraction analysis. The IR spectra of complexes 1-4 display one weak and two strong NO stretching frequencies (ν NO ) in solution, but only two strong ν NO in the solid. Density functional theoretical (DFT) calculations using complex 1 as model suggest that two spatial isomers of these complexes bear a 3 kcal energy difference in solution. Frequency calculations of the two isomers provide insight on the origin of the vibrational bands and explain the IR observation of complexes 1-4 in the solid state and in solution. Cyclic voltammetry shows two quasi-reversible, one-electron reductions for complexes 1-2 and one quasi-reversible, one-electron reduction for complexes 3-4. The paramagnetic complexes [Fe 2 (μ-RS) 2 (NO) 4 ] − (1 − -4 − ), which are prepared by the chemical reduction of neutral complexes [Fe 2 (μ-RS) 2 (NO) 4 ] (1-4), have also been investigated by EPR spectroscopy. Interestingly, the EPR spectra of complexes [Fe 2 (μ-RS) 2 (NO) 4 ] − (1 − -4 − ) exhibit an isotropic signal of g = 1.998-2.004 without hyperfine splitting in the temperature range 180-298 K. The observations are consistent with the results of the calculations, which reveal that the unpaired electron is dominantly delocalized over the two sulfur and two iron atoms. The difference of the g values between the reduced form of Roussin's red ester and the typical dinitrosyl iron complexes is explained, for the first time, by the difference in unpaired electron distributions between the two types of complexes, which provides the theoretical bases for the use of g values as a spectroscopic tool to differentiate these biologically active complexes.

Synthesis, Structures, Spectroscopic and Electrochemical Properties of Dinitrosyl Iron Complexes with Bipyridine, Terpyridine, and 1,10-Phenathroline

“…It has been demonstrated that the carbonyl groups of Fe(NO) 2 (CO) 2 can be easily replaced by phosphorus-containing or sulfur-containing ligands to produce the dinitrosyl complexes as reported in previous studies 13,18. In rare cases, it can be replaced by nitrogen-contained ligands 9,10.…”

Synthesis, characterization and crystal structure of a dinuclear iron nitrosyl complex with 2-mercapto-1-[2-(4-pyridyl)-ethyl]-benzimidazolyl

“…The characteristic nitrosyl stretching frequencies (ν NO, CH 2 Cl 2 1789 and 1732 cm −1 ) of complex 1 is similar to the reported values (ν NO, (THF) 1789 and 1736 cm −1 ) of DNICs with bidentate [N,S] ligands [7e]. When comparing with the dinuclear iron complexes [Fe 2 (μ-RS) 2 (NO) 4 ] (R = n-Pr, tBu, 6-methyl-2-pyridyl and 4,6-dimethyl-2-pyrimidyl) with [S,S] ligation [13] and cyclic tetra-nuclear iron complex [Fe(NO) 2 (Im-H)] 4 (Im = imidazole) with [N,N] ligation [7c] (Scheme 1), their IR spectra bear the same pattern but differ in the position (ν NO, (CH 2 Cl 2 ) 1789 and 1732 cm −1 for 1 vs 1797-1775 and 1763-1748 cm −1 for [Fe 2 (μ-RS) 2 (NO) 4 ] and 1801 and 1735 cm −1 for [Fe(NO) 2 (Im-H)] 4 ) and the separation of NO stretching frequencies Δν NO (~57 cm −1 for 1 vs ~29 cm −1 for [Fe 2 (μ-RS) 2 (NO) 4 ] and ~66 cm −1 for [Fe(NO) 2 (Im-H)] 4 . These results suggest that the position and Δν NO of IR spectrum may serve as an efficient tool for the discrimination of the existence of the neutral dinuclear iron nitrosyl complexes containing the variety of [N,N]/[N,S]/[S,S] ligations.…”

“…Two benzimidazolyl planes are almost parallel to each other along opposite direction and form an angle of ~113°with the 2Fe-2S plane. The Fe(1)-Fe(1a) distance of ca 4.14 Å shows that there is almost no interaction between the two iron centers, which is clearly differed from dinuclear iron complexes [Fe 2 (μ-RS) 2 (NO) 4 ] (ca 2.70 Å) [13]. The Fe-S and Fe-N(imidazole) bond distance for complex 1 is 2.3032(7) and 2.0222(16) Å, respectively, which is longer than the relevant Fe-S distance (ca 2.267 Å) in the [Fe 2 (μ-RS) 2 (NO) 4 ] [13] and Fe-N(imidazole) (ca 2.009 Å) in the [Fe(NO) 2 (Im-H)] 4 [7c].…”

Synthesis and structure of dinitrosyl iron complexes with secondary thiolate bridging ligands [Fe2(μ-SCHR2)2(NO)4], R = Me, Ph

“…Different synthetic ways are described for the dinuclear RRSE preparations: nitrosylation of Fe(II) and thiolate mixed solution with NO g [13]; S-alkylation of Roussin's red salt Na 2 [Fe 2 (l-S) 2 (NO) 4 4 ] are available with aliphatic [3,17], aromatic [18,19], peptide bound [20], and inorganic [21,22] substituents. Different ''aliphatic'' DNICs with primary [23,24] and tertiary [25,26] thiolate ligands are known, yet binuclear DNICs with secondary thiols (R 2 CH-SH) were not earlier structurally characterized by X-ray diffraction analysis. The n-propyl DNIC [Fe 2 (l-Sn Pr) 2 (NO) 4 ] was synthesized and its structure was determined by XRD crystallography to be centrosymmetric by Ref.…”

“…The n-propyl DNIC [Fe 2 (l-Sn Pr) 2 (NO) 4 ] was synthesized and its structure was determined by XRD crystallography to be centrosymmetric by Ref. [23]. A complex with benzyl substituent at the sulfur bridging atom was described by Lippard and co-workers [19] and with the ethyl substituent, by Lu et al [27].…”