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

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, π*NO−dFe, and the metal-to-ligand charge transfer, dFe−π*NO. Single-crystal X-ray structural analysis reveals that complex 1 crystallizes in the monoclinic P2(1)/n space group with two molecules per unit cell. Two parallel “chair-shaped” 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 the nitrosyl ligand with a 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 mechanism. Compound 3 shows three quasi-reversible reductions.