A New Hexanuclear Iron-Selenium Nitrosyl Cluster: Primary Exploration of the Preparation Methods Structure Spectroscopic and Electrochemical Properties

摘要

A new hexanuclear iron-selenium nitrosyl cluster, [(n-Bu)4N]2[Fe6Se6(NO)6] (>1) and a hexanuclear iron-sulfur nitrosyl cluster, [(n-Bu)4N]2[Fe6S6(NO)6] (>2) were synthesized by the solvent-thermal reactions of [(n-Bu)4N][Fe(CO)3NO] with selenium or sulfur in methanol, while a tetranuclear iron-sulfur nitrosyl cluster, (Me4N)[Fe4S3(NO)7] (>3) was also prepared by the solvent-thermal reaction of FeCl₂·4H₂O with thiourea in the presence of (CH₃)₄NCl, NaNO₂ and methanol. Complexes >1-3 were characterized by IR, UV-vis, ¹H-NMR, electrochemistry, and single crystal X-ray diffraction analysis. IR spectra of complexes >1 and >2 show the characteristic NO stretching frequencies at 1694 and 1698 cm⁻¹, respectively, while the absorptions of complex >3 appear at 1799, 1744 and 1710 cm⁻¹. The UV-vis spectra of complexes >1->3 show different bands in the range of 259-562 nm, which are assigned to the transitions between orbitals delocalized over the Fe-S cluster, the ligand to metal charge transfer (LMCT), π*_NO-d_Fe, and the metal to ligand charge transfer (MLCT), d_Fe-π*_NO. Single-crystal X-ray structural analysis reveals that complex >1 crystallizes in monoclinic P2(1) space group with two molecules per unit cell. Two parallel “chair-shape” structures, consisting of three iron and three selenium atoms, are connected by Fe-Se bonds with an average distance of 2.341 Å; each iron center is bonded to three selenium atoms and a nitrogen atom from nitrosyl ligand with pseudotetrahedral center geometry. Cyclic voltammograms of complexes >1 and >2 display two cathodic and three anodic current peaks with an unusually strong cathodic peak. Further electrochemical investigations demonstrated that the intensity of the unusually strong peak is a result of at least three processes. One is the quasi-reversible reduction and the other two are from an irreversible electrochemical process, in which the compound goes through a typical electron transfer and chemical reaction (ECE) mechanism. Compound >3 shows three quasi-reversible reductions.